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.

For large molecules, particles or whole cells, the membrane bends to engulf or release material in vesicles. ATP is required.

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: - magnitude of the concentration gradient, - temperature, - surface area, - thickness of the membrane (Fick's law).

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: - Channel proteins. Pores that open and close (e.g. potassium channels, aquaporins).

ATP-driven pumps couple ATP hydrolysis to solute movement. - Sodium-potassium pump. Moves 3 Na+ out and 2 K+ into the cell per ATP. Maintains the resting potential of neurons and muscle cells.

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. - Phagocytosis ("cell eating"). Large particles or whole cells. Used by macrophages and neutrophils to engulf pathogens and debris.

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.

Be explicit: water moves down the water potential gradient (or equivalently, towards the solution with more solute).

No ATP. The gradient drives it; the protein only provides a path.

Water still moves in both directions; the rates are equal, so the net change is zero.