AQA A level Biology Unit 5

7/24/2019 AQA A level Biology Unit 5

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NERVOUS SYSTEMOrganisms increase their chance of survival by responding to changes in their environment.Stimulus: Change in an organisms environment that can be detected by receptor cellsReceptor: Specialised cell that detects a stimulus and initiates a nerve impulse by creating a generatorpotentialKinesis:Change in the speed of random movement in response to environmental stimulus (non-directional response!"ample of Kinesis:#oodlice #ho breathe using gills$ use %inesis to stay in damp environments$ in dry

environments$ they move &uic%ly so as to e"it the dry environment and enter thedamp environment

'a"es:irected movement to#ard or a#ay from a stimulus (directional response)ositive 'a"es: Organism moves to#ards stimulus

*egative 'a"is:Organism moves a#ay from stimulus'ropism:Response to stimulus by gro#ing in a certain directionCentral *ervous System:'he brain and the spinal cord)eripheral *ervous System:)airs of nerves that go to and fromthe brain and spinal cord (the C*SSensory *eurones:Carries nerve impulses from receptors 'Othe C*S+otor *eurones:Carries nerve impulses ,RO+ C*S to theeectors$ it is of t#o types:

/oluntary *ervous System: Caries nerve impulses to bodymuscles and is voluntary

0 1utonomic *ervous System: Carries nerve impulses toglands$ smooth muscles and cardiac muscle and isinvoluntary

'he 1utonomic *ervous System is of t#o types: Sympathetic *ervous System: Stimulates eectors and therefore speeds up any activity0 )arasympathetic *ervous System: 2nhibits eectors and therefore slo#s do#n any activity

'he Sympathetic *ervous System and the )arasympathetic *ervous System are antagonistic. 1ne"ample is found in control of heart rate:'he heart is controlled by the +edulla Oblongata in the brain. 'he +edulla Oblongata speeds up heartrate via the Sympathetic nerve and slo#s do#n the heart via the )arasympathetic nerve.'he +edulla Oblongata changes the heart rate in response to t#o receptors:

Chemoreceptors in the Carotid and 1ortic arteries #hich are sensitive to a change in CO0level inblood3hen CO0levels high (e.g. during e"ercise:

'here is a change in p4 #hich is detected by chemoreceptor

Chemoreceptor sends impulses to +edulla Oblongata

+edulla Oblongata increases fre&uency of impulses do#n Sympathetic nerve to S1*

4eart beats faster 2ncreased

blood 5o#removes CO0faster

0 )ressurereceptors in

the CarotidSinus #hich are

sensitive toa change inblood pressure

2f blood pressure ishigh$ pressure

receptorscause +edulla

Oblongata toincrease fre&uency of impulses do#n the )arasympathetic nerve to slo# do#n the heart rate


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Some other receptors: )acinian Corpuscle6 it is a

pressure receptor found on thes%in and it detects SPECIFICpressures and vibrations

)ressure on s%in changes shape of pacinian corpuscle and 7amella layers are distorted

istortion of lamella layers causes Stretch +ediated Sodium Channels to open #hich areon the sensory neurone

Sodium Channel allo# positive *a8ions to enter the negative sensory neurone causing

depolarisation

'his is %no#n as a generator potential$ if it reaches the threshold it triggers an action

potential

'he amount of sodium channels that open depends on the pressure applied by the

stimulus

'herefore ob9ects #ith small surface area cause a large stimulus as they cause a lot ofpressure$ li%e a pin

0 Rod and Cone Cells6 they are photoreceptors thatare found on the retina and they detect light$ theythen send impulses to the Optic *erve #hich ta%esit to the brain

,eature Rods ConesShape Rod-shaped outer segment Con-shaped outer segment

Connections +any rods converge to one neurone(R!'2*17 CO*/!R!*C!

