Photosynthesis

Introduction

  • Photosynthesis is the process of using sunlight energy and chlorophyll to produce glucose from carbon dioxide and water.
  • Photosynthesis occurs in the chloroplast.
Chloroplasts present in a plant cell

Structure of Chloroplast

Chloroplast structure

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:
    1. Hydrogen ions (also called protons) are stored in the chloroplast – an area called the proton pool.
    2. Oxygen passes out of plant cell and leaf into the atmosphere.
    3. 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.
The process of photosynthesis (Ordinary Level)

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.
Summary of photosynthetic reactions

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)

A photosystem
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.
Pathway 1 of the light stage of photosynthesis
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.
Pathway 2 of the light stage of photosynthesis

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:
Apparatus setup for examining the effect of light intensity on the rate of photosynthesis

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.