The Cell Membrane

The main component of the membrane is the proteins. Proteins can take up about 50% of the mass of the membrane and carry out most of the functions. Some proteins have pores, which ions often pass through. Gates open and close, regulating passage of ions. One role of the proteins in the membrane is for transport of molecules and ions through the cell. The seven main functions of proteins are carriers for water soluble molecules, channels for ions, pumps which use energy to move water soluble molecules and ions, receptors which enable nerve transmitters and hormones to bind to specific cells, recognition sites that identify a cell as being of a particular type, enzymes which speed up the chemical reactions at the edge of the membrane and adhesion sites which help some cells stick together. Proteins found in the bilayer are receptor proteins. These deal with communication, recognition proteins and transport proteins that regulate the movement of water-soluble molecules through the membrane. "There are three types of cell junction. Desosomes, which anchor cells together. Tight junctions, which prevent movement of molecules between cells, forming a continuous seam. Gap junctions, which are specialised cell connections and protein channels that allow flow of ions or molecules between cells." (www.biology.arizona.edu)

There are two types of proteins in cells- carrier proteins and transport proteins. Transport proteins are either active or passive. Passive transport uses no energy and substances diffuse through channels formed by proteins. Passive transport moves molecules down a concentration gradient, examples of this are diffusion, facilitated diffusion and osmosis. A concentration gradient is necessary for diffusion as the greater a gradient, the faster diffusion can take place.

"Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration." (A New Introduction To Biology, page 24) When there is a higher concentration on one side of the membrane, a concentration imbalance is produced. Molecules continue to move across a concentration until equilibrium is reached- an equal distribution of molecules on either side of the membrane. The rate of diffusion depends on the concentration, thickness and surface area of the cell membrane and is proportional to (surface area x difference in concentration) ? thickness of surface.

Facilitated diffusion involves ion channels, which allow charged molecules to pass through the membrane. The ion channels are formed by proteins with a central pore. Carrier proteins are also involved. They carry larger molecules across the membrane by binding to the molecule e.g. glucose, which changes the shape and then releases at the other side of the membrane.

Osmosis is the movement of freely moving water molecules from their high concentration to a place of their lower concentration through a partially permeable membrane. The stronger a solution, the more negative the water potential. This shows how osmosis occurs:

Active transport is the movement of molecules and ions against the concentration gradient. It is similar to facilitated diffusion, but requires energy (ATP). This energy is needed to activate the carrier proteins, which change the shape of the molecule and release if on the other side. The energy required for this usually comes from respiration in the mitochondria. The carrier proteins in active transport are often called pumps.

Endocytosis and exocytosis are other ways of movement across the membrane. "They move substances across the membrane in bulk."(A New Introduction To Biology, page 31). Endocytosis involves the membrane wrapping around something outside the cell and forming a vesicle on the inside surface. Endocytosis brings solid and liquid materials into the cell by phagocytosis, meaning cell eating. Pinocytosis is when liquid materials are brought into the cell, meaning cell drinking. Exocytosis involves the fusion of a vesicle with the membrane and the release of its contents outside. A vesicle would separate from the edge of the Golgi apparatus in the cytoplasm and move towards the membrane where it would be pushed out of the cell. This is how hormones are secreted from cells.

Another type of lipid in the membrane is cholesterol. The amount of cholesterol varies with the type of membrane. Cell membranes have one cholesterol per phospholipid molecule. Cholesterol makes the bilayer stronger, more flexible and more permeable. Cholesterol helps to pack phospholipids in the membranes, giving more firmness. Without cholesterol, a cell would need a cell wall as it decreases permeability to small water-soluble molecules.

Another component of the membrane is carbohydrates. Glycoproteins have carbohydrates in the form of a polysaccharide attached to them in a covalent link. They are rich in sugars and store carbohydrates in animals.

The cell membrane is vital for the protection and structure of animal and plant cells. It separates the cell from its surrounding environment and maintains the homeostasis of the cell. The whole cell counts on the membrane to get rid off toxins and take in important substances like oxygen.

References:

A New Introduction to Biology: Bill Indge, Martin Rowland, Margaret Baker.

CGP GCSE Biology revision guide

www.cellsalive.com

www.biology.arizona.edu

www.cell-biology.com