AQA Unit 4 questions and marking points

Marking points for the 5 mark questions found at the end of the unit 4 paper

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Explain how the light-dependent reaction generates

May also be worded as "Explain the process of the light-dependent reaction" - Jan10, Jan13

  • Chlorophyll absorbs light energy
  • Light energy excites electrons in the chlorophyll
  • Electrons passed down the electron transport chain
  • Release energy as they are passed between carriers/involved in redox reactions
  • Energy used to join ADP + Pi to form ATP
  • Photolysis of water releases protons, electrons and oxygen*
  • NADP reduced by electrons*

*NB these answers are usually only required when asked about the process of the light-dependent reaction, not production of ATP

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Describe and explain the efficiency of energy tran

Jan10, Jan13

  • Light energy not appropriate wavelength/reflected
  • Some light energy doesn't hit chlorophyll/photosynthetic pigments
  • Low efficiency of photosynthethis/approximately 2%
  • Not all plant is eaten/energy lost through respiration/faeces
  • Energy lost in heat production
  • Carnivores use more energy
  • Efficiency of transport to consumers > efficiency of transport to producers/approx. 10%
2 of 16

Describe the light-independent reaction of photosy

Also written as "Explain how the carbon in carbon dioxide becomes carbon in triose phosphate" - Jun10

  • CO2 combines with RuBP
  • Produces 2 x GP
  • Reduced to TP
  • Requires/uses reduced NADP
  • Energy from ATP
3 of 16

Describe how changes in CO2 concentration affect p

Over 24 hours, at different heights - June10

  • No photosynthesizing at night - high CO2 conc. at night
  • Light required for light dependent reaction
  • Plants only respire at night
  • In day, plants use more CO2 than they produce
  • Rate of photosynthesis > rate of respiration 
  • Decrease in CO2 with height 
  • At ground level, fewer leaves/photosynthetic tissues so increase of CO2
4 of 16

Describe and explain how succession occurs


  • Colonization by pioneer species
  • Change in environment (e.g. formation of humus caused by present organisms)
  • Enables other species to colonize and survive
  • Change in diversity/soil development
  • Stability increases/environment less hostile
  • Forms climax community
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Describe the advantages and disadvantages of biolo



  • Specific to one pest
  • Only requires single application/reproduces
  • Maintains low pest population
  • Pests do not develop resistance
  • No chemical bioaccumulation
  • Can be used in organic farming


  • Does not totally eliminate pest
  • May become pest itself (e.g. cane toads)
  • Slow acting/lag phase
6 of 16

Explain how diversity is evolved within a species


  • Geographical isolation
  • Seperate gene pools/no interbreeding between populations
  • Variation due to mutation
  • Different selection pressures
  • Select for advantageous characteristics within one species
  • Selected organisms survive and reproduce
  • Leads to change in allele frequency
  • Occurs over a long period of time
7 of 16

Explain how intensive rearing of livestock increas


  • Slaughtered when still growing so more energy transferred to biomass
  • Fed concentrate food so higher proportion of digested food/less lost in faeces
  • Movement restricted so less respiratory loss
  • Kept inside/less heat loss/no predation
  • Genetically selected for high productivity
8 of 16

Describe the carbon cycle

Also worded as "describe how carbon compounds in a dead organism become available to cells in a leaf" - June10

  • Microorganisms are saprobiotic
  • Secrete enzymes onto dead tissue/extracellular digestion
  • Absorb products of digestion
  • Respiration of microorganisms produces carbon dioxide
  • Carbon dioxide taken into leaves
  • Absorbed through stomata
9 of 16

Describe how photosynthesis converts CO2 into orga


  • CO2 combines with RuBP
  • Produces 2 x GP
  • Reduced to 2 x TP
  • Using Reduced NADP
  • Using energy from ATP
  • TP converted back to RuBP/other organic substances (e.g. glucose)
  • In light-independent reaction
10 of 16

Explain the use of ATP in biological processes


  • Releases energy in small/managable amounts
  • Broken down in one step/single bond broken
  • Immediate source of energy for cells
  • Phosphorylates/adds phosphate
  • Lowers activation energies/makes phosphorylated substances more reactive
  • Reformed/made again
11 of 16

Describe how ATP is made in the mitochondria

Basically asking to describe respiration - Jan12

  • ATP produced in Kreb's cycle
  • Kreb's cycle produces reduced NAD/reduced FAD
  • Link reaction produces reduced NAD
  • Electrons released from reduced NAD/reduced FAD
  • Electrons passed along electron carrier chain through series of redox reactions
  • Energy released
  • Used to join ADP + Pi
  • Protons move into intermembrane space
  • Create proton gradient
  • Through ATP synthase
12 of 16

Explain the importance of ATP produced by respirat


  • No ATP production in the dark by photosynthesis
  • Some tissues unable to photosynthesise and produce ATP
  • ATP cannot be stored
  • Plants use more ATP than produced in photosynthesis
  • ATP for active transport
  • ATP for synthesis (of a named substance)
13 of 16

Explain how farming practices increase net product


  • Fertilisers added to soil
  • Add nitrates to soil (proceed to explain role of nitrates in crop growth)
  • Pesticides/biological control agents prevent damage to crops
  • Pesticides/weed killers remove competition for minerals, sunlight etc.
  • Selective breeding and genetic modification to produce high yielding crops
  • Greenhouses increase temp and CO2 concentration (explain how this increases photosynthesis and plant growth)
  • Ploughing aerates soil
  • Ploughing allows nitrification/decreases denitrification
  • Irrigation/remove water as a limiting factor
  • Protection of crops from pests
14 of 16

Describe how microorganisms produces nitrates for


  • Proteins/amino acids broken down to ammonium compounds/ammonification occurs
  • By saprobionts
  • Ammonium to nitrites
  • Nitrite to nitrates
  • By nitrifying bacteria
  • Nitrogen to ammonium compounds
  • By nitrogen-fixing bacteria in soil
15 of 16

Explain how pesticides lead to resistance in pest


  • Variation in pest species
  • Due to mutation
  • Causes allele for resistance
  • Natural selection
  • Pests with resistance survive to reproduce and produce pesticide resistant offspring
  • Increase frequency of resistant allele
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