AQA A Level Biology
Topic 5
Energy transfers in and
between organisms
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
5.1 Photosynthesis
Required practical 7
Required practical 8
5.2 Respiration
Required practical 9
5.3 Energy and ecosystems
5.4 Nutrient cycles
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
5.1 Photosynthesis
What are the stages
of photosynthesis?
1. Light dependent reaction
○ Thylakoid membrane of chloroplast
2. Light independent reaction
○ Stroma of chloroplast
Describe photoionisation in the light-dependent reaction (LDR)
● Chlorophyll absorbs light energy which excites its electrons (higher energy level)
● So electrons are released from chlorophyll (chlorophyll becomes positively charged)
Describe what happens after photoionisation in the LDR
Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP
(chemiosmotic theory):
1. Electrons move along electron transfer chain (electron carriers), releasing energy
2. This energy is used to actively pump protons from stroma into thylakoid
3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase
4. Energy used to join ADP and Pi to form ATP (photophosphorylation)
5. NADP accepts a proton and an electron to become reduced NADP
Describe photolysis of water in the LDR
● Water splits to produce protons, electrons and oxygen (H2O → ½ O2 + 2e- + 2H+)
○ Electrons replace those lost from chlorophyll
2
, More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Describe the light-independent reaction of photosynthesis (Calvin cycle)
1. CO2 reacts with ribulose bisphosphate (RuBP)
○ Catalysed by the enzyme rubisco
2. Forming 2 glycerate 3-phosphate (GP) molecules
3. GP reduced to triose phosphate (TP)
○ Using products from light-dependent reaction - reduced NADP and energy from ATP
4. Some TP converted to useful organic substances (eg. glucose)
5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)
Describe and explain how temperature affects rate of photosynthesis
1. As temperature increases, rate increases
○ Enzymes eg. rubisco gain kinetic energy
○ So more enzyme-substrate complexes form
2. Above an optimum temperature, rate decreases
○ Enzymes denature as H bonds in tertiary structure break
○ So fewer enzyme-substrate complexes form
Describe and explain how light intensity affects rate of photosynthesis
1. As light intensity increases, rate increases
○ Light-dependent reaction increases (eg. more photoionisation of
chlorophyll) so more ATP and reduced NADP produced
○ So light-independent reaction increases as more GP reduced to
TP and more TP regenerates RuBP
2. Above a certain light intensity, rate stops increasing
○ Another factor is limiting eg. temperature / CO2 concentration
3
Topic 5
Energy transfers in and
between organisms
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
5.1 Photosynthesis
Required practical 7
Required practical 8
5.2 Respiration
Required practical 9
5.3 Energy and ecosystems
5.4 Nutrient cycles
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
5.1 Photosynthesis
What are the stages
of photosynthesis?
1. Light dependent reaction
○ Thylakoid membrane of chloroplast
2. Light independent reaction
○ Stroma of chloroplast
Describe photoionisation in the light-dependent reaction (LDR)
● Chlorophyll absorbs light energy which excites its electrons (higher energy level)
● So electrons are released from chlorophyll (chlorophyll becomes positively charged)
Describe what happens after photoionisation in the LDR
Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP
(chemiosmotic theory):
1. Electrons move along electron transfer chain (electron carriers), releasing energy
2. This energy is used to actively pump protons from stroma into thylakoid
3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase
4. Energy used to join ADP and Pi to form ATP (photophosphorylation)
5. NADP accepts a proton and an electron to become reduced NADP
Describe photolysis of water in the LDR
● Water splits to produce protons, electrons and oxygen (H2O → ½ O2 + 2e- + 2H+)
○ Electrons replace those lost from chlorophyll
2
, More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Describe the light-independent reaction of photosynthesis (Calvin cycle)
1. CO2 reacts with ribulose bisphosphate (RuBP)
○ Catalysed by the enzyme rubisco
2. Forming 2 glycerate 3-phosphate (GP) molecules
3. GP reduced to triose phosphate (TP)
○ Using products from light-dependent reaction - reduced NADP and energy from ATP
4. Some TP converted to useful organic substances (eg. glucose)
5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)
Describe and explain how temperature affects rate of photosynthesis
1. As temperature increases, rate increases
○ Enzymes eg. rubisco gain kinetic energy
○ So more enzyme-substrate complexes form
2. Above an optimum temperature, rate decreases
○ Enzymes denature as H bonds in tertiary structure break
○ So fewer enzyme-substrate complexes form
Describe and explain how light intensity affects rate of photosynthesis
1. As light intensity increases, rate increases
○ Light-dependent reaction increases (eg. more photoionisation of
chlorophyll) so more ATP and reduced NADP produced
○ So light-independent reaction increases as more GP reduced to
TP and more TP regenerates RuBP
2. Above a certain light intensity, rate stops increasing
○ Another factor is limiting eg. temperature / CO2 concentration
3
