Membrane transport

Membrane transport

            According to the theory of cell, they are the main unit of organization in biology. Whether it’s a single cell or any giant with billions of cells, it’s still made of cells.
All those cells are contained by a cell membrane that keeps the pieces bound inside. When you think of a membrane, imagine it is just like a big plastic bag with some of tiny holes. That bag keeps holding all of the cell pieces and their fluids inside the cell and keeps any nasty, unwanted things outside the cell. The holes are there to let some of the things move in and out of the cell as needed.

Based on the permeability a membrane is said to be :

  • Permeable: A permeable membrane lets everything pass through.A perfect example is a cell wall in plant cells. Cell wall protection and support for plant cells. Cell walls are fully permeable to water, molecules and proteins. This allows nutrients and water to be freely exchanged between plant cells. If you can have a look at the diagram shown here in permeable membrane, both the large circles and the small stars can move freely across.
  • Impermeable: Membrane which allows only some of the substance to pass through but does not allow other substances to pass through them. This lets the cell to have some control over what enters and leaves from them. Water, small molecules and those molecules without charge can pass freely but large molecules and molecules having strong positive and negative charge must stay outside.
  • Selectively permeable: An impermeable membrane not allows anything to pass through like water and dissolved gases, most notably oxygen.

Permeability depends on:

  • Pore size of the plasma membrane.
  • Size of the substance.
  • Charges on the substance.

Substances are transported across membrane through two transport phenomena

  1. Passive transport.
  2. Active transport.

Passive transport:

 In this type of transport substances are transported without using energy(ATP) and does not require transport proteins.It is divided into
  • Diffusion: It is movement of molecules from region of higher concentration to the region of lower concentration down the concentration gradient.It is directly proportional to the concentration of substance,temperature of medium and area of diffusion pathway.And inversely proportional to the size of the substance,molecular weight of the substance and distance to which molecule to diffuse.
  • Osmosis: It is the special type of diffusion where solvent diffuses through special permeable membrane form the region of lower solute concentration to the region of higher solute concentration.It helps in absorption of water from the soil by root hairs,cell to cell movement of water and also helps in opening and closing of stomata.
  • Facilitated diffusion: Plasma membrane of most cells contains uniporters sometimes called as protein transporter that helps to transport amino acids,nucleic acids,sugars and other small molecules into and out of the cell.Such thermodynamically favoured movement is called as facilitated transport or facilitated diffusion.

Active transport:


 In this type of transport movement of molecules or ions are against concentration gradient(moves from region of lower concentration to higher concentration) with the help of energy as ATP.Active transports takes place with the help of carrier proteins present in plasma membrane.These are called as Permeases or translocases.Carrier protein binds to transportant molecule on the outer side of the plasma membrane.It results in the formation of carrier-transportant-complex.ATP molecule binds to this protein complex and hydrolysed to form ADP and pyro phosphate(pi) provides energy for for the transport.

Since, this process is an energetically unfavorable reaction, so energy is needed for this movement. Now, here the source of energy is breakdown of ATP. Here also this happens in two ways:

  • Primary Active Transport
  • Secondary Active Transport

Primary Active Transport => If the energy of ATP is used directly to pump up the molecules against their concentration gradient, then this is called as primary active transport.

Secondary Active Transport => In some cases, use of ATP may be done indirectly. Say, if a cell uses ATP to pump out the Na+ and then uses Na+ concentration gradient to bring in glucose, then this transport of glucose would be an example of secondary glucose transport

By | 2015-07-06T17:18:18+00:00 December 27th, 2012|Transport through cell membrane|0 Comments

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