Is starch and glycogen digestion intra or extracellular?

Do humans have the enzyme for starch intracellular digestion? Also, do plants have the ability to digest Glycogen? Intra or extracellular, or both?

The problem with your question is that, right now, you have three different questions:

  1. Is the starch/glycogen digestion intra or extracellular?
  2. Do humans have the enzyme for intracellular digestion of starch?
  3. Do plants have the ability to digest glycogen?

Unfortunately, that goes against Bio SE rules: you have to pick one question, otherwise your post will be closed as too broad ("Avoid asking multiple distinct questions at once.").

That being said, I'll address only this question:

Do humans have the enzyme for intracellular digestion of starch?

The answer is yes. Intracellular digestion in humans (as in any animal) is performed by lysosomes, organelles that contain hydrolytic enzymes.

Lysosomes can digest proteins, lipids, carbohydrates, nucleic acids etc. Regarding digestion of carbohydrates, this is a short list of enzymes (the complete list is way bigger) found in human lysosomes:

  • alpha-Galactosidase
  • alpha-L-Fucosidase beta-Galactosidase-1
  • beta-Glucuronidase Chitinase
  • Chondroitin B Lyase/Chondroitinase B
  • Chondroitinase
  • Cytosolic beta-Glucosidase
  • Galactosylceramidase
  • Heparanase
  • Hyaluronidase
  • Lysosomal alpha-Glucosidase
  • O-GlcNAcase
  • O-Glycosidase

Among the enzymes listed above, this is the one that answers your question: Lysosomal alpha-Glucosidase.

That enzyme, which is a α-1,4-glucosidase, breaks down starch (and glycogen) to glucose.


  • Lübke, T., Lobel, P. and Sleat, D. (2009). Proteomics of the lysosome. Biochimica et Biophysica Acta (BBA) - Molecular Cell Research, 1793(4), pp.625-635..


Starch digesting enzymes are extracellular in human beings.All starches and some glycogen are hydrolysed in the mouth by the time the food is swallowed due to the effect of ptyalin(alpha amylase)enzyme in our saliva.


Pancreatic juice containing pancreatic amylase promotes extracellular starch digestion.And almost all the starch in food is completely hydrolysed to maltose, limit dextrin and isomaltose in its effect.

The same enzyme pancreatic amylase also causes the total extracellular digestion of glycogen in the duodenum and forms maltose and other products like limit dextrin, isomaltose.


Products like limit dextrin and isomaltose is digested extacellularly in the small intestine by the enzyme isomaltase and maltose by maltase in intestinal saccus entericus to form glucose.


The principal glycogen storing organ in human is the liver and glycogen formation or hydrolysis in the organ is 'intracellular'.


Similarly in starch storing organs in plants the hydrolysis or formation is 'intracellular'.


11 Carbohydrates

Carbohydrates are macromolecules with which most consumers are somewhat familiar. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have sufficient energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar. Carbohydrates also have other important functions in humans, animals, and plants.

Figure 1 Bread, pasta, and sugar all contain high levels of carbohydrates. (“Wheat products” by US Department of Agriculture is in the Public Domain)

Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.

Answer of Question of Nutrition & Digestion

(2) extracellular digestion, and (3) intracellular digestion. A resting stage is preparative for extracellular digestion. Feeding: Bivalve molluscs are mostly suspension and filter feeders and ingest small food particles. The gills trap food particles brought into the mantle cavity along with the incurrent water. The food trapping is

unclear, but once food particles are trapped, cilia move them to the gill’s ventral margin. Cilia along the ventral margin of the gills then move food toward the mouth. Cilia covering leaf like labial palps on either side of the mouth also sort filtered food particles. Cilia carry small particles into mouth. Digestion and Absorption: The digestive
tract has a short esophagus opening into a stomach. As food enters the stomach, the
rotating crystalline style and the enzymes released by the gastric shield (both present in stomach) mechanically and enzymatically break it down. the small food particles move into the

digestive diverticulae for intracellular digestion. These diverticulae in the stomach increase the surface area for absorption and intracellular digestion. Intracellular digestion releases the nutrients into the blood and produces the fragmentation spherules that both excrete wastes and lower the pH for optimal extracellular digestion. Sorting of food separates fine particles from indigestible coarse materials which are sent on to the intestine. Partially digested food from the stomach enters a digestive gland for intracellular digestion. Cilia carry indigested wastes in the digestive gland back to the stomach and then to the intestine. There is no marked division of gut into midgut and hind gut. The intestine empties through the anus near the excurrent siphon.

Q.14. How do heterotrophic protozoa feed?

Ans. Ciliated protozoa utilize heterotrophic nutrition. Ciliary Buccal cavity action directs the food from the Cytostome environment into the buccal Food vacuole cavity and cytostome. The cytostome opens into the cytopharynx cytopharynx, which enlarges as leaves food food enters and pinches off a food containing vacuole. The detached food vacuole moves through the cytoplasm. During this movement excess water moves out of the vacuole by exosmosis, the contents are acidified and then made alkaline and lysosomes add digestive enzymes. The food particles are digested within the vacuole (intracellular digestion) in cytoplasm. The residual vacuole then excretes its wastes via the cytopyge/ cytoproct. Fig. 5.4.

Q.14a. Give an aceount of digestion in cnidarians I Hydra, and a planarian.

Ans. Cnidarians have a sac like digestive gut i.e., the gut is a closed sac called a gastrovascular cavity, Fig. 5.5(a). It has only one opening that is both entrance for food, water, and exit for wastes. Some specialized cells in the body wall lining the cavity secrete digestive enzymes that begin the extracellular digestion. Other phagocytic cells that line the cavity engulf food materials and continue intracellular digestion inside the food vacuoles. Flatworms, such as planarians have similar patterns, however the digestive gut is branched providing more surface area.

In planaria there is a gastrovascular cavity which is extensively branched. It is also an incomplete digestive tract with only one opening. When a planarian feeds, its sticks its muscular pharynx out of its mouth and sucks in food. The gastrovascular cavity is branched so as to increase the absorptive surface area. The cavity is saclike Pharyngeal glands secrete enzymes. The food is partly digested extracellularly and digested food is absorbed in cells lining the cavity. In the digestive cavity, phagocytic cells engulf small food particles, and digestion is completed in intracellular vesicles..

.15. Describe process of digestion In an insect.

Ans. Insects, such as a grasshopper have a complete digestive tract, and the digestion is extracellular. Mandibles and maxillae cut and masticate the food (leaves) mixed with saliva from salivary glands, which is taken into the mouth and passed to the crop via esophagus. Saliva lubricates the food, and enzyme amylase in it begins the digestion of carbohydrates. In the crop food is stored temporarily where digestion continues. Enzymes, carbohydrases, lipases, proteases secreted from midget enter-the crop. From the crop food passes to the stomach, where it is mechanically grinded and nutrients are stored. Large particles are returned to crop for reprocessing, small particles enter the gastric cecae, where extracellular digestion is completed. Absorption then occurs in the intestine. Undigested food then passes to rectum, where water and ions are absorbed. Solid fecal pellets then pass out of the body via the anus. Fig. 5.5

Q.16. How do vertebrate teeth reflect feeding habits of various animals?

Ans. In vertebrates teeth are specialized according to the food and feeding habits of animals. The teeth of sharks and snakes, for example slope backwards to aid in the retention of prey while swallowing. Carnivores, such as members of the dog and cat families, generally have pointed incisors and canines that can be used to kill prey and rip away pieces of flesh. The jagged premolars and molars are modified for crushing and shredding, In contrast, herbivorous mammals, such as horses deer and cows, usually have teeth with broad, ridged surfaces that work like millstones for grinding tough plant material. The incisors and canines are generally modified for biting off pieces of vegetation. Humans, being omnivores adapted for eating both vegetation and meat, have a relatively unspecialized dentition. The permanent (adult) set of teeth is 32 in number. Beginning at the midline of the upper and lower jaw are two blade like incisors for biting, a pointed canine for tearing, two premolars for grinding, and three molars. 1 2 3 for crushing. The dental formula of man is

Beavers have front teeth chisel-like to cut branches and stems of trees. Elephants have two upper front teeth (tusks) specialized as weapons and for moving objects. Carnivores, such as cats have camassial teeth.for shearing the flesh. Fig.5.6.

