Vital metabolism for survival of life in the earth Prof Adinpunya Mitra Agricultural & Food Engineering Department
THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH
THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists Autotrophs generate their own organic matter through photosynthesis Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (d) Cyanobacteria (a) Mosses, ferns, and flowering plants (b) Kelp
Light Energy Harvested by Plants & Other Photosynthetic Autotrophs 6 CO 2 + 6 H 2 O + light energy C 6 H 12 O 6 + 6 O 2
WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors Gamma rays X-rays UV Infrared Microwaves Radio waves Visible light Wavelength (nm)
WHY ARE PLANTS GREEN? Sunlight minus absorbed wavelengths or colors equals the apparent color of an object. Transmitted light
Structure of Plant Cell and Chloroplast
Image of plant cells Electron Microscopic View Light Microscopic View
WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).
Chloroplast Structure Inner membrane called the thylakoid membrane. Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural grana) Stroma is a liquid surrounding the thylakoids.
THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED Chloroplasts absorb light energy and convert it to chemical energy Light Reflected light Absorbed light Transmitted light Chloroplast
AN OVERVIEW OF PHOTOSYNTHESIS Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS
AN OVERVIEW OF PHOTOSYNTHESIS The light reactions convert solar energy to chemical energy Produce ATP & NADPH The Calvin cycle makes sugar from carbon dioxide Light Light reactions Chloroplast NADP ADP + P Calvin cycle ATP generated by the light reactions provides the energy for sugar synthesis The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose
Chloroplasts: Sites of Photosynthesis Photosynthesis Occurs in chloroplasts, All green plant parts have chloroplasts and carry out photosynthesis The leaves have the most chloroplasts The green color comes from chlorophyll in the chloroplasts The pigments absorb light energy
Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts A chloroplast contains: stroma, a fluid grana, stacks of thylakoids The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis
The location and structure of chloroplasts Chloroplast LEAF LEAF CROSS SECTION MESOPHYLL CELL Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Stroma Thylakoid Thylakoid compartment
Chloroplast Pigments Chloroplasts contain several pigments Chlorophyll a Chlorophyll b Carotenoids Figure 7.7
Chlorophyll a & b Chl a has a methyl group Chl b has a carbonyl group Porphyrin ring delocalized e - Phytol tail
Different pigments absorb light differently
Pigment Absorption
How Light is Captured?
The Light Reactions (light dependent) Photosystem I cyclic photophosphorylation Photosystem II noncyclic photophosphorylation Photolysis
The Z scheme (Light Reactions)
Cyclic Photophosphorylation Process for ATP generation associated with some Photosynthetic Bacteria Reaction Center => 700 nm
Noncyclic Photophosphorylation Photosystem II regains electrons by splitting water, leaving O 2 gas as a by-product Primary electron acceptor Primary electron acceptor Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis
Concept of Light Reaction Two types of photosystems cooperate in the light reactions ATP mill Water-splitting photosystem NADPH-producing photosystem
Plants produce O 2 gas by splitting H 2 O The O 2 liberated by photosynthesis is made from the oxygen in water (H + and e - )
How the Light Reactions Generate ATP and NADPH? Primary electron acceptor NADP Primary electron acceptor 2 Energy to make 3 Light Light Primary electron acceptor 1 Reactioncenter chlorophyll NADPH-producing photosystem Water-splitting photosystem 2 H + 1 / 2
Z-scheme of Photosynthesis
In the light reactions, electron transport chains generate ATP, NADPH, & O 2 Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up in ATP and NADPH
Chemiosmosis powers ATP synthesis in the light reactions
Chemiosmosis powers ATP synthesis in the light reactions The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H + through that membrane The flow of H + back through the membrane is harnessed by ATP synthase to make ATP In the stroma, the H + ions combine with NADP + to form NADPH
The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H + ) Light Light Thylakoid membrane Antenna molecules Stroma (low H + ) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE
A Photosynthesis Road Map Chloroplast Light Stroma Stack of thylakoids Light reactions NADP ADP + P Calvin cycle Sugar used for Cellular respiration Cellulose Starch Other organic compounds
The Calvin Cycle in Photosynthesis The Calvin cycle is the mechanism for turning CO 2 into glucose
(Ideal) 6CO 2 (Real) Calvin Cycle Strategy 6CO 2 + 6RuBP Glucose G6P + 6RuBP 6C 30C 6C 30C (Mechanism) 6CO 2 + 6RuBP 12, 3PGA
Light-independent reactions (Dark Reactions) The Calvin/Benson/C 3 cycle Has 3 phases: 1. carbon fixation phase 2. reduction of CO 2 phase 3. regeneration of RuBP phase
Calvin Cycle, earlier designated the photosynthetic "dark reactions," is now called the carbon reactions pathway: grana disks (thylakoids) Chloroplast 2 outer membranes stroma compartment The free energy of cleavage of ~P bonds of ATP, and reducing power of NADPH, are used to fix and reduce CO 2 to form carbohydrate. Enzymes & intermediates of the Calvin Cycle are located in the chloroplast stroma, a compartment somewhat analogous to the mitochondrial matrix.
