Membranes are found either on the outside of a cell or within it. The cell surface membrane surrounds the whole cell, separating the cells interior from its surroundings. It acts as a partially permeable barrier allowing very few molecules across it and fencing the majority of the chemicals inside. The membrane was first thought to be just a simple barrier, but with further research it turns out that the cell membrane is very complex and important in a wide range of cells activities and functions. Under an electronic microscope two separate layers can be seen known as the bilipid layer. This is made up of lipid and protein molecules, which form a patchwork of molecules which are free to change position, known as the Fluid Mosaic Model, which was first proposed in 1971 by S. J. Singer.
Each represents a phospholipid. The circle or head is the negatively charged phosphate group, which attracts to both the cytoplasm inside the cell and the extracellular fluid on the outside of the cell. The two tails are the two highly hydrophobic fatty acid chains of the phospholipid that does not mix with the fluid on either side of the membrane. The proteins and glycoproteins have various functions; they act as enzymes in the cells of the small intestine which is important in digestion. They also act as pores, channels and carrier proteins to aid the movement of molecules in and out of the cell. They also act as receptors for chemicals such as hormones and they can stick cells together. The cholesterol attaches to the glycoprotein and glycolipid which closes the packing of the phospholipid tails and helps regulate the fluidity of the membrane.
There are several processes that occur to aid the movement of substances across the cell membrane. Passive transport occurs in living and non-living systems and occurs with the use of natural kinetic energy of molecules or ions. There are three types of passive transport, diffusion, facilitated diffusion and a special kind of selective diffusion called osmosis.
Diffusion is the net movement of ions or molecules from a region of their higher concentration to a region of their lower concentration. This is a process in which molecules randomly move about, from an area where there is more of that molecule to where there is fewer. It is a fairly slow process as there is no energy to push the molecules through the membrane, and it only works if the molecules are small enough to actually get through the tiny pores of the membranes surface. The molecules move about freely, eventually hitting one of the tiny gaps in the membrane, any molecules that don’t find the gap, just bounce back off until they do, so it is a very random process. A man named Adolf Eugene Flick did a lot of experiments, which revealed important information about diffusion. He’s ‘law’ showed that the rate of diffusion doesn’t just happen on the difference of concentration, but also on the surface of the membrane and the thickness of the membrane. He said if the membrane has a larger surface, the molecules will find the gap much quicker and if the membrane was thicker it would take much longer for the molecules to pass through. How the molecules pass through the membrane also depends on the solubility of the molecule, weather they are fat soluble or water soluble. The proteins that form channels help water soluble molecules pass through the hydrophobic lipid interior of the membrane.
Facilitated diffusion is the diffusion of molecules across the cell membrane, assisted by other molecules such as protein carriers or pores. Certain molecules such as glucose sugar, that are needed to provide energy to the cells, need help to get through the membrane. Protein channels help bring the molecules down the concentration gradient from areas of high concentrations to low. A good example of diffusion is sodium ions. The cells in the nervous system are in an environment with a higher concentration of sodium ions than there is inside the cell. The sodium ions diffuse through the protein channel and into the cell. Another example of diffusion is that of potassium ions. There is often a higher concentration of potassium inside the cell than outside, therefore the ions move through the protein channel the other way to the outside of the cell.
Osmosis is the movement of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane. Water molecules pass freely through the membrane on both inside and outside the cell, until equilibrium is reaches and the water potential on either side is equal to each other. However they do not stop moving when equilibrium is reached, they keep moving to maintain equilibrium. Osmosis occurs in the secretion of and absorption of water molecules in the small intestine, where water diffuses through the gut wall and other substances are carried by active transport. As the human body is made up of 60 percent water, osmosis occurs in the kidneys and colon to reabsorb water molecules from waste products to be used again in the body.
Another process that occurs to aid the movement of molecules is Active Processes. These processes require energy to be expended by the cell and only work in living systems. Active transport is the movement of molecules or ions from a region of low concentration to a region of high concentration. This is the opposite of the passive transport as the molecules or ions in this process move against the concentration gradient and not with it. An example of active transport is the sodium - potassium ‘pump’ in the nerve cells. This happens by pumping sodium out of the cell and potassium into the cell, using energy in the form of ATP. It does this to create a chemical potential consisting of two concentration gradients as well as electrical potential, as three positive charges are pumped out while two negative charges are pumped in, creating a negative charge inside the cell. Other examples of active transport are the absorption of sodium molecules in the kidney tublets, and the absorption of glucose from the small intestine.
Another active process is Exocytosis. This is a process in which useful materials are secreted out of the cell from a vesicle. The vesicle, often produced by the ER or the Golgi body, fuses with the cell membrane and release its contents out. This process is used in the small intestine by discharging tiny droplets of fat into the lacteals. It also happens in the pancreas, by secreting pancreatic enzymes into the lumen where their meet and leads eventually to the pancreatic duct draining into the small intestine.
Another form is Endocytosis, which is the opposite of Exocytosis. The membrane surrounds the particles and takes it in, sealing it off to form a vesicle, trapping the material inside. The useful material is absorbed from the vesicle into the cell and the vesicle, again, fuses with the cell membrane secreting the remaining products. This is known as Phagocytotis, meaning ‘cell eating’. Another form of Endocytosis is Pinocytosis, meaning ‘cell drinking’. It is similar to Phagocytosis, but takes in droplets of moisture in liquid form, instead of molecular particles. This occurs continuously in almost all cells in the human body, allowing cells to pick up all the critical components that the cell needs.
Various substances can pass through the cell membrane, dependent on their solubility, size and contents, different processes will occur to help the substances get across, and enable cells to function properly and keep all organisms alive on Earth.
References
Pickering, W.R. As & A Level Human Biology through Diagrams, 2002, New Edition, Oxford University Press: Oxford
Websites
cellbio.utmb.edu/cellbio/membrane.html
users.rcn.com/jkimball.ma.ultranet/ BiologyPages/C/CellMembranes.html
ccgb.umn.edu/~mwd/cell_www/chapter2/membrane.html
projects.edtech.sandi.net/miramesa/Organelles/memb.html
hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html
academic.brooklyn.cuny.edu/ biology/bio4fv/page/pm_mos.htm
Other Sources
Various Classroom notes and Handouts 2005 – 2006, Len Taylor
