Lecture 9 Transport Across Membranes BIOMG 1350

Proteins.

What allows cells to selectively transfer molecules in and out?

Because of their electrical charges.

Why do ions struggle to pass through membranes?

Allow ions to pass quickly.

What do channels do in membranes?

They bind specific molecules and undergo conformational changes to carry them through the membrane (like turnstiles).

How do transporters differ from channels?

No

Does passive transport require energy?

The concentration gradient.

What determines the flow of molecules in passive transport?

Down the gradient (from high to low concentration).

In PASSIVE transport, molecules move in what direction relative to their gradient?

Yes

Does ACTIVE transport require energy?

Up/against the gradient (from low to high concentration).

In ACTIVE transport, molecules move in what direction relative to their gradient?

The electrochemical gradient.

What determines the direction of passive transport for charged molecules?

A combination of (1) the voltage (electrical gradient) and (2) the concentration (chemical gradient).

What is the electrochemical gradient?

Just the concentration gradient, since membrane potential is irrelevant.

For uncharged molecules, what is the electrochemical gradient equal to?

No

Can channels mediate active transport?

Specific molecules to diffuse across membranes.

What do channels allow?

By controlling whether channels are open or closed through conformational changes.

How do cells regulate channels?

Channels that respond to stimuli such as ligands

What are gated channels?

No

Do PASSIVE transporters require energy?

No

Do ALL transporters mediate passive transport?

None — direction is entirely determined by the electrochemical gradient.

What role do passive transporters play in determining direction of solute flow?

The glucose transporter.

What is an example of a passive transporter?

Glucose is uncharged

Why does glucose move passively across membranes?

Glucose is large; a glucose channel could accidentally allow ions to leak through

Why did cells evolve transporters for glucose instead of channels?

By using energy to push molecules against their electrochemical gradient.

How does active transport move molecules?

They couple favorable processes

How do active transport pumps couple reactions?

The sodium-potassium (Na⁺/K⁺) pump.

What is an example of an ATP-driven pump?

ATP hydrolysis releases a phosphate group → phosphate binds to pump → conformational change → 3 Na⁺ pumped out. 2 K⁺ ions bind → phosphate released → protein resets → 2 K⁺ pumped in.

How does the Na⁺/K⁺ pump work?

3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell

What ions are moved by the sodium-potassium pump and in which directions?

Light-driven pumps (use light energy to move molecules).

What other type of active pump exists besides ATP-driven pumps?

Glucose moves up its concentration gradient

What is glucose-Na⁺ coupled active transport?

Yes

Do Na⁺ and glucose need to bind simultaneously in this transporter?

1. Glucose-Na⁺ symport (active)
2. Glucose uniport (passive)
3. Na⁺/K⁺ pump (active).

What are the 3 different transporters involved in glucose uptake from food?

Two — outer and inner.

How many membranes does a mitochondrion have?

Cristae; ATP production occurs there.

What is the folded inner membrane of mitochondria called, and what occurs there?

Three H⁺ pumps in the electron transport chain use electron energy to pump H⁺ from the matrix into the intermembrane space.

How is the H⁺ gradient generated in mitochondria?

The electrochemical H⁺ gradient.

What does ATP synthase use to synthesize ATP?

Protons flow back down their gradient through ATP synthase → rotational catalytic mechanism drives phosphorylation of ADP into ATP.

How does ATP synthase generate ATP from ADP?

Without it, ATP synthase cannot function; the proton motive force provides the energy for ATP synthesis.

Why is the H⁺ gradient essential for ATP production?