Diffusion and Osmosis
Diffusion - the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM
Concentration gradient - a difference between concentrations in a space.
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Watch this animation of water molecules moving across a selectively permeable membrane.
The solute is more concentrated on the right side to start with, which causes molecules to move across the membrane toward the left until equilibrium is reached.
Selectively Permeable - membranes that allow some things through, the cell membrane is selectively permeable, water and oxygen move freely across the cell's membrane, by diffusion
Osmosis - the diffusion of water (across a membrane)
Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water.)
A simple rule to remember is:
Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.
Type of Solutions
If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side.
"ISO" means the same
The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell.
The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting
In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this.
The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction.
In plant cells, the central vacuole loses water and the cells shrink, causing wilting.
In animal cells, the cells also shrink.
In both cases, the cell may die.
This is why it is dangerous to drink sea water - its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water.
This is also why "salting fields" was a common tactic during war, it would kill the crops in the field, thus causing food shortages.
The classic demonstration of osmosis and osmotic pressure is to immerse red blood cells in solutions of varying osmolarity and watch what happens. Blood serum is isotonic with respect to the cytoplasm, and red cells in that solution assume the shape of a biconcave disk. In the images shown below, red blood cells were suspended in three types of solutions:
Isotonic - the cells were diluted in serum: Note the beautiful biconcave shape of the cells as they circulate in blood.
Hypotonic - the cells in serum were diluted in water: At 200 milliosmols (mOs), the cells are visibly swollen and have lost their biconcave shape, and at 100 mOs, most have swollen so much that they have ruptured, leaving what are called red blood cell ghosts. In a hypotonic solution, water rushes into cells.
Hypertonic - A concentrated solution of NaCl was mixed with the cells and serum to increase osmolarity: At 400 mOs and especially at 500 mOs, water has flowed out of the cells, causing them to collapse and assume the spiky appearance you see.
The key to remember about osmosis is that water flows from the solution with the lower solute concentration into the solution with higher solute concentration.
Diffusion and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy is required for the molecules to move into or out of the cell.
Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by carrier proteins - this process is called facilitated diffusion.
Active Transport - this type of transport requires that the cell use energy, because substances are moving against the concentration gradient.
Sodium Potassium Pump - the cell pumps out 3 sodium ions and takes in 2 potassium ions.
Endocytosis - cell takes in large particles by engulfing them
Exocytosis - cell gets rid of particles, opposite of endocytosis