Photosynthesis

1. Photosynthesis Learning objective To be able to explain that light energy is absorbed by electrons in chlorophyll Some excited electrons gain enough energy to leave chlorophyll The electrons are replaced by the photolysis of water • the energy from these excited electrons is used to generate ATP and reduced NADP;

2. Adaptations in a leaf Write down the adaptations in a leaf and how they facilitate photosynthesis • Thin – gases can diffuse in and out quickly • Large surface area – maximise light absorption • Thin layer of wax – to reduce evaporation from the upper surface • Rich network of vascular tissue – xylem to deliver water and minerals, phloem to remove products of photosynthesis • Cells in upper epidermis have no chloroplasts – more light gets through to pallisade layer • Tightly packed pallisade cells – larger surface area for photosynthesis • Chloroplasts move within cytoplasm – have the chance of obtaining as much light as possible • Stomata surrounded by guard cells – open in day to allow CO2 in, close at night to reduce water loss • Spongy mesophyll - creates air spaces making gas exchange more efficient between the atmosphere and pallisade mesophyll.

3. Autotrophic nutrition • Autotrophic nutrition is making your own food • Autotrophs are able to use a source of energy to make complex organis molecules from simple inorganic materials • Chemoautotrophs harness energy released in exergonic chemical reactions to synthesis their own food (mainly bacteria) • Photoautotrophs use light energy to synthesise their own organic materials (green plants and algae)

4. Structure of cholroplasts

5. Phases of Photosynthesis Light dependant reactions Oxygen Water electrons energy Light Independent Reactions Carbon Dioxide Carbohydrate

6. Light dependant reaction • Takes place in the thylakoids • Thylakoid membrane is a phospholipid bilayer with chlorophyll molecules embedded in it • Chlorophyll molecules absorb light energy and then transfer it to other protein molecules • Light dependant reaction generates ATP

7. Chlorophyll • Chlorophyll has evolved to make use of as much of the visible spectrum as possible • Chlorophyll is a mixture of compounds that fall into two basic types: • Chlorophyll • Carotenoids • The two main chlorophylls are a and b

8. Absorption and Action Spectrums • The action spectrum shows the rate of photosynthesis at different wavelengths. • The absorption spectrum shows how strongly the pigments absorb at different wavelengths.

9. Read the application box on page 6 and answer questions 1 - 3

10. Photosynthesis Learning objective To be able to explain that In the light-dependent reaction light energy excites electrons in chlorophyll; • the energy from these excited electrons is used to generate ATP and reduced NADP;

11. Thylakoid membrane • The first reactions of photosynthesis require light energy, and are called light dependant reactions which take place in the thylakoid membrane. • The thylakoid membranes have the pigments arranged in funnel shaped ‘photosystems’

12. Photosystems absorb some wavelengths of light and reflect others There are two photosystems I and II that are in close proximity to one another in the thylakoid membrane

13. Photosystem I Chl. b 650 Chl. a 670 About 300 light trapping chlorphyll molecules Chl. a 680 • Each photosystem holds the light absorbing pigments in the best position to maximize the absorbance of photons of light. • Photosystem I contains chlorophyll a and absorbs best at 700 nm (P700) • Photosystem II contains chlorophyll b and absorbs best at 680 nm (P680) Chl. a 690 Reaction centre I P700 Light reaction 1 Photosystem II Chl. b 650 Chl. a 670 About 300 light trapping chlorphyll molecules Chl. a 680 Reaction centre II P690 Light reaction 2

14. Energy captured in the Light Dependant reaction Used for 2 things: • Making ATP from ADP and Pi is called phosphorylation. In the light dependant reaction it is called PHOTOphosphorylation because it involves light • For splitting water into H+ and OH– ions –this is called photolysis • For combining electrons, photons and NADP to produce reduced NADP (NADPH) powerful reducing agent used in the light independent reactions.

15. Energy, protons and electrons used to reduce NADP Reduced NADP Production Energy used to split water into protons, electrons and oxygen Photolysis Energy used to add a phosphate group to ADP to form ATP Photophosphorylation Electrons in chlorophyll gain energy from the sun and become excited Protons Electrons Oxygen released

16. Photophosphorylation • Excited electrons passed to electron acceptor • Electrons pass along chain of electron acceptors down energy gradient • Electrons lose energy at each stage in the chain • Energy is used to add phosphate molecule to ADP - photophosphorylation

17. Photophosphorylation • Uses PSI and PSII 2e- NADPH + H+ NADP + 2H+ e- ADP + P LIGHT ATP PSI P700 LIGHT PSII P680 H2O ½ O2 + 2H+

18. Photosystem One - PS I: Its primary pigment is a molecule of chlorophyll a. What is it’s peak absorbance? • Photosystem Two - PS II: Its primary pigment is a molecule of chlorophyll b. What is it’s peak absorbance? • Where precisely in the plant does the light dependant stage occur? • Which two compounds produced in the light dependant stage are used in the light independent stage? • Which of the light dependant reactions of photosynthesis are involved in producing these compounds?

19. 1. Photosystem One - PS I:Its primary pigment is a molecule of chlorophyll a. What is it’s peak absorbance? • PSI absorption peak at 700nm. It is called P700 2. Photosystem Two - PS II: Its primary pigment is a molecule of chlorophyll b. What is it’s peak absorbance? • PSII absorption peak at 680nm. It is called P680

20. 3. Where precisely in the plant does the light dependant stage occur? In the thylakoid membranes of the chloroplasts 4. Which two compounds produced in the light dependant stage are used in the light independent stage? ATP and NADPH + H+ 5. Which of the light dependant reactions of photosynthesis are involved in producing these compounds? ATP is produced by both cyclic and non cyclic photophosphorylation NADPH is produced by non cyclic photophosphorylation