Lecture 5: Cell Membrane and Transport

What is the plasma membrane?

A phospholipid bilayer that separates extracellular fluid (ECF) from intracellular fluid (ICF) and controls molecular movement.

How thick is the plasma membrane?

Two molecules thick (a bilayer).

What is the main function of the lipid bilayer?

Forms the membrane structure, acts as a barrier to water-soluble substances, and controls membrane fluidity.

What is the structure of a phospholipid?

Hydrophilic phosphate head + hydrophobic fatty acid tails.

Why do phospholipids form a bilayer?

Hydrophobic tails avoid water while hydrophilic heads interact with water.

Name the four major phospholipids in the plasma membrane.

PE, PC, PS, PI.

What is sphingomyelin?

A membrane lipid with a ceramide core (not glycerol); abundant in myelin sheaths.

What does membrane fluidity mean?

The ability of lipids and proteins to move laterally within the membrane.

What is the role of cholesterol in the membrane?

Maintains membrane integrity and limits excessive fluidity.

What are integral membrane proteins?

Proteins embedded in the lipid bilayer.

What are transmembrane proteins?

Integral proteins that span the entire lipid bilayer.

Why must integral proteins be amphipathic?

They interact with both hydrophilic and hydrophobic regions of the membrane.

What are peripheral membrane proteins?

Proteins attached to the membrane surface or to integral proteins.

What is the function of membrane channels?

Create selective hydrophilic tunnels for water-soluble substances.

Why are channels highly selective?

Even similarly charged ions (Na⁺ vs K⁺) require different channels.

What are transporters (carriers)?

Proteins that move substances via conformational change; highly selective.

Give an example of a transporter.

Na⁺/glucose cotransporter.

What are docking marker acceptors?

Proteins that help vesicles dock and fuse with the plasma membrane.

What is the function of SNARE proteins?

Mediate vesicle docking and fusion.

What are membrane-bound enzymes?

Enzymes attached to membranes that regulate reactions (e.g., PI3 kinase).

What are receptors?

Proteins that bind extracellular ligands and trigger intracellular signaling.

Why are receptors important for specificity?

Only cells with the receptor respond to a signal.

What are cell adhesion molecules (CAMs)?

Proteins that attach cells to other cells or the extracellular matrix.

What is the glycocalyx?

Carbohydrate coat on membrane proteins and lipids involved in recognition and adhesion.

What are lipid rafts?

Cholesterol- and sphingolipid-rich membrane regions involved in signaling.

What are caveolae?

Small cave-like invaginations formed by lipid rafts.

What is cytoskeletal anchoring?

Actin, microtubules, and intermediate filaments limit protein movement in membranes.

What is the extracellular matrix (ECM)?

A gel-like network of proteins, water, and carbohydrates surrounding cells.

What is interstitial fluid?

Watery component of the ECM that allows diffusion.

What is the function of collagen?

Provides tensile strength.

What is the function of elastin?

Allows tissues to stretch and recoil.

What is the function of fibronectin?

Promotes cell adhesion and anchors cells.

What are desmosomes?

Anchor adjacent cells using cadherins and intermediate filaments.

Where are desmosomes common?

Tissues subject to stretching.

What are hemidesmosomes?

Attach epithelial cells to the basal lamina.

What are tight junctions?

Seal cells together and prevent paracellular transport.

What is the difference between paracellular and transcellular transport?

Paracellular = between cells; Transcellular = through cells.

What are gap junctions?

Connexon-based channels allowing ions and small molecules to pass between cells.

Where are gap junctions especially important?

Cardiac and smooth muscle.

What is a selectively permeable membrane?

Allows some substances to pass while restricting others.

What molecules cross membranes easily?

Gases, hydrophobic molecules, small polar molecules.

What molecules cannot cross membranes?

Ions, charged molecules, large polar molecules (e.g., glucose).

What is diffusion?

Net movement of molecules down a concentration gradient.

What drives diffusion?

Random molecular motion and concentration gradients.

What affects diffusion rate according to Fick's Law?

Concentration gradient, surface area, membrane thickness.

What is osmosis?

Movement of water down its concentration gradient.

What is osmotic pressure?

Pressure created by water movement across a membrane.

What is an isotonic solution?

No net movement of water.

What is a hypotonic solution?

Water moves into the cell.

What is a hypertonic solution?

Water moves out of the cell.