PHOTOSYNTHESIS

Photosynthesis – synthesis using light

General Equation : 6CO2 + 6H2O C6H12O6 + 6O2

Mesophyll – most active photosynthetic tissue

Photosynthetic Reactions:1. Thylakoid reactions2. Carbon fixation reactions

Nature of Light

1. Light is both a particle and wave. photon – particle quantum – amount of energy of light wavelength – distance between crests frequency – no. of wave crests per unit time

Absorption Spectrum

Absorption spectrum – a display of the amount of light energy taken up by a molecule as a function of the wavelength of light Visible region –what our eyes are sensitive to Short wavelength – high frequency, high energy Long wavelength – low frequency , low energy

Absorption Spectrum of Chlorophyll

Change in Electronic State

Upon Absorption of Light Energy: Chl + hv Chl*Pathways for Excited Chlorophyll to dispose its energy :1. Fluorescence – re-emit a photon2. Direct convertion to heat ; no emission of

photon3. Energy transfer4. Photochemistry – energy causes occurence of

chemical reactions

Action Spectrum

Photosynthetic Overview

Energy Transfer during Photosynthesis

Resonance transfer – excitation energy is conveyed from the chlorophyll that absorbs the light to the reaction center

Antenna Complexes

• Eukaryotes – within the chloroplast• Prokaryotes – plasma membrane

Light Reactions : Concepts

Quantum Yield – number of photochemical products per total number of quanta absorbed Hill reactions : Robert HillIn the light, isolated chloroplast thylakoids reduce a variety of compounds, eg. Iron salts

Enhancement effect : Robert EmersonThe rate of photosynthesis was greater when red and far-red light were given together than the sum of their individual rates

Z-scheme

Photosystems I and II : Differences1. PS l produces a strong reductant, capable of reducing

NADP, and a weak oxidant.2. PS ll produces a very strong oxidant, capable of

oxidizing water, and a weaker reductant than he one produced by PS l

3. PS l : found in the stroma lamella and edges of grana lamella PS ll : predominantly located in the grana lamella

Oxygenic organisms – Oxygen-evolving organisms

Chloroplast Structure

Electron Transfer in the Thylakoid Membrane: 4 Protein complexes

Photochemical Event1. Transfer of an electron from the chlorophyll

to an acceptor molecule: chlorophyll is in oxidized state – electron deficient Acceptor is in reduced state – electron rich2. Water is oxidized to Oxygen by PS ll 2 H2O O2 + 4H+ + 4 e-

protons – released into lumen of thylakoid, to stroma by ATP synthase

3. Pheophytin and 2 Quinones accept electrons4. Electrons flow through Cytochromes b6f complex5. Plastoquinone and Plastocyanin carry electrons between PS ll and l6. PS l Reaction Center Reduces NADP

Interference in Photosynthetic Electron Flow: Herbicides : DCM (dichlorophenyl-dimethylurea) Paraquat

Carbon Reactions

1. Calvin Cycle / Reductive Pentose Phosphate Cycle / C3 Cycle

2. C4 Photosynthetic Carbon assimilation Cycle

3. Photorespiratory Carbon Oxidation Cycle

Calvin Cycle : Stages

1. Carboxylation: CO2 + RuBP 3- Phosphoglycerate 2. Reduction of 3- Phosphoglycerate to form Glyceraldehyde-3-phosphate3. Regeneration of the CO2 acceptor , RuBP

Carbon Reactions

Rubisco- Ribulose bisphosphate carboxylase/oxygenase enzyme Competition:O2and CO2 for the substrate Ribulose

bisphosphate Effect : Limits net CO2 fixation

Autocatalytic- regeneration of biochemical intermediatesStoichiometry : 1/6 – for sucrose or starch production 5/6 – for regeneration of ribulose-1,5-bisphosphate

Regulation of the Calvin Cycle:

1. Light-dependent enzyme activation Rubisco, NADP:glyceraldehyde-3-phosphate dehydrogenase;fructose-1,6-bisphosphatase, Sedoheptulose-1,7—bisphosphatase, ribulose-5-phosphate kinase2. Increases in Rubisco activity due to light3. Light-dependent ion movements4. Light-dependent membrane transport

Photorespiration

Oxygenation – combination of Rubisco with Oxygen instead of CO2.

- results to CO2 loss

Rise in temperature effect: decrease in CO2 relative to O2 enhances the kinetic properties of Rubisco

C3 and C4 Leaf Anatomy

C4 Metabolism

1. CO2 fixation by PEP in mesophylly to form a C4 acid ( malate or aspartate)

2. Transport of C4 acids to bundle sheath cells

3. Decarboxylation of C4 acids within bundle sheath cells and generation of CO2 which is brought to Calvin cycle.

4. Transport of the C3 acid back to the mesophyll

C4 Cycle

Advantage of C4 pathway1. Concentrates CO2 in the bundle sheath cells

C4 Plants : Grasses, sugarcane, maize2. Reduces photorespiration

Crassulacean Acid Metabolism

-enables plants to improve water use efficiently1 g CO2: 400 to 500 g water loss forC3 and C4 : 50 to 100 g water los for CAM plantsTemporal and spatial separation : formation of C4 acids

Physiological and Ecological Considerationsof Photosynthesis

• Important Metabolic Steps for Optimum Photosynthesis:

1. Rubisco activity = low CO2; High light Intensity2. Regeneration of RuBP = High CO2 ; Low Light3. Metabolism of Triose PhosphatesLight Parameters:1. Spectral quality 3. Direction of Light2. Amount of Light