Providing Energy for Life PHOTOSYNTHESIS Providing Energy for Life

Do you remember food web terminology? Autotrophs = producers Photoautotrophs Chemoautotrophs Heterotrophs = consumers Primary consumers, secondary consumers, etc decomposers

Photosynthesis Photoautotrophic cells contain pigments that absorb visible light This is their source of ENERGY

Where does photosynthesis occur? Leaf: cross section

Photosynthesis Most occurs in the mesophyll layer of the leaf Each mesophyll cell contains 30 - 40 chloroplasts

CHLOROPLAST

CHLOROPLAST

CHLOROPLAST

CHLOROPLAST Light-absorbing pigments are embedded in thylakoid membranes A stack of thylakoids is called a granum Plural = grana Grana have a large surface area to allow greater diffusion of materials

CHLOROPLAST STROMA Surrounds the thylakoid membranes Contains enzymes, DNA, RNA

Electromagnetic Spectrum

Electromagnetic spectrum Visible light drives photosynthesis Substances that absorb visible light are called pigments Pigments appear different colors because they reflect different wavelengths (and thus absorb different wavelengths) Spectrophotometer measures the ability of a pigment to absorb different wavelengths of light

CHLOROPLAST PIGMENTS CHLOROPHYLL = green pigment Two forms of chlorophyll Chlorophyll a Chlorophyll b Chlorophyll absorbs light energy in the violet/blue and orange/red ranges REFLECTS in the green range

Chlorophyll The color that we see is the color that is REFLECTED, not absorbed Therefore, chlorophyll appears GREEN, because green wavelength light is reflected

Other Pigments in Chloroplasts Xanthophyll: yellow Carotene: orange Different colors indicate that they absorb (and therefore reflect) energy at different wavelengths

Absorption Spectra

Plant Pigments Located in the thylakoid membrane Absorption spectrum tells us which wavelengths are absorbed best Why is this important? HINT: it has to do with amount of energy absorbed!

PHOTOSYNTHESIS Overview Photosynthesis involves 3 energy conversions 1. absorption of light energy 2. conversion of light energy into chemical energy 3. storage of chemical energy as sugars (carbon-containing compounds)

Law of Conservation of Energy REMEMBER!!! Energy cannot be created or destroyed, only converted into different forms

PHOTOSYNTHESIS overall summary KNOW THIS!!!!! Carbon dioxide gas combines with water in the presence of light energy and chlorophyll to form sugar and oxygen gas

PHOTOSYNTHESIS

PHOTOSYNTHESIS Photosynthesis occurs in 2 groups of reactions LIGHT REACTIONS CALVIN CYCLE OR DARK REACTIONS

PHOTOSYNTHESIS I’M NOT GONNA DENY IT….THIS IS GOING TO INVOLVE MEMORIZATION!!

LIGHT REACTIONS Pigment molecules in the thylakoids absorb light energy Light energy is converted to short-lived chemical energy in carrier molecules Conversion to chemical energy has to do with transfer of electrons from one molecule to another Remember Oxidation-Reduction reactions????

Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Figure 10.4a Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 10.4 Zooming in on the location of photosynthesis in a plant. 20 m

Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum Figure 10.4b Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum Thylakoid space Inner membrane Figure 10.4 Zooming in on the location of photosynthesis in a plant. 1 m

Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 Figure 10.5 Figure 10.5 Tracking atoms through photosynthesis.

Light Reflected light Chloroplast Absorbed light Granum Figure 10.8 Light Reflected light Chloroplast Figure 10.8 Why leaves are green: interaction of light with chloroplasts. Absorbed light Granum Transmitted light

Absorption of light by chloroplast pigments Figure 10.10 RESULTS Chloro- phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) Rate of photosynthesis (measured by O2 release) Figure 10.10 INQUIRY: Which wavelengths of light are most effective in driving photosynthesis? (b) Action spectrum 400 500 600 700 Aerobic bacteria Filament of alga Engelmann’s experiment (c) 400 500 600 700

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 © 2011 Pearson Education, Inc.

Slit moves to pass light of selected wavelength. Green light Figure 10.9 TECHNIQUE Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer High transmittance (low absorption): Chlorophyll absorbs very little green light. Slit moves to pass light of selected wavelength. Green light Figure 10.9 RESEARCH METHOD: Determining an Absorption Spectrum Low transmittance (high absorption): Chlorophyll absorbs most blue light. Blue light

Hydrocarbon tail (H atoms not shown) Figure 10.11 CH3 in chlorophyll a CH3 CHO in chlorophyll b Porphyrin ring Figure 10.11 Structure of chlorophyll molecules in chloroplasts of plants. Hydrocarbon tail (H atoms not shown)

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 © 2011 Pearson Education, Inc.

The Splitting of Water Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product © 2011 Pearson Education, Inc.

Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced Photosynthesis is an endergonic process; the enery boost is provided by light © 2011 Pearson Education, Inc.

becomes reduced becomes oxidized Energy  6 CO2  6 H2O Figure 10.UN01 becomes reduced Energy  6 CO2  6 H2O C6 H12 O6  6 O2 becomes oxidized Figure 10.UN01 In-text figure, p. 188

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 H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation © 2011 Pearson Education, Inc.

H2O Light NADP ADP + P i Light Reactions Chloroplast Figure 10.6-1 Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. Chloroplast

H2O Light NADP ADP + P i Light Reactions ATP NADPH Chloroplast O2 Figure 10.6-2 H2O Light NADP ADP + P i Light Reactions ATP Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. NADPH Chloroplast O2

Calvin Cycle Light Reactions Figure 10.6-3 H2O CO2 Light NADP ADP + P i Calvin Cycle Light Reactions ATP Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. NADPH Chloroplast O2

Calvin Cycle Light Reactions [CH2O] (sugar) Figure 10.6-4 H2O CO2 Light NADP ADP + P i Calvin Cycle Light Reactions ATP Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. NADPH Chloroplast [CH2O] (sugar) O2

Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these cells allows for the chemical reactions of photosynthesis © 2011 Pearson Education, Inc.

