Transport Through The Cell Membrane PDF

TRANSPORT THROUGH THE CELL MEMBRANE The mechanisms by which substances move across the plasma membrane are essential to the life of the cell. Certain substances must move into the cell to support life whereas waste materials or harmful substances must be moved out. The processes involved in the mov...

TRANSPORT THROUGH THE CELL MEMBRANE The mechanisms by which substances move across the plasma membrane are essential to the life of the cell. Certain substances must move into the cell to support life whereas waste materials or harmful substances must be moved out. The processes involved in the movement of substances can be either passive or active. A passive process is whereby substances move across plasma membranes without assistance from the cell. The substances move on their own along a concentration gradient from an area of high concentration to an area of low concentration. Pressure differences also cause substances to move across the plasma membrane. In an active process, the cell provides energy in the form of ATP so that substances move against a concentration gradient. PASSIVE PROCESSES ACTIVE PROCESSES 1. Diffusion 1. Active Transport 2. Facilitated Diffusion 2. Endocytosis 3. Osmosis 3. Exocytosis 4. Filtration 5. Dialysis  PASSIVE PROCESSES  DIFFUSION Diffusion is the net movement of molecules or ions due to their kinetic energy from an area of higher to lower concentration until equilibrium is reached. When equilibrium is reached, they move in both directions at an equal rate. The difference between high and low concentrations is called the concentration gradient. Substances can diffuse through the cell membrane by two different methods: 1. By becoming dissolved in the lipid bilayer and then diffusing through it e.g. Oxygen, Carbon Dioxide, Fatty Acids, Alcohol 2. By diffusing through the channels within the integral proteins of the membrane e.g. Water and many ions – Na+, K+, Cl- 1  FACILITATED DIFFUSION Facilitated diffusion is the diffusion of larger molecules across a selectively permeable membrane with the assistance of integral proteins in the membrane that serve as carriers. Large molecules such as glucose are not soluble in lipids and so they cannot diffuse through the cell membrane on their own. They combine with a carrier and become soluble in the phospholipid bilayer of the membrane. In this way, glucose can pass through the membrane from an area of high concentration to one of lower concentration. No energy is used up in this process. (This process is in fact faster than simple diffusion.)  OSMOSIS Osmosis is the net movement of water molecules due to kinetic energy across a selectively permeable membrane from an area of higher to lower concentration of water until equilibrium is reached. The water molecules pass through channels in integral proteins in the membrane. The passage of water through a selectively permeable membrane generates an osmotic pressure. Osmotic pressure is the pressure required to prevent the movement of pure water into a solution containing solutes when a selectively permeable membrane separates the solutions. The greater the solute concentration, the greater the osmotic pressure. Osmotic pressure is an important force in the movement of water between various compartments of the body. 2   An isotonic solution is a solution in which the total concentration of water molecules and solute molecules are the same on both sides of the selectively permeable cell membrane.  A hypotonic solution is a solution that has a lower concentration of solutes and thus a lower concentration of water. In this condition, water molecules enter the cell faster than they can leave and the cell swells.  A hypertonic solution has a higher concentration of solutes and a lower concentration of water. In such a solution, water molecules move out of the cell faster than they can enter and the cell shrinks.  FILTRATION Filtration is the movement of solvents such as water and solute such as glucose across a selectively permeable membrane as a result of gravity or hydrostatic (water) pressure from an area of higher to lower pressure. Most small- to medium-sized molecules can be forced through a cell membrane. e.g. Filtration occurs in the kidneys. Blood pressure supplied by the heart forces water and small molecules like urea through thin cell membranes of tiny blood vessels and into the kidney tubules. Thus, protein molecules are retained whereas harmful substances such as urea are forced through to be eliminated. 3  DIALYSIS Dialysis is the diffusion of solute particles across a selectively permeable membrane in which small molecules are separated from larger ones.  ACTIVE PROCESSES  ACTIVE TRANSPORT Active transport is the movement of substances, usually ions, across a selectively permeable membrane from a region of lower to higher concentration by an interaction with integral proteins in the membrane. The process requires energy expenditure in the form of ATP.  ACTIVE TRANSPORT OF SODIUM & POTASSIUM (The sodium- potassium pump) This is a mechanism whereby a carrier actively transports sodium from inside the cell to the outside and potassium from the outside to the inside. This ‘pump’ is present in all cell membranes and uses ATP as its energy source. It normally transports 3 Na+ ions to the outside of the membrane for every 2 K+ ions transported to the inside. Therefore, an electrical potential is created across the membrane with negativity inside and positivity outside. The Na+/ K+ pump is thus an electrogenic pump.  ENDOCYTOSIS Endocytosis is the movement of large molecules and particles through plasma membranes in which the membrane surrounds the substance, encloses it and brings it into the cell. There are 2 types of endocytosis: 1. PHAGOCYTOSIS – The cytoplasm itself projects outwards to form pseudopodia. These surround the object in such a way that it becomes enclosed by the cell membrane and forms a phagocytic vesicle. This vesicle then breaks off from the cell membrane and its contents are digested. 4  2. PINOCYTOSIS – In this process, the engulfed material consists of liquid instead of solid. A very small droplet of ECF is attracted to the cell membrane. This folds around it and forms a pinocytic vesicle, which then detaches from the cell membrane and travels to the inside of the cell.  EXOCYTOSIS Exocytosis is the export of substances from the cell by reverse endocytosis. This occurs mostly in secretory cells.  5