Chapter 1: Cell and Membrane Physiology

Intro

Contents

Overview

The human body comprises several distinct organs, each of which has a unique role in supporting the life and well-being of the individual. Organs are, in turn, composed of tissues. Tissues are collections of cells specialized to perform specific tasks that are required of the organ. Although cells from any two organs may appear strikingly dissimilar at the microscopic level (compare the shape of a red blood cell with the branching structure of a nerve cell’s dendritic tree, for example, as in Figure 1.1), morphology can be misleading because it masks a set of common principles in design and function that apply to all cells.

All cells are enclosed within a membrane that separates the inside of the cell from the outside. This barrier allows the cells to create an internal environment that is optimized to support the biochemical reactions required for normal function. The composition of this internal environment varies little from cell to cell. Most cells also contain an identical set of membrane-bound organelles: nuclei, endoplasmic reticulum (ER), lysosomes, Golgi apparatuses, mitochondria.

Specialization of cell and organ function is usually achieved by adding a novel organelle or structure, or by altering the mix of membrane proteins that provide pathways for ions and other solutes to move across the barrier. This chapter reviews some common principles of molecular and cellular function that will serve as a foundation for later discussions of how the various organs contribute to maintaining normal bodily function.

Figure 1.1 Differences in cell morphology.

Cellular Environment

Cells are bathed in an extracellular fluid (ECF) that contains ionized sodium (Na+), potassium (K+), magnesium (Mg2+), chloride (Cl?), phosphate (PO4 3?), bicarbonate (HCO3 ?), glucose, and small amounts of protein (Table 1.1). It also contains around 2 mmol free calcium (Ca2+). Ca2+ is essential to life, but many of the biochemical reactions required of cells can only occur if free Ca2+ concentrations are lowered ten-thousandfold, to around 10?7 mol.

Thus, cells erect a barrier that is impermeable to ions (the plasma membrane) to separate intracellular fluid ([ICF] or cytosol) from ECF and then selectively modify the composition of ICF to facilitate the biochemical reactions that sustain life. ICF is characterized by low Ca2+, Na+, and Cl? concentrations compared with ECF, whereas the K+ concentration is increased. Cells also contain more free protein than does the ECF, and the pH of ICF is slightly more acidic.

Table 1.1:Extracellular and Intracellular Fluid Composition

Solute ECF ICF
Na+ 145 12
K+ 4 120
Ca2+ 2.5 0.0001
Mg2+ 1 0.5
Cl? 110 15
HCO3 ? 24 12
Phosphates 0.8 0.7
Glucose 5 ;1
Proteins (g/dL) 1 30
pH 7.4 7.2

Membrane Composition

Membranes comprise lipid and protein (Figure 1.2). Lipids form the core of all membranes. Lipids are ideally suited to a barrier function because they are hydrophobic: They repel water and anything dissolved in it (hydrophilic molecules). Proteins allow cells to interact with and communicate with each other, and they provide pathways that allow water and hydrophilic molecules to cross the lipid core.

Figure 1.2Membrane structure.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams & Wilkins, 2010

A. Lipids

Membranes contain three predominant types of lipid: phospholipids, cholesterol, and glycolipids. All are amphipathic in nature, meaning that they have a polar (hydrophilic) region and a nonpolar (hydrophobic) region. The polar region is referred to as the head group. The hydrophobic region is usually composed of fatty acid “tails” of variable length. When the membrane is assembled, the lipids naturally gather into a continuous bilayer (Figure 1.3). The polar head groups gather at the internal and external surfaces where the two layers interface with ICF and ECF, respectively. The hydrophobic tail groups dangle down from the head groups to form the fatty membrane core. Although the two halves of the bilayer are closely apposed, there is no significant lipid exchange between the two membrane leaflets.

Figure 1.3 Membrane lipid bilayer.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams ; Wilkins, 2010

1. Phospholipids

Phospholipids are the most common membrane lipid type. Phospholipids comprise a fatty acid tail coupled via glycerol to a head group that contains phosphate and an attached alcohol. Dominant phospholipids include phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, and phosphatidylglycerol. Sphingomyelin is a related phospholipid in which glycerol has been replaced by sphingosine. The alcohol group in sphingomyelin is choline.

