5: Cellular Membranes

Membrane Composition and Structure

· Cell membranes are bilayered, dynamic structures that perform vital physiological roles, form boundaries between cells and their molecular environments, and regulate traffic in and out of cells.

· Lipids, proteins, and carbohydrates in sundry combinations make these tasks possible.

· The lipid portion of a cellular membrane provides a permeable barrier for water-soluble molecules and establishes the physical integrity of the membrane.

· Lipids are like the water of a lake in which proteins “float.” This is called the fluid mosaic model. (See Figure 5.1.)

· Membrane proteins are embedded in the lipid bilayer.

· Carbohydrates attach to lipid or protein molecules on the membrane, generally on the outer surface.

Lipids constitute the bulk of a membrane

· Most of the lipid molecules found in biological membranes are phospholipids.

· Each has a hydrophilic (“water-loving”) side, where the phosphate groups are located, and a hydrophobic (“water-hating”) side, the fatty acid “tails.” (See Figure 5.2.)

· The lipids organize themselves into a bilayer with the hydrophilic regions facing either the outside of the cell or the interior cytoplasmic face. The hydrophobic, hydrocarbon-rich regions of each layer face each other and face away from the watery internal or external environment. This stabilizes the entire membrane structure.

· Artificial membranes can be made in the laboratory. They naturally (spontaneously) form sheets of membranes that are lipid bilayers.

· The interior of the membrane is fluid, with the consistency of lightweight machine oil, and allows some molecules to move laterally along the plane of the membrane.

· Whereas lipid molecules can move laterally and do so rapidly, they seldom move from the exterior layer to the cytoplasmic layer or vice versa.

· Such a change in position would be referred to as flip-flop.

· Flip-flop is rare without the aid of special proteins designed for the task.

· Although all biological membranes are structurally similar, some have quite different compositions of lipids and proteins.

· Some lipid membranes have 25% cholesterol, while others have no cholesterol at all.

· Cholesterol is an important part of the lipid membranes that contain it. Under some conditions, cholesterol increases membrane fluidity; at other times it decreases fluidity. The effect cholesterol has on fluidity depends on other factors, such as the fatty acid composition of the other lipids found in the membrane.

· In general, shorter fatty acids make for a more fluid membrane, as do unsaturated fatty acids.

· For any given membrane, fluidity decreases with declining temperature, and membrane function may decline in organisms that cannot maintain warm bodies. The membranes of cells that live at low temperatures tend to be high in unsaturated and short-chain fatty acids.

Membrane proteins are asymmetrically distributed

· All biological membranes contain proteins.

· On average, there is one protein molecule for every 25 phospholipid molecules found in plasma membranes.

· This ratio varies, however. The inner membranes of mitochondria have one protein for every 15 lipids. Myelin, which coats nerve cells and acts as an insulator, has about one protein for every 70 lipids. (See Figure 5.1.)

· Many proteins have both polar and nonpolar regions due to the presence of both hydrophilic and hydrophobic R groups in their amino acids.

· The association of many protein molecules with lipid molecules is not covalent; both are free to move around laterally, according to the fluid mosaic model.

· Integral membrane proteins are those that have hydrophobic regions of amino acids that penetrate or entirely cross the phospholipid bilayer.

· These proteins have long hydrophobic amino acid areas, and form an a-helical region, which tends to interact with only the hydrophobic membrane core. (See Figure 5.4.)

· The hydrophilic R groups face the aqueous areas and interact with them.

· Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer. These proteins have polar or charged groups that interact with exposed portions of integral membrane proteins or phospholipid molecules.

· Some membrane proteins are covalently attached to fatty acids or other lipid groups. The lipid portion inserts into the lipid bilayer and tethers the protein to the membrane.

· Some of the proteins and lipids can move around in the membrane.

· Experiments demonstrated that when two cells are fused to form a continuous membrane, membrane proteins distribute uniformly around this new cell.

· Two cells were stained in such a way that each cell’s proteins were a different color.

· The two cells were fused.

· Within minutes, the mixing of membrane proteins could be seen using light microscopy.

· Some proteins are restricted in movement, because of anchoring to components of the cytoskeleton.

· This tethering causes an unequal distribution of these proteins so that certain regions of the cell membrane can be specialized.

· Transmembrane proteins have a specific orientation

· One side might be shaped such that it can act as a receptor for a signaling molecule, and the other side changes shape in response to the signal binding.

· Some cells are specialized to be anchored to the extracellular matrix. The externally exposed portion of the protein locks onto extracellular polymers, while the internal region might tie to the cytoskeleton.