Q.17. How does stomach of a ruminant function?

Ans. Ruminant mammals, such as cows, sheep, and deer show some of the most unusual modifications of the stomach for storing large amounts of food which they chew later, and providing an opportunity for large number of microorganisms which digest cellulose thus compensating disability of animals to digest it. The upper portion of the stomach expands to form a large pouch, the rumen, and a smaller reticulum. The lower portion of the stomach consists of a small antechamber, the omasum, followed by abomasum which is the true stomach (contains cardia, pylorus and fundus mucosa). Food first enters the rumen which secretes copious fluid into it and churns the food.

Microorganisms, symbiotic bacteria breakdown cellulose and release fatty acids as by-products of their metabolism. This food enters the reticulum the animal periodically regurgitates and rechews the cud. which further breaks down the fibres, making them more accessible to further bacterial action. Later the pulpy mass moves into the reticulum and passed to omasum where water is removed. The cud containing great numbers of bacteria finally passes to the abomasum where digestive enzymes of the animal continue digestive process. Fig. 5.7.

Q.18. Name the component parts of the mammalian gastrointestinal tract. Name the accessory structures concerned with digestion.

Ans. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts. Following is a summary of primary and accessory organs in a human body. Fig.5.8.

Primary Organs of Digestion in Man

  1. Mouth an opening for ingress into alimentary canal, guarded by muscular lips to prevent escape of food, leads into buccal cavity.
  2. In buccal cavity, the teeth grind the food. Food is tasted, moistened, and lubricated by mixing with saliva. Posteriorly buccal cavity leads into pharynx. The tongue contain taste buds and it also help push food back into pharynx.
  3. Pharynx is the intersection that leads to both the esophagus and the windpipe (trachea). A flap-like structure, the epiglottis closes the entrance to trachea when food is swallowed.
  4. Esophagus conducts food from pharynx to the stomach.
    1. Stomach is located on the left side of the abdominal cavity, just below the diaphragm. Digestion of proteins and churning of food occurs here.
    2. Small intestine, a long tube, the first 25 cm of it is called duodenum where bile from gall bladder and pancreatic juice from pancrease are added to acidic food, the remaining part is called jejunum and ileum. Digestion and much of the absorption of food occurs in small intestine.
    3. Large intestine or colon, connected to the small intestine at a T-oshaped junction, is a wider tube where some D absorption of salts and water takes place. It opens into rectum.
    4. opens into rectum. Rectum, here feces are stored until they can be eliminated.
    1. Liver , a large brownish organ synthesizes bile salts and bicarbonates. which are Stored in gall bladder.

    Q.19. How peristalsis and segmentation differ?

    Ans. Two types of rhythmic or coordinated muscular
    movements i.e., peristalsis and segmentation mix the food material with various
    secretions and move the food from oral cavity to anus. In peristalsis, rings of circular muscles contract behind a mass of food material, and the mechanical pressure propels the material forward. As it moves, the mass expands the tube wall the expansion stimulates peristalsis, and moves down the tract in a wave-like manner. Segmentation is another type of rhythmic muscular contractions which are oscillating back-and-forth movements in the same place in small and large intestine. This movement mixes the food with digestive secretions and increases the efficiency of absorption.

    Q 20. How is gastrointestinal motility controlled?

    Ans. The gastrointestinal tract is innervated with a network of nerves in sub mucosa, and longitudinal and circular muscle layers. The nerves receive information from chemoreceptors, which respond to materials in gut i.e., carbohydrates, lipids, proteins and mechanoreceptors, which respond to distension of the walls. Sympathetic and parasympathetic nerves in gut walls work antagonistically in controlling peristalsis and segmentation. Signals from parasympathetic nerves usually increase activity in the tract. Signals from sympathetic nerves cause contraction of some sphincters and thus control rate at which materials move forward. Many different hormones help regulate secretion of enzymes, digestion, and absorption. The best known hormones are gastrin, secretin, cholecystokinin, and gastric inhibitory peptide (GIP).

    .21. What is the role of saliva?

    Ans. In human beings, more than a litre of saliva is secreted into the oral cavity each

    1. day from three pairs of salivary glands-sub-maxillary (submandibular), sublingual, and parotids. Following are the functions of saliva: (1) Dissolved in saliva is a slippery glycoprotein called mucin, which protects the soft lining of the mouth from abrasion and lubricates the food for easy swallowing. (2) saliva contains
    2. Q.22. What are the functions of stomach?
      Ans. The stomach is a muscular, distensible sac due to very elastic walls and accordion-like folds. It performs three important functions:
      It stores and mixes the food bolus received from esophagus.
      The epithelium that lines the lumen of the stomach secretes gastric juice that contains enzymes and hydrochloric acid which start digesting proteins. Mucus is also secreted which add water, and also coats the inner surface of the stomach and protects it from HCI and digestive enzymes.
      It helps regulate the passage of chyme (pulpy food) into intestine with the help of pyloric sphincter.
      Q.23. Describe process of digestion in stomach.
      Ans. The inner epithelium that lines the lumen of the stomach contains thousands of gastric glands. Three types of secretary cells are preset in these glands parietal cells secrete HCI chief cells secrete pepsinogen which the HCI coverts into pepsin and mucous cells that secrete mucus. Parietal cells and chief cells are in the pits of gastric glands, while mucous cells are at the surface epithelium surrounding the openings of the glands. Gastric secretion is controlled by a combination of nervous impulses and hormones. When we see smell, or taste food, impulses from the brain to the stomach initiate the secretion of the gastric juice. Then certain food substances (proteins) in the food stimulate the glands in stomach walls to release the hormone gastrin into the blood which when reaches back to stomach wall, the hormone stimulates further secretion of gastric juice. Fig. 5.10. Gastric pits
      buffers (bicarbonate ions) that help prevent tooth decay by neutralizing acid in the mouth, (3) saliva contains antibacterial agents such as thiocyanate Ions that kill many of the bacteria that enter the mouth with food, (4) salivary amylase, a digestive enzyme that hydrolyses the glucose polymers, starch (from plants), and glycogen (from animals), is also present in saliva.
    3. Hydrochloric acid in the gastric juice converts pesinogen to active pepsin by removing a short segment of the proteins polypeptide chain, an alteration that exposes the active site of pepsin. Pepsin hydrolyzes proteins into smaller polypeptides. Hydrolysis is incomplete because pepsin can only break peptide bonds adjacent to specific amino acids. As a result of mixing and enzyme action the meal becomes a nutrient broth called acid chyme.Q.24.Why does not gastric juice digest walls of the stomach?Ans. A coating of mucus secreted by the epithelial cells helps protect the stomach lining from being digested by the pepsin and acid in gastric juice. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Lesions in the stomach lining, called gastric ulcers, are caused mainly by bacterial (Helicobacter pylon), but they may worsen if pepsin and acid destroy the lining faster than it can regenerate.Q.25.Describe in detail digestion in the small intestine.Ans. Although some digestion of starch in oral cavity, and partial digestion of proteins in the stomach has already started, however most of the digestion of macromolecules in food occurs in the small intestine. It is about 7-8 metres in length and has 4cm diameter.The first 25cm or so of the small intestine is called the duodenum. It is here that acid chyme seeping from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall itself.The digestion of the carbohydrates, starch and glycogen begun by salivary amylase in the oral cavity continues in the small intestine. Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides, including maltose. The enzyme maltase completes the digestion of maltose, splitting it into two molecules of the simple sugar glucose. Maltase is one of a family of disaccharidases, each one specific for the hydrolysis of a different disaccharide. Sucrase, for instance, hydrolyzes table sugar (sucrose), and lactase digests milk sugar (lactose), (in general adults have much less lactase than children). The disaccharidases are built into the membranes and extracellular matrix covering the intestinal epithelium. Thus the terminal steps in carbohydrate digestion occur at the site of sugar absorption.Protein digestion in the small intestine involves completion of the work begun by pepsin in the stomach. Enzymes in the duodenum dismantle polypeptides into their component amino acids or into small peptides (fragments only two or three amino acids long). Trypsin and chymotrypsin are specific for peptide bonds adjacent to certain amino acids, and thus, like pepsin, break large polypeptides into shorter chains. Carboxypeptidase splits off one amino acid at a time, beginning at the end of the polypeptide that has a free carboxyl group. Aminopeptldase works in the opposite direction. Either aminopeptidase or carboxypeptidase alone could completely digest a protein. But teamwork among these enzymes and the trypsin and chymotrypsin that attack the interior of the proteins speeds up hydrolysis tremendously. Other enzymes called dipeptidases, attached to the intestinal lining, further hasten digestion by splitting small peptides.

