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Presentation on theme: "CHAPTER 8 LECTURE SLIDES"— Presentation transcript:

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2 Photosynthesis Chapter 8

3 Photosynthesis Overview
Energy for all life on Earth ultimately comes from photosynthesis 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts

4 Chloroplast Thylakoid membrane – internal membrane
Contains chlorophyll and other photosynthetic pigments Pigments clustered into photosystems Grana – stacks of flattened sacs of thylakoid membrane Stroma lamella – connect grana Stroma – semiliquid surrounding thylakoid membranes


6 Stages Light-dependent reactions
Require light Capture energy from sunlight Make ATP and reduce NADP+ to NADPH Carbon fixation reactions or light-independent reactions Does not require light Use ATP and NADPH to synthesize organic molecules from CO2


8 Discovery of Photosynthesis
Jan Baptista van Helmont (1580–1644) Demonstrated that the substance of the plant was not produced only from the soil Joseph Priestly (1733–1804) Living vegetation adds something to the air Jan Ingen-Housz (1730–1799) Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

9 F.F. Blackman (1866–1947) Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly Light versus dark reactions Enzymes involved

10 Pigments Molecules that absorb light energy in the visible range
Light is a form of energy Photon – particle of light Acts as a discrete bundle of energy Energy content of a photon is inversely proportional to the wavelength of the light Photoelectric effect – removal of an electron from a molecule by light


12 Absorption spectrum When a photon strikes a molecule, its energy is either Lost as heat Absorbed by the electrons of the molecule Boosts electrons into higher energy level Absorption spectrum – range and efficiency of photons molecule is capable of absorbing


14 Organisms have evolved a variety of different pigments
Only two general types are used in green plant photosynthesis Chlorophylls Carotenoids In some organisms, other molecules also absorb light energy

15 Chlorophylls Chlorophyll a Chlorophyll b
Main pigment in plants and cyanobacteria Only pigment that can act directly to convert light energy to chemical energy Absorbs violet-blue and red light Chlorophyll b Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

16 Pigments Pigments:

17 Structure of chlorophyll porphyrin ring
Complex ring structure with alternating double and single bonds Magnesium ion at the center of the ring Photons excite electrons in the ring Electrons are shuttled away from the ring

18 Carotenoids Phycobiloproteins
Carbon rings linked to chains with alternating single and double bonds Can absorb photons with a wide range of energies Also scavenge free radicals – antioxidant Protective role Phycobiloproteins Important in low-light ocean areas

19 Photosystem Organization
Antenna complex Hundreds of accessory pigment molecules Gather photons and feed the captured light energy to the reaction center Reaction center 1 or more chlorophyll a molecules Passes excited electrons out of the photosystem

20 Antenna complex Also called light-harvesting complex
Captures photons from sunlight and channels them to the reaction center chlorophylls In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins


22 Reaction center Transmembrane protein–pigment complex
When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level Light-energized electron can be transferred to the primary electron acceptor, reducing it Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule


24 Light-Dependent Reactions
Primary photoevent Photon of light is captured by a pigment molecule Charge separation Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule Electron transport Electrons move through carriers to reduce NADP+ Chemiosmosis Produces ATP Capture of light energy

25 Cyclic photophosphorylation
In sulfur bacteria, only one photosystem is used Generates ATP via electron transport Excited electron passed to electron transport chain Generates a proton gradient for ATP synthesis


27 Chloroplasts have two connected photosystems
Oxygenic photosynthesis Photosystem I (P700) Functions like sulfur bacteria Photosystem II (P680) Can generate an oxidation potential high enough to oxidize water Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

28 Photosystem I transfers electrons ultimately to NADP+, producing NADPH
Electrons lost from photosystem I are replaced by electrons from photosystem II Photosystem II oxidizes water to replace the electrons transferred to photosystem I 2 photosystems connected by cytochrome/ b6-f complex

29 Noncyclic photophosphorylation
Plants use photosystems II and I in series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with electrons obtained by splitting water Z diagram


31 Photosystem II Resembles the reaction center of purple bacteria
Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules Reaction center differs from purple bacteria in that it also contains four manganese atoms Essential for the oxidation of water b6-f complex Proton pump embedded in thylakoid membrane

32 Photosystem I Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron Passes electrons to NADP+ to form NADPH


34 Chemiosmosis Electrochemical gradient can be used to synthesize ATP
Chloroplast has ATP synthase enzymes in the thylakoid membrane Allows protons back into stroma Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

35 Production of additional ATP
Noncyclic photophosphorylation generates NADPH ATP Building organic molecules takes more energy than that alone Cyclic photophosphorylation used to produce additional ATP Short-circuit photosystem I to make a larger proton gradient to make more ATP

36 Carbon Fixation – Calvin Cycle
To build carbohydrates cells use Energy ATP from light-dependent reactions Cyclic and noncyclic photophosphorylation Drives endergonic reaction Reduction potential NADPH from photosystem I Source of protons and energetic electrons

37 Calvin cycle Named after Melvin Calvin (1911–1997)
Also called C3 photosynthesis Key step is attachment of CO2 to RuBP to form PGA Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

38 3 phases Carbon fixation Reduction Regeneration of RuBP
RuBP + CO2 → PGA Reduction PGA is reduced to G3P Regeneration of RuBP PGA is used to regenerate RuBP 3 turns incorporate enough carbon to produce a new G3P 6 turns incorporate enough carbon for 1 glucose


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41 Output of Calvin cycle Glucose is not a direct product of the Calvin cycle G3P is a 3 carbon sugar Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose Used to make starch Insoluble glucose polymer Stored for later use

42 Energy cycle Photosynthesis uses the products of respiration as starting substrates Respiration uses the products of photosynthesis as starting substrates Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria


44 Photorespiration Rubisco has 2 enzymatic activities
Carboxylation Addition of CO2 to RuBP Favored under normal conditions Photorespiration Oxidation of RuBP by the addition of O2 Favored when stoma are closed in hot conditions Creates low-CO2 and high-O2 CO2 and O2 compete for the active site on RuBP


46 Types of photosynthesis
C3 Plants that fix carbon using only C3 photosynthesis (the Calvin cycle) C4 and CAM Add CO2 to PEP to form 4 carbon molecule Use PEP carboxylase Greater affinity for CO2, no oxidase activity C4 – spatial solution CAM – temporal solution

47 C4 plants Corn, sugarcane, sorghum, and a number of other grasses
Initially fix carbon using PEP carboxylase in mesophyll cells Produces oxaloacetate, converted to malate, transported to bundle-sheath cells Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2 Carbon fixation then by rubisco and the Calvin cycle


49 C4 pathway, although it overcomes the problems of photorespiration, does have a cost
To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

50 CAM plants Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups Stomata open during the night and close during the day Reverse of that in most plants Fix CO2 using PEP carboxylase during the night and store in vacuole

51 When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
High levels of CO2 drive the Calvin cycle and minimize photorespiration


53 Compare C4 and CAM Both use both C3 and C4 pathways
C4 – two pathways occur in different cells CAM – C4 pathway at night and the C3 pathway during the day


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