Only single cone per neurone

/isual 1cuity 7o# giving unclear image1s brain cannot distinguish bet#een the

separate rods that generated theimpulse

4igh giving sharp image1s brain can distinguish #here the

impulse came from as impulse #as sentby single cone

/isual)igments

Rhodopsin 2odopsin

istribution ,ound evenly all over retina 1ll over retina but more concentrated at

fovea$ therefore #e move our heads tosee stu properlySensitivity /ery sensitive to light due to R!'2*17

CO*/!R!*C!Only functions in bright light$ i.e not as

sensitive to lightOverall

,unction;lac% and #hite vision in poor light Seeing colour and detail in bright light

R!'2*17 CO*/!R!*C!:


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Stimulation of several rods results in enough *eurotransmitters being released to reach the thresholdvalue for an action potential in the bipolar neurone in lo# lightintensities (also called spatial summation3hy #e have a high degree of visual sensitivityin lo# lightlevels:

Several rod cells connected to each bipolar cell0 1dditive eect of small amount of light stri%ing several rod

cells?m/'he +yelin Sheath:provides protection and electrical insulation


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1ction )otential: epolarisation:

*eurone stimulated causing a fe# chemical-gated *a8channels to open allo#ing *a8ions

to diuse into a"on

2f stimulus large enough to ma%e enough *a8move in that the potential changes from ->?

to -A? (threshold value$ then all the other *a8channels open that are /O7'1! gated

+ore *a8enter (positive feedbac% B change creating

more change0 Repolarisation:

3hen potential reaches 8?m/$ then K8channelsopen causing K8to rush out ma%ing potentialnegative again


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I Sodium channels on post synaptic membrane open causingan in5u" of sodium ions resulting in a depolarisation

> *eurotransmitter bro%en do#n by enEyme1cetylcholinesterase in the cleft

D 'he products are reabsorbed by pre-synaptic %nob #herethey are re-synthesis using energy from 1') inmitochondria

'emporal Summation:#here t#o or more impulses arrive in &uic% succession do#n the same neurone$due to the post synaptic membrane having ahigh threshold

Spatial Summation: #here t#o or more impulses arrive at the same time do#n dierent neurone$ dueto the post synaptic membrane having a highthreshold

4o# transmission of information in the nervous system may be modiJed by summation: Summation is the addition of a number of impulses converging on a single post synaptic neurone0 'his allo#s integration of stimuli from a variety of sources$ this is called S)1'217 SH++1'2O*


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'he mitochondria in the slo#-t#itch Jbre #ill be distributed to the edges of the Jbre because:

1llo#s rapid diusion of O"ygen

O"ygen is used in the mitochondria

A Site for Krebs Cycle and!lectron 'ransport Chain

0 ,ast-t#itch Jbres: ,OH* 2* ,2*!R+HSC7!S$ 1R+ +HSC7!S

Contract more rapidly and

provide more po#erfulcontractions over a shortperiod$ therefore for intensee"ercise

'hey are adapted to their rolefor anaer)ic respiratinby:

'hic%er and morenumerous myosinJlaments

0 4igh concentration of enEymes used in aerobic respiration


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'he +yosin: has a tail (Jbrous protein0 has a head (globular protein #hich contains 1)

1ctin:

1 globular protein #hich has 1C'2/! S2'!S that are covered by a protein called 'RO)O+FOS2*

3hen muscle contracts: 2-;ands become narro#er0 L-7ines move closer

together


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Sliding ,ilament 'heory: 1ction potential arrives through neuromuscular 9unction (motor neurone meets muscle Jbre0 1ction potential goes through tubules #hich are connected to the endoplasmic reticulum of the

muscle 'he myosin head change their angle pulling actin Jlament 2*31RS$ as they do this$ the 1) onthe myosin head is released

D 1') attaches to myosin head causing it to detach from active site of 1ctinM Ca08ions activate enEyme 1')ase$ #hich hydrolyses the 1') on the myosin head to 1) #hilst

releasing energy to allo# myosin head to retain its angle? +yosin head no# attaches itself to another active site on the actin that is further along

the actin Jlament

4o# you #rite the above in the e"am: Calcium ions bind to troponin0 'his removes the 'RO)O+FOS2* in the actin$ therefore e"posing actin binding sites )hosphocreatine allo#s regeneration of 1') #ithout respiration by releasing )i #hich combines

#ith 1) to form 1')!nergy for muscle contraction is provided by:hydrolysis of 1') to 1) and )i!nergy (1') is used for:

1ttachment bet#een actin and myosin0 )ulling of actin


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Sometimes a muscle may be so activethat O"ygen is rapidly used up andtherefore 1') has to be generated using achemical called )hosphocreatine.)hosphocreatine is stored in muscle andbrea%s do#n into 1') #hen O"ygen isbeing rapidly used up. 3hen the musclesare rela"ed$ the phosphocreatine is made$and #hen short burst e"ercise ta%esplace$ e.g. ??m sprint$ thenphosphocreatine is converted to loads of1')

