membranes and transport

Acetylcholine (ACh)

a neurotransmitter that functions in both the central and peripheral nervous systems.

Active transport

movement of particles from lower to higher concentration, using energy from ATP that has been created during respiration. Movement is through carrier proteins.

Adenosine triphosphate (ATP)

a nucleotide, present in every living cell, formed in photosynthesis and respiration from ADP and Pi; it functions in metabolism as a common intermediate between energy-requiring and energy-yielding reactions.

Adhesion

the force by which individual molecules stick to surrounding materials and surfaces.

Amphipathic

a molecule that has two different affinities - a polar end that is attracted to water (hydrophilic) and a non-polar end that is repelled by it (hydrophobic).

Aquaporin

a water channel pore (protein) in a membrane.

Cell-adhesion molecule (CAM)

proteins located on the cell surface that are involved in binding cells with other cells or the extracellular matrix.

Channel protein

a pore in the membrane that allows specific charged particles (e.g. ions) and polar substances (i.e. all hydrophilic substances) to diffuse through the membrane into or out of the cell.

Chemical signalling

the release of chemicals (ligands) that bind to a specific molecule which delivers a signal within the cell or to another cell.

Cholesterol

a lipid of animal plasma membranes; a precursor of the steroid hormones in humans, formed in the liver and transported in the blood as lipoprotein.

Concentration gradient

the difference in concentration of a substance between one area and another.

Contractile vacuole

a small vesicle in the cytoplasm of many freshwater protists that expels excess water.

Depolarization

a temporary and local reversal of the resting potential difference of the membrane that occurs when an impulse is transmitted along the axon.

Diffusion

the free passage of molecules (and atoms and ions) from a region of their higher concentration to a region of lower concentration.

Endocytosis

formation of vesicles as the plasma membrane pinches inward, taking material into the cell.

Exocytosis

vesicles fuse with the membrane and material is exported out of the cell.

Facilitated diffusion

the movement of particles from higher to lower concentration through integral proteins (carrier or channel proteins). Movement is passive.

Flaccid

plant cell that has become soft and less rigid than normal because the cytoplasm within its cells has shrunk and contracted away from the cell walls through loss of water.

Fluid mosaic model

the accepted view of the structure of the plasma membrane, comprising a phospholipid bilayer with proteins embedded but free to move about.

Gas exchange

exchange of respiratory gases (oxygen, carbon dioxide) between cells/organism and the environment.

Glycocalyx

long carbohydrate molecules attached to membrane proteins and membrane lipids.

Glycolipid

lipids with a glycocalyx attached.

Glycoprotein

membrane protein with a glycocalyx attached.

Hydrophilic

attracted to water; e.g. hydrogen bonds are readily formed between a molecule and water.

Hydrophobic

repelled by water.

Hypertonic

when the external solution is more concentrated than the cell cytoplasm and there is a net outflow of water from the cell by osmosis.

Hypotonic

when the external solution is less concentrated than the cell cytoplasm and there is a net inflow of water into the cell by osmosis.

Integral proteins

proteins that traverse from one side of a membrane to the other side.

Invagination

the intucking of a surface or membrane.

Ion channel

transmembrane protein that forms a pore across the bilayer through which specific ions can diffuse down their concentration gradients.

Isotonic

when the external solution is the same concentration as the cell cytoplasm and there is no net entry or exit of water from the cell by osmosis.

Ligand

general term for a molecule that binds to a specific site on a protein.

Ligand-gated channel

an ion channel that is stimulated to open by the binding of a small molecule such as a neurotransmitter.

Neurotransmitter

chemical released at the presynaptic membrane of an axon when an action potential arrives which transmits the action potential across the synapse.

Neurotransmitter-gated ion channel

channel protein that temporarily opens when a specific neurotransmitter bonds with it.

Osmosis

diffusion of free water molecules from a region where they are more concentrated (low solute concentration) to a region where they are less concentrated (high solute concentration) across a partially permeable membrane.

Osmotic concentration

the measure of solute concentration, defined as the number of osmoles (osmol) of solute per litre (L).

Peripheral protein

a protein that is attached to the surface of the bilayer.

Phagocytic vacuole

a vesicle formed around a particle engulfed by a phagocyte via phagocytosis.

Phagocytosis

the process by which a phagocyte engulfs and destroys foreign substances, such as bacteria.

Phospholipid

formed from a triacylglycerol in which one of the fatty acid groups is replaced by an ionized phosphate group.

Plasma membrane

the membrane of lipid and protein that forms the surface of cells; constructed as a fluid mosaic membrane.

Plasmolysis

condition when plant cells are in hypertonic solutions where the cell membrane pulls away from the cell wall due to reduced cell volume and pressure.

Pump proteins

proteins in plasma membranes that use energy to actively carry ions and/or solutes against a concentration gradient.

Receptor protein

protein that recognizes and binds with a specific chemical signal molecule on the outside of the plasma membrane.

Saturated fat

fat with a fully hydrogenated carbon backbone (i.e. no double bonds present).

Solute

dissolved molecule or ion in a solution.

Solvation

the interaction of a solvent with dissolved molecules or ions.

Solvent

a liquid in which another substance can be dissolved.

Transmembrane receptor protein

cell signalling receptor protein that is in the membrane.

Turgid

condition where the vacuole of a plant cell is full of water, pushing the cell membrane against the cell wall.

Turgor pressure

hydrostatic pressure in a cell that pushes the plasma membrane against the cell wall.

Unsaturated fat

fat with double bond(s) between carbons in the hydrocarbon chain.

Vesicle

membrane-bound sac.

Voltage-gated channels

a type of transmembrane protein that forms ion channels activated by changes in the electrical membrane potential of a neuron.

Water potential

the potential energy of water per unit volume, relative to pure water, denoted by the symbol ψ (psi) and expressed in units of pressure (kPa).

Polar molecule

a molecule where there is an unequal distribution of electrical charge: one end is slightly positive and the other end is slightly negative.