Membranes and transport across cells definitions

Membrane structure and the fluid mosaic model

Phospholipid

A lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails.

Hydrophilic

Attracted to water; polar or charged.

Hydrophobic

Repelled by water; non-polar.

Bilayer

Two layers of phospholipid with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery solutions on each side.

Intrinsic protein

A protein embedded within the bilayer, often spanning it completely.

Extrinsic protein

A protein on one surface of the bilayer that does not enter the hydrophobic core.

Glycoprotein

A protein with a carbohydrate chain attached. In the cell surface membrane, glycoproteins act in cell recognition, adhesion or as receptors.

Glycolipid

A lipid with a carbohydrate chain attached, acting in cell recognition and adhesion.

Cell signalling

Communication between cells in which one cell secretes a molecule that is transported to a target cell and binds to a receptor there.

Ligand

A molecule that binds specifically to a receptor, such as a hormone or a neurotransmitter.

Receptor

A protein, usually a glycoprotein in the cell surface membrane, whose binding site is complementary to one ligand; only cells carrying the receptor respond to the signal.

Permeability

How readily a membrane allows a substance to cross it.

Denaturation

The loss of a protein's tertiary structure when hydrogen and ionic bonds break, so its shape and therefore its function is lost.

Diffusion and osmosis

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion.

Simple diffusion

Diffusion directly through the phospholipid bilayer, available to small non-polar molecules such as oxygen and carbon dioxide.

Passive

Requiring no energy from ATP; driven by the kinetic energy of the particles themselves.

Water potential (ψ)

The tendency of water molecules to move out of a solution, measured in kPa. Pure water at atmospheric pressure has a water potential of 0 kPa; adding solute makes the solute potential negative.

Osmosis

The net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane.

Turgid

A plant cell whose contents press against the wall, giving a positive pressure potential.

Incipient plasmolysis

The point at which the pressure potential of a plant cell has just fallen to zero, so its water potential equals its solute potential.

Plasmolysis

The pulling away of the cell surface membrane from the cell wall as water leaves a plant cell.

Haemolysis

The bursting of a red blood cell when water enters it by osmosis.

Crenation

The shrinking and puckering of an animal cell when water leaves it by osmosis.

Visking tubing

A partially permeable cellulose membrane, with pores that pass water and small solutes but not large molecules, used as a non-living model of a membrane.

Active transport, co-transport and bulk transport

Active transport

The movement of a substance across a membrane against its concentration gradient, using a carrier protein and energy from the hydrolysis of ATP.

Carrier protein

An intrinsic protein with a binding site specific to one substance, which changes shape to move it across the membrane.

Co-transport

The transport of two substances across a membrane by the same carrier protein at the same time, in which one moves down its concentration gradient and provides the means for the other to move against its own.

Endocytosis

The bulk transport of material into a cell by invagination of the cell surface membrane to form a vesicle; requires ATP.

Exocytosis

The bulk transport of material out of a cell by fusion of a vesicle with the cell surface membrane; requires ATP.