Question 1

What is the fluid mosaic model and why is it called that?

Accepted Answer

The fluid mosaic model describes the structure of cell membranes as a phospholipid bilayer with embedded proteins. It's called 'fluid' because phospholipids and proteins can move laterally within the layer, and 'mosaic' because the proteins are scattered throughout the bilayer like tiles in a mosaic. This model was proposed by Singer and Nicolson in 1972 and is supported by evidence from freeze-fracture electron microscopy.

Question 2

How does temperature affect membrane permeability?

Accepted Answer

As temperature increases, phospholipids gain kinetic energy and move more, increasing membrane fluidity and permeability. However, at very high temperatures (above about 45°C), the bilayer may break down and proteins denature, causing a sharp increase in permeability. At low temperatures, phospholipids pack closely and the membrane becomes less fluid, reducing permeability. This is often demonstrated using beetroot, where pigment leakage is measured at different temperatures.

Question 3

What is the difference between channel proteins and carrier proteins?

Accepted Answer

Channel proteins form hydrophilic pores that allow specific ions or small molecules to diffuse through passively (facilitated diffusion). They are usually gated and open in response to a stimulus. Carrier proteins bind to specific molecules and change shape to transport them across the membrane; they can work by facilitated diffusion (passive) or active transport (using ATP). For example, glucose transporters are carrier proteins, while aquaporins are channel proteins for water.

Question 4

How do cells use membranes for cell signalling?

Accepted Answer

Cell membranes contain receptor proteins that bind to signalling molecules like hormones or neurotransmitters. This binding triggers a conformational change in the receptor, activating intracellular signalling pathways (e.g., via second messengers like cAMP). For instance, adrenaline binds to β-adrenergic receptors on liver cells, activating a G-protein that leads to glycogen breakdown. This allows cells to respond to external signals without the signal entering the cell.

Question 5

Why is cholesterol important in animal cell membranes?

Accepted Answer

Cholesterol is a lipid that sits between phospholipids in the bilayer. It regulates membrane fluidity: at high temperatures, it restricts phospholipid movement to reduce fluidity; at low temperatures, it prevents phospholipids from packing too tightly, thus maintaining fluidity. It also strengthens the membrane and reduces permeability to small water-soluble molecules. Cholesterol is absent in plant cell membranes, where other sterols perform similar roles.

Question 6

What is the role of glycoproteins and glycolipids in cell membranes?

Accepted Answer

Glycoproteins and glycolipids have carbohydrate chains that extend from the cell surface, forming the glycocalyx. They are involved in cell recognition (e.g., ABO blood group antigens), cell adhesion (helping cells stick together), and protection against mechanical damage. They also act as receptors for pathogens (e.g., influenza virus binds to sialic acid on glycoproteins) and are important in immune responses, such as identifying self vs non-self cells.

Biological membranes

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