Why should plants transform glucose into sucrose before transporting it to other parts?

I've learned that plants transform glucose into sucrose before sending it into phloem. But the process seems to be complex and energy comsuming. Why should plants do it? Is it really necessary?

Glucose, fructose and galactose are the three dietary monosaccharides. Glucose and Fructose are simple monosaccharides found in plants. A monosaccharide is the basic unit of carbohydrate and the simplest form of sugar, glucose are aldose and Fructose are ketose.

If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)-, and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl -(C=O)- between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.

Whereas Sucrose is a disaccharide composed of glucose and fructose. A disaccharide is more complex than monosaccharide, more complex compounds like oligosaccharides and polysaccharides exists. Sucrose synthesised within the cytosol of photosynthesizing cells is then available for general distribution and is commonly trans located to other carbon-demanding centers via the phloem.

Sucrose and starch are more efficient in energy storage when compared to glucose and fructose, but starch is insoluble in water. So it can't be transported via phloem and the next choice is sucrose, being water soluble and energy efficient sucrose is chosen to be the carrier of energy from leaves to different part of the tree. Another problem exists, glucose is highly reactive and this may result in some intermediate reactions while transporting glucose. Being a complex structure, sucrose is not as much reactive as glucose. So plants uses the sucrose as a medium to transfer energy. Inside the cells, sucrose is converted back to glucose and fructose. Energy is yielded when it is needed. So plants transfer glucose and fructose in the form of sucrose in order to:

  • Increase energy storage
  • Efficient energy transfer
  • Removing in between reactions


there is no free glucose in the photosynthesis. Stop to spread that myth. The net product is G3P. The end products of photosynthesis are sucrose and starch, but never glucose. Do you test glucose in the leaves? No… it is always for starch. ;) The G3P is converted to sucrose and other molecules, for example, thiamine. Glyceraldehyde 3-phosphate occurs as a reactant in the biosynthesis pathway of thiamine (Vitamin B1), another substance that cannot be produced by the human body. Part of sucrose is then translocated to the phloem. Starch is stored in the stroma of chloroplasts. It is also stored in the amyloplasts in the roots, stems cells after sucrose suffers a conversion to starch.

During Calvin cycle, 3 molecules of CO2 combine with RuBP acceptors to form 6 molecules of G3P. 1 G3P molecule exits the cycle and goes towards making glucose while the other 5 are recycled, regenerating 3 RuBP acceptor molecules.

Plants do synthesise glucose.

Why should plants transform glucose into sucrose before transporting it to other parts? - Biology

Yeast is a fungus and needs a supply of energy for its living and growth. Sugar supplies this energy (your body also gets much of its energy from sugar and other carbohydrates).

Yeast can use oxygen to release the energy from sugar (like you can) in the process called "respiration". So, the more sugar there is, the more active the yeast will be and the faster its growth (up to a certain point - even yeast cannot grow in very strong sugar - such as honey).

However, if oxygen is short (like in the middle of a ball of dough), then yeast can still release energy from sugar, but in these conditions, its byproducts are alcohol and carbon dioxide. It is this carbon dioxide gas which makes the bubbles in dough (and therefore in bread), causing the dough to rise.

Alcohol is a poison (for yeast as well as for people) and so the yeast is not able to grow when the alcohol content gets too high. This is why wine is never more than about 12% alcohol.

WHY does an excess of sugar inhibit the yeast?

My guess would be that the osmotic concentration of the sugar gets so great that the yeast is unable to get enough water for growth.

As fresh yeast is more than 90% water, the single substance most needed for growth is water. As osmotic concentration increases, the water potential of the sugar solution gets more and more negative until it reaches a point where is lower than the water potential of the yeast cell contents and water tends to move OUT of the cell rather than IN. I do not know whether yeast cells are able to take up water actively, by expenditure of metabolic energy to pump the water against the water potential gradient.

I imagine that up to a certain concentration, the limiting factor is the amount of sugar available for respiration and synthesis of cell materials with the yeast able to take in more water than needed for growth. As the concentration of the sugar increases, although respiration and synthesis can take place faster, the uptake of water gets slower and slower until we reach a point where the rate of uptake of water becomes the limiting factor.

Which sugar is best for yeast growth?

"I tested four sugars (fructose, glucose, sucrose, and lactose). I concluded that sucrose made the yeast cells have the most foam. My question is why? I am especially curious about why glucose didn't make the yeast have the most foaming."

I wonder what concentration of sugar you used in each case? Was each sugar solution made up to the concentration eg the same molarity?

Basically, each sugar needs to be converted to glucose to enable it to feed into respiration and it is this process which produces the gas which causes the foaming.

