C4 Photosynthesis


C.4.1  Draw and label a diagram showing the structure of a chloroplast as seen in an electron micrograph

                  2D Representation                                                           3D Representation                                                               Electron Micrograph

C.4.2  State that photosynthesis consists of the light-dependent and light-independent reactions

Photosynthesis is a two-step process:

1.  The light dependent reactions convert the light energy into chemical energy

2.  The light independent reaction uses the chemical energy to make organic molecules

Overview of Photosynthesis

C.4.3  Explain the light dependent reactions

  • The light dependent reactions occur on the thylakoid membrane and may occur by either cyclic or non-cyclic processes
  • In both processes, light excites chlorophyll (clustered in photosystems) which release electrons that pass through an electron transport chain, making ATP (photophosphorylation)

Non-Cyclic Photophosphorylation

  • Chlorophyll in photosystems I and II absorbs light, which triggers the release of high energy electrons (photoactivation)
  • The electrons from photosystem II pass along a series of carriers (electron transport chain), producing ATP via chemiosmosis 
  • The electrons from photosystem I reduce NADP+ to generate NADPH + H+
  • Electrons lost from photosystem I are replaced by electrons from photsystem II
  • Electrons lost from photosystem II are replaced by electrons generated by the photolysis of water (oxygen is produced as a by-product)

Cyclic Photophosphorylation

  • Only photosystem I is involved in cyclic photophosphorylation
  • The high energy electrons released by photoactivation pass along an electron transport chain (producing ATP) before returning to photosystem I
  • Cyclic photophosphorylation does not produce NADPH + H+, which is needed for the light independent reactions
  • Thus while cyclic photophosphorylation can make chemical energy (ATP) from light, it cannot be used to make organic molecules

Non-Cyclic versus Cyclic Photophosphorylation

C.4.4  Explain photophosphorylation in terms of chemiosmosis

  • As the electrons (released from chlorophyll) cycle through the electron transport chains located on the thylakoid membrane, they lose energy
  • This free energy is used to pump H+ ions from the stroma into the thylakoid
  • The build up of protons inside the thylakoid creates an electrochemical gradient (or proton motive force)
  • The H+ ions return to the stroma via the transmembrane enzyme ATP synthase, which uses the potential energy from the proton motive force to convert ADP and an inorganic phosphate (Pi) into ATP
  • This process is called chemiosmosis

Photophosphorylation via Chemiosmosis

C.4.5  Explain the light independent reaction

  • The light independent reaction occurs in the stroma and uses the ATP and NADPH + H+ produced by the light dependent reaction (non-cyclic)
  • The light independent reaction is also known as the Calvin cycle and occurs via three main steps:

1.  Carbon Fixation

  • The enzyme rubisco (RuBP carboxylase) catalyses the attachment of CO2 to the 5C compound ribulose bisphosphate (RuBP)
  • The unstable 6C compound that is formed immediately breaks down into two 3C molecules called glycerate-3-phosphate (GP)

2.  Reduction

  • Each GP molecule is then phosphorylated by ATP and reduced by NADPH + H+
  • This converts each GP molecule into a triose phosphate (TP) called glyceraldehyde phosphate

3.  Regeneration of RuBP

  • For every six molecules of TP produced, only one may be used to form half a sugar molecule (need two cycles to form a complete glucose)
  • The remaining TP molecules are reorganised to regenerate stocks of RuBP in a reaction that involves ATP
  • With RuBP regenerated, this cycle will repeat many times and be used to construct chains of sugars (e.g. sucrose) for use by the plant

The Light Independent Reaction (Calvin Cycle)

C.4.6  Explain the relationship between the structure of the chloroplast and its function

  • Thylakoids:  Small lumen means small changes in proton concentration have a large effect on the proton motive force
  • Grana:  Thylakoids arranged in stacks to greatly increase surface area available for light absorption (chlorophyll located in thylakoid membrane)
  • Stroma:  Contains appropriate enzymes and suitable pH for the light independent reaction to occur

C.4.7  Explain the relationship between the action spectrum and absorption spectrum of photosynthetic pigments in green plants

  • Pigments absorb light as a source of energy for photosynthesis
  • The absorption spectrum indicates the wavelengths (frequency) of light absorbed by each pigment
  • The action spectrum indicates the rate of photosynthesis for each wavelength / frequency
  • There is a strong correlation between the cumulative absorption spectrum of all photosynthetic pigments and the action spectrum
  • Both display two main peaks - a larger peak at ~450 nm (blue) and a smaller peak at ~670 nm (red) with a decrease in between (green)

Absorption Spectrum versus Action Spectrum

Chlorophyll and Photosystems

C.4.8  Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide

  • The law of limiting factors states that when a chemical process depends on more than one essential condition being favourable, its rate will be limited by the factor that is nearest its minimum value
  • Photosynthesis is dependent on a number of favourable conditions, including:

Light Intensity

  • Light is required for the light dependent reactions (photoactivation of chlorophyll and photolysis of water molecules)
  • Low light intensities results in insufficient production of ATP and NADPH + H+ (both needed for the light independent reaction)


  • Primarily affects the light independent reaction (and to a lesser extent the light dependent reactions)
  • High temperatures will denature essential enzymes (e.g. rubisco), whereas insufficient thermal energy will prohibit reactions from occurring

Concentration of Carbon Dioxide

  • Carbon dioxide is required for the light independent reaction to occur (carbon fixation of RuBP by rubisco)
  • At low levels, carbon fixation will occur very slowly, whereas at higher levels the rate will peak as all rubsico are being used

Factors Affecting the Rate of Photosynthesis

C.4.9  Analyse data relating to photosynthesis

  • Remember the equation for photosynthesis and the different roles of the light dependent and light independent reactions
  • The law of limiting factors states that the rate of a reaction will be determined by the substrate nearest its minimum value
  • Remember the factors that can affect the substrates and identify how this in turn can effect the amount of products
  • Also, remember that plants also undergo cell respiration, and that the amounts of photosynthetic products formed will be determined in part by the amounts used in cellular respiration