Osmosis is the movement (natural) of a solvent, in the case of living organisms (water) selectively through a semi-permeable membrane down a water potential gradient. In other words it is the movement of water across a selectively semi- permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration) (Bowen, 2000).
Semi-permeable membrane
A membrane is partially (semi) permeable, if it will let in water molecules but not the molecules or ions dissolved in water (the solutes such as sugar molecules). Many cell membranes function in this manner. Osmosis is there for an important mechanism in the transport of fluids in living organisms (Bowen, 2000).
Osmosis: Movement of water across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.
Key: Water, o – Solute
Osmosis is important in biological systems, as many biological membranes are semi-permeable. In general, these membranes are impermeable to organic solutes with large molecules, such as polysaccharides, while permeable to water and small, uncharged solutes. Permeability may depend on:

solubility properties,
charge, or chemistry, as well as
Solute size.

Water molecules travel through the plasma cell wall, tonoplast (vacuole) or protoplast in two ways, either by diffusing across the phospholipids’ bilayer directly, or via small transmembrane proteins similar to those in facilitated diffusion and in creating ion channels). Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its relatively hypotonic environment (Maton et al., 1997).
The process of osmosis accounts for many functions that maintain life. In relation to blood cells, blood cells placed in pure distilled water will swell and burst. If these cells are placed in a Hyper osmotic (hypertonic) solution, i.e., the solution has more dissolved particles, salts, sugar, etc., than is in the cells, they will shrivel up (a process called crenation in the case of blood cells).
The energy that drives the process of osmosis is called osmotic pressure.
In animal (human beings included), the red blood cells are very important to the survival of the organism because they transport oxygen from the gills, skin, or lungs to the cells of the various tissues (muscles, nerves, etc.). These blood cells are transported in a fluid (serum) that has approximately the same salt content as sea water.
Effects of Osmosis on Red Blood Cells
Red blood cells as in all animal cells don’t have cell walls. In cases of hypotonic solutions, red blood cells will swell up and burst (explode) .when the cell is in danger of bursting due to accumulation of too much water in it, contractile vacuoles will pump out the water out of the cell to prevent it from bursting. In hypertonic solutions, water will diffuse out of the cell due to osmosis and the cell shrinks. For the red blood cell to stay in its normal condition, it’s always surrounded by isotonic solution.
If the concentration of the cells cytoplasm is lower then medium (the medium is hypotonic) surrounding the cell, then osmosis will result by the cell gaining water, hence the cell will swell up and burst.
If the concentration of the water inside the cell is the same as that outside the cell (the medium is isotonic solution), there exists a dynamic equilibrium, meaning the number of cells getting in and leaving the cells is the same hence the cell will retain its original size. The red blood cell retains its shape because of the isotonic nature of the plasma.
If the water concentration inside the cell is higher than that of the medium (the media is a hypertonic solution), hence the number of molecules diffusing out will be more than that entering, and the cell will shrink.
The kidneys in the human body provide the necessary regulatory mechanism for the blood plasma and the concentration of water and salt removed from the blood by the kidneys, which is controlled by the hypothalamus. This process of regulating the salty and the mineral salts in the blood is called osmoregulation.
Osmosis and diffusion a have related concepts: Both processes involve the movement of materials from an area of high concentration to an area of low concentration. Diffusion involves the movement of chemical molecules from a low concentration to a higher concentration whereas osmosis involves the movement of water molecules from a high to low concentration via a semi permeable membrane.

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Importance of Osmosis in the animal/human body
Salts and minerals are transferred from water through osmosis. Osmosis transfers water through the plasma membrane (which is selective and semi permeable) of the cell. It manages the mixing of water, glucose and salts in the body cells, this is important, otherwise the cells would loose too much water and eventually die. Hence osmosis plays an important role in keeping the cells alive.
Osmosis plays an important role in the functioning of the kidneys, it is also important in the helping to transfer water and various nutrients between the blood and fluid of the cells.
People who suffer from kidney diseases depend in kidney machines to remove waste substances (products from their blood, such machines use a process called dialysis, which is similar to the process of osmosis.
Salt water fish constantly consume a lot of water, which is released to the environment through osmosis, while fresh water fish don’t drink water because their skin is responsible to absorbing water.
Diffusion of Potassium Permanganate (KMnO4)
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration across a permeable membrane as a result of kinetic energy of random motion. It is a random movement of molecules which is directional from an area of high concentration to an area of low concentration until equilibrium is achieved.
Molecules are in a constant state of motion. For example, if you dissolve KMnO4 in water so that the concentration is initially higher in one part of the water that another, diffusion will occur so that there is a net movement of KMnO4 from area if high concentration to an area of lower concentration. However, if the KMnO4 molecules have a complete even and random distribution through out the water, there will be no movement of KMnO4 in any direction.
The rate of diffusion will be affected by properties of:

The cell
The diffusing molecule
Surrounding solution

 
The rate of diffusion increases as the concentration gradient increases. When the concentration of molecules outside the cells is very high, relative to the internal concentration, the rate of diffusion will also be high. If the internal concentration are the same (low concentration gradient), the rate of diffusion will be low.