Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Topic 3.8 Photosynthesis Life on Earth is solar powered! State that photosynthesis involves the conversion of light energy into chemical energy.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Topic 3.8 Photosynthesis Photosynthesis is the process that converts solar energy into chemical energy State that photosynthesis involves the conversion of light energy into chemical energy.

Plants Unicellular protist Multicellular algaeCyanobacteria Purple sulfur bacteria 10 µm 1.5 µm 40 µm Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts are organelles that are responsible for feeding the vast majority of organisms State that chlorophyll is the main photosynthetic pigment.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast Through microscopic pores called stomata, CO 2 enters the leaf and O 2 exits State that chlorophyll is the main photosynthetic pigment.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid State that chlorophyll is the main photosynthetic pigment.

Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis can be summarized as the following equation: 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O SC.912.L.18.7 Identify the reactants, products, and basic functions of photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 SC.912.L.18.7 Identify the reactants, products, and basic functions of photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) split water, release O 2, produce ATP, and form NADPH State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

LE 10-5_1 H2OH2O LIGHT REACTIONS Chloroplast Light

LE 10-5_2 H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2

LE 10-5_3 H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Nature of Sunlight Light is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength = distance between crests of waves Wavelength determines the type of electromagnetic energy Light also behaves as though it consists of discrete particles, called photons

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis State that light from the sun is composed of a range of wavelengths (colours).

Visible light Gamma rays X-rays UV Infrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm10 6 nm 1 m (10 9 nm) 10 3 m nm Longer wavelength Lower energy Shorter wavelength Higher energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light

LE 10-7 Chloroplast Light Reflected light Absorbed light Transmitted light Granum

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

LE 10-8a White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Green light Slit moves to pass light of selected wavelength 0 100

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) Absorption spectra Absorption of light by chloroplast pigments Outline the differences in absorption of red, blue and green light by chlorophyll

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Action spectrum Rate of photo- synthesis (measured by O 2 release)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess O 2 He used aerobic bacteria clustered along the alga as a measure of O 2 production

LE 10-9c Engelmann’s experiment Aerobic bacteria Filament of algae

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excitation of Chlorophyll by Light When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

Excited state Heat Photon (fluorescence) Ground state Chlorophyll molecule Photon Excitation of isolated chlorophyll molecule Fluorescence Energy of electron e–e–

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem consists of a reaction center surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

LE Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings There are two types of photosystems in the thylakoid membrane Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I is best at absorbing a wavelength of 700 nm The two photosystems work together to use light energy to generate ATP and NADPH State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH Explain the light-dependent reactions.

STROMA (Low H + concentration) Light Photosystem II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H2OH2O O2O2 +2 H + 1/21/2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H+H+ i [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells In addition to food production, photosynthesis produces the oxygen in our atmosphere

LE Light CO 2 H2OH2O Light reactionsCalvin cycle NADP + RuBP G3P ATP Photosystem II Electron transport chain Photosystem I O2O2 Chloroplast NADPH ADP +P i 3-Phosphoglycerate Starch (storage) Amino acids Fatty acids Sucrose (export)