Yeast is able to synthesise a range of enzymes to do this:-

Sucrose is a disaccharide: GLUCOSE-FRUCTOSE = SUCROSE
Sucrase will split sucrose.
Isomerase will convert Fructose to Glucose.

Thus, 0.1M sucrose will yield 0.2M glucose (when ALL is converted to glucose).

Lactose is a disaccharide: GLUCOSE-GALACTOSE = LACTOSE
Lactase will split lactose and Transacetylase will convert Galactose to Glucose.
However, I believe yeast does not have the gene for lactase and this is why the lactose sugar remains intact in 'Milk stout'.
So, I predict that lactose was bottom of your list, with the least foaming.

If a sugar is too concentrated, it will slow down the reaction (this is why honey does not normally ferment), so, you should be careful to only use dilute solutions in your experiment.

So, I suspect sucrose came out best in your test because it yielded twice as much glucose as the "same concentration" of glucose.

Energy and Photosynthesis

Photosynthesis begins with Earth's ultimate energy source: the sun. The chlorophyll in plants uses the sun's energy to combine carbon dioxide with water to form a simple sugar molecule while releasing oxygen as a byproduct, according to University of Michigan. Chemically, the photosynthetic equation represents the combination of six carbon dioxide (CO2) molecules with six water (H2O) molecules to form one sugar (C6H12O6) molecule, releasing six oxygen (O2) molecules. This process is usually abbreviated as 6CO2 + 6H2O = C6H12O6 + 6O2, in which the plus (+) sign separates the chemicals in the process. The equal (=) sign is read as "yields," indicating the reaction taking place. Only about 1 percent of the sun's energy that hits the leaf drives the process of photosynthesis, but that single percent supports most life on Earth.

The leaf immediately uses some of the sugar created during this reaction as energy for the leaf to maintain its metabolic processes. Some of the sugar is immediately converted to starch granules, which are stored in the leaf or converted to cellulose to form cell walls as the plant grows. Excess amounts of sugar move through the plant to provide energy for metabolic processes in other parts of the plant. The sugar that isn't needed for energy or building cellulose is converted to starch that's stored in stems, leaves, roots and seeds. The process by which glucose is converted to starch is known as "dehydration synthesis." A water molecule is released as each of the simple sugar molecules of glucose are added to the starch molecule, according to Biology Online.

Cellular respiration, also referred to as the "dark cycle," begins when sunlight isn't available. Plants still need to continue their metabolic processes during these times, so the starch grains stored in the leaf provide the needed energy. The stored starch within the leaf converts back to sugar, which combines with oxygen to release carbon dioxide, water and energy in the form of adenosine triphosphate (ATP). Essentially the reverse of photosynthesis, this chemical equation is written as C6H12O6 + 6O2 = 6H20 + 6CO2 + ATP, which means that one glucose molecule combines with six oxygen molecules, breaking the sugar molecule apart into six water molecules and six carbon dioxide molecules to release the sun's stored energy, reports Georgia State University's HyperPhysics.

Why should plants transform glucose into sucrose before transporting it to other parts? - Biology

The main source of carbon for plants comes from the atmosphere as carbon dioxide and fixation of this carbon dioxide by leaves and other
green parts of the shoot system in the presence of light as an energy source - the process of photosynthesis . Some of this fixed carbon is
converted into fuels for respiration, such as glucose, some is stored as compact materials like starch, and some is used in building blocks,
such as proteins, nucelotides and phospholipids, to build the plant body.

Fixation is an odd word, and one that stems all the way back to alchemy. It makes perfect sense, however, when you think of gases as
mobile, with their molecules constantly moving around by diffusion due to their thermal kinetic energy. Fixation means to render less
mobile, and was historically depicted by clipping the 'wings' of the gaseous or volatile substance (by converting it into a solid or
non-volatile liquid, for example).

However, it is a little known fact that roots can also fix carbon. Roots do not photosynthesise, however, and they fix carbon in the absence
of light with the use of enzymes. Ordinarily roots only fix enough carbon for their own secretory activities, with the bulk of carbon fixation
occurring in the shoot system. Root-fixed carbon is used, for example, to produce root secretions which leak into the soil around the roots
(the rhizosphere). These secretions probably have various functions, but appear to encourage plant-friendly microbes, signaling to
mycorhizal fungi, for example, advertising the root and encouraging the fungus to form a symbiosis with it. These secretions are also
thought to be important in helping to 'mobilise, plant nutrients - many nutrients are locked onto soil particles in an insoluble form and so
can not be absorbed by the roots until they have been dissolved or mobilised. In some herbaceous plants, it has been shown that roots
can fix enough surplus carbon to contribute to shoot carbon, that is the roots export carbon that they fix and which is in excess of their own
needs, into the shoot system. To what extent the roots contribute to the stem carbon of woody plants is not known.

