AQA Unit 4 questions and marking points

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

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%

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

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

Jan11

• 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

June11

Advantages

• 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

Disadvantages

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

Jan11

• 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

Jan10

• 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

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

June11

• 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

Jan12

• 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

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

Jan12

• 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)

June12

• 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

June12

• 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

June12

• 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