8.1 Essential ideas

8.1.3 Photosynthesis

During photosynthesis, light energy is converted into chemical energy:

Figure 8.1.3a – Photosynthetic equationFigure 8.1.3a – Photosynthetic equation

  • Photosynthesis takes place in the chloroplasts. The process can be divided into two parts: light-dependent reactions and light-independent reactions.

Figure 8.1.3b – Overview of light-dependent and light-independent reactions in the chloroplastFigure 8.1.3b – Overview of light-dependent and light-independent reactions in the chloroplast

The light-dependent reactions

  • Light-dependent reactions take place in the membrane and intermembrane space of thylakoids.
  • Large protein complexes, called Photosystem II (PS II) and Photosystem I (PS I), are embedded in the thylakoid membranes. The photosystems contain the photosynthetic pigments, which make them characteristically green in colour.
  •  When pigments absorb light within the photosystems, electrons are excited and jump to a higher energy level. This is called photoactivation, and occurs in the reaction centre.

Figure 8.1.3c – Photoactivation occurs when light absorbed by the photosynthetic pigments reaches the reaction centre of the photosystemFigure 8.1.3c – Photoactivation occurs when light absorbed by the photosynthetic pigments reaches the reaction centre of the photosystem

  • The reaction centre of Photosystem II absorbs light at wavelengths of 680nm, while the reaction centre of Photosystem I absorbs light at 700nm.
  • Photosystem II replaces the excited electrons by splitting water into protons, electrons, and elemental oxygen. This is called photolysis.

Figure 8.1.3d – Photolysis of waterFigure 8.1.3d – Photolysis of water

  • Photolysis of water generates all the electrons for use in the light-dependent reactions. Oxygen is released as a waste product of photolysis, while the protons accumulate in the thylakoid space.

Figure 8.1.3e – Detail of the thylakoid membrane during the light-dependent reactionsFigure 8.1.3e – Detail of the thylakoid membrane during the light-dependent reactions

  • As long as photoactivation occurs, electrons are transferred through a chain of electron transport proteins in the thylakoid membrane. As they are transferred, the electrons release energy that is used to pump protons into the thylakoid space from the stroma.
  • In this way, the excited electrons from Photosystem II are responsible for the proton gradient.
  • ATP synthase takes advantage of the proton gradient to generate ATP, using the principle of chemiosmosis, as protons diffuse out into the stroma. (see 8.1.2). ATP generation in the light-dependent reactions is called photophosphorylation.
  • When light hits the reaction centre of Photosystem I, excited electrons are replaced from the electron transport chain originating at Photosystem II.
  • High-energy electrons from Photosystem I are used to reduce the electron carrier molecule NADP, nicotinamide adenine dinucleotide phosphate.
  • NADP is reduced to NADPH + H+. This is achieved through the enzyme NADP reductase.

The light-independent reactions

  • A metabolic pathway called the Calvin Cycle makes up the light-independent reactions. The Calvin cycle takes place in the stroma.
  • The Calvin cycle is simplified in the diagram below to show three turns of the cycle.

Figure 8.1.3f – Summary of the Calvin cycleFigure 8.1.3f – Summary of the Calvin cycle

  • At the beginning of the cycle, carbon dioxide molecules combine with ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme RuBP carboxylase, or Rubsico. 
  • The resulting 6-carbon molecules immediately split into 3-carbon molecules called glycerate 3- phosphate (3-PGA).
  • Glycerate 3-phosphate is reduced to triose phosphate (G3P), using the electrons from NADPH and the energy from ATP. For every three turns of the cycle, six molecules of triose phosphate are produced.
  • One molecule of triose phosphate is released from the Calvin cycle. It will be used in the synthesis of glucose or other organic compounds like amino acids.
  • The five remaining triose phosphate molecules are used to regenerate RuBP. RuBP regeneration requires phosphates from ATP.

Figure 8.1.3g – Chloroplast micrographFigure 8.1.3g – Chloroplast micrograph
How is the structure of a chloroplast adapted to its function?

Key questions

  • Compare Photosystem II and Photosystem I.
  • List the products of the light-dependent reactions.
  • Outline the function of two named enzymes in photosynthesis.
  • Explain the terms photoactivation, photolysis and photophosphorylation.
  • Identify substances from the Calvin cycle that are associated with the following terms: carboxylation, reduction, regeneration.

Language tools

  • 'Light independent' indicates that these reactions occur independent of light conditions. Refrain from calling them 'dark reactions'.
  • Different abbreviations are used in different textbooks for the metabolic intermediates associated with the Calvin cycle. Always begin with the full names of the molecules, and provide consistent abbreviations in an exam. (e.g. Glycerate 3- phosphate, 3-PGA).
  • Photosystem II operates before Photosystem I during the light-dependent reactions. The confusing nomenclature has to do with the fact that PSI was discovered first.

Figure 8.1.3h – Compare photosystemsFigure 8.1.3h – Compare photosystems
Details of the roles of PSII and PSI.

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