'hey could give a vague statement li%e:4o# energy released in mitochondriaduring respiration produces contraction ofa muscle Jbril:

4ydrolysis of 1') releases energy0 !nergy used to form cross-bridges

bet#een actin and myosin


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!ctotherms (cold blooded:control their body temperature by behavioural means only$ *Ophysiological cooling=heating mechanism$ therefore usually 1ir 'emperature N;ody 'emperature

'emperature is detected by hypothalamus #hich has a @heat loss and @heat gain centre'he s%in plays an important role for regulating temperature as it serves as a receptor.3hen #e are too cold$ @heat gain centre in hypothalamus detects this via a receptor in the s%in andnhibits the @heat loss centre$ then the @heat gain centre sends nerve impulses to appropriate part ofthe body to do the follo#ing:

Shivering: creates heat0 /asoconstriction: constriction of arteriole #alls allo#s less heat in blood to be lost from s%in


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'herefore fe# collisions and thus fe#er enEyme-substrate comple"es formed$ therefore too slo#reactions

'he above t#o situations are both *!1'2/! ,!!;1CK3hy the activity of !ctotherms that live in deserts varies so much:

'heir body temperature varies #ith that of environment0 'emperature of desert 5uctuates greatly


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R!+!+;!R:1s mentioned$ the cells that contain the glucagon receptors are only those #hich containglycogen$ one of the places #here these cells are found is in the 72/!R

'he above t#o situations #ere e"amples of *!1'2/! ,!!;1CK$ the high glucose concentration isreduced and lo# glucose concentration is increased through negative feedbac%.Hsually$ after eating something$ your glucagon levels #ill do do#n as glucose levels #ill be going up.!ven though people #ith diabetes have high glucose levels$ their glucose levels can still go do#n#ithout insulin$ this is through:

'he loss of glucose in urine0 lucose gets used up in cell respiration

3hen a diabetic person ta%es an insulin in9ection$ and then does not eat a meal$ his glucose levels donot fall dangerously lo# because:

lucagon is still active #hich produces glucose using lycogenolysis0 )erson is not active so little glucose used up in respiration

'here is one other hormone that can increase glucose levels:1drenaline: 1ctivates an enEyme called 1denyl Cyclase that causes the brea%do#n of glycogen toglucose

0 lucose produced is then usedfor respiration (this is #hy you canrun much faster #ith adrenaline


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'he corpus 7uteum is essential as it produces )rogesterone #hich maintains the uterus lining.1lthough fertilisation happens after ovulation$ se"ual intercourse can ta%e place a day before ovulationas sperm can survive for a fe# days.f egg is not fertilised:

Corpus 7uteum degenerates and stops producing progesterone0 *o more progesterone causes uterus lining to brea% and also allo#s ,S4 to be released #hich

causes follicle development$ thus restarting the cycle!vidence that fertilisation has not occurred: )rogesterone levels drop0 1nother follicle develops

f egg is fertilised: 1 hormone called 4uman Chorionic onadotrophin (4C acts as a signal to Corpus 7uteum to

%eep producing progesterone0 )rogesterone maintains uterus lining

4o# #ould you %no# from a graph that fertilisation has occurred=not occurred:See if the )rogesterone concentration has been maintained= has fallen4o# ,S4 release is controlled by negative feedbac% (i.e. ho# do #e ma%e sure ,S4 doesnt %eeprising:

,S4 stimulates development of follicles #hich release Oestrogen0 Oestrogen inhibits ,S4

4o# 74 concentration is controlled by negative feedbac% (i.e. ho# do #e ma%e sure ,S4 doesnt %eeprising:

74 rise causes a rise in )rogesterone0 )rogesterone inhibits 74

Role of 74 and ,S4 in the +enstrual Cycle: ,S4 stimulates gro#th of a follicle #hich produces Oestrogen0 74 causes ovulation


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'herefore there #ill remain a high concentration of ,S4 (#hich #ont do anything becausefollicles are inactive

,or a girl$ the Jrst ovulation ta%es place late in puberty$ this is advantageous because:!nsures se" organs are mature before conception can occur

*.;. 3henmale s#eat

s e"posed toa female$ the

pheromonesn the male

s#eat #illstimulatethe

4ypothalamus to produce ,S4.

