Active Transport

It is used to import and export substances into and out of the cell. Active transport is:

• movement of substances


• usually against a concentration gradient


• across a cell membrane
  
• uses energy (usually from ATP provided by respiration in the mitochondria)

Different carrier proteins carry specific molecules or ions through the cell membrane. There are three types, since they move:

1. Uniport carriers: a single substance in a single direction.
2. Symport carriers: two substances in the same direction.
3. Antiport carriers: two substances in opposite directions.

The precise mechanism of active transport is unclear. There are two different hypothesis:

Hypothesis 1: Cotransport

The plumping of one substance indirectly drives the transport of one or more other substances against a concentration gradient.

Hypothesis 2

Protein molecules change shape to transport solutes form one side of the membrane to the other. To do this, ATP is hydrolysed to ADP. The phosphate group attaches directly onto the protein, causing it to change shape.

Cytosis

Active transport which involves infolding and outfolding of sections of the cell membrane.

• Exocytosis (transport out of a cell). Vesicles and vacuoles move to the cell surface membrane, fuse with it and release their cargo to the outside world.


• Endocytosis (transport into a cell). There are three types:

*Phagocytosis (cellular eating). Solid substances brought inside the cell by invagination. A vacuole is formed.

*Pinocytosis (cellular drinking). Smaller infoldings.

*Receptor-mediated endocytosis: receptor molecules on the cell surface membrane bind with a specific substance from the extracellular environment. As receptor sites are filled, the surface folds inwards until a coated vesicle finally separates from the cell surface membrane. (Following Fig)

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