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5.0: Prelude to Structure and Function of Plasma Membranes - Biology

5.0: Prelude to Structure and Function of Plasma Membranes - Biology


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The plasma membrane, which is also called the cell membrane, has many functions, but the most basic one is to define the borders of the cell and keep the cell functional. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment for crossing.


MEMBRANES

Membrane fluidity -- according to the fluid mosaic model, proteins and lipids diffuse in the membrane.

  • preventing ion flux
  • active transport of ions from side to side of the plasma membrane.
  1. Types of molecules that can cross membranes by diffusion:
    • Water and small lipophilic organic compounds can cross.
    • Large molecules ( e.g. proteins) and charged compounds do not cross.
  2. Direction relative to the concentration gradient: movement is DOWN the concentration gradient ONLY (higher concentration to lower concentration).
  3. Rate of diffusion depends on
    • charge on the molecule -- electric charge prevents movement.
    • size -- smaller molecules move faster than larger molecules.
    • lipid solubility -- more highly lipid-soluble molecules move faster.
    • the concentration gradient -- the greater the concentration difference across the membrane, the faster the diffusion.
  4. Direction relative to the membrane: molecules may cross the membrane in either direction, depending only on the direction of the gradient.
  1. Ion channels exist for Na + , K + and Ca ++ movement. These channels are specific for a given ionic species.
  2. Channels consist of protein, which forms a gate that opens and closes under the control of the membrane potential.
  3. Ion movement through channels is always down the concentration gradient.
  1. A carrier must be able to perform four functions in order to transport a substance.
    • Recognition -- to specifically bind the substance that is to be transported.
    • Translocation -- movement from one side of the membrane to the other.
    • Release -- on the other side of the membrane
    • Recovery -- return of the carrier to its original condition so it can go through another cycle of transport.
  2. Terminology: Carriers are also variously called "porters,""porting systems,""translocases,""transport systems" and "pumps."
  3. Carriers resemble enzymes in some of their properties.
    • They are NOT enzymes, as they do NOT catalyze chemical reactions.
    • They are enzyme-like in the following ways. They are specific. They have dissociation constants for the transported substances which are analogous to Km of enzymes. Transport can be inhibited by specific inhibitors. They exhibit saturation, like enzymes do. Diffusion, in contrast, is not saturable, and its rate increases with increasing concentration.
  4. A general model for transport is that the carrier is a protein which changes conformation during the transport process.
  5. Sometimes carriers move more than one molecule simultaneously. Nomenclature:
    • Uniport: a single molecule moves in one direction.
    • Symport: two molecules move simultaneously in the same direction.
    • Antiport: Two molecules move simultaneously in opposite directions.
  1. The characteristics of a carrier operating by passive mediated transport.
    • Faster than simple diffusion
    • Movement is down the concentration gradient only (like diffusion)
    • No energy input is required -- the necessary energy is supplied by the gradient.
    • The carrier exhibits specificity for the structure of the transported substance saturation kinetics specific inhibitability
  2. Examples of passive mediated transport.
    • Glucose transport in many cells. A uniport system Can be demonstrated by the fact that adding substances with structures that resemble the structure of glucose can inhibit glucose transport specifically. It is specific for glucose. The K m for glucose is 6.2 mM (a value in the neighborhood of the blood concentration of glucose, 5.5 mM) The K m for fructose is 2000 mM The transport process involves attachment of glucose outside the cell. Conformational change of the carrier protein. Release of the glucose inside the cell. There is no need to change K m for glucose, since the glucose concentration in the cell is very low.
    • Chloride-bicarbonate transport in the erythrocyte membrane. This is catalyzed by the band 3 protein seen previously. An antiport system: both ions MUST move in opposite directions simultaneously. The system is reversible, and can work in either direction. Movement is driven by the concentration gradient.
  1. There are two sources of energy for active transport.
    • ATP hydrolysis may be used directly.
    • The energy of the Na + gradient may be used in a symport mechanism. The energy of the Na + going down its gradient drives the movement of the other substance. But since the Na + gradient is maintained by ATP hydrolysis, ATP is the indirect source of energy for this process.
  2. The characteristics of a carrier operating by active transport.
    • Can move substances against (up) a concentration gradient.
    • Requires energy.
    • Is unidirectional
    • The carrier exhibits specificity for the structure of the transported substance saturation kinetics specific inhibitability
  3. How can the substance be released from the carrier into a higher concentration than the concentration at which it bound in the first place?
    • The affinity of the translocase for the substance must decrease, presumably by a conformational change of the translocase.
    • This process may require energy in the form of ATP.
  4. Examples of active mediated transport.
    • Ca ++ transport is a uniport system, using ATP hydrolysis to drive the Ca ++ movement. There are two Ca ++ translocases of importance.
      • In the sarcoplasmic reticulum, important in muscle contraction.
      • A different enzyme with similar activity in the plasma membrane.
    • The Na + -K + pump (or Na + -K + ATPase).
      • An antiport system.
      • Importance: present in the plasma membrane of every cell, where its role is to maintain the Na + and K + gradients.
      • Stoichiometry: 3 Na + are moved out of the cell and 2 K+ are moved in for every ATP hydrolyzed.
      • Specificity: Absolutely specific for Na + , but it can substitute for the K + .
      • The structure of the Na + -K + pump is a tetramer of two types of subunits, alpha 2 beta 2 . The beta-subunit is a glycoprotein, with the carbohydrate on the external surface of the membrane.
      • The Na + -K + ATPase is specifically inhibited by the ouabain, a cardiotonic steroid. Ouabain sensitivity is, in fact, a specific marker for the Na + -K + ATPase.
      • The proposed mechanism of the Na + -K + ATPase shows the role of ATP in effecting the conformational change.
        • Na + attaches on the inside of the cell membrane.
        • The protein conformation changes due to phosphorylation of the protein by ATP, and the affinity of the protein for Na + decreases.
        • Na + leaves.
        • K + from the outside binds.
        • K + dephosphorylates the enzyme.
        • The conformation now returns to the original state.
        • K + now dissociates.
    • Na + linked glucose transport is found in intestinal mucosal cells. It is a symport system glucose is transported against its gradient by Na + flowing down its gradient. Both are transported into the cell from the intestinal lumen. Na + is required one Na + is carried with each glucose. The Na + gradient is essential it is maintained by the Na + -K + ATPase.
    • Na + linked transport of amino acids, also found in intestinal mucosal cells, works similarly. There are at least six enzymes of different specificity that employ this mechanism. Their specificity is as follows. Short neutral amino acids: ala, ser, thr. Long or aromatic neutral amino acids: phe, tyr, met, val, leu, ile. Basic amino acids and cystine: lys, arg, cys-cys. Acidic amino acids: glu, asp Imino acids: pro and hypro Beta-amino acids: beta-alanine, taurine.
  1. There are four types of signals.
    • Nerve transmission
    • Hormone release
    • Muscle contraction
    • Growth stimulation
  2. There are four types of messenger molecules.
    • steroids
    • small organic molecules
    • peptides
    • proteins
  3. The messenger may interact with the cell in either of two ways.
    • Entry into the cell by diffusion through the cell membrane (the steroid hormones do this).
    • Large molecules or charged ones bind to a receptor on the plasma membrane.
  4. The events associated with communication via these molecules may include the following.
    • Primary interaction of the messenger with the cell (binding by a receptor).
    • A secondary event, formation of a second messenger. (this is not always found).
    • The cellular response (some metabolic event).
    • Termination (removal of the second messenger).
  1. Steroids are lipid soluble, and can diffuse through the plasma membrane.
  2. Cells which are sensitive to steroid hormones have specific receptor proteins in the cytosol or nucleus which bind the steroid.
  3. The receptor-hormone complex then somehow causes changes in the cell's metabolism, typically by affecting transcription or translation.
  4. The mechanism of termination is unclear, but involves breakdown of the hormone.
  1. Membrane receptors bind specific messenger molecules on the exterior surface of the cell. Either of two types of response may occur.
    • Direct response: binding to the receptor directly causes the cellular response to the messenger.
    • Second messenger involvement: Binding to the receptor modifies it, leading to production of a second messenger, a molecule that causes the effect.
    • In each case messenger binding induces a conformational change in the receptor protein. Binding of the messenger resembles binding of a substrate to an enzyme in that there is a dissociation constant inhibition (by antagonists) which may be competitive, noncompetitive, etc.
  2. A variety of messengers can bind to various tissues.
    • Various cellular responses may occur, depending on the tissue.
    • Either positive or negative responses may occur, even in the same tissue, depending on the type of receptor.
  3. The response of a cell to a messenger depends on the number of receptors occupied.
    • A typical cell may have about 1000 receptors.
    • Only a small fraction (10%)of the receptors need to be occupied to get a large (50%) response.
    • Receptors may have a dissociation constant of about 10 exp -11 this is the concentration of messenger at which they are 50% saturated. Thus very low concentrations of messengers may give a large response.
  1. The receptor is a complex pentameric protein which forms a channel through the membrane.
  2. Mechanism of action.
      Binding of acetylcholine, a small molecule, at the exterior surface causes the channel to open. (Binding)
  3. Na + and K + flow through the channel, depolarizing the membrane. (Response)
  4. The esterase activity of the receptor then hydrolyses the acetylcholine, releasing acetate and choline, and terminating the effect. (Recovery)
  5. The process can now be repeated.
  1. Definition: This intracellular mediator is called a second messenger .
  2. Effect of second messenger formation: Since a receptor usually forms many molecules of second messenger after being stimulated by one molecule of the original effector, second messenger formation is a means of amplifying the original signal.
  3. The formation and removal of the second messenger can be controlled and modulated.