The Role of Root Pressure

As well as contributing a small amount to the pressure gradient which drives xylem sap part-way up a tree, or perhaps to the top of some
short plants, or even small trees, in certain conditions, root pressure possibly has several important functions. In very humid climates
where evapotranspiration is too low to drive xylem flow, root pressure ensures that some xylem travels up the plant to deliver mineral
nutrients from the roots to the shoot system. This root pressure may cause droplets of water to exude from vessels at the edges and tips
of leaves ( guttation ) and some plants have special pores ( hydathodes ) at the ends of the veins to allow this water out (after the minerals
are extracted from it by the cells). This flow due to root pressure also occurs in many plants at night, contributing to early morning due. It is
also important in very early spring, supplying growing buds with minerals before the leaves have opened enough for evapotranspiration to
take over as the main driving force. Root pressure may also help unblock cavitated vessels.

In maples and birches, cold-tolerant trees, xylem sap is driven up the stem in winter and carries sucrose with it to fuel the developing
flowers which open early before the leaves. The sucrose is loaded into the xylem from ray parenchyma and other storage cells in the
xylem. This sap ascent can not be utilising the transpiration stream as no leaves are available in winter to drive it. It occurs on warmer
days that follow cold nights and is thought to involve a night-freeze, warm-day cycle of pressure changes in the trunk. At night the xylem
sap freezes and this is thought to trap and compress gases in the xylem as the sap freezes. The daytime heat melts the ice in the xylem,
expanding the trapped gases and generating a positive pressure to drive xylem sap flow up the trunk. At this point the sugary xylem sap
can be collected for use in making syrup and wines. In the evenings, the colling temperatures dissolve more gases in the xylem, seeded
from adjoining tissues such as parenchyma, ready to form compressed gases when the xylem freezes at night. A good article, on an
external web site, describing this process in maple can be found here: maple syrup .

It should be remembered that plants can also absorb nutrients through their leaves. Gases, liquids and solid mineral dusts falling on the
leaf can all lead to increased nutrient absorption. The nutrients are absorbed through the plasmodesmata and also to some extent
through the cuticle. Apparently, a European forest may absorb up to 30% of its nitrogen through the canopy in this way (Thomas, 2000).

Schulz, 1992. Living sieve cells of conifers as visualized by confocal, laser-scanning fluorescence microscopy. Protoplasma 166: 153-164.

Scheirer, 1978. Cell wall chemistry and fine structure in leptoids of Dendroligotrichum (Bryophyta): The end wall. Amer. J. Bot. 65:

Esau, 1976. Anatomy of Seed plants, second edition.

Thomas, P. 2000. Trees: their natural history. Cambridge University Press.

Other sources to be added.

Article last updated: 27/2/15, 25 Aug 2020 (some broken links fixed)

We've all sat under enormous old trees, whether it's been for a picnic, or for shade, or just to have a quiet place to sit and
think. Many of us vaguely remember our school science classes and can recall the basics of photosynthesis and the life-cycle
of a tree. However, there's actually a lot more going on under that bark than you might realize. Take another look at how a tree
waters, feeds and grows and all the intricate mechanisms it has developed to do it and you may find it quite fascinating! At the
least, it'll certainly give you something to consider the next time you lean up against a 100 year old oak tree to consider your
finances .

Oak trees, like the one illustrated above (a 3D Pov-Ray model) may take-up more than 400 litres of water each day! This water
moves up the stem in the outermost rings of xylem (wood). Some of this water becomes (temporarily) incorporated into the cells
have a vast surface area of leaves to catch enough sunlight for photosynthesis and these leaves need carbon dioxide which is
reacted with water to form the organic building blocks of the plant's cells, using the energy harnessed by sunlight. These
building blocks include amino acids and sugars . The much needed carbon dioxide comes from the atmosphere and leaves
have closable pores called stomata (singular stoma) to take up this carbon dioxide by diffusion. Leaves are generally born on
stalks which have a hinge at their base (called the pulvinus) and this allows the leaves to vibrate in the wind, stirring the air to
help the leaves obtain a fresh supply of carbon dioxide (diffusion in still air is slow and a zone of stagnant carbon dioxide
depleted air would surround each leaf). A general equation for photosynthesis is given below:

Left: a varnish cast of the undersurface of a leaf showing the
stomata and their accompanying guard cells.

