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aqa alevel biology energy transfer in and between organisms
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the complete revision guide to unit 5 to use alongside your notes and practice questions. There is mini checklist included which match the specification and notes underneath

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  • May 9, 2024
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  • 2023/2024
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3.5 Energy transfers in and between organisms
Contents


3.5.1 Photosynthesis (A-level only)
An overview of photosynthesis
The light-dependent stage
The light-independent stage (Calvin cycle)


3.5.2 Respiration (A-level only)
An overview of aerobic respiration
Glycolysis
Anaerobic respiration
The link reaction
The Krebs cycle
Oxidative phosphorylation
Exam question examples

3.5.3 Energy and ecosystems (A-level only)
Main principles and measuring biomass
Gross and net primary production (GPP and NPP)
Rates of productivity units
Energy transfer between trophic levels is inefficient
Farming practices to increase energy transfer efficiency

3.5.4 Nutrient cycles (A-level only)
Basic nutrient cycles
Role of saprobionts in decomposition
Role of mycorrhizae
The nitrogen cycle
Importance of the nitrogen cycle
The phosphorus cycle
The use of natural and artificial fertilisers
Environmental issues with the use of fertilisers
Eutrophication
Exam question examples



Material of @medic.coffee (not to be redistributed without permission)

,3.5.1 Photosynthesis (A-level only)

What the specification says:
The light-dependent reaction in such detail as to show that:
chlorophyll absorbs light, leading to photoionisation of chlorophyll
some of the energy from electrons released during photoionisation is conserved in the production of
ATP and reduced NADP
the production of ATP involves electron transfer associated with the transfer of electrons down the
electron transfer chain and passage of protons across chloroplast membranes and is catalysed by
ATP synthase embedded in these membranes (chemiosomotic theory)
photolysis of water produces protons, electrons and oxygen.

The light-independent reaction uses reduced NADP from the light- dependent reaction to form a
simple sugar. The hydrolysis of ATP, also from the light-dependent reaction, provides the additional
energy for this reaction.

The light-independent reaction in such detail as to show that:
carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-
phosphate (GP). This reaction is catalysed by the enzyme rubisco
ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate
some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
some of the triose phosphate is converted to useful organic substances.

Students should be able to:
Identify environmental factors that limit the rate of photosynthesis
Evaluate data relating to common agricultural practices used to overcome the effect of these limiting
factors.

Summary of what you need to know:

The light-dependent stage of photosynthesis
The light-independent stage of photosynthesis (Calvin cycle)
Environmental factors limiting the rate of photosynthesis
Evaluate data related to agricultural practices used to overcome effects of limiting factors




Material of @medic.coffee (not to be redistributed without permission)

,An overview of photosynthesis


At GCSE, you will have learnt that photosynthesis occurs in chloroplasts of plant cells where…

…and that was just about as much detail as
Carbon dioxide + water → glucose + oxygen
you needed to know for photosynthesis!


At A-level, you learn that photosynthesis is split into 2 stages:

1. The light-dependent stage
2. The light-independent stage (‘The Calvin cycle’)


An overview of photosynthesis:

The light-dependent stage

- Occurs in the thylakoid membrane of chloroplasts If you need a reminder of the structure of
a chloroplast, have a read up on 3.2.1.1!
- Results in the production of: ATP, reduced NADP and oxygen


The light-independent stage (Calvin cycle)

- Occurs in the stroma of chloroplasts
- Results in the production of hexose sugars and other organic substances



The light-dependent stage of photosynthesis


As mentioned above, this stage occurs in the thylakoid membrane of chloroplasts. Make sure you
specify in the exam that it occurs in the thylakoid membrane, not just the thylakoid.

As it suggests in the name, this stage requires light energy from the sun!
There are 4 stages overall:

- Photoionisation
- Production of ATP and reduced NADP We will have a look at each stage in a bit more
- Chemiosmosis detail on the next few pages!
- Photolysis



Material of @medic.coffee (not to be redistributed without permission)

, Diagram of what is happening in the thylakoid membrane:

(reduction)



Stroma +
NADP + 2H NADPH



H+
light energy light energy
H + H+ ADP + Pi ATP
H+
2e-



2e- 2e-


2e-
2e-


PSII PSI
+ H+
H
H2O
Enzyme - ATP
synthase

1 O2 + 2H+
light energy 2




Thylakoid lumen



This may look really confusing at first since there is a lot going on. But don’t worry, we’ll go through
each stage slowly and help you make sense of the big picture.

The labels on the diagram are there to help you too and the pink boxes to highlight some of the key
reactions which are occurring in the light dependent stage!

Overview of the key products of the light-dependent reaction:

- ATP → used later on in the light-independent reaction
- Reduced NADP → used later on in the light-independent reaction
- Oxygen → leaves cell as a by-product or used in respiration



Material of @medic.coffee (not to be redistributed without permission)

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