Wjec a2 biology student unit guide unit by4 metabolism, microbiology and homeostasis

Tai Lieu Chat Luong STUDENT UNIT GUIDE WJEC A2 Biology Unit BY4 Metabolism, Microbiology and Homeostasis Andy Clarke I would like to thank Alex Cook and Phil Evans for their help and advice in writing this book Philip Allan, an imprint of Hodder Education, an Hachette UK company, Market Place, Deddington, Oxfordshire OX15 0SE Orders Bookpoint Ltd, 130 Milton Park, Abingdon, Oxfordshire OX14 4SB tel: 01235 827827 fax: 01235 400401 e-mail: education@bookpoint.co.uk Lines are open 9.00 a.m.–5.00 p.m., Monday to Saturday, with a 24-hour message answering service You can also order through the Philip Allan website: www.philipallan.co.uk © Andy Clarke 2013 ISBN 978-1-4441-8297-2 First printed 2013 Impression number Year 2015 2014 2013 All rights reserved; no part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any means, electronic, mechanical, photocopying, recording or otherwise without either the prior written permission of Philip Allan or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS Cover photo: Fotolia Typeset by Integra Software Services Pvt Ltd., Pondicherry, India Printed in Dubai Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and made from wood grown in sustainable forests The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin P2197 This material has been endorsed by WJEC and offers high quality support for the delivery of WJEC qualifications While this material has been through a WJEC quality assurance process, all responsibility for the content remains with the publisher Contents Getting the most from this book�� About this book�� Content Guidance Energy and living things�� Respiration�� Photosynthesis�� 16 Microbiology�� 24 Populations�� 33 Excretion�� 41 The nervous system�� 51 Responses in plants�� 65 Questions & Answers Q1 ATP and respiration�� 71 Q2 Respiration�� 73 Q3 Photosynthesis�� 75 Q4 Microbiology�� 78 Q5 Populations�� 81 Q6 The kidney�� 83 Q7 The nervous system�� 86 Q8 The nitrogen cycle�� 89 Knowledge check answers�� 91 Index�� 93 Getting the most from this book Examiner tips Advice from the examiner on key points in the text to help you learn and recall unit content, avoid pitfalls, and polish your exam technique in order to boost your grade Knowledge check Rapid-fire questions throughout the Content Guidance section to check your understanding Summary Knowledge check answers Turn to the back of the book for the Knowledge check answers Summaries l Each core topic is rounded off by a bullet-list summary for quick-check reference of what you need to know Questions & Answers The nitrogen cycle Exam-style questions Question Question The nitrogen cycle Describe the nitrogen cycle Include the form that nitrogen takes in each part and the role of bacteria (giving names wherever possible) Any diagrams included in your answer must be fully annotated Total: 10 marks Examiner comments on the questions Tips on what you need to to gain full The last question on every WJEC exam paper is an essay-style question worth 10 marks There are always two alternatives and you are required to answer one Although the subject matter of the questions will differ, on the whole these questions are testing recall with understanding (AO1) Therefore, if you have revised the whole of the unit and are well prepared you should gain most of the marks on this type of question You may include diagrams within your answer and you are strongly advised to this Biology is a visual subject and your notes will probably contain many diagrams to help you understand the biology You will not gain any credit for just drawing a diagram However, if you annotate it then the annotations will gain credit Drawing diagrams will also help you to construct a coherent answer icon Sample student answers When plants and animals die decomposers b release ammonia into the soil c l The ammonia is then converted into nitrates by bacteria d l The plants can then absorb the nitrates to make more proteins e l Other bacteria change nitrates into nitrogen gas f which enters the atmosphere g l Some plants, such as peas, can absorb the nitrogen directly h each answer would be awarded in the exam and 3/10 marks awarded The diagram of the nitrogen cycle is incomplete and there are six statements about the cycle, demonstrating a lack of preparation a , c g These points gain marks for correctly identifying the locations of different forms of nitrogenous compound b, d, e, f There is no reference made to any of the processes involved (putrefaction, nitrification, denitrification and nitrogen fixation) or to the names of the bacteria involved h This statement is incorrect and shows a lack of understanding then read the examiner comments (preceded by the icon Student B Plan: N2 NO3– ) following each student answer Plants NO2– NH4+ Animals Decomposers Essay: Plants absorb nitrates from the soil a to provide a source of nitrogen for the synthesis of amino acids and proteins b Animals consume the plants, digest the proteins into amino acids, which they then use to make proteins Unit BY4: Metabolism, Microbiology and Homeostasis Find out how many marks plants and animals a then look at the student each set of questions answers Nitrogen is an important element and is found in proteins and nucleic acids in l Practise the questions, answers that follow on sample student Student A l marks, indicated by the Examiner commentary 89 WJEC A2 Biology About this book This guide will help you to prepare for BY4, the examination for WJEC A2 Biology Unit 4: Metabolism, Microbiology and Homeostasis Your understanding of many of the principles in Unit may be re-examined here as well Content Guidance The Content Guidance section covers all the concepts you need to understand and facts you need to know for the BY4 exam It also includes examiner tips and knowledge checks to help you prepare for BY4 The order in which topics appear in the guide follows the order of the specification with the exception of the detail of chemiosmosis, which is included in respiration and photosynthesis, rather than with ATP The concepts in each topic are presented first followed by details of the processes and adaptations of the various structures involved You are advised to familiarise yourself with the key ideas before attempting to learn the associated facts The A2 biology course is more demanding than AS and includes stretch-andchallenge and synoptic aspects Stretch and challenge: At A2 you have to develop a greater understanding of biological concepts and demonstrate a greater ability to apply your knowledge and understanding (AO2) The