    The protein digesting enzymes, including trypsin, chymotrypsin, and carboxypeptidase, are secreted as inactive zymogens by the pancreas. An intestinal enzyme called enteropeptidase triggers activation of these enzymes within the lumen of the small intestine.
    The digestion of nucleic acids involves a hydrolytic assault similar to that mounted on proteins. A team of enzymes called nucleases hydrolytizes DNA and RNA in food into their component nucleotides. Other hydrolytic enzymes then break nucleotides down further into nucleosides, nitrogenous bases, sugars, and phosphates.
    Nearly all the fat in a meal reaches the small intestine completely undigested. Hydrolysis of fats is a special problem, because fat molecules are insoluble in water. Bile salts secreted into the duodenum coat tiny fat droplets and keep them from coalescing, a process called emulsification. Because the droplets are small, there is a large surface area of fat exposed to lipase, an enzyme that hydrolyzes the fat molecules.
    Thus, the macromolecules from food are completely hydrolyzed to their component monomers as peristalsis moves the mixture of chyme and digestive juices along the small intestine. Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, the jejunum and ileum, function mainly in the absorption of nutrients.

    Q.26. Give an account of hormonal control of digestion in humans.

    Ans. At least four regulatory hormones help ensure that digestive secretions are present only when needed. We have already seen that gastrin is released from the stomach lining in response to the presence of food. The acidic pH of the chyme that enters the duodenum stimulates the intestinal wall to release a second hormone, secretin. This hormone signals the pancreas to release bicarbonate, which neutralizes the acid chyme. A third hormone, cholecystokinin (CCK), produced by cells in the lining of the duodenum, causes the gallbladder to contract and release bile into the small intestine. CCK also triggers the release of pancreatic enzymes. The chyme, particularly if rich in fats, also causes the duodenum to release a fourth hormone, enterogastrone, which inhibits peristalsis in the stomach, thereby slowing down the entry of food into the small intestine. Fig. 5.11. Let’s now follow the action of enzymes from the pancreas and intestinal wall in digesting macromolecules.

    Q.27. Give an account of absorption of products of digestion in small intestine.

    Ans. To enter the body, nutrients that accumulate in the lumen when food is digested must cross the lining of the digestive tract. A limited number of nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. Fig. 5.12.

    The lining of the small intestine has a huge surface area of about 300 m 2 . Large circular folds in the lining bear fingerlike projections called villi, and each of the epithelial cells of a villus as many microscopic appendages called microvilli, which are exposed to the lumen of the intestine. Commonly called a brush border for its bristlelike appearance, the huge microvillar surface is an adaptation well suited to the task of absorbing nutrients.

    Only two single layers of epithelial cells separate nutrients in the lumen of the intestine from the blood stream. Penetrating the core of each villus is a net of microscopic blood vessels, (capillaries) and a small vessel of the lymphatic system called a lacteal, (in addition to their circulatory system that carries blood, vertebrates have an auxiliary system of vessels — the lymphatic system — which carries a clean fluid called lymph. Nutrients are absorbed across the epithelium and then across the unicellular wall of the capillaries or lacteals. In some cases, the transport is passive. The simple sugar fructose, for example, is apparently absorbed by deffusion down its concentration gradient from the lumen of the intestine into the epithelial cells, then out of the epithelial cells into capillaries. Other nutrients, including amino acids, small peptides, vitamins, glucose, and several other simple sugars, are pumped against gradients by the epithelial membranes. The absorption of some nutrients seems to be coupled to the active transport of sodium across the membranes of the epithelial cells. The membrane pumps sodium out of the cell and into the lumen, and the passive reentry of the

    Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined within those cells to form fats again. The fats are then mixed with cholesterol and coated with special proteins, forming small globules called chylomicrons, which arb transported by exocytosis out of the epithelial cell and into a lacteal.

    The capillaries and veins that drain nutrients away from the villi all converge into a single circulatory channel, the hepatic portal vessel, which leads directly to the liver. The rate of flow in this large vessel, about 1L per minute, ensures that the liver, which has the metabolic versatility to interconvert various organic molecules, has first access to nutrients absorbed after a meal is digested. The blood that leaves the liver may have a very different balance of nutrients from the blood that entered via the hepatic portal vessel. From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.

    Q.28. What role does pancreas play in the body of humans?

    Ans. Pancreas is an important gland in the body.. It has both endocrine and exocrine functions.

    Endocrien Functions Islets of langerhans release two important hormones i.e., insulin from p cells which lowers blood sugar level, and glucagon from a cells which increases blood sugar level by breaking down glucagon into glucose.

    Exocrine Functions

    Exocrine cells (pancreatic acini) secrete a number of digestive enzymes into pancreatic duct which merges with the hepatic duct from the liver to form a

    common bile duct that enters duodenum. The enzymes in active form (zymogens) released from pancreas are procarboxypeptidase, chymotripsinogen, and trypsinogen. An enzyme called enteropeptidase which is bound to the intestinal epithelium converts trypsinogen to trypsin, which then activates procarboxypeptidase to carboxypeptidase, and chymotrypsinogen to active chymotrypsin. These enzymes i.e. trypsin, carboxypeptidase and chymotrypsin digest proteins into small peptides and individual amino acids. Pancreatic lipases split triglycerides to glycerol and free fatty acids. Amylases convert polysaccharides to disaccharides and monosacharides. Pancreas also secretes bicarbonate that help neutralize acid food coming from stomach i.e. raise pH from 2 to 7.

    Q.29. What is the function of large intestine?

    Ans. Large intestine has no circular folds, villi, or microvilli. Small intestine opens into large intestine near a blind sac, the secum, with an extension called appendix both are storage sites. Appendix has lymphoid tissue and functions as part of the immune system. The major functions of large intestine include the absorption of water, and minerals, and formation and storage of feces. When water reabsorption is insufficient, diarrhoea results ,and when reabsorption is too Much, constipation results. Bacterial (escherichia cob) and fungi exist as symbionts. They secrete amino acids and vitamin K, which the host’s gut absorbs. Feces are expelled out through anus.

    Q.30. Describe role of liver and gall gladder.

    Ans. The liver is the largest organ in the _mammalian body. In the liver, millions of

    specialized cells called hepatocytes take up nutrients absorbed from intestines and release them into the bloodstream. Some major functions of the liver include:

    An Accelerated Review of the Digestive System

    During intracellular digestion, the breaking down of macromolecules takes place within the cell. During extracellular digestion, macromolecules are broken down in places outside the cell (in the extracellular space, in the surrounding area, in the lumen of digestive tracts, etc.)

    The evolutionary development of extracellular digestion allowed organisms to benefit from a greater variety of foods. The breaking down of larger molecules into smaller ones outside the cell permitted the use of other foods that, due the size of their molecules, could not be interiorized by diffusion, phagocytosis or pinocytosis.

    3. How is extracellular digestion related to cell and tissue specialization?

    A variety of specialized cells and tissues appeared as a result of extracellular digestion to provide enzymes and special structures for the breaking down of dietary macromolecules.

    This phenomenon allowed other cells to be used for other tasks and differentiations while benefiting from nutrients distributed via circulation.

    Complete Digestive Systems

    4. What is the difference between a complete digestive system and an incomplete digestive system? How are these types of digestive systems related (or not) to extracellular digestion?

    Animals with an incomplete digestive system are those in which the digestive tract has only one opening (cnidarians, platyhelminthes). Animals with a complete digestive system are those in which the digestive tract has two openings, a mouth and an anus (including all other animal phyla, with the exception of poriferans, which do not have any digestive tract).

    In animals with incomplete digestive tracts, digestion is mixed. It begins in the extracellular space and finishes in the intracellular space. In animals with complete digestive systems, extracellular digestion within the digestive tract predominates.

