Cell Membrane Transport

Cell Membrane Transport

Based on the permeability a membrane is said to be:

 Permeable: If a substance passes readily through the membrane

 Impermeable: If a substance does not pass through the membrane

 Selectively permeable: If the membrane allows some of the substances to pass through but does not allow all the substances to pass through it.

The permeability of a membrane depends on 1)the size of pores in the Plasma membrane. 2)The size of the substance molecules 3)The charge on the substance molecules.

All the biological membranes are selectively permeable. Its permeability properties ensure that essential molecules such as glucose, amino acids and lipids readily enter the cell, metabolic intermediates remain in the cell and waste compounds leave the cell. In short it allows the cell to maintain a constant internal environment.

Substances are transported across the membrane either by:

1. Passive Transport or 2. Active Transport

Passive Transport

Physical processes

Passive Transport of materials across the membrane requires no energy by the cell and it is unaided by the transport proteins. The physical processes through which substances get into the cell are 1.Diffusion 2.Osmosis

1. Diffusion

The rate of diffusion is directly proportional to

           the concentration of the substance

           temperature of the medium

           area of the diffusion pathway

The diffusion is inversely proportional to

           the size of the substance molecules

           the molecular weight of the substance molecule

           the distance over which the molecules have to diffuse

Diffusion through Biomembranes

Gases and small hydrophobic molecules diffuse directly across the phospholipid bilayer at a rate proportional to their ability to dissolve in a liquid hydro carbon. Transport of molecules takes place along the concentration gradient and no metabolic energy is expended in this process. This can be described as 'down hill transport' . Diffusion through the bio membrane takes place in two ways.

   Diffusion of fat-soluble substances through plasma membrane simply by dissolving in the lipid bilayer.

   Diffusion of water soluble substances and ions: This takes place through pores in the membranes.

Diffusion of charged particles water soluble substances and ions such as K+ Cl- and HCO3- diffuse through the pores in the membranes. An ion diffuses from the side richer in like charges to the side with an excess of opposite charges. The difference of electrical charges between the two sides of a membrance is called electro chemical gradient.

The integral proteins of the membrane act as protein channels extending through the membrane. The movement of gas molecules occurs down its pressure gradient.

2. Osmosis

It is the special type of diffusion where the water or solvent diffuses through a selectively permeable membrane from a region of high solvent concentration to a region of low solvent concentration.

Role of Osmosis

It helps in absorption of water from the soil by root hairs.

 Osmosis helps in cell to cell movement of water.

 Osmosis helps to develop the turgor pressure which helps in opening and closing of stomata. (For more about Osmosis see unit 5.4)

Uniporter Catalyzed Transport

The plasma membrane of most cells (animal or plant) contains several uniporters that enable amino acids, nucleosides, sugars and other small molecules to enter and leave cells down their concentration gradients. Similar to enzymes, uniporters accelerate a reaction that is thermodynamically favoured. This type of movement sometimes is referred to as facilitated transport or facilitated diffusion.

Three main features distinguish uniport transport from passive diffusion. 1. the rate of transport is far higher than predicted 2.transport is specific 3.transport occurs via a limited number of transporter proteins rather than through out the phospholipids bilayer.

Active transport

It is vital process. It is the movement of molecules or ions against the concentration gradient. i.e the molecules or ions move from the region of lower concentration towards the region of higher concentration. The movement of molecules can be compared with the uphill movement of water.

Energy is required to counteract the force of diffusion and the energy comes from ATP produced by oxidative phosphorylation or by concentration gradient of ions. Thus active transport is defined as athe energy dependent transport of molecules or ions across a semi permeable membrane against the concentration gradient.

Active transport takes place with the help of carrier proteins that are present in the plasma membrane. In the plasma membrane there are a number of carrier molecules called permeases or translocases present. For each type of solute molecule there is a specific carrier molecule. It has got two binding sites; one for the transportant and other for ATP molecule. The carrier proteins bind the transportant molecule on the outer side of the plasma membrane. This results in the formation of carrier-transportant-complex. As the ATP molecule binds itself to the other binding site of the carrier protein it is hydrolysed to form ADP and energy is released. This energy brings conformational change in the carrier transportant-complex and the transportant is carried through the channel on the other side of the membrane. The carrier molecule regains its original form and repeats the precess.

There are two forces which govern the movement of ions across selectively permeable membranes, the membrane electric potential and the ion concentration gradient. ATP driven ion pumps generate and maintain ionic gradients across the plasma membrane.

Endocytosis and exocytosis

Endocytosis and exocytosis are active processes involving bulk transport of materials through membranes, either into cells(endocytosis) or out of cells (exocytosis).

Endocytosis occurs by an in folding or extension of the plasma membrane to form a vesicle or vacuole or vauole. It is of two types.

1.      Phagocytosis:(cell eating)-Substances are taken up in solid form. Cells involving in this process are called phagocytes and said to be phagocytic. (eg.) some white blood cells. A phagocytic vacuole is formed during the uptake.

2.      Pinocytosis (cell drinking)-Substances are taken up in liquid form. Vesicles which are very small are formed during intake. Pinocytosis is often associated with amoebiod protozozns, and in certain kidney cells involved in fluid exchange. It can also occur in plant cells.

Exocytosis is the reverse of endocytosis by which materials are removed from cells such as undigested remains from food vacuoles.