Key Words

It is important that you know the meanings of all the key scientific words as this will make it easier for you to understand what quesitons are asking and you may be tested on the meanings of some of these.

Look at the words below. Do you know a meaning for each? Click/Tap on the word to check that you know the correct definition.


Active transport

The process of moving a substance against a concentration gradient using energy from the hydrolysis of ATP molecules.


A channel protein that allows water to diffuse into the cell rapidly.


A protein, glycoprotein, glycolipid or sugar molecule found attached to a cell membrane which lymphocytes use for identification of cells.


2 layers.  How the phospholipids are found in the cell.

Carrier protein

An intrinsic protein which can change shape when a molecule interacts with it (usually with the glycoprotein attached to it). This shape change allows a substance to enter the cell or could be used as a receptor to cause a chemical reaction.

Cell signalling

Communication between cells where cells may work together to trigger a response or where cells identify themselves to each other (eg antigens)

Channel protein

A protein which allows small molecules such as ions to travel through the pore that runs through the centre of it.

Counter current flow

When two fluids move in opposite directions – eg the water moves in the opposite direction of blood flow in a fish.


A substance that stabilises the plasma membrane and maintains its fluidity.


The word used to describe an animal cell which has been placed into a solution of lower water potential and has lost water.


The folded membrane inside the mitochondria which contains many protein enzymes embedded on the inside layer for some of the reactions in respiration.


When an animal cell is placed into a solution with a less negative water potential which causes the cell to swell and burst.


The movement of particles from an area of high concentration to an area of low concentration.


The process by which large molecules are engulfed in the membrane and enter the cell.
Phagocytosis is an example of this.


The process in which large molecules leave a cell.


A protein found only on one side of the plasma membrane.

Facilitated diffusion

A type of diffusion which is aided. For example, using carrier proteins to help move molecules into the cell.


The name given to an animal cell which has lost water when it has been placed into a solution of lower water potential.

Fluid mosaic model

The model of the plasma membrane proposed by Singer and Nicholson.


The name of the chains attached to the outside of the plasma membrane.  These are either glycolipids or glycoproteins.


A molecule found in the plasma membrane composed of carbohydrate and protein.


Mixes well with water.


Does not mix with water.


A protein that spans the plasma membrane, some of these proteins act as channel and others as carrier proteins.


This is the overall movement. No net movement means no overall movement even though individual particles will be moving in both directions.


A substance, usually a molecule that does not have a charge and can diffuse through the phospholipids in the plasma membrane.


The movement of water from a higher (less negative) water potential to a lower (more negative) water potential.

Partially permeable

A substance that is permeable to some substances but not to others.


A process that does not require energy.


The ability of a membrane to allow a substance through.


A type of endocytosis used by to take in large solid molecules.


The type of lipid found in the plasma membrane. One of the fatty acid chains has been replaced by a phosphate group.


The endocytosis of liquid particles into the cell.

Plasma membrane

Made out of a bilayer of lipids, proteins, glycoproteins, glycolipids and cholesterol. It is 7nm thick and our best model of it is called the fluid-mosaic model.


The name given to a plant cell which has lost water when it has been placed into a solution of lower water potential.


A molecule or ion that has a charge.


Something that is made from a solute which dissolves into a solvent.


The folded inner membrane of the chloroplasts which contain many protein enzymes embedded on the inside layer for some of the reactions of photosynthesis.


The name given to a plant cell which has swollen when it has been placed into a solution of higher water potential.

Water potential

The measure of an ability of a substance to make water move. This always has a negative value or zero.
Return to the A Level menu