Structure and function of plasma membrane in cells Essay
Structure and function of plasma membrane in cells
‘The structure and importance of the plasma membrane found within and around all cells’ The plasma membrane surrounds all eukaryotic and prokaryotic cells. Eukaryotic cells have membrane bounded organelles whereas prokaryotic cells do not. The plasma membrane forms the boundary between the cell cytoplasm and the environment. Its function are to allow different environments to be established inside and outside the cell. It also controls the movement of substances into and out of the cell.
The cell surface membrane which surrounds all cells consists of many components, which together is referred to the fluid mosaic model. It is called fluid because the individual phospholipids can move relative to one another, making the membrane flexible so it can constantly change shapes. It is called mosaic as the different types of proteins are embed in the membrane in different shapes, sizes and patterns, so it resembles a mosaic. Phospholipids are important components to the structure of the plasma membrane.
It forms a bilayer sheet, one layer of the phospholipids has its hydrophilic head (the phosphate which is attracted to water) pointing inwards so that it interacts with the water in the cell cytoplasm and the other layer of phospholipids has its hydrophilic head pointing outwards to interact with the water surrounding all cells. The hydrophobic tail (the fatty acid end of the phospholipid which orients itself away from water and towards fat) of both the phospholipid layers points into the centre of the membrane, protected from the water.
The phospholipid bilayer has important roles in the plasma membrane, such as allowing lipid-soluble substances to enter and leave the cell via diffusion and to stop water soluble substances from entering and leaving the cell. This is also what makes the membrane flexible. Another component of the plasma membrane are proteins. There are two types; extrinsic and intrinsic. Extrinsic proteins occur on the surface of the bilayer or only partly embedded in it. These proteins can work in conjunction with glycolipids, acting as a recognition site. Glycolipids are an essential part of the cell membrane.
They help determine our blood groups. The glycolipids act as receptors at the surface of the red blood cell, this is important as we can use this to classify our blood type which is critical during blood transfusions, as if we give the incorrect blood type the recipient’s immune system may detect these differences and treat the donated blood as not theirs and so the patient will die. Glycolipids can also act as cell receptors for molecules such as hormones during embryonic development. The importance of the plasma membrane in keeping certain substances inside the cell is shown when someone has cholera.
The bacteria, vibrio cholerae, starts to produce a toxic protein which has two parts. One of the parts binds to a specific carbohydrate receptor (a glycolipid) on the cell-surface membrane. Only the epithelial cells of the small intestines have the specific receptor which binds to the toxic, hence why it only effects this part of the body. The other toxic part enters the epithelial cells, which causes the ion channel of the cell-surface membrane to open, so the chorine ions which normal are contained within the epithelium cells floods into the lumen of the small intestines.
This has serious effects on the body as it results in water loss from the blood and tissues which causes symptoms such as diarrhoea and dehydration. Intrinsic proteins completely span the phospholipid bilayer from one side to the other. Some of these proteins are enzymes e. g permeases. Permeases are membrane transport proteins which are class of mulitpass transmembrane proteins that makes the diffusion of a specific molecule in or out of a cell by passive transport easier. There are four types of passive transport; diffusion, facilitated diffusion, filtration and osmosis.
Active transport is not passive as it requires energy to move substances against the concentration gradient. Diffusion, osmosis and active transport of substances in and out of the membrane is very important for all types of cells. One example is the root hair cell. These cells are the exchange surface in plants which are responsible for the absorption of water and mineral ions so without osmosis and active transport this would not be possible. The water is taken up by osmosis through the partially permeable membrane.
The root hair cells are surrounded by a soil solution which contains small quantities of mineral ions but mainly water, so has a high water potential (slightly less than zero). The root hair cells themselves contain a high quantity of amino acids, mineral ions and sugars inside them (low water potential). Therefore water will move by osmosis from the soil solution and into the root hair cells, going down the water potential gradient. Cells need to let water-soluble ions and molecules, like glucose and amino acids into them from the environment.
However these molecules diffuse through the phospholipid bilayer of the plasma membrane very slowly, so they use another form of passive transport to move these types of molecules into and out of the cell; facilitated diffusion. Facilitated diffusion occurs at specific points on the plasma membrane where there are special types of proteins molecules, these molecules form water filled channels across the membrane to allow water soluble substances through. This pathway also allows large molecules into the cell which are too big to diffuse through the bilayer.
The intrinsic proteins are very important in the axon plasma membrane. The phospholipid bilayer of the axon membrane prevents sodium and potassium ions diffusing across it so they diffuse across the membrane using intrinsic proteins instead. These proteins contain ion channels which pass through them. Some channels have gates which can open or close to allow sodium or potassium ions to move through them at specific times where as some are open all the time. Sodium and potassium have different gated channels. Some intrinsic proteins actively transport potassium ions into the axon membrane and sodium ions out of the axon.
This is called the sodium-potassium pump and is essential process for the creation of the nerve impulse. The epithelial cells lining the villi possess microvilli, which are finger like projections of the cell surface membrane. Microvilli are 0. 6 micrometres in length. This increases the surface area of the membrane to allow maximum absorption of the products of digestion like glucose. Through these points I have shown that the there are many different features of the plasma membrane which are all important in helping the plasma membrane with the functions in all the different cells of the body and plants.