  1. Structure of cAMP: an internal (cyclic) 3', 5'-phosphodiester of adenylic acid.
  2. The mechanism of action of cAMP is to activate an inactive protein kinase.
    • Animated activation sequence.
    • Since an active protein kinase which acts on many molecules of its substrate is produced, this process is an amplification of the original signal.
    • Since the protein kinase is activated by cAMP it is called protein kinase A.

    The reaction ATP < -> cAMP + PPi is reversible, but subsequent hydrolysis of the PPi

  • G-proteins are a class of proteins that are so named because they can react with GTP. There are G-proteins in addition to the ones under consideration here.
  • G s and G i are so named because they stimulate and inhibit, respectively, adenyl cyclase.
  • Structure: G-proteins are complexes of three different subunits, alpha, beta and gamma. Beta and gamma are similar in the G s and G i proteins. The alpha-subunits are different, and are called alpha s and alpha i , respectively.
  • Mechanism: Receptor-messenger interaction stimulates binding of GTP to the alpha-subunits. The alpha-subunit with its bound GTP then dissociates from the beta-gamma complex. The alpha-subunit with its bound GTP then acts on adenyl cyclase. alpha s -GTP stimulates adenyl cyclase. alpha i -GTP inhibits adenyl cyclase.
  • The alpha-subunit of the G-protein has GTPase activity. After it cleaves the GTP it reassociates with the beta-gamma complex to form the original trimer.
  • cAMP already formed is cleaved by cAMP phosphodiesterase.
  • The hormone gradually and spontaneously dissociates from the receptor.
  1. Animated activation sequence.
  2. IP 3 and DG are synthesized by the enzyme, phospholipase C, which has phosphatidylinositol 4,5-bisphosphate (PIP 2 ) phosphodiesterase activity. PIP 2 is a normal minor component of the inner surface of the plasma membrane.
  3. The phosphodiesterase is controlled by a G-protein in the membrane, which activates the phosphodiesterase.
  4. Mechanism: IP 3 and DG have separate effects.
    • IP 3 releases Ca ++ from the endoplasmic reticulum. The Ca ++ then activates certain intracellular protein kinases.
    • DG activates protein kinase c, a specific protein of the plasma membrane.
    • Note that both IP 3 and DG activate protein kinases, which in turn phosphorylate and affect the activities of other proteins.
  5. Termination of the signal occurs at several levels.
    • IP 3 is hydrolyzed.
    • Ca ++ is returned to the endoplasmic reticulum or pumped out of the cell.
    • The GTPase activity of the G-protein hydrolyses the GTP, terminating the activity of the phospholipase C.
  6. Many systems respond to changes on IP 3 and DG. Be aware of the large number of systems affected.

