2 Marking scheme: End-of-chapter test - GryphonScience

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Original material © Cambridge University Press 2008. 1. 2 Marking ... 1 a A = phospholipid; B = protein (allow extrinsic/external/enzyme/receptor); ... 2 There are three main ways in which receptors in the cell membrane are involved in cell.
2 Marking scheme: End-of-chapter test 1

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A = phospholipid; B = protein (allow extrinsic/external/enzyme/receptor); C = channel/carrier/intrinsic/transport/transmembrane protein; D = glycoprotein; E = glycolipid. [5] Allows passage of substances through the membrane/forms pore/channel. [1] Fluid because individual phospholipid (and protein) molecules can move around in the membrane; mosaic refers to the scattered protein molecules present in the membrane. [2] The presence of more unsaturated fatty-acid tails makes the membrane more fluid. [1] Form H-bonds with water molecules outside the cell, stabilising the membrane structure; act as receptor molecules (e.g. for hormones or neurotransmitters); act as antigens for cell recognition. [max. 2]

There are three main ways in which receptors in the cell membrane are involved in cell signalling. The receptor may form an ion channel. For example, the transmitter substance acetylcholine is released by nerve cells at a synapse. At the membrane of the next nerve cell, it binds to a receptor and causes sodium ions to flood into the second cell, starting a new nerve impulse. A second mechanism is when the receptor itself acts as an enzyme. For example, the hormone insulin binds with the enzyme tyrosine kinase, causing glucose to be changed into glycogen in liver cells. A third mechanism involves a substance in the membrane called a G-protein. When this is activated, it triggers a series of reactions. The hormone adrenaline is detected by liver cells in this way, causing the conversion of glycogen into glucose.

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The net movement of molecules or ions from a region of their higher concentration to a region of their lower concentration (or down a concentration gradient). diffusion occurs directly through the phospholipid bilayer; facilitated diffusion occurs through specific protein channels. The movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. The amount by which the solute molecules in a solution lower the water potential (compared with pure water). The positive pressure inside a plant cell when the cell contents push against the cell wall. The process that occurs when a plant cell is placed in a solution of lower water potential than the inside of the cell, when the cell protoplast pulls away from the cell wall.

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There would be a net movement of water into the cell by osmosis because distilled water has a higher water potential than the cell. There would be a net movement of water out of the cell by osmosis because the sucrose solution has a lower water potential than the cell.

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Cells take in water and burst releasing their contents (haemoglobin) therefore a red solution/no precipitate is formed. In tube B, the cells would appear normal (biconcave disc shape) in C they would be crinkly/crenated (due to loss of water).

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COAS Biology 1 Teacher Resources

Original material © Cambridge University Press 2008

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2 Marking scheme: End-of-chapter test

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A solution with the same concentration of salts as the blood plasma. A solution that is hypotonic/too dilute might cause the blood cells to burst; a solution that is hypertonic/too concentrated might cause cells to lose water by osmosis/become crenated.

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Both occur through special transport proteins; active transport occurs against a concentration gradient while facilitated diffusion occurs down a concentration gradient; active transport requires energy supplied by ATP, whereas facilitated diffusion does not.

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The vesicles move to the cell surface membrane; they fuse with the membrane; the enzymes are released from the vesicles to the region outside the cell.

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Active transport; the uptake of nitrate is going against a concentration gradient; nitrate ions are negatively charged, so are also entering against an electrical gradient. [3]

COAS Biology 1 Teacher Resources

Original material © Cambridge University Press 2008

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