6.6 Chromatography Subtopics 6.1 Overview of photosynthesis 6.2 Absorption spectrum of photosynthetic pigments 6.3 Light dependent reaction 6.4 Light independent reaction/ Calvin cycle 6.5 Alternative mechanisms of carbon fixation: Hatch-Slack /C4 and Crassulacean Acid Metabolism/CAM pathways Topic 6: Photosynthesis
6.1 Overview of photosynthesis Objectives: a) Outline the complete process of photosynthesis: Light dependent and light independent reactions
Definition • The conversion of light energy to chemical energy that is stored in sugar/glucose or other organic compounds. (Campbell 11th ed., pg. 259) • Occurs in plants, algae & certain prokaryotes. • Using molecular formulas, the complex series of chemical reactions can be summarized with this chemical equation (Campbell 11th ed., pg. 262) • water appears on both sides of equation because 12 molecules are consumed and 6 molecules are newly formed during photosynthesis
• The equation is simplified by indicating only the net consumption H2O • For photosynthesis, green plants use - sunlight as an energy source, - CO2 & H2O as raw materials. • The light energy trapped by green plant - is converted to chemical energy & - stored in the bonds of organic molecules. - O2 is released as a byproduct.
• Photosynthesis occurs in the chloroplasts. • Chloroplasts are found mainly in mesophyll cells. • O2 exits & CO2 enters the leaf through microscopic pores, stomata, in the leaf. • A typical mesophyll cell has 30 - 40 chloroplasts.
• Photosynthesis occurs in the chloroplasts. • Structure of Chloroplast – The thylakoid membranes & grana increase the surface area for attachment of chlorophyll molecules, accessory pigments & electron carriers involved in the light dependent reactions. – The thylakoid space contains H+ involved in chemiosmotic synthesis of ATP.
– The thylakoid membranes are surrounded by semi-fluid; stroma. – Stroma contains circular DNA, ribosomes & enzymes which catalyse light-independent reactions, starch granules & lipid granules. ▪ Photosynthetic pigments are embedded in the thylakoid membranes. ▪ Function of the pigments: to trap / absorb the light energy.
Photosystem • A large complex into which the lightabsorbing pigments are organized with proteins and other molecules” • Located in the thylakoid membrane of the chloroplast • Involved in the light-dependent reaction • Composed of a reaction centercomplex • The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor.
• Each light-harvesting complex consists of various pigment molecules (which may include chlorophyll a, chlorophyll b and multiple carotenoids) bound to proteins • light-harvesting complexes act as an antenna for the reactioncenter complex Two types of photosystem: • Photosystem I (PSI) • Photosystem II (PSII)
6.2 Absorption Spectrum of Photosynthetic Pigments Objectives: a) State the photosynthetic pigments involved in photosynthesis.
Introduction • Chlorophyll is the green pigment that absorbs light energy during photosynthesis. – gives the green colour to the leaves. • When light meets matter, it may be reflected, transmitted or absorbed. • Different pigments absorb photons of different wavelengths. – A leaf looks green because chlorophyll (dominant pigment) • absorbs red & blue light, while transmitting & reflecting green light. – Photon: small particles, packets of energy in the light.
Photosynthetic pigments • Chlorophyll a – blue-green pigment – absorbs best in the red & blue-violet light and reflecting green light – The most abundant photosynthetic pigment – Exists in several forms; depend on its red absorption peak; may be at 680nm or 700nm – act as reaction center
• Chlorophyll b - yellow-green pigment - absorbs best in the blue wavelength - acts as accessory / antenna molecule - absorbs light energy but have to transfer the energy to chlorophyll a. • Carotenoids - yellow, orange, red or brown pigments - absorb violet and blue green light - acts as accessory pigments / antenna molecules - because they pass the light energy to chlorophyll a in the reaction centre.
Chromatography • Is a technique used to separate photosynthetic pigments • Pigments are extracted from the leaf using acetone & separated on a chromatography paper using specific solvent • Pigments identified: a) Chlorophyll a (blue-green) b) Chlorophyll b (yellow-green) c) Xanthophyll (yellow) d) Carotene (orange) e) Pheophytin (grey)
Electromagnetic Spectrum • The entire range of electromagnetic radiation is the electromagnetic spectrum. • The segment most important for life is the narrow band between 380 to 750 nm. – visible light: because it is detected as various colours by human eye
6.3 Light Dependent Reaction Objectives: a) Explain the cyclic and noncyclic photophosphorylation. (C3) Will be discussed in detail during tutorial.
