4.5 - Active Transport
- 1The process of active transport
- 2How different factors affect the rate of active transport
- 3The process of endocytosis
- 4The process of exocytosis
Active transport
Active transport is the movement of particles from an area of lower concentration to an area of higher concentration. This process requires energy from respiration in the form of ATP.
Energy is needed as particles move up (or against) a concentration gradient.
As a result, active transport is described as an active process.
Carrier proteins
Like facilitated diffusion, active transport involves the use of carrier proteins to transport molecules or ions across membranes. The process works in the following way:
- The molecule or ion binds to the carrier protein.
Factors affecting the rate of active transport
There are four key factors that affect the rate of active transport:
- Temperature - At higher temperatures, particles have more kinetic energy and travel faster. Respiration also increases with temperature. Very high temperatures denature carrier proteins, decreasing the rate of active transport.
- Thickness of membrane - Particles travel shorter distances through thin exchange surfaces, so travel faster.
- Number of carrier proteins - The more of these proteins, the faster the rate of active transport.
- Rate of respiration - The more respiration, the more ATP available for active transport.
Bulk transport
Large molecules such as enzymes or hormones cannot be transported through channel or carrier proteins. Instead, they are moved into and out of cells using another form of active transport, known as bulk transport.
There are two types of bulk transport:
- Endocytosis - This transports materials into cells.
- Exocytosis - This transports materials out of cells.
Endocytosis
Endocytosis is the process in which large molecules, or many molecules at once, are transported into cells.
The cell-surface membrane engulfs the materials to form a vesicle which is then moved into the cytoplasm where the materials can be processed.
Endocytosis takes two forms:
- Phagocytosis - The uptake of solid materials.
- Pinocytosis - The uptake of liquid materials.
Exocytosis
Exocytosis is the process in which large molecules, or many molecules at once, are transported out of cells.
Vesicles (mostly formed by the Golgi apparatus) move towards and fuse with the cell-surface membrane where materials are released outside the cell.