Introduction
- Photosynthesis is the process of using sunlight energy and chlorophyll to produce glucose from carbon dioxide and water.
- Photosynthesis occurs in the chloroplast.

Structure of Chloroplast

General Process of Photosynthesis
- Sunlight is absorbed by chlorophyll (found in thylakoid membranes of the chloroplast).
- Chlorophyll molecules are arranged into photosystems.
- This absorbed energy is used to split water into three different components:
- Hydrogen ions (also called protons) are stored in the chloroplast – an area called the proton pool.
- Oxygen passes out of plant cell and leaf into the atmosphere.
- Electrons are given to chlorophyll.
- Chlorophyll that has energy because of sunlight transfers that energy to its electrons, creating high-energy electrons.
- These high-energy electrons are combined with protons and carbon dioxide to make glucose.


Detailed Process of Photosynthesis
Photosynthesis occurs in the chloroplast in two stages:
- Light stage – requires the direct input of light and occurs in the thylakoid membranes.
- Dark stage (light-independent stage OR Calvin Cycle) – does not require the direct input of light and occurs in the stroma.

Light Stage
- Sunlight photon strikes a cluster of chlorophyll molecules called a photosystem.
- The chlorophyll molecules transfer the energy to a reaction center chlorophyll (RCC).
- The energy is absorbed by an electron which becomes a ‘high-energy electron‘.
- The energised electron is released from the RCC and can take one of two paths:
1. Cyclic pathway (pathway 1)
2. Non-cyclic pathway (pathway 2)

1. Cyclic pathway
- The energised electron is picked up by an electron acceptor.
- It is passed from electron acceptor to electron acceptor losing energy along the way.
- This energy is used to power the production of ATP from ADP and a phosphate.
- Once the electron has been passed through it is taken back up by the RCC (chlorophyll).
- The ATP is passed onto the next stage of photosynthesis – the dark stage.

2. Non-cyclic pathway (pathway 2)
- The energised electron is picked up by an electron acceptor.
- It is passed from electron acceptor to electron acceptor losing energy along the way.
- This energy is used to power the production of ATP from ADP and a phosphate.
- The photosystem is deficient in electrons and splits water into electrons, protons and oxygen gas (photolysis).
- The electrons are taken up by the photosystem, the protons are stored in a proton pool within the chloroplast and the oxygen gas is either released into the atmosphere or used in respiration.
- The electrons that passed through the electron acceptors are now low-energy electrons and are now passed onto another photosystem.
- Light strikes this second photosystem and the electrons are re-energised.
- The electrons are released and captured by NADP+ to become NADP–.
- Protons are attracted towards and taken up by NADP– to become NADPH.
- NADPH and ATP are passed onto the next stage of photosynthesis – the dark stage.

Dark Stage (Calvin cycle)
- NADPH and ATP from the light stage are used to reduce (addition of protons and electrons) carbon dioxide in the stroma of the chloroplast.
- Glucose is formed in this reaction.
- As a result, NADPH is converted back to NADP+ and ATP is converted back to ADP and a phosphate.

Mandatory Experiment: to investigate the effect of light intensity OR carbon dioxide concentration on the rate of photosynthesis
Equipment:
- Pondweed (Elodea)
- Metre stick
- Pond water
- Thermometer
- Paperclip
- Sodium hydrogen carbonate
- Stopwatch
- Backed blade
- Strong fluorescent light source
- Beaker and test tube
Method:
- Obtain fresh pondweed.
- Cut a small section using the backed blade and crush the cut end slightly between your fingers.
- Place pondweed in a test tube with pond water.
- Ensure the cut end is facing upwards and weigh the pondweed down by attaching a paperclip.
- Shine the strong light source on the pondweed for 5 minutes to allow the pondweed to adjust.
- Move the light source various distances from the pondweed.
- Allow the pondweed to adjust to each new light intensity for 5 minutes before counting the number of bubbles per minute.
- Count the number of bubbles per minute at each distance.
- Calculate the light intensity at each distance by using the following formula: 1/d2 (where d is the distance from the light source)
- Fill in the table below:


Result:
- As the light source is moved further away from the pondweed, the rate of bubbles per minute decreases.

Conclusion:
- The rate of photosynthesis increases with light intensity
- The rate of photosynthesis stops increasing, levelling off when the plant has become light saturated.