There are two main ways in which substances can be absorbed in bulk across a plasma membrane:

• Endocytosis: this is where the plasma membrane forms a vesicle around the substances. Once the load has been released the membrane seals back up again. Pinocytosis involves fluids being brought into the cell while phagocytosis involves solids.
• Exocytosis: this is when a vesicle is formed within the cell in the cytoplasm, sometimes from the Golgi apparatus. The vesicle then fuses with the plasma membrane and the contents are released outside of the cell. This is how insulin is secreted from cells.

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Inhibitors

Inhibitors slow down the rate of a reaction by interfering with an enzyme. The effect can be either temporary or permanent.

Competitive inhibitors have a similar shape to the substrate and so prevent the ES complex from forming by fitting into the active site of the enzyme. No reaction takes place because even though they fit they still don’t have exactly the same shape as the substrate. The rate of reaction then decreases because less substrate can bind with the enzymes.

Usually this kind of inhibition is temporary as the inhibitor leaves the active site at some point. Therefore, the rate of reaction is dependent on the ratio of substrate to competitive inhibitor as they are both competing for the active site.

Non-competitive inhibitors prevent the EP complex from being formed and, therefore, the product. They bind onto part of the enzyme other than the active site which is known as an allosteric site. Once bound they change the tertiary structure of the enzyme thereby distorting the shape of the active site which means that the substrate can no longer fit. As this kind of inhibitor isn’t competing for the same site as the substrate it isn’t affected by the concentration of substrate available.

Most non-competitive inhibitors bond permanently to their allosteric site and so denature the enzyme. However, there is a number of whose effects are reversible and, in fact, are found in organisms for specific metabolic functions.