Cell surface membrane.

his focuses specifically on IAL Edexcel A Level Biology.

what describes the structure of the cell surface membrane the best?

A bilayer of phospholipids with proteins embedded.

why is the cell surface membrane partially permeable?

the phospholipid bilayer allows small, non polar molecules like oxygen and carbon dioxide to diffuse freely but large or charged molecules require transport proteins or active processes.

Why do unsaturated fatty acids make membranes more fluid?

the unsaturated fatty acid tails have kinks (double bonds) so the phospholipids cannot pack tightly and therefore are more spread out. this results the membrane more fluid and molecules move more easily

the role of cholesterol

cholesterol fits between the phospholipids, restricting their movement. the cholesterol prevents the membrane from becoming too fluid at high temperature and too rigid at low temperature, helping to main stability and permeability.

Beetroot discs are placed in water at increasing temperatures. Explain why more pigment leaks out at higher temperatures.

at high temperature, heating increases kinetic energy of phospholipids, the bilayer becomes more fluid the membrane proteins denature and the membrane loses its structure and becomes leaky. this allows betalain pigment to diffuse out.

Which transport process requires carrier proteins but not energy?

facilitated diffusion because it uses protein channels or protein carriers for passive movement down concentration gradient.

Why can’t ions diffuse through the bilayer?

The fatty acid tails are hydrophobic, while ions are charged and therefore hydrophilic. The ions are repelled by the fatty acid tails and cannot cross the bilayer. They require protein channels or carrier proteins to pass through.

What does the Fluid Mosaic Model mean?

fluid: the phospholipids and proteins can move laterally within the layer.

mosaic: proteins of different shapes and sizes are scattered throughout the bilayer, like tiles in a mosaic.

A student adds ethanol to a red onion cell. Predict what will happen to the membrane and explain why.

Ethanol dissolves the phospholipids in the bilayer, disrupting the membrane structure. This increases membrane permeability, so substances such as pigments leak out of the cell.