)lant ro#th ,actors: 'hey are a type of hormone$ but unli%e animal hormones$ they are made by cells throughout the

plant rather than by particular organs0 Hnli%e animal hormones$ some plant gro#th factors actually aect the tissues that release them


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0 ,ertilisation is random4o#ever plants gro#n from tissue culture #ill be clones as they are produced via mitosis.

)aracrine Signalling: communication bet#een close cells using chemicals called 7ocal Chemical+ediators

Chemical mediators are released into tissue 5uid but not into the blood6 therefore they onlyaect cells that are in their immediate vicinity!"amples of chemical mediators are 4istamine and )rostaglandins

.N' TR'NSCRIPTION , TR'NS('TION*1: made up of t#o polynucleotides containing se&uences of nucleotide bases that determine the

se&uence of amino acids)rotein is made in the ribosomes$ but *1 is too large to leave the nucleus$ therefore mR*1 ta%es thecode out of the nucleus through the nuclear pores.ene: 1 speciJc section of a chromosome that codes for an amino acid'R2CK H!S'2O*: #hy are the percentages of bases from the middle part of the chromosome and theend part dierentG'hey are dierent genesQ 'herefore they #ill have dierent base se&uences


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enetic Code:Se&uence of nucleotide bases on the mR*1 that code for amino acidsenetic Code:

Hniversal6 same codons code for same amino acid0 *on-overlapping6 each base is part of only one codon


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0 1cts as a template for a ne# strand in *1 Replication

3hy some errors during copying of a se&uence of bases are not severe: Some amino acids have more than one code$ therefore error may not aect amino acid0 2f amino acid is changed$ the ne# amino acid may not aect the functioning of the protein$ i.e.

the changed amino acid #as not an important amino acid in terms of the structure and functionof the protein

'ranslation: mR*1 leaves nucleus via nuclear pore and enters ribosome #here proteins are made #hich are

to be used in the cell0 !ach codon on the mR*1 has a complimentary anticodon on the tR*1


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'he active site shape no longer complimentary to shape of substrate

4o# mutation of base may cause the production of a complete ne# protein (a bit li%e the above: ;ase se&uence is changed0 ierent mR*1


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-ENE MO.IFIC'TION , ITS PROCESSenetic +odiJcation:

2solation:Jnding *1 fragments that have the desired gene that produces the desired protein$then separating it

0 2nsertion:putting the desired *1 fragments into a vector


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therefore they #ill use el !lectrophoresis to separate the fragments. (el !lectrophoresis #ill bee"plained shortly Once the fragments are separated$ they #ill use a *1 )robe (#ill be e"plainedshortly #hich #ill bind to a complimentary base se&uence in the gene$ the 5uorescence=radioactivityof the *1 )robe #ill tell us #hich fragment contains the desired gene. 'hen #e #ill movie ontonsertion$ etc.'he overall isolation using Restriction !ndonucleases:

Restriction enEymes cut *1 at speciJc base se&uences /21 4FRO7FS2S forming a stic%y end0 Same restriction enEyme also cuts *1 into #hich gene is inserted forming another stic%y end


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0 'herefore dierent proteins #ill be made


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isadvantages of using a virus to introduce genes into cells: +ay cause disease0 2mmunity may develop to the virus

n /itro'his process is automated$ ma%ing it both rapid and ecient*1 polymerase:1n enEyme that 9oins free nucleotides #ith their complimentary bases on the *1$

but it does #or% for the start and end se&uence of the *1 strand)rimers:have base se&uence complimentary to that of the start and end se&uence of the *1 strandPROCESS PO(YMER'SE C*'IN RE'CTION03

'he desired *1 fragments$ primers and *1 polymerase are placed in a vessel in athermocycler

0 'he temperature is increased to MAoC$ causing the t#o strands of the desired *1 fragments toseparate


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4o# )CR diers fromtranscription:

'ranscription usesR*1 )olymerase$ )CR uses*1 )olymerase

0 2n transcription$ thetemplate is one

strand$ in )CR$ the template ist#o strands

One use of )CR:

)rovides multiple copies of a*1 fragment

*umber of *1 moleculesproduced N Original number of*1 molecules 0*umber of )CR Cycles

