The bilayer is described as fluid since the lipids within it are able to slide around and exchange places with each other freely. This feature allows processes such as phagocytosis to occur.
There are five main ways in which transport across cells can occur:
Simple Diffusion;
The only substances that can diffuse directly through the membrane are either those that are lipid soluble or those that are very small. No energy is required for this kind of diffusion and the movement of these substances can only be down their concentration gradient. The speed of diffusion, according to Fick’s Law, could be increased if there was a larger concentration gradient, a thinner surface to diffuse across and a larger surface area.
Osmosis;
This is the diffusion of only water across membranes. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.
Within the cell are solutions of a number of different solutes. The more solutes there are, the less water molecules there are. This way, the concentration of water can also be described as water potential.
Solute particles have a tendency to attract water molecules to themselves, reducing the potential of the water molecules to move. Therefore, the more solutes there are, the less potential the water molecules have of moving. Water potential then is simply a measure of the tendency of water molecules to move (measured in kPa).
Pure water has 0 kPa, so as the concentration of solutes increases, water potential will get increasingly negative. Therefore, water will always ‘move’ from a low to high water potential, where 0 kPa is the highest possible water potential.
Different water potentials have different effects on animal and plant cells. Since an animal cell does not possess a cell wall, it is then effectively more vulnerable to the effects osmosis. Animal cells in a hypertonic solution (higher w.p outside cell) will have water moving out of it, leaving it shriveled and crenated. Animal cells in a hypotonic solution (lower w.p. outside cell) will have water moving inwards until it eventually burst. Plant cells however, will not burst in a hypotonic solution due to the protection of its cell wall. It will instead swell and become turgid. In a hypertonic solution, the cytoplasm becomes playsmolysed.
In an isotonic solution (equal w.p outside cell), all cells will stay the cell due to no net movement of water through membranes.
Facilitated Diffusion
This is diffusion that can only occur with the aid of trans-membrane proteins. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. No energy is needed and again substances can only move down their concentration gradient. There are two types of transport proteins:
Channel proteins – have a fixed shape and form a pore or channel through the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.
Carrier proteins – have a flexible shape and have a binding site for a specific solute. The site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration, the protein will mould around the substance and transport them to where they are at a low concentration.
The higher the concentration of ion/solute molecules, the greater its chance of binding.
Active Transport
Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape and moulds itself around the molecule, and releases it on the other side. There is a different protein pump for each molecule to be transported since they are highly specific globular proteins. The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP, producing ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.
Endocytosis and exocytosis via vesicles.
These are the only processes where larger molecules can be moved into and out of a cell.
Endocytosis is the transport of substances into the cell. As the molecules gets near the cell, the membrane forms a slight dip or pit. When they are close enough, they are enclosed by a fold of the membrane, which then pinches shut to form a closed vesicle. As the vesicle moves into the cell, the molecules are digested and the product molecules released. When the materials are small and often liquid particles, endocytosis is known as pinocytosis. When the materials are large and often solid particles, the process is known as phagocytosis.
Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. The molecules to be transported have to be enclosed in a membrane vesicle first, usually from the Golgi apparatus and RER. It then moves to the membrane and fuses with it, forcing the substances out.
These processes are controlled and require energy.