Concept 10.2: 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 © 2011 Pearson Education, Inc.

Photon (fluorescence) Figure 10.12 Excited state e Heat Energy of electron Photon (fluorescence) Photon Figure 10.12 Excitation of isolated chlorophyll by light. Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center © 2011 Pearson Education, Inc.

THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.13a Photosystem STROMA Photon Light- harvesting complexes Reaction- center complex Primary electron acceptor e Thylakoid membrane Figure 10.13 The structure and function of a photosystem. Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light

A primary electron acceptor in the reaction center accepts excited electron and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions © 2011 Pearson Education, Inc.

There are two types of photosystems in the thylakoid membrane Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called P680 © 2011 Pearson Education, Inc.

Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 © 2011 Pearson Education, Inc.

Linear Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and linear Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy © 2011 Pearson Education, Inc.

A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+) © 2011 Pearson Education, Inc.

P680+ is a very strong oxidizing agent Figure 10.14-2 P680+ is a very strong oxidizing agent H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680 O2 is released as a by-product of this reaction Primary acceptor 2 e H2O 2 H + 3 1/2 O2 e e P680 1 Light Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH. Pigment molecules Photosystem II (PS II) 53

Diffusion of H+ (protons) across the membrane drives ATP synthesis Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H+ (protons) across the membrane drives ATP synthesis © 2011 Pearson Education, Inc.

Diffusion of H+ (protons) across the membrane drives ATP synthesis Figure 10.14-3 Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H+ (protons) across the membrane drives ATP synthesis Primary acceptor 4 Electron transport chain Pq 2 e H2O 2 H Cytochrome complex + 3 1/2 O2 Pc e e 5 P680 1 Light ATP Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH. Pigment molecules Photosystem II (PS II) 55

Figure 10.14-4 Primary acceptor Primary acceptor 4 Electron transport chain Pq e 2 e H2O 2 H Cytochrome complex + 3 1/2 O2 Pc e e P700 5 P680 Light 1 Light 6 ATP Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Pigment molecules 56

Figure 10.14-5 Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e 2 e 8 e e H2O NADP 2 H Cytochrome complex NADP reductase + H + 3 1/2 O2 NADPH Pc e e P700 5 P680 Light 1 Light 6 ATP Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH. Pigment molecules Photosystem I (PS I) The electrons are passed down a second electron transport chain through the protein Ferrodoxin(Fd). This etc. does not create a proton gradient , no ATP 57

Mill makes ATP NADPH ATP Photosystem II Photosystem I e e e e e Figure 10.15 e e e Mill makes ATP NADPH e e e Photon Figure 10.15 A mechanical analogy for linear electron flow during the light reactions. e ATP Photon Photosystem II Photosystem I 58

Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH No oxygen is released Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle © 2011 Pearson Education, Inc.

Primary acceptor Primary acceptor Fd Fd NADP + H Pq NADP reductase Figure 10.16 Primary acceptor Primary acceptor Fd Fd NADP + H Pq NADP reductase Cytochrome complex NADPH Pc Figure 10.16 Cyclic electron flow. Photosystem I Photosystem II ATP 60

Some organisms such as purple sulfur bacteria have PS I but not PS II Cyclic electron flow is thought to have evolved before linear electron flow Cyclic electron flow may protect cells from light-induced damage © 2011 Pearson Education, Inc.

Light Reactions

STROMA (low H concentration) Cytochrome complex NADP reductase Figure 10.18 STROMA (low H concentration) Cytochrome complex NADP reductase Photosystem II Photosystem I Light 3 Light 4 H+ NADP + H Fd Pq NADPH 2 Pc H2O 1 1/2 O2 THYLAKOID SPACE (high H concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.18 The light reactions and chemiosmosis: the organization of the thylakoid membrane. Thylakoid membrane ATP synthase ADP + P i ATP STROMA (low H concentration) H+ 63

ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH © 2011 Pearson Education, Inc.

Figure 10.UN08 Figure 10.UN08 Appendix A: answer to Test Your Understanding, question 9

PHOTOSYNTHESIS: SUMMARY

CALVIN CYCLE Energy molecules from the light reactions are used to make sugar (ATP and NADPH) Also known as the DARK REACTIONS

Memorize the Players of the CS!! Carbon dioxide 1C Ribulose biphosphate (RuBP) 5C 3 Phosphoglycerate (3PGA) 3C 1,3-biphosphoglycerate (3BPGA) 3C Glyceraldehyde 3-phosphate (G3P) 3C

Calvin Cycle

What colors of light will drive photosynthesis by green plants most efficiently? red only yellow only green only blue only red and blue Answer: e Concept 10.2

Photosynthesis and Organic Carbon The organic carbon in a tree comes primarily from soil. water. air. organic fertilizer (manure, detritus). light. Answer: c—more specifically, carbon dioxide in the air This question relates to Concept 10.1. It is another follow-up question to the van Helmont experiment and to the previous question on biomass. These three questions could be asked in sequence, and the question on van Helmont resampled, if students come to progressively realize that the tree’s weight comes from the carbon dioxide in the air and only minimally from mineral nutrients in the soil.

Origin of Oxygen Gas Which experiment will produce 18O2? both experiment 1 and experiment 2 neither experiment Answer: a This question relates to Concept 10.1.

Summary Photosynthesis Video