1. 7.2 Photosynthesis Topic 7 Cell Respiration & Photosynthesis

2. Chloroplast Structure Chloroplasts have an envelope, inner and outer membranes. Inside, thylakoid membranes form stacks called grana (granum – sing) Thylakoids provide a large surface area for light absorption. Thylakoids also have a small internal space. Chloroplasts have smaller ribosomes (70S) and circular DNA. The fluid filling the chloroplast is called the stroma. It contains the enzymes for the Calvin Cycle.

3. Chloroplast Structure Ref: Biology for the IB Diploma, Allott

4. Chloroplast Structure Ref: Biology for the IB Diploma, Allott

5. Photosynthesis Photosynthesis is the process that plants, algae and some bacteria use to produce all of the organic compounds they need. Photosynthesis is a metabolic pathway, consisting of many chemical reactions. Photosynthesis consists of; The light dependent reactions produce intermediate compounds for the light independent reactions The light independent reactions where glucose, amino acids and other organic compounds are made.

6. Light Dependent Reactions Chlorophyll absorbs light and the energy from the light raises an electron in the chlorophyll molecule to a higher energy level. An excited electron The chlorophyll is photoactivated Chlorophyll is located in the thylakoid membrane and it is arranged in groups of hundreds of molecules called photosystems. There are two types of photosystem: Photosystem I Photosystem II

7. Light Dependent Reactions These occur in the thylakoid membranes Photosystem II absorbs light energy. This energy is used to excite electrons from chlorophyll to a higher energy level. The excited electron from photosystem II is passed along a chain of carriers in the thylakoid membrane. The electrons lost from photosystem II are replaced by splitting water – photolysis. Oxygen gas is released and the protons can either combine with NADP + at the end of the pathway or be pumped into the thylakoid interior. As the electrons are passed from one carrier to the next, they give up some of their energy. One of these is cytochrome, which acts as a proton pump to pump protons into the thylakoid interior

8. Light Dependent Reactions The protons then diffuse back through ATP synthase, making ATP. This is called non-cyclic photophosphorylation. The coupling of electron transport to ATP synthesis is by Chemiosmosis (like in the mitochondrion). The electrons are then passed to photosystem I, where light energy is absorbed to boost them to a higher level. The electrons are passes through a short chain of carrieres to NADP +, in the stroma, forming NADPH + H +.

9. Light Dependent Reactions Alternatively the electrons can fall back through carriers to photosystem I. As the electrons flow along a chain of carriers they cause pumping of protons across the thylakoid membrane. A proton gradient is formed and this allows production of ATP by ATP synthase. ATP synthesised in this way is called cyclic photophosphorylation.

10. Ref: Biology for the IB Diploma, Allott

11. Ref: IB Biology Higher Level, OSC

12. Photophosphorylation This is the synthesis of ATP using the energy from light. The principle is the same as for oxidative phosphorylation in the mitochondria. Electrons flow through the carriers, including photosystem II and I As the pass through the cytochrome carrier, protons are pumped into the interior of the thylakoid. The thylakoid interior is small to increase the concentration of protons. The protons then flow back by diffusion down a concentration gradient into the stroma through ATP synthase ATP is generated from ADP + P

13. Photophosphorylation Ref: IB Biology Higher Level, OSC

14. Cyclic and Non-cyclic Photophosphorylation The difference between cyclic and non-cyclic photophosphorylation is that with non-cyclic, the electrons continue on the end in NADP +, whereas in cyclic they are returned to the chlorophyll in photosystem I. The cyclic pathway has TWO advantages: It provides additional ATP needed to drive the light independent reactions. The Calvin Cycle requires ATP and NADPH + H + in the ratio of 3:2. Non-cyclic photophosphorylation gives a ratio of 1:1. It provides ATP that can be used for other processes such as stomatal opening by guard cells.

15. Light Independent Reactions The light independent reactions are driven by the ATP and NADPH + H + made in the light dependent reactions. Both sets of reactions continue at the same time. The light independent reactions occur in the stroma. It is a cyclic system called the Calvin Cycle.

16. Ref: Biolgy, Campbell 7 th ed.

17. Light Independent Reactions Carbon Fixation: CO 2 is an essential substrate in the Light In-dependent reactions. It enters the chloroplast by diffusion. At the start of the Calvin cycle, CO 2 combines with ribulose bisphosphate (RuBP), a five carbon compound. This is a carboxylase reaction and the enzyme is called rubisco. The product of this reactions is a 6 carbon compound which immediately splits to form TWO molecules of glycerate 3- phosphate.

18. Light Independent Reactions Carbohydrate Synthesis: Glycerate 3-phosphate is converted into a carbohydrate by a reduction reaction. Hydrogen is supplied by NADPH. Energy is also required in the form of ATP. Both NADPH and ATP were synthesised in the Light Dependent Reactions. Gylcerate 3-phosphate is reduced to a 3 carbon sugar, triose phosphate. 2 triose phosphate molecules are joined together to form glucose phosphate. Condensation reactions join the glucose phosphate molecules together to form Starch.

19. Light Independent Reactions Not all of the triose phosphate sugar molecules are used to produce glucose. Some are used to resynthesise ribulose bisphosphate (RuBP). In fact 5/6 of the triose phosphate molecules are used to resynthesise RuBP. Only 1/6 of the triose phosphate molecules produce is use to make glucose phosphate. Thus, 2 turns of the calvin cycle produce 1 molecule of glucose phosphate.

20. Ref: IB Biology Higher Level, OSC

21. Ref: Biology for the IB Diploma, Allott

22. Action Spectrum of Photosynthesis A spectrum is a range of wavelengths of electromagnetic radiation (the radiation emitted from the sun). The spectrum of visible light is the range of wavelengths from 400nm to 700 nm. Each wavelength is colour of light: 400-525 violet-blue 525-625 green-yellow 625-700 orange-red The efficiency of photosynthesis is not the same in all wavelengths. The Action Spectrum shows the efficiency of the different wavelengths of light that are used in photosynthesis.

23. Action Spectrum of Photosynthesis

24. The Absorption Spectrum of Photosynthesis The Absorption Spectrum of Photosynthesis shows how much of a particular wavelength of light that are absorbed by the photosynthetic pigments. The two most common pigments are chlorophyll a and chlorophyll b. The action spectrum and absorption show strong similarities: Greatest absorption in violet-blue range. Also a high level of absorption in the red range. The lowest absorption in the yellow-green range. There is a close correlation between the action spectrum and the absorption spectrum.

25. Limiting Factors There are 3 main factors that can affect the rate of photosynthesis: Light intensity Temperature Concentration of Carbon dioxide. Changes to one of these factors can affect the rate of photosynthesis. The factor that is nearest to its minimum is called the Limiting Factor.