Structure: The insulin receptor is a tetramer with two kinds of subunits, alpha and beta. Disulfide bridges bind them together.


References

Singer, S. J. & Nicolson, G. L. The fluid mosaic model of the structure of cell membranes. Science 175, 720–731 (1972).

Liu, Y., Engelman, D. M. & Gerstein, M. Genomic analysis of membrane protein families: abundance and conserved motifs. Genome Biol. 3, 0054.1–0054.12 (2002).

Doura, A. K., Kobus, F. J., Dubrovsky, L., Hibbard, E. & Fleming, K. G. Sequence context modulates the stability of a GxxxG-mediated transmembrane helix-helix dimer. J. Mol. Biol. 341, 991–998 (2004).

Adams, P. D., Engelman, D. M. & Brunger, A. T. Improved prediction for the structure of the dimeric transmembrane domain of glycophorin A obtained through global searching. Proteins 26, 257–261 (1996).

Stenberg, F. et al. Protein complexes of the Escherichia coli cell envelope. J. Biol. Chem. 280, 34409–34419 (2005).

Wong, W., Scott, J. D. AKAP signalling complexes: focal points in space and time. Nature Rev. Mol. Cell Biol. 5, 959–970 (2004).

Grigorieff, N., Ceska, T. A., Downing, K. H., Baldwin, J. M. & Henderson, R. Electron-crystallographic refinement of the structure of bacteriorhodopsin. J. Mol. Biol. 259, 393–421 (1996).

Brown, D. A. & London, E. Functions of lipid rafts in biological membranes. Annu. Rev. Cell Dev. Biol. 14, 111–136 (1998).

Zaccai, G. How soft is a protein? A protein dynamics force constant measured by neutron scattering. Science 288, 1604–1607 (2000).

Petrache, H. I. et al. Hydrophobic matching mechanism investigated by molecular dynamics simulations. Langmuir 18, 1340–1351 (2002).

Mitra, K., Ubarretxena-Belandia, I., Taguchi, T., Warren, G. & Engelman, D. M. Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol. Proc. Natl Acad. Sci. USA 101, 4083–4088 (2004).

Williamson, I. M., Alvis, S. J., East, J. M. & Lee, A. G. Interactions of phospholipids with the potassium channel KcsA. Biophys. J. 83, 2026–2038 (2002).

Perozo, E., Kloda, A., Cortes, D. M. & Martina, B. Physical principles underlying the transduction of bilayer deformation forces during mechanosensitive channel gating. Nature Struct. Biol. 9, 636–637 (2002).

Abramson, J. et al. Structure and mechanism of the lactose permease of Escherichia coli. Science 301, 610–615 (2003).

Fu, D. et al. Structure of a glycerol-conducting channel and the basis for its selectivity. Science 290, 481–486 (2000).

Luecke, H., Schobert, B., Richter, H. T., Cartailler, J. P. & Lanyi, J. K. Structure of bacteriorhodopsin at 1.55 A resolution. J. Mol. Biol. 291, 899–911 (1999).

Zheng, L., Kostrewa, D., Berneche, S., Winkler, F. K. & Li, X. D. The mechanism of ammonia transport based on the crystal structure of AmtB of Escherichia coli. Proc. Natl Acad. Sci. USA 101, 17090–17095 (2004).

Abramson, J. et al. The structure of the ubiquinol oxidase from Escherichia coli and its ubiquinone binding site. Nature Struct. Biol. 7, 910–917 (2000).

Iverson, T. M., Luna-Chavez, C., Cecchini, G. & Rees, D. C. Structure of the Escherichia coli fumarate reductase respiratory complex. Science 284, 1961–1966 (1999).

Long, S. B., Campbell, E. B. & Mackinnon, R. Crystal structure of a mammalian voltage-dependent Shaker family K + channel. Science 309, 897–903 (2005).

Kurisu, G., Zhang, H., Smith, J. L. & Cramer, W. A. Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity. Science 302, 1009–1014 (2003).

Miyazawa, A., Fujiyoshi, Y. & Unwin, N. (2003). Structure and gating mechanism of the acetylcholine receptor pore. Nature 423, 949–955.

Stock, D., Leslie, A. G. & Walker, J. E. Molecular architecture of the rotary motor in ATP synthase. Science 286, 1700–1705 (1999).

Stroebel, D., Choquet, Y., Popot, J. L. & Picot, D. An atypical haem in the cytochrome b(6)f complex. Nature 426, 413–418 (2003).

Xia, D. et al. Crystal structure of the cytochrome bc1 complex from bovine heart mitochondria. Science 277, 60–66 (1997).

Zouni, A. et al. Crystal structure of photosystem II from Synechococcus elongatus at 3.8 Å resolution. Nature 409, 739–743 (2001).

Binda, C., Newton-Vinson, P., Hubalek, F., Edmondson, D. E. & Mattevi, A. Structure of human monoamine oxidase B, a drug target for the treatment of neurological disorders. Nature Struct. Biol. 9, 22–26 (2002).

Bracey, M. H., Hanson, M. A., Masuda, K. R., Stevens, R. C. & Cravatt, B. F. Structural adaptations in a membrane enzyme that terminates endocannabinoid signaling. Science 298, 1793–1796 (2002).

Ferguson, K. M. et al. EGF activates its receptor by removing interactions that autoinhibit ectodomain dimerization. Mol. Cell 11, 507–517 (2003).

Newton, A. C. Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm. Biochem. J. 370, 361–371 (2003).

Kusumi, A. et al. Paradigm shift of the plasma membrane concept from the two-dimensional continuum fluid to the partitioned fluid: high-speed single-molecule tracking of membrane molecules. Annu. Rev. Biophys. Biomol. Struct. 34, 351–378 (2005).

Saxton, M. J. & Jacobson, K. Single-particle tracking: applications to membrane dynamics. Annu. Rev. Biophys. Biomol. Struct. 26, 373–399 (1997).

