Cell Membrane Transport: Lipid Bilayers, Ions, and Proteins

Are lipid bilayers permeable to ions and most uncharged polar molecules?

No, they are impermeable to ions and most uncharged polar molecules.

Which small, non-polar molecules can easily diffuse across the cell membrane without proteins?

O₂ and CO₂

Can small, uncharged polar molecules cross the lipid bilayer?

Yes, molecules like water and ethanol (EtOH) can diffuse, but diffusion slows as molecule size increases.

Can ions or charged molecules diffuse across the lipid bilayer?

No, they are virtually blocked regardless of size.

How do things get across the membrane?

Integral, transmembrane transport proteins.

channel

forms a pore across the bilayer through which specific inorganic ions or, in some cases, polar organicmolecules can diffuse - discriminates based on size and electrical charge.

transporter

undergoes a series of conformational changes to transfer small solutes across the lipid bilayer - has a specific binding site that recognizes molecules or ions

Passive transport

molecules cross cell membrane depending on the concentration gradient (High->Low)

Active transport

carried out by transporters (pumps)and requires energy. Required to transport molecules against gradient

Which ions do cells control across the membrane?

Na⁺, K⁺, Ca²⁺, and H⁺.

Which ion is most plentiful outside the cell? Inside?

Outside = Na⁺; Inside = K⁺ ("Kin").

How is Na⁺ electrically balanced? How is K⁺ balanced?

Na⁺ by Cl⁻; K⁺ by negative anions (nucleic acids, proteins, metabolites).

What is membrane potential?

A minor imbalance of electrical charge across the cell membrane (difference in electric potential inside vs. outside).

What is the state of the membrane when the cell is unstimulated?

Resting potential.

What 2 forces act on charged molecules across membranes?

Concentration gradient + electrical (charge-dependent) force.

what is the inside of the cell usually charged as, and how does it affect ions?

Inside is negative → pulls in positive ions, repels negative ions.

What is the combined effect of concentration + electrical forces called?

Electrochemical gradient.

What happens if concentration and electrical forces act in the same direction? Example?

Strong driving force; example = Na⁺.

What happens if concentration and electrical forces oppose each other? Example?

Weak driving force; example = K⁺.

How does the glucose transporter move molecules?

In either direction, but not simultaneously.

After a meal, where does glucose move and why?

Into the cell; glucose is higher outside.

After fasting, where does glucose move and why?

Out of the cell (to brain); glucose is higher inside (from glycogenolysis).

What type of transport does the glucose transporter use?

Passive transport (follows concentration gradient).

What is required to move molecules against their electrochemical gradient?

Active transport (requires energy).

How does a gradient-driven (coupled) pump work?

Uses one molecule moving with its gradient to power another against its gradient.

How does an ATP-driven pump work?

Hydrolyzes ATP to force movement against a gradient.

How does a light-driven pump work? Example?

Uses captured light energy to pump molecules against gradient; example = bacteriorhodopsin.

What does the Na⁺-K⁺ pump do?

Pumps Na⁺ out (against gradient) and K⁺ in (against small gradient).

What type of transport is the Na⁺-K⁺ pump, and what powers it?

Antiport transport, ATP-dependent (hydrolyzes ATP for energy).

How does the Na⁺-K⁺ pump operate?

Through conformational changes; each step depends on the one before.

Plants and microorganisms don't use the Na+-K+ pumps at all What do they use?

protons as we use sodium, to create coupled transport

Two important features of Ion Channels

1. Are Ion-selective

2. Most ion channels are gated

Which ion's leak channels make the plasma membrane most permeable at rest?

K⁺ leak channels.

What happens as K⁺ leaves the cell following its concentration gradient?

Negative charge builds inside, pulling K⁺ back in.

What is resting membrane potential? Typical value in neurons

Steady-state membrane potential, usually negative; about -60 mV in neurons.

Which ion most strongly influences resting potential, and why?

K⁺, because the membrane is most permeable to K⁺ at rest.

What do mechanically gated ion channels respond to? Example?

Mechanical vibration/pressure; e.g., sound waves, touch.

What do ligand-gated channels require to open? Example?

Binding of a molecule (e.g., cAMP, hormone, acetylcholine).

What controls voltage-gated channels?

Voltage sensor sensitive to membrane potential changes.

What method allows recording ion flow across a single channel in real time?

Patch-clamp recording.

What do dendrites do

Receive signals from axons of other neurons.

What does the axon do?

Conducts signals away from the cell body.

What is an action potential?

A traveling wave of electrical excitation carrying a signal without weakening.

What triggers an action potential?

Depolarization opening voltage-gated Na⁺ channels.

What causes repolarization?

Voltage-gated K⁺ channels opening (slower than Na⁺).

What is a synapse?

A specialized junction where neurons communicate with target cells

What happens at the axon terminal when an action potential arrives?

Voltage-gated Ca²⁺ channels open → Ca²⁺ influx triggers neurotransmitter release.

In a synapse, what type of signal is converted to what?

Electrical → chemical → electrical.

How do neurotransmitters affect the postsynaptic cell?

By binding to transmitter-gated ion channels.

Example of an excitatory neurotransmitter? What does it do?

Acetylcholine → Na⁺ influx → depolarization → activates postsynaptic cell.

Example of an inhibitory neurotransmitter? What does it do?

GABA or Glycine → Cl⁻ influx → hyperpolarization → inhibits postsynaptic cell.

How is the neurotransmitter signal stopped?

Neurotransmitters are destroyed by enzymes or reuptaken by transporters.

What category of membrane transport are channels a part of?

facilitated diffusion