Tut 3 Membranes Protein trafficking(MS Annotated)

St Andrew’s Junior College The Cell and Biomolecules of Life / Membranes & Protein Trafficking 2021 H1 Biology 1 Full Name: Civics group: 21S Index no.: Date: Core Idea 1: The Cell and Biomolecules of Life Membranes & Protein Trafficking Tutorial 3 (Annotated MS) MCQ ANSWERS 1 The structure of phospholipids includes the following: 1 glycerol linked to fatty acids (does not explain why phospholipid is amphipathic) 2 hydrophobic fatty acid chains (explains why phospholipids form a bilayer with hydrophobic tails facing inwards) 3 ester bonds (does not explain why phospholipid is amphipathic) 4 phosphate group attached to glycerol (explains why phospholipids form a bilayer with phosphate heads facing outwards) Which structure(s) enable formation of a lipid bilayer in cell surface membranes? A 1 and 3 B 2 and 3 C 2 and 4 D 1, 2 and 4 1 2 3 4 5 6 7 8 9 10 C D C D C C C A B C Ester bonds phosphoester bond

St Andrew’s Junior College The Cell and Biomolecules of Life / Membranes & Protein Trafficking 2021 H1 Biology 2 2 The cell surface membrane structure is described as a ‘fluid mosaic’. Which statement describes the ‘mosaic’ part of the cell surface membrane? A the different patterns that are obtained by the moving phospholipid molecules B the random distribution of cholesterol molecules within the phospholipid bilayer C the regular pattern produced by the phospholipid heads and membrane proteins D the scattering of the different proteins within the phospholipid bilayer 3 The diagram below shows a section of a cell surface membrane. Which of the following statements are correct? 1 Structure A is found only on the extracellular surface of the membrane. 2 Structure B may contain a channel that is hydrophilic to allow ions to move across the membrane. 3 Structure C allows transport of substances across the membrane only in the presence of ATP. ➔ Carrier protein should be a transmembrane protein to allow transport of substances across membrane. Even if structure C is a carrier protein, it may not be limited to active transport as the statement implied. Carrier proteins can also refer to those which are in facilitated diffusion which can occur without hydrolysis of ATP as a source of energy. 4 Structure C is found only on one surface of the membrane. ➔ integral protein can be found in either surface of the membrane –facing the cytoplasm (inside cell) or facing the external environment (outside the cell). A 1, 2 and 4 B 2, 3 and 4 C 1 and 2 D 2 and 3 ‘mosaic’ refers to the proteins embedded in a random manner ‘fluid’ refers to proteins & phospholipids being able to move within the membrane Oligosaccharide or short carbohydrate chain transmembrane proteins (also integral proteins) integral protein channel protein which is an example of integral protein Membrane components Membrane model

St Andrew’s Junior College The Cell and Biomolecules of Life / Membranes & Protein Trafficking 2021 H1 Biology 3 Note: Structure B is an e.g of a transmembrane protein with water-filled channel to allow glucose which is polar and ions which are charged (e.g K+ or Ca2+ ions) to pass through the phospholipid bilayer. The hydrophobic core which is made up of the non-polar hydrocarbon chain of fatty acids of phospholipids prevents such hydrophilic (glucose or ions) molecules from entering the cell. 4 The cell surface membranes of plants adapted to cold conditions change as the weather gets colder , allowing the plants to carry out exocytosis. (cell surface membrane needs to be fluid to allow fusion of membrane of vesicles in exocytosis to occur). Which change occurs? A a decrease in the ratio of proteins to saturated phospholipids  FALSE. should be unsaturated phospholipids as saturated phospholipids will only allow tight packing of phospholipids to occur and hence membrane will be less fluid. Also proteins move relatively very slowly and do not contribute much to membrane fluidity. B a decrease in the ratio of unsaturated phospholipids to saturated phospholipids ➔ FALSE; This means less unsaturated phospholipids and these will make membrane less fluid at lower temperature. C an increase in the ratio of proteins to unsaturated phospholipids ➔ FALSE Proteins move relatively very slowly and do not contribute much to membrane fluidity . Cells rarely change their protein content of their cell membranes in response to temperature changes. At a given protein concentration , unsaturated phospholipids will be lower compared to proteins when there is an increase in the ratio of proteins to uns aturated phospholipids Decreased unsaturated phospholipids decrease membrane fluidity at low temperature. D an increase in the ratio of unsaturated phospholipids to saturated phospholipids ➔ TRUE More unsaturated phospholipids (more kinks in tails) prevent close packing, thus hindering membrane solidification. How did you deduce the answer? ➢ Exocytosis depends upon membrane / lipid fluidity and phospholipids become less fluid at lower temperatures. ➢ Membrane fluidity is influenced by the amount of unsaturated phospholipids (phospholipids with unsaturated hydrocarbon chain of fatty acids) in comparison to amount of saturated phospholipids, stated as a ratio here ➢ higher unsaturated phospholipids ➔ membrane more fluid because of C=C double bonds in the hydrocarbon tails, allowing kinks to form to prevent the close packing of phospholipid molecules together. (Note: proteins are held in membrane by weak bonds and are “floating” in the “sea of phospholipids”. Proteins move very slowly and contribution to the fluidity of membrane is less compared to phospholipid due to large numbers of phospholipids compared to proteins.) Membrane fluidity regulation

St Andrew’s Junior College The Cell and Biomolecules of Life / Membranes & Protein Trafficking 2021 H1 Biology 4 5 The cell membrane of an African desert rat and that of the Arctic red fox are compared to investigate their composition. Which of the following is the most probable makeup of the respective membranes? Amount of saturated fatty acids Amount of cholesterol Arctic red fox Desert rat Arctic red fox Desert rat A Low High Low High B High Low Low Low C Low High High High D High Low High Low How did you deduce the answer? ➢ African desert rat lives in a hot climate (relative ly high temperature) whereas Arctic red fox lives in a cold climate (relatively low temperature). ➢ As such, the African desert rat must reduce the membrane fluidity of their cells while the Arctic red fox must increase the membrane fluidity of their cells. ➢ High amount of saturated fatty acids –> membrane become s less fluid at low temperature. Hence membrane of cells in organisms living at low temperature tend to have less saturated fatty acids to maintain fluidity of membrane. Reverse arguments for more saturated fatty acids in membranes of cells in organisms living at high temperature. Explain the role of cholesterol ➢ High amount of cholesterol – can either reduce or increase membrane fluidity ➢ For the African desert rat, high cholesterol level would reduce membrane fluidity by restricting the movement of phospholipid molecules (in hot climate). ➢ For the Arctic red fox, high cholesterol level would increase membrane fluidity by preventing the close packing of phospholipids (in cold climate). 6 What are the features of facilitated diffusion? uses proteins in membrane uses ATP molecules move down a concentration gradient A √ √ √ B x √ √ C √ x √ D √ √ x The following information serves as a revision: Diffusion is defined as net movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gr