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
- Passive transport.
- Active transport.
- 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.
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