Life Science 7A - Membrane Transport (Week 2a)

Why are biological models useful but potentially misleading?

Models simplify complex systems so patterns are easier to see, but may omit scale, motion, chemical detail, diversity, or exceptions. Ask what the model includes and what it leaves out.

Why is the phospholipid bilayer semipermeable?

Its hydrophobic core lets some small nonpolar molecules cross relatively easily but strongly restricts ions and most large or polar molecules unless a transport pathway is available.

Which molecules cross a pure lipid bilayer most easily, and which cross poorly?

Small nonpolar molecules such as O2 and CO2 cross readily. Ions and large polar molecules cross poorly. Water can cross slowly, but aquaporins can greatly increase its movement.

Define diffusion.

Net movement of particles down their concentration gradient due to random motion, from higher concentration toward lower concentration, until equilibrium is approached.

What is a concentration gradient?

A difference in concentration across space or a membrane. It can drive net movement when the molecule can cross the barrier.

What is simple diffusion?

Passive movement directly through the lipid bilayer, down a concentration gradient, without a transport protein or direct energy input.

What is facilitated diffusion?

Passive movement down a concentration or electrochemical gradient through a membrane protein, such as a channel or carrier. It does not directly require ATP.

How do channels and carriers differ?

Channels create a selective passageway through the membrane. Carriers bind a solute and change shape to move it. Both can mediate facilitated diffusion; carriers often show saturation.

What is active transport?

Transport that moves a substance against its concentration or electrochemical gradient and therefore requires an energy source, directly or indirectly.

What is primary active transport? Give an example.

A transporter uses energy directly, often from ATP hydrolysis, to move a substance against its gradient. Example: the sodium-potassium pump.

What is secondary active transport?

A transporter uses the energy stored in one substance's electrochemical gradient to move another substance against its gradient. The gradient was established by another energy-driven process.

How do symport and antiport differ?

Symport moves two substances in the same direction across a membrane; antiport moves them in opposite directions.

What is osmosis?

Net movement of water across a selectively permeable membrane toward the side with higher effective solute concentration (lower water potential), assuming the solute cannot freely cross.

Define isotonic, hypertonic, and hypotonic relative to a cell.

Isotonic: same effective solute concentration, so no net water movement. Hypertonic: higher effective solute concentration than the cell, so water tends to leave it. Hypotonic: lower effective solute concentration than the cell, so water tends to enter it.

A cell is placed in a hypertonic solution. Predict water movement and cell-volume change.

Water moves out of the cell; the cell shrinks. An animal cell may crenate; a plant cell may lose turgor and plasmolyze.

A cell is placed in a hypotonic solution. Predict water movement and cell-volume change.

Water moves into the cell; an animal cell may swell and potentially lyse. A plant cell becomes turgid because its cell wall resists expansion.

A cell is placed in an isotonic solution. Does water stop moving?

No. Water continues moving both directions, but the rates are equal, so there is no net water movement and average cell volume remains stable.

How do you determine tonicity from a diagram with solute particles on each side of a membrane?

First identify which solutes can cross. Compare the concentrations of nonpenetrating (effective) solutes on each side. Water tends to move toward the side with more effective solute.

A membrane is permeable to water but not to solute X. Side A has 2 units of X and side B has 6 units in equal volumes. Which way is net water movement?

From side A toward side B, because side B has the higher concentration of nonpenetrating solute.

Why is tonicity not always identical to total solute concentration?

Tonicity depends on solutes that cannot freely cross the membrane and therefore sustain an osmotic effect. Permeant solutes may not maintain a lasting difference.

What factors influence the rate and direction of passive transport?

Direction is set by the relevant concentration/electrochemical gradient. Rate depends on gradient size, membrane permeability, surface area, temperature, molecule properties, and availability/number of transport proteins.

Why can a charged ion not simply diffuse through the hydrophobic membrane core?

Moving a charge into the nonpolar core is energetically unfavorable. Ions generally need channels or transporters.

A solute is polar and relatively large, but its concentration is higher outside the cell. Will it necessarily enter by diffusion?

No. A favorable gradient does not overcome the bilayer's permeability barrier by itself. It may need a specific channel or carrier.

A molecule's concentration is equal on both sides of a membrane. What is the net diffusion rate?

Net diffusion is zero at equilibrium, although molecules continue moving randomly in both directions.

A carrier-mediated transport graph plateaus as solute concentration rises. What does the plateau suggest?

Transport is saturating because a limited number of carriers are occupied or cycling at their maximum rate.

How can a cell move glucose into itself against glucose's gradient without directly using ATP at the glucose transporter?

Secondary active transport can couple glucose uptake to movement of an ion down its electrochemical gradient, such as sodium moving down its gradient.

Define metabolism, anabolism, and catabolism.

Metabolism is the sum of chemical reactions in a cell or organism. Anabolism builds complex molecules and generally requires energy. Catabolism breaks molecules down and often releases usable energy.