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Photosynthesis

Explore the process of photosynthesis, from light absorption to organic compound generation, distinguish between C3 and C4 pathways, and learn about autotrophs and heterotrophs. Delve into the structure of chloroplasts, the Calvin cycle, and photophosphorylation.

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Photosynthesis

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  1. Photosynthesis Department of Botany The Open University of Sri Lanka

  2. Content • Learning Outcomes • Autotrophic organisms • Heterotrophs • Photosynthesis • Light absorption by the chloroplast • The stages of photosynthesis • The C4 plants • Factors affecting photosynthesis • The measurement of photosynthesis

  3. Learning Outcomes After studying this session, you should be able to • explain the mode of obtaining their energy requirement • for maintenance of their biological activities. • describe the structure of leaf, chloroplast and different • pigments associated with photosynthesis • describe the portions of photosynthetically active areas • in the electromagnetic spectrum • explain that light energy is converted to chemical energy, • and carbon is fixed into organic compounds.

  4. describe how light is captured by the plants by means of • pigments and the structural adaptation to do so. • distinguish between the two processes involved in the • photosynthesis i.e., Phase I and Phase II. • explain photophosphorylation, the process in which • NADPH and ATP are generated. • briefly describe the Calvin cycle and its important • reactions. Distinguish between the C3 and C4 pathways of photosynthesis.

  5. Howdowe get energy to do our work? • From food of course. Can we synthesize our food within our bodies? • No we cannot. So we are referred to as • “heterotrophs”. But there are organisms having the • ability to synthesize their own food within their • bodies.Theyare called “autotrophs”.

  6. Categorization of organisms depending on their food habits Live within another organism and obtain nutrition from the host Obtain food from decaying matters Use energy of a chemical reaction for the synthesis of food Eg. Some bacteria like Ferrous bacteria Use energy of sun light for the synthesis of food Eg. All green plants, Cyanobacteria And some bacteria

  7. What is this process of synthesis of food within the bodies of all plants? • It is called “photosynthesis”. And it occurs within • cells of plants. Sun light 6O2 6CO2 C6H12O6 + + 6H2O Glucose Oxygen Carbon dioxide Water Chlorophyll

  8. Where does photosynthesis occur? “chloroplast”. • In higher green plants or eukaryotes this • reaction takes place in the • But in prokaryotic organism photosynthesis • occur in photosynthetic lamella dispersed in the • cytoplasm. • As you know prokaryotes like bacteria and • cyanobacterialack membrane bound organelles.

  9. What is the structural unit of photosynthesis? • It is the “thylakoid” • This structure has the form of a flattened sac or • vesicle.

  10. Do prokaryotes have thylakoids? • In eukaryotes the thylakoid may form a part of • the internal membrane structure of the chloroplast. • To give you a clear picture of the thylakoid • let’s look at the detailed structure of the, • chloroplast.

  11. Do prokaryotes have thylakoids? • Yes. They do, however their thylakoidmembrane • may form a part of the cell membrane or they • may occur in the cytoplasm • In cyanobacteria the thylakoid may be a part • of the elaborate internal structure.

  12. Chloroplast Starch grain External membrane Stroma Inter Membranous space Channel interconnecting thylakoid Internal membrane Grana Thylakoid

  13. Chloroplast • Chloroplasts like mitochondria are bound by • double membranes which are separated by inter- • membrane space. • Look at the picture once again and trace the • membrane. • Inner membrane is smooth in chloroplast • unlike that of mitochondria.

  14. Chloroplast • Thylakoids are formed by a third membranous • system which is the interior of the chloroplast. • Surrounding the thylakoid is a dense solution • which fills the interior of the chloroplast. • This is called the stroma and it’s composition • differs from that of the solution surrounding • other organelles in the cytoplasm.

  15. Chloroplast • You know now that thylakoids are flattened • sacs. They are enclosed by additional • compartments called thylakoidspaces which • also contain another solution with yet another • composition.

  16. Chloroplast • Under the high power of a light microscope • this is how the chloroplasts appear. Chloroplasts • But under an electron microscope we can see • that thylakoids are present in stacks.

