Cambridge International A Level Biology Revision Guide

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Cambridge International AS Level Biology Answers to self-assessment questions Answers to SAQs Chapter 4 1 The interior of the membrane is made up of the tails of phospholipid molecules which provide a hydrophobic environment. If a hydrophobic molecule reaches the cell surface membrane, it can easily cross the membrane. 2 Large number of possible reasons: for example, to gain nutrients, to remove waste products such as carbon dioxide from respiration, to gain oxygen for respiration, to secrete hormones, to secrete enzymes, to maintain constant pH and ionic concentration. 3 Cube A SA 6, vol 1; SA : vol ratio 6 : 1 Cube B SA 24, vol 8; SA : vol ratio 3 : 1 Cube C SA 54, vol 27; SA : vol ratio 2 : 1 4 Larger organisms cannot rely on diffusion as a means of transport for essential requirements like oxygen and nutrients. This means that special long distance transport systems must be present, such as the blood system in animals and the vascular system in plants. This is discussed further at the beginning of Chapter 7. 5 Water potentials are equal. 6 a the pure water or dilute solution b the concentrated solution c the solution with the same concentration as the red cell 7 The animal cell does not have a cell wall. Plasmolysis is the pulling away of cytoplasm from the cell wall. 8 a from A to B b Water molecules can move from A to B and from B to A, but more move from A to B in a given time period. Overall, therefore, A loses water and B gains water – the overall movement is the net movement. c A has a higher water potential than B (−250 is less negative than −400) and water always moves from regions of higher to lower water potential. Water crosses a partially permeable cell surface membrane every time it enters or leaves a cell – this process is called osmosis. d i Pure water has a water potential of zero, which is higher than that of cells A and B. There is therefore a net movement of water into cells A and B by osmosis through their partially permeable cell surface membranes. As water enters, the volume of the protoplasts will increase, exerting pressure on the cell walls and raising the pressure potential of the cells. This increases the water potential of the cells. This will continue until an equilibrium is reached when the contents of the cells reach the same water potential as the water, namely zero. The cells will then be turgid. ii 1 mol dm –3 sucrose solution has a lower water potential than that of cells A and B. There is therefore a net movement of water out of cells A and B by osmosis through their partially permeable cell surface membranes. As water leaves the cells, the protoplasts shrink and the pressure they exert on the cell walls drops; in other words, the pressure potential of the cells decreases. This decreases the water potential of the cells. Eventually, the pressure potential drops to zero and the cells are at incipient plasmolysis. As shrinkage continues, the protoplasts pull away from the cell walls – this is plasmolysis. The sucrose solution can pass freely through the permeable cell walls and remains in contact with the protoplasts. As water leaves the cells, the contents of the protoplasts get more and more concentrated and their water potential gets lower and lower (more and more negative). Equilibrium is reached when the water potential of the cells equals that of the sucrose solution. Cambridge International AS and A Level Biology © Cambridge University Press 2014
Cambridge International AS Level Biology Answers to self-assessment questions Answers to SAQs Chapter 5 1 By removal of tubulin subunits (depolymerisation). 2 a 92 chromatids 3 a b 92 DNA molecules (each chromatid contains one DNA molecule) c 92 kinetochores d 46 chromatids e 92 chromatids 4 To hold chromatids together and to attach chromosomes to the spindle. 5 Of 75 000 cells, 9 were undergoing mitosis. Therefore, mitosis occupies 9/75 000 of the cell cycle. Mitosis lasts one hour. Therefore, cell cycle is 75 000/9 hours long = 8333 hours = 8333/24 days = 347 days. (Cell cycles vary in length in adult animals from less than eight hours to more than one year.) 6 It suggests that specialisation is not due to different genes being present in different cells, but that different genes are expressed (switched on) in different cells. This is indeed the case. metaphase b anaphase Cambridge International AS and A Level Biology © Cambridge University Press 2014
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Mary Jones, Richard Fosbery, Jennif
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University Printing House, Cambridg
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Cambridge International AS Level an
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Cambridge International AS Level Bi
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Chapter 1: Cell structure Ultrastru
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Chapter 1: Cell structure The nucle
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Chapter 2: Biological molecules a b
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Chapter 1: Cell structure Chapter 3
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Chapter 3: Enzymes Mammals such as
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Chapter 3: Enzymes QUESTION 3.2 Why
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Chapter 3: Enzymes Enzyme inhibitor
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Chapter 7: Transport in plants a b
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Chapter 10: Infectious diseases Ver
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Chapter 10: Infectious diseases 8 a
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Chapter 11: Immunity Smallpox was f
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Chapter 11: Immunity Phagocytes Pha
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Chapter 11: Immunity 3 Which of the
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Chapter 11: Immunity 10 a Rearrange
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Chapter P1: Practical skills for AS
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Chapter 14: Homeostasis The hypotha
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Chapter 14: Homeostasis 8 An invest
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Chapter 15: Coordination 329 Learni
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Chapter 16: Inherited change Katydi
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Cambridge International A Level Biology Revision Guide

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