Membrane Structure and Function - 1 The Cell Membrane and Interactions with the Environment As mentioned earlier, the boundary between any cell and its environment is the plasma membrane. Each cell must interact with its environment in a number of ways. Each cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, salts, etc.) from the environment, maintain water balance with its surroundings, and remove waste materials from the cell. The plasma membrane can do its job because it is differentially, or selectively permeable. The membrane permits some materials to enter and leave easily, some with the assistance of membrane molecules, and other substances are prohibited from entering or leaving. The plasma membrane has a number of functions: Serves as the boundary between the cytoplasm of the cell and the external environment. Maintains the cell's environment by regulating materials that enter or leave the cell. Provides mechanisms for cell-to-cell communication. Has genetically unique cell recognition markers to provide mechanisms for a cell to recognize "self" versus "non-self" (foreign materials), important to the immune system and defense of the organism. Note that although the plasma membrane forms the boundary of the cell, and surrounds the cell, many internal structures of eukaryotic cells also have their own membrane boundaries. Much of what we say about membrane structure and function at this time applies to all membranes, not just the plasma membrane. The Fluid Mosaic Membrane Structure The typical membrane structure consists of a phospholipid bilayer. Recall that phospholipids are molecules with both hydrophilic (polar) and hydrophobic (non polar) regions (in other words, they are amphipathic). The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they form hydrogen bonds with surrounding water molecules.
Membrane Structure and Function - 2 Because the individual phospholipid molecules are not bonded to each other, a membrane is flexible (or fluid ) particularly to lateral movement of the fatty acids, important to its functions. The membrane is held together, for the most part, by the hydrophobic interactions. The phospholipid molecules are not bonded to each other, so they tend to move along the plane of the membrane. The rate of movement can be measured, averaging two micrometers per second. Phospholipid Movement Unsaturated/Saturated With Cholesterol The saturation of fatty acids affects the fluidity -- the more saturated, the less movement. Cholesterol, found in membranes of animal cells, reduces fluid movement of the phospholipids at normal temperatures. Membranes will solidify as temperature decreases. The temperature at solidification depends on the saturation of the fatty acids (just as it does with fats and oils). Solidified membranes do not function well. Research (reported in Nature) showed that brain cell membranes of ground squirrels become more solid during hibernation. Proteins migrate to more fluid regions of the membrane so they can continue to function. In caribou, circulation is reduced in the lower legs to prevent excess heat loss during cold winters. The membranes of the lower legs have more unsaturated fatty acids than those of the upper legs to retain more fluidity in reduced temperatures.
Membrane Structure and Function - 3 Membrane Proteins Interspersed throughout a membrane s phospholipid layer are a number of amphipathic proteins. The orientation of the proteins is such that hydrophobic regions of the proteins are within the fatty acid regions of the phospholipids and hydrophilic regions of the proteins are at the aqueous interfaces of the membrane (interior and exterior). This orientation is important to how the membrane proteins function. Some proteins are mobile within the membrane (probably moved by motor molecules of the cytoskeleton) and others are fixed in position. The membrane is associated with a network of supporting cytoskeletal filaments, some of which help shape the cell and some help anchor proteins in the membrane. Membranes also contain some carbohydrates (glycoproteins and proteoglycans) and glycolipids on the exterior side. The resultant membrane structure (proteins scattered throughout the fluid phospholipid layers) resembles a mosaic, hence the name fluid mosaic membrane. Proteins in membranes determine how the specific membrane functions. Recall that membrane is manufactured in the endoplasmic reticulum. The orientation of membrane proteins and lipids is determined at the manufacturing site. Molecules on the inside of the ER and Golgi vesicles become exterior membrane molecules. Membrane Protein Categories Membrane proteins are divided into two categories, integral and peripheral, depending on their location. That is the easy part. Biologists further identify the membrane proteins by function and there are many! Integral (Transmembrane) Proteins Proteins that go through the membrane are called integral or transmembrane proteins. They have hydrophobic (non-polar amino acids with alpha helix coiling) regions within the interior of the membrane and hydrophilic regions at either membrane surface. Peripheral Proteins Peripheral proteins are attached to the surface of the membrane, often to the exterior hydrophilic regions of the transmembrane proteins. On the interior surface, peripheral proteins typically are held in position by the cytoskeleton. On the exterior, proteins may attach to the extracellular matrix. For animal cells, these attached proteins help give the membrane strength.
