In the Name of God, the Most Merciful, the Most Compassionate. Movement of substances across the plasma membrane

Transcription

1 *Quick Revision: In the Name of God, the Most Merciful, the Most Compassionate Movement of substances across the plasma membrane Passive transport (doesn't require metabolic energy) Active transport (requires metabolic energy) Simple diffusion (downhill) Through phospholipid bilayer Facilitated diffusion (downhill by carriers) Through protein channels Osmosis Primary active transport (energy is derived directly from breakdown of ATP) Secondary active transport (energy is derived indirectly from the ionic concentration gradient created originally by primary active transport) Vesicular transport Co-transport (symport) sodium and other substance move in the same direction Counter-transport (antiport) sodium and other substance move in opposite directions 1 P a g e

2 *The doctor emphasized some information from the last lecture: 1. Fructose enters the cell from the intestinal lumen via facilitated diffusion. 2. Some carriers for facilitated diffusion of glucose require insulin. 3. Active transport can only use carrier proteins, but carrier proteins can be used in active or passive transport depending on what type of carrier protein it is! 4. Soduim dependent carrier = secondary active transport. Sodium independent carrier = facilitated diffusion. 5. In case the sodium potassium pump doesn t function properly, high rates of sodium will remain in the cells, and potassium outside the cells. As a result, water collects inside the cell and ruptures it. In this case, secondary active transport is not expected to happen! 6. In hydrogen potassium pump, both hydrogen and potassium are transported against their concentration gradients. 7. In secondary active transport, one of the solutes (usually Na+) is transported downhill and provides energy for the uphill transport of the other solute. *Vesicular Transport: Vesicular transport is an active process in which materials move into or out of the cell enclosed as vesicles. There are multiple pathways of vesicular transport: 1. From endoplasmic reticulum (ER) to Golgi apparatus. 2. From Golgi apparatus to the plasma membrane. Recall that many cytoplasmic vesicles move along microtubules with the help of motor proteins! *Bulk Transport across the Plasma Membrane: Water and small solutes enter and leave the cell by diffusing through the lipid bilayer of the plasma membrane or by being pumped or moved across the membrane by transport proteins. However, large molecules, such as proteins and polysaccharides, as well as larger particles, 2 P a g e

3 generally cross the membrane in bulk by mechanisms that involve packaging in vesicles. Like active transport, these processes require energy. Endocytosis Exocytosis Phagocytosis (cellular eating) Pinocytosis (cellular drinking) Endocytosis: the taking in of biological molecules by forming new vesicles from the plasma membrane. 1. Phagocytosis: engulfing of particles such as bacteria, dead cells, or tissue debris. It occurs in the following steps: The cell membrane receptors attach to the surface ligands of a particle. The edges of the membrane around the points of attachment evaginate outward to surround the entire particle forming a closed phagocytic vesicle with the help of a protein called clathrin protein located on the inside of the cell membrane. The vesicle is pushed to the interior. Then, one or more lysosomes become attached to the vesicle and empty their enzymes to the inside of it, forming a digestive vesicle which digests the engulfed particles. 2. Pinocytosis: gulping droplets of extracellular fluid into tiny vesicles called pinocytotic vesicles. It occurs in much the same way as phagocytosis. Exocytosis: the secretion of biological molecules by fusion of vesicles with the plasma membrane. This process is highly controlled; there are a lot of secretory cells that don t release their contents until receiving the proper stimulus. For example: the release of neurotransmitter from nerve endings occurs by exocytosis from the synaptic vesicles, initiated by the arrival of action potential (the stimulus), which activates the voltagegated Ca+2 channels and opens them. This is followed by a rise in intracellular Ca+2 which increases the rate of exocytosis. 3 P a g e

4 Notes: 1. Transocytosis is the vesicular transport of macromolecules from one side of the cell to the other. 2. Chemical-gated channels are activated by the binding of a ligand to a receptor in the plasma membrane, whereas voltage-gated channels are activated by action potential! *Membrane Potentials: Electrical potentials exist across the membranes of virtually all cells of the body. In addition, some cells, such as nerve and muscle cells, are capable of generating rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. Plasma membrane works as capacitor that separates ions on the two sides of the membrane from moving down their concentration gradient through phospholipids. So, These ions move down their gradient only through protein channels, creating an ionic current (ionic current is different from electronic current which is created by the electric charge flow, from lower to higher electrical potential). For example, let us assume that the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane, and that the membrane in this instance is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from inside toward outside, there s a strong tendency for extra numbers of potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical changes to the outside, thus creating electropositivity outside the membrane and electronegativity inside. Within a millisecond or so, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior. This leads to electrochemical equilibrium (different from the chemical equilibrium, which depends on the concentration gradient, and the electrical equilibrium, which depends on the electrical potential). The potential across the membrane can be measured using a galvanometer. Also, we can calculate it using Nernst equation: 4 P a g e

5 ln (x) = log (X) E = electromotive force (volt) R = gas constant = (J/mol.k) T = absolute temperature Z = valence F = Faraday s constant = ^4 (C/mol) Concentration (C) of ions is measured in mm Derivation of Nernst equation: Electrochemical potential = zero Δ G conc. + Δ G volt. = zero Δ G conc. = - R T ln (C outside/ C inside) So, z F E - R T ln (C outside/ C inside) = zero The equation becomes: Δ G volt. = z F E Nernst equation can be used to calculate the Nernst potential for any univalent ion at normal body temperature of (37) degrees. By calculating the numeric values of the equation s constants, we get: E = 61 millivolt log (concentration outside/concentration inside) Or E = - 61 millivolt log (concentration inside/concentration outside) 5 P a g e

6 Note: The sign of the potential (electromotive force) is positive (+) if ions inside the cell are positively charged, and ions outside the cell are negatively charged. The sign is negative (-) if ions inside the cell are negatively charged, and ions outside the cell are positively charged. Please don t forget to refer to the slides! Best wishes from me to you! Your colleague: Aseel Nsairat. "The fact that the mind rules the body is, in spite of its neglect by biology and medicine, the most fundamental fact which we know about the process of life"!! Dr. Bernie S. Siegel Love, Medicine & Miracles. 6 P a g e