Hessa, T. et al. Recognition of transmembrane helices by the endoplasmic reticulum translocon. Nature 433, 377–381 (2005).

Picot, D., Loll, P. J. & Garavito, R. M. The X-ray structure of the membrane protein prostaglandin H2 synthase-1. Nature 367, 243–249 (1994).


Biological Membranes

Cell surface membranes are pretty important - they hold the cell together and control what goes in and out. Without them, our bodies would just be a soupy mess. Substances can move through the cell membrane in different ways, through osmosis, diffusion or active transport.

Fluid mosaic model

The structure of the plasma membrane is made up of a bilayer of phospholipids with proteins and cholesterol interspersed throughout the structure. The fluid mosaic model is used to describe the arrangement of molecules in the membrane - ‘fluid’ because the phospholipids are constantly moving around and ‘mosaic’ because protein molecules are scattered throughout the phospholipids like tiles in a mosaic. We refer to this concept as a ‘model’ because it is the best representation of membrane structure based on the evidence which is currently available. As we learn more about the structure of the plasma membrane, the fluid mosaic model may be updated.

Components of the plasma membrane

Phospholipids: consist of a hydrophilic head group which faces the intracellular / extracellular fluid and two hydrophobic tails which point towards each other, away from water. They are the main component of the plasma membrane and form a barrier to anything which is not lipid-soluble (such as ions and glucose).

Glycoproteins: these are proteins with sugar molecules attached. They act as recognition sites and antigens - antigens are like little ‘flags’ on the surface of our cells which allows our body to detect which cells are our own and which cells are foreign.

Glycolipids: these are phospholipids with sugar molecules attached. They have a similar function to glycoproteins - they also act as recognition sites and antigens. They also increase membrane stability by forming hydrogen bonds with water molecules.

Cholesterol: cholesterol is a lipid which slots in between the phospholipid tails, pushing them closer together. It regulates the stability and fluidity of the plasma membrane.

Intrinsic proteins: these are proteins which span both bilayers of the plasma membrane. They act as channels or carrier proteins to transport water-soluble molecules.

Extrinsic proteins: these are proteins which are found on the surface of the plasma membrane. They usually function as enzymes and catalyse chemical reactions inside the cell.

Transport across cell membranes

Molecules can make their way across the plasma membrane in one of three ways: osmosis (if the molecule is water), diffusion (if it is a molecule moving down its concentration gradient) or active transport (if it is a molecule moving against its concentration gradient. For larger substances to get into or out of the cell, such as proteins or carbohydrates, they will rely on processes called endocytosis and exocytosis.

Osmosis: osmosis is the movement of water molecules down its concentration gradient across a partially permeable membrane. It is a passive process so does not require energy in the form of ATP. Osmosis is responsible for the movement of water molecules into the root hair cells of plants, for example.

Simple diffusion: this is the movement of molecules down their concentration gradients. When molecules move by simple diffusion, they pass directly through the phospholipid bilayer. It is a passive process which means that no energy is required. Oxygen and carbon dioxide move by simple diffusion when they pass from the alveoli into the bloodstream during gas exchange.

The process of active transport.

Facilitated diffusion: facilitated diffusion involves the movement of molecules down their concentration gradients. It differs from simple diffusion in the fact that a carrier protein or a channel protein within the cell membrane helps them get from one side to the other. This is also a passive process. An example of facilitated diffusion is the movement of glucose molecules into liver cells through glucose transporter proteins embedded in the plasma membrane.

Active transport: when molecules move against their concentration gradients (so from a region of low concentration to a region of high concentration), they do so by active transport. This involves a carrier protein which carries the molecule from one side of the membrane to the other. It is an active process and uses ATP to release energy. An example is the transport of glucose from the villi of the intestine into the bloodstream.

Endocytosis: if substances are too large to cross the membrane, they enter the cell by endocytosis. The cell surrounds the substance and folds its membrane around it. The membrane then pinches off to engulf the substance, which causes a vesicle to form inside the cell containing the ingested substance. This is an active process so will require energy in the form of ATP. An example of endocytosis is when phagocytes carry out phagocytosis, in which the phagocyte engulfs a whole bacterium in order to destroy it.

Exocytosis: when large substances need to leave the cell, such as hormones and digestive enzymes, they do so by exocytosis. These substances will be contained inside vesicles which move towards the plasma membrane and fuse with it. This causes the substances to either be released outside of the cell or they will be inserted straight into the membrane (for example, if the substance is a membrane protein). Exocytosis is an active process which requires ATP.

Investigating cell membrane structure

The permeability of cell membranes is affected by things like temperature, pH and ethanol. You may be asked to describe an experiment to determine the effect of one of these factors on membrane permeability. These experiments using involve plant cells which contain a coloured pigment, such as beetroot, since we can measure the amount of membrane permeability depending on how much pigment leaks out of the cells and into the surrounding solution. The method for this type of experiment is outlined below:

  • Prepare eight cylinders of beetroot of equal size. Make these samples as similar as possible, e.g. by cutting from the same part of each plant. Rinse each piece to remove any pigment released during cutting.
  • If you are investigating the effect of temperature, prepare eight water baths of varying temperatures ranging from 0-70 o C.
  • Prepare a series of test tubes containing the same volume of water (e.g. 10 cm 3 ). Place the tubes in different water for five minutes.
  • Place a single sample of beetroot into each of the eight test tubes. Leave for 15 minutes.
  • Use forceps to remove the pieces of beetroot from each tube. Keep the coloured liquid and transfer into a cuvette.
  • Use a colorimeter to measure how much light is absorbed by each liquid. The darker the solution (i.e. the more permeable the membrane), the more light is absorbed.
  • Draw a graph plotting absorbance against temperature.

Temperature and membrane permeability

At temperatures below freezing, the permeability of cell membranes increases since the proteins in the membrane unfold and become deformed. The molecules in the membrane have low amounts of energy so cannot move around much. The phospholipids become closely packed together which makes the membrane rigid. When temperatures fall low enough for ice crystals to form, these can puncture the membrane which increases its permeability when the cell membrane rethaws, damaging the cell.

