CHAPTER 8Photosynthesis
Chapter 8Photosynthesis
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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
*except for that which doesn’t…chemosynthesis
Chloroplast Structure
• 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
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Elodea Chloroplasts
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Stages
• Light-dependent reactions– Require light
1.Capture energy from sunlight
2.Make ATP and reduce NADP+ to NADPH
• Carbon fixation reactions or light-independent reactions– Does not require light
3.Use ATP and NADPH to synthesize organic molecules from CO2
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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)
• F.F. Blackman (1866–1947)– Came to the
startling conclusion that photosynthesis is a multistage process, only one portion of which uses light directly
– Light versus dark reactions
– Enzymes involved
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• C. B. van Niel (1897–1985)– Found purple sulfur bacteria do not release O2
but accumulate sulfur– Proposed general formula for photosynthesis
• CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
– Later researchers found that the O2 produced comes from water, not CO2
• Robin Hill (1899–1991)– Demonstrated Niel was right that light energy
could be harvested and used in a reduction reaction
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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
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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
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• 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
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Chlorophylls
• Chlorophyll a– 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
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Pigments
Pigments:
• 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
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• Action spectrum– Relative effectiveness of different
wavelengths of light in promoting photosynthesis
– Corresponds to the absorption spectrum for chlorophylls
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• Carotenoids– 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
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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
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
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Antenna Complex
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
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Light-Dependent Reactions
1. Primary photoevent– Photon of light is captured by a pigment molecule
2. Charge separation – Energy is transferred to the reaction center; an
excited electron is transferred to an acceptor molecule
3. Electron transport– Electrons move through carriers to reduce NADP+
4. Chemiosmosis– Produces ATP
Cap
ture
of
light
ene
rgy
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• In sulfur bacteria, only one photosystem is used
• Generates ATP via electron transport
• Anoxygenic photosynthesis (no O2 released)
• Excited electron passed to electron transport chain
• Generates a proton gradient for ATP synthesis
Cyclic photophosphorylation
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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
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• 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
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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
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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
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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
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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
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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 ATP40
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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
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Calvin cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to form PGA
• Uses the enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco
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3 phases
1. Carbon fixation– RuBP + CO2 → PGA
2. Reduction– PGA is reduced to G3P
3. 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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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
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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
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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
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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
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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
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• 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
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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
• 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
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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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