    5. What are some of the evolutionary advantages among animals with a complete digestive tract?

    A complete digestive tract allows animals to continuously feed without waiting for waste to be eliminated before beginning to digest new foods. In this way, the absorption of larger amounts of nutrients is possible and therefore bigger and more complex species can develop. Digestive tracts with two openings also make digestion more efficient, since they provide different sites with different physical and chemical conditions (mouth, stomach, bowels) for the action of different complementary digestive enzyme systems.

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    Mechanical Digestion

    6. What is mechanical digestion? In general, in molluscs, arthropods, earthworms, birds and vertebrates, which organs are involved in this type of digestion?

    Mechanical digestion is the fragmentation of food aided by specialized physical structures, such as teeth, prior to਎xtracellular਍igestion. The mechanicalਏragmentation of food helps digestive enzyme reactions, because it provides a larger total area for the contact between enzymes and their substrates.

    In some molluscs, mechanical fragmentation is carried out by the radula (a tooth-like structure). Some arthropods, such as lobsters and dragonflies, have mouthparts that carry out the mechanical digestion of food. In earthworms and birds, mechanical digestion is carried out by an internal muscular organ. In mandibulate vertebrates, mandibles and chewing muscles exist to triturate food prior to chemical digestion.

    Chemical Digestion

    7. Concerning extracellular digestion, what is meant by chemical digestion?

    Chemical digestion is the series of enzymatic reactions used to break down macromolecules into smaller ones. 

    8. Which type of chemical reaction is the breaking down of macromolecules into smaller ones that occurs during digestion? What are the enzymes that participate in this process called?

    The reactions of extracellular digestion are hydrolysis reactions or rather, the breaking down of molecules with the help of water. The enzymes that participate in digestion are hydrolytic enzymes.

    Human Digestive Tissues and Organs

    9. Which organs of the body are part of the human digestive system?

    The digestive system, also known as “systema digestorium”, or the gastrointestinal system, is composed of the digestive tract organs plus the digestive adnexal glands. The digestive tract is composed of the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus. 

    10. What are peristaltic movements? What is their role in human digestion?

    Peristalsis is the process of synchronized contractions of the muscular wall of the digestive tract. Peristaltic movements may occur starting at the esophagus up until and including the bowels.

    Peristaltic movements are involuntary and they have the function of moving and mixing food along the digestive tube. Peristaltic movement deficiency (in the event of injuries of the innervation of the muscular wall of the digestive tract caused by Chagas’ disease, for example) can lead to the interruption of food traffic inside the bowels, leading to severe clinical consequences such as megacolon (abnormal enlargement of the colon) and megaesophagus (enlargement of the esophagus). 

    11. From the lumen to the external surface, what tissues form the digestive tract wall?

    From the internal surface to the external surface, the digestive tract wall is made of mucosa (epithelial tissue responsible for intestinal absorption), submucosa (connective tissue beneath the mucous membrane where blood, lymphatic vessels and neural fibers are located), muscle layers (smooth muscle tissue, two layers, one interior circular layer and another exterior longitudinal layer, structures responsible for peristaltic movements), and the serous membrane (epithelial and connective tissue which form the external surface of the organ). In the bowels, the serous membrane extends to form the mesentery, a serosa that encloses blood vessels and supports the bowels within the abdominal cavity. 

    The Mouth and Salivary Glands

    12. Where are salivary glands located in humans?

    There are 6 major salivary glands in humans, one of which located in each parotid gland, two beneath the jaws (submandibular) and two at the base of the tongue (sublingual). More than 700 other minor salivary glands exist dispersed on the lip mucosa, gums, palate and pharynx.

    13. What is the approximate pH of saliva secretion? Is it an acidic or alkaline fluid? What are the main functions of saliva?

    The pH of saliva is approximately 6.8. Therefore, it is a slightly acidic pH.

    Saliva lubricates the food and starts its enzymatic extracellular digestion. It also works as a buffer for the pH of the mouth, as well as plays an important role in protecting the body against pathogens, due to the presence of IgA antibodies in it (also present in tears, the colostrum, mother’s milk and in the mucosae of the intestine and airways).

    14. What digestive enzyme is contained in saliva? Which type of food does it digest and into which smaller molecules does it break down the food?

    Salivary hydrolase is known as salivary amylase, or ptyalin. Ptyalin digests carbohydrates by breaking down starch and glycogen, glucose polymers, into maltose (a glucose disaccharide) and dextrin.

    The Esophagus

    15. Why doesn't food enter the trachea instead the esophagus?

    When food is swallowed, the swallow reflex is activated and the larynx elevates and closes to stop food particles from entering the trachea, preventing the aspiration of foreign materials into the bronchi.

    16. Is the esophagus a muscular organ? Why can food reach the stomach even if someone is lying down flat on a bed?

    The esophagus is a predominantly muscular organ. It is a muscular tube, which consists of striated muscle tissue in its upper third, mixed muscle tissue (striated and smooth) in its middle third and smooth muscle tissue in its lower third. The peristalsis of the esophagus causes the food to move towards the stomach even without the help of gravity.

    The Stomach

    17. What route does ingested food follow, from the time of swallowing until it reaches the duodenum?

    Until reaching the duodenum, food enters the mouth, passes through the pharynx, goes down the esophagus and passes through the stomach.

    18. What is the valve that separates the stomach from the esophagus called? What is its function?

    The valve that separates the stomach from the esophagus is called the cardia. It has the function of preventing the acidic contents of the stomach from entering back the esophagus once again. The improper functioning of this valve causes gastroesophageal reflux, a disease in which patients complain of bloating and heartburn (retrosternal burning).

    19. What is the valve that separates the duodenum from the stomach called? What is its function?

    The valve that separates the stomach from the duodenum is the pylorus. It has the function of keeping food within the gastric cavity for enough time to allow gastric digestion to take place. It also has the function of preventing intestinal contents from going back into the stomach.

    20. What is the pH inside the stomach? Why is it necessary to maintain that pH level? How is it maintained? What cells produce that pH?

    The normal pH of gastric juice is around 2. Therefore, it is an acidic pH.

    It is necessary for the gastric pH to be maintained acidic for the transformation of pepsinogen (a proenzyme secreted by gastric chief cells) into pepsin, the digestive enzyme that acts only under low pHs. This pH level is attained through the secretion of hydrochloric acid (HCl) by parietal cells. 

    21. Besides being necessary for the activation of the main gastric digestive enzyme, how is HCl directly involved in digestion?

    With its corrosive effect, HCl also helps rupture the bond between food particles, facilitating the digestive process.

    22. How are the gastric mucosa protected from the acidic pH of the stomach?

    The gastric epithelium is mucus secretory tissue, meaning that it produces mucus. The mucus covers the stomach wall, preventing its corrosion by gastric juice.

    23. What digestive enzyme acts within the stomach? Which type of food does it digest? What cells produce that enzyme?

    The digestive enzyme that acts in the stomach is pepsin. Pepsin has the function of breaking proteins down into smaller peptides. The gastric cells that produce pepsinogen (the zymogen precursor of pepsin) are chief cells.

    The Small Intestine

    25. What are the three parts of the small intestine?

    The small intestine is divided into three parts: the duodenum, jejunum and ileum. 

    26. Which of carbohydrates, fats or proteins have already undergone chemical digestion upon reaching the pylorus (upon exiting the stomach)?

    Upon exiting the stomach, carbohydrates have already undergone chemical digestion in the mouth and proteins have already undergone the chemical digestion process from the enzymes in the stomach.ꃊrbohydrates are changed under the effects of salivary amylase (ptyalin) and proteins are changed under the action of the enzyme pepsin in gastric juice. Fats do not undergo chemical digestion until reaching the duodenum.

    The Liver and Gallbladder

    27. What substance produced in the liver is involved in digestion in the small intestine? What is the role of this substance in the digestive process?

    Bile, an emulsifier liquid, is made by the liver and later stored in the gallbladder and released in the duodenum.

    Bile is composed of bile salts, cholesterol and bile pigments. Bile salts are detergents, amphiphilic molecules, or rather, molecules with a polar water-soluble portion and a non-polar fat-soluble portion. This feature allows bile salts to enclose fats inside water-soluble micelles in a process called emulsification. Through this process, fats come into contact with intestinal lipases, enzymes that break them down into simpler fatty acids and glycerol.

