Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. Gas exchange in gills and lungs operates by this process. Carbon dioxide is produced by all cells as a result of cellular metabolic processes. Since the source is inside the cell, the concentration gradient is constantly being replenished/re-elevated, thus the net flow of CO2 is out of the cell.

This Flow of material is done in diffusion through the material molecules moving around, which they are moving around they will eventually start to bang into each other will shall send some of these molecules across the cells membrane this continues to happen until the concentration which was one the outside of the cell membrane is the same as the concentration of molecules on the instead of the cell membrane the reason why this would stop then is due to the fact that when there is an even amount of them on either sides there are not enough molecules to bang into each other or that when a molecule does get his there is not enough repeated blows to give it the momentum to send over the cell membrane.

There is another type of diffusion this is call Osmosis and is when a net movement of water molecules occurs by diffusion across a cell membrane, the cell membrane in this case has to be selectively permeable as it will allow the passage of water into or out of the cell depending on the solute concentrations on the two sides of the membrane, so if the concentration on one side of the cell is high solute concentration but the other side has a low solute concentration, the membrane will allow the water to flow between both sides of the Equilibrium is reached on both sides of the membrane This equilibrium of concentration is reached after sufficient water has moved to equalize the solute concentration on both sides of the membrane, and at that point, net flow of water ceases.

These two types of passive cell movements are very similar thy both are used to move things form an area of high concentration to an area of low concentration until both sides reach equilibrium. But in the case of osmosis which deals only with transport of water there is no molecules being “thrown over the cell wall” instead the to concentrations merge until they are both equal in concentration .

The above examples are al examples of Passive transport which is the transport materials across cells which does not require any energy expenditure to transport port the materials, the following transport systems endo/exocytosis and the process of pinocytosis and phagocytosis which are two different way in which endocytosis will take place. All require the use of energy or ATP in order to move the materials into or out of the cell.







The first active transport to be addressed is Endocytosis. This is the process in which a cell will engulf material from out side the cell membrane and enclose it is a sac in which the material shall be digested. It does this enclosing by the plasma membrane enclosing around the material which needs to be taken into the cell, then the cell membrane closes together stopping the material from escaping and moves the material into the cytosol. After the material has reached the cytosol a vesicle will form and move away from the cell membrane in order to be absorbed into the cell.

The two other things which happen during Endocytosis is either Phagocytosis or pinocytosis. Phagocytosis is simply what happens when a solid piece of material in brought into the cell the vesicle in which the solid piece of material is in is called a phagosome or food vacuole. Further more Phagocytosis only happens in a few cells with in the body especially those which are used to destroy bacteria like white blood cells which use Phagocytosis to destroy bacteria and other foreign bodies. Also in organisms like amoeba Phagocytosis is used to feed the amoeba as the amoeba will use it to capture a small organism and they digest it.


The other thing is Pinocytosis which means s cell drinking it involves the take taking in of smaller particles into a cell while Phagocytosis is used to take in large particles into the cell. The particles which have been taken into the cell are contained in small vesicles which will then fuse with lysomes in order to break down the particles contain within the vesicles (This also happens in phagocytosis). Pinocyotosis unlike phagocytosis is used primarily for the in take of extra cellular fluids; it also differs in the fact that it creates smaller vesicles.

Exocytosis is the transport of materials out side of the cell it is the opposite of endocytosis it does this by a vesicles in the cytosol fusing with the cell membrane. When this happens the vesicle will start to fuse with the cell membrane which lead’s to it opening up thus expelling the materials out of the cell. The materials which are usually expelled from the cell are contents from the Golgi body or waste products from the digestion of some matter which has been taken into the cell.

The similarities between all 4 of these transport system is that they are all their to provide net transport of materials around the body, in order to supply it with materials or move materials from the cell. Where they differ is in the way they do this the two passive transport processes differ between each other as while they are both use in order to create a equilibrium in the concentration gradient they use different ways or that they provide equilibrium for different things as diffusion is done by molecules using each other to go across the cell membrane while osmosis is dealing with the movement of water across membranes. These in turn differ between Endocytosis and Exocytosis due to the fact that both endo and exo use energy to transport material into or out of the cells also the two cyotosis are not their to create an equilibrium instead they are being used to go against the concentration gradient thus they have to use energy to force through concentration gradient.