Chapter 7

Explain how small hydrophobic molecules cross cell membranes.

Hydrophobic molecules can dissolve in the lipid bilayer which makes it easy for them to cross.

Distinguish among hypertonic(osmotic), hypotonic(osmotic), and isotonic(osmotic) solutions.

Hypertonic solutions have higher solute concentrations. Hypotonic solutions have lower solute concentrations. Isotonic solutions have equal solute concentrations.

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.

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.

Describe the process of cotransport. Is it a type of active transport? Explain how it is indirectly ATP dependent.

A mechanism through which the active transport of solute is indirectly driven by an ATP-powered pump that transports another substance against its gradient. As that transported substance then diffuses back down down its concentration gradient through a contransporter, the solute is carried against its concentration gradient across the membrane.

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.

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.

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.

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.

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.

Explain how cholesterol resists changes 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.

Define diffusion. Explain why diffusion is a spontaneous process.

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.

Describe the fluidity of the components of a cell membrane and explain how membrane fluidity is influenced by temperature and lipid 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.

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.

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.

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.

Describe the processes of and distinguish among phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Phagocytosis: A form of endocytosis in which pseudopodia wrap around a food particle, creating a vacuole that then fuses with a lysosome containing hydrolytic enzymes. Pinocytosis:Droplets of extracellular fluid are taken into the cell in small vesicles. Receptor mediated:Allows a cell to acquire specific substances from extracellular fluid.

Explain why phospholipids are amphipathic molecules.

Phospholipids are amphipathic because they have hydrophilic heads and hydrophobic tails.

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.

Explain why a concentration gradient of a substance across a membrane represents potential energy.

The concentration gradient drives diffusion (potential energy drives kinetic energy).

Explain how large molecules are transported across a cell membrane.

The process of exocytosis secretes large molecules by the fusion of vesicles with the plasma membrane.