2014 HCI H1 Biology Prelims P1&P2 Answers

HWA CHONG INSTITUTION / 2014 JC2 H1 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 1 HWA CHONG INSTITUTION 2014 JC2 H1 BIOLOGY PRELIMINARY EXAMINATION MARK SCHEME PAPER 1 Qn No. Correct Answer Qn No. Correct Answer Qn No. Correct Answer 1 B 11 A 21 D 2 B 12 C 22 A 3 B 13 D 23 C 4 A 14 D 24 A 5 C 15 A 25 D 6 D 16 D 26 B 7 B 17 A 27 D 8 B 18 B 28 A 9 D 19 A 29 C 10 A 20 A 30 C PAPER 2 QUESTION 1 (a) (i) Name and state the function of organelles A and B [2] A: Mitochondria, site of cellular respiration to generate ATP B: Nucleus, to store genetic material / DNA. (ii) Explain why there is an abundance of organelle C in the plasma cell. [2] 1. There is an abundance of structure C which is studded with ribosomes. 2. As ribosome is the site of polypeptide synthesis 3. and the ER lumen is the site of protein folding 4. Both are necessary to make large quantity of proteins (b) Explain how the structure of the ce ll membrane relates to its role in maintaining the internal environment of the plasma cell. [3] 1. Structure D is composed of a phospholipid bila yer, with hydrophilic phosphate head and hydrophobic hydrocarbon tails 2. The hydrophobic hydrocarbon tails creates a hydrophobic core 3. The cell membrane is selectively permeable, allo wing desirable substances to be kept within and undesirable substances kept out of the cell 4. It defines the boundary of the cell, keeping the interior of the cell physically separated from the surrounding environment 5. The membrane is fluid, which allows both phospholipids and proteins to move about laterally 6. This allows cell membrane to form pseudopodia to engulf foreign particles 7. Proteins are randomly distributed in the phospholipid bilayer 8. Carrier and channel proteins help to regulate the transport of substrate into and out of the cells 9. Cholesterol is commonly found wedged between phospholipids in cell membrane 10. and regulates membrane fluidity

HWA CHONG INSTITUTION / 2014 JC2 H1 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 2 (c) Describe how macrophages engulf and digest cellular debris. [3] 1. Macrophage engulfs the cellular debris through endocytosis / phagocytosis 2. The plasma membrane of macrophage ,invaginates / ex tends outwards to enclose, the debris to form vesicles 3. The vesicles pinch off the cytoplasm 4. And may fuse with lysosome which contains hydrolytic enzymes to digest the cellular debris [Total: 10 marks] QUESTION 2 (a) Describe how light is harvested at structures A and B. [3] 1. Accessory pigments embedded in the thylakoid membrane forms the antenna complex of photosystems 2. Range of pigments allow for a range of wavelengths to be absorbed 3. Photons or light energy is absorbed by these pigment molecules 4. Electrons in the pigment molecules are excited / moves to higher energy level 5. These energy is passed from one pigment to another via resonance energy transfer 6. Resonance energy is captured by the excitation of an electron of special chlorophyll a in the reaction center (b) Explain the effect that the herbicide bi nding to this protein will have on photosynthesis. [2] 1. Triazine prevents non-cy clic photophosphorylation 2. Resulting in no electrons available to form reduced NADP 3. As ATP is produced by cyclic photo phosphorylation, ATP is still produced 4. Resulting in no / less , ATP and no reduced NADP available for Calvin cycle / light independent reaction / conversion of GP to TP (c) (i) Using Fig 2.2, describe and explain the effects an increase in oxygen concentration will have on photosynthesis [ 3 ] 1. An increase in oxygen concentration reduces t he rate of photosynthesis / increases the rate of photorespiration 2. This i s less Rubisco is available for CO 2 / more oxygen competing with CO 2 for Rubisco; / more O 2 binding to Rubisco/ O2 outcompetes CO2 for Rubisco 3. This results in less CO 2 , fixation for Calvin cycle, / excess CO2 given off 4. There is less, glycerate 3-phosphate / GP / TP , pr oduced and less RuBP being, regenerated / formed (c)(ii) Suggest why the process outlined in Fig 2.2 is known as photorespiration. [2] 1. The process uses oxygen and, excretes / produces , carbon dioxide 2. Light energy / non-cyclic photophosphorylation / light dependent reaction / products of the light dependent reaction / ATP and NADPH, is required for this process to occur 3. The same photosynthetic enzyme / Rubisco is used and allows the Calvin cycle to continue (d) Suggest why these plants do not show photorespiration. [1] 1. oxygen’s 3D conformation is not complementary to PEP carboxylase, thus oxygen is not a substrate for / cannot bind to / will not compete for PEP carboxylase 2. PEP carboxylase , is only specific to carbon dioxide [Total: 10 marks]

HWA CHONG INSTITUTION / 2014 JC2 H1 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 3 QUESTION 3 (a) Outline what is meant by “ descent with modification by natural selection”. [3] 1. Descent with modification: present-day species arose from a succession of ancestors 2. Accumulation of diverse modifications, adaptations over millions of years 3. Shared ancestry 4. Natural selection is the mechanism that brings about descent with modification 5. Natural environment “selects” fo r certain traits to be propagated (b) (i) Explain why variation is important in selection. [3] 1. Inherited allelic variation exist in the individuals present in the populations 2. Natural selection occurs where different habitats each exerts its own specific/ different selection pressure 3. Thus different habitats select for favourable phenotypes 4. resulting in differential survival and reproductive abi lities of individuals in the population on the island/ increase chances of survival till reproductive age 5. Individuals that are selected for pass on alleles to their offspring (ii) Sketch a graph on the axes below to show the distri bution in size of seahorses as a result of disruptive selection. [1] 1. two peaks 2. dip in middle connected (iii) Explain how disruptive selection has been maintained in this species of seahorse. [3] 1. mates selected by size 2. few intermediates mate 3. habitat for intermediate size no longer available / difference in predation 4. intermediates selected agai nst / extremes selected for 5. alleles for extreme phenoty pes (more likely to be) passed on 6. increase in allele frequencies for extreme phenotypes [Total: 10 marks] QUESTION 4 (a) (i) Explain the significance of forming more stem cells. [2] 1. To preserve a population of undifferentiated cells 2. By mitotic division / allowing long-term self-renewal (ii) Suggest how a cell differentiates to form a specialised cell. [2] 1. signals that switch on / off certain genes 2. Thus, different proteins are produced in order to change the cell (b) Suggest two ethical concerns on the use of embryonic stem cells. [2] 1. Blastocyst must be destroyed when the cells ar