LIGHT DEPENDENT REACTION • Occurs in the thylakoid membrane • Involve photoactivation of chlorophyll resulting in the conversion of light energy into ATP through a complex and produce NADPH + H+ • ATP is produced through photophosphorylation What exactly happen when chlorophyll and other pigments absorb light? Light absorption • Photons are absorbed by accessory photosynthetic pigments molecules (antenna).
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2 routes for electron flow Noncyclic Photophosphorylation (noncyclic electron flow) Cyclic Photophosphorylation (cyclic electron flow)
Noncyclic photophosphorylation (noncyclic electron flow) I. Noncyclic Photophosphorylation (Linear electron flow)
Cyclic photophosphorylation (Cyclic electron flow)
The light dependent reactions: organization of the thylakoid membrane.
Differentiate between Cyclic and Noncyclic Photophosphorylation Criteria Cyclic photophosphorylation Non-cyclic photophosphorylation Photosystem involved • Involve Photosystem I • Involve Photosystem I and Photosystem II Products • ATP only • ATP, NADPH + H+ and O2 Electron flow • Cyclic electron flow • Non-cyclic electron flow Photolysis of water • Photolysis of water does not occur • Photolysis of water occur
Differentiate between Cyclic and Noncyclic Photophosphorylation Criteria Cyclic photophosphorylation Non-cyclic photophosphorylation Oxygen • No oxygen release • Oxygen release from water Electron donor • First electron donor is PSI/ none • First electron donor is water Final electron acceptor • Last electron acceptor: none/ PS I • Last electron acceptor: NADP+
6.4 Light independent reaction / Calvin Cycle Objectives: a) Explain Calvin cycle involving carbon fixation, reduction and regeneration of RuBP Will be discussed in detail during tutorial.
• Uses ATP and NADPH to convert CO2 to glucose/sugar - ATP: source of energy - NADPH: reducing agent • Occurs in the stroma of the chloroplast and each stage is mediated by an enzyme • Consist of three stages: 1. Carbon fixation 2. Reduction 3. Regeneration of CO2 acceptor (RuBP) Light independent reaction (Calvin cycle)
Light independent reaction (Calvin cycle)
6.5 Alternative Mechanism of Carbon Fixation: C4 and Crassulacean Acid Metabolism (CAM) Pathways Objectives: a) Explain photorespiration (C3) b) State the alternative mechanism of carbon fixation (C4 and Crassulacean Acid Metabolism (CAM) pathways) (C1) c) Explain carbon fixation in C3, C4 and CAM plants (C3) C3 subtopics will be discussed in detail during tutorial.
Hatch-Slack (C4 ) Pathway
Differences C3, C4 and CAM Pathways C3 plant C4 plant CAM plant 1. Definition Major type of photosynthesis produces 3-carbon compound in Calvin cycle A type of photosynthesis produces intermediate 4-carbon compound, then split into 3-carbon compound in Calvin cycle A type of photosynthesis which gathers sunlight during the day and fix CO2 at night 2. Location Occur in mesophyll cell Occur in bundle sheath cell and mesophyll cell Occur in mesophyll cell 3. First stable product 3-Phosphoglycerate (3-PGA) Oxaloacetate (OAA) 3-PGA at daytime and OAA at night 4. Optimum temperature 15 – 25 oC 30 – 40 oC > 40 oC 5. Carboxylation enzyme RUBP carboxylase oxygenase (Rubisco) PEP carboxylase in mesophyll, Rubisco in bundle sheath cell PEP carboxylase in the dark, Rubisco in the daytime 6. Photorespiration High Minimize by performing CO2 fixation and Calvin cycle in separate cell Minimize by performing CO2 fixation and Calvin cycle in separate times 7. Examples of plants Wheat, rice, potato Sugarcane, maize, Sorghum Cactus, orchid, vanilla
Reference Websites https://www.youtube.com/watch?v=zWO-bTi6u8M https://www.youtube.com/watch?v=SnnmmKApT-c https://www.youtube.com/watch?v=IRg9NJAS2Q8 https://www.youtube.com/watch?v=c2ZTumtpHrs https://www.youtube.com/watch?v=13h5oC4jIsk