2. Cholesterol

Cholesterol is the second most common membrane lipid. It is hydrophobic but contains a polar hydroxyl group that draws it to the bilayer’s outer surface, where it nestles between adjacent phospholipids (Figure 1.4). Between the hydroxyl group and the hydrocarbon tail is a steroid nucleus. The four steroid carbon rings make it relatively inflexible, so adding cholesterol to a membrane reduces its fluidity and makes it stronger and more rigid.

Figure 1.4Cholesterol location with the membrane.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams ; Wilkins, 2010

3. Glycolipids

The outer leaflet of the bilayer contains glycolipids, a minor but physiologically significant lipid type comprising a fatty acid tail coupled via sphingosine to a carbohydrate head group. The glycolipids create a carbohydrate cell coat that is involved in cell-to-cell interactions and that conveys antigenicity.

B. Proteins

The membrane’s lipid core seals the cell in an envelope across which only lipid-soluble materials, such as O2, CO2, and alcohol can cross. Cells exist in an aqueous world, however, and most of the molecules that they need to thrive are hydrophilic and cannot penetrate the lipid core. Thus, the surface (plasma) membrane also contains proteins whose function is to help ions and other charged molecules across the lipid barrier. Membrane proteins also allow for intercellular communication and provide cells with sensory information about the external environment. Proteins are grouped on the basis whether they localize to the membrane surface (peripheral) or are integral to the lipid bilayer (Figure 1.5).

Figure 1.5 Membrane proteins.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams ; Wilkins, 2010

1. Peripheral

Peripheral proteins are found on the membrane surface. Their link to the membrane is relatively weak and, thus, they can easily be washed free using simple salt solutions. Peripheral proteins associate with both the intracellular and extracellular plasma membrane surfaces.

a. Intracellular

Proteins that localize to the intracellular surface include many enzymes; regulatory subunits of ion channels, receptors, and transporters; and proteins involved in vesicle trafficking and membrane fusion as well as proteins that tether the membrane to a dense network of fibrils lying just beneath its inner surface. The network is composed of spectrin, actin, ankrin, and several other molecules that link together to form a subcortical cytoskeleton (see Figure 1.5).

b. Extracellular

Proteins located on the extracellular surface include enzymes, antigens, and adhesion molecules. Many peripheral proteins are attached to the membrane via glycophosphatidylinositol ([GPI] a glycosylated phospholipid) and are known collectively as GPI-anchored proteins.

2. Integral

Integral membrane proteins penetrate the lipid core. They are anchored by covalent bonds to surrounding structures and can only be removed by experimentally treating the membrane with a detergent. Some integral proteins may remain localized to one or the other of the two membrane leaflets without actually traversing its width. Others may weave across the membrane many times (transmembrane proteins) as shown in Figure 1.6. Examples include various classes of ion channels, transporters, and receptors.

Figure 1.6Membrane-spanning proteins.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams & Wilkins, 2010


Clinical Application 1.1: Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, inherited disease caused by a defect in the gene that encodes phosphatidylinositol glycan A. This protein is required for synthesis of the glycophosphatidylinositol anchor used to tether peripheral proteins to the outside of the cell membrane. The gene defect prevents cells from expressing proteins that normally protect them from the immune system. The nighttime appearance of hemoglobin in urine (hemoglobinuria) reflects red blood cell lysis by immune complement. Patients typically manifest symptoms associated with anemia. PNH is associated with a significant risk of morbidity, in part, because patients are prone to thrombotic events. The reason for the increased incidence of thrombosis is not well delineated.


Diffusion

Movement across a membrane requires a motive force. Most substances cross the plasma membrane by diffusion, their movement driven by a transmembrane concentration gradient. When the concentration difference across a membrane is unfavorable, however, then the cell must expend energy to force movement “uphill” against the concentration gradient (active transport).

A. Simple diffusion

Consider a container filled with water and divided into two compartments by a pure lipid membrane (Figure 1.7). Blue dye is now dropped into the container at left. Initially, the dye remains concentrated and restricted to its small entry area, but molecules of gas, water, or anything dissolved in water are in constant thermal motion. These movements cause the dye molecules to distribute randomly throughout the entire chamber, and the water eventually becomes a uniform color, albeit lighter than the original drop.

The example shown in Figure 1.7 assumes that the dye is unable to cross the membrane, so the chamber on the right remains clear, even though the difference in dye concentrations across the barrier is very high. However, if the dye is lipid soluble or is provided with a pathway (a protein) that allows it to cross the barrier, diffusion will carry the molecules into the second chamber, and the entire tank will turn blue (Figure 1.8).

