Function of Membranes & Fluid Mosaic Model

What is the main function of membranes?

To separate the contents of cells from their environment and create distinct compartments within cells.

What is compartmentalisation?

The formation of separate membrane-bound areas in a cell to allow specific conditions for reactions and protect vital components.

Why is compartmentalisation vital?

It allows incompatible metabolic reactions to occur simultaneously in different parts of the cell.

What is the plasma membrane?

The cell surface membrane that separates the cell from its external environment.

What is the basic structure of all membranes?

A phospholipid bilayer.

Describe the phospholipid bilayer.

Hydrophilic phosphate heads face outwards;

hydrophobic fatty acid tails face inwards, forming a hydrophobic core.

Why is the phospholipid bilayer suited to its function?

Because both sides of the membrane can interact with the aqueous environments inside and outside cells.

What is the hydrophilic part of a phospholipid?

The phosphate head.

What is the hydrophobic part of a phospholipid?

The fatty acid tail.

What did electron microscopy reveal about membranes in the 1950s?

Two parallel dark lines representing the lipid bilayer.

Who proposed the fluid mosaic model and when?

Singer and Nicolson in 1972.

Describe the fluid mosaic model.

Phospholipids can move within the layer (fluid), and proteins of various shapes and sizes are embedded in the bilayer (mosaic).

Why is membrane fluidity important?

It allows flexibility and movement of proteins within the membrane.

Name the main components of the plasma membrane.

Phospholipids, cholesterol, glycolipids, glycoproteins, intrinsic and extrinsic proteins.

What are intrinsic proteins?

Transmembrane proteins embedded through both layers of the bilayer.

How are intrinsic proteins held in place?

By interactions between hydrophobic R-groups and the hydrophobic core of the membrane.

What are channel proteins?

Intrinsic proteins that form hydrophilic channels for passive transport of ions and polar molecules down a concentration gradient.

What are carrier proteins?

Intrinsic proteins that change shape to transport molecules, used in both passive and active transport.

What are glycoproteins?

Intrinsic proteins with attached carbohydrate chains.

Functions of glycoproteins?

Act as receptors for chemical signals and in cell adhesion (forming tight junctions).

What is cell signalling?

The process by which cells communicate using chemical signals binding to receptors to trigger a response.

Give an example of glycoprotein receptor action.

Neurotransmitter receptors (e.g. acetylcholine) or peptide hormone receptors (insulin, glucagon).

How do some drugs work on receptors?

By binding to them, altering cell responses (e.g., β-blockers reduce heart response to stress).

What are glycolipids?

Lipids with attached carbohydrate chains acting as cell markers or antigens.

What is the function of glycolipids?

They enable immune system recognition of self and non-self cells.

What are extrinsic proteins?

Peripheral proteins on one side of the bilayer.

How do extrinsic proteins interact with the membrane?

Through hydrophilic R-groups interacting with phospholipid heads or intrinsic proteins.

What is cholesterol?

A lipid molecule with hydrophilic and hydrophobic ends found between phospholipids.

What is the role of cholesterol in membranes?

It regulates membrane fluidity and stability.

How does cholesterol stabilise membranes?

It binds to phospholipid heads and tails, pulling them together, preventing them from becoming too fluid or too rigid.

How does cholesterol prevent membranes from becoming too solid?

It stops phospholipids from grouping too closely and crystallising.

What are the three main roles of membranes at the cell surface?

1) Partially permeable barrier

2) Site of cell signalling/communication,

3) Site of chemical reactions.

What are the roles of membranes within cells?

To compartmentalise organelles, act as partially permeable barriers, and provide sites for metabolic reactions.

Give an example of a membrane’s role in respiration.

Mitochondrial cristae membranes contain ATP synthase and electron carriers for ATP production.

Give an example of a membrane’s role in photosynthesis.

Chloroplast thylakoid membranes contain enzymes for photosynthesis.

Why do alcohol, caffeine, and nicotine act quickly on the body?

They are lipid-soluble and diffuse rapidly through membranes.

Why are some organelle membranes highly folded?

It increases surface area for chemical reactions and enzyme attachment.

Define partially permeable membrane.

A membrane that allows certain molecules to pass through but not others.

Name the two main types of membrane protein.

Intrinsic and extrinsic proteins.