Transport Across Membrane PDF

Dr. Yousef Eshawee Plasma membrane Lecture 1 The plasma membrane surrounds the cell and has the property of selective permeability. Selective permeability: is the ability of the plasma membrane to let some substances in and keep others...

Dr. Yousef Eshawee Plasma membrane Lecture 1 The plasma membrane surrounds the cell and has the property of selective permeability. Selective permeability: is the ability of the plasma membrane to let some substances in and keep others out and it is essential for maintaining cellular homeostasis. In 1972, Singer and Nicolson developed the fluid-mosaic model of membrane structure. According to this model, 1) A cell membrane is a double layer (bilayer) of phospholipids surround one layer of proteins. - Each phospholipid molecule has one polar end (head) and one nonpolar end (tail).  The nonpolar tail (lipid portion) is repelled by water (hydrophobic).  The polar heads (phosphate portion) is water attracting (hydrophilic). 1 2) Cholesterol molecules are embedded in the interior of the cell membrane and function in Help to make the membrane less permeable to water-soluble substances. The relatively rigid structure of the cholesterol molecules helps to stabilize the membrane. 3) The membrane proteins are either: - Peripheral proteins: are situated on the inner or outer surface of the cell membrane - Intrinsic proteins: embedded in membrane surface and span the entire width of the membrane The membrane’s proteins have several functions: a) Form channels, carrier enzymes or transporter to permit movement of materials across membrane. (Intrinsic P.) b) With oligosaccharides on cell outer surface, they constitute antigens (markers that identify the cells of an individual as “self”). (Peripheral P.) c) Serve as receptor sites for hormones. (Intrinsic P.) d) Attach the membrane to the cell’s inner scaffolding (the cytoskeleton). (Peripheral P.) 2 Functions of plasma membrane The cell membrane acts as a selective barrier, regulating the flow of substances in and out of the cell. It maintains the internal environment of the cell by allowing certain molecules to enter and leave, ensuring optimal conditions for cellular activities. It provides structural support and helps maintain cell shape. It facilitates cell signaling, allowing cells to communicate and interact with their external environment. The cell membrane enables the transport of essential molecules across the membrane, ensuring the proper balance of ions, nutrients, and waste products. 3 4  Plasma membrane 5  Movement across membrane 1- Simple diffusion The features of this process: - It transports materials from an area of higher concentration to that of lower concentration. - It doesn’t require energy for transport. - It transports small and lipophilic molecules. - It continues until equilibrium. An example: Transport of O2 and CO2 6 2- Facilitated diffusion The features of this process: - It transports materials from an area of higher concentration to that of lower concentration. - It doesn’t require energy for transport. - It involves transporter, channel, and carrier proteins to transport molecules. - It transports large and hydrophilic molecules. An example: Transport of glucose and amino acids inside the cell 7 8 3- Osmosis It is the diffusion of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. Thus, water will move from an area with more water present to an area with less water. It occurs when there is a difference in solute concentration between the intracellular and extracellular space. In other word, Water molecules move from an area of lower solute concentration to an area of higher solute concentration until equilibrium is achieved. In general, there are three types of solution according to solute concentration in water. 9 a) Isotonic solution: - A solution with the same salt concentration as that in cells. - Thus, there is no movement of water in either direction. - E.g. The blood plasma is isotonic to red blood cells. b) Hypotonic solution: - A solution with a lower salt concentration than that in cells. - Thus, water moves into a cell, which swells and may burst. - E.g. Distilled water (0% salt) is hypotonic to human cells. c) Hypertonic solution: - A solution with a higher salt concentration than that in cells. - Thus, water moves out of a cell, which shrinks. - E.g. Seawater is hypertonic to human cells. 10 Isotonic solution Hypertonic solution Hypotonic solution 11 4- filtration The process of filtration also requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure (hydrostatic pressure). Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure. The blood pressure in capillaries is higher than the pressure of the surrounding tissue fluid. Thus, it forces plasma (water) and dissolved materials through the capillary membranes into the surrounding tissue spaces. This is how cells receive nutrients. 12 13 5- Active transport Active transport process moves molecules and other substances across plasma membrane from an area of low concentration to one of high concentration (against a concentration gradient). Molecules require carrier proteins (pumps) to pass across membrane. It requires ATP energy. Examples of active-transport mechanism include: The calcium pump that keeps the calcium concentration hundreds of times lower inside the cell than outside. The sodium-potassium pump found in nerve and muscle cells. 14 15 6- Endocytosis Endocytosis involves bulk movement of materials across the plasma membrane, rather than movement of individual molecules. It is a process by which cells engulf substances from the external environment The three forms of endocytosis are: 1) Pinocytosis: - It is nonspecific uptake of small droplets of extracellular fluid. - These cells are stationary and may take in small molecules from extracellular space. - The cells of the kidney tubules reabsorb small proteins by pinocytosis so that the protein is not lost in urine. 2) Phagocytosis: - It is similar to pinocytosis, except that the cell takes in solid material rather than liquid. - An example of phagocytosis is a white blood cell engulfing and digesting bacteria. 3) Receptor-mediated endocytosis: - Involves a specific receptor protein on the plasma membrane that “recognizes” an extracellular molecule and binds with it. 16 17 7- Exocytosis Cells are also capable of sending materials out of the cell, enabling the secretion or release of molecules from the cell into the extracellular environment. The transport of materials outside the cell membrane occurs through sac or vesicle. Example: Neurons release neurotransmitters through exocytosis to transmit signals to neighboring cells 18 19 20