Are the cytosol and extracellular fluids electrically neutral?

I've found several sources that state that overall, the cytosol of a cell is electrically neutral. The extracellular fluid is also purportedly electrically neutral. How can that be when we have membrane potentials?

Also in that regard, how do the ion transporters that set up the membrane potential know when to stop pumping ions to get a certain potential set up?

Membrane potentials are caused by different concentrations of ions on opposite sides of membranes in micro-environments along the membrane. The membrane can be, overall, electrically and chemically neutral with small areas that have differences in electrochemical gradients.

For cells to produce these potentials they have proteins embedded in the lipid membrane. Ion pumps are the main players in creating potentials and gradients; through active transport (the use of ATP) the protein moves ions from one side to the other. When the concentration of that area of membrane is at one concentration the proteins are held in one conformation (active or inactive) and when the gradient is changed or meets a threshold the protein changes conformation and becomes activated or inactivated. So the actual electrochemical gradient dictates when the proteins are transporting.

There are also other proteins/messengers that are produced which can interact with the transmembrane proteins that keep them in one configuration or another to facilitate the production of an action potential. Once again, when this specific concentration is met the signaling molecule/transporter will become inactivate or activated.

This is just stuff I recall from some classes. Honestly, wikipedia is a fine general knowledge source. And more detailed than my explanation.

What Is Cytosol? How Is It Different From Cytoplasm?

Cytosol is the fluid found inside living cells. More specifically, it&rsquos the water-based fluid in which organelles, proteins and other structures of the cell live. Also known as the cytoplasmic matrix, it constitutes most of the intracellular fluid (ICF). Cytosol is often confused with cytoplasm, however, which is an entirely different entity within a cell.

Cytosol is often confused with cytoplasm. They both start with &lsquocyto&rsquo and seem to refer to the same thing in most cellular biology textbooks. However, though they appear to be interchangeable, they are two separate terms and their usage can provide different information.

While cytoplasm consists of all the contents found inside a cell (excluding the nucleus), cytosol is just the liquid or aqueous part of the cytoplasm. In other words, cytoplasm is the area of space outside the nucleus that consists of cytosol and other organelles.

What is Cytoplasm?

The cytoplasm is a transparent semisolid or gelatinous fluid. Both prokaryotic cell and eukaryotic cell contain a cytoplasm. The cytoplasm is the whole content that lies within the plasma membrane of a prokaryotic cell. However, this is slightly different in a eukaryotic cell. The eukaryotic cell has a nucleus. Hence, the cytoplasm of a eukaryotic cell is the content which lies between the plasma membrane and nuclear membrane. The cytoplasm contains cytosol, inclusions, and organelles such as Golgi apparatus, mitochondria, and ribosomes. These organelles are membrane-bound components, which have special functions. Cytoplasmic inclusions are insoluble small particles, including pigments, granules, droplets, and crystals.

Figure 01: Cytoplasm

Almost all the cellular activities take place in the cytoplasm. Some examples of these activities are cell division, glycolysis, and many biochemical reactions. Moreover, the catabolism of macromolecules takes place in the cytoplasm by enzymatic reactions. Not only that, cytoplasm participates in cell expansion and cell growth as well.

Examples of intracellular fluid in the following topics:

Body Fluid Composition

  • The cytosol or intracellularfluid consists mostly of water, dissolved ions , small molecules, and large water-soluble molecules (such as proteins).
  • The pH of the intracellularfluid is 7.4.
  • The concentrations of the other ions in cytosol or intracellularfluid are quite different from those in extracellular fluid.
  • Ocular fluid in the eyes contrasts cerebrospinal fluid by containing high concentrations of proteins, including antibodies.
  • Describe the composition of intracellular and extracellular fluid in the body

Fluid Compartments

  • The major body fluid compartments include: intracellularfluid and extracellular fluid (plasma, interstitial fluid, and trancellular fluid).
  • The intracellularfluid of the cytosol or intracellularfluid (or cytoplasm) is the fluid found inside cells.
  • Although water forms the large majority of the cytosol, its mainly functions as a fluid medium for intracellular signaling (signal transduction) within the cell, and plays a role in determining cell size and shape.
  • It is the intravascular fluid part of extracellular fluid (all body fluid outside of cells).
  • Examples of this fluid are cerebrospinal fluid, and ocular fluid, joint fluid, and the pleaural cavity which contain fluid that is only found in their respective epithelium-lined spaces.