Between temperatures of O o C and 45 o C, membranes are partially permeable. As temperature increases, the components in the membrane gain kinetic energy and move around more. The more fluid the membrane is, the more substances it allows through.

As temperatures exceed 45 o C, permeability increases rapidly because proteins in the membrane become denatured and start to unravel. In addition, water inside the cell cytoplasm expands, putting pressure on the cell membrane and creating gaps within the bilayer.


Oxygen transport capacity

Whereas only 0.03 ml O2 * L 𢄡 * mmHg 𢄡 PO2 at 37ଌ can be transported in blood in physical solution, one gram of Hb can bind

1.34 ml of O2. Thus, the presence of a normal amount of Hb per volume of blood increases the amount of O2 that can be transported about 70-fold, which is absolutely essential to meet the normal tissue O2 demand. It is therefore apparent that an increased amount of Hb also increases the amount of O2 that can be delivered to the tissues (Figure ​ (Figure1). 1 ). In fact, the O2 transport capacity was found to correlate directly with aerobic performance as can be seen from an increase in performance after infusion of red blood cells (Berglund and Hemmingson, 1987) and by the strong correlation between total Hb and maximal O2 uptake (VO2,max) in athletes (for review see Sawka et al., 2000 Schmidt and Prommer, 2010). Calbet et al found that acute manipulations of the O2 carrying capacity also vary performance (Calbet et al., 2006). Thus, it is a clear advantage for aerobic athletic performance to have a high O2 transport capacity.

Parameters required to evaluate O2 transport capacity are the Hb concentration in blood (cHb) and hematocrit (Hct), as well as total Hb mass (tHb) and total red blood cell volume (tEV) in circulation. cHb and Hct are easy to measure with standard hematological laboratory equipment. Together with SO2 they indicate the amount of O2 that can be delivered to the periphery per unit volume of cardiac output. tHb and tEV indicate the total amount of O2 that can be transported by blood. A large tHb and tEV allows redirecting O2 to organs with a high O2 demand while maintaining basal O2 supply in less active tissues. Because they are affected by changes in plasma volume (PV) cHb and Hct allow no conclusion on tHb and tEV, respectively.

Results on cHb, Hct and red blood cell count in athletes and their comparison with values obtained in healthy, sedentary individuals are conflicting due to the fact that red blood cell volume and PV change independently and due to the many factors affecting each of these parameters (see below). Establishing normal values for tHb and tEV for athletes is hampered further by the possibility of use of means to increase aerobic capacity such as blood and erythropoietin (EPO) doping.

Hematocrit in athletes

Many but not all studies show lower Hct in athletes than in sedentary controls (Broun, 1922 Davies and Brewer, 1935 Ernst, 1987 Sawka et al., 2000). However, several studies also report higher than normal Hct. A highly increased Hct increases blood viscosity and increases the workload of the heart (El-Sayed et al., 2005 Böning et al., 2011). It therefore bears the risk of cardiac overload.

Many studies showed that Hct tended to be lower in athletes than in sedentary individuals (Broun, 1922 Davies and Brewer, 1935 Remes, 1979 Magnusson et al., 1984 Selby and Eichner, 1986 Ernst, 1987 Weight et al., 1992). This was verified by Sharpe et al. (2002) in the course of establishing reference Hct and Hb values for athletes. The found that out of

1100 athletes from different countries 85% of the female and 22% of the male athletes had Hct values below 44%. A tendency for an inverse correlation of Hct with training status, indicated by VO2,max, was also shown (Heinicke et al., 2001). However, a small proportion of sedentary controls and athletes has higher than normal Hct. Sharpe et al. (2002) found that 1.2% of all females and 32% of all males in their study had an Hct > 47%. When following female and male elite athletes and controls over a study period of 43 months Vergouwen (Vergouwen et al., 1999) found 6 males controls and 5 males athletes with a Hct > 50% and 5 females controls but no female athletes with a Hct > 47%.

Hematocrit during exercise

Changes in Hct occur rapidly. Hct increases during exercise due to a decrease in PV when fluid replacement during exercise is insufficient (Costill et al., 1974). There is fluid loss due to sweating, a shift of plasma water into the extracellular space due to the accumulation of osmotically active metabolites, and filtration as a consequence of an increased capillary hydrostatic pressure (Convertino, 1987). The resultant increase in plasma protein increases oncotic pressure and thus moderates fluid escape (Harrison, 1985). Changes appear less pronounced during swimming than running exercise, where immersion and the re-distribution of blood volume seem to cause shifts in PV independent of volume regulatory hormones (Böning et al., 1988). An increase in hematocrit due to catecholamine-induced sequestration of red blood cells from spleen is unlikely in humans but has been found in other species (Stewart and McKenzie, 2002).

Long-term changes of hematocrit

In a recent review, Thirup (2003) reports a within-subject variability of

3% when reviewing 12 studies on more than 600 healthy, non-smoking, mostly sedentary individuals, and when measurements were repeated in sampling intervals ranging from days to

2 months. Sawka et al. summarized data from 18 investigations and found that PV and blood volume increased rapidly after training sessions, whereas red cell volume remained unchanged for several days before it began to increase indicating that Hct values were decreased for several days (Sawka et al., 2000). The magnitude of Hct change seems to depend on exercise intensity during training sessions and the type of exercise (strength vs. endurance for review see Hu and Lin, 2012). A few weeks after the training intervention a new steady state had established, and Hct had returned to pre-training values (Sawka et al., 2000). The post-training increase in PV and the increased PV in highly trained athletes (e.g., Hagberg et al., 1998 Sawka et al., 2000 Heinicke et al., 2001 Schumacher et al., 2002) is likely caused by aldosterone dependent renal Na + reabsorption, and by water retention stimulated by elevated antidiuretic hormone in compensation for the water loss during individual training sessions (Costill et al., 1976 Milledge et al., 1982).

There appear to be quite large seasonal variations in Hct (relative change up to 15%) with lower values in summer than in winter that might result in season-to-season changes from

42% in summer and 48% in winter as found among several thousand study participants. Seasonal changes depend on climatic effects with larger differences in countries closer to the equator (Thirup, 2003). Studies of seasonal changes in Hct of athletes are sparse but indicate that Hct might be decreased by another 1𠄲% in summer by addition of a training effect.