Membrane Structure and Function - 4 Membrane Protein Functions Transport Proteins Transport Proteins are transmembrane proteins that serve as carriers for specific substances that need to pass through the membrane by providing a hydrophilic channel. Transport proteins have binding sites that attract specific molecules. Most of our ions (Ca ++, Na +, Cl -, K +, etc.), along with amino acids, sugars and other small nutrient molecules are moved through transport proteins. When a molecule binds to the carrier protein, the protein changes shape moving the substance through the membrane. This process may require energy (ATP), and the ATP complex is a part of the transport protein. When ATP is involved with actively moving molecules through the protein channel the process is called Active Transport. Enzymatic Proteins Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface. The active site of the enzyme will be oriented in the membrane for the substrate to attach. Enzymes needed for metabolic pathways can be aligned adjacent to each other to act like an assembly line for the reactions. Signal Transduction (Receptor) Proteins Signal transduction proteins have attachment sites for chemical messengers, such as hormones. The signal molecule, when it attaches to the protein, promotes a conformational change that relays the message into the cell to trigger some cell activity. Chemical messaging in cells is the subject of a later chapter. These proteins are also called receptor proteins.
Membrane Structure and Function - 5 Recognition Proteins Glycoproteins (carbohydrate-protein hybrids) serve as surface receptors for cell recognition and identification. They are important to the immune system so that immune system cells can distinguish between one s own cells and foreign cells. Recognition proteins are also used to guide cell attachments/adhesions in developmental processes. Cell Adhesion (Intercellular Joining) Proteins. Some proteins are responsible for the cell junctions such as tight junctions and gap junctions that permit cells to adhere to each other. Attachment Proteins Attachment proteins attach to the cytoskeleton or extracellular matrix to help maintain cell shape (particularly for animal cells) and fix into position some membrane proteins. Some proteins attach to the cytoskeleton on the interior of the membrane; others attach to the extracellular matrix of glycoproteins. Collagen is an important glycoprotein of the extracellular matrix. Some attachment proteins, the integrins, attach to both the extracellular matrix and to the cytoskeleton in the interior of the cell, spanning the membrane.
Membrane Structure and Function - 6 Structure of the Membrane Proteins Relative to Functions Anchoring proteins have non-polar α helix regions that fix the protein into the phospholipid bilayers. Polar regions are on either side of the non-polar regions, attracted to the hydrophilic phospholipid regions. Anchor proteins are also used to attach to the fibrous network of the cytoskeleton to give shape and strength to some cells. Channel proteins will have non-polar α helix segments traversing the lipid bilayers many times forming a channel through which the target substance can pass. Often a carrier molecule or "pump" will be embedded in the protein matrix. Pores are formed when non-polar β pleated sheet regions of proteins create "tunnels" in the membrane lipid bilayers. Anchoring Protein Channel Protein Pore Protein
Membrane Structure and Function - 7 Moving Materials Through Membranes A significant part of membrane activity involves transporting materials through it in one direction or the other. Mineral ions, water, amino acids, monosaccharides and other nutrients are constantly passing through membranes. The cell membrane is selectively, or differentially, permeable. This means that: Some materials freely pass - the membrane is permeable to such molecules and whether they are inside or outside of the cell depends on other factors. Some materials are excluded Some materials enter or leave the cell only by using energy For example, small hydrophobic molecules, such as CO 2, O 2 and small lipids, dissolve in the membrane and pass through readily. Tiny polar molecules, such as H 2 O and alcohol, can also minimally slip between the phospholipid molecules. Ions and most nutrient molecules do not move freely through membranes, but are often carried by the transport protein channels, either with or without the use of energy. Most large molecules are excluded and must be manufactured within the cell, or moved by significant alterations of the membrane itself. Before we talk about how molecules move through membranes, it is useful to have some definitions: Fluid Any substance that can move or change shape in response to external forces without