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

Why does a cell need a plasma membrane?

It separates the cell from its surroundings and selectively controls what enters and leaves.

What does selectively permeable mean?

The membrane allows some substances to cross more easily than others.

Why is a phospholipid bilayer semipermeable?

Its hydrophobic core favors some small nonpolar molecules but restricts many ions and polar molecules.

Which molecules cross a pure lipid bilayer most easily?

Small nonpolar molecules, such as oxygen (O₂).

Which molecules cross a pure lipid bilayer poorly?

Charged ions and large polar molecules, such as Na⁺ and glucose.

Clicker Q1: Which listed molecule is least likely to cross a pure lipid bilayer?

Na⁺, the sodium ion (answer C), because its charge is unfavorable in the hydrophobic core.

What is diffusion?

Net movement of particles down their concentration gradient, from higher to lower concentration.

What is a concentration gradient?

A difference in a substance's concentration between two regions.

Does diffusion require a membrane?

No. Diffusion can occur with or without a membrane if particles can move between regions.

What happens to net diffusion at equilibrium?

Net movement is zero, though particles continue moving randomly in both directions.

What determines the direction of net diffusion?

The concentration gradient, provided the substance can cross the barrier.

What is simple diffusion?

Passive movement directly through the lipid bilayer down a concentration gradient.

What is facilitated diffusion?

Passive movement down a gradient through a specific membrane protein.

Do simple and facilitated diffusion directly require ATP?

No. Both are passive transport and do not directly use cellular energy.

How do channels and carriers differ?

Channels form selective passageways; carriers bind a solute and change shape to move it.

Why might a solute need a transport protein even when its gradient favors movement?

The solute may be too large, polar, or charged to cross the lipid bilayer core.

What is active transport?

Transport that uses energy to move a substance against its concentration or electrochemical gradient.

What is primary active transport?

Transport that uses energy directly, often from ATP hydrolysis.

What is secondary active transport?

Transport that uses energy stored in one substance's ion gradient to move another substance.

Which transporter directly uses ATP: primary or secondary?

Primary active transport (clicker Q6, answer B).

What powers a secondary active transporter?

A substance moving down its electrochemical gradient, powering another substance's transport.

What is symport?

Coupled transport of two substances in the same direction across a membrane.

What is antiport?

Coupled transport of two substances in opposite directions across a membrane.

What is osmosis?

Net movement of water across a selectively permeable membrane toward the side with higher effective solute concentration.

What is tonicity?

A solution's effect on a cell's water movement and volume, based on effective solutes.

What is a hypertonic solution relative to a cell?

It has a higher effective solute concentration; water tends to leave the cell.

What is a hypotonic solution relative to a cell?

It has a lower effective solute concentration; water tends to enter the cell.

What is an isotonic solution relative to a cell?

It has equal effective solute concentration; there is no net water movement.

Does water stop moving in an isotonic solution?

No. Water moves both ways at equal rates, so there is no net movement.

What happens to an animal cell in a hypertonic solution?

Water leaves and the cell shrinks.

What happens to an animal cell in a hypotonic solution?

Water enters; the cell swells and may burst.

What happens to a plant cell in a hypotonic solution?

Water enters, making the cell turgid as the cell wall resists expansion.

What happens to a plant cell in a hypertonic solution?

Water leaves; the cell loses turgor and may plasmolyze.

How do you predict net water movement from a diagram?

Compare effective solute concentrations; water moves toward the side with more nonpenetrating solute.

Why must you consider whether a solute can cross when predicting tonicity?

Permeant solutes may not maintain a lasting osmotic difference; nonpenetrating solutes drive tonicity.

If side A has 2 units of nonpenetrating solute and side B has 6 in equal volumes, which way does water move?

From A to B, toward the higher effective solute concentration.

Clicker Q2: Lettuce cells swell after soaking. What does that indicate about water movement?

There was net water movement into the plant cells (answer A).

Clicker Q3: If water moves into plant cells, how are the cells toned relative to the solution?

The cells are hypertonic relative to the surrounding solution (answer A).

A membrane diagram shows higher amino acid concentration outside than inside. Which side is higher?

The exterior; this matches clicker Q4, answer A.

A membrane diagram shows higher H⁺ concentration outside than inside. Which side is higher?

The exterior; this matches clicker Q5, answer A.

Clicker Q7: What powers the secondary transporter shown in the lecture?

Na⁺ moving from high to low concentration (answer C).

Clicker Q8: In the illustrated secondary transport system, where is Ca²⁺ concentration higher?

In the cytoplasm (answer B), according to the clicker key.

What is an electrochemical gradient?

The combined effect of an ion's concentration difference and the electrical charge difference across a membrane.

Why do ions generally need channels or transporters to cross membranes?

Their charge makes entry into the hydrophobic membrane core energetically unfavorable.

How can a cell take up a solute against its gradient without ATP being used directly by that transporter?

Secondary active transport can couple uptake to an ion moving down its electrochemical gradient.

What happens to transport rate as a concentration gradient becomes steeper?

Net passive transport rate generally increases, if permeability and other conditions remain constant.

What happens if a membrane lacks the specific transporter a solute needs?

Transport may be very slow or negligible even if the concentration gradient favors movement.

What does a plateau in a carrier-mediated transport graph suggest?

Transporter saturation: available carriers are occupied or cycling near their maximum rate.

How do you distinguish passive from active transport in a model?

Passive transport moves down a gradient without direct energy input; active transport uses energy to move against a gradient.

How do you interpret arrows in a transporter diagram?

Track each substance separately, compare its concentrations on both sides, and identify which gradient supplies energy.

If solute concentration is higher outside, will it necessarily enter the cell?

No. Its gradient may favor entry, but membrane permeability and the right transporter also matter.

What is the difference between concentration gradient and transport direction?

The gradient predicts the favored direction only when the membrane permits that substance to cross.

What is the key difference between primary and secondary active transport?

Primary transport directly uses an energy source such as ATP; secondary transport uses an existing ion gradient.

What is the key difference between osmosis and diffusion of a solute?

Osmosis is net water movement; solute diffusion is net movement of the solute down its gradient.

A cell is placed in a solution with more nonpenetrating solute outside. Predict water movement and cell volume.

Water moves out; the cell loses volume.

A cell is placed in a solution with less effective solute outside. Predict water movement and cell volume.

Water moves in; the cell gains volume.

How should you rank molecules for crossing a pure lipid bilayer?

Generally, small nonpolar molecules cross best; large polar molecules and charged ions cross poorly.