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Photosynthesis. Chapter 8. Photosynthesis Overview. Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts.

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Chapter 8

Photosynthesis overview

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



  • 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



  • 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


Discovery of photosynthesis

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 in fact a multistage process, only one portion of which uses light directly

    • Light versus dark reactions

    • Enzymes involved


  • 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 O2 produced comes from water

  • Robin Hill (1899–1991)

    • Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction



  • 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

Absorption spectrum

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

Many pigments

Many pigments

  • 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



  • 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

Structure of chlorophyll

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

Action spectrum

Action spectrum

  • Relative effectiveness of different wavelengths of light in promoting photosynthesis

  • Corresponds to the absorption spectrum for chlorophylls

Other pigments

Other Pigments

  • 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

Photosystem organization

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

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

Reaction center

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

Light dependent reactions

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

Cyclic photophosphorylation

Cyclic photophosphorylation

  • In sulfur bacteria, only one photosystem is used

  • Generates ATP via electron transport

  • Anoxygenic photosynthesis

  • Excited electron passed to electron transport chain

  • Generates a proton gradient for ATP synthesis

Chloroplasts have two connected photosystems

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

The connection

The connection

  • 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

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

Photosystem ii

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

Photosystem i

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



  • 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

Production of additional atp

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

Carbon fixation calvin cycle

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

Calvin cycle

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 ribulosebisphosphatecarboxylase /oxygenaseor rubisco

3 phases

3 phases

  • Carbon fixation

    • 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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Output of calvin cycle

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

Energy cycle

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



  • 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

Types of photosynthesis

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

C 4 plants

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

The cost of doing business this way

The cost of doing business this way

  • 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

Cam plants

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

Cam plants in action

CAM plants in action

  • 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

Compare c 4 and cam

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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