844683_AQA A-Level Biology_001-048.indd

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ContentsSection 5

Energy transfers in and between organismsPhotosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Energy and ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Nutrient cycles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Section 6

Organisms respond to changes in their internal and external environmentsStimuli, both internal and external, are detected and lead to a response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Nervous coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Skeletal muscles are stimulated to contract by nerves and act as effectors . . . . . . . . . . . . . . . . . . . . . . . . . . 37Homeostasis is the maintenance of a stable internal environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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This workbook will help you to prepare for your AQA A-level exam.

Your exam is 2 hours long for each of three A-level papers.

For each topic there are:• stimulus materials, including key terms and

concepts• short-answer questions that build up to

exam-style questions• spaces for you to write or plan your answers• questions that test your mathematical skills

Answering the questions will help you to build your skills and meet the assessment objectives AO1 (knowledge and understanding), AO2 (application) and AO3 (analysis).

You still need to read your textbook and refer to your revision guides and lesson notes.

Mark schemes are provided for some of the questions so that you can see what the examiner wants and where the marks come from.

Timings are given for the exam-style questions to make your practice as realistic as possible.

Answers are available at: www.hoddereducation.co.uk/workbookanswers

844683_AQA A-Level Biology_001-048.indd 2 13/04/2016 17:45

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Stim

uli, both internal and

external, are detected

and lead

to a response

SE

CT

ION

6Section 6Organisms respond to changes in their internal and external environments

Stimuli are changes in the environment, which may be the external environment or the organism’s internal environment. Stimuli are detected by receptors, and the information is passed on to a coordinator. This determines a suitable response, which is carried out by an effector. Receptors are specific to particular kinds of stimuli. Nerve cells, or neurones, carry electrical impulses

along their whole length. Nerve impulses are specific to the target cell only. Nerves produce a fast, short-lived and localised response. Hormones, by contrast, travel round the body in the blood. They stimulate specific receptors on their target cells, producing slow, long-lasting and more widespread responses. Plant responses are controlled by hormone-like growth substances.

Stimuli, both internal and external, are detected and lead to a responseSurvival and responseThe ability to respond to changes in the environment increases an organism’s chances of survival. In flowering plants, specific chemical growth factors move from growing regions to other regions. Here they stimulate a directional growth response to a directional stimulus. One chemical growth substance is indoleacetic acid (IAA). This causes a growth response in roots and shoots in response to light and gravity.

Many simple animals show taxes and kineses. These are simple movement responses that keep the organism in a favourable environment.

Vertebrates have simple, three-neurone reflexes involving the spinal cord.

● 1 What is a tropism? (AO1) 2 marks

● 2 A plant root was described as being positively geotropic and negatively phototropic. What does this mean? (AO1) 2 marks


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● 3 The diagram shows two important investigations into phototropism using coleoptiles.

Coleoptile

Mica barrier inserted on one side Apex cut off and then replaced witha gelatine block underneath it

Investigation A: Darwin (1880)

Investigation B: Boysen-Jensen (1913)

Directionof light

Apex cut off Apex shielded withlightproof cap

Intact

1 2 3

4 5

a What conclusions could Darwin make from investigation A? (AO3) 3 marks

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2

3

b Mica does not let chemicals or electrical signals pass through. Gelatine allows chemicals to pass through but not electrical signals.

i What conclusions could Boysen-Jensen make from experiment 4 in investigation B? (AO3) 2 marks

ii What conclusions could Boysen-Jensen make from experiment 5 in investigation B? (AO3) 3 marks


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Stim

uli, both internal and

external, are detected

and lead

to a response

SE

CT

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6

● 4 In a different investigation, a scientist removed the tip of the coleoptile and replaced it on one side of the shoot in uniform light conditions, as shown.

Sketch the result you would expect to see after 2 days. Explain your answer. (AO3) 3 marks

● 5 The diagram shows the growth of a young root and shoot from a germinating seed.

Shoot Root

a Describe the response shown by the root and shoot to gravity. (AO2) 1 mark

The graph shows the effects of different auxin (IAA) concentrations on root and shoot growth.

0

10–6 10–5 10–4 10–3 10–2 10–1 1 10 100 1000

Deg

ree

of

stim

ula

tio

nD

egre

e o

f in

hib

itio

n

Auxin concentration/parts per million

Roots

Shoots

Tip removed Tip put on oneside of coleoptile


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b Use the graph to explain the different responses of the root and shoot to gravity. (AO2) 4 marks

● 6 IAA (auxin) loosens the bonds between fibres in the cell wall. Use your knowledge of osmosis to explain how this can lead to cell elongation. (AO2) 3 marks

● 7 What is:

a taxis

b kinesis

(AO1) 2 marks

● 8 Ants navigate to and from their nests by keeping a constant angle between their direction of movement and the direction of the Sun. What type of behaviour is this? (AO2) 1 mark

● 9 The flatworm, Dendrocoelum lacteum, increases its rate of turning as light intensity increases.

a Name this type of behaviour. (AO1) 1 mark

b Suggest the advantage of this behaviour to the flatworm. (AO2) 2 marks


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844683_AQA A-Level Biology_001-048.indd

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