Quick questions on Cell Membrane and Transport - TCE Biology (Tasmania)

The cell membrane (plasma membrane) is built mainly from a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. In water the molecules arrange themselves with heads facing out toward the watery surroundings and tails tucked inward, forming a double layer.

Passive transport needs no energy from the cell. The simplest form is diffusion, the net movement of particles from a region of high concentration to a region of low concentration, down the concentration gradient. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the bilayer. Diffusion continues until the concentrations are even (equilibrium).

Osmosis is the diffusion of water across a partially permeable membrane, from a region of higher water concentration (more dilute solution) to a region of lower water concentration (more concentrated solution). It is just diffusion of water, but it is named separately because of its importance to cells.

Active transport moves substances against their concentration gradient, from low to high concentration. Because this is the opposite of the natural direction, the cell must supply energy, usually from ATP. Carrier proteins act as pumps, changing shape to move the substance across. An example is the absorption of mineral ions by root cells even when the soil concentration is lower than inside the cell.

A cell constantly exchanges materials with its surroundings: taking in oxygen and nutrients, removing carbon dioxide and waste, and maintaining the right internal conditions. The combination of passive and active mechanisms lets it do this selectively. Active transport is especially important where the cell must move substances against a gradient, which is why cells doing a lot of active transport (such as kidney or root cells) contain many mitochondria to supply ATP.