Lecture 2 - Membrane Biophysics

Learning Objective 1: Membrane Composition and Substance Entry

What are the main components of biological membranes?

Lipids, proteins, and a small amount of carbohydrates.

What determines the ratio of protein to lipid in a membrane?

Metabolic activity; more active membranes have more proteins.

What is the role of cholesterol in membranes?

Maintains membrane fluidity across temperature ranges; inserts between phospholipid heads.

What are lipid rafts?

Specialized membrane microdomains rich in sphingolipids and lipid-anchored proteins involved in protein trafficking and signal transduction.

What is the glycocalyx?

A protective layer of carbohydrates (glycoproteins/glycolipids) on the cell surface, involved in immune recognition.

How does membrane composition affect polar and non-polar substance entry?

Non-polar (hydrophobic) substances pass through easily by diffusion; polar (hydrophilic) substances require transport proteins.

Learning Objective 2: Types of Transport

Define diffusion.

Passive movement of molecules down their concentration gradient (high concentration to low concentration) due to random motion.

Define facilitated diffusion.

Passive transport of molecules via specific carrier proteins or channels.

Define mediated transport.

Transport of substances across membranes with the help of membrane proteins (includes both passive and active transport).

Define active transport.

Movement of substances against their concentration gradient using energy (usually ATP).

Learning Objective 3: Types of Transporters

What are the types of passive transporters?

Ion channels, aquaporins (water channels), and solute carriers (facilitated diffusion).

What are ion channels classified by?

Selectivity, conductance, and gating mechanism.

What are aquaporins and their function?

Water channels facilitating rapid water movement; regulated by the number of channels inserted into the membrane.

What are the three types of solute carriers?

Uniporters (single molecule), symporters (two molecules, same direction), antiporters (two molecules, opposite directions).

Q: What are the key properties of carrier-mediated transport?

Specificity – each carrier transports only specific molecules or types of molecules.
Saturation – carriers have a maximum rate; once all are occupied, transport can't increase.
Competition – similar molecules may compete for the same carrier, reducing transport efficiency.

Q: What is an example of competition in carrier-mediated transport?

A: Galactose competes with glucose for GLUT2, reducing glucose uptake.

Q: What is primary active transport?

A: Uses ATP directly (e.g., Na+/K+ ATPase pump).

Q: What is secondary active transport?

A: Uses energy stored in ion gradients (e.g., SGLT uses Na+ gradient to import glucose).

Q: Difference between GLUT and SGLT transporters?

A: GLUT: facilitated diffusion; SGLT: secondary active transport using Na+ gradient.

Q: What is endocytosis?

A: a cellular process where a cell takes in material from its surroundings by engulfing it with its cell membrane

Q: Types of endocytosis?

A: Phagocytosis (large particles), pinocytosis (fluid), receptor-mediated endocytosis (specific ligands via clathrin-coated pits).

Q: What is exocytosis?

A: Export of substances out of the cell via fusion of vesicles with the plasma membrane.

Q: What protein assists in vesicle fusion during exocytosis?

A: SNARE proteins.

Q: What is autophagy?

A: Cellular process of degrading internal components for recycling, important in homeostasis and disease.

Q: What virus uses autophagy and endosomal pathways for replication?

A: Hepatitis B virus (HBV), involving RAB5A protein.