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Photosynthesis. Photosynthesis: An Overview. Electrons play a primary role in photosynthesis In eukaryotes, photosynthesis takes place in chloroplasts. Overview. Autotrophs use energy to make their own organic molecules from CO 2 and inorganic sources such as water

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Photosynthesis: An Overview

  • Electrons play a primary role in photosynthesis

  • In eukaryotes, photosynthesis takes place in chloroplasts


  • Autotrophs use energy to make their own organic molecules from CO2 and inorganic sources such as water

    • Photoautotrophs use light as an energy source

  • Heterotrophs consume or decompose organic molecules


  • Two stages of photosynthesis:

    • Light-dependent reactions – energy from sunlight is converted into chemical energy to replenish ATP and NADPH

    • Light-independent reactions (Calvin cycle) – excess energy is stored by building high-energy sugar molecules to be used when sunlight is not available


  • The two stages of photosynthesis are linked together and occur at the same time

Fig. 9-2, p. 179

Animation: Photosynthesis overview


  • Leaves have a complex internal structure to optimize light interception and penetration of gases for photosynthesis

Fig. 9-3b, p. 180


  • In eukaryotes, photosynthesis takes place in the chloroplasts

Fig. 9-3c, p. 180

Cutaway of a small

section from the leaf

One of the photosynthetic

cells, with green chloroplasts

Leaf’s upper surface

Photosynthetic cells

Large central




The leaf’s surfaces enclose many photosynthetic cells.

Stomata are minute openings through which O2 and CO2

are exchanged with the surrounding atmosphere.

Fig. 9-3b, p. 180

Cutaway view of a chloroplast

Outer membrane

Inner membrane


• light absorption by chlorophylls and carotenoids

• electron transfer

• ATP synthesis by ATP synthase

Stroma (space around thylakoids)

• light-independent reactions


Stromal lamellae





Fig. 9-3c, p. 180

Animation: Sites of photosynthesis

Chloroplast Structure

  • Outer and inner membranes

    • Intermembrane compartment

  • Fluid within the inner membrane is the stroma

Chloroplast Structure

  • Thylakoid membranes are a complex of flattened internal membrane compartments

    • Stacks of membranes called grana

    • Tubular lamellae connections between the grana

    • Compartment enclosed by thylakoids called the thylakoid lumen

The Light-Dependent Reactions of Photosynthesis

  • Electrons in pigment molecules absorb light energy in photosynthesis

  • Chlorophylls and carotenoids cooperate in light absorption

  • The photosynthetic pigments are organized into photosystems in chloroplasts

  • Electrons flow from water to photosystem II to photosystem I to NADP+ leading to the synthesis of NADPH and ATP

  • Electrons can also drive ATP synthesis by flowing cyclically around photosystem I

  • Experiments with chloroplasts helped confirm the synthesis of ATP by chemiosmosis

Light-Dependent Reactions

  • Two main processes:

    • Light absorption

    • Synthesis of ATP and NADPH


  • Light is a form of electromagnetic radiation

Fig. 9-4b, p. 181

Visible Light

  • Visible light ranges from wavelengths of about 400 nm (violet) to 700 nm (red)

  • Photons are discrete units of energy carried in light

  • The shorter the wavelength, the greater the energy

Fig. 9-4a, p. 181


  • Pigment molecules absorb certain wavelengths and transmit or reflect other wavelengths

  • Certain electrons in the pigment can absorb energy from photons in light

    • Electron starts in the ground state (low energy)

    • Electron absorbs photon energy and jumps to a higher energy level (excited state )


  • After excitation, one of three events occurs:

Fig. 9-5, p. 182

De-Excitation Events

  • Energy released as heat or as light of a longer wavelength (fluorescence), electron returns to the ground state

  • Excited electron transferred to a nearby electron-accepting molecule called a primary acceptor

De-Excitation Events

  • Energy from excited electron in one pigment molecule is transferred to a neighboring pigment molecule by inductive resonance

  • Transfer excites second pigment and the first pigment returns to the ground state


  • Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria

Fig. 9-6a, p. 183


  • Chlorophyll absorbs blue and red wavelengths most strongly

Fig. 9-7a, p. 184


  • Carotenoids expand the range of wavelengths absorbed

Fig. 9-6b, p. 183


  • Photosystems are large complexes of light-absorbing pigments and proteins embedded in the thylakoid membranes to absorb light efficiently

Fig. 9-8, p. 184

Synthesis of ATP and NADPH Summary

  • Energy from sunlight is captured in excited electrons and used to synthesize more useful ATP and NADPH

  • Electron transfer systems are used to extract energy from excited electrons

Synthesis of ATP and NADPH Summary

  • Some of the energy is used to create a gradient of H+ across the thylakoid membranes that provides energy for ATP synthesis

  • Electrons are ultimately passed along to reduce NADP+ to NADPH

Water Splitting

  • Water splitting complex provides a source of electrons to be excited during light-dependent reactions

    2 H2O → 4 H+ + 4 e– + O2

  • Located close to photosystem II

Noncyclic Electron Flow

Fig. 9-10, p. 187

Light-Independent Reactions

  • Light-independent reactions (Calvin cycle) store some of the energy captured from sunlight in the form of high-energy molecules such as sugar

  • When sunlight is not available, stored carbohydrates are broken down by aerobic respiration in the mitochondria to replenish ATP and NADH

Calvin Cycle

  • During the Calvin cycle, CO2 is reduced and converted into organic substances

    • NADPH provides electrons and hydrogen

    • ATP provides additional energy

  • Carbon fixation involves capturing CO2 molecules with the key enzyme rubisco (RuBPcarboxylase/‌oxygenase)

Calvin Cycle

  • Each turn of the Calvin cycle captures one CO2 molecule

  • Three turns of the Calvin cycle are needed to capture the three carbons in one G3P molecule

  • Six turns are needed to make a six-carbon sugar such as glucose

    6 CO2 + 12 NADPH + 18 ATP↓C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi

Calvin Cycle Summary

  • Carbon fixation – CO2 added to RuBP to produce two 3PGA molecules

  • Reduction – NADPH and ATP used to convert 3PGA into G3P, a higher energy molecule used to build sugars

  • Regeneration – remaining G3P molecules are used to recreate the starting material RuBP

Fig. 9-13a, p. 192

Animation: Calvin-Benson cycle


  • As rubisco provides the source of organic molecules for most of the world’s organisms

    • 100 billion tons of CO2 are converted into carbohydrates each year

    • Represents 50% or more of the total protein in leaves

Photosynthetic Products

  • Surplus G3P formed in the Calvin cycle can be the starting material for many organic molecules

    • Carbohydrates, lipids, proteins, nucleic acids

  • Sucrose (disaccharide) is used to circulate photosynthetic products from cell to cell in plants

  • Starch is used for longer storage of energy

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