Water potential and diffusion


Diffusion is the net movement of a substance from a region where it is in a higher concentration to a region where it is in lower concentration. This continues until the molecules are equally distributed. It’s results from the random movement of molecules and doesn’t require additional energy. Small lipid soluble molecules can diffuse acroos the plasma membrane through the phospholipid  bilayer.


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Facilitated diffusion

Molecule that are soluble in water or charge molecules such as ions cant diffuse through the phospholipid bilayer. These molecule have to diffuse through the plasma membrane with help from proteins. This kind of diffusion is called facilitated diffusion. Some of these are protein channels that are permanently open. The protein channel is lined with hydrophilic amino acids and water.

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facilitated diffusion

Molecules can also difuse through the membrane by binding to a carrier protein.the molecule binds to the carrier protein. This causes the protein to change shape and release the molecule on the other side of the membrane . no additional energy is used.

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facilitated diffusion

facilitated diffusion

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Some solutes cant pass freely through call membranes and this allows osmosis. Osmosis is a really special kind of diffusion. To explain osmosis the words water potential are used. Water potential is the tendency for a solution to gain or lose water. Pure water has the highest possible water potential of zero. Adding solutes to water decrease the water potential ie. Makes the water potential more negative. Water molecules move by osmosis from a region of higher water potential to a region of lower water potential across a selectively permeable membrane . this occurs until the water potential is the same on both sides of the membrane.

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Water can cross a membrane in two ways. All membranes are permeable to water to a small degree. Although it is a polar molecule, it is very small, so water can slip between the phospholipids bilayer. However some membranes can be up to 100 times more permeable to water due to membrane channels. A solution with the same water potential as a cell is said to be isotonic with the cell.

A solution with a lower water potential than the cell is said to be hypertonic

A solution with a higher water potential than the cell is said to by hypotonic  

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Keeping the osmotic balance

Glucose and other solutes will dissolve in blood plasma and lower the water potential but it is mainly the concentration of electrolytes in plasma and in cells that is responsible for maintaining water potential balance. Electrolytes are ions with positive or negative charge. Positively charged ions are called carions and those with a negative charge are called anions.

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keeping the osmotic balance

A test for electrolytes includes the measurement of sodium, potassium, chloride and bicarbonate ions but other plasma ions can also be tested for in the plasma such as calcium, magnesium and phosphate. Levels of electrolytes in the body fluids such as plasma and urine are kept within a narrow range. Monitoring these levels is necessary for the diagnosis of many conditions such as diabetes and kidney disease.

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