Photosynthesis: Light-Dependent Stage (Edexcel International A Level Biology)

• Photosynthesis takes place in two distinct stages
  • The light-dependent reactions, which rely on light directly
  • The light-independent reactions, which do not use light directly, though do rely on the products of the light-dependent reactions
• Both these sets of reactions take place within the chloroplast
  • The light-dependent reactions take place across the thylakoid membrane
  • The light-independent reactions take place in the stroma
• Light energy in the light-dependent reactions enables the splitting of water molecules in a reaction known as photolysis
  • Photolysis of one molecule of water, or H₂O, produces
    • 2 hydrogen ions (2H⁺), also known as protons
    • 2 electrons (2e^-)
    • One atom of oxygen (O)
  • The hydrogen ions and electrons are used during the light-dependent reactions while the oxygen is given off as a waste product
• During the light-dependent reactions light energy is converted into chemical energy in the form of ATP and reduced NADP
  • NADP is a type of molecule called a coenzyme; its role is to transfer hydrogen from one molecule to another
  • When NADP gains hydrogen it is reduced, and can be known as either reduced NADP or NADPH
  • Remember that
    • Reduction is gain of electrons, gain of hydrogen, or loss of oxygen
    • Oxidation is loss of electrons, loss of hydrogen, or gain of oxygen
  • Reduced NADP can reduce other molecules by giving away hydrogen
  • NADP can oxidise other molecules by receiving hydrogen
• The useful products of the light-dependent reactions, ATP and NADPH, are transferred to the light-independent reactions within the chloroplast

The products of the light-dependent reaction are ATP, NADPH, and oxygen. Oxygen is given off as a waste product while ATP and NADPH pass to the light-independent reactions. The ADP and NADP produced during the light-independent reaction can pass back to the light-dependent reactions to allow more ATP and NADPH to be produced.

Production of ATP and NADPH

• ATP and NADPH are produced during the light-dependent reactions as a result of a series of events that occur on the thylakoid membrane known as photophosphorylation
  • Photo = light
  • Phosphorylation = the addition of phosphate; in this case to ADP to form ATP
• Two types of photophosphorylation take place
  • Non-cyclic photophosphorylation
    • This produces both ATP and NADPH
  • Cyclic photophosphorylation
    • This produces ATP only
• Both cyclic and non-cyclic photophosphorylation involve
  • A series of membrane proteins which together make up the electron transport chain
    • Electrons pass from one protein to another along the electron transport chain, releasing energy as they do so
  • Chemiosmosis
    • The energy released as electrons pass down the electron transport chain is used to produce ATP

Non-cyclic photophosphorylation

• Light energy hits photosystem II in the thylakoid membrane 
  • It is slightly confusing that photosystem II comes first in this sequence; the numbers simply reflect the order in which the photosystems were discovered
• Two electrons gain energy and are said to be excited to a higher energy level 
• The excited electrons leave the photosystem and pass to the first protein in the electron transport chain
  • As the excited electrons leave photosystem II they are replaced by electrons from the photolysis of water
• The electrons pass down the chain of electron carriers known as an electron transport chain 
• Energy is released as the electrons pass down the electron transport chain which enables chemiosmosis to occur
  • H­­⁺ ions are actively pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane 
  • H­­⁺ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane
  • The movement of H­­⁺ ions causes the ATP synthase enzyme to catalyse the production of ATP
• At the end of the electron transport chain the electrons from photosystem II are passed to photosystem I
• Light energy also hits photosystem I, exciting another pair of electrons which leave the photosystem
• The excited electrons from photosystem I also pass along an electron transport chain
• These electrons combine with hydrogen ions from the photolysis of water and the coenzyme NADP to form reduced NADP

H⁺ + 2e^- + NADP⁺ → NADPH

• The reduced NADP and the ATP pass to the light-independent reactions 

Non-cyclic photophosphorylation involves photosystems I and II and produces both ATP and NADPH

Cyclic photophosphorylation

• Light hits photosystem I 
• Electrons are excited to a higher energy level and leave the photosystem
• The excited electrons pass along the electron transport chain, releasing energy as they do so
• The energy released as the electrons pass down the electron transport chain provides energy to drive the process of chemiosmosis
  • H­­⁺ ions are actively pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane 
  • H­­⁺ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane
  • The movement of H­­⁺ ions cause the ATP synthase enzyme to catalyse the production of ATP
• At the end of the electron transport chain the electrons rejoin photosystem I in a complete cycle; hence the term cyclic photophosphorylation
• The ATP produced enters the light-independent reaction

Cyclic photophosphorylation involves Photosystem I and produces ATP