Lecture 12 — Membrane Structure & Transport

What is the fluid mosaic model?

Membranes are fluid structures made from many different molecules.

What are the major components of cell membranes?

Phospholipids, cholesterol, proteins, carbohydrates, lipid rafts.

How are phospholipids arranged in the bilayer?

Polar heads face water; nonpolar tails face inward.

Why do phospholipids form bilayers?

They are amphipathic.

What does amphipathic mean?

Having hydrophilic and hydrophobic regions.

What is membrane fluidity?

The ability of membrane molecules to move within the bilayer.

How do unsaturated fatty acids affect membrane fluidity?

Their double-bond kinks prevent tight packing and increase fluidity.

How do saturated fatty acids affect membrane packing?

Straight tails can pack more tightly.

What does cholesterol do in membranes?

Stabilizes membrane fluidity.

How does cholesterol stabilize membranes?

Prevents phospholipids from packing too closely or drifting too far apart.

What is an integral membrane protein?

Protein embedded in the lipid bilayer.

What is a peripheral membrane protein?

Protein attached to membrane surface without entering hydrophobic core.

Where are membrane carbohydrates usually located?

Outside the cell.

What are important functions of membrane carbohydrates?

Cell recognition and cell adhesion.

What does semipermeable mean?

Some molecules cross the membrane more easily than others.

What is simple diffusion?

Molecules move directly through membrane down their concentration gradient.

Does simple diffusion require a transport protein?

No.

What is facilitated diffusion?

Passive transport through a channel or transporter protein.

What is passive transport?

Movement down an electrochemical gradient without energy.

What is active transport?

Movement against an electrochemical gradient using energy.

If ATP is shown and molecules move low → high concentration, what process is occurring?

Active transport.

What is an electrochemical gradient?

Combined concentration and electrical gradients.

What is a channel protein?

Transmembrane pore allowing specific solutes to diffuse across.

What is a transporter protein?

Protein that binds solute and changes conformation to move it across.

What is a symporter?

Moves two substances in the same direction.

What is an antiporter?

Moves two substances in opposite directions.

How does the Na+/glucose symporter work?

Na+ moves down its gradient while glucose moves against its gradient.

Does the Na+/glucose symporter directly use ATP?

No; it uses energy stored in the Na+ gradient.

What does the Na+/K+ pump do?

Pumps Na+ out and K+ into the cell using ATP.

What gradients does the Na+/K+ pump maintain?

High Na+ outside and high K+ inside.