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Partially permeable

Permeable to some substances but not to others

Fluid mosaic model

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.

Neurotransmitters

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.

Active transport

The process which uses energy to move substances against a concentration gradient or across a partially permeable membrane using a special transport protein.

Nuclear membrane

The thin, flexible structure enclosing the contents of the nucleus in a cell.

Phosphate group

A molecule containing phosphorus and oxygen.

Cell membrane

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.

Immune system

The body's natural defence mechanism against infectious diseases.

Unit membrane

Bilayer of polar lipid molecules in an aqueous environment - the basis of the structure of the cell membrane.

Glycoproteins

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.

Phospholipid

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

Mitochondria

Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse

Carbohydrate

Energy producing organic compounds which are made of carbon, hydrogen and oxygen. Examples of food containing carbohydrate are rice, pasta, bread and potatoes

Cholesterol

A lipid which can be measured in the blood. High levels are linked to an increased risk of cardiovascular disease

Hydrophilic

Molecules that absorb or dissolve in water - usually polar molecules.

Hydrophobic

Insoluble in water, repel water.

Glycolipids

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.

Respiration

The biochemical process by which the cells in the body releases energy

Transplant

The process of replacing a damaged or diseased organ with a healthy organ from a dead or living donor.

Cellulose

A complex carbohydrate which makes up plant cell walls

Antigen

The protein markers found on the surface of a cell that causes the immune system to produce antibodies against it.

Protein

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.

Receptors

Protein molecules attached to cells that only bind to specific molecules with a particular structure

Hormone

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

Cancer

A mass of abnormal cells which keep multiplying in an uncontrolled way.

Lipid

Molecules which contain a lot of stored energy built up of fatty acids and glycerol. Lipids include oils and fats

Organ

A structure with a particular function which is made up of different tissues.

Nerve

A bundle of neurones - it may be all sensory neurones, all motor neurones or a mixture of both

Messenger RNA

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

Adenosine triphosphate

Molecule which acts as the common energy currency in all cells, providing the energy needed to drive chemical reactions in cells.

Cell membranes

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.

The structure of the cell membrane

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

The phospholipid bilayer in aqueous solution that forms the backbone of the cell membrane

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

The fluid mosaic model of the cell membrane.

  • A Phospholipids: lipid molecules with a hydrophilic ‘head’ region around the ionic phosphate group and a long hydrophobic hydrocarbon tail that form a bilayer in aqueous solutions.
  • B Cholesterol: a lipid with a steroid ring structure, and hydrophilic and hydrophobic regions. It makes up part of the membrane structure - there may be up to one cholesterol molecule for every two phospholipids. Cholesterol makes the membrane stiffer and more rigid – so the amount of cholesterol in the structure affects the rigidity of the membrane.
  • C Glycolipids: 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.
  • D Proteins: a wide variety of molecules that carry out many of the very specific functions of the cell membrane. There are integral proteins and peripheral proteins. They can form temporary and permanent channels in the membrane, allowing different molecules to pass in and out of the cell. They may be enzymes involved in active transport systems or enzymes linked to biochemical pathways such as photosynthesis or respiration. Proteins also act as receptor molecules for other molecules such as hormones and neurotransmitters
  • E Glycoproteins: 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

Functions 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.

  • Membranes form partially permeable barriers between the cell and its environment, between organelles and the cytoplasm and within organelles. They control the movement of substances both into and out of the cell and into and out of organelles. Permanent and temporary protein pores are involved in this control, as well as temporary and permanent active transport systems. Some channels are gated – they can be opened or closed depending on conditions inside or outside of the cell as described on the next page.
  • Membranes are the site of many chemical reactions because the enzymes involved are embedded in the membrane structure. Reactions take place both on the cell surface membrane and on the membranes in organelles such as mitochondria and chloroplasts.
  • Membranes are important in the development of chemical and electrochemical gradients – for example those involved in nerve impulses and in the production of ATP by chemiosmosis.
  • Membranes are the site of cell identification. The carbohydrate markers attached to glycoproteins and glycolipids along with some membrane proteins act as antigens, identifying one cell to other cells. For example this system enables the cells of the immune system to identify pathogens, cells from other organisms of the same species (eg after an organ transplant), abnormal body cells (eg cancer cells) and toxins produced by pathogens
  • Membranes are the site of cell communication. Cell signalling takes place between cells through the protein receptor molecules in the cell surface membrane and within cells; for example in the passing of hormone messages from the body to the nucleus of the cell and the movement of mRNA out of the nucleus through nuclear membrane pores.This process is described in more detail later.
TEM nuclear pore

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))

Activity:

Use materials of your choice – anything from plasticine to plastic bottles and beyond – make a three dimensional model of the cell membrane that can be used to explain the structure and functions of this amazing structure.