breaking apart. Gases and liquids are fluids. Concentration The number of molecules of a substance in a given volume Gradient A physical difference between two regions so that molecules will tend to move from one of the regions toward the other. Concentration, pressure and electrical charge gradients are common in cells. In general, the movement of any substance is subject to physical rules of molecule behavior. All molecules are in motion (their intrinsic kinetic energy which is called thermal motion). One effect of this motion is that atoms and molecules make random collisions with other molecules. However, when the distribution of molecules is not equal, and we have a gradient, there is a net movement of molecules along the gradient. Many gradients exist between a cell's environment and the cytoplasm of the cell. These gradients are important in moving materials through membranes, both passively (without the use of energy by the cell) and actively (transport requiring cell energy). Movement of most substances takes place by simple diffusion, facilitated diffusion and active transport. We shall discuss all three. Simple and facilitated diffusion are means of passive transport. Active transport consumes energy to move substances against a gradient.
Membrane Structure and Function - 8 Passive Transport Moving things through membranes without the expenditure of cell energy down gradients. Simple Diffusion Diffusion is the movement of a substance from where there is more of it along a concentration gradient to where there is less of it, until molecules are equally distributed (and the gradient no longer exists), a state of equilibrium. Strictly speaking, we say that molecules will move from where there is more free energy to where there is less free energy. Equilibrium means that there in no net movement. Molecules can and will continue to move, but for every forward movement there will be a matching reverse movement. Diffusion is a means of passive transport, since no additional energy is expended for the process. Molecules are moving down an energy gradient, so the movement is spontaneous. In terms of cellular activity, diffusion: Requires no energy But the cell has no control over diffusion, and the rate of diffusion is pretty slow and can not cover much distance. The Rate of Diffusion can be affected by: Temperature (Higher temperature, faster molecule movement) Molecule size (Smaller molecules often move more easily) Concentration (Initial rate faster with higher concentration) Gradient of the two regions (Greater the gradient differential, the more rapid the diffusion (again, initially))
Membrane Structure and Function - 9 Materials that may move through membranes by passive diffusion include: H 2 O (water) (although much moves via facilitated diffusion) CO 2 (carbon dioxide) O 2 (oxygen) Some lipid-soluble molecules (alcohol) Note: The movement of water through a differentially permeable membrane in response to solute concentrations, the phenomenon of osmosis, is a special case of diffusion that we shall discuss later. Facilitated Diffusion Most molecules cannot move freely through the membrane, but can pass through membranes with the help of membrane transport proteins, some of which temporarily bind to the substance to be moved through the membrane, a process called facilitated diffusion. No energy is involved, so it is still a passive process. Transport proteins are specific, and are limited in number in membranes. The rate of movement of materials is dependent on the availability of transport proteins as well as the concentration of the substance to be moved. In addition, transport proteins can be blocked by some molecule (not the target molecule) that may be attracted to the binding site, but does not move through it. There are a number of different ways in which transport proteins work, and the precise mechanisms of movement are not fully understood. Some have binding sites that attract a polar target molecule; as the target molecule builds up in concentration it moves through the open hydrophilic protein channel along its gradient. Channels for specific ions are common in membranes. Much water movement through membranes also involves facilitated diffusion. There are special channel proteins, called aquaporins, that facilitate the movement of water at a rate needed for cell activities. Some transport proteins have channels with gates. The gate opens to let the target molecule pass through when it receives an electrical or chemical signal. For example, neurotransmitter chemicals serve as signal molecules to open the gates for sodium to flow into the nerve cell. Facilitated diffusion also occurs with carrier molecules, substances to which the target molecule to be transported temporarily binds, resulting in a conformational change that moves the target substance through the membrane. Facilitated Diffusion Models
