What happens in the light-independent reactions of photosynthesis?

Understanding:

•  Absorption of light by photosystems generates excited electrons

•  Transfer of excited electrons occurs between carriers in thylakoid membranes

The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)

These reactions occur within specialised membrane discs within the chloroplast called thylakoids and involve three steps:

• Excitation of photosystems by light energy
• Production of ATP via an electron transport chain
• Reduction of NADP+ and the photolysis of water

Step 1:  Excitation of Photosystems by Light Energy

• Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane
• Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm)
• When a photosystem absorbs light energy, delocalised electrons within the pigments become energised or ‘excited'
• These excited electrons are transferred to carrier molecules within the thylakoid membrane

Understanding:

•  Excited electrons from Photosystem II are used to contribute to generate a proton gradient

•  ATP synthase in thylakoids generates ATP using the proton gradient

      
Step 2:  Production of ATP via an Electron Transport Chain

• Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane
• As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the thylakoid
• This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
• The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis)
• ATP synthase uses the passage of H+ ions to catalyse the synthesis of ATP (from ADP + Pi)
• This process is called photophosphorylation – as light provided the initial energy source for ATP production
• The newly de-energised electrons from Photosystem II are taken up by Photosystem I

Understanding:

•  Excited electrons from Photosystem I are used to reduce NADP

•  Photolysis of water generates electrons for use in the light dependent reactions

     
Step 3:  Reduction of NADP+ and the Photolysis of Water

• Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+
• This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
• The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II
• The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
• Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product)

Overview of the Light Dependent Reactions

• The light dependent reactions occur within the intermembrane space of the thylakoids
• Chlorophyll in Photosystems I and II absorb light, which triggers the release of high energy electrons (photo activation)
• Excited electrons from Photosystem II are transferred between carrier molecules in an electron transport chain
• The electron transport chain translocates H+ ions from the stroma to within the thylakoid, creating a proton gradient
• The protons are returned to the stroma via ATP synthase, which uses their passage (via chemiosmosis) to synthesise ATP
• Excited electrons from Photosystem I are used to reduce NADP+ (forming NADPH)
• The electrons lost from Photosystem I are replaced by the de-energised electrons from Photosystem II
• The electrons lost from Photosystem II are replaced following the photolysis of water
• The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions

Light Dependent Reactions Analogy

Z Scheme

The energy changes (oxidation / reduction) that occur during photosynthesis may be represented as a Z scheme:

• First vertical bar:  Photosystem II electrons are energised by light (electrons replaced by photolysis of water molecules)
• Diagonal bar:  Electrons lose energy as they pass through an electron transport chain (synthesising ATP)
• Second vertical bar:  Photosystem I electrons are energised by light (electrons used to reduce NADP+)

Understanding:

•  In the light independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate

        
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules

• The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma

The light independent reactions are collectively known as the Calvin cycle and involve three main steps:

• Carboxylation of ribulose bisphosphate
• Reduction of glycerate-3-phosphate
• Regeneration of ribulose bisphosphate

Step 1:  Carbon Fixation

• The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
• An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
• The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)
  
• A single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP

Understanding:

•  Glycerate-3-phosphate is reduced to triose phosphate using reduced NADP and ATP

        
Step 2:  Reduction of Glycerate-3-Phosphate

• Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
• Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy
  
• Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule

Understanding:

•  Triose phosphate is used to regenerate RuBP and produce carbohydrates

•  Ribulose bisphosphate is reformed using ATP

    
Step 3:  Regeneration of RuBP

• Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule
• Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch
• The remaining five TP molecules are recombined to regenerate stocks of RuBP  (5 × 3C = 3 × 5C) 
• The regeneration of RuBP requires energy derived from the hydrolysis of ATP