• 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