Membrane Structure and Function - 1 0 Problems can arise when transport proteins are genetically or developmentally non-functional. Their target substance cannot be transported, and in some cases, serious problems result. Materials that move through membranes by facilitated diffusion include: Glucose Many small ions Amino acids Energy-Requiring Transport Across Membranes All cells need to move some substances through membranes in a direction counter to the gradient, maintain concentrations of molecules that are not at equilibrium with the external environment by constantly pumping them into or out of the cell, and move substances that are too large or bulky be moved without the use of cell energy. Cells have a number of ways to move things with the use of energy. Active Transport Some transport proteins can move substances through the membrane against the concentration gradient. Active transport typically requires two active sites on the carrier protein, one to recognize the substance to be carried, and one to release ATP to provide the energy for the protein carriers or "pumps". Often, ATP transfers its phosphate to the transport protein, changing the protein s shape so that the target substance can be carried across the membrane. Much energy is expended by the cell to do this! The sodium/potassium pump, which maintains the appropriate Na + /K + ion balance in typical animal cells, is one such example. Note that with the Na + /K + pump, the carrier protein is exchanging sodium and potassium ions. The change brought about by the phosphorylation of the transport protein moves sodium; the release of sodium permits the binding of potassium on the other side of the membrane. The release of the phosphate changes the conformation so that the potassium is carried through the membrane and released. In some cases, concentration gradients of ions, typically H + or Na + ions, can be used to provide the energy needed to move something through a membrane. This mechanism works because the charges on the ions create an electrochemical gradient that can be measured as a membrane voltage potential. Most cells are negatively charged relative to their external surroundings.
Membrane Structure and Function - 1 1 Electrogenic pumps use both the charge gradient and the concentration gradient to facilitate movement. Transport proteins that use charged pumps are called electrogenic pumps. The sodium-potassium pump is one example. In the hydrogen proton pump ATP is used to pump H + across a membrane that builds up both a concentration and a charge on the other side. The hydrogen proton pump is important in generating a positive change in the extracellular fluid of many plants, bacteria and fungi. ATP synthesis uses H + pumps. Electrogenic Pump Cotransport Coupled Transport Electrogenic pumps are also used in the processes of coupled transport. The substance to be moved is "coupled" to the concentration of a different substance that is being transported down a gradient in a protein channel after that substance (typically Na+ or H+) has been actively pumped through the membrane to create a force. In cotransport, proton pumps actively move H + through a membrane that creates a gradient on the other side. Transport proteins then facilitate the movement of the H + back through the membrane along its gradient. This H + gradient is coupled to the movement of some other substance against its gradient in the same direction on the transport protein through the membrane. The transport protein has two binding sites on the same side of the membrane: one for the ion and one for the target substance. For example, in plants, while H + is moving "down" through a transport protein channel, amino acids may be transported "up" along with it. The energy gradient of the H + provides the energy needed to move the amino acids. Loading sucrose into phloem for translocation in plants uses H + cotransport. Sodium ions are also used in cotransport, in particular to move amino acids and sugars. One of the most important metabolic processes of life, ATP synthesis, typically involves cotransport. H + is actively pumped to one side of a membrane, building up concentration, charge and ph gradients. As the accumulated H + move back through a membrane transport protein (the ATP synthase complex), their force is used to synthesize ATP. This specific process is called chemiosmosis, something we shall discuss later. In countertransport, the substances coupled move in the opposite direction in the membrane. The target substance to be moves binds to the opposite side of the coupled transport protein as the coupling ion whose gradient will provide the force to move the target substance.
