A. Introduction
1. Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.
2. The main macromolecules in membranes are lipids and proteins, but include some carbohydrates.
a. The most abundant lipids are phospholipids.
b. The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model.
1. A membrane is a fluid structure composed of proteins embedded in a double layer of phospholipids.
B. Membrane Structure
1. Membrane model
a. The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water. (Fig. 8.1)
b. Membranes differ in thickness, appearance when stained, and percentage of proteins.
c. In 1972, S.J. Singer and G. Nicolson presented a model of membrane structure that proposed that the membrane proteins are dispersed and individually inserted into the phospholipid bilayer.
1. In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water and the hydrophobic regions are in a nonaqueous environment. (Fig. 8.2)
2. Membranes are fluid.
a. Most of the lipids and some proteins can drift laterally in the plane of the membrane. (Fig. 8.4)
b. The lateral movements of phospholipids are rapid, about 2 microns per second.
c. Membranes rich in unsaturated fatty acids are more fluid than those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent tight packing.
d. The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells.
1. At warm temperatures, it restrains the movement of phospholipids and reduces fluidity.
2. At cool temperatures, it maintains fluidity by preventing tight packing.
e. Many larger membrane proteins move more slowly but do drift.
f. Other proteins never move, anchored by the cytoskeleton. (Fig. 8.6)
3. Membranes are mosaics of structure and function.
a. Proteins determine most of the membrane�s specific functions.
b. The plasma membrane and the membranes of the various organelles each have unique collections of proteins.
c. There are two populations of membrane proteins.
1. Peripheral proteins are not embedded in the lipid bilayer at all-they are loosely bound to the surface of the membrane, often connected to integral proteins.
2. Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane. (Fig. 8.7)
a. Where integral proteins contact the membrane core, they have hydrophobic regions with nonpolar amino acids, often coiled into alpha helices.
b. Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids.
d. One role of membrane proteins is to reinforce the shape of a cell and provide a strong framework.
1. On the cytoplasmic side, some membrane proteins connect to the cytoskeleton.
2. On the exterior side, some membrane proteins attach to the fibers of the extracellular matrix.
e. The proteins in the plasma membrane provide a variety of major cell functions. (Fig. 8.9)
1. Transport
2. Enzymes
3. Signal transduction
4. Cell-cell adhesion
5. Cell-cell recognition
6. Attachment to the inside and outside of the cell
4. Membrane carbohydrates are important for cell-cell recognition.
a. The membrane plays a role in cell-cell recognition.
1. This attribute is important in cell sorting and organization into tissues and organs during development.
2. It is also the basis for rejection of foreign cells by the immune system.
b. Cells recognize other cells by keying on surface molecules, proteins or carbohydrates, on the plasma membrane.
1. Membrane carbohydrates are usually short branched polysaccharides with fewer than 15 sugar units.
2. They may be covalently bonded either to lipids, forming glycolipids, or, more commonly, to proteins, forming glycoproteins.
3. The polysaccharides on the external side of the plasma membrane vary from species to species, individual to individual, and even from cell type to cell type within the same individual.
a. This variation marks each cell type as distinct.
b. The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells. (Textbook Activity 8A)
C. Traffic Across Membranes
1. A membrane�s molecular organization results in selective permeability.
a. What substances enter and exit the cell?
1. A steady traffic of small molecules and ions moves across the plasma membrane in both directions.
2. For example, sugars, amino acids, and other nutrients enter a cell and metabolic waste products leave.
3. The cell absorbs oxygen and expels carbon dioxide.
4. It also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane.
5. However, substances do not move across the barrier indiscriminately; membranes are selectively permeable.
b. Permeability of a molecule through a membrane depends on the interaction of that molecule with the hydrophobic core of the membrane.
1. Hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.
2. Ions and polar molecules pass through with difficulty.
a. This includes small molecules, like water, and larger critical molecules, like glucose and other sugars.
b. Ions, whether atoms or molecules, and their surrounding shell of water also have difficulties penetrating the hydrophobic core.
c. Proteins can assist and regulate the transport of ions and polar molecules. (see examples below) (Textbook Activity 8B)
2. Passive transport is diffusion across a membrane.
a. Diffusion is the tendency of molecules of any substance to spread out in the available space.
b. Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules.
c. Movements of individual molecules are random but movement of a population of molecules may be directional. (Fig. 8.10)
d. Dye will cross a permeable membrane until both solutions have equal concentrations of the dye.
1. At this dynamic equilibrium as many molecules pass one way as cross in the other direction.
e. In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.
f. Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.
g. The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.
