Structure & function of cell membrane
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The plasma membrane is a semi-permeable barrier that regulates material flow in and out of cells, composed of a lipid bilayer with embedded proteins, as explained by the fluid mosaic model proposed in 1972. Key discoveries about its structure and function were made by various scientists over the years, highlighting the roles of membrane proteins in selective permeability and transport mechanisms like active and facilitated diffusion. Additionally, cell junctions play crucial roles in intercellular communication and structural integrity in tissues.
Structure & function of cell membrane
- 2. The Plasma membrane or the cell membrane is a thin, biological membrane present in all eukaryotic and prokaryotic cells that forms a boundary between the cell and its environment and regulating the flow of materials in to and out of cell.
- 4. The cells maintain an approprite amount of all molecules within them to function effectively. So, this plasma membrane acts as a semi permeable membrane allowing the entry and exits of certain materials. It is like a gaurd at a gated community who inspects those who enter and leave, to make sure that only people and things needed in the community are there.
- 6. The cell membrane was discovered by a Swiss botanist Carl Naegeli and C Carmer in 1855. Despite the existance microscopes from 1600, no-one thought that the cell membrane existed because all they could see was the cell wall. Carl Naegeli and Carmer noted that the surface of the cell was not continuous and that it was impermeable to pigments added to the solution around the cell.
- 7. They also found that the photoplasmic surface was more dense and viscous when compared to the cytoplasm. They called this surface as the plasma membrane.
- 8. • The first insight into chemical nature of the membrane was obtained by Ernst Overton in 1890s. • He knew that the nonpolar solutes dissolved very easily in the non polar solvents than polar solvents and the polar solutes had opposite solubility. • So, he realised that the substances entering the cell had to be dissolved in the outer boundary of cell is due to lipids.
- 9. • Irvin Langmuir, in 1917, during his research in nature of oil film, found that the membrane was made of monolayer of lipids and they were arranged vertically with hydrocarbon chains away from water and carboxyl groups in contact with the surface of water. • This finding was a key in understanding the lipid bilayer and cell membrane structure.
- 10. • The two Dutch scientists E. Gorter and F. Grendel in 1925 were the first to find that the membrane was made of two layers of lipids ( lipid bilayer) with hydrophilic heads and hydrophobic tails, but they could not explain about the solute permeability or the surface tension.
- 11. • In 1935, Hugh Davson and James Danielle proposed that the membrane is made of lipid bilayer and on both outer and inner surface there was a lining of globular proteins. • In 1950 they found that selective permeability was because of the presence of protein lined pores within the lipid bilayer, which allowed the passage of polar solutes and ions into and out of cell.
- 12. • It was in 1972 that S. Jonathan singer and Garth Nicholson proposed the Fluid Mosaic Model which is considered as the central dogma of membrane biology. • It describes the structure of cell membrane as a lipid bilayer with proteins embeded in it and which is free to move laterally within the membrane.
- 13. • It was first proposed by S. J. Singer and G. Nicholson in 1972 to describe the structure of the plasma membrane. • The fluid mosaic model describes the plasma membrane as that which surrounds the cell, which is made up of two layers of phospholipids and at body temperature is fluid.
- 14. • Embedded within this membrane is variety of protein molecules that acts as channels and pumps. • It contains carbohydrates, cholesterol and other lipids. • The protein and other substances such as cholesterol become embedded in the lipid bilayer, giving the membrane the look of the mosaic.
- 15. • Since the plasma membrane has the consistency of vegetable oil at body temperature, the proteins and other substances are able to move freely laterally. • That is why the plasma membrane is described as a fluid mosaic model. • The fulid mosaic model thoery thereby states that plasma membrane structure is a lipid bilayer with mosaic of proteins embedded in it and moves freely parallel to the surface of the membrane.
- 16. • The fluidity of lipid bilayer was shown by the technique of fluorescence recovery. • The fluorescent dye is used to tag the lipids and a high density laser beam is used to bleach the dye in a tiny spot on the cell surface. • When observed under fluorescent microscope, it is seen that within seconds the bleached spot became fluorescent again. • This explained the lateral diffusion of phospholipids.
- 18. 1. MEMBRANE LIPIDS 2. MEMBRANE PROTEINS
- 22. 2. MEMBRANE PROTEINS INTEGRAL MEMBRANE PROTEINS PERIPHERAL PROTEINS GLYCOPROTEINS
- 25. • To maintain cell functions, many biological molecules enter and leave the cell. • All materials that the cell gets from its environment or sends to the environment, Passes through this semipermeable plasma membrane. • Membrane transport is esseltial for cellular life.
- 28. Chemicals that can pass through the membrane are:- Water Carbon dioxide Oxygen Small polar molecules such as ammonia Lipids such as cholesterol Chemicals that cannot pass through the membrane are:- All ions including hydrogen ions Large polar molecules like glucose Aminoacids Macromolecules such as proteins, polysachharides
- 33. Small uncharged polar molecules like water, urea, ethanol, have an exceptions as they can diffuse through the lipid bilayer. There are certain factors that affect the diffusion across the cell membrane: Size of solute Solute polarity Temperature Lipid solubility
- 37. The substances to be moved binds to these proteins and this complex will bind to a receptor site and then be transported across the membrane. This process does not require energy as molecules are moving down the concentration gradient. Polar and charged solutes such as glucose, fructose, galactose and some vitamins are transported by facilitated diffusion.
- 39. A solution with lower solute concentration than inside of cell is called hypotonic solution. It causes the cell to swell and burst as it causes movement of water to inside of cell. A solution with higher solute concentration than inside of cell is called hypertonic solution. This causes osmosis of water from inside of cell to outside leading to shrinkage of cell.