    28. What is the adnexal organ of the digestive system in which bile is stored? How does this organ react to the ingestion foods high in fat?

    Bile is concentrated and stored in the gallbladder.

    When foods high in fat are ingested, the gallbladder contracts to release bile into the duodenum. (This is the reason why patients with gallstones must not ingest fatty foods, as the reactive contraction of the gallbladder may move some of the stones to the point of blocking the duct that drains bile into the duodenum, causing pain and possible severe complications.) 

    29. What are the digestive functions of the liver?

    Besides making bile for release in the duodenum, the liver has other digestive functions.

    The network of veins that absorbs nutrients from the intestines, called mesenteric circulation, drains its blood content almost entirely to the hepatic portal vein. This vein irrigates the liver with materials absorbed from digestion. Therefore, the liver is involved in storing, processing and inactivating nutrients.

    Glucose is polymerized into glycogen in the liver. This organ also stores many vitamins and the iron absorbed in the intestine. Some important metabolic molecules, such as albumin and clotting factors, are made in the liver from dietary amino acids. In the liver, ingested toxic substances, such as alcohol and drugs, are also inactivated. 

    The Pancreas

    30. In addition to the liver, what other adnexal gland of the digestive system releases substances involved in extracellular digestion into the duodenum?

    The other adnexal gland of the digestive system is the pancreas. This organ produces the digestive enzymes that digest proteins (proteases), lipids (lipases) and carbohydrates (pancreatic amylases). Other digestive enzymes, such as gelatinase, elastase, carboxypeptidase, ribonuclease and deoxyribonuclease are also secreted by the pancreas.

    31. How does pancreatic juice participate in the digestion of proteins? What enzymes are involved?

    The pancreas secretes trypsinogen which, upon being subject to the action of the enzyme enterokinase, which is secreted by the duodenum, is transformed into trypsin. Trypsin in turn catalyzes the activation of pancreatic chymotrypsinogen into chymotrypsin. Trypsin and chymotrypsin are proteases that break proteins down into smaller peptides. The smaller peptides are then broken down into amino acids by the enzyme carboxypeptidase (also secreted by the pancreas in zymogen form and activated by trypsin) with the help of the enzyme aminopeptidase, which is produced in the intestinal mucous membrane.

    32. How does pancreatic juice proceed with the digestion of carbohydrates? What enzyme is involved?

    Carbohydrate digestion begins with the action of salivary amylase (ptyalin) in the mouth and continues in the duodenum through the action of pancreatic juice. This juice contains the enzyme pancreatic amylase, or amylopsin, which breaks down starch (amylum) into maltose (a disaccharide made of two glucose molecules).

    33. How does pancreatic juice help digest lipids? What enzyme is involved?

    The enzyme pancreatic lipase is present in pancreatic juice. This enzyme breaks down triacylglycerol (triglyceride) into fatty acids and glycerol.

    Digestive Enzymes, Digestive Secretions and pH

    34. In addition to pancreatic juice in the intestine, enteric juice containing digestive enzymes is also secreted. What are these enzymes and which type of molecule do each of these enzymes break down?

    Enteric juice is secreted by the small intestine mucosa. The enzymes of enteric juice and their respective functions are described as follows:

    Enterokinase: enzyme that activates trypsinogen into trypsin. Saccharase: enzyme that breaks down sucrose (saccharose) into glucose and fructose. Maltase: enzyme that breaks਍own maltose into two glucose molecules. Lactase: enzyme that breaks down  lactose into glucose and galactose. Peptidases: enzymes that break਍own oligopeptides into amino acids. Nucleotidases: Enzymes that break਍own nucleotides into their components (nitrogenous bases, phosphates and pentoses).

    35. Coming from the acidic pH of the stomach, what pH is present when chyme enters the duodenum? Why is it necessary to maintain that pH level in the small intestine? What organs are responsible for that pH level and how is it maintained?

    Upon entering the duodenum, chyme comes into contact with pancreatic juice with a pH of approximately 8.5. The neutralization of the acidity of the chyme is necessary to maintain the adequate pH level for the functioning of the digestive enzymes that act in the duodenum. Without the neutralization of the acidity of the chyme, mucous membrane of the intestine would be damaged.

    When stimulated by the acidity of the chyme, the duodenum produces a hormone called secretin. Secretin stimulates the pancreas to release pancreatic juice and also signals the gallbladder to expel bile in the duodenum. The pancreatic secretion, rich in bicarbonate ions, is released in the duodenum and neutralizes the chyme acidity this acidity is also neutralized by the secretion of bile in the duodenal lumen.

    36. What are the five human digestive secretions? Which of them is the only one that does not contain digestive enzymes?

    The human digestive secretions are: saliva, gastric juice, bile, pancreatic juice and enteric juice. Among these secretions, only bile does not contain digestive enzymes.

    37. Why do protease-producing cells of the stomach and of the pancreas produce only the precursors to active proteolytic enzymes?

    The stomach and the pancreas make zymogens of the proteases pepsin, chymotrypsin and trypsin and these zymogens are released into the gastric or duodenal lumen for activation. This is to prevent the digestion of these organs' (stomach and pancreas) own cells and tissues by the active form of the enzymes. Therefore, the production of zymogens is a protective strategy against the natural effects of proteolytic enzymes.

    Intestinal Villi and Microvilli

    38. After digestion, the next step is absorption by cells of the mucous membrane of the intestine. For this to happen, a large absorption surface is an advantage. How is it possible for the small internal space of the body of a pluricellular organism to contain a large intestinal surface?

    Evolution tried to solve this problem in two ways. The simplest way is the long and tubular shape of the bowels (approximately eight meters in length), made possible by the closely folded and numerous loops of the small intestine. More efficient solutions are intestinal villi and the microvilli of the mucosal membrane cells.

    The intestinal wall is not smooth. The mucous membrane, together with its submucosa, projects into the gut lumen like gloved fingers, forming invaginations and villi that multiply the available surface for absorption. In addition, the epithelial cells that cover these villi contain numerous hair-like projections called microvilli on the external surface (lumen surface) of their plasma membrane. The absorptive area of the intestines is thus increased hundreds of times through these solutions.

    The jejunum and ileum contain folds that also have the function of increasing the absorption surface.

    The Colon

    39. In which part of the digestive tract is water mainly absorbed? What about mineral ions and vitamins?

    The majority of water, vitamins and mineral ions are absorbed by the small intestine. The large intestine, however, is responsible for the reabsorption of nearly 10% of ingested water, a significant amount that gives consistency to feces (colon diseases can cause diarrhea).

    The Route from Digestion to Tissues

    40. From the intestinal lumen to tissues, what is the route of nutrients after digestion?

    Monosaccharides, amino acids, mineral salts and water are absorbed by the intestinal epithelium and collected by the capillary vessels of the intestinal villi. From the capillaries, nutrients go to the mesenteric circulation, a system of blood vessels that drains the intestinal loops. The blood from the mesenteric circulation is drained to the hepatic portal vein and some nutrients are processed by the liver. From the liver, nutrients are gathered by the hepatic veins, which discharge their blood content into the inferior vena cava. Blood from the inferior vena cava then gains the right chambers of the heart and is pumped to the lungs for oxygenation. From the lungs, the blood then returns to the heart, where it is pumped to tissues, thus distributing nutrients and oxygen.

    Chylomicrons and Cholesterol

    41. What is the special route that lipids follow during digestion? What are chylomicrons?

    Triglycerides emulsified by bile within micelles are subject to the action of lipases, which break them down into fatty acids and glycerol. Fatty acids, glycerol and cholesterol are absorbed by the intestinal mucosa. In the interior of the mucosal cells, fatty acids and glycerol form triglycerides once again, which, along with cholesterol and phospholipids, are packed in small vesicles covered by proteins called chylomicrons. The chylomicrons are released in minuscule lymphatic vessels as opposed to blood vessels, and enter into lymphatic circulation. Therefore, the lymphatic system plays an important role in the absorption of lipids.

    Lymphatic circulation drains its content into venous blood circulation. In that way, chylomicrons reach the liver, where their lipid content is processed and released into the blood in the form of protein-containing complexes called lipoproteins, such as HDL, VLDL and LDL.