Membrane carbohydrates are recognition sites

· Some cells have carbohydrates associated with their external surfaces.

· Carbohydrate-bound lipid is called glycolipid.

· Most of the carbohydrate in the membrane is covalently bonded to proteins, forming glycoproteins.

· The carbohydrate is added in the endoplasmic reticulum, and modifications occur in the Golgi apparatus.

· Plasma membrane glycoproteins enable cells to be recognized by other cells and proteins, utilizing an “alphabet” of single sugars to create a diversity of messages.

Cell Adhesion

· An example of cell adhesion is found in sponges.

· A sponge is a multicellular marine animal.

· If the sponge is treated in such a way as to cause its cells to disaggregate, what was a single animal is now thousands of individual cells. If the cells are left together in a solution for a few hours, they reaggregate into a sponge. (See Figure 5.5.) This is an example of species-specific cell adhesion.

· If two different species of sponges are disaggregated into individual cells and the two types of cells are placed together, the cells will reaggregate the way they were: Two sponges will appear. The cells from one species will aggregate only with cells from its own species. However, if the cells from two sponges of the same species are combined, just one large sponge will form upon reaggregation.

· This species-specific aggregation occurs because there are species-specific cell adhesion molecules that fit together to create the tissues of the sponge.

· The adhesion involves plasma membrane recognition proteins.

Cell adhesion involves recognition proteins

· The membrane proteins responsible for the cell–cell recognition in sponges were the first ever to be identified and purified.

· The recognition protein is a large glycoprotein, composed of 80% sugar. It is partially embedded in the plasma membrane.

· The species-specific recognition portion faces outward and is exposed to the environment.

· There are two general ways that cell adhesion molecules work.

· One is called homotypic (homo means same; typic, type) and occurs when both cells possess the same type of cell surface receptor and their interaction causes them to stick together. This is how the sponge cell adhesion proteins work.

· The other is called heterotypic and is like a plug and socket.

· It has been found that most cases involve the homotypic style of cell–cell adhesion.

· The heterotypic binding has been found to be the case for mammalian sperm and egg binding.

Specialized Cell Junctions

· Complex multicellular organisms use special cell–cell recognition proteins to allow specific kinds of cells to adhere to each other and create tissues. These connections are called cell junctions. They are particularly common in epithelial tissues, which line body cavities and cover body surfaces.

· The three main major categories of cell junctions in animals are tight junctions, desmosomes, and gap junctions. (See Figure 5.6. The Instructor’s Resource CD-ROM includes micrographs of various cell junctions.)

Tight junctions seal tissues and prevent leaks

· The purpose of tight junctions is to prevent substances from moving through the intercellular space.

· Tight junctions also restrict the migration of membrane proteins and phospholipids and therefore can help polarize and stabilize the cell.

· Sealing the space between epithelial cells forces materials to pass through cells by entering them. This provides the opportunity to regulate what passes through.

Desmosomes hold cells together

· Desmosomes have dense plaques on the cytoplasmic surface of plasma membranes.

· The plaques are attached both to cytoplasmic fibers and to membrane cell adhesion proteins.

· Desmosomes act like spot welds on adjacent cells.

· Cytoplasmic fibers that connect to the plaques of desmosomes are intermediate filaments. The intermediate filaments, made of a protein called keratin, span throughout the cell, connecting plaques to plaques via a network of fibers.

· The desmosomes of one cell connect its fibrous network to the fibrous networks of other cells.

· Intermediate filaments are extremely strong and provide great mechanical stability to epithelial cells. Some epithelial cells, like those that form our skin, need this stability to resist wearing.

Gap junctions are a means of communication

· Gap junctions are connections that facilitate communication between cells.

· Gap junctions are made up of specialized protein channels called connexons.

· These connexons span the plasma membranes of two adjacent cells and protrude from them slightly.

· Each connexon makes a pore.

· Connexons are made of proteins called connexins, which snap together to generate the pore.

· The pores of a cell connect to the pores of another. Molecules the sizes of ions, amino acids, and nucleotides can get through, but large polymers are excluded.

· Connexons can open or close.

Passive Processes of Membrane Transport

· Biological membranes are selectively permeable. They allow some substances to pass, while other substances are restricted in their movement.

· Some substances can move by simple diffusion through the phospholipid bilayer.

· Some must travel through proteins to get in, but the driving force is still diffusion. This process is called facilitated diffusion.

· There are two kinds of proteins involved in facilitated diffusion: channel and carrier proteins.