Membrane Structure and Function - 1 2 Membrane Interactions with the Environment Larger substances may require changes in membrane shape and the fusion of membranes to move things into or out of cells. Exocytosis Materials can be exported from the cell by fusing vesicles with the plasma membrane, a process called exocytosis. Typically, materials for export are packaged in the Golgi body and the vesicles formed travel along the cytoskeleton until they reach the plasma membrane. Once the vesicle membrane and plasma membrane fuse, the contents of the vesicle are freed from the cell. For example, insulin, made in cells of the pancreas, leaves the cells of the pancreas by exocytosis. New wall material in plants is secreted via exocytosis. Endocytosis Substances which enter the cell using membrane modifications move by endocytosis. There are three methods of moving by membrane modification: Pinocytosis, receptor-mediated endocytosis and phagocytosis. Pinocytosis Membrane invaginates, substances "fall" in cavity, used for moving fluids into or out of a cell. Whatever molecules were in the fluid will be moved into the cell.
Membrane Structure and Function - 1 3 Receptor-Mediated Endocytosis Highly specific receptor molecules in the membrane attract the substance to be moved into the cell, creating a membrane depression in that area (a coated pit). When sufficient molecules have been attracted, the pocket will be pinched off forming a coated vesicle in the cytoplasm. Molecules that bind to receptor sites are called ligands. (It s a general term that simply means something that attaches to a receptor.) Cholesterol is transferred from LDLs to individual cells via receptor-mediated endocytosis. Phagocytosis Membrane pseudopodia surround and engulf particulate objects, packaging them in a membrane-bounded vacuole. Phagocytosis is used for solids large objects, such as prey engulfed by Amoeba, and bacteria by white blood cells.
Membrane Structure and Function - 1 4 Now to a Complication of water, membranes and diffusion: Osmosis Osmosis is the movement (diffusion) of water across a differentially permeable membrane in response to solute (dissolved substances) gradients maintained by the membrane. The "force" to move water through membranes is called osmotic pressure. It is comparable to physical pressure. Osmotic pressure may be resisted by the cell membrane (if it is strong enough) or the cell wall (in organisms that have cell walls). The wall or membrane exerts a mechanical pressure. The difference in the osmotic pressure and the wall or membrane pressure is known as water potential. Water potential is very important in a number of processes. For the process of osmosis: A membrane separates two solutions and the proportion of solutes to water is unequal on the two sides of the membrane. A water gradient exists, in part because dissolved substances always lower the concentration of water in a solution. (Pure water would have the highest concentration of water any substance that is added to pure water will displace some water molecules, lowering the proportional content of the water.) Moreover, solutes attract water to their surfaces forming hydration shells and when the solutes move along their gradient, the attracted water moves, too, attracting more water. The membrane permits water passage. The membrane is not permeable to the solute(s), which are substances that can "bind" to water, affecting the free flow of water. Since osmosis depends of the differences in the concentration of water, the specific types of solutes do not matter; it's their collective effect on the concentration of water than counts. Or, it's not so much the number of molecules, or volume of molecules, but the proportions of solutes to water.
Membrane Structure and Function - 1 5 There are terms that are used to describe the ratio of water to solutes, and they are always used to describe the comparative proportion of water to solutes on both sides of the membrane (or inside of and outside of the cell). When using these terms we must be careful to define higher and lower relative to location (inside of or outside of the cell, for example). Hyperosmotic (Hypertonic) When discussing cells, if the external solution has a higher solute concentration (less water) than the internal solution, it is a hypertonic solution. There is more water (relative to solutes) inside of the cell. Strictly speaking, the solution that has the higher proportion (concentration) of solutes is said to be hyperosmotic or hypertonic. Hyperosmotic solutions will cause water to leave cells by osmosis, and cells may shrink. Hypoosmotic (Hypotonic) The external solution (again, speaking of cells) has a lower solute concentration (more water) than the internal solution of the cell. Hypoosmotic solutions will cause water to enter cells by osmosis, causing the cells to swell. Isosmotic (Isotonic) Isotonic solutions will have equal proportions of solutes to water on both sides of the membrane. Isosmotic solutions are osmotically balanced and there is no net movement of water. Water will move through the membrane, but equal amounts of water will be moving in both directions. Human Red Blood Cells Typical Plant Cell Hypoosmotic Isosmotic Hyperosmotic It is important to understand that the solutions do not have to be identical in osmotic activity. You can have completely different solute substances on either side of the membrane. It's the total proportion of solutes (which bind to water molecules inhibiting their movement) to water that affects osmosis, not the specific chemicals.