Water Content in the Body

  • A significant percentage of the human body is water, which includes intracellular and extracellular fluids.
  • Water also provides a fluid environment for extracellular communication and molecular transport throughout the body.
  • The water in the body is distributed among various fluid compartments that are interspersed in the various cavities of the body through different tissue types.
  • In diseased states where body water is affected, the fluid compartments that have changed can give clues to the nature of the problem.

Introduction to Osmoregulation

  • The intake is balanced by more or less equal excretion of fluids by urination, defecation, sweating, and, to a lesser extent, respiration.
  • The solutes in body fluids are mainly mineral salts and sugars.
  • The body's fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body.
  • Mammalian systems have evolved to regulate osmotic pressure by managing concentrations of electrolytes found in the three major fluids: blood plasma, extracellular fluid, and intracellularfluid.
  • Water movement due to osmotic pressure across membranes may change the volume of these fluid compartments.

Types of Receptors

  • Receptors, either intracellular or cell-surface, bind to specific ligands, which activate numerous cellular processes.
  • The toxin then enters these intestinal cells, where it modifies a G-protein that controls the opening of a chloride channel and causes it to remain continuously active, resulting in large losses of fluids from the body that can lead to potentially fatal dehydration.
  • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme.
  • In some cases, the intracellular domain of the receptor itself is an enzyme or the enzyme-linked receptor has an intracellular domain that interacts directly with an enzyme.
  • Many intracellular receptors are transcription factors that interact with DNA in the nucleus and regulate gene expression.

Short-Term Chemical Control

  • When stimulated a signal transduction cascade leads to increased intracellular calcium from the sarcoplasmic reticulum through IP3 mediated calcium release, as well as enhanced calcium entry across the sarcolemma through calcium channels.
  • The rise in intracellular calcium complexes with calmodulin, which in turn activates myosin light chain kinase.
  • Once elevated, the intracellular calcium concentration is returned to its basal level through a variety of protein pumps and calcium exchangers located on the plasma membrane and sarcoplasmic reticulum.
  • Localized tissues utilize multiple ways to increase blood flow including releasing vasodilators, primarily adenosine, into the local interstitial fluid which diffuses to capillary beds provoking local vasodilation.
  • Dephosphorylation by myosin light-chain phosphatase and induction of calcium symportersand antiporters that pump calcium ions out of the intracellular compartment both contribute to smooth muscle cell relaxation and therefore vasodilation.

Microbial Growth at Low or High pH

  • The pH of different cellular compartments, body fluids, and organs is usually tightly regulated in a process called acid-base homeostasis.
  • Most acidophile organisms have evolved extremely efficient mechanisms to pump protons out of the intracellular space in order to keep the cytoplasm at or near neutral pH.
  • Therefore, intracellular proteins do not need to develop acid stability through evolution.

Type IV (Delayed Cell-Mediated) Reactions

  • Historically, the immune system was separated into two branches: humoral immunity, for which the protective function of immunization could be found in the humor (cell-free bodily fluid or serum) and cellular immunity, for which the protective function of immunization was associated with cells.
  • 1. activating antigen-specific cytotoxic T-lymphocytes that are able to induce apoptosis in body cells displaying epitopes of foreign antigen on their surface, such as virus-infected cells, cells with intracellular bacteria, and cancer cells displaying tumor antigens
  • It is most effective in removing virus-infected cells, but also participates in defending against fungi, protozoans, cancers, and intracellular bacteria.
  • Activated CD8+ T cells destroy target cells on contact, whereas activated macrophages produce hydrolytic enzymes and, on presentation with certain intracellular pathogens, transform into multinucleated giant cells.


  • The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.
  • This literally means "cell drinking" and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid.
  • In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid.
  • Instead, it will stay in those fluids and increase in concentration.
  • In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off.

Polycystic Kidney Disease

  • The cysts are numerous and are fluid-filled, resulting in massive enlargement of the kidneys.
  • Gene PKD-1 is located on chromosome 16, and codes for a protein involved in regulation of cell cycle and intracellular calcium transport in epithelial cells it is responsible for 85% of the cases of ADPKD.
  • As the cysts accumulate fluid, they enlarge, separate entirely from the nephron, compress the neighboring renal parenchyma, and progressively compromise renal function.
  • Under the function of gene defect, epithelial cells of renal tubule turn into epithelial cells of cyst wall after phenotype change and begin to have the function of secreting cyst fluid, which leads to continuous cysts enlargement.
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The cytosol (cf. cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a portion of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways and glycolysis. They also act as intracellular receptors and form part of the ribosomes, enabling protein synthesis.