The physical nature of diffusion

· Molecules in solution vibrate, rotate, and move from place to place. The greater the temperature, the more quickly they move.

· The random movement of molecules results in the molecules distributing evenly around an area. Once they are distributed evenly, they are said to be at equilibrium. (See Figure 5.7.)

· Diffusion is the process of random movement toward the state of equilibrium. Although individual particles move randomly, in diffusion the net movement is directional until equilibrium is reached.

· Diffusion is therefore the net movement from regions of greater concentrations to regions of lesser concentrations.

· Imagine that the classroom has a glass pane that separates one half from the other. Imagine that balls are bouncing around in one half of the room.

· The balls are kept in one half of the room because there is no way for balls to leave that area and enter the other.

· Imagine that someone makes a hole in the glass pane large enough for a ball to escape and enter the other side.

· Eventually balls will escape through the hole until both halves of the room have approximately the same number of balls. At this point, the likelihood of a ball leaving one side of the room is about the same as the likelihood of a ball entering. This state is equilibrium. Note that balls continue to pass through the hole in both directions.

· Diffusion over short distances is very fast. Small molecules and ions may move from one end of an organelle to another in a millisecond.

· Diffusion over larger distances is very slow. Diffusion across more than a centimeter may take an hour or more; diffusion across meters may take years.

· Diffusion is not adequate to distribute materials over the length of the human body. However, within our cells—across layers of membranes or from one cell to another—diffusion is rapid enough to distribute small molecules and ions almost instantaneously.

· In a solution, diffusion rates are determined by temperature, the physical size of the solute, the electrical charge of the diffusing material, and the concentration gradient.

· The insertion of a biological membrane affects the movement of chemicals in solution according to the membrane’s properties. It may be permeable to some molecules and impermeable to others. If permeable, the concentration of the diffusing substance eventually reaches equilibrium on both sides of the membrane.

Simple diffusion takes place through the membrane bilayer

· Substances that can move freely through the lipid bilayer move by a passive process.

· Some substances can move by simple diffusion through the phospholipid bilayer.

· Membranes can influence diffusion rates.

· How quickly molecules move across a lipid membrane depends on how permeable the membrane is to the solute. For example, ethanol moves much more rapidly than glycerol across the membrane, although both will eventually achieve equilibrium.

· Substances to which the membrane is permeable move across from the outside to the inside and from the inside to the outside. If the concentration is greater outside, more molecules will move from the outside to the inside, because there are more molecules to move in that direction.

· Molecules to which the membrane is impermeable cannot move across the membrane and so stay on one side or the other.

· Polar and charged molecules, such as amino acids, sugars, and ions do not pass readily across the lipid bilayer.

· The hydrophobic interior of the membrane tends to exclude hydrophilic substances.

· One notable exception is water, which can pass through the lipid bilayer.

Osmosis is the diffusion of water across membranes

· Water diffusion across membranes is a special case and is referred to as osmosis.

· It is a completely passive process and requires no metabolic energy.

· If you place a plant cell in a solution with a high concentration of sucrose (table sugar), the plant cell membrane will shrink inside the cell wall. (See Figure 5.8. The Instructor’s Resource CD-ROM includes micrographs of plasmolyzed cells—cells with cell walls in hypertonic solutions.)

· If you put the plant cell in pure water, the cell membrane will expand (build turgor pressure) until it presses very firmly against the cell wall.

· According to the rule of diffusion, molecules move from areas of greater concentration to lesser concentration. When pure water is outside the cell, the concentration of water is greater there, and so the net movement is into the cell.

· Water will continue to move until the concentration is the same on both sides of the membrane, unless something prevents this movement from occurring.

· The three terms used to compare the solute concentrations of solutions are isotonic, hypertonic, and hypotonic.

· Isotonic solutions have equal solute concentrations. In other words, when all the individual particles outside the cell are totaled and then adjusted for a certain volume, for example, per ml, they will equal the total number of all particles inside the cell, per ml.

· With all diffusion, it is the concentration and not the total number of molecules that determines the net direction of movement, because the probability of molecules moving from one point to another depends on how many molecules there are per unit area.

· Imagine the ball example, again. If a small area of the classroom was walled off by the glass panel, then a hole in the glass is made and the balls given time to distribute, there would be fewer balls in the smaller area, but the balls per unit volume would be about equal.

· This is because the probability of the balls moving from one area to another is influenced by the density of the balls, not the total number.

· A hypertonic solution has a greater total solute concentration than the solution to which it is being compared.