2n /ivo 2n /itro

1dvantages

Hseful #hen #e #ish to introduce a geneinto another organism

0 *o ris% of contamination


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Changes to genetic ma%e-up of individual may aect normal developmentConcerns regarding ene 'herapy:

4igh cost compared #ith conventional treatments0 Hn%no#n side-eects


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I 2n this #ay *1 fragments of dierent lengths are separatedn the e"am$ for el !lectrophoresis you #ill #rite:

Current s#itched on0 ,ragments move due to electrical attraction


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0 7ose #eight )attern uni&ue to every individual

Southern ;lotting is the method used for: !*!'2C ,2*!R)R2*'2*6 the dierences in *1 bet#een humans is found in the non-coding

*1$ #hich is repetitive'he probability of t#o organisms having the same Jngerprints is very lo#6 about in ?M

0 'o identify restriction fragments containing a particular gene


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0 !mbryo has potential for development to person therefore could be regarded as murder

Completely irrelevant point you should remember:3hen substance comes out in faeces$ #e say it hasnot been absorbed$ but #hen a substance comes out inurine$ #e say it has been absorbed but not used for gro#th.

R!+!+;!R: 7ysosomes are substances that brea% do#n tissue

SYNOPTIC ESS'Y TOPICSStructure of lucose 4o# heart beat is initiated and coordinated


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,ormation of disaccharide from 0monosaccahrides using condensation

(removing #ater

3hat are pathogens

,ormation of 0 monosaccahrides from adisaccharide using hydrolysis (adding #ater

4o# valves in heart maintain one #ay 5o# ofblood

Starch igestion )hysical barriers against pathogens7actose 2ntolerance )hagocytosis

,ormation of dipeptide from 0 amino acidsusing condensation (removing #ater

Cell-+ediated response

,ormation of 0 amino acids from dipeptideusing hydrolysis (adding #ater

4umoral response and ho# it involves a primaryand secondary response

Structure of amino acid 1ntigens and antigenic variabilityStages of globular protein B primary$

secondary$ tertiary$ &uaternary1ntigen-1ntibody comple" (remember

antibodies are made by ;-7ymphocytes!nEyme-substrate comple"es 3hat are +onoclonal 1ntibodies

!ect on temperature=p4=substrateconcentration on !nEyme

)rocess of /accination and ethical issuesconcerning it

Competitive and *on-competitive inhibitors Causes of variation (mutations$ meiosis$ fusionof gametes$ environmental in5uences and

diversity inde"

'est for starch$ lipids$ reducing sugars$ non-reducing sugars$ protein

Sampling (involves bias$ chance$ sample siEe$etc.

Organelles of a cell and their functions Structure of *1 and ho# it is related to itsfunction

)rocess of ultracentrifugation 4omologous chromosomes (structure andfunction and alleles (#hat it is and #here its

found+agniJcation 'riplet Code

'!+ and S!+ )rocess of +eiosis and ho# it provides variationStructure of partially permeable membrane of

cell/ertical ene 'ransmission and 4oriEontal(con9ugation ene 'ransmission and ho#

antibiotics #or%Structure of the 0 7ipids$ 'riglycerides and)hospholipids B i.e. ho# they are made of

fatty acids and glycerol molecule

Selective ;reeding$ enetic ;ottlenec% and,ounder !ect and ho# they reduce genetic

diversity,unction of cell membrane 'hrombosis

!"trinsic proteins and 2ntrinsic proteins andtheir involvement #ith facilitated diusion

Structure of 4aemoglobin

3hat is 7ipid diusion through )hospholipidbilayer

Role of 4aemoglobin (readily associate #ith O0at a gas e"change site and readily disassociate

at respiration site3hat is osmosis ;ohr !ect

3hat is active transport and ho# its relatedto carrier proteins only

iseases associated #ith 4aemoglobin

4o# products of starch are absorbed byepithelial cells$ role of epithelial cells and

adaptations

)roperties and components of Starch$ lycogenand Cellulose

Organelles of pro%aryotic cell and itsdierences from eu%aryotic cell

enetic comparison$ 1mino 1cid se&uencecomparison and 2mmunological comparison to

compare speciesCholera 'a"onomy and use of 4ierarchy

Oral Rehydration 'herapy Courtship behaviour and its importance3hat is ventilation