In prokaryotes, all chemical reactions take place in the cytosol. In eukaryotes, the cytosol surrounds the cell organelles this is collectively called cytoplasm. In plants, the amount of cytosol can be reduced because of the large tonoplast (central vacuole) that takes up most of the cell interior volume. The portion of cytosol in the nucleus is called nucleohyaloplasm.

The cytosol also surrounds the cytoskeleton, which is made of fibrous proteins (e.g. microfilaments, microtubules, and intermediate filaments). In many organisms, the cytoskeleton maintains the shape of the cell, anchors organelles, and controls internal movement of structures (e.g. transport vesicles).

The cytosol is a "soup" with free-floating particles, but is highly organized on the molecular level. As the concentration of soluble molecules increases within the cytosol, an osmotic gradient builds up toward the outside of the cell. Water flows into the cell, making the cell bigger. To prevent the cell from bursting apart, molecular pumps in the plasma membrane, the cytoskeleton, the tonoplast or the cell wall (if present), are used to counteract the osmotic pressure.

Cytosol mostly consists of water, dissolved ions, small molecules, and large water-soluble molecules (such as protein). It contains about 20% to 30% protein.

Normal human cytosolic pH is (roughly) 7.0 (i.e. neutral), whereas the pH of the extracellular fluid is 7.4.

Life: The Science of Biology. Purves, Sadava, Orians, Heller. Sunderland, MA. Sinauer Associates, Inc. 2004. ISBN 0-7167-9856-5 (ILM USA)[hide]
v • d • e
Urinary system, physiology: renal physiology and acid base physiology
Filtration Ultrafiltration - Countercurrent exchange
Hormones affecting filtration Antidiuretic hormone (ADH) - Aldosterone - Atrial natriuretic peptide
Endocrine Renin - Erythropoietin (EPO) - Calcitriol (Active vitamin D) - Prostaglandins
Assessing Renal function / Measures of dialysis Glomerular filtration rate - Creatinine clearance - Renal clearance ratio - Urea reduction ratio - Kt/V - Standardized Kt/V - Hemodialysis product
Acid base physiology Fluid balance - Darrow Yannet diagram - Body water - Interstitial fluid - Extracellular fluid - Intracellular fluid/Cytosol - Plasma - Transcellular fluid - Base excess - Davenport diagram - Anion gap
Buffering/compensation Bicarbonate buffering system - Respiratory compensation - Renal compensation
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2.4 Chemical Bonds, Molecules, And Compounds

A chemical bond is an attraction between atoms. This attraction may be seen as the result of different behaviors of the outermost or valence electrons of atoms. Chemical bonds between atoms were explained by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons. As the chemical properties of the elements were known to largely repeat themselves according to the periodic law, in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of electron shells about the nucleus.

Figure 2.5: Examples of Lewis dot-style representations of chemical bonds between carbon (C), hydrogen (H), and oxygen (O). Lewis dot diagrams were an early attempt to describe chemical bonding and are still widely used today.

A chemical bond can be a covalent bond, an ionic bond, a hydrogen bond or just because of Van der Waals force. All bonds can be explained by quantum theory, but, in practice, simplification rules allow chemists to predict the strength, directionality, and polarity of bonds.

Covalent bonding is a common type of bonding in which two or more atoms share valence electrons more or less equally. The simplest and most common type is a single bond in which two atoms share two electrons. Other types include the double bond, the triple bond. In non-polar covalent bonds, the electrons are shared equally between the bonding atoms. Molecules that are formed primarily from non-polar covalent bonds are often immiscible in water or other polar solvents, but much more soluble in non-polar solvents such as hexane. A polar covalent bond is a covalent bond with a significant ionic character. This means that the two shared electrons are closer to one of the atoms than the other, creating an imbalance of charge.

Atoms will share valence electrons in such a way as to create a noble gas electron configuration (eight electrons in their outermost shell) for each atom. Atoms that tend to combine in such a way that they each have eight electrons in their valence shell are said to follow the octet rule. However, some elements like hydrogen and lithium need only two electrons in their outermost shell to attain this stable configuration these atoms are said to follow the duet rule, and in this way they are reaching the electron configuration of the noble gas helium, which has two electrons in its outer shell. In a polar covalent bond, one or more electrons are unequally shared between two nuclei.

