Membrane Transport
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There are two types of transport in the membrane:
- Active Transport: it is the movements of substances across the membranes using the energy from ATP. it moves the concentrations from a region of lower to a region of higher concentration. For this to happen, protein pumps are utilized and they only transport particular substances so there is a control of what is being absorbed and expelled, also, they work in one direction; the substance only goes in through one side and goes out through another. This process allows the cell to maintain interior concentrations of molecules that are different from exterior concentrations.
- Since active transport requires work and energy to be performed, the processes of endocytosis and exocytosis, fall in this category of membrane transport:
- Endocytosis occurs when a portion of the plasma membrane pinches off to enclose the macromolecules. This causes a change in the shape of the membrane; a vesicle is formed and then enters the cytoplasm of the cells. Afterwards the membrane goes back to its original shape due to its fluid nature.
- Exocytosis is the reverse of endocytosis. It starts in the ribosomes of the rough endoplasmic reticulum and then goes through four steps:
1. The protein of the ribosomes enters the lumen of the ER
2. Protein then exits the ER and enters the cis side (receiving side) of the Golgi apparatus and a vesicle is involved
3. In the Golgi apparatus, the protein is modified and exits through the trans side (discharging side) inside the vesicle
4. The vesicle with the modified protein moves to and fuses with the plasma membrane and is afterwards secreted.
- Exocytosis is the reverse of endocytosis. It starts in the ribosomes of the rough endoplasmic reticulum and then goes through four steps:
1. The protein of the ribosomes enters the lumen of the ER
2. Protein then exits the ER and enters the cis side (receiving side) of the Golgi apparatus and a vesicle is involved
3. In the Golgi apparatus, the protein is modified and exits through the trans side (discharging side) inside the vesicle
4. The vesicle with the modified protein moves to and fuses with the plasma membrane and is afterwards secreted.
- Passive Transport: it occurs when there are areas or different concentration of a particular substance. Substances move from an area of higher concentration to an area of lower concentration and it does not require energy in the form of ATP. There are three types of passive transport:
- Facilitated diffusion: Since the center of the membrane is hydrophobic, ions with positive or negative charges can not easily pass through, therefore, in order for ions and other particles to diffuse there has to be channels for them through the plasma membrane. These channels are narrow and consist of protein. The size ensures that only one type of particle passes through instead of two or more. Because these channels help particles go from and area of higher concentration to an area of lower concentration, this process is called facilitated diffusion.
- Osmosis: net movement of more water molecules moving in one direction across a partially permeable membrane (not everything passes through). Osmosis is different from diffusion because water is the solvent, which is the liquid in which particles dissolve, and the dissolved particles are called the solutes; therefore, the direction in which the water moves is due to the concentration of solutes, instead of the concentration of water molecules. It is because of the attraction between solutes particles and water molecules that water moves to regions of higher solute concentration.
Applications and skills:
- Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
- Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
- Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.