Explanation:
All eukaryotic cells have surface cell membrane also known as plasma membrane. It is a selectively permeable membrane that helps hold the cell together and controls the substances that go in and out of the cell. It has a phospholipid bilayer with em embedded proteins. Movement of substances can either be passive ( without energy) or active (requires energy). The following processes are employed in transporting molecules in and out of the cell:
1. Passive diffusion and osmosis: Molecules like CO2, O2 and small ions can move across the plasma membrane by diffusion. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on two sides of the membrane. Such a concentration gradient across a semi-permeable membrane sets up an osmotic flow.
2. Passive transport through channel proteins: Another way is to have molecules diffuse through protein channels or permeases that fits specific molecules. This process is called facilitated diffusion. An example of protein channel is aquaporin, which allows water to freely pass through the cell membrane. Since water cannot mix with oil and the cell membrane is oily (due to lipid layer), water cannot freely pass in and out of the cell. Aquaporins allow water molecules flow into cells on a single file.
3. Active transport: letting molecules diffuse down their gradient does not require energy, but pumping these molecules in other directions against gradient requires energy. Active transport involves pumping molecules against a concentration gradient requiring energy in form of Adenosine triphosphate (ATP).
4. Endocytosis and Exocytosis: Large molecules which cannot be pumped or controlled by one channel can be moved in and out of the cell through fusion of membrane pouches.
Endocytosis is when the cell membrane forms an inward deformation i.e. forms an invagination in order to engulf a molecule outside the cell. The deformation pinches off from the membrane on the inside of the cell creating a vesicle containing the captured substance. This process requires energy, it is therefore an active process.
Exocytosis: To remove undigested residues brought in by endocytosis, the secretory vesicle is moved by cytoskeleton from the interior of the cell to the surface coming in contact with the plasma membrane. The collision with the plasma membrane causes the vesicle to break and release its contents outside of the cell.
