Quick questions on Diffusion, osmosis and active transport across membranes (QCE Biology Unit 1)

Passive processes move substances down their concentration gradient. No metabolic energy is needed; the gradient supplies the driving force.

Active processes move substances against their concentration gradient and require metabolic energy.

Net movement of particles from high to low concentration until evenly distributed. Across membranes, only small non-polar molecules (O2, CO2, urea, ethanol, steroid hormones) cross the bilayer directly. Rate depends on:

Net movement of water across a selectively permeable membrane from a region of higher water potential (more dilute, lower solute) to lower water potential (more concentrated, higher solute). Water passes through the bilayer slowly and through aquaporin channels rapidly.

Polar and charged solutes (ions, glucose, amino acids) cannot cross the bilayer directly. They move down their concentration gradient through specific membrane proteins:

ATP-driven pumps couple ATP hydrolysis to solute movement.

A solute is moved against its gradient by piggybacking on the gradient of another solute (typically Na+). Example: SGLT1 in the small intestine couples Na+ entry to glucose entry against the glucose gradient.

The plasma membrane invaginates and pinches off a vesicle inside the cell.

Secretory vesicles fuse with the plasma membrane and release their contents to the outside. Used to secrete hormones, neurotransmitters, digestive enzymes and to add new membrane material.

Distinguish between facilitated diffusion and active transport with respect to direction and energy source. [2 marks]

Potato discs were weighed before and after 30 minutes in 0.0, 0.2, 0.4, 0.6 and 0.8 mol/L sucrose, with mass changes of +9, +3, -1, -5 and -10 percent. Identify the sucrose concentration isotonic to potato cytoplasm and explain the trend. [3 marks]

Refer to the human gut epithelial cell. (a) Identify the transport process by which glucose is absorbed against its gradient and name the energy source. (b) Predict the effect of a drug that blocks the basolateral Na+/K+ ATPase.