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BIOL1040 OBJECTIVES MODULE 1: PRINCIPLES OF CELL FUNCTION LECTURE 1: CELL MEMBRANE STRUCTURE & FUNCTION 1) Describe the structural elements of cell membranes and their functions Cell membranes are composed of four different types of molecules: phospholipids, cholesterol, proteins and carbohydrates. Molecule Structure Function Phospholipids Amphipathic- hydrophilic head, hydrophobic tail Head contains phosphate group Tails contains two strings are hydrocarbons Arranged to form lipid bilayer Most important property of lipid bilayer is that it is a highly impermeable structure Cholesterol Composed of four rings of hydrogen and carbon atoms Hydrophobic- found among tails Fluidity buffer: reduces membrane fluidity at moderate temperatures by reducing phospholipid movement. At low temperatures it hinders solidification by disrupting the regular packing of phospholipids. Proteins Composed of amino acids which make polypeptide chains Membrane proteins have six main functions. Include transport, acting as receptors and enzymes. They determine most of the membranes functions. FLUID MOSAIC MODEL In the fluid mosaic model the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a bilayer or phospholipids. Lateral movement (side-by-side) of phospholipids occurs 107 times per second however, flipflopping is rare because in order to do so the hydrophilic head must cross the hydrophobic interior of the membrane (once a month) Unsaturated hydrocarbon (C=C) tails prevent packing due to the kink in the tail, enhancing fluidity Saturated hydrocarbon (C-C) tails pack together, increasing viscosity THE SLIDING FILAMENT MODEL OF MUSCLE CONTRACTION Myosins head is bound to ATP and is in its lowenergy configuration. ATP is hydrolyzed to ADP and inorganic phosphate. It is converted to its high-energy form The myosin head binds to actin, forming a crossbridge. Pulls thin filament to center of sarcomere Crossbridge is broken when a new molecule of ATP binds to myosin head The filaments do not change length when the sarcomere shortens. The thin and thick filaments slide past each other, increasing their overlap The myosin filaments have heads that bind and pull actin rapidly Each myosin has a long “tail” region and a globular “head” region Calcium ions and regulatory proteins bound to actin play crucial roles in both muscle cell contraction and relaxation o Tropomyosin (regulatory protein) and the troponin complex are bound to actin strands 1) 2) 3) 4) 5) 6) 7) Acetylcholine (neurotransmitter) released at synaptic terminal diffuses across synaptic cleft and binds to receptor proteins on muscle fiber’s plasma membrane Action potential is propagated along plasma membrane and down T tubules Action potential triggers release of calcium ions from sarcoplasmic reticulum Calcium ions bind to troponin in thin filament; myosin binding sites exposed Cycles of myosin cross bridge formation and breakdown coupled with ATP hydrolysis, slide thin filament Cytosolic calcium ions removed by active transport into SR after action potential ends Tropomyosin blockage of myosin binding sites is restored; contraction ends, and muscle fiber relaxes 1) Make the link between cellular level events and how these relate to gross force production and movement Two basic mechanisms by which the nervous system produces graded contractions of whole muscles (1) by varying the number of muscle fibers that contract and (2) by varying the rate at which muscle fibers are stimulated (mechanical summation) o Force increases as more motor neurons controlling the muscle are activated (recruitment) Tropic hormones are hormones that activate another endocrine gland to produce other hormones Non-tropic hormones are hormones that target non-endocrine tissues Growth hormone has tropic and non-tropic effects. It exerts diverse metabolic effects to raise blood glucose (non-tropic) and acts on the liver to release insulin-like growth factors (tropic) Example: ADH (ANTIDIURETIC HORMONE AKA VASOPRESSIN) Stimulus: osmolarity of blood increases due to body losing water via sweating Osmoreceptors in hypothalamus detect changes in blood osmolarity stimulating the release of ADH ADH enters bloodstream and acts on target cell in the kidney tubules ADH makes epithelium more permeable to waterincreased renal absorption of water (retain as much water as possible)reduces urine volumemake urine more concentrated Mechanism via action of aquaporins Hypothalamus also increases feeling of thirst Drinking water decreases blood osmolarity 1) Rationalize how hormones mediate the short-term and long-term responses to stress Adrenal medulla secretes epinephrine and norepinephrine (short term) Adrenal cortex secretes mineralocorticoids and glucocorticoids (long term) Stress is the stimulus and is detected by the hypothalamus which sends nervous impulse via spinal cord to the adrenal medulla. The release of catecholamines (adrenaline and noradrenaline) acts on alphaadrenoreceptors and beta-adrenoreceptors in target tissues. Adrenaline that binds to beta-receptors in liver cells causes an increase in blood glucose levels while adrenaline that binds to beta-receptors in smooth muscle cells in the wall of blood vessels that supply skeletal muscle causes blood vessels to dilate, increasing flow to skeletal muscle. Adrenaline that binds to alpha-receptors in the smooth muscle cells in wall of blood vessels that supplies intestines causes blood vessels to constrict. Short term effects Long term effects Effects of epinephrine and norepinephrine Effects of mineralocorticoids Retention of sodium ions and water by kidneys Increased blood volume and blood pressure Glycogen broken down to glucose, increase in blood glucose Increases blood pressure Increased breathing rate Increased metabolic rate Change in blood flow patterns leading to increased alertness and decreased digestive, excretory, and reproductive system activity Effects of glucocorticoids Proteins and fats broken down and converted to glucose, increased blood glucose Possible suppression of immune system