    42. What are the so-called “good” and “bad” types of cholesterol?

    Lipoproteins are complexes made of lipids (triglycerides and cholesterol) and proteins. Lipoproteins present different densities according to the ratio of their protein to lipid quantities, since lipids are less dense than proteins. Low-density lipoproteins (LDL) are those with a low protein/lipid ratio high-density lipoproteins (HDL) have a high protein/lipid ratio another group is very low-density lipoproteins (VLDL) with a very low protein/lipid ratio.

    LDL is known as “bad cholesterol” because it transports cholesterol from the liver to tissues and, as a result, contributes to the formation of atheroma plaques inside blood vessels, a condition called atherosclerosis (not to be confused with arteriosclerosis), which can lead to severe circulatory obstructions such as acute myocardial infarction, cerebrovascular accidents and thrombosis. HDL is known as “good cholesterol” because it transports cholesterol from tissues to the liver (to be eliminated with bile). A high amount of HDL in the blood reduces the risk of atherosclerosis. (VLDL transforms into LDL after losing triglycerides in the blood).

    The Digestive function of Vegetable Fibers

    43. Why does the ingestion of vegetable fibers improve the regularity of the bowel movements in people who suffer from hard stool?

    Some types of plant fibers are not absorbed by the intestine but play an important role in the functioning of the organ. They retain water inside the bowels and therefore contribute to the softening of the feces. Softer feces are easier to eliminate during defecation. People who eat less dietary fiber may suffer from hard stool and constipation.

    Intestinal Bacterial Flora

    44. What are the main functions of the bacterial flora within the human gut?

    Bacteria that live inside the gut play an important role in digestion. Some polysaccharides such as cellulose, hemicellulose and pectin are not digested by digestive enzymes secreted by the body instead, they are broken down by enzymes released by bacteria in the gastrointestinal tract. Intestinal bacterial flora also produce substances vital to the functioning of the bowels, facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Some gut bacteria are the main source of vitamin K for the body and, as a result, they are essential for the blood clotting process.

    The intestinal flora contains useful but also potentially harmful bacteria. It is estimated that more than 100 trillion bacteria live in a human gut. Some bacteria are useful because they compete with other species, preventing the excessive proliferation of other bacteria.

    Digestive Hormones

    45. The release of digestive secretions is controlled by hormones. What hormones participate in this regulation?

    The hormones that participate in the regulation of digestion are gastrin, secretin,਌holecystokininਊnd enterogastrone.

    46. How is gastrin produced and what is its function in the digestive process?

    The presence of food in the stomach stimulates the secretion of gastrin, which in turn triggers the release of gastric juice.

    47. Where is secretin produced and what is its ਏunction in the digestive process?

    Secretin is produced in the duodenum. The acidity of ਌hyme causes the duodenum to release this hormone, which in turn stimulates the secretion of pancreatic juice.

    48. How is cholecystokinin produced and what is its function in the digestive process?

    The fat level of the chyme detected in the duodenum stimulates the secretion of਌holecystokinin (CCK). CCK acts by stimulating the secretion of pancreatic juice and the releasing of bile by the gallbladder.

    49. Where is enterogastrone produced and what is its function in the digestive process?

    When chyme is too fatty, the duodenum secretes enterogastrone. This hormone reduces the peristalsis of the stomach, thus slowing the entrance of food into the duodenum (as the digestion of fats takes more time).

    Avian and Ruminant Digestive Systems

    50. What are the special structures of the avian digestive tract and their respective functions?

    The digestive tract of birds contains special structures, which occurs in this order: the crop, the proventriculus and the gizzard.

    The crop has the function of the temporary storage of ingested food and is a more dilated area of the avian esophagus. The proventriculus is the chemical stomach of birds, in which food is mixed with digestive enzymes. The gizzard is a muscular pouch that serves as a mechanical stomach, in which food is ground up to increase the exposure area of the food particles to digestive enzymes. 

    51. Compared to mammals, do birds absorb more or less water in their digestive system? Why is this phenomenon an adaptation to flight?

    Bird feces are more liquid than mammal feces, meaning that less water is absorbed by the avian digestive system. The more frequent elimination of feces in birds due to their less solid feces is an adaptation to flight, since their body weight is maintained lower.

    52. What is meant by “the mutualistic digestion of cellulose”, a phenomenon that occurs in some mammals and insects?

    Herbivorous animals eat large amounts of cellulose, a substance not digested by their digestive enzymes. In these animals, regions of the digestive tract are colonized by microorganisms that digest cellulose. As a result, a mutualistic ecological interaction between animals and microorganisms occurs. This interaction is present in horses, cows, rabbits and in some insects, such as termites.

    53. Cows swallow their food once and then this food goes back to the mouth to be chewed again. How can this phenomenon be explained?

    The food ingested by cows and other ruminant animals first passes through two compartments of the digestive tract called the rumen and the reticulum. Within them, the food is subject to the action of digestive enzymes released by microorganisms that live there in a mutualistic ecological interaction. In the reticulum, the food is broken down. After passing through reticulum, the food (cud) is regurgitated to the mouth to be chewed and swallowed once again in a process called rumination. The food then enters the omasum, where it is mechanically mixed. After that, the food goes to the abomasum, the organ where chemical digestion takes place. After leaving the abomasum (the true stomach). the food gains the intestine.

    Now that you have finished studying Digestive System, these are your options:

    Biochemistry. 5th edition.

    Glycogen is a readily mobilized storage form of glucose. It is a very large, branched polymer of glucose residues (Figure 21.1) that can be broken down to yield glucose molecules when energy is needed. Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds. Branches at about every tenth residue are created by α-1,6-glycosidic bonds. Recall that α-glycosidic linkages form open helical polymers, whereas β linkages produce nearly straight strands that form structural fibrils, as in cellulose (Section 11.2.3).

    Figure 21.1

    Glycogen Structure. In this structure of two outer branches of a glycogen molecule, the residues at the nonreducing ends are shown in red and residue that starts a branch is shown in green. The rest of the glycogen molecule is represented by R.

    Glycogen is not as reduced as fatty acids are and consequently not as energy rich. Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids? Glycogen is an important fuel reserve for several reasons. The controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen's role in maintaining blood-glucose levels is especially important because glucose is virtually the only fuel used by the brain, except during prolonged starvation. Moreover, the glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.

    The two major sites of glycogen storage are the liver and skeletal muscle. The concentration of glycogen is higher in the liver than in muscle (10% versus 2% by weight), but more glycogen is stored in skeletal muscle overall because of its much greater mass. Glycogen is present in the cytosol in the form of granules ranging in diameter from 10 to 40 nm (Figure 21.2). In the liver, glycogen synthesis and degradation are regulated to maintain blood-glucose levels as required to meet the needs of the organism as a whole. In contrast, in muscle, these processes are regulated to meet the energy needs of the muscle itself.

    Figure 21.2

    Electron Micrograph of a Liver Cell. The dense particles in the cytoplasm are glycogen granules. [Courtesy of Dr. George Palade.]

    21.0.1. An Overview of Glycogen Metabolism:

    Glycogen degradation and synthesis are relatively simple biochemical processes. Glycogen degradation consists of three steps: (1) the release of glucose 1-phosphate from glycogen, (2) the remodeling of the glycogen substrate to permit further degradation, and (3) the conversion of glucose 1-phosphate into glucose 6-phosphate for further metabolism. The glucose 6-phosphate derived from the breakdown of glycogen has three fates (Figure 21.3): (1) It is the initial substrate for glycolysis, (2) it can be processed by the pentose phosphate pathway to yield NADPH and ribose derivatives and (3) it can be converted into free glucose for release into the bloodstream. This conversion takes place mainly in the liver and to a lesser extent in the intestines and kidneys.

    Figure 21.3

    Fates of Glucose 6-Phosphate. Glucose 6-phosphate derived from glycogen can (1) be used as a fuel for anaerobic or aerobic metabolism as in, for instance, muscle (2) be converted into free glucose in the liver and subsequently released into the blood (more. )

    Glycogen synthesis requires an activated form of glucose, uridine diphosphate glucose (UDP-glucose), which is formed by the reaction of UTP and glucose 1-phosphate. UDP-glucose is added to the nonreducing end of glycogen molecules. As is the case for glycogen degradation, the glycogen molecule must be remodeled for continued synthesis.