O H 2 C OPO 3 2- C -O C O H C OH H C OH H C OH H 2 C OPO 3 2- H 2 C OPO 3 2- Ribulose-1,5-bisphosphate (RuBP) 3-Phosphoglycerate (3PG) Ribulose Bisphosphate Carboxylase (RuBP Carboxylase), catalyzes CO 2 fixation: ribulose-1,5-bisphosphate + CO 2 2 3-phosphoglycerate Because it can alternatively catalyze an oxygenase reaction, the enzyme is also called RuBP Carboxylase/Oxygenase (RuBisCO). It is the most abundant enzyme on earth.
carbon fixation phase
Reduction of CO2 phase
Regeneration of RuBP phase
3-PGA ATP ADP 36c Carbon Balance in Calvin Cycle 1,3 bispo 4 glycerate NADPH + H + NADP + Glyceraldehyde-3-PO 4 36c 24c DHAP 6c Calvin Calvin 12c Fructose 1,6bisPO 4 Fructose-6-PO 4 Glucose-6-PO 4 6c 18c Calvin 12c
Synthesis of Sucrose and Starch F6P G6P G1P ATP PP i ADP-glucose (glucose) n UTP PP i UDP-glucose Cytosol fructose-6-po 4 Stroma ADP + (glucose) n+1 Starch (amylose) UDP + Sucrose-6-PO 4 H 2 O P i Sucrose
Thank goodness for photosynthesis
Summary of Calvin Cycle O C carbon dioxide O H C H 2 C CHO OH OPO 3 2 glyceraldehyde- 3-phosphate CO 2 + ATP + NADPH Glyceraldehyde-3-P + ADP + P i + NADP + Glyceraldehyde-3-P may be converted to other CHO: metabolites (e.g., fructose-6-p, glucose-1-p) energy stores (e.g., sucrose, starch) cell wall constituents (e.g., cellulose). Glyceraldehyde-3-P can also be utilized by plant cells as carbon source for synthesis of other compounds such as fatty acids & amino acids.
Summary of Calvin Cycle
Photorespiration: O 2 can compete with CO 2 for binding to RuBisCO, especially when [CO 2 ] is low & [O 2 ] is high. O O C H C OH H 2 C OPO 3 2 3-phosphoglycerate O C H 2 C O 2 OPO 3 phosphoglycolate When O 2 reacts with ribulose-1,5-bisphosphate, the products are 3-phosphoglycerate plus the 2-C compound 2-phosphoglycolate. This reaction is the basis for the name RuBP Carboxylase/Oxygenase (RuBisCO). Photorespiration is a wasteful process, substantially reducing efficiency of CO2 fixation, even at normal ambient CO2
O 2 O 2 O 2 O 2 PHOTORESPIRATION Definition 1: The aberrant use of oxygen by chloroplasts Definition 2: O 2 O 2 An interference with carboxylation caused by the deviant interaction of RUBISCO with oxygen A process that leads to only one 3PGA being produced in the dark reaction in chloroplasts
Strategy for Preventing Photorespiration PLAN Avoid RUBISCO Fix CO 2 in an environment shielded from O 2 Use an enzyme that does not react with O 2
Strategy (cont.) SOLUTION CO 2 fixation occurs in Mesophyll cells CO 2 fixing enzyme is not RUBISCO CO 2 fixing enzyme is PEP carboxylase PEP carboxylase will not react with O 2 Take Home RUBISCO never changed Instead plant anatomy changed
OAA (4C) Malate PEP (3C) Pyruvate (C3) CO 2 ADP ATP Separation in space (C4 cycle) Mesophyll cell surrounding bundle sheath CO 2 sugar C3 Bundle sheath cell Vascular tissue
C 3 vs C 4 Plants A Lesson in Photoefficiency C 3 C 4 CO 2 directly RuBP recipient RUBISCO open O 2 can interfere Photorespiration likely CO 2 indirectly PEP recipient RUBISCO shielded O 2 cannot interfere No photorespriation
C 3 and C 4 Plants C 3 C 4 soybean wheat rice sugar beet alfalfa spinach tobacco sunflower corn sorghum sugar cane millet crab grass Bermuda grass pigweed
What did you Learn? Photorespiration is to be avoided RUBISCO is an oxidase/carboxylase Oxygen cuts photoefficiency in half C 3 and C 4 plants differ in anatomy Mesophyll cells fix CO 2 to PEP Bundle sheath cells have Calvin cycle C 4 plants grow more ferociously
OAA (4C) Malat PEP (3C) Pyruvate (C3) CO 2 ADP ATP Separation in time (CAM cycle) In the dark CO 2 sugar C3 In the light Mesophyll
Overview of the Dark Reactions Carbon dioxide is broken and fixed into glucose or fructose molecules in the CALVIN CYCLE!!!! Glucose subunits can make cellulose or other polysaccharides, such as fruit sugars. The carbon skeleton in glucose also helps to synthesize other important biochemical compounds such as, lipids, amino acids, and nucleic acids.
Overview of Photosynthesis
Summary Diagram of Photosynthesis
It's not that easy bein' green but it is essential for life on earth!
QUIZ Why are leaves green? What is chlorophyll? Which cell organelle contains chlorophyll? What are the units of light? How are photons captured? What s produced in PS I?
Quiz What are the products of the Calvin cycle? Where in the chloroplast the Calvin cycle reactions occur? What is the most prevalent enzyme on the planet? Why is it so important?