Content Guidance section contains boxes detailing investigations carried out on particular aspects of biology The specification does not require you to know the details of these investigations, but they will give you an idea of the sort of information you could be provided with to assess AO2 Synoptic element: You need to start piecing together the topics you have studied so far and try to see the links between them; this is the synoptic element In Unit BY1 you learnt the ‘core concepts’ in biology — the fundamentals of biochemistry and cell biology This knowledge underpins all aspects of A2 biology To ensure you have a good understanding of Unit BY4 it is essential that you revisit these concepts Synoptic links are highlighted throughout the Content Guidance section Questions and Answers This section will help you to: l familiarise yourself with the question styles you can expect in the unit test l understand what the examiners mean by terms such as ‘describe’ and ‘explain’ l interpret the question material — especially any data that the examiners give you l write concise answers to the questions that the examiners set It would be impossible to give examples of every kind of question in one book, but these should give you a flavour of what to expect Two students, Student A and Student B, attempt each question in this section Their answers, along with the examiner comments, should help you to see what you need to to score a good mark — and how you can easily not score a mark even if you understand the biology Unit BY4: Metabolism, Microbiology and H omeostasis Content Guidance Energy and living things Key concepts you must understand l Most energy available to living organisms is derived directly or indirectly from the sun l Autotrophic organisms (e.g plants) convert light energy into chemical energy during photosynthesis This chemical energy is locked up within organic molecules l All organisms, both autotrophic and heterotrophic, break down these organic molecules during respiration to produce adenosine triphosphate (ATP) l ATP is the only source of immediate energy within the cell for processes such as active transport, muscle contraction and the synthesis of organic molecules, such as proteins l ATP is often referred to as the ‘universal energy currency’ because it transfers energy for biochemical reactions in the cells of all living organisms Examiner tip Synoptic link to BY1: In Unit BY1 you studied the structure of nucleotides and the functions of nucleic acids Revisiting these topics will help you with aspects of energy transfer Adenosine triphosphate (ATP) The structure of ATP is shown in Figure It is a free RNA nucleotide consisting of a ribose sugar, the base adenine and three phosphate groups (adenosine = ribose + adenine) NH2 Adenine N H C C N C C N C N O O H2C H Ribose Examiner tip Make sure that you get the name right You could be given a diagram similar to Figure and asked to name it or label it Many students get this wrong by labelling the base adenosine, or calling the molecule adenine triphosphate or adenosine triosephosphate H H OH P O H H O O O P O O O P O O Phosphate groups OH Figure A molecule of ATP As energy is released when ATP is hydrolysed to adenosine diphosphate (ADP) and inorganic phosphate (P i), it is an exergonic reaction (see Figure 2) This reaction is catalysed by the enzyme ATPase and involves the removal of the terminal phosphate group The reaction is always coupled with an energy-requiring reaction (endergonic reaction) so that energy is transferred ATP acts as an energy carrier and is suited to its function because: l the molecule is soluble and can be transported within the cell (but cannot leave the cell), transferring chemical energy to energy-requiring processes WJEC A2 Biology Respiration the hydrolysis of ATP releases small quantities of energy (30.6 kJ mol−1) that are matched closely to the energy required in the coupled reaction l the energy is transferred quickly as the hydrolysis of ATP requires only one enzyme ATP Energy from respiration or photons of light Energy for cellular work ADP + P i Figure The interconversion of ATP, ADP and P i ATP is reformed from ADP and P i by a condensation reaction This requires the input of energy, i.e it is an endergonic reaction The energy required can come from cellular respiration or from the transduction of light energy during photosynthesis This reaction is catalysed by the enzyme ATP synthase (also known as ATP synthetase) After studying this topic you should be able to: l understand the importance of chemical energy in biological processes l Examiner tip The ‘law of conservation of energy’ states that energy can neither be created nor destroyed However, energy can be converted from one form to another When answering questions relating to the hydrolysis of ATP, you must refer to energy being released You will not gain credit for stating that energy is produced Knowledge check (a) Give three examples of cellular activities that require ATP (b) Describe three advantages of ATP for its function as the universal source of energy recognise the structure of ATP and describe its role as an energy carrier and its use in the liberation of energy for cellular activity Summary l Respiration Key concepts you must understand l Respiration is a process that occurs within the cells of all living organisms It can be represented by the following simple chemical equation: C6H12O6 + 6O2 6CO2 + 6H 2O l Respiration releases chemical energy from the oxidation of organic molecules, such as glucose, to synthesise ATP l There are three ways in which molecules can be oxidised or reduced: Oxidation Reduction Gaining oxygen Losing oxygen Losing hydrogen Gaining hydrogen Losing electrons (e−) Gaining electrons (e−) Unit BY4: Metabolism, Microbiology and H omeostasis Examiner tip In biology it is more helpful to think about oxidation in terms of loss of hydrogen and loss of electrons and reduction in terms of gain of hydrogen and electrons Content Guidance l Oxidation and reduction reactions always take place together because as one molecule is oxidised another molecule is reduced These chemical reactions are called redox reactions l Figure represents a typical step in the respiratory pathway Note that the coenzyme (NAD) is reduced as the organic molecule is oxidised NAD Reduced NAD Oxidised organic molecule Organic molecule Figure Oxidation of an organic molecule coupled with the reduction of a coenzyme l The oxidation reactions involved in respiration are exergonic The energy released from the organic molecules is used to reduce the coenzymes NAD and FAD, as these reactions