    The regulation of these processes is quite complex. Several enzymes taking part in glycogen metabolism allosterically respond to metabolites that signal the energy needs of the cell. These allosteric responses allow the adjustment of enzyme activity to meet the needs of the cell in which the enzymes are expressed. Glycogen metabolism is also regulated by hormonally stimulated cascades that lead to the reversible phosphorylation of enzymes, which alters their kinetic properties. Regulation by hormones allows glygogen metabolism to adjust to the needs of the entire organism. By both these mechanisms, glycogen degradation is integrated with glycogen synthesis. We will first examine the metabolism, followed by enzyme regulation and then the elaborate integration of control mechanisms.


    Signal cascades lead to the mobilization of glycogen to produce glucose, an energy source for runners. [(Left) Mike Powell/Allsport.]

    • 21.1. Glycogen Breakdown Requires the Interplay of Several Enzymes
    • 21.2. Phosphorylase Is Regulated by Allosteric Interactions and Reversible Phosphorylation
    • 21.3. Epinephrine and Glucagon Signal the Need for Glycogen Breakdown
    • 21.4. Glycogen Is Synthesized and Degraded by Different Pathways
    • 21.5. Glycogen Breakdown and Synthesis Are Reciprocally Regulated
    • Summary
    • Problems
    • Selected Readings

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    Useful Notes on Polysaccharides (With Diagram)

    The polysaccharides (or glycans) are composed of long chains of sugars and can be divided into two main functional groups the structural polysaccharides and the nutrient polysaccharides.

    The structural polysac­charides serve primarily as extracellular or intercellu­lar supporting elements.

    Included in this group are cellulose (found in plant cell walls), mannan (found in yeast cell walls), chitin (in the shells of arthropods and the cell walls of some fungi), hyaluronic acid, keratan sulfate, and chondroitin sulfate (in connective tis­sue) and the peptidoglycans of bacteria.

    The nutrient polysaccharides serve as reserves of mono-saccharides and are in continuous metabolic turnover. Included in this group are starch (plant cells and bacteria), glyco­gen (animal cells), and paramylum (in certain proto­zoa). On chemical bases, the polysaccharides can be di­vided into two broad classes: the homopolysac- charides and the heteropolysaccharides.

    In the homopolysaccharides, all the constituent sugars are the same. Included in this class are cellulose, starch, glycogen, and paramylum. In the heteropolysac­charides, the constituent sugars may take different forms included here are hyaluronic acid, keratan sul­fate, and chondroitin sulfate.

    Some polysaccharides are un-branched (i.e., linear) chains whose structure may be ribbon like or helical (usually a left-handed spi­ral). Other polysaccharides are branched and, like many proteins, assume a globular form. A description of some of the more important polysaccharides fol­lows.

    Cellulose is not only the most common of all polysac­charides it is also the most abundant organic sub­stance in the living world. Indeed, it is estimated that more than half of all organic carbon is present as cellu­lose. Cellulose is the major component of plant cell walls, where it plays a structural role. Cellulose is an un-branched polymer of glucose in which the neighbor­ing mono-saccharides are joined by β1→4 glycosidic bonds (Fig. 5-12).

    Chain lengths vary from several hundred to several thousand glucosyl units (in the al­gae Valonia, a single molecule of cellulose may con­tain more than 20,000 glucosyl units). Successive pyranose rings in cellulose are rotated 180° relative to one another so that the chain of sugars takes on a “flip-flop” appearance.

    In plant cell walls, large numbers of cellulose mole­cules are organized into cross-linked, parallel micro­ fibrils (Fig. 5-13), whose long axis is that of the indi­vidual chain. The cellulose micro fibrils may be coated with hemicellulose (a smaller polysaccharide formed from xylose, mannose, or galactose).

    The plant cell wall is composed of a series of layers of these micro-fi­brils, with the walls of neighboring cells cemented to­gether by pectin (a polymer of galacturonic acid). Vast amounts of cellulose may be deposited between neighboring cells as is- vividly seen in the scanning electron photomicrographs of wood tissue shown in Figure 5-13 Cellulose has also been identified in the cell walls of algae and some fungi.

    Chitin is an extracellular structural polysaccharide found in large quantities in the body covering (cuticle) of arthropods and in smaller amounts in sponges, mollusks, and annelids. Chitin has also been identified in the cell walls of most fungi and some green algae. The chemical structure of chitin is closely related to that of cellulose the difference is that the hydroxyl group of each number 2 carbon atom is replaced by an acetamido group. Hence, chitin is an unbranched polymer of N-acetylglucosamine (Fig. 5-14) containing sev­eral thousand successive aminosugar units linked by β→4 glycosidic bonds.

    Hyaluronic Acid, Keratan Sulfate, and Chondroitin Sulfate:

    Cartilage tissue contains the related polysaccharides hyaluronic acid, keratan sulfate, and chondroitin sulfate. Hyaluronic acid is an unbranched heteropoly-saccharide containing repeating disaccharides of N- acetylglucosamine and glucuronic acid (Fig. 5-15). Glucuronic acid is linked to N-acetylglucosamine in each disaccharide by a 1→3 glycosidic bond, but suc­cessive disaccharides are 1→4 linked. In addition to cartilage, hyaluronic acid is found in other connective tissues, in the synovial fluid of joints, in the vitreous humor of the eyes, and also in the capsules that en­close bacteria.

    Keratan sulfate, like hyaluronic acid, is a repeating disaccharide forming an unbranched chain. Each di­saccharide unit of the polysaccharide consists of galactose and sulfated N-acetylglucosamine. Chon­droitin sulfate is a repeating disaccharide consisting of alternating glucuronic acid and sulfated N- acetylgalactosamine residues. Hyaluronic acid, kera­tan sulfate, and chondroitin sulfate, together with a number of oligosaccharides and proteins, form the proteoglycan (see below) that gives cartilage its un­usual tensile strength and resilience.

    Inulin is an unbranched nutrient polysaccharide found in the bulbs of such plants as artichokes, dahl­ias, and dandelions. It consists of repeating fructose units in β2 → 1 linkage (Fig. 5-16).

    Glycogen is a branched nutrient homopolysaccharide containing glucose in α1→4 and α1→6 linkages. It is found in nearly all animal cells and also in certain pro­tozoa and algae. In view of its ubiquitous occurrence, it is an extremely important polysaccharide and has been the subject of numerous studies. In man and other vertebrates, glycogen is stored primarily in the liver and muscles and is the principal form of stored carbohydrate. In an unstarved animal, as much as 10% of the liver weight may be glycogen.

    Glycogen undergoes almost continuous biosynthesis and degra­dation, especially in liver tissue. Liver glycogen serves as a reservoir for glucose under starvation conditions (it may be almost completely depleted during 24 hours of fasting) and during muscular exertion. Glycogen is quickly resynthesized from newly ingested carbohy­drate.

    Glycogen molecules exist in a continuous spectrum of sizes, with the largest molecules containing many thousands of glucosyl units. A small portion of a gly­cogen molecule is shown in Figure 5-17. The glucosyl units that are linked by α1 → 4 glycosidic bonds are or­ganized into long chains the chains are intercon­nected at branch points by α1→6 glycosidic bonds.

    This yields the “bush”- or “tree”-like structure de­picted in Figure 5-18. It should be noted that in a gly­cogen molecule there is only one glucose unit whose number 1 carbon atom bears a hydroxyl group. All of the other 1-OH groups are involved in α1 → 4 and α1 → 6 glycosidic bonds. The single free 1-OH group is called the “reducing end” of the molecule and is noted by the letter R in Figure 5-18. In contrast, numerous “non-reducing ends” are present (i.e., free 4-OH and 6-OH groups) at the terminals of the outermost chains.

    In Figure 5-18 the individual glucose units are rep­resented by circles and the branch points (i.e., the α1→6 linkages) by heavier connections. In this model of glycogen, a number of different kinds of chains may be distinguished. A chains are attached to the mole­cules by a single 1 → 6 linkage (chains of open circles in Fig. 5-18) and B chains (gray circles) bear one or more A chains. Each glycogen molecule contains only one C chain (colored circles), and this is the chain that ends in the free reducing group.