The decreased Hct in athletes has been termed “sports anemia.” For a long time it had been explained with increased red blood cell destruction during exercise and thus appeared to be the same phenomenon as the well-known march hemoglobinuria (Broun, 1922 Kurz, 1948 Martin and Kilian, 1959). Intravascular destruction of red blood cells occurs at shear stresses between 1000 and 4000 dyn/cm 2 (Sutera, 1977 Sallam and Hwang, 1984), values well above physiological values at rest (Mairbäurl et al., 2013). It is related to the intensity and the kind of exercise (Yoshimura et al., 1980 Miller et al., 1988). Foot strike in runners has been the most often reported reason for intravascular hemolysis (Telford et al., 2003), which can be prevented by good shoe cushioning (Yoshimura et al., 1980 Dressendorfer et al., 1992). It also occurred during mountain hiking (Martin et al., 1992), in strength training (Schobersberger et al., 1990), karate (Streeton, 1967), in swimmers (Selby and Eichner, 1986 Robinson et al., 2006), basketball, Kendo-fencing, and in drummers (Schwartz and Flessa, 1973 Nakatsuji et al., 1978). Running exercise has been found to increase plasma hemoglobin from

120 mg/liter indicating that about 0.04% of all circulating red blood cells were lyzed (Telford et al., 2003). Exercise had been shown to alter red blood cell membrane appearance in correlation with elevated haptoglobin (Jordan et al., 1998). Senescent red blood cells may be particularly prone to exercise induced intravascular hemolysis as indicated by a decreased mean red blood cell buoyant density and a density distribution curve that was skewed toward younger, less dense cells in trained individuals indicated by increased levels of pyruvate kinase activity, 2,3-DPG and P50, higher reticulocyte counts (Mairbäurl et al., 1983). Other possible reasons for “sports anemia” under discussion are nutritional aspects such as insufficient protein intake and altered profile of blood lipids (for review see Yoshimura et al., 1980), and iron deficiency (Hunding et al., 1981).

Total hemoglobin mass (tHb) and total red blood cell volume (tEV)

As indicated above, PV is prone to acute changes, whereas changes in total red blood cell mass (or volume) are slow due to slow rates of erythropoiesis (Sawka et al., 2000). Therefore, total hemoglobin and/or red blood cell volume has to be measured in addition to cHb and Hct to obtain a reliable measure of the oxygen transport capacity. Several methods have been applied to determine these parameters.

Grehant and Quinquard (1882) were the first to describe blood volume measurements by use of carbon monoxide (CO)-rebreathing. This method is based on the much higher affinity of Hb to CO than to O2 (for review see Mairbäurl and Weber, 2012), which allows using CO in an indicator dilution method. It has been used to measure the fraction of blood mass relative to body mass by Arnold et al. (1921). The technique has been improved considerably by Sjostrand by advancing the method to estimate carboxy-hemoglobin (Sjostrand, 1948). To date CO rebreathing or inhalation has been further improved (Godin and Shephard, 1972 Schmidt and Prommer, 2005). MCHC is then used to calculate tEV, and Hct to estimate total blood volume. Total red blood cell volume can be determined directly after injection of 99m Tc-labeled red blood cells (Thomsen et al., 1991). By indirect means, total red blood cell volume can also be calculated from Hct after measuring the PV using Evans blue (T-1824), which binds to albumin, and by injection of 125 iodine-labeled albumin. Several of these methods have been compared by Thomsen et al. (1991) who reported a correlation of r = 0.99 between PV measured by 125 I-albumin and Evans blue, and showed that PV calculated from measuring tEV with labeled red blood cells was about 5�% lower than that from labeling albumin.

Applying these techniques Kjellberg et al. found that trained individuals had increased tHb (Kjellberg et al., 1949), a result that has been confirmed many times thereafter both by comparing groups of individuals with different training status and by measuring tEV before and after prolonged training periods (for a recent review see Sawka et al., 2000). Schmidt and Prommer summarized recently that different training modalities vary in their effects on tHb, where they put the main emphasis on training in hypoxia (Schmidt and Prommer, 2008). In summary, these studies show that an increase in tHb by 1 g achieved e.g., by administration of erythropoietin, increased VO2,max by

3 ml/min (Parisotto et al., 2000 Schmidt and Prommer, 2008). From the correlation shown by Heinicke et al. (2001) it can be derived that an increase in 1 g of tHb per kg body weight (g/kg) increased VO2,max by

5.8 ml/min/kg, where non-athletes (though with a rather high VO2,max of 45 ml/min/kg) had a tHb of 11 g/kg and their best athletes (average VO2,max = 71.9 ml/kg) had a tHb of 14.8 g/kg (Heinicke et al., 2001). Their findings fit well to the results reported by Kjellberg, who found a 37% higher tHb in elite athletes than in untrained individuals (Kjellberg et al., 1949). Schmidt and Prommer (2008) combined results from several of their studies and found a change in VO2,max of 4.2 ml/min/kg in males and of 4.4 ml/min/kg in females per change in tHb of 1 g/kg with very high correlation coefficients (r

0.79), whereas there was no correlation between VO2,max and Hb or Hct. However, there are also reports on a lack of difference in tHb between sedentary and trained individuals (Green et al., 1991). As mentioned above all these studies bear the burden of uncertainty that athletes may have taken measures to increase performance, which makes it difficult to establish “normal values” of tHb and tEV for athletes.

Different duration of exercise training (weeks vs. months) appear to explain the diverging results in the studies on tHb and training. Sawka et al. (2000) found no increase when training lasted less than 11 days. Also most studies on 4� months of training showed no or only small effects their own longitudinal study on “leisure sportsmen” resulted in an increase in tHb by

6% in the course of a 9-month endurance training (summarized in Schmidt and Prommer, 2008) indicating that adjustments of tHb and tEV by training are slow, and that a pronounced increase may require several years of training.