Click here to see more about leaf morphology and vasculature.

Stomata will close to conserve water at night-time when photosynthesis can no longer continue, and they will close if the plant is
losing too much water. Stomata will open and close according to a circadian rhythm as well as in direct response to light and dark.
Stomata may also close in response to wounding - plants can lose a lot of water through open wounds and some plants, e.g.
tomato plants, react rapidly to damage by transmitting electrical signals throughout their leaves which trigger the stomata to close.

Despite these measures, plants lose a lot of water through their stomata and this water needs to be replaced. Xylem vessels carry
water from the roots, up the stem and to the leaves. Plants exploit this situation, making the most of a 'bad thing' and they utilise
the so-called transpiration stream of sap ascending in the xylem to carry mineral nutrients, obtained by the roots from the soil,
with it. Transpiration from the leaves also helps to keep them cool (important as they are purposefully exposed to the sunlight!).

The xylem sap is carried into each leaf, along the central vein in the midrib and into the leaf's vascular network . Water then
traverses the leaf from cell-to-cell and evaporates into the air spaces. How does water travel from cell-to-cell? In plant tissues like
leaf mesophyll, neighbouring cells are connected by pores called plasmodesmata . These are tiny pores that traverse the cell
walls and cell membranes of neighbouring cells. They are membrane-lined and the cytoplasms of neighbouring cells are
continuous as the cytoplasm fills the plasmodesmatal channels. Thus, the cytoplasms of plant cells form a connected continuous
system called the symplast . Water can travel through the cytoplasm from cell-to-cell through the symplast. Water can, however,
also move through the extracellular spaces and cell walls that together form the apolast . The cell walls are principally made of
cellulose microfibrils which form a porous mesh. Water can move through the apoplast by capillarity (and probably moves faster
than through the symplast).

Water will move upwards inside a very narrow glass tube (capillary tube) for a short distance. Water does this because it is
sticky. Water molecules are electrically charged and dipolar (one end or pole of the molecule is negatively charged, the other
pole positively charged). Opposite charges attract and water molecules bond or stick to each other (by hydrogen bonds in a
process called cohesion - like sticking to like) and they are also electrically attracted to other materials, like the glass wall of a
capillary tube, or the cellulose microfibrils of the apoplast (in a process called adhesion - two unlike materials sticking
together). This attraction pulls water along inside narrow spaces (this is how a sponge can passively soak-up water). The
force that drives this movement of water is the capillary force of cohesion-adhesion. This force drives the movement of water
through plant apoplasts. Capillarity is of key importance in water transport along the outside of moss stems in ectohydric
mosses .

Water moves from the xylem across the leaf to the air spaces by the apoplast and symplast and then evaporates through the
stomata (transpiration).

Click here and here for more information on the structure of xylem and wood.

It is the transpiration of water from leaves which is the main driving force for the movement of
water in xylem. Loss of water from the leaves creates a negative suction pressure that draws
water up the stem. Peak flow rates in xylem are about 1 mm/s, though maximum velocities as high
as 0.8 m/s have been reported. Flow in the xylem stops at night and then velocities rise in the
morning, peaking around midday. Wider vessels have larger maximum velocities but are more
prone to cavitation (the formation of air bubbles that block xylem vessels) in cold weather. For
this reason, the evergreen cold-tolerant conifers have narrower vessels and deciduous trees
produce narrower vessels in Autumn and wider vessels in Spring and Summer - this annual cycle
in vessel size creates the annual growth ring seen in trees. Cavitation occurs when the water
column breaks - the water is being pulled up to a great height and may break under its won
weight, especially if cold and 'brittle' and fracture of the water column is also more likely in wider
vessels. The fact that columns of water can be lifted up against gravity at all is due to cohesion:
water molecules stick to one another and so they move up the xylem as a continuous column.
Water will also adhere to the inside of the vessel wall. This physical explanation of xylem sap
ascent is called the cohesion-tension theory , as the water is pulled up the tree (and so is in
tension). The movement of sap in the xylem is called the transpiration stream .

Before considering the phloem transport system, let's look at water uptake in roots.

Water potential (given the Greek letter psi as its symbol) is simply the potential energy possessed by a unit
volume (a set volume, e.g. 1 metre cubed) of water. It is the sum total of the various kinds of potential energy
the water can possess. The most familiar is the gravitational potential energy. Take a ball and raise it in the air
and you have increased its gravitational potential energy. Energy is the ability to cause change and this
gravitational energy is potential energy because it has the potential to cause change, but will not do so until the
ball is released. When released, the ball falls to the ground as it loses gravitational potential energy which is
turned into kinetic energy (the energy of movement).

