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When an animal cell or a plant cell is placed in a medium, which is a water solution , the possible consequences are listed below.
When a plant cell is placed in a hypertonic solution , the water from inside the cell?s cytoplasm diffuses out and the plant cell is said to have become "flaccid". When a plant cell is placed in an isotonic solution, a phenomenon called incipient plasmolysis is said to occur. In a multi-component system, dialysis is a process by which only certain compounds including both the solvent molecules and small solute molecules are able to pass through the selectively permeable dialysis membrane but other larger components such as large colloidal molecules like proteins cannot pass through pores in the dialysis membranes .
The most important medical application of dialysis is in dialysis machines, where hemodialysis is used in the purification of blood from patients suffering from renal malfunction. Reverse osmosis is the process by which the liquid solvent moves across the semi-permeable membrane against its concentration gradient , i.e. This process is also known as hyperfiltration as it is one of the best filtration methods known .The removal of particles as small as ions from a solution is made possible using this method.
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When the plant cell is placed in a hypotonic solution , it takes up water by osmosis and starts to swell, but the cell wall prevents it from bursting. If the plant cell is then observed under the microscopic, it will be noticed that the cytoplasm has shrunk and pulled away from the cell wall .This phenomenon is called plasmolysis. In hypotonic solutions, animal cells swell up and explode as they cannot become turgid because there is no cell wall to prevent the cell from bursting. Dialysis can therefore be used for separation of proteins from small ions and molecules and hence is used for purification of proteins required for laboratory experiments. Blood from the patient is circulated through a long cellophane dialysis tube suspended in an isotonic solution called the dialysate which is an electrolyte solution containing the normal constituents of blood plasma. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.


For example, the red blood cell in the blood plasma retains its shape because of the isotonic nature of the plasma.
The process is reversed as soon as the cells are transferred into a hypotonic solution (deplasmolysis). When the cell is in danger of bursting, organelles called contractile vacuoles will pump water out of the cell to prevent this. Examples of membranes through which dialysis occurs are animal bladders, parchment and cellophane (cellulose acetate). The toxic end products of nitrogen metabolism such as urea from the blood pass through the dialysis membrane where they are removed while cells, proteins and other blood components are prevented by their size from passing through the membrane. It can also be used to purify fluids such as ethanol and glycol, which will pass through the reverse osmosis membrane, while rejecting other ions and contaminants from passing. There are a number of ways in which this may happen.Passive TransportWhen placed in a system, a substance will tend to even out - it will move from a region of high concentration to a region of low concentration. When this happens the green parts of the plant droop and are unable to hold the leaves up into the sunlight. In hypertonic solutions, water diffuses out of the cell due to osmosis and the cell shrinks.
Also, the dialysate concentration can be controlled so that salt , water and acid-base imbalances in electrolytes are corrected. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, etc. The pressure inside the cell rises until this internal pressure is equal to the pressure outside. This liquid or hydrostatic pressure called the turgor pressure prevents further net intake of water . In the human body, the kidneys provide the necessary regulatory mechanism for the blood plasma and the concentration of water and salt removed from the blood by the kidneys is controlled by a part of the brain called the hypothalamus.
In cells, a number of processes may ensure that the Concentration Gradient is maintained and the Equilibrium is not met.


Turgidity is very important to plants as it helps in the maintenance of rigidity and stability of plant tissue and as each cell exerts a turgor pressure on its neighbor adding up to plant tissue tension which allows the green parts of the plant to "stand up" into the sunlight.
The process of regulating the concentration of water and mineral salts in the blood is called osmoregulation Animals which live on dry land must conserve water and so must animals which live in the salty sea water, but animals which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis.
Water can diffuse in this way, as can Carbon Dioxide and Oxygen.Some charged molecules normally are unable to pass through Biological Membranes since they are repelled by the Hydrophobic Phosphate 'heads'. However, special Proteins, called Channel Proteins, provide pores though which these molecules can diffuse.
These Proteins are often shaped in such a way as to only allow certain molecules to pass through.Larger molecules, like Glucose, are too big to diffuse through the Phospholipid Bilayer unaided. Once a specific molecule collides with the Carrier Protein, it 'flip flops' carrying the molecule to the other side of the membrane.Active ProcessesSometimes substances are required to be moved against the Concentration Gradient, or faster than they would by Passive Transport. For example, plants need to take up Magnesium Ions in their roots against the Concentration Gradient. In these cases, Active Processes are used, which require energy.Active TransportActive Transport involves the movement of molecules across a Membrane using Proteins in the Bilayer similar to Carrier Proteins.
These use energy in the form of ATP to 'pump' molecules in one direction across a membrane.The Carrier Proteins involved in Active Transport are Complementary to the molecule they transport. Once the molecule collides with the Protein, it is transported across the membrane and released the other side. This is done via the movement of Vesicles within the cell, which can easily fuse of be 'pinched off' from a membrane.The movement of substances into a cell is called Endocytosis, and the movement of substances out of a cell is called Exocytosis. The prefixes 'phago' and 'pino' can be used to donate the movement of solid and liquid material respectively.Energy, in the form of ATP, is used in the movement of Vesicles around the cell and fusion with, and pinching off from, the membrane.



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