IBDP Biology (2025+): Membranes and Transport SL only

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.

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.

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.

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.

Diffusion

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

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.

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.

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.

Peripheral protein

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

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.

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.

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.

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.