Permeable to some substances but not to others
Our current model of membrane structure consisting of a fluid phospholipid bilayer with many other molecules (including cholesterol, glycolipids, proteins and glycoproteins) floating or embedded in the lipid sea, all with different functions.
Chemicals which are released in a synapse when an action potential reaches the end of one neurone. They cross the synaptic gap and trigger and impulse in the next neurone.
The process which uses energy to move substances against a concentration gradient or across a partially permeable membrane using a special transport protein.
The thin, flexible structure enclosing the contents of the nucleus in a cell.
A molecule containing phosphorus and oxygen.
The membrane which forms the boundary between the cytoplasm of a cell and the medium surrounding it and controls the movement of substances into and out of the cell.
The body's natural defence mechanism against infectious diseases.
Bilayer of polar lipid molecules in an aqueous environment - the basis of the structure of the cell membrane.
Proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system.
A lipid molecule with a hydrophilic "head" region around the ionic phosphate group and a long hydrophobic hydrocarbon tail that forms a bilayer in aqueous solutions
Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse
Energy producing organic compounds which are made of carbon, hydrogen and oxygen. Examples of food containing carbohydrate are rice, pasta, bread and potatoes
A lipid which can be measured in the blood. High levels are linked to an increased risk of cardiovascular disease
Molecules that absorb or dissolve in water - usually polar molecules.
Insoluble in water, repel water.
Lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.
The biochemical process by which the cells in the body releases energy
The process of replacing a damaged or diseased organ with a healthy organ from a dead or living donor.
A complex carbohydrate which makes up plant cell walls
The protein markers found on the surface of a cell that causes the immune system to produce antibodies against it.
A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.
Protein molecules attached to cells that only bind to specific molecules with a particular structure
A chemical messenger produced by a particular gland or cells of the endocrine system. Hormones are transported throughout the body in the blood stream but they produce a response only in specific target cells
A mass of abnormal cells which keep multiplying in an uncontrolled way.
Molecules which contain a lot of stored energy built up of fatty acids and glycerol. Lipids include oils and fats
A structure with a particular function which is made up of different tissues.
A bundle of neurones - it may be all sensory neurones, all motor neurones or a mixture of both
The molecule which transcribes the DNA code and carries it out of the nucleus through the pores in the nuclear membrane to the ribosomes in the cytoplasm which synthesise the required proteins
Molecule which acts as the common energy currency in all cells, providing the energy needed to drive chemical reactions in cells.
Cell membranes are vital to the way cells function. In animal cells they form the outer layer of the cell, the ultimate barrier between the inside of the cell and its surroundings. In plant cells the cell surface membrane is inside a relatively rigid cellulose cell wall but the properties of the membrane still control much of what moves into and out of the cell. Most of the organelles inside a eukaryotic cell are also membrane bound. Understanding the properties of cell membranes is key to understanding how cells work.
Our current model of the cell membrane has been built up over many years by a combination of experimental data and electron microscopy
The unit membrane
The basic structure of the cell membrane is a bilayer of phospholipids. Phospholipid molecules have a hydrophilic ‘head’ region around the ionic phosphate group and a long hydrophobic hydrocarbon tail. These polar lipids form a bilayer in aqueous solutions with the hydrophilic heads pointing outwards and the hydrophobic tails forming a hydrophobic layer in the middle. This bilayer is known as a unit membrane.
The phospholipid bilayer in aqueous solution that forms the backbone of the cell membrane
The cell membrane, however, is more than a simple unit membrane. Our current model is of a fluid phospholipid bilayer with many other molecules associated with it, floating or embedded in the lipid sea. These other molecules include cholesterol, glycolipids, proteins and glycoproteins and they all have different functions in the membrane. This is the fluid mosaic model of membrane structure and it explains many of the properties of membranes that we can observe experimentally.
The fluid mosaic model of the cell membrane.
Many of the functions of the surface cell membrane and membranes around cell organelles are similar, although there are some which are specific to the outer membrane.
The pores in the nuclear membrane allow chemicals to move into the nucleus and mRNA to move out into the cytoplasm. (Image courtesy of Don W. Fawcett/Hector E. Chemes/Bernard Gilula (CC BY-NC-ND 3.0))