class 1 2 chloroplast structure and function and light dependent reaction.pptx

A2 respiration and chloroplast presentation

1. Photosynthesis wednesday 18/9/2024 3 classes Chapter 13

2. 13 Photosynthesis 13.1 Photosynthesis as an energy transfer process Learning outcomes Candidates should be able to: 1 describe the relationship between the structure of chloroplasts, as shown in diagrams and electron micrographs, and their function 2 explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules.

3. 3 state that within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light-dependent stage and the stroma is the site of the light-independent stage. 4 describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in thylakoids 5 interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis

4. 6 describe and use chromatography to separate and identify chloroplast pigments (reference should be made to Rf values in identification of chloroplast pigments) 7 state that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis 8 explain that in cyclic photophosphorylation: • only photosystem I (PSI) is involved • photoactivation of chlorophyll occurs • ATP is synthesised

5. 9 explain that in non-cyclic photophosphorylation: • photosystem I (PSI) and photosystem II (PSII) are both involved • photoactivation of chlorophyll occurs • the oxygen-evolving complex catalyses the photolysis of water • ATP and reduced NADP are synthesised

6. 10 explain that during photophosphorylation: • energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected) • the released energy is used to transfer protons across the thylakoid membrane • protons return to the stroma from the thylakoid space by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected)

7. 13.1 Photosynthesis as an energy transfer process Light energy absorbed by chloroplast pigments in the light dependent stage of photosynthesis is used to drive reactions of the light independent stage that produce complex organic compounds. Chapters subtopics: 13.2 Investigation of limiting factors Environmental factors influence the rate of photosynthesis. Investigating these shows how they can be managed in protected environments used in crop production.

8. 13.1 An energy transfer process In Chapter 12 you learnt how all organisms use ATP as their energy currency. They make ATP by using the chemical potential energy contained in glucose and other organic molecules to join a phosphate group to ADP. This is done by respiration, which is a series of enzyme controlled reactions that take place in every living cell. In this chapter you will find out how the glucose that is used in respiration is produced. It is done by photosynthesis, in which energy in sunlight is transferred to carbohydrates such as glucose. Like three of the stages in respiration, photosynthesis happens in specialised organelles inside cells.

9. The process of photosynthesis occurs in two stages, which happen in separate parts of the chloroplast. Figure 13.1 shows an overview of the complete process of photosynthesis. These are chloroplasts and they are found in In eukaryotic cells, such as green plants and in many protoctists, such as Euglena.

10. Chloroplast structure and function In eukaryotic organisms, the photosynthetic organelle is the chloroplast. In dicotyledons, chloroplasts can be seen with a light microscope, it is about 3-10 μm in diameter There may be only a few chloroplasts in a cell or many as 100 in some palisade mesophyll cells. Chloroplasts are double-membrane-bound organelles with an oval shape measuring approximately 7–10µm in length https://www.youtube.com/watch?v=OF_F9cAvLlQ

11. The structure of a chloroplast is shown in Figure 13.2.

12. https://www.youtube.com/watch?v=YxUG5EuBI-E

13. Each chloroplast is surrounded by an envelope of two phospholipid membranes . The membrane system is the site of the light dependent reactions of photosynthesis. The grana (singular granum) are made from thylakoid membranes. The thylakoid membranes are the sites where light energy is harnessed to make ATP. Each pigment passes energy to the next member of the cluster, finally feeding it to the chlorophyll a reaction center ( primary pigment)

14. The membranes of the grana provide a large surface area, which holds the pigments, enzymes and electron carriers needed for the light dependent reaction. The membranes make it possible for a large number of pigment molecules to be arranged so that they can absorb as much light as necessary. The pigment molecules are also arranged in particular light- harvesting clusters for efficient light absorption. In each photosystem, the different pigments are arranged in the thylakoid in funnel like structures

15. The membranes of the grana hold ATP synthase and are the sites of ATP synthesis by chemiosmosis

16. The stroma is the site of enzyme-catalysed anabolic reactions that use carbon dioxide for the synthesis of carbohydrate. Which is the site of the light independent reactions , it contains the enzymes of the Calvin cycle , sugar and organic molecules A system of membranes also runs through the ground substance, or stroma. Also within the stroma are small (70 S) ribosomes, a loop of DNA, lipid droplets and starch grains. The loop of DNA codes for some of the chloroplast proteins, which are made by the chloroplasts ribosomes.

17. However , other chloroplast proteins are coded for by the DNA in the plant cell nucleus. https:// www.youtube.com/watch?v=ULXqFtlgKf4&list=RDCMUC8pOYw9kw8z9uOKgAZ7ki8w&index=2

18. The light-dependent stage The light-dependent stage of photosynthesis occurs in the thylakoids. The light dependent stage : • uses energy from light • synthesises ATP • reduces a coenzyme called NADP • uses water to supply hydrogen and electrons • produces oxygen as a waste product. Figure 13.4 shows an overview of the light-dependent stage.

19. There are pigments located within the thylakoid membranes that absorb light. Each pigment absorbs some wavelengths better than others. The use of more than one pigment enables more wavelengths of light to be absorbed, so more efficient use can be made from sunlight. The important pigments are divided into two groups: 1 . the chlorophylls – chlorophyll a and chlorophyll b 2 . the carotenoids – carotene and xanthophyll. Photoactivation: Trapping light energy : https:// www.youtube.com/watch?v=k_R6i6Rmq7k Class 2

20. Chlorophylls a and b mainly absorb the violet blue (400 –500 nm) and red wavelength (600 -700 nm). They reflect green. Carotenoids absorb mainly in the blue- violet and green  regions (500-600 nm) . Pigment Colour Chlorophyll a Green Chlorophyll b Blue β carotene orange xanthophyll yellow

21. The pigments are organised into groups called photosystems within the membrane. https:// www.youtube.com/watch?v=k_R6i6Rmq7k There are two photosystems: photosystem I and photosystem II https:// www.youtube.com/watch?v=jTnNGIx5-P8

22. The graph in Figure 13.5 is called an absorption spectrum as it shows the relative absorption of each pigment across the wavelengths present in visible light. The absorption spectrum is made using separate, purified, pigments. Photoactivation: Trapping light energy :

23. The graph in Figure 13.6 has a line called an action spectrum because it shows the rate of photosynthesis at different wavelengths.

