AP Biology Chapter 7 Notes
Terms in this set (21)
Explain why phospholipids are amphipathic molecules.
Phospholipids have both hydrophobic regions and hydrophilic regions. The molecules in its bilayer are arranged so the hydrophobic fatty acid tails are sheltered from water while the hydrophilic fatty acids interact with water.
Explain what freeze-fracture techniques reveal about the arrangement of proteins in membranes.
The freeze fracture method splits a membrane along the middle of the phospholipid bilayer. When it is viewed through an electron microscope, protein particles are interspersed in a smooth matrix.
Describe the fluidity of the components of a cell membrane and explain how membrane fluidity is influenced by temperature and membrane composition.
Many of the membrane proteins drift in the phospholipid bilayer. Some move directional while others never move because they are anchored to the cytoskeleton. When temperatures become cooler, the membranes become solid and the phospholipids are packed more tightly. Membranes with more unsaturated fats are more fluid than those with more saturated fats because the double bonds in unsaturated fatty acids prevent tight packing.
Explain how cholesterol resists change in membrane fluidity with temperature change.
Cholesterol is wedged between phospholipid molecules in plasma membrane of animal cells. During warmer temps, the cholesterol restrains phospholipid movement, reducing fluidity. At colder temps, it maintains fluidity by preventing tight packing.
Distinguish between peripheral and integral membrane proteins.
Peripheral proteins are not embedded in the lipid bilayer at all, but are loosely bound to the surface of the protein and can be connected to integral proteins. Integral proteins penetrate the hydrophobic core of the lipid bilayer and have hydrophilic regions of amino acids when in contact with the aqueous environment.
List six major functions of membrane proteins.
1. Transport of specific solutes in/out cell.
2. Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway.
3. Signal transduction, relaying hormonal messages to the cell.
4. Cell-to-cell recognition, allowing other proteins to attach two adjacent cells together.
5. Intercellular joining of adjacent cells with gap or tight junctions.
6. Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizing the location of certain membrane proteins.
Explain the role of membrane carbohydrates in cell-cell recognition.
Cells can recognize other cells by binding to the surface molecules (usually carbohydrates) on the plasma membrane. The carbohydrates are usually branched oligosaccharides that form glycolipids when bonded with lipids and glycoproteins when formed with proteins.
Explain how hydrophobic molecules cross cell membranes.
Hydrophobic molecules can dissolve in the lipid bilayer which makes it easy for them to cross.
Distinguish between channel proteins and carrier proteins.
Channel proteins are transport proteins that have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane (Ex: aquaporins for water). Carrier proteins are transport proteins that bind to molecules and change shape to shuttle them across the membrane.
Define diffusion and explain why is it spontaneous.
Diffusion is the tendency of molecules of any substance to spread out in the available space. It is driven by heat (kinetic energy) of molecules. Diffusion decreases free energy and increases entropy by creating a randomized mixture.
Explain why a concentration gradient of a substance across a membrane represents potential energy.
The concentration gradient drives diffusion (potential energy drives kinetic energy).
Distinguish among hypertonic, hypotonic, and isotonic solutions.
Hypertonic solutions have higher solute concentrations. Hypotonic solutions have lower solute concentrations. Isotonic solutions have equal solute concentrations.
Define osmosis and predict the direction of water movement based on differences in solute concentrations.
Osmosis is the diffusion of water across a selectively permeable membrane. Water movement goes from hypotonic solution to hypertonic solution. When isotonic, the water moves at equal rates.
Describe how living cells with and without cell walls regulate water balance.
Cells without cell walls are immersed in an isotonic environment that experiences no net water movement. Water molecules move at the same rate in each direction through osmosis (in hypertonic environment the cell would shrivel and die; in hypotonic, the cell would expand and burst). Cells with cell walls osmoregulate to control water balance using the contractile vacuole to force water out of the cell. The cell wall keeps the cell from bursting, but cannot keep it from shrinking.
Explain how transport proteins facilitate diffusion.
Channel proteins provide hydrophilic corridors for specific molecules. Ion channels open/close depending on chemical/physical stimulus. Other transport proteins translocate the solute binding site and solute across the membrane while the protein changes shape.
Distinguish among osmosis, facilitated diffusion, and active transport.
Osmosis is the diffusion of water across a selectively permeable membrane. Facilitated diffusion is the passive movement of molecules down their concentration gradient via transport proteins. Active transport requires the cell to expend metabolic energy (usually ATP) to enable the cell to maintain internal concentrations of small molecules that would otherwise diffuse across the membrane.
Describe the two forces that combine to produce an electrochemical gradient.
1. Chemical force based on an ion's concentration gradient.
2. Electrical force based on the effect of the membrane potential on the ion's movement.
Explain how an electrogenic pump creates voltage across a membrane.
By pumping electrons across the membrane, the side where the electrons are being taken away has a more positive charge, while the side getting more electrons has a negative charge.
Describe the process of cotransport.
Other molecules aid the target molecule to get through the barrier. More molecules can link up to the target on the way (think of a free ride.)
Explain how large molecules are transported across a cell membrane.
Large molecules use vesicles to cross the membrane. The vesicle is moved by the cytoskeleton to the plasma membrane. When the two membranes come in contact, the bilayers fuse and split the contents to the outside.
Distinguish between pinocytosis and receptor-mediated endocytosis.
Pinocytosis is when a cell creates a vesicle around a droplet of extracellular fluid and all included solutes are taken into the cell. Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment. This happens when ligands bind to special receptors on the membrane surface and the receptor proteins are in regions coated with coat proteins that bring the bound substances into the cell.
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