Water potential is a useful concept for describing water movement in plants. It is often said that water will
always move from a region of high water potential to a region of low water potential (down a water potential
gradient) - like the falling ball. However, this is not strictly true, water will move down a water potential gradient
of its accord if allowed to do so. However, it is possible to move water from a region of low water potential to a
region of high water potential, by supplying energy (as you did when raising the ball) - the water will then gain
potential energy. In xylem water does move from high to low water potential. The plant expends none of its own
energy to move the water, it simply opens the stomata and transpiration does the rest and water moves from
high water potential in the roots to low water potential in the air above the canopy. (transpiration creates a
negative pressure potential at the top of the tree, sucking the water up the stem). However, in phloem water
can move from low to high water potential because the plant uses cellular energy (in the form of ATP) to
actively pump water up or down the stem (it creates positive pressure that pushes the water along in the
phloem). In both xylem and phloem it is the pressure which drives water movement.

The phloem contains a system of vessels for transporting photoassimilates (the products of photosynthesis
like amino acids and sugars) around the plant. Sugar is transported mainly as sucrose and nitrogen as amino
acids (the building blocks of proteins). The reason why sucrose is transported rather than glucose is probably
because sucrose is harder for bacteria to metabolise and so transporting sucrose reduces the risk of infection.
(Similarly insects transport the sugar trehalose and seaweeds mannitol. Mammals, however, transport glucose
in their blood stream since this can be rapidly utilised by nerve cells and having such large brains necessitates
a ready fuel supply. This, combined with their constantly warm temperatures, makes mammals very prone to
infection and so they have evolved highly sophisticated immune systems. Still, however, major wounds to large
animals in nature often result in death, not from the wound, but from the ensuing infection).

Phloem is a tissue comprising several different cell types, including parenchyma, tough sclerenchyma fibres,
and the phloem vessels, called sieve tubes, which are made of sieve-tube members or elements (sieve tube
cells). Phloem carries sugars (and other photoassimilates) from sources where they are released (e.g.
photosynthesising leaves where they are made, or storage organs like bulbs germinating in Spring) to sinks
where they used (e.g. growing fruit, shaded leaves, storage organs like roots which store the sugars as starch).
This process is called translocation (a name sometimes also used to describe movement of materials in xylem).
It is now known that it is not just the sugars and other solutes that move through the phloem, but the water
moves with it - the whole phloem sap moves in bulk flow ( mass flo w or mass transport).

In photosynthesising leaves, the sugars can be transported toward the phloem sieve tubes in the veins by
either the symplast or apoplast pathways, as shown below:

Note that the apoplastic pathway involves the sugars crossing the cell membranes of specialised parenchyma
cells called companion cells (there is typically one companion cell per sieve element). This cell-surface
membrane can regulate the transport of the sugar and also drives it by actively pumping sugar from the
neighbouring parenchyma cells into the phloem sieve tubes (using membrane protein pumps that require
cellular energy in the form of ATP). This pumping of sugars into the companion cell/sieve tube element
creates an osmotic gradient, or put another way, lowers the solute potential of the companion cell and hence
lowers its water potential. (Solute potential is highest for pure water which is given the value zero, so other
solutions will have a lower or more negative solute potential). Water then follows the sugar as it moves from
high to low water potentials and enters the sieve tubes. As the phloem travels along the sieve tubes in the
source, passing from element to element, it is given a push as more sugars are loaded in, creating a pulsatile
pressure (positive pressure potential) that pushes the phloem along.

Unloading of the sugars occurs at the sink. Here companion cells actively pump the sugars out from the
phloem, causing water to follow. This unloading adds to the pressure gradient in the sieve tubes - the phloem
is pushed along by sugar loading at the source and pulled along by sugar unloading at the sink. This
movement from source to sink can occur in any direction within the plant, but one of the major routes will be
from the leaves, down the stem, to the roots. Roots can not and do not photosynthesise and need sugars to
meet their own energy needs, they also store excess sugars as starch , safely underground and away from
browsing herbivores!

Adjacent sieve tube members are separated by porous sieve plates with pore diameters ranging from about 1 micrometre to 14 micrometres.
responsible for providing most of the energy needed to pump materials into and out of the phloem tubes to neighbouring parenchyma cells.
Large single plasmodesmata and groups of plasmodesmata, called sieve areas, connect the cytoplasm of the companion cell to its sieve tube
element. 'Sieve area' is a general term for an area of pores connecting a sieve tube cell to another cell. When these pores are especially
large and confined to a distinct piece of cell wall, as in the end-walls of the sieve tube cells, they are called sieve plates.