are coupled During each oxidation reaction a small Examiner tip Synoptic link to BY1: Respiration is a series of enzyme-catalysed reactions Therefore factors that affect enzymes affect the rate of respiration The most important factor influencing the rate of respiration is temperature quantity of energy is, in effect, transferred to these coenzymes l During glycolysis, the link reaction and Krebs cycle, organic molecules are repeatedly oxidised and therefore most of the energy contained within glucose is transferred to the reduced coenzymes NADH and FADH When the coenzymes are re-oxidised the stored energy is used to synthesise ATP via oxidative phosphorylation l The series of oxidation reactions in respiration brings about the gradual release of chemical energy from organic molecules in a series of small steps (as opposed to combustion which is the uncontrolled release of energy in a single step) l Aerobic respiration occurs in the presence of oxygen Respiration that takes place in the absence of oxygen is called anaerobic respiration Aerobic respiration Mitochondria are present in all eukaryotic cells and they are involved in synthesis of ATP during aerobic respiration Figure shows the structure of a mitochondrion The organelle is composed of a double membrane enclosing a fluid-filled matrix The inner membrane is highly folded to form cristae This increases the surface area for the synthesis of ATP Cristae Outer membrane Matrix Inner membrane Figure Structure of a mitochondrion Figure shows the location of the four stages of respiration ATP is synthesised mainly in the mitochondria WJEC A2 Biology Respiration Glycolysis The link reaction Krebs cycle (formation of acetyl coenzyme A) Electron transport chain Glucose Acetyl coenzyme A Krebs cycle Electron transport and chemiosmosis ATP 34 ATP Pyruvate ATP Figure Outline of the stages of aerobic respiration Glycolysis (splitting of glucose) Glycolysis occurs in the cytoplasm — this is where the enzymes for glycolysis are located The main stages in the pathway are shown in Figure Glucose 6C ATP ADP Hexose bisphosphate 6C × triose phosphate 3C NAD ADP + Pi NADH2 ATP × pyruvate 3C Figure The steps involved in glycolysis l Step 1: Two molecules of ATP are required for the phosphorylation of glucose to produce hexose bisphosphate The energy from the hydrolysis of ATP activates glucose and makes the molecule more reactive l Step 2: Hexose bisphosphate is split (lysis) producing two molecules of triose phosphate Unit BY4: Metabolism, Microbiology and H omeostasis Questions & Answers Student A (a) (i) Day 10 a (ii) Days to 14 b (b) (i) Intraspecific competition c (ii) 240 d (iii) Carry out the experiment in a bigger container so they have more space e (iv) The numbers would increase f as the P aurelia start to die and there is more food for P caudatum g (c) (i) Density-dependent factors — larger populations have more of an effect on the factor than a smaller population h and with a density-independent factor the size of the population is irrelevant i (ii) A natural disaster such as an earthquake j 4/10 marks awarded a , b Correct c Incorrect d The student has read the correct value from the graph but loses the mark by not including the units e Incorrect If the experiment could be carried out with animals such as lions feeding on zebra, it would be easier to appreciate that the maximum number of lions is dependent upon the number of zebra and that making the ‘enclosure’ bigger will not increase the number of lions (it will just make it harder for them to catch the zebra!) f, g Correct h, i This shows confusion and implies that the population is affecting the factor, when it is the factor that affects the population j The student has not read the question carefully and has given an implausible answer Student B (a) (i) Day 10 a (ii) Days to 14 b (b) (i) Interspecific competition c (ii) 240 per cm3 d (iii) Provide the paramecium with more food (B pyocyaneus) e (iv) The numbers would increase f as there would be fewer P aurelia and therefore less competition g (c) (i) The effect of a density-dependent factor on the population increases with increasing population density h The effect of a density-independent factor on the population is the same regardless of the population density i (ii) Temperature j 9/10 marks awarded a – f Correct g The statement is correct but incomplete as there is no reference to food/bacteria h, i Correct In both definitions the student states clearly that it is the factor that affects the population j Correct in many respects this is the best example of a densityindependent factor It is equally applicable to the natural environment and experimental situations If you are well prepared you should be able to gain most of the marks available on this type of question Student A’s answers show a lack of preparation The student has not learnt the key terms and easy marks have been lost for not including units and by not reading the question carefully Student A gains marks (grade U) In contrast, Student B gains marks (grade A) The information provided has been used correctly and the precise answers demonstrate that time has been spent learning the biology 82 WJEC A2 Biology The kidney Question Question The kidney (a) The diagram below is of a kidney nephron C Proximal convoluted tubule B Bowman’s capsule Y X Glomerulus A F Distal convoluted tubule Descending limb of loop of Henlé D G Collecting duct Ascending limb of loop of Henlé E (i) In which region of the kidney the Bowman’s capsules occur? (ii) Explain why the urea concentration is higher in the region labelled Y than it is in the region labelled X (iii) Match any of the letters A–G from the diagram with the statements shown in the following table Statement (1 mark) (1 mark) (4 marks) Letter(s) Main site of selective reabsorption Areas involved in ultrafiltration Sodium ions actively pumped from this region Antidiuretic hormone acts on this region (iv) The water potential of the blood changes as it passes through the glomerulus and the water potential of the filtrate changes as it passes along the nephron Using a tick [✓ ] complete the table below to show if the water potential increases or decreases as it passes along (4 marks) each of the regions labelled in the diagram Flowing along region Water potential increasing Water potential decreasing A C D E Unit BY4: Metabolism, Microbiology and H omeostasis 83 Questions & Answers (b) The kangaroo rat, Dipodomys spectabilis, is found in desert regions of North America It does not drink water and feeds on dry seeds and other dry plant material It produces very little urine (i) Explain how