How does the cell membrane control what enters and leaves a cell?
Describe the structure of the cell membrane and explain the mechanisms of membrane transport.
The fluid mosaic model of the membrane and the mechanisms of diffusion, osmosis, facilitated diffusion, active transport, and bulk transport, for TCE Biology Unit 2.
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Structure of the membrane
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.
Embedded in this bilayer are proteins, cholesterol, and carbohydrate chains. The arrangement is described by the fluid mosaic model: fluid because the phospholipids can move sideways, and mosaic because of the varied proteins scattered through it. The proteins act as channels, carriers, receptors, and markers.
Passive transport: diffusion
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).
The rate of diffusion increases with a steeper concentration gradient, higher temperature, larger surface area, and shorter distance to travel.
Osmosis
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.
Facilitated diffusion
Some substances cannot cross the bilayer easily because they are large or charged, for example ions and glucose. They move passively through specific channel or carrier proteins, a process called facilitated diffusion. It still moves substances down their concentration gradient and needs no energy, but it depends on the right transport protein being present.
Active transport
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.
Bulk transport
Very large molecules and particles are moved in or out of the cell in membrane-bound sacs called vesicles, using energy:
- Endocytosis brings material into the cell by folding the membrane inward to form a vesicle. Taking in solids is phagocytosis; taking in liquids is pinocytosis.
- Exocytosis releases material from the cell as a vesicle fuses with the membrane, for example secreting enzymes or hormones.
Linking transport to cell needs
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.