When a sieve tube is cut or wounded, the P-protein spreads out (unwinds) rapidly to fill the lumen and block the pores in the sieve plates, so
blocking the vessel and preventing further loss of phloem sap.

Gymnosperms, including conifers like the pine tree, do not have sieve tubes. Instead they have sieve cells that are connected together by
sieve areas with smaller pores (the pores are generally less than 0.8 micrometres in diameter), and so do not form an open tube. Sieve cells
are much narrower (less than about 5 micrometres in diameter, compared to sieve tube cells which are up to about 50 micrometres in
diameter) and several times longer than sieve tube cells. Specialised parenchyma cells function much like companion cells, but are called
albuminous cells in gymnosperms. (In development, a sieve tube cell and its companion cell are produced from the same parent cell by cell
division, whereas sieve cells and albuminous cells do not have a common parent cell). Sieve cells in gymnosperms retain their SER (smooth
endoplasmic reticulum) as a network that terminates on either side of the sieve are pores, and do not have P-protein. When a gymnosperm
sieve cell is wounded, the SER swells up and expands to block the pores. This SER network may also have some role in transport and it is
possible that translocation in gymnosperm phloem operates by a different mechanism to that described for angiosperm sieve tubes.

Mosses and algae also have phloem-like tissues. In mosses elongated leptoid cells are connected by fields of plasmodesmata (with pore
diameters of 0.12 to 0.15 micrometres in Dendroligotricum ) and so resemble sieve cells. In seaweeds , trumpet hyphae are made up of sieve
cells connected by sieve areas or sieve plates, with pore diameters ranging from 0.04-0.09 micrometres in Laminaria , to 6.0 micrometres in
the giant kelp Macrocystis . In gymnosperms the pore diameter is typically less than 0.8 micrometres, and 6.5 micrometres in Fagus ( beech ).
The general pattern seems to be that as plants moved onto land and grew larger, so did their pore sizes, which reduces resistance to sap
movement and increases the maximum rate of translocation in a vessel of a given diameter. Gymnosperms may form very large trees and are
at odds with this trend and more will be said about this later.

which expand to fill all the space right up to the rigid cell walls ('boxes') that contain them. This will happen if the cells are exposed to a
sugar solution of low concentration - water will move into the cells until the cells are full. Right - cells which have lost water by osmosis,
causing their protoplasts to shrink away from the cell walls, a phenomenon called plasmolysis (the cells are said to be plasmolysed ). This
happened because the tissue was soaked in a sugar solution of high concentration, causing water to diffuse out from the cells by osmosis.
As water makes up some 70% of the volume of cells, the protoplasts have shrunk and come away from their surrounding cell walls.

This phenomenon is important in plants. In soft green or fleshy plant parts, when the plant has sufficient water, the cells are full and the
protoplasts push against the cell walls, keeping them rigid (rather like tyres filled with water at high pressure) and giving the plant parts
support - the cells and plant parts are said to be turgid (swollen and rigid). This is important for keeping leaves and green stems upright to
intercept sunlight - the tissues (especially parenchyma) functions as a pressurised cellular solid. A common example of a pressurised
cellular solid is polystyrene - tiny balls of foam filled with air which when packed together become rigid - bending the polystyrene slightly
squeezes the air inside the balls which resist bending. If a plant is dehydrated (due to water loss by evaporation) however, the cells
plasmolyse and lose pressure and the cell walls lose rigidity and the cells become flaccid and the plant wilts. This is also protective - as
leaves droop so they become less-exposed to the drying sun and/or wind. Of course, the plant cannot survive in this desiccated state
indefinitely, as water is vital to cell chemistry, but they can recover quickly if the plant receives water.

Animal cells, such as mammalian red blood cells, similarly shrink if they lose water by osmosis to a concentrated solution. However, if
immersed in distilled water, which is fairly pure, then water enters the cells by osmosis, causing them to swell and burst - mammalian cells
will explode within seconds on contact with distilled water. Single-celled organisms that resemble animal cells, like amoeba , can survive
because they have contractile vacuoles, which swell up with water, as excess water enters the cell, and then contract to expel the water.

