It is important for cells to control what enters and exits the cell. The transport of materials to and fro is controlled by the selectively permeable cell membrane. This means that it will allow spontaneous passage of some materials but others must use special processes to get across.
There are two main types of cell transport:
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It is seen that molecules constantly collide and tend to move according to concentration gradients existing. The movement of like molecules down a concentration gradient that is from a region of high concentration to a region of lower concentration is called as diffusion. Diffusion is a passive process, as it does not need energy in the form of ATP. Most molecules like are able to cross the cell membrane passively simply diffusing across it. Passive transport follows a concentration gradient.
The ability of cells to get oxygen to fuel the electron transport depends on the ability of oxygen to diffuse through the membranes into the cell. Similarly the carbon dioxide which is created as a product of cellular respiration passively diffuses out of the cell through the cell membrane. Small polar uncharged molecules can pass through easily. This process moves with the concentration gradient.
Facilitated diffusion is also a passive process of diffusion but this process uses membrane proteins called carrier proteins to help in the process. There are three types of transport proteins-uniport, symport and antiport.
Uniport proteins carry a single solute across the membrane.
Symport proteins translocate two different solutes simultaneously in the same direction.
Antiport proteins exchange teo solute by transporting one into the cell and one out of the cell.
Active transport is the net movement of dissolved particles against the concentration gradient with the help of transport proteins. This process needs energy in the form of ATP. The ATP donates a phosphate to a specific gateway molecule which then “pumps” the desired molecule across the membrane, even if it goes against a concentration gradient. Thus ATP energy is used to drive the pump.
Vesicles, small sacs made of membranes, can transport and store substances within the cytoplasm. Active transport is used to transport large molecules by a process called as:
Endocytosis is the process in which the cell membrane is invaginated forming a vesicle that contains extracellular medium. In endocytosis the particles to be transported are enclosed in small portions of the cell membrane to form vesicles. This enclosed particle is then bought into the cell by using energy in the form of ATP.
Exocytosis is used by the cells to release secreted proteins to the exterior. Expulsion of waste products takes place by exocytosis. This process is the reverse of endocytosis in which the cell membrane fuses with the particle and releases the contents outside.