It is the outer limiting membrane of both prokaryotic and eukaryotic cells. It is an ultra thin, elastic, living membrane. Plasma membrane is a dynamic and selective transport barrier. Since the plasma membrane is ultra thin, it could be observed only under electron microscope. Structure of the membrane is studied by isolating the same from the cell and conducting biochemical investigations. In 1895 Overton suggested that the membrane is made of fatty substances. Other workers later concluded that two layers of lipid were present in the cell membrane. According to a model proposed by Danielli and Davson in 1935, the lipid bilayer of the membrane was coated on either side with protein.
Ultra structure of the cell membrane
Cell membranes are about 75A0 thick. Under the electron microscope they appear to consist of 3 layers.
1. an outer electron dense layer of about 20Ao thick
2. an inner electron dense layer of about 20Ao thick
3. a middle pale coloured layer about 35Ao thick
In 1960, Robertson using electron micrographs proposed a unit membrane hypothesis. According to this hypothesis the two outer layers of protein are about 2 nm thick and appear densely granular. They enclose a clear central area of about 3.5 nm wide consisting of lipids. The lipids are mainly phospholipid molecules.
Singer and Nicholson (1972) have proposed a fluid mosaic model for the plasma membrane. The fluid mosaic membrane is a dynamic structure. In this structure much of the protein molecules float about. Some of them are anchored to the organelles within the cell. Lipid molecules also move about. ‘Fluid mosaic model’ is applied to all biological membranes in general.
The cell membrane controls the passage of materials both into and out of the cell. It regulates the passage of water and dissolved substances. Water passes through the membrane by Osmosis. Water soluble substances cross the membrane by diffusion or by active transport. Many water soluble solutes are transported by carrier proteins. Lipid soluble compounds pass more quickly by dissolving in the phospholipid layer.
The outer and inner layers are formed of protein molecules whereas the middle one is composed of two layers of phospho lipid molecules. Such a trilaminar structure is called “Unit membrane” which is a basic concept of all membranes.
Fluid mosaic Model
The lipid molecules form a continuous bilayer. The protein molecules are arranged as extrinsic proteins on the surface of lipid bilayer and as intrinsic proteins that penetrate the lipid bilayer either wholly or partially. The lipid bilayer is formed of a double layer of phospholipid molecules. They are amphipathic molecules i.e. they have a hydrophilic and hydrophobic part. The arrangement of phospholipids forms a water resistant barrier. So that only lipid soluble substances can pass through readily but not water soluble substances. The phospholipid bilayer forms the basic structure of all biomembranes which also contain proteins, glycoproteins, cholesterol and other steroids and glycolipids. The presence of specific sets of membrane proteins permits each type of membrane to carry out distinctive functions.
Proteins are arranged in two forms.
1. Extrinsic or peripheral proteins: These are superficially attached to either face of lipid bimolecular membrane and are easily removable by physical methods.
2. Intrinsic or Integral proteins: These proteins penetrate the lipid either wholly or partially and are tightly held by strong bonds. In order to remove them, the whole membrane has to be disrupted. The integral proteins occur in various forms and perform many functions.
Functions of plasma membrane
In all cells the plasma membrane has several essential functions to perform. These include transporting nutrients into and metabolic wastes out of the cell preventing unwanted materials from entering the cell. In short, the intercellular and intra cellular transport is regulated by plasma membrane. The plasma membrane maintains the proper ionic composition pH (~7.2) and osmotic pressure of the cytosol. To carry out all these functions, the plasma membrane contains specific transport proteins that permit the passage of certain small molecule but not others. Several of these proteins use the energy released by ATP hydrolysis to pump ions and other molecules into or out of the cell against concentration gradients. Small charged molecules such as ATP and amino acids can diffuse freely within the cytosol but are restricted in their ability to leave or enter it across the plasma membrane. In addition to these universal function, the plasma membrane has other important functions to perform. Enzymes bound to the plasma membrane catalyze reactions that would occur with difficulty in an aqueous environment. The plasma membranes of many types of eukaryotic cells also contain receptor proteins that bind specific signaling molecules like hormones, growth factors, neurotransmitters etc. leading to various cellular responses. Like the entire cell, each organelle in eukaryotic cells is bounded by a unit membrane containing a unique set of proteins essential for its proper functioning.
Based on the permeability, a membrane is said to be:
1. Permeable: If a substance passes readily through the membrane
2. Impermeable: If a substance does not pass through the membrane
3. 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 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
Diffusion is the movement of molecules of any substance from a region of it’s higher to a region of it’s lower concentration (down its own concentration gradient) to spread uniformly in the dispersion medium on account of their random kinetic motion.
The rate of diffusion is directly proportional to
1. the concentration of the substance
2. temperature of the medium
3. area of the diffusion pathway
The diffusion is inversely proportional to
1. the size of the substance molecules
2. the molecular weight of the substance molecule
3. 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.
1. Diffusion of fat-soluble substances through plasma membrane simply by dissolving in the lipid bilayer.
2. 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 membrane is called electric 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.
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
1. It helps in absorption of water from the soil by root hairs.
2. Osmosis helps in cell to cell movement of water.
3. Osmosis helps to develop the turgor pressure which helps in opening and closing of stomata.
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.
It is a 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 the 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 though the channel on the other side of the membrane. The carrier molecule regains its original form and repeats the process.
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. 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 amoeboid protozoans, 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.