🧬 BIOLOGY OF THE CELL — Cell Membranes & Transport

the phospholipid bilayer

What is the basic structure of all cell membranes?

A molecule with both hydrophilic and hydrophobic regions.

What is meant by “amphipathic”?

Lipids, proteins, and carbohydrates.

What are the three main components of the plasma membrane?

What is the function of membrane lipids?

To form the structural barrier separating the cell from its environment

Phospholipids and cholesterol.

Which lipids make up the bilayer?

A fluid, dynamic structure with proteins floating in a lipid sea.

: What is the fluid mosaic model?

Temperature, fatty acid tail saturation, and cholesterol content.

What affects membrane fluidity?

prevents fluidity and high temperatures , stabilises the membranes, prevents the phospholipids from packing tightly at low temperatures

How does cholesterol affect membrane fluidity

Phosphatidylcholine and sphingomyelin

Which phospholipids are mainly on the outer layer ? 

Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.

Which phospholipids are mainly on the inner leaflet?

 Negatively charged head groups of PS and PI.

What gives the cytosolic side of the membrane a negative charge?

Microdomains rich in cholesterol and sphingolipids that organise signalling and endocytosis.

What are lipid rafts?

Flask-shaped invaginations of the membrane involved in caveolin-mediated endocytosis.

What are caveolae?

Integral (transmembrane) and peripheral

What are the two main types of membrane proteins?

Transport, receptors, structural support, and enzymatic activity.

What are the roles of membrane proteins?

Alpha-helices made of hydrophobic amino acids.

What structure allows transmembrane proteins to span the membrane?

The extracellular side only.

On which side of the membrane are carbohydrates found?

The carbohydrate “sugar coat” on cell surfaces involved in protection and cell recognition

What is the glycocalyx

Glycoproteins, glycolipids, and proteoglycans.

What types of molecules contribute to the glycocalyx?

Q: What is endocytosis?
A: Uptake of materials into the cell via vesicle formation.

Q: What is exocytosis?
A: Release of materials from the cell by vesicle fusion with the plasma membrane.

Q: What protein drives vesicle scission in endocytosis?
A: Dynamin.

Q: What is clathrin’s role in endocytosis?
A: Forms a coated pit around cargo for receptor-mediated uptake.

Q: What triggers vesicle fusion in neurotransmitter release?
A: Increased intracellular calcium.

Q: What are SNARE proteins?
A: Proteins that mediate vesicle docking and fusion during exocytosis.

Lipids – form the bilayer structure (mainly phospholipids + cholesterol).

Proteins – channels, transporters, receptors, enzymes.

Carbohydrates – attached to proteins/lipids on the outer surface (glycoproteins, glycolipids).

Name three components of the plasma membrane.

• Synaptobrevin (V-SNARE): on the vesicle

• Syntaxin (T-SNARE): on the target (plasma) membrane

• SNAP-25 (T-SNARE): also on the target membrane

3 Proteins in the Snare Complex

Certain neurotoxins (like botulinum toxin and tetanus toxin) cut or destroy SNARE proteins.

What Neurotoxins Do ? 

Vesicles can’t dock to the membrane.

Vesicles can’t fuse, so neurotransmitters can’t be released.

The nerve signal stops — muscles don’t receive stimulation → muscle paralysis/relaxation.

What happens when  Snares are cleaved ? 

Vesicles dock via snare proteins

Ca²⁺ triggers fusion via synaptotagmin

Neurotransmitter released

Muscle contracts normally

What happens during exocytosis ?

Process by which cells internalise materials by engulfing them in membrane-bound vesicles.

What is endocytosis?

Clathrin-mediated and caveolae-mediated endocytosis.

Name two main types of endocytosis.

Nutrient uptake, receptor recycling, membrane turnover, and signalling.

What is the general purpose of endocytosis?

Dynamin.

Which enzyme is needed for vesicle scission from the membrane?

Clathrin is a protein that helps the cell form vesicles during endocytosis 

What is Clathrin?

Cargo binds receptor → clathrin coat forms → vesicle pinches off (dynamin) → uncoats → fuses with endosome.

what happens during cell mediated endocytosis