Endocytosis and exocytosis:

Endocytosis:

Molecules that need to be taken into the cell bind with receptors. This causes the cell membrane to form a cavity – a process known as invagination. The cell membrane forms a cavity around the molecules and it eventually breaks away from the membrane as a vesicle containing the substances. There are two types of endocytosis which are pinocytosis and phagocytosis.

Pinocytosis and examples:

Pinocytosis is for the movement of many small particles, often dissolved in water. It is used by the cell to take in molecules that are too large to get through the membrane in the usual way, for example proteins, glycoprotein, vitamin B12 etc. All cells are capable of pinocytosis.

Exocytosis and why it is needed:

Exocytosis is the movement of large molecules from inside the cell to the outside of the cell. Many cells produce substances that are needed elsewhere in the body, so they need to be moved out of the cell into a duct or into the blood to be carried where they are needed. Examples include extracellular enzymes used for digestion, hormones, mucus and antibodies. The substances have been made by the ribosomes and are finished off in the Golgi apparatus. Vesicles bud off from the Golgi apparatus and make their way towards the cell membrane where they will undergo exocytosis and leave the cell. The membrane of the vesicle merges with the cell membrane and the substances are released on the outside of the cell.

Mechanism and need for energy:

Energy in the form of ATP is needed to move the vesicle either to or from the cell membrane. This process uses protein molecules actin and myosin. When calcium ions are released into the cell they contract and move the vesicle much like they contract in muscle.

Surface area

1x1x6 =6

2x2x6 = 24

Volume

1x1x1 = 1

2x2x2x = 8