the kidney of this mammal is adapted to reduce the volume of urine produced. (2 marks) (ii) Suggest how desert animals are able to obtain water from dry seeds. (2 marks) Total: 14 marks Questions on the kidney tend to be structured, although you might be asked a 10-mark question on the topic, particularly with regard to osmoregulation Part (a) tests recall with understanding (AO1) and there are some easy marks to gain However part (a)(iv) could prove tricky as students get confused with water potential Part (b) tests both AO1 and AO2 (application of knowledge and understanding) and there is a synoptic element to the question, with a link to BY1 Student A (a) (i) Medulla a (ii) This is because water is being reabsorbed from the filtrate b (iii) Statement Letter(s) Main site of selective reabsorption Cc Areas involved in ultrafiltration Bd Sodium ions actively pumped from this region D and E e Antidiuretic hormone acts on this region Gf (iv) Flowing along region A Water potential increasing ✓g C D E Water potential decreasing ✓h ✓i ✓j (b) (i) It would have a larger loop of Henle k, which is an adaptation to living in the desert l (ii) When the seeds are digested, water is released and absorbed into the blood m 4/14 marks awarded a Incorrect b, c Correct d No mark awarded as both the glomerulus (A) and Bowman’s capsule (B) are involved in ultrafiltration e E is correct, but D is wrong so no mark is given f Although ADH acts on both the distal convoluted tubule (F) and the collecting duct (G) the focus on the specification is the collecting duct and the wording of the question implies one region, therefore the student gains the mark g Incorrect h Correct i, j Incorrect k The student states that the loop of Henle is ‘larger’ This is not the same as longer so the student fails to score l The answer is too vague and is a poor attempt at an explanation m The student demonstrates a complete lack of understanding of some basic biology 84 WJEC A2 Biology The kidney Question Student B (a) (i) Cortex a (ii) This is because water is being reabsorbed from the filtrate and the urea is concentrated into a smaller volume of filtrate b (iii) Statement Letter(s) Cc Main site of selective reabsorption Areas involved in ultrafiltration A and B d Sodium ions actively pumped from this region Ee Antidiuretic hormone acts on this region F and G f (iv) Flowing along region Water potential increasing Water potential decreasing A ✓g C ✓h D ✓i E ✓j (b) (i) As the rat lives in arid conditions it has longer loops of Henle k This creates a high solute concentration in the tissues of the medulla l allowing more water to be reabsorbed into the blood m (ii) The seeds contain lipids n which can be used as a respiratory substrate o by the rat This releases metabolic water p 14/14 marks awarded a , Correct b This is an excellent answer that explains fully the increase in concentration c – f Correct g –j Correct The student understands the change in water potential as the filtrate passes through the nephron k– m The student has linked structure to function and given a full explanation linking the length of the loop with the effect on the solute concentration and the volume of water reabsorbed n Student B uses knowledge from BY1 o and links this to knowledge of respiration p to explain one of the functions of lipids This is an excellent answer The function of the kidney relies heavily on the principles of movement of molecules across membranes Solutes are actively transported out of the filtrate lowering the water potential — therefore water follows by osmosis This question demonstrates the need to have a full understanding of these aspects of biology If you are well prepared, and have reviewed your notes from BY1 you should be able to gain most of the marks available Student A has gained some of the marks available for simple recall about the different regions of the nephron The mark in part (a) (iv) was probably gained by chance There is no evidence that the student has spent time learning the detail involved in kidney function or the underlying principles involved Student A gains marks (grade U) In contrast, Student B gains full marks (grade A) for linking structure to function and cause and effect The student has spent time learning the biology and shows an excellent understanding of the fundamental principles involved in kidney function Unit BY4: Metabolism, Microbiology and H omeostasis 85 Questions & Answers Question The nervous system (a) A diagram of a section of the spinal cord is shown B Grey matter A Receptor C D Effector White matter (i) Identify structures A–D (4 marks) (ii) On the diagram above, draw a sensory neurone, a relay neurone and a motor neurone The sensory neurone should enter at one side of the spinal cord and the motor neurone should exit on the other side The neurones should link the receptor to the effector Label (3 marks) each neurone (b) Pacinian corpuscles are receptors found in the skin and consist of a single sensory neurone surrounded by connective tissue They respond to changes in pressure The Pacinian corpuscle was stimulated and the electrical activity across the membrane of the sensory neurone was recorded using a microelectrode as shown Potential difference –20 across membrane/ –40 –60 mV Cathode ray oscilloscope Potential difference –20 across membrane/ –40 –60 mV Microelectrode Pressure Oscilloscope readings Light pressure Medium pressure Axon Pacinian corpuscle Myelin sheath Potential difference across membrane/ mV +40 +20 –20 –40 –60 Heavy pressure (i) Explain the change in potential difference shown by the microelectrode after light pressure (3 marks) was applied (ii) Explain the change in potential difference across the membrane shown by the micro (6 marks) electrode when heavy pressure was applied Total: 16 marks 86 WJEC A2 Biology The nervous system Question Questions on the nervous system involving spinal reflexes are common Students often lose marks when asked to complete diagrams showing the neurones involved The spinal cord can be shown orientated in different ways and you must make sure that you use any information given in the text Part (a) of this question tests both recall with understanding (AO1) and application of knowledge and understanding (AO2) However, these are relatively easy marks to obtain if you are well prepared Part (b) tests AO2 and gives a scenario about receptor cells (which you probably have not been taught about because it is not required on this specification) However on careful inspection of the information given it becomes apparent that the question is simply testing knowledge of nerve impulses Note that