AQA biology a-level paper 2
describe and explain the steps in the light dependent reaction of photosynthesis -
ANS 1. photoionisation: light reaches chlorophyll in PSII, which is absorbed by
an electron, which becomes excited and moves to a higher energy level.
2. the electron passes to a carrier protein in the thylakoid membrane, and is
passed down a series of carrier molecules called an electron transfer chain.
3. as the electron moves down, energy is lost from the electron and is released as
ATP.
4. the loss of electron from PSII is 'refilled' by an electron produced by photolysis,
which also produces hydrogen and oxygen.
5. the lost electron reaches PSI, which absorbs light energy and boosts another
electron to a higher energy level (excitation).
6. this electron also goes down an electron transport chain.
7. this reaches the final electron acceptor which is a proton. they combine to form
H and reduce NADP to NADPH.
describe and explain the steps in the light independent reaction pf
photosynthesis. - ANS 1. CO2 diffuses into stroma and combines with ribulose
bisphosphate (RuBP) using the enzyme rubisco.
2. this forms an unstable 6 carbon molecule, which splits into 2 3 carbon
molecules, glyercate-3-phosphate (G3P) .
3. G3P is reduced by NADPH to triose-phosphate (TP), which is aided by ATP for
energy.
4. TP can be converted into useful organic substances.
5. TP can also be reformed into RuBP using ATP.
describe glycolysis in respiration. give net formation. - ANS 1. glucose is
converted into phosphorylated glucose by 2ATP. this makes it very reactive, so it
splits into 2 triose phosphate (TP).
2. 2TP is then oxidised by 2NAD and 4 ATP is formed to form pyruvate.
3. NET: 2ATP, 2Pyruvate, 2NADH, 2H+
describe links reaction in respiration. give net formation. - ANS 1. pyruvate
diffuses into the matrix of mitochondria.
2. pyruvate is oxidised by NAD. CO2 is lost. this forms acetate.
3. acetate and co-enzyme A combine to form acetyl co-enzyme A.
4. NET: CO2, reduced NAD, acetyl co-enzyme A
,describe krebs cycle in respiration. give net formation. - ANS 1. acetyl
co-enzyme A combines with 4 carbon molecule (oxaloacetate) to form 6 carbon
citric acid.
2. CO2 is lost (decarboxylation), molecule is oxidised by NAD and ATP is
produce. this forms 5 carbon compound.
3. it is oxidised by 2NADH and FAD, and is decarboxylated.
4. this forms 4 carbon molecule again.
describe oxidative phosphorylation in respiration. - ANS 1. reduced coenzyme
passes its H to a carrier protein in the ETC. this splits into a proton and electron.
2. the protons pass through the space between inner and outer mitochondrial
membrane.
3. electrons pass through proteins on ETC.
4. protons return back via ATP synthase in the membrane, producing ATP.
5. the protons and electrons recombine to form H, which combines with O to form
water.
6. oxygen is the last electron acceptor in the ETC.
define biomass - ANS the total mass of organisms in a given area
what is the 'gross primary production' - ANS the chemical energy stored in a
plants biomass
what is the 'net primary production' - ANS the chemical energy stores in a
plants biomass after respiratory losses have been considered. this energy is
available to consumers.
how can you calculate the net primary production? - ANS NPP = GPP - R
why is converting sunlight energy into biomass in producers inefficient? - ANS
some light isn't the correct wavelength to be absorbed
some light doesnt hit chloroplast
some light is converted into heat energy
some light energy is reflected
describe the nitrogen cycle. - ANS fixation:
atmospheric nitrogen can be fixed by rhizbium bacteria.
if struck by lightning, it becomes reactive and combines with oxygen to form NO.
ammonification:
, saprobionts feed on organic matter and release ammonia, which then forms
ammonium ions in the soil.
nitrification:
nitrifying bacteria convert ammonium ions into nitrite ions and then to nirate
ions.
denitrification:
anaerobic denitrifying bacteria convert soil nitrates into gaseous nitrogen.
homeostasis - ANS the maintenance of an internal environment within restricted
limits in organisms. all cells are in an environment that meets their requirements
and allows them to function normally despite external changes.
why is homeostasis important? - ANS 1. the enzymes that control biochemical
reactions in cells are sensitive to change e.g. in pH or temperature, which can
cause them to denature. homeostasis allows enzyme controlled reactions to take
place at a suitable rate.
2. homeostasis allows a constant blood glucose concentration to ensure a
constant water potential, so cells don't shrink or burst.
3. homeostasis allows organisms to be more independent of external changes.
list the parts of control mechanisms in homeostasis. - ANS 1. optimum
temperature
2. receptor- detects any deviation from the optimum temperature
3. coordinator- info from receptor to effector
4. effector- often a muscle/gland, brings about change to return the system to
optimum level
5. feedback mechanism
what is negative feedback? - ANS this is when the change produced by the
system leads to a change in the stimulus detected by the receptor, turning the
system off.
what is positive feedback? - ANS this is when a deviation from the optimum
causes changes that result in an even greater deviation from the norm
describe and explain the steps in the light dependent reaction of photosynthesis -
ANS 1. photoionisation: light reaches chlorophyll in PSII, which is absorbed by
an electron, which becomes excited and moves to a higher energy level.