An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and the ionic bond is the electrostatic force of attraction between them. For example, sodium (Na), a metal, loses one electron to become an Na + cation while chlorine (Cl), a non-metal, gains this electron to become Cl−. The ions are held together due to electrostatic attraction, and that compound sodium chloride (NaCl), or common table salt, is formed.

Figure 2.7: Formation of an ionic bond. Sodium and fluorine atoms undergoing a redox reaction to form sodium fluoride. Sodium loses its outer electron to give it a stable electron configuration, and this electron enters the fluorine atom exothermically. The oppositely charged ions – typically a great many of them – are then attracted to each other to form a solid.

A molecule (from French molécule, from New Latin molecula (“a molecule”), diminutive of Latin moles (“a mass”) see mole + -cule.) is a group of two or more atoms held together by chemical bonds. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with two atoms in the oxygen molecule (O2) or it may be heteronuclear, a chemical compound composed of more than one element, as with water (two hydrogen atoms and one oxygen atom H2O).

Molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the “molecule” a charge, the result is sometimes named a molecular ion or a polyatomic ion.

The “inert” or noble gas elements (helium, neon, argon, krypton, xenon and radon) are composed of lone atoms as their smallest discrete unit, but the other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various pharmaceuticals.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as ionic compounds and network solids, are organized in such a way as to lack the existence of identifiable molecules per se. Instead, these substances are discussed in terms of formula units or unit cells as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as table salt), solids like carbon and diamond, metals, and familiar silica and silicate minerals such as quartz and granite.

One of the main characteristics of a molecule is its geometry often called its structure. While the structure of diatomic, triatomic or tetra-atomic molecules may be trivial, (linear, angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.

Are the cytosol and extracellular fluids electrically neutral? - Biology

Last class we introduced a sensitive, multichannel voltage meter called the EKG.

Recall that depolarization literally means "reversing polarity". In the case of all bioexcitable cells, this means going from a negative membrane potential to a positive membrane potential. The influx of positive ions creates a voltaage (called a generator potential) which is what provides the initial push that starts the current through the heart on its way.

Recall that depolarization of all excitable cells is repeated over and over as the current progresses through all of the myocardia. The depolarizations are necessary to recreate the generator potential required to keep pushing the current along the muscle cells (what would happen if depolarizations simply stopped happening along the way?).

  • The drop in voltage Q sometimes seen in an EKG is due to the lateral current near the exit of the septum of the heart.
  • The voltage spike up at QR is due to the emergence of the current from the septum.
  • At this point, the heart has both a large numbers of cells depolarizing and is directed right at the apex.
    • The P curve, by contrast, was more diffuse in direction. This, along with the lower density of myocardia, contributed to the lower voltage and the lack of sharpness of the peak of the P wave.
    • The current is mainly moving away from the apex. A change in current direction causes measured voltage to change its sign.

    The Nature of Bioelectricity (this is a repetition of part of the last reading).

    We have established that there is such a thing as bioelectricity, that it is real electricity, and that it is essential in our lives. Last time we began a look at the conditions that allow bioelectricity to occur and that allow an electric current to pass through biological tissue (an inherently poor conductor). How can there be such a thing as bioelectricity?

    The electricity that we use is out "outer world" (our surroundings) comes from generators (basically, spinning a magnet around a wire coil). Clearly we don't do that in our bodies. But the electricity that we produce is just as truly electric as anything we get from a generator.

    Last class we refined our definition of an action potential (AP). Previously, the AP was just some sort of electrical signal that was passed along heart muscle cells that made them contract.

    • Most cells are not electrically neutral across their cell membranes (i.e., from inside the cell to outside).
      • This is because there are different concentrations of ions and charged proteins inside the cell (in the cytosol) and outside of the cell (in the extracellular fluid).
        • For example, in excitable cells, such as neurons and muscle cells, there is a much greater concentration of potassium (K) and much smaller concentrations of sodium (Na) and chloride (Cl) in the cytosol than in the extracellular fluid.
        • Muscle and nerve cells are called "excitable" because they have specialized channels that allow ions to diffuse into and out of the cell.
          • There is a lot more Na + in the extracellular fluid than in the cytosol (the fluid within the cell).
          • When Na channels open, the Na outside of the cell rushes in.
            • The positive current thus produced is perpendicular to the direction that the current moves within the cell.
            • The influx of + charge (from the extracellular fluid) creates a positive charge gradient along the interior of the cell.
            • The positive charge gradient along the interior of the cell is what pushes the positive current along the interior of the cell.
            • When the current reaches the next set of Na + channels, it causes them to open, depolarizing the cell membrane there and allowing another cohort of Na + to enter from the extracellular fluid.
            • This process, which is repeated along the length of muscle (and nerve) cells, allows the electrical current to continue from one end of the cell to the other.
              • The out-to-in Na + current boosts the positive current along the cell's long axis.
              • The interior current is what is passed through the heart.
              • We can see voltage change at the membrane (-70 to +30 mV) propagate along the membrane of the muscle cell.
              • The progression of the depolarization (the "action potential") is what the EKG records.