part (b)(ii) is worth marks and so you need to make sure that you provide a detailed answer Student A (a) (i) A = motor neurone a; B = central canal b; C = sensory neurone c; D = peripheral nervous system d (ii) e, f and g A B Grey matter Receptor C D White matter Effector (b) (i) When light pressure was applied the potential difference rose from −60 mV to about −50 mV h This is due to sodium ions entering the axon i and then the membrane repolarises back to −60 mV j (ii) This is a threshold stimulus k The heavy pressure causes the sodium channels to open l and the sodium ions are actively transported into the axon m The inside of the axon is now about +45 mV n The membrane then repolarises o due to the potassium ions diffusing out p 8/16 marks awarded a Incorrect b Correct c, d Incorrect e – g The student has ignored the information provided and attempted to draw a normal reflex pathway However the sensory neurone and motor neurone are entering and leaving the spinal cord via the wrong roots and no credit can be given for the relay neurone as there are no labels h The student describes the trace i and gives a correct explanation for the depolarisation j The concept of threshold is being tested here and as the student makes no reference to this so only marks are awarded k , l and n – p The student gives a concise answer and correctly links the change in potential difference with the movement of sodium and potassium ions, for marks m The student loses a mark for incorrectly stating that active transport is involved in the inward movement of sodium ions Unit BY4: Metabolism, Microbiology and H omeostasis 87 Questions & Answers Student B (a) (i) A = dorsal root ganglion a; B = central canal b; C = ventral root c; D = dorsal root d (ii) e, f and g A B Grey matter Receptor Sensory neurone C D White matter Relay neurone Motor neurone Effector (b) (i) When the receptor is stimulated with light pressure the membrane starts to depolarise h as sodium ions diffuse into the axon i However the threshold is not reached j and an action potential not generated k (ii) An action potential is produced l The heavy pressure causes the gated sodium channels to open m and the sodium ions rapidly diffuse into the axon n The inside of the axon is now positively charged o The sodium channels then close and the potassium channels open p The membrane then repolarises due to the potassium ions diffusing out at a faster rate q 15/16 marks awarded a – c Correct d Both students failed to recognise this as a spinal nerve e – g The student has used the information provided and correctly drawn the position of the three neurones and labelled them h – k This is an excellent answer The data from the graph are linked clearly with knowledge of the nerve impulse and the concept of the threshold The student earns all marks l – q This is a well-structured answer using knowledge of the nerve impulse to explain the data from the graph Student B gains all marks There will always be questions that contain information about unfamiliar situations If you take your time reading the information you should be able to pick out the relevant biology and be able to answer the question If you spend time looking in different textbooks at drawings of reflex arcs, being asked to complete one in the exam should not pose a problem Student A shows a complete lack of understanding of the structure of reflex pathways and an inability to follow the instructions provided However in part (b) the student scores of the marks available with good knowledge of the biology of nerve impulses Overall, student A scores marks (grade E) In contrast Student B gains 15 marks (grade A) for analysing the data and providing detailed, coherent answers The student has clearly spent time learning the biology 88 WJEC A2 Biology The nitrogen cycle Question Question The nitrogen cycle Describe the nitrogen cycle Include the form that nitrogen takes in each part and the role of bacteria (giving names wherever possible) Any diagrams included in your answer must be fully annotated Total: 10 marks The last question on every WJEC exam paper is an essay-style question worth 10 marks There are always two alternatives and you are required to answer one Although the subject matter of the questions will differ, on the whole these questions are testing recall with understanding (AO1) Therefore, if you have revised the whole of the unit and are well prepared you should gain most of the marks on this type of question You may include diagrams within your answer and you are strongly advised to this Biology is a visual subject and your notes will probably contain many diagrams to help you understand the biology You will not gain any credit for just drawing a diagram However, if you annotate it then the annotations will gain credit Drawing diagrams will also help you to construct a coherent answer Student A l Nitrogen is an important element and is found in proteins and nucleic acids in plants and animals a l When plants and animals die decomposers b release ammonia into the soil c l The ammonia is then converted into nitrates by bacteria d l The plants can then absorb the nitrates to make more proteins e l Other bacteria change nitrates into nitrogen gas f which enters the atmosphere g l Some plants, such as peas, can absorb the nitrogen directly h 3/10 marks awarded The diagram of the nitrogen cycle is incomplete and there are six statements about the cycle, demonstrating a lack of preparation a , c g These points gain marks for correctly identifying the locations of different forms of nitrogenous compound b, d, e, f There is no reference made to any of the processes involved (putrefaction, nitrification, denitrification and nitrogen fixation) or to the names of the bacteria involved h This statement is incorrect and shows a lack of understanding Student B Plan: N2 NO3– Plants NO2– NH4+ Animals Decomposers Essay: Plants absorb nitrates from the soil a to provide a source of nitrogen for the synthesis of amino acids and proteins b Animals consume the plants, digest the proteins into amino acids, which they then use to make proteins Unit BY4: Metabolism, Microbiology and H omeostasis 89 Questions & Answers During decomposition by saprophytic bacteria the proteins within the dead plants and animals are broken down c and ammonium ions are released d into the soil These ions are converted to nitrites e by Nitrobacter f and then to nitrates g by Nitrosomonas h Under anaerobic conditions, denitrifying bacteria convert nitrates into atmospheric nitrogen gas i which is unavailable to plants Nitrogen-fixing bacteria can convert nitrogen gas into ammonium