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  • In cellular biology, ‘membrane transport’ refers to the collection of mechanisms that regulate the passage of solutes, such as ions and small molecules, through biological membranes (lipid bilayers that have proteins embedded in them).
  • The regulation of passage through the membrane is a result of selective membrane permeability – a characteristic of biological membranes which allows them to separate substances of distinct chemical nature. In other words, they can be permeable to certain substances but not to others.
  • There are several different types of membrane transport, depending on the characteristics of the substance being transported and the direction of transport.
  • In simple diffusion, small non-charged molecules or lipid soluble molecules pass between the phospholipids to enter or leave the cell, moving from areas of high concentration to areas of low concentration (they move down their concentration gradient). Oxygen, carbon dioxide and most lipids enter and leave cells by simple diffusion.

  • Osmosis is a type of simple diffusion in which water molecules diffuse through a selectively permeable membrane from areas of high water concentration to areas of lower water concentration. (Note that the more particles there are dissolved in a solution, the less water there is in it, so osmosis is sometimes described as the diffusion of water from areas of low solute concentration to areas of high solute concentration).
  • Osmotic pressure is the pressure that causes the diffusion of water through semi-permeable membranes. It increases due to an increase in the concentration of solutes in the solution.
  • There are three types of osmosis solutions: the isotonic solution, hypotonic solution, and hypertonic solution.
  • An isotonic solution is when the solute concentration is balanced with the concentration inside the cell. In an isotonic solution, the water movement still moves between the solution, but the rates are the same in both directions, thus the water movement is balanced between the inside of the cell and the outside of the cell.
  • A hypotonic solution is when the solute concentration is lower than the concentration inside the cell. In a hypotonic solution, the water moves into the cell and can cause the cell to swell; cells that don’t have a cell wall, such as animal cells, could explode in this type of solution.
  • A hypertonic solution is when the solute concentration is higher than the concentration inside the cell. In a hypertonic solution, the water moves out of the cell and causes the cell to shrivel.

Effects of osmosis in plant cells

  • Plant cells are enclosed by a rigid cell wall. 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. The plant cell is said to have become ‘turgid’, i.e. swollen and hard. The pressure inside the cell rises until this internal pressure is equal to the pressure outside. This liquid (or hydrostatic pressure called ‘turgor pressure’) prevents the further net intake of water.
  • Turgidity is very important to plants as it helps with the maintenance of rigidity and stability of plant tissue and, as each cell exerts a turgor pressure on its neighbor, it creates plant tissue tension which allows the green parts of the plant to ‘stand up’ into the sunlight.
  • When a plant cell is placed in a hypertonic solution, the water from inside the cells cytoplasm diffuses out and the plant cell is said to have become ‘flaccid’. If the plant cell is then observed under a microscopic, it will be noticed that the cytoplasm has shrunk and pulled away from the cell wall. This phenomenon is called plasmolysis. The process is reversed as soon as the cells are transferred into a hypotonic solution (deplasmolysis).
  • When a plant cell is placed in an isotonic solution, a phenomenon called ‘incipient plasmolysis’ is said to occur. ‘Incipient’ means ‘about to be’. Although the cell is not plasmolsysed, it is not turgid either. When this happens, the green parts of the plant droop and are unable to hold the leaves up in the sunlight.

Effects of osmosis in animal cells

  • Animal cells do not have cell walls so, in hypotonic solutions, animal cells swell up and explode. They cannot become turgid because there is no cell wall to prevent the cell from bursting. When the cell is in danger of bursting, organelles called contractile vacuoles will pump water out of the cell to prevent this from happening.
  • In hypertonic solutions, water diffuses out of the cell due to osmosis and the cell shrinks. Thus, the animal cell always has to be surrounded by an isotonic solution. In the human body, the kidneys provide the necessary regulatory mechanism for the blood plasma. The concentration of water and salt removed from the blood by the kidneys is controlled by a part of the brain called the hypothalamus. The process of regulating the concentration of water and mineral salts in the blood is called osmoregulation.
  • Animals that live on dry land must conserve water, as do animals that live in salty sea water. Animals that live in freshwater have the opposite problem; they must get rid of excess water as fast as it enters into their bodies by osmosis.