The plasma membrane acts as a semi-permeable barrier between the cell and the extracellular environment. This permeability must be highly selective to ensure that essential molecules such as glucose, amino acids and lipids can readily enter the cell so that these molecules and metabolic intermediates remain in the cell and waste compounds leave the cell. The selective permeability of the plasma membrane allows the cell to maintain a constant internal environment i.e. homeostasis. In all types of cells there exists a difference in ionic concentration with the extracellular medium. Similarly, the organelles within the cell often have a different internal environment from that of the surrounding cytosol and organelle membranes maintain this difference. Transport across the membrane may be passive or active. It may occur through the phospholipid bilayer or with the help of specific integral membrane proteins, called permeases or transport proteins. There are various properties of membrane transport that includes

Polar and charged solutes will not cross cell membrane effectively without specific protein carriers.

Each protein carrier will only bind and transport one (or a few very similar) type of solute.

Ions are typically transported by special proteins that form membrane channels.

Passive transport

It is a type of diffusion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gradient i.e. they move from high concentration to low concentration. No metabolic energy is consumed in passive transport. Passive transport is of following four types

  1. Osmosis The plasma membrane is permeable to water molecules. The to and fro movement of water molecules through the plasma membrane occurs due to the differences in the concentration of the solute on its either sides. The process by which the water molecules pass through a membrane from a region of higher water concentration to the region of lower water concentration is known as osmosis. The process in which the water molecules enter into the cell is known as endosmosis, while the reverse process which involves the exit of the water molecules from the cell is known as exosmosis. In plant cells due to excessive exosmosis the cytoplasm along with the plasma membrane shrinks away from the cell wall. This process is known as plasmolysis.
  2. A cell contains variety of solutes in it, for instance, the mammalian erythrocytes contains potassium (K+), calcium (Ca2+), phosphate (PO43-), dissolved hemoglobin and many other substances. If the erythrocyte is placed in a 0.9% solution of sodium chloride (NaCl), then it neither shrinks nor swells. In such case, because the intra-cellular and extra-cellular fluids contain same concentration and no osmosis takes place. This type of extra-cellular solution or fluid is known as isotonic solution or fluid. If the concentration of NaCl solution is increased above 0.9% then the erythrocytes shrink due to excessive exosmosis. The solutions which have higher concentrations of solutes than the intracellular fluids are known as hypertonic solutions. Further, if the concentration of NaCl solution decreases below 0.9%, the erythrocytes will swell up due to endosmosis. The extra-cellular solutions having less concentration of the solutes than the cytoplasm are known as hypotonic solutions. Due to endosmosis or exosmosis the water molecules come in or go out of the cell. The amount of the water inside the cell causes a pressure known as hydrostatic pressure. The hydrostatic pressure which is caused by the osmosis is known as osmotic pressure. The plasma membrane maintains a balance between the osmotic pressure of the intra-cellular and inter-cellular fluids.
  3. Simple diffusion is where a small molecule in aqueous solution dissolves into the phospholipids bilayer, crosses it and then dissolves into the aqueous solution on the opposite side. There is little specificity to the process. The relative rate of diffusion of the molecule across the phospholipids bilayer will be proportional to the concentration gradient across the membrane.
  4. Facilitated diffusion This is a special type of passive transport, in which ions or molecules cross the membrane rapidly because specific permeases in the membrane facilitate their crossing. Facilitated diffusion does not require the metabolic energy and it occurs only in the direction of a concentration gradient. Facilitated diffusion is characterized by the following special features.

The rate of transport of the molecule across the membrane is far greater than would be expected from a simple diffusion.

This process is specific; each facilitated diffusion protein (called protein channel) transports only a single species of ion or molecule.

There is a maximum rate of transport, i.e., when the concentration gradient of molecules across the membrane is low, an increase in concentration gradient results in a corresponding increase in the rate of transport.