ions j Rhizobium are symbiotic bacteria that live in the root nodules of legumes k The plant uses the ammonium ions to make amino acids Azotobacter is another type of nitrogen-fixing bacteria that is free-living in the soil l 10/10 marks awarded The student gains full marks even though the answer is incomplete and contains a couple of errors The answer is well-structured and includes detail of the processes and the names of the bacteria involved although f and h Nitrosomonas and Nitrobacter are mixed up The answer demonstrates that the student is well prepared and has taken the time to learn the detail of the cycle If you are well prepared you should be able to gain most of the marks available on this type of question There are usually about 15 marking points on the mark scheme, which allows you to achieve maximum marks without knowing ‘everything on the topic’; as demonstrated by Student B who gains 10 marks (grade A) Quality of written communication is important and you must ensure that you give a coherent answer that is detailed and contains precise biological terminology Student A’s answers lack the detail expected at this level and the student gains only marks (grade U) Many students shy away from practising the 10-mark essay questions from past papers However it is worth bearing in mind the following: 1 The grade boundaries for this unit tend to be or marks apart Therefore the quality of your answer to the 10-mark question can have a big effect on your overall grade 2 The majority, if not all, of the marks are for recall of knowledge with understanding (AO1) You can gain full marks without getting everything right As part of your revision, when you have finished reviewing each topic you are advised to answer at least one appropriate past paper essay question This will allow you to assess how appropriate your knowledge is If you are well prepared you will get most, if not all, of the marks available! 90 WJEC A2 Biology Knowledge check answers Knowledge check answers (a) Active transport, muscle contraction, synthesis of organic molecules, e.g proteins, mitosis/meiosis (b) ATP is soluble and easily transported around the cell; ATP is an immediate source of energy as only one enzyme is needed to hydrolyse it; ATP releases energy in useable amounts that are matched closely to the energy required in a coupled reaction 2 The metabolic pathway does not involve oxygen 3 The glucose molecules cannot enter the mitochondria and as the mitochondria have been isolated from the cell there are no enzymes present to convert the glucose into pyruvate 4 Oxygen is the terminal electron acceptor in the electron transport chain It accepts electrons and protons and is reduced to water; this allows the coenzymes NAD and FAD to be regenerated (a) Any two molecules from pyruvate, NADH2, ADP or inorganic phosphate (Pi) (b) Any two molecules from water, NAD or ATP (a) The rate of ATP synthesis would increase The enzymes involved in glycolysis, the link reaction and Krebs cycle would have more kinetic energy and would therefore increase the rate of the reactions involved Therefore, more NADH2 and FADH2 would be produced, leading to more ATP being produced via oxidative phosphorylation More ATP would also be produced via substrate-level phosphorylation during glycolysis and the Krebs cycle (b) The rate of ATP synthesis would fall Less ATP would be produced via substrate-level phosphorylation during the Krebs cycle Less ATP would be produced via oxidative phosphorylation as less NADH2 and FADH2 would be produced during the link reaction and Krebs cycle (a) oxygen (b) pyruvate (c) ethanal 8 By containing several pigments the chloroplast is able to absorb a greater range of wavelengths of light, therefore absorbing more light energy and increasing the rate of photosynthesis 9 Plants appear green because the photosynthetic pigments reflect the green wavelengths of light 10 On the thylakoid membranes 11 (a) the reaction centre (b) the antenna complex 12 ATP and NADPH2 13 Its cells are spherical and it has a simple cell wall composed of a thick layer of peptidoglycan 14 Carbon: to be able to synthesise organic molecules Nitrogen: to be able to synthesise amino acids, proteins and nucleic acids Phosphorus: to be able to synthesise nucleotides for the production of ATP and nucleic acids, DNA and RNA 15 Gram negative: it has a complex cell wall composed of a thin layer of peptidoglycan and an outer layer of Unit BY4: Metabolism, Microbiology and H omeostasis 16 17 18 19 20 21 22 23 24 25 26 lipopolysaccharides These bacteria stain pink as the cell wall does not retain the crystal violet stain Facultative anaerobe: they grow better in the presence of oxygen but can survive in anaerobic conditions 19 colonies counted from a dilution factor of 10−4 therefore 19 × 104 = 190 000/1.9 × 10 bacteria A total count includes both living and dead cells and can therefore overestimate the population A viable count includes only living cells and is based on the assumption that one cell gives rise to one colony However, if clumping of cells occurs then the colonies will merge, giving an underestimate of the population (a) The cooling water jacket prevents overheating due to microbial respiration (b) The sparging ring provides oxygen, allowing aerobic respiration to occur As the oxygen bubbles through the culture it mixes the microbes with the nutrients (c) The air filters prevent contamination from airborne microbes The lag phase in the rat population is caused by a low number of individuals of reproductive age The lag phase in the bacterial population is due to the switching on of genes and the synthesis of the enzymes necessary for the bacteria to utilise their culture medium (a) Carrying capacity is the maximum population size that the environment can support indefinitely (b) Exponential growth is the doubling of the population per unit time Density-dependent factors include competition for food/water/ mates, predation, disease and the accumulation of toxic waste Density-independent factors include climatic factors such as temperature It is acting in a density-independent manner as 20% of the beetles are dying at each population density In broad-leafed woodland the two species are in direct competition and the grey squirrels have out-competed the red squirrels, which have become locally extinct The nitrates provide a source of nitrogen to synthesise amino acids/proteins and nucleotides/nucleic acids (a) Draining waterlogged fields reduces anaerobic conditions and therefore reduces denitrification by denitrifying bacteria (b) Ploughing fields aerates the soil so that nitrifying bacteria