3. Structure and function of cell membrane (Biochemistry)

Structure and function of cell membrane

1. CELL MEMBRANE STRUCTURE AND FUNCTION

2. Plasma membrane was discovered by Schwann (1838) Named as cell membrane by Nageli and Cramer (1855) Membrane was given the name of Plasmalemma by Plowe (1931) Chemically a biomolecule consists of lipid (20-40%), proteins (59-75%) and carbohydrates (1-5%) Important lipids of the membrane is Phospholipids (some 100 types), sterols (e.g., cholesterol), glycolipids, sphingolipids (e.g., sphingomyelin, cerebrosides).

3. Carbohydrates present in the membrane are branched or unbranched oligosaccharides, e.g., hexose, fructose, hexosamine, sialic acid, etc. Protein can be fibrous or globular, structural, carrier, receptors or enzymatic. Several types of models explained structure of plasma membrane: Lamellar models/ sandwich model ( Danielli and Davson , 1935) Robertson model (David Robertson, 1959) Fluid-Mosaic model (Singer and Nicolson, 1972)

4. Fluid-Mosaic model The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model, was first proposed by Singer and Nicolson in 1972. According to the fluid mosaic model, the plasma membrane is a mosaic of components-primarily, phospholipids, cholesterol, and proteins-that move freely and fluidly in the plane of the membrane. The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrate groups that are attached to some of the lipids and proteins.

5. A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group. Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer. Cholesterol , another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane. Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins , or lipids, forming glycolipids.

6. PHOSPHOLIPIDS

7. Phospholipids, arranged in a bilayer, make up the basic fabric of the plasma membrane. They are well-suited for this role because they are amphipathic. The hydrophilic, or "water-loving," portion of a phospholipid is its head, which contains a negatively charged phosphate group as well as an additional small group which may also or be charged or polar. The hydrophilic heads of phospholipids in a membrane bilayer face outward, contacting the aqueous (watery) fluid both inside and outside the cell. Since water is a polar molecule, it readily forms electrostatic (charge-based) interactions with the phospholipid heads.

8. The hydrophobic, or "water-fearing," part of a phospholipid consists of its long, nonpolar fatty acid tails. The fatty acid tails can easily interact with other nonpolar molecules, but they interact poorly with water. The phospholipid bilayer formed by these interactions makes a good barrier between the interior and exterior of the cell, because water and other polar or charged substances cannot easily cross the hydrophobic core of the membrane.

9. PROTEINS

10. Proteins are the second major component of plasma membranes. There are two main categories of membrane proteins: integral and peripheral. The portions of an integral membrane protein found inside the membrane are hydrophobic, while those that are exposed to the cytoplasm or extracellular fluid tend to be hydrophilic. Some integral membrane proteins form a channel that allows ions or other small molecules to pass. Peripheral membrane proteins are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids.

11. CARBOHYDRATES

12. Carbohydrates are the third major component of plasma membranes. In general, they are found on the outside surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Along with membrane proteins, these carbohydrates form distinctive cellular markers, sort of like molecular ID badges, that allow cells to recognize each other.

13. Carbohydrates are the third major component of plasma membranes. In general, they are found on the outside surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Along with membrane proteins, these carbohydrates form distinctive cellular markers, sort of like molecular ID badges, that allow cells to recognize each other.

14. TIGHT JUNCTION

15. Tight junctions, also known as occluding junctions or zonulae occludentes (singular, zonula occludens ) are multiprotein junctional complexes whose general function is to prevent leakage of transported solutes and water and seals the paracellular pathway. Tight junctions may also serve as leaky pathways by forming selective channels for small cations, anions, or water. Tight junctions (TJ) are specialized membrane structures found in cell-cell contact areas where the membranes of the neighboring cells come into a close proximity. Tight junctions are present only in vertebrates.

16. Tight junctions are composed of branching network of sealing strands, each started acting independently from the others. Therefore, the efficiency of the junction in preventing ion passage increases exponentially with the number of strands. There are at least 40 different proteins composing the tight junctions. These proteins consist of both transmembrane and cytoplasmic proteins. The three major transmembrane proteins are occludin, claudins, and junction adhesion molecule (JAM) proteins. Structure of tight junction

17. They hold cells together. Barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes such as material transport and maintenance of osmotic balance: Aims to preserve the transcellular transport. Tight junctions prevent the passage of molecules and ions through the space between plasma membranes of adjacent cells, so materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. Function of tight junction

18. TRANSPORT MECHANISM

19. The cell membrane is one the great multitasker of biology . It provides structure for the cell, protects cytosolic contents from the environment, and allow cells to act as specialized unit. This phospholipid bilayer can determine that what molecule can move into or out of the cell, and so is in large part responsible for maintaining the delicate homeostasis of each cell. Transport: Transport is any process in which movement of matter and/or energy occurs from one part of a system to another.

20. Diffusion Diffusion is the net movement of material from an area of high concentration of that substance to an area with lower concentration of that substance. Simple Diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane. Simple diffusion of lipid – soluble substances can take place through the lipid bilayer , its rate dependent on how highly lipid soluble it is (E.g. oxygen , carbon dioxide , nitrogen , alcohol ).

21. Water & lipid -insoluble substances simply diffuse through protein channels, the number and size of openings available determining its rate. The protein channels involved in simple diffusion are distinguished by 2 important characteristics: They are often selectively permeable to certain substances. Many of the channels can be opened or closed by gates. Facilitated diffusion is also called carrier-mediated diffusion because a substance transported in this manner diffuses through the membrane using a specific carrier protein to help.

22. Fick’s Law of Diffusion: The net diffusion rate of a gas across a fluid membrane is proportional to the difference in concentration, to the surface area of the membrane, to the permeability of the membrane to the substance and inversely proportional to the thickness of the membrane and molecular weight of the molecule. 

23. The osmotic pressure is defined to be the pressure required to maintain equilibrium, with no net movement of solvent . Osmotic pressure depends on the molar concentration of the solute but not on its identity . It is the exact amount of pressure required to stop osmosis. The tonicity of a solution refers to the effect on cell volume of the concentration of non – penetrating solutes in the solution surrounding the cell.