Osmosis is the flow of water through a semi permeable membrane that blocks the transport of salts or other solutes through it. It is also defined as the net movement of water from an area of high concentration to a region of low concentration. Osmosis is a fundamental effect in all biological systems. When two water volumes are separated by a semi permeable membrane water will flow from the side of low solute concentration to the side of high solute concentration. If external pressure is applied on the area of high concentration then reverse osmosis may occur. The movement of solute molecules within a solvent is determined by the solvent molecules that surround it.
Collisions of solute molecules determine movement. For example, in a u-tube, the two vessels are separated by a semi permeable membrane. The system will stabilize when the osmotic pressure is balanced. The solute molecules generate pressure on all solution boundaries, including the membrane. Osmosis is significant is plant and animal cells. In a plant cell, the water moves to the vacuole. In a hyperosmotic solution, that is, there is a lower concentration of water, if water permeates out of an animal cell, the cell becomes crenated or shriveled. In an isosmotic solution (equal concentrations) water enters and leaves the cell sufficiently, allowing the call to function properly. In a hypoosmotic solution (higher water concentration) the cell becomes lysed as too much water enters the call. When a plant cell is in a hyperosmotic solution, water leaves the cell and it becomes plasmolyzed. In an isosmotic solution, the flow of water in and out of the plant call is flaccid. In a hypoosmotic solution, the large amount of water that enters the cell causes to become turgid. Osmosis is a significant inn the functioning of cells.
Another essential type of cellular transport is facilitated diffusion. Defined as the net movement of molecules, ions and atoms from a region of high concentration to low. All particles are constantly in motion, henceforth diffusion involves the movement of particles. Diffusion results from the random motion and/or collisions of particles. It is the act measured of particles as a result of a concentration gradient, where concentration is the number of particles per unit volume and concentration gradiet is a difference in concentration of a substance across a space. Diffusion will continue until the particles become uniformly distributed in the medium in which they are dissolved. The diffusion rate increases as temperature increases therefore an increased temperature increases the motion and particle collisions. Diffusion rate also increases as the concentration gradient increases, and an increased concentration causes an increase in particle collisions. Diffusion is responsible for a drop of blue dye becoming evenly distributed in a glass of water. This is because there is movement of particles between regions of different concentrations. Some small, hydrophilic organic molecules, like sugars, can pass through call membranes by facilitated diffusion. For example, the outer membrane of E. coli form pores which allow disaccharides and other related molecules to diffuse into the cell. The plasma membrane of human red blood cells contain transmembrane proteins that permit the diffusion of glucose from the blood into the cell. Facilitated diffusion occurs through selective channels.
Active transport requires the cell to spend energy, it is the movement of molecules or ions from a region that is in low concentration to a region of high concentration or movement against its concentration gradient. An example is the thyroid gland. The iodine found in our blood is used it make thyroxin. The iodine must be actively transported from the blood to the thyroid against the concentration gradient. The thyroid makes thyroxin which regulates metabolism. Another example is the sodium potassium pump. The binding of cytoplasmic sodium to the protein stimulates phosphorylation by ATP. Then the phosphorylation causes the protein to change its conformation. This conformation change expels sodium to the outside and the extracellular potassium binds. The potassium binding triggers the release eof the phosphate group. The loss of the phosphate restores the original conformation. The potassium is released and the sodium sites are receptive again, and then the cycle repeats. Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. This requires a protein and a source of energy, generally ATP. Some transporters bind ATP directly and use the energy of its hydrolysis to drive active transport. Even large molecules can be channeled through the membranes. Active transport can occur only in closed membranes. These membranes may consist of different compartments such as a vacuole etc. the transport direction is determined by coupling the transport with the gradient.
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