· A hypotonic solution has a lower total solute concentration than the solution to which it is being compared. Cells placed into solutions that are hypotonic (relative to the cell’s cytoplasm) swell as water moves into the cell, down its concentration gradient.

· The integrity of cells such as red blood cells depends absolutely on the maintenance of constant solute concentrations in the plasma.

· Plants can be exposed to pure water because their rigid cell walls limit the amount of water that can enter. Animal cells, lacking cell walls, may continue to take on pure water and eventually burst. (See Figure 5.8.)

Diffusion may be aided by channel proteins

· Polar substances, such as amino acids and sugars, and charged substances such as ions, do not diffuse across lipid bilayers.

· How do these substances, which are important raw materials, enter cells?

· One way they enter is through a process referred to as facilitated diffusion. Facilitated diffusion involves proteins that are embedded in the plasma membrane.

· The two kinds of facilitated diffusion across biological membranes depend on two kinds of proteins: channel proteins and carrier proteins.

· Channel proteins are integral membrane proteins that form channels lined with polar amino acids. Nonpolar (hydrophobic) amino acids face the outside of the channel, toward the fatty acid tails of the lipid molecules. (See Figure 5.9.)

· The best-studied protein channels are the ion channels. Hundreds of different ion channels have been identified.

· Ion channels are gated. The gates can be either open or closed. There are various mechanisms for controlling the opening and closing of ion channels.

· Once the channel is opened, millions of ions can rush through it per second. Just how fast these ions move and in which direction depends on the concentration gradient.

· Water can move through ion channels as well. Another way water enters a cell rapidly is through protein-lined membrane channels called aquaporins.

Carrier proteins aid diffusion by binding substances

· Another kind of facilitated diffusion uses carrier proteins and involves not just opening a channel but actually binding the transported substance.

· Carrier proteins allow diffusion in both directions. This is one way sugars and amino acids are transported. (See Figure 5.10.)

· The concentration gradient can be kept by metabolizing the transported substance once it enters the cell. For example, as soon as glucose enters the cell, it is metabolized; therefore, the cell’s glucose concentration stays low, and the movement of glucose continues.

· Again, the rate of movement through carrier proteins is dependent on concentration, but only to a point, because the carrier must bind the substance it transports.

· If the limited number of carrier protein molecules are loaded with solute molecules, the carrier proteins are said to be saturated.

· Saturation can limit the number of molecules that can move into the cell per unit of time.

Active Transport

· In contrast to diffusion, active transport requires the expenditure of energy.

· Ions or molecules are moved across the membrane from regions of lesser concentration to regions of greater concentration.

· This is movement against the concentration gradient.

· ATP is the energy currency used either directly or indirectly to achieve active transport.

Active transport is directional

· Three characteristically different protein-driven systems involved in active transport are uniport, symport, and antiport. (See Figure 5.11.)

· Uniport transporters move a single type of solute, such as calcium ions, in one direction.

· Symport transporters move two solutes in the same direction.

· For example, amino acid transport might be coupled to sodium ion transport.

· Sodium ions move from greater concentration outside the cell, down the concentration gradient to the inside of the cell, and as that occurs, an amino acid is moved up the concentration gradient and into the cell as well.

· Antiport transporters move two solutes in opposite directions, one into the cell, and the other out of the cell.

· An example is the sodium–potassium pump, which moves sodium out of the cell and potassium into it. (See Figure 5.12.)

· For each molecule of ATP used, three sodium ions are pumped out and two potassium ions are pumped in.

Primary and secondary active transport rely on different energy sources

· If ATP is used directly for the pumping system, such as in the sodium-potassium pump, the system is a primary active transport system.

· Only cations, like sodium, potassium, and calcium, are transported directly by pumps that use a primary active transport system.

· Secondary active transport systems are systems that use established gradients to move substances. (See Figure 5.13.)

· This form of transport uses ATP indirectly. The ATP molecules are consumed to establish the ion gradient.

· The gradient is then used to move a substance, as described for the symport and antiport systems.

· Sodium and potassium ions are pumped by the primary active transport system, while glucose gains entry to the cell via a secondary active transport system.

· An example is the symport system found in intestinal cells, which moves glucose up its concentration gradient, while moving sodium ions down its ion concentration gradient. Both sodium and glucose enter the cell. The sodium must be pumped out again, but the energy harvested from the glucose more than pays for this energy expense.

