A membrane s structure and functions are determined by its constituents: lipids, proteins, and carbohydrates. The general design of membranes is known as the fluid mosaic model. Phospholipids form a continuous bilayer which is like a lake in which a variety of proteins float.
The lipid molecules are usually phospholipids with two regions: Hydrophilic regions electrically charged heads associate with water molecules Hydrophobic regions nonpolar fatty acid tails that do not dissolve in water
A bilayer is formed when the fatty acid tails associate with each other and the polar heads face the aqueous environment.
Membranes may differ in lipid composition; there are many types of phospholipids. Phospholipids may differ in: Fatty acid chain length Degree of saturation Kinds of polar (phosphate-containing) groups present
Cholesterol is an important component of animal cell membranes. (up to 25% of lipid content) Hydroxyl groups interact with the polar heads of phospholipids. Cholesterol is important in modulating membrane fluidity; other steroids function as hormones.
The fatty acids make the membrane somewhat fluid. This allows some molecules to move laterally within the membrane. Membrane fluidity is influenced by: Lipid composition short, unsaturated chains increase fluidity Temperature fluidity decreases in colder conditions
All biological membranes contain proteins; the ratio of proteins to phospholipids varies. Not all amino acid R groups are the same. Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. Integral membrane proteins are at least partly embedded in the phospholipid bilayer.
Anchored membrane proteins have hydrophobic lipid components that anchor them in the bilayer. Proteins are asymmetrically distributed on the inner and outer membrane surfaces. Transmembrane proteins extend through the bilayer; they may have domains with different functions on the inner and outer sides of the membrane.
Some membrane proteins can move within the phosopholipid bilayer; others are restricted. Cell fusion experiments illustrate this migration. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.
Diverse carbohydrates are located on the outer cell membrane and play a role in communication. Glycolipid carbohydrate covalently bonded to a lipid Glycoprotein one or more oligosaccharides covalently bonded to a protein Proteoglycan protein with more and longer carbohydrates bonded to it
Cells can adhere to one another through interactions between cell surface carbohydrates and proteins.
Membranes are constantly forming, transforming into other types, fusing, and breaking down. Though membranes appear similar, there are major chemical differences among the membranes of even a single cell.
Selective permeability: biological membranes allow some substances, but not others, to pass Two processes of transport across membranes: 1. Passive transport does not require metabolic energy. A substance moves down its concentration gradient.
2. Active transport does require input of metabolic energy. A substance moves against its concentration gradient.
Passive transport can occur by: Simple diffusion through the phospholipid bilayer Facilitated diffusion through channel proteins or aided by carrier proteins
Diffusion is the process of random movement toward equilibrium; a net movement from regions of greater concentration to regions of lesser concentration.
Speed of diffusion depends on three factors: Diameter of the molecules smaller molecules diffuse faster. Temperature of the solution higher temperatures lead to faster diffusion. Concentration gradient the greater the concentration gradient, the faster a substance will diffuse.
Cell cytoplasm is an aqueous solution, as is the surrounding environment. Diffusion of each solute depends only on its own concentration. A higher concentration inside the cell causes the solute to diffuse out; higher concentration outside causes the solute to diffuse in.
Some molecules cross the phospholipid bilayer by simple diffusion: O 2, CO 2, and small, nonpolar, lipid-soluble molecules. Polar (hydrophilic) molecules do not pass through they are not soluble in the hydrophobic interior of the membrane. Amino acids, sugars, ions, water
Osmosis is the diffusion of water across membranes through special channels. It depends on the concentration of water molecules on either side of the membrane water moves down its concentration gradient. The higher the total solute concentration, the lower the concentration of water molecules.
Osmotic pressure: pressure that must be applied to a solution to prevent flow of water across a membrane by osmosis Π = crt c = total solute concentration R = the gas constant T = absolute temperature
The higher concentration of a substance on one side of a membrane represents stored energy. If a membrane allows water, but not solutes, to pass through, the net movement of water molecules will be toward the solution with the higher solute concentration and the lower concentration of water molecules.
When comparing two solutions separated by a membrane: A hypertonic solution has a higher solute concentration. Isotonic solutions have equal solute concentrations. A hypotonic solution has a lower solute concentration.
Concentration of solutes in the environment determines the direction of osmosis in all animal cells. In other organisms, cell walls limit the volume of water that can be taken up. Turgor pressure is the internal pressure against the cell wall as it builds up, it prevents more water from entering.
Facilitated diffusion: Channel proteins are integral membrane proteins that form channels across the membrane through which some substances can pass. Substances can also bind to carrier proteins to speed up diffusion. Both processes operate in either direction.
Ion channels: channel proteins that allow specific ions to pass through Most are gated channels they open when a stimulus causes the protein to change shape. Ligand-gated the stimulus is a ligand, a chemical signal. Voltage-gated the stimulus is a change in electrical charge difference across the membrane.
Water crosses membranes at a faster rate than simple diffusion. It may hitchhike with ions such as Na + as they pass through ion channels. Aquaporins are channels that allow large amounts of water to move along its concentration gradient.
Carrier proteins in the membrane facilitate diffusion by binding substances. Glucose transporters are carrier proteins in mammalian cells. Glucose molecules bind to the carrier protein and cause the protein to change shape it releases glucose on the other side of the membrane.
Cells maintain an internal environment with a different composition than the outside environment. This requires work energy from ATP is needed to move substances against their concentration gradients (active transport). Specific carrier proteins move substances in only one direction, either into or out of the cell.
The sodium potassium (Na + K + ) pump is an integral membrane protein that pumps Na + out of a cell and K + in. One molecule of ATP moves two K + and three Na + ions.
Macromolecules are too large or too charged to pass through biological membranes, so instead they cross within vesicles. To take up or to secrete macromolecules, cells must use endocytosis and exocytosis.
Exocytosis moves materials out of the cell in vesicles. The vesicle membrane fuses with the cell membrane and the contents are released into the environment. Exocytosis is important in the secretion of substances made by cells such as digestive enzymes and neurotransmitters.
Endocytosis brings macromolecules and particles into eukaryotic cells. The cell membrane invaginates, or folds around the particle and forms a vesicle. The vesicle then separates from the membrane.
Endocytosis depends on receptors proteins that bind to specific molecules (ligands). The receptors are integral membrane proteins on the cell membrane. The resulting vesicle includes both the receptor and its ligand, plus other substances present near the site of invagination.
Phagocytosis ( cellular eating ): a specialized cell engulfs a large particle or another cell A food vesicle (phagosome) forms and usually fuses with a lysosome, where the contents are digested. Pinocytosis ( cellular drinking ): vesicles are smaller and bring in fluids and dissolved substances
Receptor endocytosis brings specific large molecules into a cell via specific receptors. This allows cells to control internal processes by controlling location and abundance of each type of receptor on the cell membrane. It also plays a role in cell signaling.