Membranes, Diffusion and Osmosis

A membrane contains a phospholipid bilayer with hydrophilic heads facing water and hydrophobic tails forming the interior. Proteins are embedded or associated with it; glycoproteins and glycolipids have carbohydrate components involved in recognition and other interactions. Some components move laterally, making the membrane dynamic rather than a fixed tiled surface.

Cholesterol may be present and restrict movement of other molecules, helping regulate membrane properties. Channel proteins provide hydrophilic pathways and carrier proteins bind particular solutes and change conformation. Proteins are not all pores, and the entire membrane is not made only from phospholipids.

Simple diffusion is net movement down a concentration gradient through the bilayer where the substance can pass sufficiently readily, as for oxygen and carbon dioxide. Facilitated diffusion also moves down an appropriate gradient but uses channels or carriers, allowing ions or polar solutes to cross the hydrophobic barrier.

Neither passive process directly requires ATP hydrolysis. A larger surface area, steeper gradient, shorter diffusion path or more suitable transport proteins can increase the rate. Carrier-mediated rates can reach a maximum when the available carriers are occupied; this does not turn facilitated diffusion into active transport.

Osmosis is net movement of water through a partially permeable membrane from higher water potential to lower water potential. Dissolved solutes lower water potential; higher does not mean “more negative”. Pure water at the reference conditions has a water potential of zero, while many biological solutions have negative values.

Water entering an animal cell may cause swelling and lysis because it lacks a rigid wall. A plant cell becomes turgid as pressure against its wall limits further net entry. Water loss can reduce turgor and eventually cause plasmolysis, when the protoplast withdraws from the wall. At equal water potentials water still moves both ways, but there is no net movement.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.