can convert ammonium ions into nitrates for plant growth (c) Planting leguminous crops such as clover allows symbiotic nitrogen-fixing bacteria to convert atmospheric nitrogen to ammonium ions (d) Ploughing crops such as clover into the soil encourages putrefaction by saprophytic bacteria, which increases the concentration of ammonium ions (and ultimately nitrate ions) in the soil Although they have access to fresh water, they excrete uric acid They are reptiles and therefore they lay eggs The low toxicity of uric acid means that it can accumulate inside the eggs without damaging the embryos 91 Knowledge check answers 27 (a) Any three from water, glucose, amino acids, urea, fatty 28 29 30 31 32 3 34 35 36 37 38 39 92 acids, glycerol, small proteins, inorganic ions (e.g Na+) (b) Large plasma proteins The efferent arteriole has a narrower lumen than the afferent arteriole, which causes a build-up of blood in the capillaries, which increases the hydrostatic pressure The microvilli provide a large surface area for absorption and they contain many mitochondria to provide ATP for active transport The otter is a mammal that lives in and around fresh water Therefore it has short loops of Henle because it does not need to produce urine with a low water potential The camel is a mammal that lives in deserts It has long loops of Henle, which enable the generation of a high solute concentration in the tissues of the medulla so that the camel can reabsorb more water Loop of Henle The detectors (receptors) are the osmoreceptors in the hypothalamus; the coordinator is the posterior lobe of the pituitary gland (and ADH); the effectors are the walls of the distal convoluted tubule and collecting ducts of the nephron Muscles and glands An axon transmits impulses away from the cell body; a dendrite transmits impulses towards a cell body The Na+/K+ pump actively transports three Na+ ions out of the axon in exchange for two K+ The membrane of the axon is more permeable to K+ ions so they diffuse out more rapidly than the Na+ ions This creates an uneven distribution of charge and the membrane is polarised with the inside of the axon being negatively charged When the neurone is stimulated the sodium channels open, increasing the permeability of the membrane to Na+ The Na+ ions diffuse rapidly into the axon, depolarising the membrane; the inside of the axon, becomes positively charged (a) The myelin sheath provides electrical insulation to the neurone by preventing ion exchange across the membrane This increases the length of local circuits and the action potential moves by saltatory conduction This speeds up the transmission of a nerve impulse (b) The increase in diameter reduces the longitudinal resistance of the cytoplasm in the axon This increases the length of the local electrical circuits in the axon, increasing the speed of transmission The mitochondria provide ATP for the resynthesis of neurotransmitter and for the Na+/K+ pump Synapses transmit impulses in only one direction because the neurotransmitter is found on the presynaptic side and the neurotransmitter receptors are found on the postsynaptic membrane The refractory period prevents the nerve impulse from travelling in two directions along an axon 40 The insecticide is an excitatory drug It inhibits acetylcholinesterase and therefore acetylcholine cannot be broken down The acetylcholine remains in the synapse and causes continuous stimulation of the postsynaptic membrane White matter contains myelinated axons; the grey matter is composed of cell bodies 42 Stimulus Hot surface Receptor Thermoreceptors and pain receptors in the skin Coordinator CNS (spinal cord) Effector Arm (biceps) muscles Response Contraction of muscle to remove the hand/arm 43 Nerve net Vertebrate neurones One type of neurone Three types of neurone Neurones are shorter Neurones are longer Neurones branched Neurones not branched Neurones can transmit impulses in both directions Neurones can transmit impulses in one direction only Impulses pass in all directions from point of stimulation Impulses follow a unidirectional pathway from the point of stimulation Neurones are nonmyelinated Neurones may be myelinated Many synapses involved Few synapses involved Slower transmission of impulses Faster transmission of impulses 44 (a) The leaf/stem/bud (b) It would cause the rapid conversion to Pr (c) It would cause the slow conversion to Pr (d) Expose the plant to sunlight/white light/red light Phloem WJEC A2 Biology Index Note: Bold page numbers indicate antibiotics 24, 25, 30 defined terms penicillin production 31–32 C Calvin cycle 22–23 antidiuretic hormone (ADH) 49, 50 A Calvin’s lollipop 23 aseptic techniques 26–27, 30 carbon cycle 39 absorption spectrum 18 assessment objectives 70 carbon fixation 22 acetylcholine (ACh) 59 ATP (adenosine triphosphate) 6–7 carotenoids 17, 18 acetyl coenzyme A (acetyl coA) 9, ATPase 6 carrying capacity 28, 33, 34–35 ATP synthase/synthetase 7, 12, 21 cell bodies, neurones 52, 59, 10–11, 13, 14 action potentials 54, 55–57 autoradiograms 23 62, 64 generation of 55 autotrophic organisms 6, 16 cell walls, bacteria 26, 32 propagation of 58–59 axons 52, 54 chemical control of pests 37 and withdrawal reflex 63 action potentials 55–56 action spectrum 18 membrane structure 54 active neurones 55–56 myelinated 52, 53, 59, 62 active transport 47, 48, 54, 55 and nerve impulse adenine 6 transmission 57–58 adenosine triphosphate (ATP) 6–7 questions & answers 71–72 and respiration 8–15 synthesis 11–12, 20–21 ADH (antidiuretic hormone) 49, 50 aerobic respiration 8–9 chemiosmotic channel protein 12, 21 chemiosmotic theory oxidative phosphorylation 12 photophosphorylation 20–21 non-myelinated 57–58 chlorophylls 17, 18, 19, 20 permeability of 55 chloroplasts 16, 17, 19 Azotobacter 41 cholinergic synapse 60 B chromatography 17, 23 bacteria cell walls 26, 32 electron transport chain 11–12 classification 24–26 energy yield 15 culturing 26–27 glycolysis 9–10 denitrifying 40 Krebs cycle 10–11 Gram-negative 25, 26 link reaction 10 Gram-positive 25, 26 summary 12–14 nitrifying 40 classification of bacteria 24–26 cocaine 62 coenzymes 8, 9, 10, 11, 13 collecting duct 44–45, 49, 50–51 competition 34–36, 81–82 continuous fermentation 30 coordinators 49, 51, 52 countercurrent-multiplier system 48 all-or-nothing law 57 and nitrogen cycle 40–41 ammonia 42, 43 nitrogen-fixing 41 ammonification 40 penicillin’s effect on 32 anaerobic conditions, population growth 27–30 D saprophytic 40 day-neutral plants 66 denitrification 40 anaerobic respiration 14–15 energy yield 15 animals culturing bacteria 26–27 cyclic phosphorylation 21 batch fermentation 