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It is important that the cell is supplied with all the substances it needs (e.g. oxygen) and that waste substances (e.g. carbon dioxide), or substances for export, leave the cell. There are various processes by which this can happen...
This is the process that is used in oxygen entering a cell, and carbon dioxide leaving.
These molecules will move from where they are at a high concentration to where they are at a lower concentration. i.e. they diffuse down a concentration gradient.
The blood system in humans continually brings more oxygen to the cell and takes carbon dioxide away. This maintains a high concentration gradient.
Since the movement is always down the concentration gradient, it requires no energy. The small molecules pass from one side of the membrane to the other by moving between the lipid molecules.
Fick's law is used to measure the rate of diffusion. It states that:
(The symbol α means 'proportional to')
The larger the area and difference in concentration and the thinner the surface, the quicker the rate.
So, for example, in the lung the surface area is made very large by the presence of many alveoli. The difference in concentration is maintained by breathing, which brings in air with a high oxygen concentration and removes the air with a high carbon dioxide concentration and by a good blood supply. The capillaries surrounding the alveoli take away the oxygenated blood and replace it with blood with a high carbon dioxide concentration. The walls of the alveoli are only one cell thick, so the surface across which diffusion occurs is thin and the rate is high.
In plants, a good example would be root hair cells. They have a very large surface area due to the drawing out of the cytoplasm to produce a very fine root hair. Water continues to enter the root by osmosis because there is a high concentration of mineral salts in the cells and the water is moved up the plant by the xylem. Water only has to penetrate one cell in order to enter the plant and so again the rate of diffusion is high.
This is a special case of diffusion in which we are concerned only with the movement of water.
If two solutions are separated by a semi-permeable membrane, which only allows certain sized molecules through (as in a plasma membrane), there will be a net (overall) movement of the water molecules, from the less concentrated solution (the one with more water molecules), to the solution which is more concentrated (has more solute molecules). This is because as in ordinary diffusion the molecules move to even-out any difference in concentration.
However, because of the semi-permeable membrane, which does not allow the larger solute molecules to cross, only the water molecules can move. The water molecules will continue to cross the semi-permeable membrane until an equilibrium is reached, where the two solutions are of equal concentration.
This is a measure of the tendency of water molecules to move from one place to another. The symbol used for water potential is the Greek letter psi, Ψ
Water always moves from a region of higher water potential to one of lower water potential, or down the concentration gradient. So we can define osmosis as the movement of water molecules from a region of higher water potential to a region of lower water potential through a semi-permeable membrane.
The water potential of a cell is dependent upon the combination of its solute and pressure potentials. The water potential of pure water is zero and since adding solutes lowers water potential, they make the water potential less than zero, i.e. negative. The more solute, the more negative the water potential becomes. The amount that the solute molecules lower the water potential is called the solute potential. It always has a negative value and is given the symbol, Ψs
Pressure also has a role to play in determining water potential. The greater the pressure inside a cell, the greater the tendency will be for water to leave it. This contribution to water potential is called the pressure potential. It always has a positive value because it increase water potential and is given the symbol Ψp
If the water potential surrounding an animal cell is higher than that of the cell, it will gain water, swell and burst. If the surrounding solution's water potential is lower than that of the cell, it will lose water and shrivel up. This is why it is so important to maintain a constant water potential inside the bodies of animals.
In animal cells:
Water potential = Solute potential
Ψ = Ψs
Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid. When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall. The cell wall gives very little and so pressure starts to build up inside the cell. This has the tendency to stop more water entering the cell and also stops the cell from bursting. When a plant cell is fully inflated with water, it is called turgid.
So in plant cells the equation used to calculate the water potential of a cell is therefore:
Water potential = Solute potential + Pressure potential
Ψ = Ψs + Ψp
If a plant cell is placed in a solution with a lower water potential, it will loose water. The living part of the cell or protoplast will shrink and pull away from the cell wall. At this point the pressure potential is zero and so the water potential of the cell is equal to its solute potential. This process is called plasmolysis and the cell is said to be plasmolysed. The point at which the protoplast is just about to pull away from the cell wall is called incipient plasmolysis.
If charged particles or large molecules are to move across the membrane, another process needs to be found, as they are less soluble (or even insoluble) in lipid. They move through protein-lined pores.
These line a water-filled pore in the membrane so water-soluble molecules can easily pass through.
Different channels allow different substances to pass through (the channels are selective). Some channels are gated (they will only open when appropriately stimulated).
In this case, the substance actually combines with a protein and is carried from one side of the membrane to the other. (The exact details of this process remain unclear.) These proteins are specific for a particular substance.
In both these cases, substances are moving down the concentration gradient so no energy is required.
Sometimes substances need to be moved from where they are at a lower concentration to where they are at a higher concentration - against the concentration gradient. This allows cells to take up essential molecules even when they are at a low concentration outside.
Because molecules are moved against the concentration gradient, it requires energy.
It is thought that active transport uses carrier proteins similar to those involved in facilitated diffusion.
If very large molecules or groups of molecules need to enter or exit a cell, they do so using vesicles.
The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the cell surface membrane and the contents leave. This is called exocytosis (see Golgi apparatus earlier).
Materials entering the cell can do so when the plasma membrane invaginates to surround the material. The membrane seals off to form a vesicle, which can then move into the cell. This is endocytosis.
If the material is fluid, minute vesicles are formed. This type of endocytosis is called pinocytosis.
If the material is relatively large, and is digested by enzymes after fusion of the vesicle with a lysosome, it is called phagocytosis. This occurs in white blood cells that ingest bacteria and other foreign bodies.