Topic 8.2 Photosynthesis

1. Topic 8.2 Photosynthesis

2. Assessment Statements 8.2.1 Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs. 8.2.2 State that photosynthesis consists of light-dependent and light independent reactions. 8.2.3 Explain the light-dependent reactions. 8.2.4 Explain photophosphorylation in terms of chemiosmosis. 8.2.5 Explain the light-independent reactions. 8.2.6 Explain the relationship between the structure of the chloroplast and its function. 8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants. 8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.

3. Internal membranes called thylakoids are the location of the light dependent reaction. Chloroplast Structure Stroma surrounds the thylakoids and inside the double membrane. This is the location of the light independent reaction that includes the Calvin cycle. The stroma often contains starch grains and oil droplets both products of photosynthesis.

4. LSA for absorption of light Concentrates H+ ions Compartmentalisation of enzymes for Calvin cycle

5. stroma thylakoids grana (sing. granum)

6. Stages of Photosynthesis

7. Used in respiration, stored as starch, converted to cellulose cell walls and other products In the light independent reaction (LIR) carbon is fixed using NADPH+ ATP 6CO2 + 6H2O → C6H12O6 + 6O2 Water is split in the light dependent reactions (LDR) to produce H+ ions which are used to generate ATP and NADPH

8. Light dependent reaction Thylakoid membranes Uses light - absorbed by chlorophyll Uses H2O and produces O2 Coupled to reduction of ADP to ATP and coenzyme NADP+ to NADPH + H+ Involves photolysis and photophosphorylation

9. The Light Independent Reaction Takes place in the stroma Enzymes involved Temperature sensitive Uses CO2 - carbon fixation produces glucose Uses products of LDR - ATP and NADPH

10. Light has four functions It excites the electrons, which then allows photophosphorylationto occur. It excites the electrons, which then allows the reduction of NADP to NADPH. It triggers the opening of the stomata so that CO2can enter. It initiates the photolysis of water.

11. Photophosphorylation The addition of phosphate using light energy. e.g. ADP + Pi ATP e.g. when light energy is used in the Z scheme to form ATP from ADP and phosphate.

12. Photolysis The splitting of water into H+,e– and oxygen using light energy. e- e- e- e-

13. Reduction Reduction is loss of oxygen or gain of H+ or electrons (e-). e.g. whenNADP + H+ NADPH NADP is reduced to NADPH by the addition of H+ and electrons. The opposite of reduction is oxidation

14. Light dependent reaction Chlorophyll molecules are arranged clusters called photosystems and embedded in the thylakoid membranes

15. Light Dependent Reaction

16. Photoactivation When light hits the chlorophyll molecule the light energy is transferred to electrons. The electrons become excited, and break their bonds. This is called photoactivation. excited or activated state Light is absorbed and activates the electron Electron returns to ground state emitting a packet of energy

17. ATP Production This generates ATP Excited electron leaves the chlorophyll molecule And is passed along a series of electron acceptors (ETC) to PSI Exciting an electron in the chlorophyll molecule Protons pass through ATP Synthase by chemiosmosis H+ produced by photolysis accumulate in the thylakoid space Light (680nm) strikes PSII Electrons are replaced through photolysis

18. Chemiosmosis The diffusion of ions across a partially permeable membrane through ATP Synthase. As electrons pass along the ETC energy is released. This energy is used to pump protons across the thylakoid membrane into the thylakoid space. As protons build up a gradient is created. The flow of electrons from the thylakoid to the stroma generates ATP.