1. Oxygen and carbon dioxide both diffuse directly through the cell membrane down their concentration gradients.
2. Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins. (See facilitated diffusion below.) (Textbook Activity 8C)
3. Osmosis is the passive transport of water.
a. Differences in the relative concentration of dissolved materials in two solutions
1. The solution with the higher concentration of solutes is hypertonic.
2. The solution with the lower concentration of solutes is hypotonic.
3. These are comparative terms.
a. Tap water is hypertonic compared to distilled water but hypotonic when compared to seawater.
4. Solutions with equal solute concentrations are isotonic.
b. Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar. (Fig. 8.11)
1. The hypertonic solution has a lower water concentration than the hypotonic solution.
2. Water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.
a. This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.
3. Osmosis continues until the solutions are isotonic.
c. The direction of osmosis is determined only by a difference in total solute concentration-the kinds of solutes in the solutions do not matter.
d. When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis.
4. Cell survival depends on balancing water uptake and loss.
a. Animal cells
1. An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane. (Fig. 8.12)
a. Water flows across the membrane, but at the same rate in both directions.
b. The volume of the cell is stable.
2. The same cell in a hypertonic environment will lose water, shrivel, and probably die.
3. A cell in a hypotonic solution will gain water, swell, and burst.
4. For a cell living in an isotonic environment (for example, many marine invertebrates) osmosis is not a problem.
5. Similarly, the cells of most land animals are bathed in an extracellular fluid that is isotonic to the cells.
6. Organisms living in either a hypertonic or hypotonic environment must have adaptations for osmoregulation to maintain their internal environment (such as pumping water into or out of the cell).
b. Plants, prokaryotes, fungi, and some protists
1. The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell�s water balance.
2. A plant cell in a hypotonic solution will swell until the elastic wall opposes further uptake.
a. At this point the cell is turgid, a healthy state for most plant cells.
b. Turgid cells contribute to the mechanical support of the plant.
3. If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt.
4. In a hypertonic solution, the plant cell loses water and its volume shrinks.
a. Eventually, the plasma membrane pulls away from the wall-this plasmolysis is usually lethal. (Textbook Activity 8D)
5. Facilitated passive transport of water and selected solutes (facilitated diffusion).
a. Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.
b. The passive movement of molecules down their concentration gradients via a transport protein is called facilitated diffusion.
c. Properties of passive transport proteins
1. They only have one or a few specific solutes they carry.
2. Transport proteins can become saturated when they are translocating passengers as fast as they can.
3. Transport proteins can be inhibited by molecules that resemble the normal solutes they carry.
a. When these inhibitors bind to the transport proteins, they outcompete the normal substrate for transport.
4. Passive transport requires no energy because solutes are moving down their concentration gradients.
d. Types of passive transport proteins (Fig. 8.14)
1. Many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane.
a. These channel proteins allow fast transport.
b. For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
2. Some channel proteins, gated channels, open or close depending on the presence or absence of a physical or chemical stimulus.
a. The chemical stimulus is usually different from the transported molecule.
b. For example, when neurotransmitters bind to specific gated channels on the receiving neuron, these channels open.
1. This allows sodium ions into a nerve cell.
2. When the neurotransmitters are not present, the channels are closed.
3. Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape.
a. These shape changes could be triggered by the binding and release of the transported molecule.
6. Active transport is the pumping of solutes against their gradients.
a. Some facilitated transport proteins can move solutes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated.
b. This active transport requires the cell to expend its own metabolic energy.
c. Active transport is critical for a cell to maintain its internal concentrations of small molecules that would otherwise diffuse across the membrane.
d. Active transport is performed by specific proteins embedded in the membranes.
e. ATP supplies the energy for most active transport.
1. Often, ATP powers active transport by shifting a phosphate group from ATP (forming ADP) to the transport protein.
2. This may induce a conformational change in the transport protein that translocates the solute across the membrane. (Fig. 8.16)
7. Exocytosis and endocytosis transport large molecules.
a. Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles.
b. During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane.
1. When the two membranes come in contact, the bilayers fuse and spill the contents to the outside.
c. During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane.
1. A small area of the plasma membrane sinks inward to form a pocket.
a. As the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell.
2. One type of endocytosis is phagocytosis, "cellular eating."
a. In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole.
b. The contents of the vacuole are digested when the vacuole fuses with a lysosome. (Fig. 8.19)
3. In pinocytosis, "cellular drinking," a cell creates a vesicle around a droplet of extracellular fluid. (Textbook Activity 8G)