              The existence of excitable cells, and the electrical current that excitable cells make possible, form the basis for our ability to move, to sense our surroundings, and even to think. These processes, and others, are possible because excitable cells are able to create, transport, and use bioelectricity.

              • We know that the SA node creates a voltage (a potential . called a generator potential).
              • The voltage does what all voltages do when connected to a conductor (even a poor conductor): it generates an electric current that passes through the heart muscle cells.
              • We know that there is an instrument, called an electrocardiogram, that measures voltage changes along lines of sight that are parallel to surfaces of the heart.
                • The voltages occur because there is a current passing through the heart.
                • The size of the voltages (the peaks of the P, QRS, and T waves) are proportional to the number of cells that , at that moment, have the current passing through them.

                The Cell and the Parts of a Cell that We Will Focus On

                • Cells have their own births, deaths, and lives.
                • Everything that happens in your body happens because of Cells.
                • The 75 trillion living entities cooperate to make you a living person.
                • has an outer membrane that consists of a phospho-lipid bilayer
                • is filled with fluid chemicals, and a series of highly organized structures (organelles)
                • is composed of 70 to 85 percent water and 15 to 30 percent proteins, lipids, carbohydrates, and charged electrolytes (e.g., sodium, potassium, calcium, chloride, etc.) that maintain osmotic pressure and, in some cases, actively participate in cellular functions.

                Quick Review of Cellular Structure in Animals (animal cells are eukaryotic here's a quick review of eukaryotic and prokaryotic cells)


                Several studies have been published about the effect of extracellular pH on complement activation. Hammer et al. [54] showed acid-induced activation of C5 when combined with C6, resulting in a lytic complex called C5b,6 a . They proposed that during acid activation of C5 and C6, the high local H + concentration alters the tertiary structure of either or both of these components, resulting in formation of the C5,6 a complex formation and its subsequent cleavage to generate lytic capacity. Fishelson et al. [55] investigated the effect of pH on the alternative complement pathway and demonstrated superior lysis of sheep erythrocytes at pH 6.4 compared with 7.4, in addition to an increase in the generation of the two C3 convertases and increased binding of complement proteins to human erythrocytes. They concluded that the optimal pH for the initiation and amplification of the alternative pathway and for the formation of the membrane attack complex is 6.4. Sonntag et al. [56] investigated the effect of lactate and of hydrochloric acid [57] on selected complement proteins in blood from healthy volunteers and found significantly increased levels of activated products C3a and C5a in blood and plasma compared with untreated controls. They concluded that neither cellular interaction nor contact with destroyed cells is necessary to initiate the complement system in acidosis [56] and that acidosis per se rather than lactate is the trigger for activation of complement in vitro [57]. In a clinical study of neonatal hypoxic-ischaemic acidosis by the same group, similar increases in C3a and C5a, along with increased factor X11a, were shown, commensurate with complement activation [58]. However, they also demonstrated a reduction in median-complement function as measured by lysis of sensitized sheep erythrocytes by activated plasma-complement factors and a reduction in levels of the C1 inhibitor, Clq, and factor B compared with healthy controls. They concluded that the activation of complement was probably a result of cellular disintegration because of ischaemia, causing the release of subcellular constituents such as mitochondrial proteins, which activate the complement cascade in vitro and in vivo. No explanation was offered for the decrease in functional activity. Miyazawa and Inoue [59] demonstrated activation of the complement system by C-reactive protein in mildly acidic conditions via a pH-dependent, conformational change of the protein.