30 deamination 42 bioaccumulation 37 death/decline phase 28 biological control of pests 37–38 death rates/deaths 33, 34–35 anaerobic respiration 14, 15 biomagnification 37 decarboxylation 10 excretory products 42–43 birth rates/births 33, 34–35 dehydrogenation reactions 10 nervous system 52–64 Bowman’s capsule 45–46 dendrites 52, 60 Unit BY4: Metabolism, Microbiology and H omeostasis 93 Index use in penicillin denitrification 40 denitrifying bacteria 40 density of populations 34–35 diameter of axon and impulse speed 57 dilution plating 29–30 Diplococci 25 distal convoluted tubule 44–45, 49, 50 dopamine 62 drugs, effect on synaptic transmission 61–62 E light-dependent stage, production 31–32 photosynthesis 20–21 light-independent stage, G photosynthesis 22–23 genetic resistance 37 glomerulus 45–46 glycolysis 9–10 Gram stain procedure 25–26 limiting factors, growth 28 link reaction 10, 13, 14 log phase, population growth 28 long-day plants 66, 67–69 loop of Henle 44, 45, 47–49 H hexose bisphosphate 9, 13 M homeostasis 49–51 mammals Hydra 64 kidneys 43–49 effectors 49, 51, 52, 64 hydrostatic pressure 46 nervous system 52–64 electrochemical nature of nerve hyperpolarisation 56, 61 marijuana 62 hypothalamus 50 membrane structure 53 impulses 53–54 electron transport chain 11–12, 14 endergonic reaction energy carrier, ATP 6–7 energy yield, respiration 15 excitatory drugs 62 excretion 41–42 homeostasis 49–51 mammalian kidney 43–49 nitrogenous waste 42–43 exergonic reaction 6, 10 exponential growth 28 F facultative anaerobes 29 far-red light 66–67 fermentation 30–32 florigen 69 industrial fermentation, penicillin 31–32 inhibitory drugs 62 inhibitory synapses 61 insecticides 62 integrated pest management 38 intercellular spaces 47 interspecific competition 35, 36 intraspecific competition 35, 36 involuntary (of reflexes) 62 94 population growth 27–30 questions & answers 78–80 microvilli 46 mitochondrial matrix 10 mitochondrion, structure of molecular filter, ultrafiltration 45 motor neurones 52–53 myelinated axons 62 N K negative feedback 49 kidneys gross anatomy 43–44 nephron 44–49 questions & answers 83–85 Krebs cycle 10–11, 13 nephron 44–45 Bowman’s capsule 45–46 collecting duct 48–49 glomerulus 45–46 loop of Henle 47–49 proximal convoluted freshwater fish, ammonia anaerobic respiration 15 penicillin production 30–32 jellyfish, nerve net 64–65 manipulation of 68–69 fungi culturing bacteria 26–27 myelin sheath 53, 58 effect of photoperiod 66–67 excretion 42 bacterial classification 24–26 J flowering of plants fossil fuels and carbon cycle 39 microbiology 24 I L tubule 46–47 lag phase, population growth 27–28 nerve impulses 53–57 transmission of 57–59 WJEC A2 Biology Index nerve nets, jellyfish 64–65 oxidative phosphorylation 11, 12–13 nervous system 51 phytochrome 66–67, 69 pigments, photosynthetic 17–19 jellyfish 64–65 oxygen and bacterial growth 29 pituitary gland 50, 51 mammalian 52–64 P plants questions & answers 86–88 neural pathway 52 neurones 52–53, 59–64 neurotransmitters 59, 61–62, 63 nitrification 40 nitrifying bacteria 40 nitrogen cycle 40–41 questions & answers 89–90 nitrogen fixation 41 nitrogen-fixing bacteria 41 nitrogenous waste, excretion of 41–43 nodes of Ranvier 52, 53, 58–59 non-competitive inhibitor 12 non-cyclic phosphorylation 20–21 non-myelinated neurones 57–58 non-selective pesticides 37 noradrenaline 59 nucleus 52 nutrient cycles 39–41 nutrients and growth 28 Paramecium 35–36 penicillin production 30–32 Penicillium notatum 31 peripheral nervous system 52 permeability ADH affecting 49, 50 of axon membranes 55, 56 loop of Henle, nephron 47–48 persistent pesticides 37 pest control 33 biological 37–38 chemical 37 integrated 38 pesticides 37 pests 33, 37–38 phases of population growth, bacteria 27–28 pH and bacterial growth 29 anaerobic respiration 14–15 photoperiodism 66–69 photosynthesis 16–24 podocytes 45–46 population growth measuring 29–30 optimum conditions 28–29 phases of 27–28 populations 33 growth of 27–30, 33–34 limits to growth 34–36 nutrient cycles 39–41 pests and pest control 37–38 questions & answers 81–82 posterior pituitary gland 50 potential difference 53–54, 55, 56 primary metabolites 30 proximal convoluted tubule 44, 45, 46–47 phosphorylation 9 oxidative 11–13 substrate-level 10 photolysis of water 20, 21 psychoactive drugs 62 putrefaction 40 pyruvate (pyruvic acid) 9, 10, 13, 14–15 photoperiod 66 O obligate aerobes 29 obligate anaerobes 29 optimum conditions, bacterial growth 28–29 organophosphorus insecticides 62 detection of 66–69 photoperiodism 66–69 photophosphorylation 16, 20 chemiosmotic theory 20–21 R reabsorption of water 49 receptors 49, 51, 52, 62, 63 red light 66, 67 photoreceptors 64 redox reactions 7–8 photosynthesis 16–17 reduction 7–8 ornithine cycle 43 light-dependent stage 20–21 reflex arc 62, 63, 64 oscilloscope readings 54 light-independent stage 22–23 reflexes 62–64 osmoreceptors 50, 51 photosynthetic pigments 17–19 refractory period 57 osmoregulation 50–51 questions & answers 75–77 relay neurones 52, 63, 64, 86, 88 osmosis 46–47, 48, 49 photosynthetic pigments 17–19 oxidation 7–8, 10 photosystems 18–21 Unit BY4: Metabolism, Microbiology and H omeostasis repolarisation, axon membranes 56 95 Index spinal cord 62–63, 64, 86–87 thylakoid membranes 18, 19, 20–21 aerobic 8–14 spinal reflexes 62–64 total count, population growth 29 anaerobic 14–15 Staphylococcus aureus 25 touch-sensitive nerve cells 64 questions & answers 73–74 stationary phase, population triose phosphate 9–10, 13, respiration 7–8 respiratory substrates, alternative 15 responses in plants 65–69 stimuli 51 stimulus–response control systems 51 resting neurones 55 resting potential 54, 55, 56, 57 Streptococci 25 Rhizobium 41 substrate-level phosphorylation 10, 12 ribose 6 Rubisco 22 summation, synapses 61 synapses 59 S 14–15, 22 growth 28 U ultrafiltration 45–46 universal energy currency, ATP 6 urea, excretion of 42, 43 uric acid 43 urinary tract anatomy 43–44 effect of drugs 61–62 saltatory conduction 58–59 V functions of 61 viable count 29 saprophytic bacteria 27–28, 40 transmission via 59–61 W Schwann cells 53, 59 synaptic knob 59 secondary metabolites 30, 31 synaptic vesicles 59 selective reabsorption 46, 47 sensory neurones 52, 62, 63, 64 T temperature serial dilution 29–30 and impulse speed 58 short-day plants 66 and population growth 29, 35 water, photolysis of 20, 21 water potential of blood, control of 50–51 water potential gradients, loop of Henle 48–49 wavelengths of light 18, 66 spatial summation 61 temporal summation 61 Z speed of nerve impulses 57–59 threshold, action potentials 57 Z-scheme 20 96 WJEC A2 Biology