Cell theory began in the 17th century following observations of cells with newly invented microscopes. It was the consensus that plant and animal tissues were composed of cells, but due to the low resolution of the microscope it was impossible to see any barriers around the animal cell, although it was accepted that there must be one to allow compartmentalisation of cellular reactions.
By the 20th century, the chemical nature of the membrane was being investigated following observations in which membranes formed spheres after being placed in water so it was suggested that the membrane was lipid based. As a result, the chemical but not structural basis of the membrane was known.
A number of theories were put forward following the discovery of the lipid basis of the structure surrounding the cell, but it was not until 1972 when Singer and Nicolson put forward the theory that a partially-permeable membrane was present around the outside of cells. Their theory of the fluid-mosaic model of membrane structure, although overly simplistic is important and is referenced today. It suggests that proteins are able to float on and in a movable lipid bilayer.
Plasma or cell-surface membranes surround all prokaryotic and eukaryotic cells. Plasma membranes are composed largely of phopsholipids which are amphipathic molecules with a hydrophillic (or water-loving) phosphate head group, and a hydrophobic (or water hating) fatty-acid tail. The amphipathic nature of phospholipids means that the phospholipids arrange themselves into a bilayer, with the hydrophobic tails on the inside and the hydrophilic heads on the outside. Some of the phospholipids also have a carbohydrate group attached, a glycolipid. Glycolipids play a role in cell-to-cell communication and recognition.
In addition to the phospholipids, the membrane also has a number of proteins embedded in it. Most of these proteins can move freely within the membrane and as such are not fixed in place. This makes the membrane fluid. The proteins are involved with transport of substances such as ions across the membrane. Furthermore, cholesterol can be found in the membrane, its role being to maintain membrane fluiditiy.
All cells need to interact with their environment. They must obtain oxygen and nutrients, maintain osmotic pressure and be able to remove waste products such as carbon dioxide from inside. The plasma membrane acts as a partially-permeable barrer which can control what enters and leaves the cell.
The membrane acts as a barrier, separating the cell cytoplasm from the external environment. It also ensures that the cell's internal environment can stay constant by selectively controlling what can and cannot enter or leave the cell. In addition, the membrane can provide a means of cell-to-cell communication through the use of genetically unique receptor molecules that are present on the surface of the membrane. This cell-to-cell communication is particularly important in the recognition of self and non-self in terms of an immune response.
The membranes of animal cells contain four types of major phospholipid which are arranged in a bilayer. These major types of phospholipid are phosphatidylcholine, which has a choline incorporated into its headgroup, phosphatidylethanolamine, which has an ethanolamine incorporated into its headgroup, phosphatidylserine, which has serine incorporated into its headgroup and sphingomyelin. Sphingomyelin is the only phospholipid in eukaryotic cell membranes that is not derived from glycerol.
In addition to the phospholipids, which account for over 50% of the membrane, there are other components. Glycolipids - lipids that are linked to a polar head group which contains carbohydrates - are found exclusively in the outer membrane with the carbohydrate part exposed to the cell surface. Cholesterol is an additional component, which is a lipid that consists of four hydrocarbon rings and it is involved with maintaining membrane fluidity.
Cholesterol is an amphipathic molecule which, like phospholipids, means that it contains a hydrophobic portion and a hydrophilic portion. The hydroxyl portion of cholesterol aligns with the phosphate head of the phospholipids as it is hydrophilic, whereas the hydrophobic hydrocarbon tail portion remains tucked inside the membrane.
Cholesterol is abundant in the cell membrane accounting molecule for molecule for almost 50% of the cell membrane. However, because it is a small molecule, cholesterol only accounts for 20% of the membranes mass.
Despite its small mass, cholesterol plays an important role in maintaining the integrity of the membrane and is also involved with cell to cell signalling. Without cholesterol membranes would become too fluid in hot environments or would become not fluid enough in cold environments.
There are three main types of protein found in the plasma membrane of most cells.
Transmembrane proteins span the entire membrane, i.e. go from one side of the membrane to the other side. Transmembrane proteins are amphipathic in nature which means that they have hydrophobic regions which are on the inside of the membrane, away from water, and hydrophillic regions, which are in contact with the cytoplasm.
Integral proteins are solely inside the bilayer and are permanantly attatched to the biological membrane. Integral membrane proteins can only be separated from the bilayer with the use of detergents.
Peripheral proteins are only on the exterior of the membrane. These proteins adhere only temporarily to the membrane by attatching to integral membrane proteins or by penetrating the outer regions of the membrane.
Channel proteins and other transport proteins found in the membrane are needed for the movement of molecules across the membrane. Some molecules, such as oxygen can diffuse across the membrane without the need for transport proteins. However larger, less soluble substances need to be transported. Channel proteins allow small, water soluble molecules to pass through the membrane. This does not require energy. Whereas other transport proteins do require energy, in the form of ATP as they transport molecules such as glucose and ions across the membrane against their concentration gradient.
There are other proteins that are found in the membranes of some cells, these proteins are less common, but are still very important to cellular function. Cell-to-cell communication proteins are essential to the formation of gap junctions. Such junctions can be found in heart muscle cells to allow the coordination of heart contraction.
Receptor and recognition proteins are also important features of membranes. These proteins allow for cell recognition and are involved with cell communication.
Some molecules can move by diffusion. This requires no additional input of energy and is the spontaneous, net movement of molecules, down their concentration gradient. This means from an area of high concentration to an area of low concentration. Diffusion is affected by the extent of the concentration gradient and by the temperature of the environment. Molecules, such as oxygen, that can diffuse across plasma membranes do not require the presence of transport proteins.
Osmosis is the diffusion of water. Although the movement of water does not require the input of energy, it does require specialised proteins in the plasma membrane. These proteins are called aquaporins and they control the amount of water that can move in and out of the cell to ensure that the osmotic pressure in the cell is maintained.
Facilitated diffusion of molecules also does not require energy. This form of transport is needed for large molecules such as glucose that cannot easily diffuse across the membrane.
Active transport, however, does require energy. This involves the movement of molecules across the membrane against their concentration gradient from an area of high concentration to an area of low concentration. Charged ions such as calcium need to be moved by active transport. Some active transport pumps couple the movement of two substances across the membrane. For example the sodium-potassium pump couples the movement of potassium ions into the cell and sodium ions out of the cell.
Membranes are essential to cell function. Without membranes cells could not perform their specialised functions. In additions many cells would rupture due to high pressures. As a result membranes are essential to life.
> NetBiochem, 1995, Membranes [online] Available at: www.library.med.utah.edu/NetBiochem/membrane.htm [Acessed 01/10/12]
> Biological Membranes, 2006, Carnegie Mellon University [online] Available at: www.telstar.ote.cmu.edu/biology/animation/Membranes.html [Acessed 01/10/12]
> Biological Membranes [online] Available at: www.ks.uiuc.edu/services/class/BIOPHYSS490M [Acessed 02/10/12]
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