Membrane Structure and Function - 1 6 Effects of Osmosis Osmosis has a tremendous impact on living organisms that are continuously exposed to a variety of solutes in their extracellular mediums. Cells cannot afford to either lose water or gain excess water. They must maintain an equal proportion of solutes both inside and outside of the cells, a condition called osmotic balance, to function. The process by which organisms regulate their osmotic balance is called osmoregulation. Here are some examples: Hyperosmotic Environments An environment which has a higher proportion of solutes than found inside the cell will cause water to leave the cell. Salt water, for example, is hypertonic to the cells of many organisms. The cells of an organism placed in sea water will lose water and shrivel, a phenomenon called plasmolysis, unless it has special mechanisms to prevent this. Salt water organisms have a variety of such mechanisms. Sharks circulate urea that increases the solute concentration in their extracellular fluids to approximate that of sea water. Most marine invertebrates are isotonic to sea water. (They would not survive in fresh water.) Many salt water mammals rely on impervious surfaces to prevent water loss. A hyperosmotic environment for terrestrial organisms is common, for the substrate and atmosphere often have a lower proportion of water than the internal cellular environment. Plant cells plasmolyze when placed in a hypertonic environment and lose turgor, causing the plant to "wilt". This routinely happens when their substrate lacks sufficient moisture. Fortunately, adding water to the substrate reverses the osmotic gradient, creating a hypoosmotic environment so that water moves back into cells. Regrettably, after too long a period of time in a plasmolyzed condition a plant enters a state of permanent wilt, and does not recover. Some call this death. Plasmolysis in Plants
Membrane Structure and Function - 1 7 Hypoosmotic Environments An environment which has a lower proportion of solutes than found inside the cell will cause water to enter the cell. Fresh water, for example, is hypotonic to the cells of all organisms. The extracellular spaces in plants are typically saturated with water vapor as water diffuses into roots and is drawn upward through the xylem tissue to be distributed to cells. Plant cells take advantage of osmotic pressure, using the cell wall and central plant vacuole. As mentioned earlier, stored substances in the vacuole attract water, which increases fluid pressure within the vacuole. This hydrostatic pressure, called turgor pressure, forces the cytoplasm against the plasma membrane and cell wall, balancing the osmotic pressure to move water into the cell. These balanced forces keep the cell rigid, maintaining turgor. Turgor provides support and strength for herbaceous plants and other plant parts lacking secondary cell walls. Animal cells may swell to bursting when placed in fresh water, a hypoosmotic environment. Animal cells, therefore, require some method to prevent this and maintain osmotic balance. Many fresh water protists have contractile vacuoles, structures which collect the water which moves into their cell from the environment, and periodically expel the collected water to the external environment by contracting the vacuole though a pore, hence the name, contractile vacuole. Full Contractile Vacuole in the Paramecium Empty Fresh water fishes continuously excrete a very dilute urine to remove excess water that enters through their gills. Note: Most terrestrial animals maintain an isosmotic environment by surrounding the cells with an osmotically balanced extracellular fluid. Animals have systems to maintain osmotic balance such as the kidney (and other regulatory structures) of humans. Even so, most terrestrial organisms are at risk of dehydration. Body surfaces typically have protective layers to minimize this risk, and most organisms take in water to compensate for water that diffuses (or is excreted) out of the body.