                One study examined the effect of chronic, compensated acidosis and alkalosis on antibody synthesis in rats and showed decreased synthesis of antibody with acidosis [60]. A pH dependence of immunoglobulin G (IgG) binding by the neonatal Fc receptor has also been shown, and high-affinity binding was observed at pH 6–6.5 and weak or no binding at pH 7.5 [61]. The differential binding reflects the physiological variations in pH between the gut and bloodstream of the neonate, and the lower pH predominates in the gut where binding occurs. In another study, acid-pH-treated TB sera resulted in significantly greater titres of antibodies toMycobacterium tuberculosis and higher antigen-binding ability of the former [62]. The changes were shown to be irreversible. Recently, Lopez et al. [63] have shown that acidic pH increases the avidity of human IgG binding to human neutrophils, monocytes, and NK cells. There are several structural and molecular studies on the pH dependence of antibody/antigen association however, their functional relevance remains to be evaluated, and, therefore, they will not be reviewed here. In conclusion, there is a growing body of evidence suggesting a positive effect of acidic pH on complement activation, and more research is required to clarify the effects of ambient pH on antibody synthesis and binding.

                Are the cytosol and extracellular fluids electrically neutral? - Biology

                The cytosol (as opposed to cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a large part of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways, glycolysis, and they act as intracellular receptors and ribosomes. In prokaryotes, all chemical reactions take place in the cytosol. In eukaryotes, the cytosol contains the cell organelles. In plants, the amount of cytosol can be reduced due to the large tonoplast (central vacuole) that takes up most of the cell interior volume.

                The cytosol is not a "soup" with free-floating particles, but is highly organized on the molecular level. The cytosol also contains the cytoskeleton. This is made of fibrous proteins (microfilaments, microtubules, and intermediate filaments) and (in many organisms) maintains the shape of the cell, anchors organelles, and controls internal movement of structures, e.g., transport vesicles.

                As the concentration of soluble molecules increases within the cytosol, an osmotic gradient builds up toward the outside of the cell. Water flows into the cell, making the cell larger. To prevent the cell from bursting apart, molecular pumps in the plasma membrane, the cytoskeleton, the tonoplast or the cell wall (if present), are used to counteract the osmotic pressure.


                In physiological fluids calcium ion takes part in many processes. Among these are muscle contraction, microtubule formation, hormonal responses, exocytosis, fertilization, neurotransmitter release, blood clotting, protein stabilization, intercellular communication, mineralization, and cell fusion, adhesion, and growth. Most of these Ca 2+ related activities occur by interactions with proteins, which Ca 2+ may stabilize, activate, and modulate.

                In extracellular fluids the free or weakly bound Ca 2+ concentration is about 1 mM. Within many cells the free Ca 2+ concentration in the cytosol is only 0.1 μM, 10 −4 times less than in extracellular fluids. Cell membranes contain pumps, Ca-ATPases, that aid in maintaining the extraordinary concentration gradient. However, a substantial amount of Ca 2+ occurs within cells, some of it bound tightly to proteins. In response to a stimulus the free Ca 2+ concentration may increase about 10 times. Thus proteins that participate in these responses possess Ca 2+ dissociation constants in the μM range. The cytosolic Ca 2+ concentration change is achieved rapidly, and free Ca 2+ serves as a messenger or trigger for other interactions.

                Ca 2+ sites in proteins are composed of negatively charged and neutral oxygen donors nitrogen donors seem unlikely, and none have been found. Protein oxygen donors derive from carboxylate groups, carbonyl oxygens of the amide backbone, and hydroxy groups of serine and threonine side chains.

                Ca 2+ varies in its coordination number and bond lengths. The frequency of Ca 2+ coordination numbers decreases in the order 8 > 7 > 6 > 9. Coordination about Ca 2+ is basically ionic and spherical. Ca 2+ O bond distances range from 2.3 to 2.6 Å. In solution, even within a single complex, there may be variability in bond distances and, in many cases, coordination number. To bind Ca 2+ , proteins provide a pocket of appropriate size and shape with two or more negatively charged carboxylate side chains. Specific applications of these general principles appear in other papers in this symposium and in Vol. 17 of ‘Metal Ions in Biological Systems’, H. Sigel, ed.

                Except for the charge difference, usually not crucial, tripositive lanthanide ions mimic many Ca 2+ properties. Energy transfer from a nearby excited aromatic chromophore produces Tb 3+ luminescence. The spectrum and relative intensity compared to the total luminescence intensity of the circularly polarized luminescence from parvalbumin and troponin-C are nearly identical. Specific Ca 2+ binding sites in the two kinds of proteins are therefore similar. The excitation spectrum identifies the donor group in the energy transfer process as a phenylalanine side chain in parvalbumin and a tyrosine side chain in troponin-C. The two amine acids comprise homologous pairs in the two proteins.

                Watch the video: Cytosol vs cytoplasm; Whats The Difference? (January 2022).