Sedentary high altitude residents have an increased tHb in comparison to their low altitude counterparts, where blood volume has been found to be increased from

100 ml/kg (Hurtado, 1964 Sanchez et al., 1970). Results on sojourners to high altitude indicate that, similar to training, the increase in tHb and blood volume is also slow requiring weeks to months of high altitude exposure. At high altitude, the increase may be masked by a decrease in PV (Reynafarje et al., 1959). Therefore, a short-term stay at moderate and high altitude will not increase tHb and tEV (Myhre et al., 1970). A summary of 14 different studies Sawka et al. (2000) shows that several studies found no change in tEV upon ascent whereas some did, and explained discrepancies with the difference in the duration of exposure to high altitude. A gain in tEV between 62 and 250 ml/week was found when the sojourn lasted about 3 weeks.

Based on the raise in tEV upon ascent to high altitude and by training in normoxia it was concluded that effects of training and high altitude exposure on tHb might be additive, and that training at simulated altitude or by ascent to moderate or high altitude should cause an even further increase than training in normoxia. However, results are inconsistent ranging from no effect (Svedenhag et al., 1997 Friedmann et al., 1999) to a pronounced increase after 3𠄴 weeks of training at altitudes between 2100 and 2400 m (Levine and Stray-Gundersen, 1997 Friedmann et al., 2005 Heinicke et al., 2005). Lack of effects has in part been explained with lower training intensities at high than at low altitude, which is due to the decrease in performance with increasing altitude (Cerretelli and DiPrampero, 1985). Several strategies have been developed aimed at improving the training efficiency while still 𠇌onsuming” adjustments to hypoxia, one being the “sleep-high-train-low” protocol. Current concepts and concerns are reviewed in (Richalet and Gore, 2008 Stray-Gundersen and Levine, 2008 Robach and Lundby, 2012). Results are unclear, and often show no effect on tHb [e.g., in a well-designed, Placebo-controlled study by Siebenmann et al. (2012)]. A thorough analysis reveals that more than 14h per day of exposure to hypoxia seem to be required to attain a detectible increase in tHb and tEV (analysis in Schmidt and Prommer, 2008).

Control of erythropoiesis

It has been recognized by Bert (1882) that live at high altitude corresponds with increased hemoglobin, and later that Hct, Hb, and tHb are increased (Reynafarje et al., 1959 Hurtado, 1964 Sanchez et al., 1970), which was later recognized to be associated with elevated levels of erythropoietin (Mirand and Prentice, 1957 Scaro, 1960 Siri et al., 1966). The elevated tEV is thought to compensate for the decreased arterial O2-content when the inspired PO2 is low. Stimulation of vascularization by the vascular endothelial growth factor, VEGF, is another means warranting tissue O2 supply in chronic hypoxia (for review see e.g., Marti, 2005). Both processes depend on sensing hypoxia within typical target cells and specific signaling pathways that adjust the expression of specific genes.

One such oxygen dependent mechanism is the control of expression by hypoxia inducible factors, HIF (Semenza, 2009). Active HIF consists of alpha and beta subunits. The beta subunit (HIF-β, also called ARNT) is expressed constitutively and is not directly affected by oxygen levels (Semenza, 1999). There are several isoforms of the alpha subunit, where HIF-1α seems to mainly control metabolic adjustments such as glycolysis (Hu et al., 2003), and HIF-2α has been identified as the major regulator of erythropoiesis (Scortegagna et al., 2005 Gruber et al., 2007). In hypoxia, the hydroxylation of HIF-alpha subunits by prolyl-hydroxylases (PDH) is prevented due to the lack of O2 required as a direct substrate, which then prevents the hydroxylation-dependent poly-ubiquitinylation by the Van Hippel-Lindau tumor suppressor pVHL-E3 ligase and subsequent proteasomal degradation (Schofield and Ratcliffe, 2004) resulting in increased protein levels of HIF alpha subunits. Upon stabilization, alpha subunits enter the nucleus, where they dimerize with HIF-β. The dimer binds to a specific base sequence in the promoter region of genes called hypoxia response element, HRE, to induce the expression of genes (for recent reviews see (Semenza, 2009 Haase, 2010)). Besides stabilization, HIF-alpha subunits are also controlled at the transcriptional level (Görlach, 2009 Semenza, 2009).

In his review Haase (2010) nicely summarizes the experiments that led to the conclusion that HIF-2α is the major regulator of EPO production in liver (fetal) and kidney (adults), but that there are also a variety of different direct and indirect mechanisms. As shown in the scheme provided by Semenza (2009), although at that time related to actions of HIF-1α rather than HIF-2α, it can be seen that hypoxia controlled gene expression regulates not only the expression of EPO but also the expression of proteins whose action is a prerequisite for erythropoiesis such as EPO-receptors, iron transporters mediating intestinal iron reabsorption, and transferrin and transferrin receptors required for iron delivery to peripheral cells.

In the adult, the oxygen sensor controlling EPO production is in the kidney, where the cells producing EPO have been shown to be peritubular fibroblasts in the renal cortex (Maxwell et al., 1993 Eckardt and Kurtz, 2005). EPO production can be induced by two kinds of hypoxia: one is a decreased PO2 in the kidney and in other tissues while the hemoglobin concentration is normal such as in hypoxic hypoxia. The other is called anemic hypoxia, where the hemoglobin concentration is decreased and but arterial PO2 is normal resulting in a decreased venous PO2 (Eckardt and Kurtz, 2005). There appears no difference in the effectiveness to produce EPO between these two situations. A mixture of these conditions might be a situation causing a decreased blood flow to the kidney at normal PO2 and hemoglobin concentration, which should also result in decreased capillary and venous PO2. The exact mechanisms controlling EPO production by the fibroblasts is not fully understood but appears to involve hypoxia-dependent recruitment of fibroblasts located in juxta-medullary and cortical regions (Eckardt et al., 1993).

EPO released into blood has many functions other than stimulating erythropoiesis (for review see Sasaki, 2003). In the bone marrow EPO binds to EPO receptors on progenitor cells in erythroblastic islands (Chasis and Mohandas, 2008), where it stimulates proliferation and prevents apoptotic destruction of newly formed cells (Lee and Percy, 2010). This increases the amount of red blood cells released from the bone marrow per time resulting in increased tEV when the rate of release exceeds red blood cell destruction (see above, sports anemia).

