- Osmosis
- Diffusion
- Facilitated diffusion
- Active transport
Osmosis is the net movement of water molecules from a region of high concentration to a region of low concentration through a partially permeable membrane, which separates the two concentrations. The solvent (e.g. water) will pass from the less concentrated solution to the one of higher until the two concentrations are equal. Once the water molecules are evenly distributed equilibrium occurs. Now the smaller solvent molecules have moved across the membrane they get impeded by the larger solute molecules (e.g. glucose) that are not able to cross the membrane. Because there are no glucose molecules on the right of the membrane, water molecules move easily to the left in the reverse direction. Osmosis is a passive process, this means that movement happens across a cell membrane but no energy is needed to make it happen, this is why it is a passive process. Osmosis is vital in the transport of fluids in living organisms e.g. the transport of water from the roots up the stem of plants.
Diffusion is the process in that molecules and ions move across a membrane if they are soluble in lipids. These solute molecules move randomly in water and some hit the phospholipid bilayer and are able to pass through it. But diffusions main principle is that it is the movement of a substance from a high concentration to a region of low concentration of the same substance. An example of the body using diffusion is in the lungs as the oxygen diffuses into the alveoli. And diffusion is also a passive process, as the use of energy is not needed.
The rate of diffusion depends on the concentration gradient, the size and nature of the different molecule and the area which the diffusion takes place.
The greater the difference in concentration gradient between the two regions that the substance is diffusing across, the greater the rate of diffusion. Small molecules diffuse faster than larger ones and fat-soluble ones diffuse more rapidly through the cell membrane than water soluble ones. The larger the surface area of which diffusion takes place means there is a greater rate of diffusion, the area of the surface is increased by the introduction of villi and microvilli.
Another type of diffusion is facilitated diffusion. The word facilitated means to help and this is how this process differs as it allows a quicker exchange of particles from a high concentration to a low concentration, it is also a reversible reaction depending on the levels of concentration on both sides. It is also a passive process as no energy is involved so material is moved along the concentration gradient.
In facilitated diffusion particles pass through the membrane but unlike diffusion they use the intrinsic protein channels to pass through, these channels for water filled connections across the bilayer, which allow water-soluble substances to move through. When a substance is not soluble in a lipid they use this form of diffusion. If the molecule or ion has the right size and shape they are able to just pass straight through a hole in the protein channel, whilst other more larger molecules and ions have to change shape to allow them to diffuse through the cell surface membrane. This happens by the diffusing molecule combining with the carrier protein causing a shift in shape allowing the molecule to pass through the channel.
Certain characteristics are needed to allow diffusion to happen:
- Temperature - The higher the temperature gives the ions and molecules more energy so they are moving faster and therefore will diffuse faster.
- Concentration gradient – A greater difference on either side determines how many collisions that are made between the molecules and the bilayer.
- Surface area of the membrane – If you increase the surface are you are increasing the chances for ions to collide.
Active transport is an example of some life cycles when the cell is already more concentrated inside than whet it is on the outside. So active transport is a different kind of transport between cells as it is not passive and requires energy to work. This metabolic energy from the cell is needed because the molecules and ions are transported against the concentration gradient as they move from a region of low concentration to a region of high.
ATP is the energy used in this process, it stands for Adenosine Triphosphate and it is produced in the mitochondria during respiration with glucose. So when respiration stops then active transport also does so cells that are involved with this process generally have a large number of mitochondria.
Active transport depends on glycoproteins in the surface membrane; these are called carrier proteins. The proteins take up molecules like glucose from outside of the cell membrane. The glucose molecules attach themselves to the carrier protein and ATP binds to the membrane on the outside of the cell. Because the glucose has joined to the protein a change of shape occurs, this allows the molecules to be able to go into the inside of the cell but there is no access to the ones that are still on the outside. The glucose molecules are then released into the inside of the cell this causes the protein to repeat to its original position so active transport can take place once again.
From these four different examples of the transportation of materials and the movement of these in and out of cell you can see how the surface membrane is structured to allow the passing of different molecules, solvents and particles. Whether its using a passive or active processes the cell is designed to cope with the different materials it need to move through the membrane to help make our body function and work properly.