24. Figure 13.6 is a comparison of an absorption spectrum and an action spectrum for the same leaf.

25. An action spectrum is often shown for the whole leaf or for whole chloroplasts , as photosynthesis will not occur only in the presence of a purified pigment. This is because a variety of other molecules are required to harness the energy captured by the pigment. Light of the correct wavelength is absorbed by a complex of very many photosynthetic pigment molecules. The energy transferred to these pigment molecules is, in turn, passed to a chlorophyll molecule. An electron from chlorophyll uses this energy from the light to leave its orbital. This is called photoexcitation.

26. The excited electron is taken by an electron acceptor. As the electron has a negative charge, it leaves behind a positively charged chlorophyll ion. The event where the electron is taken by an electron acceptor is called photoionisation. It is the energy of these electrons that is used for ATP synthesis.

27. Experimental skills 13.1: Chromatography of photosynthetic pigments Health and safety : propanone liquid and vapour is highly flammable, causes drowsiness and dizziness if vapour is inhaled, and causes dryness and cracking of skin on repeated contact with liquid. Use as small a volume as possible. Use in a well-ventilated location. Do not use near naked flames.

28. Wear eye protection and avoid contact with skin. A student obtained a green leaf from spinach and a green seaweed. You can easily extract chloroplast pigments from leaf to see how many pigments are present, by using paper chromatography https://www.youtube.com/watch?v=ej2zXOwASVI Then You can calculate the R f value ( retention factor) for each pigment, using this equation:

29. R f = distance travelled by pigment spot/ distance traveled by solvent these will vary depending on the solvent used, but in general carotenoids have R f value close to 1, while chlorophyll b has a much lower R f value and chlorophyll a has an R f value between those of carotenoids and chlorophyll b

30. The student extracted the photosynthetic pigments from each of these samples using the following procedure . 1-Some green tissue, some sand and a small volume of propanone (acetone) were crushed together using a pestle and mortar. 2- Chromatography paper was prepared by drawing a pencil line close to the bottom of the paper. The letter X was placed, using a pencil, at the positions on the line where the samples from the spinach and the seaweed would be placed.

31. 3- A small volume of propanone was placed into a beaker to a depth less than the height of the pencil line from the bottom of the chromatography paper. 4- The green extract from the crushed sample was spotted onto the pencil X using the head of a large  pin. 5- Each spot was allowed to dry before applying more of the extract to the same position. This was repeated many times, to make a small very dark green spot of pigment. 6-The chromatography paper was placed into the beaker as shown in Figure 13.11.

32. 7-The chromatography paper was removed when the propanone from the bottom of the beaker had travelled almost to the top of the paper. https://www.youtube.com/watch?v=vxhp8SfJUWU

33. The position of the X is called the origin and the position on the chromatography paper reached by the propanone is called the solvent front . The pigments are carried different distances from the origin. This is due to differences in solubility in the solvent, differences in molecular sizes and differences in attraction to the paper. The results, called chromatograms, are shown in Figure 13.12.

34. Photophosphorylation The use of light energy to phosphorylate ADP forming ATP is called photophosphorylation . This process can be cyclic or non-cyclic, depending on the fate of the electron that was released in the photoionisation step. Cyclic photophosphorylation uses only photosystem I (P700). Light causes electrons to be excited and removed from chlorophyll a . These electrons are passed through a chain of electron carriers before returning to the chlorophyll a . Class 3

35. During the electron transfer process in the carriers, enough energy is released by the electrons to later enable the formation of ATP from ADP and inorganic phosphate. The process is called cyclic because electrons are removed from chlorophyll a and the electrons return to chlorophyll a . Cyclic photophosphorylation is summarised in Figure 13.8.

36. Non-cyclic photophosphorylation uses both photosystems. (PSI,PSII)./ ( P700,P680). It is called non-cyclic because the electrons are not returned to the pigment from which they were removed. Photolysis occurs first in photosystem II. This is where a water molecule is split, as shown in the equation H 2 O → 2H+ + 2e– + ½O 2 Photolysis is catalysed by the oxygen-evolving complex. This is an enzyme that is surrounded by the proteins of photosystem II.

37. When light energy is absorbed by the pigments in photosystem II, this energy is passed to the oxygen-evolving complex to enable the splitting of water. Photolysis is the source of oxygen, which is a waste product of photosynthesis. The two electrons are transferred to photosystem II. When it absorbs light energy, the electrons become ‘excited’. Instead of returning to photosystem II, they pass through an electron transport chain to photosystem I. As they do so they yield energy which will later enable the formation of ATP.

38. When light is absorbed by photosystem I, the electrons are excited again and this time used, with the hydrogen ions from water, to reduce NADP. Non-cyclic photophosphorylation is summarised in Figure 13.9.

39. In the case of both cyclic and non-cyclic photophosphorylation, chemiosmosis is used to synthesise ATP. Energy from the electron transport chains is used to pump protons across the thylakoid membranes and into the spaces between these membranes. This builds up a high concentration of hydrogen ions in the thylakoid space. Protons are then allowed to flow out into the stroma of the chloroplast by facilitated diffusion through ATP synthase, which turns ADP into ATP. This process is summarised in Figure 13.10.