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Photosynthesis Notes AQA A Level Biology, Summaries of Biology

Biology

Photosynthesis Notes AQA A Level Biology - made using the specification.

Typology: Summaries

2025/2026

Uploaded on 05/30/2026

shiv-31
shiv-31 🇬🇧

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3.5.1 Photosynthesis
Photosynthesis takes place in chloroplasts - these are double membrane organelles.
The leaf is the main photosynthetic structure in the plant. It has:
- A large surface area to absorb as much light as possible
- Thin to maintain a short diffusion pathway
- Many stomata for increased gas exchange
- Many air spaces
- A network of xylem and phloem
- Many compact, packed mesophyll cells at the top
Thylakoids
These are disc shaped structures in the chloroplast with a folded membrane. They contain
chlorophyll which is a photosynthetic protein.
They also have electron carrier proteins embedded within their membranes which are
involved in the light dependent reaction
Stroma
Fluid centre of the chloroplast that contains enzymes involved in the light independent
reactions (otherwise known as the Calvin Cycle).
Inner and Outer Membranes
These membranes control the exit and entrance of substances in the chloroplast
Chlorophyll → it is located in the photosystems (which are large protein complexes on the
thylakoid membranes that photosynthetic pigments attach to) - there are a mix of coloured
proteins that can absorb light (pigments).
There are different proportions of each pigment which gives leaves slightly different colours.
Each pigment absorbs a different wavelength of light - this is advantageous as a
wider range of wavelengths of light can be absorbed → this means that the amount
of light energy absorbed increases so more energy can be used in light dependent
reactions.
Light Dependent Reactions LDR
This is the first stage of photosynthesis and it occurs in the thylakoid membranes. It
requires light.
Light energy and water are used to create ATP and reduced NADP (nicotinamide
adenine dinucleotide phosphate) (also known as NADPH) which are then needed in the
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3.5.1 Photosynthesis

Photosynthesis takes place in chloroplasts - these are double membrane organelles. The leaf is the main photosynthetic structure in the plant. It has:

  • A large surface area to absorb as much light as possible
  • Thin to maintain a short diffusion pathway
  • Many stomata for increased gas exchange
  • Many air spaces
  • A network of xylem and phloem
  • Many compact, packed mesophyll cells at the top

Thylakoids

These are disc shaped structures in the chloroplast with a folded membrane. They contain chlorophyll which is a photosynthetic protein. They also have electron carrier proteins embedded within their membranes which are involved in the light dependent reaction

Stroma

Fluid centre of the chloroplast that contains enzymes involved in the light independent reactions (otherwise known as the Calvin Cycle).

Inner and Outer Membranes

These membranes control the exit and entrance of substances in the chloroplast Chlorophyll → it is located in the photosystems (which are large protein complexes on the thylakoid membranes that photosynthetic pigments attach to) - there are a mix of coloured proteins that can absorb light (pigments). There are different proportions of each pigment which gives leaves slightly different colours. Each pigment absorbs a different wavelength of light - this is advantageous as a wider range of wavelengths of light can be absorbed → this means that the amount of light energy absorbed increases so more energy can be used in light dependent reactions.

Light Dependent Reactions LDR

This is the first stage of photosynthesis and it occurs in the thylakoid membranes. It requires light. Light energy and water are used to create ATP and reduced NADP ( nicotinamide adenine dinucleotide phosphate ) (also known as NADPH) which are then needed in the

next stage of photosynthesis – this is essentially the purpose of LDR - converting light energy into chemical energy and storing it there so that it can be used to form sugars in the Calvin cycle. NADP+^ + 2e-^ + H+^ →NADPH (reduced NADP) The purpose of NADPH is to act as a reducing agent - it is able to donate high energy electrons to other molecules - needed in the LIR Photosystem – pigment-protein complex The protein holds the chlorophyll in the correct position The chlorophyll is what absorbs the light energy and excites electrons (its own electrons)

There are 3 key stages for LDRs :

1) Photoionisation of chlorophyll Light energy is absorbed by the chlorophyll (chlorophyll is embedded in the photosystem) and the energy results in electrons becoming excited and going up an energy level to leave the chlorophyll. Therefore chlorophyll has been ionised by light PSI and PSII are the photosystems in the membrane and the electrons leave here and help in the next stage. Chlorophyll a molecules lose 2 electrons in this process - PSII becomes highly oxidising as it has lost an electron and it pulls electrons from water leading to photolysis 2) Chemiosmosis (2 branches remember) The excited electrons from the photoionisation stage are passed down an electron transport chain - this is made up of a series carrier proteins (such as plastoquinone and plastocyanin) embedded in the thylakoid membrane. Excited electrons have more energy but as they pass through the ETC, they lose this energy. This energy is then used to pump H+ ions from the stroma into the thylakoid lumen to create a proton gradient. Photolysis of water adds to this proton gradient. This creates an electrochemical gradient from the lumen of the thylakoid to the stroma (high potential energy). Protons move down this gradient via ATP synthase and this movement releases energy that ATP synthase can use to combine ADP and Pi to form ATP → this is the chemiosmotic theory.

6 carbon product that is immediately broken down). This reaction is catalysed by the enzyme rubisco.

  1. GP is reduced to form triose phosphate using energy from the hydrolysis of ATP and by accepting a proton and 2 electrons from reduced NADPH
  2. After every 3 turns, 6 molecules of TP are made, 1 of these TP is converted to useful organic substances such as hexose sugars
  3. The rest of the TP molecules are used to regenerate RuBP using energy released from the hydrolysis of ATP (5 molecules of TP left - 15 carbon - 3 RuBP) ( 6 turns of the calvin cycle are needed to produce 1 molecule of glucose per CO2 molecule) Glucose is produced - this can go on to form different organic substances such as sucrose, cellulose, starch. It can also be converted into glycerol and thus combined with fatty acids to make lipids for the plant. Factors affecting the rate of photosynthesis
  4. Temperature - the light independent reaction depends on the correct temperature

due to the presence of the enzyme RuBisCo ( photosynthesis is an enzyme

controlled reaction ). If the temperature is too low, the enzyme and substrates

have less kinetic energy and are less likely to collide to form an ESC and form GP. If the temperature is too high, RuBisCo denature and tertiary structure changes as R group interactions are broken- active site changes - it is no longer complementary to the substrates CO2 and RuBP so GP can no longer form so TP is not formed so organic molecules are not produced.

  1. CO2 concentration - a lower concentration of CO2 means that less GP and less TP is made per unit of time - this is because CO2 is less likely to collide with Rubisco so the enzyme will catalyse fewer reactions so less products will be formed.
  2. Light intensity - lower light intensity means that photolysis and photoionisation will occur at a slower rate so the products involved in photosynthesis will form more slowly. The rate of photosynthesis is directly proportional to light intensity when light intensity is limiting You will need to refer to limiting factors. The rate of photosynthesis may plateau because another factor becomes limiting.

The light compensation point is the point at which there is no net gas exchange; this occurs when the CO2 released from respiration is equal to the CO2 taken up during photosynthesis. # The specification references agricultural practices and how farmers overcome limiting factors. This could include:

  • Increasing light intensity by providing artificial lights
  • Keeping plants in a greenhouse with a heater to increase the temperature
  • Burning fuels to release CO2 or using liquid CO You may be asked to evaluate farmer’s choices or ideas based on given data. The modifications specified above share the major downside of cost. If the increase in yield is significant enough and outweighs the costs, it should go ahead. Otherwise it will not be cost effective. You must think in terms of profit