2. the electron passes to a carrier protein in the thylakoid membrane, and is
passed down a series of carrier molecules called an electron transfer chain.
3. as the electron moves down, energy is lost from the electron and is released as
ATP.
4. the loss of electron from PSII is 'refilled' by an electron produced by photolysis,
which also produces hydrogen and oxygen.
5. the lost electron reaches PSI, which absorbs light energy and boosts another
electron to a higher energy level (excitation).
6. this electron also goes down an electron transport chain.
7. this reaches the final electron acceptor which is a proton. they combine to form
H and reduce NADP to NADPH.
describe and explain the steps in the light independent reaction pf
photosynthesis. - ANS 1. CO2 diffuses into stroma and combines with ribulose
bisphosphate (RuBP) using the enzyme rubisco.
2. this forms an unstable 6 carbon molecule, which splits into 2 3 carbon
molecules, glyercate-3-phosphate (G3P) .
3. G3P is reduced by NADPH to triose-phosphate (TP), which is aided by ATP for
energy.
4. TP can be converted into useful organic substances.
5. TP can also be reformed into RuBP using ATP.
describe glycolysis in respiration. give net formation. - ANS 1. glucose is
converted into phosphorylated glucose by 2ATP. this makes it very reactive, so it
splits into 2 triose phosphate (TP).
2. 2TP is then oxidised by 2NAD and 4 ATP is formed to form pyruvate.
3. NET: 2ATP, 2Pyruvate, 2NADH, 2H+
describe links reaction in respiration. give net formation. - ANS 1. pyruvate
diffuses into the matrix of mitochondria.
2. pyruvate is oxidised by NAD. CO2 is lost. this forms acetate.
3. acetate and co-enzyme A combine to form acetyl co-enzyme A.
4. NET: CO2, reduced NAD, acetyl co-enzyme A
,describe krebs cycle in respiration. give net formation. - ANS 1. acetyl
co-enzyme A combines with 4 carbon molecule (oxaloacetate) to form 6 carbon
citric acid.
2. CO2 is lost (decarboxylation), molecule is oxidised by NAD and ATP is
produce. this forms 5 carbon compound.
3. it is oxidised by 2NADH and FAD, and is decarboxylated.
4. this forms 4 carbon molecule again.
describe oxidative phosphorylation in respiration. - ANS 1. reduced coenzyme
passes its H to a carrier protein in the ETC. this splits into a proton and electron.
2. the protons pass through the space between inner and outer mitochondrial
membrane.
3. electrons pass through proteins on ETC.
4. protons return back via ATP synthase in the membrane, producing ATP.
5. the protons and electrons recombine to form H, which combines with O to form
water.
6. oxygen is the last electron acceptor in the ETC.
define biomass - ANS the total mass of organisms in a given area
what is the 'gross primary production' - ANS the chemical energy stored in a
plants biomass
what is the 'net primary production' - ANS the chemical energy stores in a
plants biomass after respiratory losses have been considered. this energy is
available to consumers.
how can you calculate the net primary production? - ANS NPP = GPP - R
why is converting sunlight energy into biomass in producers inefficient? - ANS
some light isn't the correct wavelength to be absorbed
some light doesnt hit chloroplast
some light is converted into heat energy
some light energy is reflected
describe the nitrogen cycle. - ANS fixation:
atmospheric nitrogen can be fixed by rhizbium bacteria.
if struck by lightning, it becomes reactive and combines with oxygen to form NO.
ammonification:
, saprobionts feed on organic matter and release ammonia, which then forms
ammonium ions in the soil.
nitrification:
nitrifying bacteria convert ammonium ions into nitrite ions and then to nirate
ions.
denitrification:
anaerobic denitrifying bacteria convert soil nitrates into gaseous nitrogen.
homeostasis - ANS the maintenance of an internal environment within restricted
limits in organisms. all cells are in an environment that meets their requirements
and allows them to function normally despite external changes.
why is homeostasis important? - ANS 1. the enzymes that control biochemical
reactions in cells are sensitive to change e.g. in pH or temperature, which can
cause them to denature. homeostasis allows enzyme controlled reactions to take
place at a suitable rate.
2. homeostasis allows a constant blood glucose concentration to ensure a
constant water potential, so cells don't shrink or burst.
3. homeostasis allows organisms to be more independent of external changes.
list the parts of control mechanisms in homeostasis. - ANS 1. optimum
temperature
2. receptor- detects any deviation from the optimum temperature
3. coordinator- info from receptor to effector
4. effector- often a muscle/gland, brings about change to return the system to
optimum level
5. feedback mechanism
what is negative feedback? - ANS this is when the change produced by the
system leads to a change in the stimulus detected by the receptor, turning the
system off.
what is positive feedback? - ANS this is when a deviation from the optimum
causes changes that result in an even greater deviation from the norm