Endocytosis and Exocytosis

· There are a few systems designed to transport polymers, as opposed to simply monomers or ions. Macromolecules in general enter the cell via endocytosis and exit via exocytosis. (See Figure 5.14.)

Macromolecules and particles enter the cell by endocytosis

· Macromolecules such as proteins, polysaccharides, nucleic acids, and triglycerides are too large and too charged to enter through the membrane.

· Endocytosis is a general term for a group of processes that bring macromolecules, large particles, small molecules, and even other cells into the eukaryotic cell.

· There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. In all three, the plasma membrane invaginates toward the cell interior while surrounding the materials on the outside.

· The vesicle is formed when the pocket of membrane deepens, pinches off, and migrates with its contents to the cell’s interior.

· During phagocytosis, which involves the largest vesicles, entire cells can be engulfed. Phagocytosis is common among unicellular protists. White blood cells in humans and other animals also use phagocytosis to defend the body against invading foreign cells.

· The Instructor’s Resource CD-ROM includes photographs of phagocytosis (in Chapters 4 and 18).

· Pinocytosis, which means “cellular drinking,” involves vesicle formation as well, but the vesicles are far smaller. (See Figure 5.15.)

· Dissolved substances and fluids are brought into the cell.

· There is little or no specificity as to what the cell brings in.

· In humans, the single layer of cells separating blood capillaries from surrounding tissue uses pinocytotic vesicles to acquire fluids from the blood.

Receptor-mediated endocytosis is highly specific

· Receptor-mediated endocytosis is similar to pinocytosis, but it is highly specific. Animal cells use this type of endocytosis to transport specific macromolecules from the environment.

· Receptor proteins are exposed on the outside of the cell in regions called coated pits.

· Clathrin molecules form the “coat” of the pits. The forming vesicles are the “pits.” (See Figure 5.16.)

· As the receptors bind specific macromolecules outside the cell, the receptors’ cytoplasmic sides associate with clathrin molecules.

· Coated vesicles form with the macromolecules trapped inside.

· After the vesicles form and are embedded in the cell cytoplasm, they lose the clathrin coat. Depending on the particular substance retrieved, they might end up at specific locations within the cell or be digested in lysosomes.

· Receptor-mediated endocytosis is the method by which cholesterol is taken up by mammalian cells.

· Cholesterol, which is water insoluble, is synthesized in the liver and transported throughout the body within low-density lipoproteins (LDL).

· The LDL binds to the receptors found on the surface of the cell in coated pits.

· Clathrin-coated pits cause an invagination of bound receptors.

· Vesicles containing LDL are brought into the cell.

· The LDL receptors are recycled back to the cell’s surface. The LDL ends up in a lysosome and is digested, freeing the cholesterol for use by the cell.

· Some humans inherit a condition in which their LDL receptors are deficient; this results in dangerously high levels of cholesterol in their blood.

Exocytosis moves materials out of the cell

· Exocytosis is the process by which materials packaged in vesicles are secreted (cast out) from the cell. This happens when the vesicle membranes fuse with the plasma membrane and release vesicle contents (wastes, enzymes, hormones, etc.) to the environment.

Membranes Are Not Simply Barriers

· The plasma membrane is not simply a barrier. (See Figure 5.17a.)

· Plasma membranes are involved in information processing, to initiate, modify, or turn off a cell function.

· Membranes are important in energy transformation.

· The inner mitochondrial membrane helps convert the energy of fuel molecules to the energy in ATP.

· The thylakoid membranes of chloroplasts are involved in the conversion of light energy into chemical bond energy. (See Figure 5.17b.)

· Membranes are involved in organizing chemical reactions into “assembly line” reactions that proceed more rapidly and efficiently. (See Figure 5.17c.)

Membranes Are Dynamic

· Membranes actively participate in numerous cellular processes.

· Membranes continually form, move, and fuse. (See Figure 5.18.)

· Eukaryotic cells form their membranes through a series of activities.

· The smooth endoplasmic reticulum synthesizes and distributes the phospholipid molecules.

· Ribosomes form the protein molecules, which then are inserted into rough endoplasmic reticulum.

· Within cells, segments of membrane move about, change their structures, and fuse with other membranes.

· Each organelle modifies its membranes to carry out specific functions.

· Despite the similar appearance and interconvertibility of membranes, they show major chemical differences depending on the location in the cell and the functions they serve.

· Dynamic in both structure and activity, membranes are central to life.

Photos on the Instructor’s Resource CD-ROM include cross-sectional and freeze-fracture TEMs of various cell membranes, cell junctions, and plasmolyzed cells.