Membrane transport-1.pdf

Membrane Transport May Release Energy or Cost Energy Process that ensures the smooth movement of various molecules, ions and substances across cell membranes Empowers the cell to maintain an internal equilibrium, known as homeostasis Assumes a central role in nutrient absorption, waste elimination,...

Membrane Transport May Release Energy or Cost Energy Process that ensures the smooth movement of various molecules, ions and substances across cell membranes Empowers the cell to maintain an internal equilibrium, known as homeostasis Assumes a central role in nutrient absorption, waste elimination, intercellular exchange… Separates the cell’s inner realm from the external surroundings Plasma membrane:  phospholipid bilayer  Semi permeable  Contain proteins The interior of the cell is chemically different from the exterior Concentrations of some dissolved substances (solutes) are higher inside the cell than outside Likewise, the inside of each organelle in eukaryotic cells may be chemically quite different from the solution in the rest of the cell Gradient: Describes any such difference between two neighboring regions Concentration gradient: a solute is more concentrated in one region than in a neighboring region If a substance moves from an area where it is more concentrated to an area where it is less concentrated, it is said to be ‘moving down’ or ‘following its concentration gradient As the solutes move, the gradient dissipates/ disappears, unless energy is expended to maintain it Cell transport encompasses the movement of substances across the cell membrane to uphold cellular balance and necessary functions, with two main types: Passive transport & Active transport Passive transport: Does not require energy input A substance moves across a membrane without the direct expenditure of energy There are three types of passive transport: Simple diffusion: a form of passive transport in which a substance moves down its concentration gradient without the use of a carrier molecule or protein. Substances may enter or leave cells by simple diffusion only if they can pass freely through the membrane, e.g. lipids, oxygen, carbon dioxide. Several factors affect the rate of diffusion. Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes. Mass of the molecules diffusing: More massive molecules move more slowly because it is more difficult for them to move between the molecules of the substance they are moving through; therefore, they diffuse more slowly. Temperature: Higher temperatures increase the energy and therefore the movement of the molecules increasing the rate of diffusion. Solvent density: As the density of the solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. 2. Facilitated diffusion: Polar molecules, along with ions, navigate the cell membrane through the utilization of integral membrane proteins functioning as channels or carriers. Also does not require energy. Two types: Channel mediated diffusion and Carrier mediated diffusion Differ in the type of protein used to move the substances across the membrane Channel mediated: Leak channel ( Always open) and Gated channel (Open due to stimulus) Carrier mediated: By carrier protein 3. Osmosis: This intriguing process entails the diffusion of water molecules across a selectively permeable membrane, responding to discrepancies in solute concentrations. This depends on tonicity of the environment: The ability of the solution to cause water movement Isotonic solution same solute concentration as the cell. When cell is placed in an isotonic solution no nett movement of water is found, i.e. the amount of water moving into the cell is equal to the amount flowing out. The cell size will remain constant. Hypotonic solution has less solutes and is less concentrated resulting in a higher water potential. When cell is placed in hypotonic solution water will move into cell resulting in cell swelling. Hypertonic solution has more solutes and is more concentrated resulting in a lower water potential. When cell is placed in hypertonic solution water will move out of cell resulting in cell shrinking. Active Transport Requires Energy Input In Active Transport, the cell uses a Transport Protein to move a substance against its concentration gradient. Energy for active transport comes from ATP. Example is the Sodium-Potassium Pump Cells must contain high concentrations of potassium and low concentrations of sodium to perform many functions. e.g. Nerve and muscle functioning The Na+- K+ pump is responsible for example for the flow of impulses along nerve and muscle cells. ATP provides the energy for this movement via a carrier protein. During the resting stage the carrier protein has a shape that can accommodate 3 Na+. When the ATP splits the inorganic phosphate (P) attaches to the carrier protein causing a Conformational change (change in physical structure) of the carrier protein. The 3 Na+ are then released on the outside. The carrier protein can then take up 2 K + from the outside. With the release of the P, the carrier protein again changes shape resulting in the release of the K+ on the inside and the whole cycle is repeated again For every 3 Na+ moved out of the cell, 2K+ are moved into the cell. Large Particles Enter or Leave the Cell with the Help of Vesicles Endocytosis allows a cell to engulf fluids and large molecules and bring them into the cell. Exocytosis, the opposite of endocytosis, uses vesicles to transport fluids and large particles out of cells. Vesicles within the cells move to the cell membrane and join with it, releasing the substance outside the membrane