Some of the major functions of cell membrane of eukaryotic cell are as follows:
1. Compartmentalization 2. Selective Permeability 3. Cellular recognition and adhesion 4. Cellular movements 5. Vital functions 6. Receptors 7. Enzymes 8. Disease.
1. Compartmentalization:
The cell membrane encloses the protoplasm and maintains the individuality of the cell. Prokaryotic cell is a uni-compartment system while eukaryotic cell has multi-compartment system in which the internal membrane bound compartments are organelles containing different set of chemicals.
2. Selective Permeability:
The cell membrane serves as selective permeability barrier allowing the entry or exists of some ions and molecules through it. The membrane proteins (carriers and channels) provide sites at which molecules cross the membrane either actively or passively.
3. Cellular recognition and adhesion:
Glycoproteins and glycolipids of the cell membrane act as cell surface markers and help in recognizing self from non-self. The RBCs have surface antigens that determine the various blood group systems. The HLA antigens (Human Leucocyte Antigens) on cell membrane are recognized by immune system.
4. Cellular movements:
The cell membrane from pseudopodia, cilia and flagella which help in cellular movements.
5. Vital functions:
(i) The membranes are the location of vital processes like respiration, photosynthesis, synthesis of cell wall constituents, lipids, transmission of nerve impulses etc.
(ii) The fluid nature of the cell membrane is helpful various functions like cell division, cell growth, secretin, endocytosis and formation of intercellular junctions.
6. Receptors:
Cell membranes have receptors for certain hormones.
7. Enzymes:
Cell membranes possess enzymes for performing certain reactions on their surface, e.g., ATPase (for ATP synthesis and release of energy from ATP), phosphatases, esterase’s etc..
8. Disease:
Defects in the organisation of cell membrane may cause certain disease, e.g., Bernard Soulier syndrome, a type of bleeding disorder in human. The term membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and molecules through biological membranes. As few molecules are able to diffuse through a lipid bilayer the majority of the transport processes involve transport proteins.
Another Question
Selective transport of substances through cell membrane occurs by three methods:
(i) Passive transport
(ii) Active transport and
(iii) Bulk transport.
(I) Passive transport. (Downhill transport):
In this type of transport, substances cross the membrane without any energy expenditure. The driving force for passive transport requires the concentration gradient. Passive transport is of two types: Simple diffusion and Facilitated diffusion.
(a) Simple diffusion:
It occurs through phospholipid bilayers of the membranes, where no membrane proteins involved. Diffusion is a slow passive transport of molecules that occurs from higher concentration to lower concentration due to their own kinetic energy until equilibrium is reached. The rate of diffusion of substances across a membrane depends upon the concentration gradient, temperature, pressure and, size and lipid solubility of substances.
The lipid soluble substances (O2, N2, H2, CH4, NH3, benzene) and small uncharged polar molecules (CO2, urea and glycerol) diffuse through the membranes by dissolving in lipid matrix. The diffusion of solutes rather than solvents through the semi permeable membrane is called dialysis.
(b) Facilitated diffusion:
It involves the use of membrane proteins (Channels and Carriers) to facilitate the movement of molecules in either direction across a membrane. In some cases, molecules pass through channels within the protein. In other cases, the protein changes shape, allowing molecules to pass through.
Channel proteins form open pores through the membranes, allowing the transport of any molecules of the appropriate size. For example, ion channels allow specific type of ions (cations and anions) to diffuse through the membrane. Aquaporin’s are water channels in biological membranes for passive; transport of water.
In contrast to channel proteins, Carrier proteins transport ions as well as solutes like sugar and amino acids across the membranes by physically binding to them and then undergo conformational change to release the same to the other side of the membrane.
Some carrier proteins allow transport only if two types of molecules transport together. This is called cotransport which is of two types i.e. symport and antiport. In symport, two molecules move together in the same direction. In antiport, two molecules move in opposite direction. When a carrier protein transports a single molecule across a membrane in one direction, the process is called uniport. The transport by carrier proteins can be either active or passive, whereas transport by channel proteins is always passive.
(II) Active transport (Uphill transport):
In this case, the substances are transported against their concentration gradient i.e. from lower to higher concentration. This form of transport requires energy and carriers. In primary active transport, the energy obtained by ATP hydrolysis used directly for transport, e.g. Na+-K+ pump, Cat+pump. In secondary active transport, indirect energy source is required, e.g. transport of glucose and amino acids is coupled to active transport of Na+.
(III) Bulk transport or vesicular transport:
Active transport of materials in large quantity (bulk) through vesicles is called bulk or vesicular transport. It is very common in secretory and excretory cells. Bulk transport occurs by two processes i.e. endocytosis and exocytosis.
A. Endocytosis:
It is the bulk import of materials into the cells by vesicles. Vesicle (bleb) formation or blobbing occurs by in-folding of the cell membrane. It does not occur in plant cells due to rigid cell wall and internal turgor.
Endocytosis is of three types:
(i) Phagocytosis;
(ii) Pinocytosis and
(iii) Recep-tormediated endocytosis.
(i) Phagocytosis [“Cell eating”):
It involves the ingestion of relatively large, solid particles, such as bacteria or cellular debris, via large vesicles pinched off from plasma membrane. These vesicles are called phagosomes. The phagosome fuses with lysosome to form a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole containing the indigestible vacuole is called residual vacuole. The undigested food particles are thrown out by the process of exocytosis (Fig. 3.12).
Many one-celled organisms, such as amoebas, feed in this way, as do plasmodial slime molds and cellular slime molds. In mammals, macrophages & neutrophils are phagocytic.
(ii) Pinocytosis (“Cell drinking”):
It involves taking in of bulk amount of fluid and substances dissolved in it by cells across the cell membrane by forming small detachable vesicles called pinosome. The pinosome migrates towards the interior where it liberates the fluid either in the cytoplasm or a vacuole. Lysosomes are required if digestion of solutes is involved (Fig. 3.13).
Unlike phagocytosis, which is carried out only by certain specialized cells, pinocytosis is believed to occur in all eukaryotic cells, as the cells continuously and indiscriminately “sip” small amounts of fluid from the surrounding medium.
(iii) Receptor-mediated endocytosis (RME):
The cells that undergo RME have coated pits, where specific receptors are localized. These coated pits are depressions of the plasma membrane coated with protein clathrin. The substance being transported attaches to the receptors in the coated pit. Shortly thereafter the coated pit invaginates and pinches off to form a coated vesicle. Within the cell, the coated vesicles shed their coats and then fuse with some other membrane-bound structure (e.g., Golgi bodies or small vacuoles), releasing their contents in the process. For exam pie, transport of iron & cholesterol into the cells by RME.
B. Exocytose:
It is the reverse of endocytosis by which bulk materials exit the cells with the help of vesicles. The vesicles are formed internally from Golgi apparatus and moved by cytoskeleton to the cell surface where they fuse to expel their contents. This is called ephagy, cell vomiting or emeiocytosis. It occurs during cell secretion, excretion, removal of undigested remains from food vacuoles, release of neurotransmitters from nerve cells etc.