Osmosis and Plasmolysis

Not all scientists define water potential in the same way. We have adopted the definition in terms of potential energy per unit volume. More or
less equivalently, some define it as potential energy per mole (one mole being about 6.022 x 10^23 particles, or molecules of water in this
case). Other definitions refer to water potential as the free energy of water per unit volume or per mole. Free energy is the energy available
to do work, and so is essentially equivalent to potential energy in most cases. However, free energy is sometimes defined as internal energy
only. A system, such as a mass of water, has both internal energy (due to the movements of its molecules relative to each other) and
external energy (due to the movement of the body of water as a whole). In plant biology, we need to include both external and internal
energy - external energy is important when we consider movement of water up a tree - as it ascends, the water as a whole gains gravitational
potential energy, which has the potential to do work should the water be released and allowed to fall to the ground. Internal energy is
important as this drives diffusion.

Diffusion is the net or overall movement of molecules from a region of higher concentration to a region of lower concentration. (By
concentration we mean the number of molecules in a given volume, such as the number of molecules per cubic meter). This movement
occurs because molecules are in constant thermal motion (they move about and the hotter they are, the more they move) and this
movement is random. If a membrane separates the regions of higher and lower concentration, then diffusion will only take place if the
membrane is permeable to the substance diffusing - for example if it is porous and the pores are large enough to allow the molecules
through. The cell-surface membrane works in this way - it is a selectively-permeable membrane, allowing only certain substances to cross it.
Water is one such substance. However, because cell membranes are made principally of lipids (fats/oils) in the form of phospholipids, water
does not cross easily (oil and water do not mix - water does not dissolve well in fat and vice versa) and will only slowly leak across a
phospholipid membrane. However, cells need water and so cell membranes have protein pores, called aquaporins, that are the right size to
allow water molecules to cross easily. This is an example of facilitated diffusion - the protein pores help or facilitate the diffusion of water
across the membrane. Diffusion of water across cell membranes is so important that it is given its own special name - osmosis .

In biology we rarely deal with 'pure' water, since biological fluids are all solution. A solution is a mixture of materials, in which certain
substances, called solutes, are dissolved in a solvent (which is the main component of the mixture). Most often in biology the solvent of
interest is water and this will contain a variety of dissolved solutes such as salts (like sodium chloride or common table salt, NaCl, and slats of
potassium, calcium and iron and other metals) sugars and amino acids. When salt, NaCl, dissolves in water, it splits up into ions of sodium,
Na+ and chloride, Cl- (an ion is an atom or molecule that gains one or more units of net electric charge, either positive or negative). In such
a solution it is inconvenient to think of the concentration of water as this is hard to measure, and we usually think in terms of concentration of
solute. Since ions, atoms or molecules of solvent occupy space that would otherwise be occupied by water molecules, the higher the total
solute concentration (of all solutes) the lower the concentration of water. Thus water will diffuse from a region of lower solute concentration
(higher water concentration) to a region of higher solute concentration (lower water concentration).

Osmosis across plant cell membranes can be easily demonstrated using plant tissues. A classic experiment involves taking strips of onion
epidermis (such as the translucent tissue-paper like layer on the inside of the fleshy leaves of the onion bulb which is one-cell thick and so
easily observed under the light microscope) and immersing them for twenty minutes or so in different concentrations of solution, typically a
solution of sucrose sugar. Some results from such an experiment are shown below:

Photosynthesis within the Chloroplast

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope composed of an outer membrane and an inner membrane. Within the double membrane are stacked, disc-shaped structures called thylakoids.

Embedded in the thylakoid membrane is chlorophyll, a pigment that absorbs certain portions of the visible spectrum and captures energy from sunlight. Chlorophyll gives plants their green color and is responsible for the initial interaction between light and plant material, as well as numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen. A stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is the stroma or &ldquobed.&rdquo

Figure: Structure of the Chloroplast: Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

Discussion & Explanation

The hypothesis was supported in that all forms of sugar produced energy and that glucose was the most efficient.

The carbon dioxide produced can be directly related to the energy produced through fermentation because carbon dioxide is a by-product of ethanol fermentation (Cellular, 54). The control that contained no sugar produced no energy because a source of sugar is required for glycolysis and fermentation to occur.

Glucose had the greatest rate of energy production because its rate of carbon dioxide production was the largest. Sucrose had the second-highest rate of production while fructose had the lowest rate out of the three sugars. Glucose’s rate of energy production was more than three times that of fructose.

Glucose was directly used in the glycolysis cycle and did not require any extra energy to convert it into a usable form (Freeman, 154). This supported why glucose was the most efficient.

Sucrose required an enzyme and energy input to break it down into glucose and fructose in order for it to be processed in glycolysis (Freeman, 189). Fructose also could not be used immediately in the glycolysis chain but had to be altered to enter the chain as one of the intermediates (Berg, 2002).