  17. Chloroplast • They are called “grana” • In some thylakoidmembranes there are • extensions that interconnect grana through • stromawhich separate them. • Now look at the diagram of the chloroplast • again and try to distinguish all the parts.

  18. Chloroplast • The thylakoid of the chloroplast is oriented • parallel to each other. Thus by swinging towards • the light chloroplast simultaneously aims all of • its million of pigment molecules for optimum • reception. Photosynthetic pigments • You have observed that most of the leaves are • green in color.

  19. Why is that ? • Because leaves have pigments What are pigments? • They are substances which absorb light

  20. What are these pigments? • They are , Chlorophyll a Chlorophyll b Carotene Xanthophylls

  21. What are these pigments? • Among these, chlorophyll is the major and • the most important pigment for • photosynthesis. • Out of chlorophyll, chlorophyll a is present in • all photosynthesizing organisms and is • essential for the process of photosynthesis.

  22. Where are these pigments present? • They are arranged on the lamella of chloroplast in • a very thin layer to facilitate maximum • absorption of light. • They are more concentrated in the thylakoid • disc of the grana.

  23. How can you relate the presence of pigments and photosynthesis? • Presence of pigment is essential for the • photosynthesis to take place. • Because these pigments must absorb light. • Mainly sunlight to provide energy for the reaction • to proceed.

  24. Transverse section of a plant leaf Cuticle Upper epidermis Palisade sheath cell Spongy mesophyll cell Lower epidermis Stoma Guard cell Sub-stomatal air space

  25. Absorption Spectrum of Chlorophyll Chlorophyll a Chlorophyll b 400 500 600 700 800 Wavelength (nm)

  26. Process of photosynthesis inside the Chloroplast CO2 H2O NADP+ ADP RUBP 3-Phosphoglycerate ATP NADPH G3P Starch O2 Amino acid Fatty acid Sucrose

  27. Photosynthesis in summarized form CO2 ATP Light Reaction Dark Reaction Glucose NADPH O2 H2O H2O

  28. CO2 1C Ribulose 1,5-biphosphate 3-phosphoglycerate 5C 3C 6ATP 3ATP 6ADP 3ADP 1,3-diphosphoglycerate Ribulose 5- phosphate 3C 5C C3 Cycle 6NADPH 2Pi 6NADP 6Pi Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate 3C 3C Glyceraldehyde 3-phosphate Sugar, Fatty acid, Amino acids 3C

  29. Electromagnetic spectrum Visible range Gamma rays X-rays Ultraviolet rays Infrared rays Short wave Radar FM TV AM 10-6 102 10-14 10-12 10-10 10-8 10-4 10-2 1 Wavelength (m) 400 500 600 700 Wavelength (nm)

  30. CO2 concentration and rate of photosynthesis Rate of photosynthesis Carbon dioxide concentration Photosynthetic rate in relation to light intensity Rate of photosynthesis Light intensity

  31. The phase of light reaction X - 0.4 ATP FRS Cyclic electron flow ADP + P Q Ferredoxin reductase Cytochrome b Ferredoxin PS II 0 NADP- Cytochrome b Plasto -Quinone ATP +0.4 CytoChrome f ADP + P PS I P 700 O2 + 4H+ Plastocyanin 2H2O Non cyclic electron flow Sun light +0.8 Z P 680 Sun light B

  32. Structure of C4 plant leaf Vascular membrane (Vein) Bundle sheath cell Air space beneath stoma Mesophyll cells

  33. Measurement of Photosynthesis There are two methods by which the rate of photosynthesis can be measured. • Harvest Method • Gas Exchange method

  34. Harvest Method • In this method the rate is determined by the increase • of dry weight of the plant, assuming that when CO2 • is fixed to form sugars the dry weight is increased. • By measuring the dry weight of the plant at the • beginning and at the end of a given period the rate • can be determined.

  35. Harvest Method • To get accurate results weights have to be obtained • over a long period of time. • However this method has its draw backs because • respiration, the oxidation of the produced sugars can • take place simultaneously, leading to decrease in dry • weight.

  36. Gas Exchange method Air bubbles Hydrilla plant

  37. Author – IndraniAmarasinghe Web Content Developer – ChameeraKendaragama Produced by The Open University of Sri Lanka 2014

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