Figure 1.7 Simple diffusion in water.

Figure 1.8 Diffusion through a lipid bilayer.

B. Fick law

The rate at which molecules such as blue dye cross membranes can be determined using a simplified version of the Fick law:

where J is diffusion rate (in mmol/s), P is a permeability coefficient, A is membrane surface area (cm2), and C1 and C2 are dye concentrations (mmol/L) in compartments 1 and 2, respectively. The permeability coefficient takes into account a molecule’s diffusion coefficient, partition coefficient, and the thickness of the barrier that it must traverse.

1. Diffusion coefficient

Diffusion rates increase when a molecule’s velocity increases, which is, in turn, determined by its diffusion coefficient. The coefficient is proportional to temperature and inversely proportional to molecular radius and the viscosity of the medium through which it diffuses. In practice, small molecules diffuse quickly through warm water, whereas large molecules diffuse very slowly through cold, viscous solutions.

2. Partition coefficient

Lipid-soluble molecules, such as fats, alcohols, and some anesthetics can cross the membrane by dissolving in its lipid core, and they have a high partition coefficient. Conversely, ions such as Na+ and Ca2+ are repelled by lipids and have a very low partition coefficient. A molecule’s partition coefficient is determined by measuring its solubility in oil compared with water.

3. Distance

Net diffusion rate slows when molecules have to traverse thick membranes compared with thin ones. The practical consequences of this relationship can been seen in the lungs (see 22?II?C) and fetal placenta (see 37?III?B), organs designed to maximize diffusional rates by minimizing diffusional distance between two compartments.

4. Surface area

Increasing the surface area available for diffusion also increases the rate of diffusion. This relationship is used to practical advantage in several organs. The lungs comprise 300,000,000 small sacs (alveoli) that have a combined surface area of ~80 m2 that allows for efficient O2 and CO2 exchange between blood and the atmosphere (see 22?II?C). The lining of the small intestine is folded into fingerlike villi (Figure 1.9), and the villi sprout microvilli that, together, create a combined surface area of ~200 m2 (see 31?II). Surface area amplification allows for efficient absorption of water and nutrients from the gut lumen. Efficient exchange of nutrients and metabolic waste products between blood and tissues is ensured by a vast network of small vessels (capillaries) whose combined surface area exceeds 500 m2 (see 19?II?C).

5. Concentration gradient

The rate at which molecules diffuse across a membrane is directly proportional to the concentration difference between the two sides of the membrane. In the example shown in Figure 1.8, the concentration gradient (and, thus, the dye diffusion rate) between the two compartments is high initially, but the rate slows and eventually ceases as the gradient dissipates and the two sides equilibrate.

Note that thermal motion causes the dye molecules to continue moving back and forth between the two compartments at equilibrium, but net movement between the two is zero. If there were a way of removing dye continually from the chamber on the right, the concentration gradient and diffusion rate would remain high (we would also have to keep adding dye to the left chamber to compensate for movement across the barrier).

Figure 1.9 Intestinal villi.

From Mills, S.E. Histology for Pathologists. Third Edition. Lippincott Williams ; Wilkins, 2007


O2 movement between the atmosphere and the pulmonary circulation occurs by simple diffusion, driven by an air–blood O2 concentration gradient. Blood carries away O2 as fast as it is absorbed, and breathing movements constantly renew the O2 content of the lungs, thereby maintaining a favorable concentration gradient across the air–blood interface (see 23?V).


C. Charged molecules

The prior discussion assumes that the dye is uncharged. The same basic principles apply to diffusion of a charged molecule (an electrolyte), but electrolytes are also influenced by electrical gradients. Positively charged ions such as Na+, K+, Ca2+, and Mg2+ (cations) and negatively charged ions such as Cl? and HCO3 ? (anions) are attracted to and will move toward their charge opposites. Thus, if the dye in Figure 1.8 carries a positive charge, and an electrical gradient is imposed across the container, dye molecules will move back through the membrane toward the negative electrode (Figure 1.10).

The electrical gradient in Figure 1.10 was generated using a battery, but the same effect can be achieved by adding membrane-impermeant anions to Chamber 1. If Chamber 1 is filled with cations, the dye molecules will be repelled by the positive (like) charge and will accumulate in Chamber 2 (Figure 1.11). Note that the electrical gradient causes the dye concentration gradient between the two chambers to reform.

