Photosynthesis: The Light Dependent Stage

1. Photosynthesis: The Light Dependent Stage • state that the light-dependent stage takes place in thylakoid membranes and that the light-independent stage takes place in the stroma; • outline how light energy is converted to chemical energy (ATP and reduced NADP) in the light-dependent stage (reference should be made to cyclic and non-cyclic photophosphorylation, but no biochemical detail is required); • explain the role of water in the light-dependent stage; • outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP) (reference should be made to ribulosebisphosphate (RuBP), ribulosebisphosphatecarboxylase (rubisco) and glycerate 3-phosphate (GP), but no other biochemical detail is required);

2. Light Dependent Stage • Takes place in the thylakoid membranes of the chloroplast • Photosystems are embedded in these membranes • Photosystem I mainly in the intergranal lamellae • Photosystem II almost exclusively on the granal lamellae • Photosystems trap light energy to be converted to chemical energy

3. Photosynthesis • We will look at what happens with the raw materials in photosynthesis Water + carbon dioxide  glucose + oxygen 6H2O + 6CO2  C6H12O6 + 6O2

4. Stage 1: Splitting Water • Known as photolysis • Enzymes in Photosystem II split water with light into H+ and e- • 2H2O 4H+ + 4e- + O2 • Some oxygen used for respiration • Some diffuses out into the air • In this way, water is one of the raw materials in photosynthesis

5. Stage 2: Electron Transport • Just like the one in Respiration, Photosynthesis begins with an electron transport chain called photophosphorylation (as opposed to oxidative phosphorylation in respiration) • It involves Photosystems I and II • A photon of light hits a chlorophyll molecule which excites 2 electrons that have come from water • The electrons are captured by electron acceptors and passed along electron carriers embedded in the thylakoid membranes • Again, the electron carriers are proteins containing iron (like in respiration)

6. Stage 2: Electron Transport • Energy is released as the electrons are passed from carrier to carrier • This pumps protons across the thylakoid membranes into the thylakoid space where they accumulate • A proton gradient forms • Chemiosmosis takes place and the H+ move through ATP synthase and ATP is generated • The Hydrogen acceptor is a co-enzyme called NADP which becomes reduced NADP • This is called Non- Cyclic Photophosphorylation

8. Task • Underneath your copy of the diagram, use the textbooks to describe the sequence of events happening in non-cyclic photophosphorylation

9. Non- Cyclic Photophosphorylation (Z- Scheme) • Photons of light hit photosystem II and move electrons to a higher energy level • ATP is made as the electrons are passed from carrier to carrier • The electrons are passed along a chain of electron carriers to photosystem I • Light energy also hits photosystem I which again, excites electrons which combine with Hydrogen ions and pass to a different electron acceptor (NADP) to become NADPH

10. Cyclic Photophosphorylation • This is another way that ATP is formed, it involves: • Only Photosystem I • Excited electrons from water pass to an electron acceptor, then back to the chlorophyll molecule from which they were lost • Small amounts of ATP are made • It is known as cyclic photophosphorylation

12. Photosynthesis: The Light Independent Stage(Calvin Cycle) • outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP) (reference should be made to ribulosebisphosphate (RuBP), ribulosebisphosphatecarboxylase (rubisco) and glycerate 3-phosphate (GP), but no other biochemical detail is required); • explain the role of carbon dioxide in the light-independent stage (Calvin cycle); • state that TP can be used to make carbohydrates, lipids and amino acids; • state that most TP is recycled to RuBP;

13. Calvin Cycle • Takes place in the stroma of the chloroplast • Products of light dependent stage (photophosphorylation) are used

15. Calvin Cycle • Carbon dioxide diffuses into leaf through stomata on underside of leaf and enters stomata • Combines with 5-carbon rubulosebisphosphate (RuBP) helped by the enzyme rubisco • Two 3-carbon glycerate 3-phosphates (GP) created • GP is reduced and phosphorylated by NADP and ATP to make triose phosphate (TP) • Triose phosphate goes on to make sugars e.g. glucose • RuBP is reformed

16. How are the Products of the Calvin Cycle Used? • GP used for amino acids and fatty acids • TP used to make sugars e.g. glucose, sucrose, fructose • TP can be converted to glycerol and combined with fatty acids formed from GP to make lipids • So... • TP can be used to make carbohydrates, lipids and amino acids; • But... most TP is recycled to RuBP

17. Questions • Where do the light dependent and light independent reactions take place? • Outline how light energy is converted to chemical energy (cyclic and non cyclic) • Explain the role of water in the light-dependent stage • Explain the role of carbon dioxide in the light independent stage (calvin cycle) • What happens to most of the triose phosphate produced in the Calvin Cycle? • What else can triose phosphate make?

18. Questions • Where do the light dependent and light independent reactions take place? • Outline how light energy is converted to chemical energy (cyclic and non cyclic) • Explain the role of water in the light-dependent stage • Explain the role of carbon dioxide in the light independent stage (calvin cycle) • What happens to most of the triose phosphate produced in the Calvin Cycle? Most is recycled to RuBP • What else can triose phosphate make? Carbohydrates, lipids and amino acids