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

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:

Distinguish between simple diffusion, facilitated diffusion and active transport with respect to the use of membrane proteins and ATP. [3 marks]

Potato cubes were weighed before and after 30 minutes in three sucrose solutions: 0.0 mol/L (gained 8 percent mass), 0.3 mol/L (gained 1 percent), 0.8 mol/L (lost 7 percent). Explain the trend using the term osmosis, and estimate the sucrose concentration that matches the cytoplasm's water potential. [3 marks]

Refer to the cystic fibrosis CFTR protein. (a) Classify CFTR as a passive or active transporter and justify. (b) Explain why blocked chloride transport leads to thick mucus in airways.