Effects of exercise and training on erythropoiesis

The increased tHb and tEV in trained athletes indicates that exercise stimulates erythropoiesis. An additional marker is the elevation of reticulocytes counts which can be observed within 1𠄲 days (Schmidt et al., 1988) after endurance (Convertino, 1991) and strength training units (Schobersberger et al., 1990). Despite apparent effects of single training units on red blood cell production several studies show that reticulocyte counts in athletes are not much different from sedentary controls (Lombardi et al., 2013) and values appear quite stable over years (Banfi et al., 2011 Diaz et al., 2011). There is, however, significant variation of reticulocyte counts in athletes during the year showing in general higher reticulocyte counts at the beginning of a season but lower values after intensive training sessions, competitions, and at the end of a season (Banfi et al., 2011). Nevertheless, markers of pre-mature forms of reticulocytes are increased in athletes, which is indicative of stimulated bone marrow (Diaz et al., 2011 Jelkmann and Lundby, 2011).

Whereas the control of erythropoiesis in hypoxic and anemic hypoxia is well-understood, the signals stimulating erythropoiesis upon training in normoxia are unclear. Exposure to hypoxia causes a fast increase in EPO (Eckardt et al., 1989), but no or only minor changes in EPO have been observed after exercise of different modalities in untrained and trained individuals (Schmidt et al., 1991 Bodary et al., 1999), whereas the time course of change in reticulocyte count is similar to effects of high altitude (Schmidt et al., 1988 Mairbäurl et al., 1990). The higher reticulocyte counts, a decreased mean red blood cell buoyant density and mean corpuscular hemoglobin concentration, and increased levels of other markers of a decreased mean red blood cell age (higher 2,3-DPG and P50, higher red blood cell enzyme activities and creatine) have been found in peripheral blood from trained individuals (Mairbäurl et al., 1983 Schmidt et al., 1988), which are all indicators of an increased red blood cell turnover (Schmidt et al., 1988 Smith, 1995) and thus stimulated erythropoiesis. These newly formed red blood cells ease the passage of blood through capillaries because of a higher membrane fluidity and deformability of (Kamada et al., 1993).

Arguments for hypoxia as the relevant trigger for exercise induced erythropoiesis are sparse, and are at best indirect. Even during heavy exercise there is only a small decrease in arterial PO2 (see chapter 2, arterial O2 loading) that by itself will barely be sufficient to cause relevant renal EPO production. There is, however, a considerable decrease in renal blood flow with increasing exercise intensity that decreases renal O2 supply (for an excellent review on splanchnic blood flow regulation in exercise see Laughlin et al., 2012). The O2 supply to renal tubules might be further decreased, because renal cortical arteries and veins run parallel allowing exchange diffusion of O2 that may cause arterial deoxygenation. PO2 in cortical veins is low because of the high oxygen consumption required for Na + and water reabsorption of renal cortical epithelial cells (Eckardt and Kurtz, 2005). It can therefore be speculated that the decreased flow during exercise further decreases renal cortical PO2 to a level causing significant hypoxia of the peritubular, EPO producing fibroblasts during exercise, and that this effect is aggravated as exercise intensity increases. Interestingly, training attenuates the decrease in renal blood flow, which seems more pronounced following endurance than high-intensity interval sprint training in rats (Musch et al., 1991 Padilla et al., 2011), which might explain the weak erythropoietic response in highly trained athletes.

A variety of humoral factors known to affect erythropoiesis also change during exercise. Androgens are long known for their stimulatory effect on erythropoiesis by stimulation of EPO release, increasing bone marrow activity, and iron incorporation into the red cells, which is best indicated by polycythemia as a consequence of androgen therapy (Shahidi, 1973 Shahani et al., 2009). Endurance exercise and resistance training cause a transient increase in testosterone levels in men and women (Hackney, 2001 Enea et al., 2009). Post-exercise values vary with exercise intensity in both genders. Interestingly, post-exercise testosterone levels also directly change with mood (win vs. loss), which seems more pronounced in men than women (for review see Shahani et al., 2009).

Stress hormones such as catecholamines and cortisol stimulate the release of reticulocytes from the bone marrow and possibly also enhance erythropoiesis (Dar et al., 2011 Hu and Lin, 2012). Erythropoiesis is also stimulated by growth hormone and insulin-like growth factors (Kurtz et al., 1988 Christ et al., 1997) which also increase during exercise (Hakkinen and Pakarinen, 1995 Schwarz et al., 1996).


Author information

Yuyong Tao and Lily S. Cheung: These authors contributed equally to this work.

Affiliations

Department of Molecular and Cellular Physiology, 279 Campus Drive, Stanford University School of Medicine, Stanford, 94305, California, USA

Yuyong Tao, Shuo Li, Yan Xu & Liang Feng

Department of Plant Biology, Carnegie Institution for Science, 260 Panama Street, Stanford, 94305, California, USA

Lily S. Cheung, Joon-Seob Eom, Li-Qing Chen & Wolf B. Frommer

Center of Growth, Metabolism and Aging, Key Laboratory of Bio-Resource and Eco-Environment of Ministry of Education, College of Life Sciences, Sichuan University, Chengdu, 610014, China

NE-CAT and Dep. of Chemistry and Chemical Biology, Cornell University, Building 436E, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, 60439, Illinois, USA


Difference between Plant and Animal cells

Another important distinction in our fundamental unit of life class 9 notes is between the plant cell and the animal cell. Refer to the table mentioned below to understand it better-

Plant Cells Animal Cells
A rigid cell wall encapsulates the plasma membrane No cell wall present
Larger than animal cells Much smaller than plant cells
Contain plastids Do not contain plastids (except protozoan Euglena)
Vacuoles are large and permanent Vacuoles are small and temporary
Do not contain centrosomes and centrioles Do contain centrosomes and centrioles

Hopefully, through these fundamental unit of life class 9 notes, you are well versed with this chapter of biology. Consult Leverage Edu experts for career counselling and have a better understanding of which career path to choose.


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