State two examples of situations where active transport occurs

When something needs to be transported against its concentration gradient, or a molecule is too big to simply diffuse through a membrane, active transport has to be used. So active transport requires energy (ATP).

Examples:

1. Antiport pumps: transport one substance in one direction while transporting another substance the other way. An example of this is the sodium-potassium pump.

2. Symport pumps - uses a substance that wants to move from an area of high concentration to low concentration and carries another substance against its concentration gradient. An example of this is the sodium-glucose transport protein

3. Endocytosis - large amounts of extra cellular fluid can be taken into a cell - needs ATP to bind proteins in the cell membrane and causes them to change their shape...which eventually causes the surrounding of the fluid to be brought in (a vesicle).

4.exocytosis - opposite of above

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.