Dye molecules will continue migrating from Chamber 2 back to Chamber 1 until the concentration gradient becomes so large that it equals and opposes the electrical gradient, at which point an electrochemical equilibrium has been established. As discussed in Chapter 2, most cells actively expel Na+ ions to create an electrical gradient across their membranes. They then use the power of the combined electrical and chemical gradient (the electrochemical gradient) to move ions and other small molecules (e.g., glucose) across their membranes and for electrical signaling.

Figure 1.10 Charge movement induced by an electrical gradient.

Figure 1.11 Repellent effects of like charges.

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Pores, Channels, and Carriers

Small, nonpolar molecules (e.g., O2 and CO2) diffuse across membranes rapidly, and they require no specialized pathway. Most molecules common to the ICF and ECF are charged, however, meaning that they require assistance from a pore, channel, or carrier protein to pass through the membrane’s lipid core.

A. Pores

Pores are integral membrane proteins containing unregulated, water-filled passages that allow ions and other small molecules to cross the membrane. Pores are relatively uncommon in higher organisms because they are always open and can support very high transit rates (Table 1.2). Unregulated holes in the lipid barrier potentially can kill cells by allowing valuable cytoplasmic constituents to escape and Ca2+ to flood into the cell from the ECF.

Aquaporin (AQP) is a ubiquitous water-selective pore. There are 13 known family members (AQP0–AQP12), three of which are expressed widely throughout the body (AQP1, AQP3, and AQP4). AQP is found wherever there is a need to move water across membranes. AQPs play a critical role in regulating water recovery from the renal tubule (see 27?V?C), for example, but they are also required for lens transparency in the eye (AQP0), keeping skin moist (AQP3), and mediating brain edema following insult (AQP4). Because AQP is always open, cells must regulate their water permeability by adding or removing AQP from the membrane.

Table 1.2:Approximate Transit Rates for Pores, Channels, and Carriers

Pathway Example Molecule(s) Moved Transit Rate (Number/s)
Pores Aquaporin-1 H2O 3 × 109
Channels Na+ Na+ 108
ClC1 Cl? 106
Carriers Na+-K+ ATPase Na+, K+ 3 × 102

Clinical Application 1.2: Staphylococcus aureus

Staphylococcus aureus is a skin-borne bacterium that is a leading cause of community- and hospital-acquired, bloodborne “staph” infections (bacteremia). The incidence of such infections has been rising steadily in recent decades and is of growing concern because of the high associated mortality rates and the increasing prevalence of antibiotic-resistant strains such as MRSA (methicillin-resistant Staphylococcus aureus).

The bacterium’s lethality is due to its producing a toxin (?-hemolysin) that kills blood cells. The toxin targets the plasma membrane, where toxin monomers assemble into a multimeric protein with an unregulated pore at its center. The cell loses control of its internal environment as a result. The ion gradients dissipate, and the cell swells and then blisters and ruptures, releasing nutrients that are believed to nourish the bacterium as it proliferates.

Skin lesions caused by Staphylococcus aureus.

From Fleisher, G.R., Ludwig, W., and Baskin M.N. Atlas of Pediatric Emergency Medicine. Lippincott Williams ; Wilkins, 2004


B. Channels

Ion channels are transmembrane proteins that assemble so as to create one or more water-filled passages across the membrane. Channels differ from pores in that the permeability pathways are revealed transiently (channel opening) in response to a membrane-potential change, neurotransmitter binding, or other stimulus, thereby allowing small ions (e.g., Na+, K+, Ca2+, and Cl?) to enter and traverse the lipid core (Figure 1.12).

Ion movement is driven by simple diffusion and powered by the transmembrane electrochemical gradient. Ions are forced to interact with the channel pore briefly so that their chemical nature and suitability for passage can be established (a selectivity filter), but the rate at which ions traverse the membrane via channels can be as high as 108 per second (see Table 1.2). All cells express ion channels, and there are numerous types, including the voltage-gated Na+ channel that mediates nerve action potentials and voltage-gated Ca2+ channels that mediate muscle contraction. Ion channels are discussed in detail in Chapter 2.

Figure 1.12 Ion channel opening.

Modified from Chandar, N. and Viselli, S. Lippincott’s Illustrated Reviews: Cell and Molecular Biology. Lippincott Williams & Wilkins, 2010