- 41. Eg, transportation of sodium out of cell and potassium into the cell. There are two forms of active transports:-
- 42. When the process uses chemical energy in the form of ATP, redox energy or photon energy to transport substances across the membrane, it is called primary active transport. The energy is derived directly from the breaskdown of ATP or some other high energy phosphate compounds. The proteins act as pumps to transport ions.
- 43. Most of the enzymes that perform this transport are transmembrane ATP-ase. A primary ATP-ase which is universal to all animal cells is sodium- potassium pump which maintains the cell potential.
- 44. When the process uses electrochemical gradient to transport substances, it is called secondary active transport. Here the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of a membrane, created in the first place by primary active transport.
- 45. The pore forming proteins act as channels across the cell membrane for transporting substances. The energy stored in Na+ , H+ concentration gradient is used to transport other solutes or ions.
- 49. This pump is called a P-type ion pump because the ATP interactions phosphorylate the transport protein and causes a change in its confirmation.
- 50. It is an antiporter enzyme located in the plasma membrane of the cells, which transport potassium ions from the extra cellular fluid to the cytoplasm and sodium ions from the cytoplasm to outside of the cell. The pump is present in all the cells of the body, and it is responsible for maintaining the sodium and potassium concentration difference across the cell membrane as well as establishing a negative electrolyte potential inside the cells.
- 51. It was discovered by Danish scientist Jens Christian Skou in 1950. It was investigated by the passage of radioactively labelled ions across the plasma membrane. It showed that the sodium and potassium ions on both sides were interdependent which suggested that the same carrier protein transported both the ions. This carrier protein is a complex of two globular proteins namely αsubunit andβsubunit which has receptor sites for transport of three sodium ions out of cell for every two potassium ions pumped in.
- 53. 4. Now, two potassium ions binds at the receptor sites present on the portion of protein that is near to outside of the carrier protein. 5. The ATP is then activated and the energy released causes confirmational change in the protein causing potassium ions to be released into the cell. 6. The returns to its first stage-steady to receive new sodium ions, so that the cycle can begin all over again.
- 58. • It is the movement of substances out of the cell in the form of the secondary vesicles, which fuses with the plasma membrane and then releases its contents into the extracellular fluid. • It is important in the expulsion of waste materials out of the cell, and the secretion of enzymes and hormones. • Neurotransmitters, digestive enzymes, hormones are released from cell by exocytosis.
- 59. • It is the movement of substances from extra cellular fluid into cell in the form of vesicles. • The large polar molecules that cannot pass through the plasma membrane enters the cell by endocytosis. • This process requires energy in the form of ATP.
- 60. Pinocytosis
- 66. It attracts the substance tobe absorbed by forming a membrane depression or a coated pit on the membrane. When sufficient molecules have been attracted, the pocket will pinch off forming a coated vesicle in the cytoplasm. Inside the cytoplams the vesicle shed off their coats and then fuse with other membrane bound structures releasing their contents. E.g, Uptake of iron, cholesterol by the cell occurs by receptor mediated endocytosis.
- 68. • Cell junction is a type of structure that exists in the tissues and organs. • It is a multi-protein complex that occurs between the neighbouring cells which helps in communication between them. • There occurs a specialized modification of the plasma membrane at the point of contact, forming a function or a bridge.
- 70. • Also known as occluding junction, is the closest contact between adjacent cells providing a tight seal, preventing the leakage of mlecules cross the cells. • It is found just beneath the apical region (portion of cell exposed to lumen is apical surface) of cell around the cell circumference.
- 71. • Since they are tight seals limiting the passage of molecules and ions, most materials actually enter the cells by diffusion or active transport. • The tight junction is formed by proteins called claudins and occludins which are arranged in strands along the line of junction creating a tight seal. • It is usually seen in epithelial cells, ducts of liver, pancreas and urinary bladder.
- 73. • These are protein complexes that occur at cell to cell junction in epithelial and endothelial tissues which provides strong mechanical attachments between adjacent cells. • It is built from proteins cadherins and catenins.
- 74. • The cytoplasmic face of the cell has actin filaments and these actin bundules of one cell joins with the actin bundles of the neighbouring cells providing a strong mechanical attachment. • The space between the neighbouring cell membranes are about 20-25 nm. • This kind of junction is seen in heart muscles and they hold the cardiac muscles together when it expands and contracts.
- 76. • They are specialized intracellular channels which are brought into intimate contact with a gap of about 2-3 nm between the adjacent cells. • They directly form a connection between the cytoplasm of adjacent cell so that molecules, ions, electrical impulse pass directly from cell to cell.
- 77. • The intracellular channels are like hollow cylinders and they are called as connexons. • These connexons are madeup of proteins called connexin. • The two adjcent connexons form a hydrophilic channel of 3 nm diameter and it is through this channel that the ions and molecule pass. • Gap junction is seen in muscles and nerves. In heart tissue helps in regular heart beat, in brain it is seen in cerebellum and it helps in muscular activity.
- 80. • These are intracellular junctions which form a strong adhesion between adjacent cells. • It enables the cell to resist any stress. • The intermediate filaments (presents intracellularly) of adjacents cells join with eachother to form the strong adhesions so that they can function as a single unit. • They are usually seen in orgns subjected to mechanical stress like skin, heart and neck of uterus
- 82. • It is a type of cell junction seen in plants. • These are microscopic channels that connect the cytoplasm of adjacent cells. • It penetrates through the cell wall and it provides an easy route for movement of ions, small molecules like RNA and proteins.