19. ATP Synthase

20. Production of NADPH Accepted by ferredoxin and passed down the ETC Electrons are re-excited and leave chlorophyll Used to reduce NADP with a H+ ion NADPH and ATP are passed to the LIR Light (700nm) strikes PSI

21. Non-cyclic Phosphorylation

22. Non-cyclic phosphorylation makes ATP and NADPH (needed for the LIR) Cyclic phosphorylation only makes ATP You need more ATP than NADPH2 for the LIR

23. doesn’t return to same molecule returns to same molecule PSI water PSI and PSII PSI only NADP PSI

27. Light Independent Reactions Takes place in the stroma Uses light energy trapped in the LDR (ATP and reduced NADP) The process is controlled by enzymes Ribulose Bisphosphate Carboxylase (Rubisco) Metabolic cycle Involves carbon fixation

28. The Calvin Cycle - 3 stages CO2 is added to a 5 carbon compound called ribulose bisphosphate (RuBP) 1 Carbon fixation 2 Reduction 3 Regeneration RuBP splits to form glycerate 3 phosphate (G3P) G3P is reduced to triose phosphate (TP) ⅙ TP molecules is used to make hexose bisphosphate ⅚ TP molecules are used to regenerate RuBP

29. CO2 unstable 6C compound P P carbon fixation glycerate 3 phosphate (G3P) P P P ribulose bisphosphate (RuBP) ADP + Pi NADPH2 Regeneration ATP NADP hexose bisphosphate P Reduction P P triose phosphate (TP)

30. Chlorophyll Found within chloroplasts Absorb and capture light Made up of a group of five pigments Chlorophyll a Chlorophyll b Carotenoids: xanthophyll and carotene Phaetophytin Chlorophyll a is the most abundant Proportions of other pigments accounts for varying shades of green found between species of plants

31. Absorption Spectrum PSII absorbs at 680nm PSI absorbs at 700nm Action Spectrum Blue light is high energy and used in photosynthesis Green light is reflected and not absorbed

32. Different pigments absorb different wavelengths of light, making photosynthesis more efficient. The greater the range of wavelengths of light that can be absorbed, the greater the light energy obtained. Primary pigments excite electrons. e.g.chlorophyll a (NB there are different forms of chlorophyll a, e.g. 720 nm and 680 nm). Accessory pigments channel electrons to primary pigment for photoexcitation. e.g.: chlorophyll b, carotene, xanthophyll.

33. Review

34. Limiting Factors The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature. Under a given set of conditions only one factor will affect the rate of photosynthesis this factor is at its minimum and is called the limiting factor The overall rate of photosynthesis is determined by the step that is proceeding most slowly (rate-limiting step). Each factor e.g. light, temperature etc. can become the limiting factor in any on the rate-limiting steps.

41. (a) Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope (4) Award [1] for each of the following clearly drawn and correctly labelled. double/inner and outer membrane/envelope—shown as two concentric continuous lines close together; granum/grana —shown as a stack of several disc-shaped subunits; (intergranal) lamella — shown continuous with thylakoid membrane; thylakoid — one of the flattened sacs; stroma; (70S) ribosomes/(circular) DNA / lipid globules / starch granules /thylakoid space;

42. Effect on the concentration of TP, GP and RuBP

44. Outline the light-dependent reactions of photosynthesis (6) (chlorophyll/antenna) in photosystem II absorbs light; absorbing light/photoactivation produces an excited/high energy/ free electron; electron passed along a series of carriers; reduction of NADP+ / generates NADPH + H+; absorption of light in photosystem II provides electron for photosystem I; photolysis of water produces H+ / O2; called non-cyclic photophosphorylation; in cyclic photophosphorylation electron returns to chlorophyll; generates ATP by H+ pumped across thylakoid membrane / by chemiosmosis / through ATP synthetase/synthase;

45. Explain the effect of light intensity and temperature on the rate of photosynthesis.(8) both light and temperature can be limiting factors; other factors can be limiting; graph showing increase and plateau with increasing light / description of this; graph showing increase and decrease with increasing temperature /description of this; light: affects the light-dependent stage; at low intensities insufficient ATP; and insufficient NADPHH + H+ produced; this stops the Calvin cycle operating (at maximum rate); temperature: affects light-independent stage / Calvin cycle; temperature affects enzyme activity; less active at low temperatures / maximum rate at high temperatures; but will then be denatured (as temperature rises further); Award [5 max] if only one condition is discussed.