Quick questions on Plasma membrane structure and transport: VCE Biology Unit 1

The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates. It is "fluid" because the components move laterally; it is a "mosaic" because the embedded molecules are scattered through the bilayer.

The membrane lets some substances through but not others.

Driven by concentration, pressure or electrochemical gradients. The cell does not spend ATP.

Moves substances against their concentration gradient, from low to high concentration, through a carrier protein (pump) that uses ATP to change shape.

For substances too large for any protein channel.

Each phospholipid has a hydrophilic phosphate head (water-loving) and two hydrophobic fatty-acid tails (water-fearing). In water, they self-assemble into a bilayer with heads on the outside and tails packed in the middle. This is the structural basis of the membrane.

Integral (transmembrane) proteins span the bilayer; many act as channels, carriers, receptors or enzymes. Peripheral proteins sit on one face, often anchored to integral proteins, and act in signalling and structural roles.

Found in animal cell membranes (not plant). Sits between phospholipids and regulates fluidity: at high temperatures it stiffens the membrane; at low temperatures it prevents the fatty acid tails from packing too tightly.

Attached to outer-face proteins (glycoproteins) and lipids (glycolipids). Form a "glycocalyx" that is the basis for cell recognition, immune signalling and cell adhesion.

Movement of small non-polar molecules directly across the bilayer, down their concentration gradient. Example: oxygen entering a respiring cell.

Movement of polar molecules or ions through a channel protein (always open or gated) or carrier protein (changes shape), down their concentration gradient. Example: glucose entering a red blood cell through the GLUT1 carrier; Na+ entering a nerve cell through a sodium channel.

Movement of water across a semi-permeable membrane from a region of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). Aquaporins greatly accelerate this. In an animal cell:

It is a phospholipid bilayer, with embedded proteins, cholesterol and carbohydrates. Markers want fluid mosaic.

Osmosis is specifically the movement of water across a semi-permeable membrane. Diffusion can refer to any substance.

It can, slowly through the bilayer and quickly through aquaporins.