1.4 - C - Active Transport
Active transport is a means of membrane transport that requires the use of ATP molecules. It is used to allow the cell to transport materials against the gradient from a low concentration to high concentration. One form of active transport is carried out by a class of membrane proteins called protein pumps.
Cells have a wide range of protein pumps, which allows the cell to precisely control the contents of the cytoplasm. Typically, the molecule or ion can only reach the center of the pump if a conformational change takes place using ATP. This change in shape allows the molecule/ion to be moved against their concentration gradient.
A vesicle is small membrane sac that transports droplets of fluid inside. They are small, spherical and normally found in eukaryotic cells. They can happen because of the fluidity of membranes, which allows structures surrounded by membrane to change shape and move.
To form a vesicle, a small portion of the membrane is pulled in and pinched off. This is carried out by proteins in the membrane, which requires the use of energy from ATP. The vesicle formed contains contents from the extracellular environment (see above). The process of taking in materials this way is called endocytosis.
Vesicles taken in by endocytosis contain solutes from the environment as well as larger particles that are unable to be taken in through other methods.
Some cell types are able to ingest other organisms using endocytosis. Phagocytes do so to eliminate infections and single-celled organisms like Amoeba do so to obtain food.
The image to the right shows an ameoba cell ingesting a particle by means of endocytosis.
Exoctyosis is the reverse process of endocytosis and involves the release of material from the cell into the environment. Vesicles from the cytoplasm fuse with the cell membrane and release their contents. Expelled substances can be useful (neurotransmitters) or the waste products of digestion.
Contents of vesicles in exocytosis are synthesized in the rough endoplasmic reticulum, processed in the golgi apparatus and then carried to the cell membrane for expulsion.
Cells can also use exocytosis to form contractile vesicle, which contain water from the cytoplasm. These can be expelled to control the overall water content of the cell.
The diagram below shows how endocytosis, exoctyosis and vesicles can be used to benefit the cell. On the left, food particles are taken in via endocytosis and the resulting vesicle fuses with a lysosome that contains digestive enzymes. On the right, molecules are synthesized in the rough ER an then modified in the Golgi apparatus. The resulting product is secreted into the environment via exocytosis.