    Exterior chains are those portions of individual chains between the non-reducing end groups and the outermost branch points. Finally, those parts of individual chains between branch points are called interior ‘chains. The exterior chains of glycogen are usually six to nine glucosyl units long, whereas interior chains contain only three to four glucosyl units. Approximately 8 to 10% of all glycosidic linkages are the 1→6 type.

    Glycogen molecules are often sufficiently large to be studied by electron microscopy. Although the mole­cules can be seen in transmission electron photomicro­graphs of osmium tetroxide-fixed tissues , morphological studies of glycogen are usually carried out with material that has been negatively stained with phosphotungstic acid and os­mium tetroxide (Fig. 5-19).

    Starch is a nutrient polysaccharide found in plant cells, protists, and certain bacteria and is similar in many respects to glycogen. (In fact, glycogen is often referred to as “animal starch.”) Starch usually occurs in cells in the form of granules visible by both light and electron microscopy.

    In plant cells (such as potato or corn) these granules may be several micrometers in diameter and may account for more than half of the cell’s dry weight. In microorganisms the starch gran­ules are smaller, having diameters of only 0.5 to 2 μm. Starch granules contain a mixture of two different polysaccharides, amylose and amylopectin, and the relative amounts of these two polysaccharides vary ac­cording-to the source of the starch.

    The amyloje component of starch is an unbranched 1→4 polymer of glucose and may be several thousand glucosyl units long. The polysaccharide chain exists in the form of a left-handed helix containing six glucosyl residues per turn (Fig. 5-20). The familiar blue color that is produced when starch is treated with iodine is believed to result from the coordination of iodine ions in the interior of the helix. The polysaccharide must contain at least six helical turns (i.e., 36 glucosyl resi­dues) to produce the characteristic blue color when treated with iodine.

    Amylopectin is a branched polysaccharide containing 1→4 and 1→6 linked glucosyl units in this re­spect it is similar to glycogen. However, amylopectin has a more open structure with fewer 1→6 linkages and longer chain lengths. Some of the characteristics of amylopectin and glycogen are compared in Table 5-1.

    Other Polysaccharides:

    In addition to the polysaccharides already described, several others should be briefly reviewed. Mannan, a homopolymer of mannose, is found in the cell walls of yeast and is also stored intracellularly in some plants. In yeast, the mannan has a branched structure, whereas in plants it is a linear molecule.

    Parts of the Alimentary Canal [back to top]

    1. Mouth (Buccal cavity) [back to top]

    The teeth and tongue physically break up the food into small pieces with a larger surface area, and form it into a ball or bolus. The salivary glands secrete saliva, which contains water to dissolve soluble substances, mucus for lubrication, lysozymes to kill bacteria and amylase to digest starch. The food bolus is swallowed by an involuntary reflex action through the pharynx (the back of the mouth). During swallowing the trachea is blocked off by the epiglottis to stop food entering the lungs.

    2. Oesophagus (gullet) [back to top]

    This is a simple tube through the thorax, which connects the mouth to the rest of the gut. No digestion takes place. There is a thin epithelium, no villi, a few glands secreting mucus, and a thick muscle layer, which propels the food by peristalsis. This is a wave of circular muscle contraction, which passes down the oesophagus and is completely involuntary. The oesophagus is a soft tube that can be closed, unlike the trachea, which is a hard tube, held open by rings of cartilage.

    3. Stomach [back to top]

    This is an expandable bag where the food is stored for up to a few hours. There are three layers of muscle to churn the food into a liquid called chyme. This is gradually released in to the small intestine by a sphincter, a region of thick circular muscle that acts as a valve. The mucosa of the stomach wall has no villi, but numerous gastric pits (10 4 cm 𔂬 ) leading to gastric glands in the mucosa layer. These secrete gastric juice, which contains: hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact it hinders it by denaturing most enzymes) mucus to lubricate the food and to line the epithelium to protect it from the acid and the enzymes pepsin and rennin to digest proteins.

    4. Small Intestine [back to top]

    This is about 6.5 m long, and can be divided into three sections:

    (a) The duodenum (30 cm long). Although this is short, almost all the digestion takes place here, due to two secretions: Pancreatic juice, secreted by the pancreas through the pancreatic duct. This contains numerous carbohydrase, protease and lipase enzymes. Bile, secreted by the liver, stored in the gall bladder, and released through the bile duct into the duodenum. Bile contains bile salts to aid lipid digestion, and the alkali sodium hydrogen carbonate to neutralise the stomach acid. Without this, the pancreatic enzymes would not work. The bile duct and the pancreatic duct join just before they enter the duodenum. The mucosa of the duodenum has few villi, since there is no absorption, but the submucosa contains glands secreting mucus and sodium hydrogen carbonate.

    (c) The ileum (4 m long). These two are similar in humans, and are the site of final digestion and all absorption. There are numerous glands in the mucosa and submucosa secreting enzymes, mucus and sodium hydrogen carbonate.

    The internal surface area is increased enormously by three levels of folding: large folds of the mucosa, villi, and microvilli. Don't confuse these: villi are large structures composed of many cells that can clearly be seen with a light microscope, while microvilli are small sub-cellular structures formed by the folding of the plasma membrane of individual cells. Microvilli can only be seen clearly with an electron microscope, and appear as a fuzzy brush border under the light microscope.

    Circular and longitudinal muscles propel the liquid food by peristalsis, and mix the contents by pendular movements - bi-directional peristalsis. This also improves absorption.

    5. Large Intestine [back to top]

    This comprises the caecum, appendix, ascending colon, transverse colon, descending colon and rectum. Food can spend 36 hours in the large intestine, while water is absorbed to form semi-solid faeces. The mucosa contains villi but no microvilli, and there are numerous glands secreting mucus. Faeces is made up of plant fibre (cellulose mainly), cholesterol, bile, mucus, mucosa cells (250g of cells are lost each day), bacteria and water, and is released by the anal sphincter. This is a rare example of an involuntary muscle that we can learn to control (during potty training).


    Antonio Blanco , Gustavo Blanco , in Medical Biochemistry , 2017


    When starch is partially hydrolyzed by the action of acids or enzymes (amylases), it is degraded to maltose, maltotriose , and an oligosaccharide called dextrin. One type of dextrin, known as “limit dextrin” is one of the products after digestion with amylase. Since this enzyme catalyzes the hydrolysis of α-1→4 but does not affect links α-1→6, the digestive action of amylase stops at the starting points of the starch branches. These nonhydrolyzed sections of the starch molecule represent the limit of the action of amylase, giving them their name.

    Absorption of amino acids and of monosaccharides

    Processes involved in absorbing amino acids and monosaccharides into a capillary from the small intestine
    What name is given to the part of the lining cell shown by a zigzag line at the left of the diagram?
    > brush border/microvilli
    What advantage does this provide in the absorption process?
    > gives larger surface area/more channel proteins

    Amino acids and monosaccharides are absorbed by secondary active transport, which has several stages.

    Sodium ions enter cells lining the ileum together with either of these products of digestion, via a channel protein, of which there are several sorts. This process - facilitated diffusion - is an example of co-transport.

    Sodium ions are pumped out of the cell and into the interstitial fluid using an ATP-powered protein pump. This establishes a concentration gradient of Na + ions between the lumen and the lining cell. Arguably it could be said that it also establishes a concentration gradient of digestion products.

    Each product of digestion leaves the cell via a different carrier protein/permease channel.

    They then enter the blood capillary by diffusion. This is assisted by the steady movement of blood which maintains a concentration gradient by taking away the digestion products.


    A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. All of the monosaccharides are connected together by covalent bonds. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

    Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Basically, starch is a long chain of glucose monomers. Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

    Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

    Figure 4 Amylose and amylopectin are two different forms of starch. Amylose is composed of unbranched chains of glucose monomers. Amylopectin is composed of branched chains of glucose monomers. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen (not shown) is similar in structure to amylopectin but more highly branched.

    Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers (Figure 5).

    Figure 5 In cellulose, glucose monomers are linked in unbranched chains. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.

    Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 6). This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.

    Figure 6 Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide. (credit: Louise Docker)

    Watch the video: Carbohydrates: Starch and Glycogen. A-level Biology. OCR, AQA, Edexcel (January 2022).