2nspiration and !"piration!ssential features of gas e"change and ho#

the lung is adapted for gas e"changeStructure of leaf )alisade Cell and its adaptation

for )hotosynthesis)ulmonary 'uberculosis Semi Conservative *1 Replication

)ulmonary ,ibrosis )rocess of gas e"change in insects1sthma Counter-current system in Jshes

!mphysema 4o# Jsh maintain continuous 5o# of #ater over


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gillsRis% factors of Chronic Obstructive )ulmonaryisease B li%e smo%ing$ genetic ma%e-up$ air

pollution

+itosis (2nterphase$ )rophase$ +etaphase$1naphase$ 'elophase and components of each

stage4o# 1trial systole and /entricular systole ta%e

place$ i.e. heart cycleSmall surface area to volume ratio and large

surface area to volume ratio and theirimplications

1theroma and ho# it causes heart attac% Structure of a plant leaf and its adaptation forgas e"change

,eatures of transport system Structure of +yoJbril

Structure of blood vessels (arteries$arterioles$ capillaries$ veins and ho# each

one is related to its function

)rocess of Sliding ,ilament 'heory

,ormation of tissue 5uid and its return tocirculatory system

Comparison of hormonal system #ith nervoussystem

+ovements of #ater through roots(1poplastic path#ay and Symplastic

path#ay$ absorption of #ater through roots

)lant gro#th factors

'ranspiration and factors aecting it(movement of #ater out of leaf

)rocess of 'ranscription and all that it involvesand its importance in protein synthesis

;iotic and 1biotic ,actors )rocess of 'ranslation and all that it involves andits importance in protein synthesis

7imiting factors of )hotosynthesis Comparison of R*1 #ith *1 and mR*1 #ithtR*1

reenhouses and #hat they do inside them 'ypes of mutations;iological and Chemical pesticides )rocess of controlling cell division

Hsing transect$ interrupted belt-transect$continuous belt transect$ point transect and

line transect for measuring abundance

)rocess of controlling the e"pression of a gene(SiR*1$ Oestrogen and 'ranscriptional ,actors

+ar% Release Capture$ ho# you do it andthings you ensure #hilst doing it

2solation (use of Restriction !ndonucleases andReverse 'ranscriptase

)opulation changes (demographic transitionand #hy they ta%e place 2n /ivo 2nsertion (use of *1 7igase

2nterspeciJc and 2ntraspeciJc competition 2n /ivo 'ransformation (use of Ca08ions'ransfer of energy through ecosystem andho# farmers ta%e measures to increase this$

*) N ) B R7

2n /ivo 2dentiJcation ( types of gene mar%ers

3hy 1') is a good source of energy and itsimportance

2n /ivo ro#th

1') synthesis from 1) and )i 2n /itro )CR7ight ependent Reaction and everything it

involves)rocess of locating genes using *1 )robes

7ight 2ndependent Reaction (C17/2* CFC7!

and everything it involves

3hat is gene therapy$ ho# could it be done and

is it eectivelycolysis and 7in% Reaction )rocess of Jnding the base se&uence of a gene

using el !lectrophoresisKrebs Cycle enetic screening and its implications

!lectron 'ransport Chain )rocess of enetic ,ingerprinting1naerobic Respiration and its implications enetically modiJed crops$ pros and cons

Carbon Cycle Hsing embryos in genetic modiJcation$ pros andethical arguments

*itrogen Cycle 2mportance of refractory period!utrophication Synaptic 'ransmission

eforestation and Conservation$ ethical

arguments for conservation and againstdeforestation

Structure of neurone and process of action

potential

)rocess of succession and role ofdecomposers

3rite any e"tra topics you need to learnhere:

+onohybrid 2nheritance (homoEygous andheteroEygous and its in5uence on genetic

disease li%e 4aemophilia


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4ardy-3einberg )rinciple and e&uationStabilising selection and irectional selection

)rocess of Speciation*ervous System and the basis of ho# it

#or%s (stimulus ...!ector)rocess of controlling 4eart Rate (negative

feedbac%)rocess of regulating body temperature

(negative feedbac%)rocess of regulating glucose concentration

(negative feedbac%)rocess of menstrual cycle (negative and

positive feedbac%)acinian Corpuscle and Rods=Cones (Retinal

ConvergenceRe5e" 1rc and its importance

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AQA A level Biology Unit 5