These processes required to convert the non-glucose sugars into a usable form reduced their efficiency when compared to glucose. The largest source of error for the experiment was the start time of fermentation. The yeast was added to the fructose solution well after the glucose and fructose yeast solutions began fermenting.

Fermentation takes time to reach its maximum rate of energy production so the time gap left glucose and sucrose further ahead than fructose in the fermentation process (Berg, 2002). The data on the rate of carbon dioxide production was therefore skewed because the start of fermentation was not controlled.

Glucose and sucrose appear far more efficient than fructose because of this error. If this experiment were to be repeated, extra care would be taken to ensure that fermentation began at the same time. The measurements of sugars would be measured in equal molarity and not by percent in a solution so that the sugar molecules are equal across all of the tests.

Other follow-up experiments may include testing other types of yeasts to see how fermentation rates are impacted. The results of these experiments could impact what sugars are the most efficient in alcohol fermentation. This could determine what types of sugar brewers should use for the most efficient production of alcohol.

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The Problem With Lactose

Disaccharides have their place in a healthy diet, but not all disaccharides are well-received. Lactose is a disaccharide found in milk. This particular disaccharide requires a digestive enzyme called lactase to break it down into its monosaccharides, glucose and galactose.

Whether due to genetics or aging, your body may not produce enough lactase or any at all, which means your body can't break down lactose. If left intact, the disaccharide causes digestive issues such as abdominal pain, bloating and diarrhea. If you're lactose intolerant, you want to avoid foods that contain lactose, drink lactose-free milk or use digestive enzymes to prevent the side effects.

Conclusions and perspectives

Plant innate immunity does not involve straightforward pathways but arises as a highly complicated network including many signalling molecules and various cross-talks. In this intricate network, sugar signals may contribute to immune responses and probably function as priming molecules. It is likely that these putative roles also depend greatly on coordinated relationships with hormones and light status. Today, plant protection against a vast range of invasive pathogens and pests needs promising strategies to produce various agrochemicals to confer crop resistance ( Rahnamaeian, 2011). However, producing agrochemicals with no environmental risks is almost impossible. Moreover, plant resistance breeding programmes are time-consuming, and conferred resistance may be lost in a relatively short time. Therefore, there is a strong need to find biodegradable and cheap alternatives. More fundamental research is needed towards sugar-mediated plant immunity in order to explore further the possibilities of using biodegradable sugar-(like) compounds as alternatives to toxic agrochemicals.

Digestion and Absorption

Digestion is the mechanical and chemical break down of food into small organic fragments. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections.


The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.

The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body. The steps in carbohydrate digestion are summarized in Figure 1 and Table 1.

Figure 1. Digestion of carbohydrates is performed by several enzymes. Starch and glycogen are broken down into glucose by amylase and maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and lactase, respectively.

Table 1. Digestion of Carbohydrates
Enzyme Produced By Site of Action Substrate Acting On End Products
Salivary amylase Salivary glands Mouth Polysaccharides (Starch) Disaccharides (maltose), oligosaccharides
Pancreatic amylase Pancreas Small intestine Polysaccharides (starch) Disaccharides (maltose), monosaccharides
Oligosaccharidases Lining of the intestine brush border membrane Small intestine Disaccharides Monosaccharides (e.g., glucose, fructose, galactose)


A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes—trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure 2 and Table 2.

Figure 2. Protein digestion is a multistep process that begins in the stomach and continues through the intestines.

  • Pepsin
  • Trypsin
  • Elastase Chymotrypsin
  • Carboxypeptidase
  • Aminopeptidase
  • Dipeptidase


Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.

Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases (enzymes that break down lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure 3.

Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate “heads” of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.

Figure 3. Lipids are digested and absorbed in the small intestine.


Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.

Figure 4. Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum.

Practice Question

Which of the following statements about digestive processes is true?

  1. Amylase, maltase, and lactase in the mouth digest carbohydrates.
  2. Trypsin and lipase in the stomach digest protein.
  3. Bile emulsifies lipids in the small intestine.
  4. No food is absorbed until the small intestine.

Free Response

What is the overall outcome of the light reactions in photosynthesis?

The outcome of light reactions in photosynthesis is the conversion of solar energy into chemical energy that the chloroplasts can use to do work (mostly anabolic production of carbohydrates from carbon dioxide).

Why are carnivores, such as lions, dependent on photosynthesis to survive?

Because lions eat animals that eat plants.

Why are energy carriers thought of as either “full” or “empty”?

The energy carriers that move from the light-dependent reaction to the light-independent one are “full” because they bring energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. There is not much actual movement involved. Both ATP and NADPH are produced in the stroma where they are also used and reconverted into ADP, Pi, and NADP + .