CAMPBELL BIOLOGY IN FOCUS Urry Cain Wasserman Minorsky

Overview: The Process That Feeds the Biosphere § Photosynthesis is the process that converts solar energy into chemical energy § Directly or indirectly, photosynthesis nourishes almost the entire living world © 2014 Pearson Education, Inc.

§ Autotrophs sustain themselves without eating anything derived from other organisms § Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules § Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules © 2014 Pearson Education, Inc.

Figure 8. 1 © 2014 Pearson Education, Inc.

§ Heterotrophs obtain their organic material from other organisms § Heterotrophs are the consumers of the biosphere § Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2 © 2014 Pearson Education, Inc.

§ Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes § These

40 m Figure 8. 2 (d) Cyanobacteria 10 m (b) Multicellular alga (c) Unicellular

Concept check: T or F • All autotrophs do cellular respiration • All autotrophs do photosynthesis • Purple sulfur bacteria were the first photosynthetic organisms • Eukaryotes obtained photosynthetic ability by engulfing a photosynthetic prokaryote © 2014 Pearson Education, Inc.

Figure 8. 2 a (a) Plants © 2014 Pearson Education, Inc.

Figure 8. 2 b (b) Multicellular alga © 2014 Pearson Education, Inc.

10 m Figure 8. 2 c © 2014 Pearson Education, Inc. (c) Unicellular eukaryotes

40 m Figure 8. 2 d © 2014 Pearson Education, Inc. (d) Cyanobacteria

1 m Figure 8. 2 e © 2014 Pearson Education, Inc. (e) Purple sulfur

Concept 8. 1: Photosynthesis converts light energy to the chemical energy of food § The structural organization of photosynthetic cells includes enzymes and other molecules grouped together in a membrane § This organization allows for the chemical reactions of photosynthesis to proceed efficiently § Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria © 2014 Pearson Education, Inc.

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 § Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf § Each mesophyll cell contains 30– 40 chloroplasts © 2014 Pearson Education, Inc.

§ CO 2 enters and O 2 exits the leaf through microscopic pores called stomata § 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 interior fluid © 2014 Pearson Education, Inc.

Figure 8. 3 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO 2 Chloroplast Mesophyll cell Thylakoid Granum Thylakoid Stroma space Outer membrane Intermembrane space Inner membrane 1 m © 2014 Pearson Education, Inc. 20 m

Concept Check: T or F • Gases such as carbon dioxide enters the leaf through the stroma • The light reactions take place in the thylakoids • The stroma contains chlorophyll, a light capturing pigment • Photosynthesis occurs in the mitochondria, which contains an inner and outer membrane just like the chloroplast • All autotrophs contain mesophyll tissue © 2014 Pearson Education, Inc.

Figure 8. 3 a Leaf cross section Chloroplasts Vein Mesophyll Stomata © 2014 Pearson

Figure 8. 3 b Chloroplast Mesophyll cell Thylakoid Granum Thylakoid Stroma space Outer membrane

Figure 8. 3 c Mesophyll cell 20 m © 2014 Pearson Education, Inc.

Figure 8. 3 d Granum Stroma 1 m © 2014 Pearson Education, Inc.

Tracking Atoms Through Photosynthesis: Scientific Inquiry § Photosynthesis is a complex series of reactions that can be summarized as the following equation 6 CO 2 12 H 2 O Light energy C 6 H 12 O 6 6 O 2 6 H 2 O © 2014 Pearson Education, Inc.

The Splitting of Water § Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product © 2014 Pearson Education, Inc.

Figure 8. 4 Reactants: Products: © 2014 Pearson Education, Inc. 6 CO 2 C

Photosynthesis as a Redox Process § Photosynthesis reverses the direction of electron flow compared to respiration § Photosynthesis is a redox process in which H 2 O is oxidized and CO 2 is reduced § Photosynthesis is an endergonic process; the energy boost is provided by light © 2014 Pearson Education, Inc.

Figure 8. UN 01 becomes reduced becomes oxidized © 2014 Pearson Education, Inc.

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 H 2 O § Release O 2 § Reduce the electron acceptor, NADP , to NADPH § Generate ATP from ADP by adding a phosphate group, photophosphorylation © 2014 Pearson Education, Inc.

§ 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 Animation: Photosynthesis © 2014 Pearson Education, Inc.

Figure 8. 5 CO 2 H 2 O Light NADP Pi Light Reactions Calvin

Figure 8. 5 -1 H 2 O Light NADP Pi Light Reactions Chloroplast ©

Figure 8. 5 -2 H 2 O Light NADP Pi Light Reactions ATP NADPH

Figure 8. 5 -3 H 2 O CO 2 Light NADP Pi Light Reactions

Figure 8. 5 -4 H 2 O CO 2 Light NADP Pi Light Reactions

Concept 8. 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 © 2014 Pearson Education, Inc.

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 is the distance between crests of waves § Wavelength determines the type of electromagnetic energy © 2014 Pearson Education, Inc.

§ The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation § Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see § Light also behaves as though it consists of discrete particles, called photons © 2014 Pearson Education, Inc.

Figure 8. 6 10− 5 nm 10− 3 nm Gamma rays 103 nm 1 nm X-rays UV 1 m (109 nm) 106 nm Infrared Microwaves 103 m Radio waves Visible light 450 380 500 Shorter wavelength Higher energy © 2014 Pearson Education, Inc. 550 600 650 700 750 nm Longer wavelength Lower energy

Misconception Check: T or F • Gamma rays are the most harmful to living things • The electromagnetic spectrum is the range of energy that comes from the sun • X rays are not harmful to cells • visible light can be found between 380 and 750 nm • Chloroplasts can absorb all the energy in the electromagnetic spectrum © 2014 Pearson Education, Inc.

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 Animation: Light and Pigments © 2014 Pearson Education, Inc.

Figure 8. 7 Light Reflected light Chloroplast Absorbed light Granum Transmitted light © 2014

Concept check: T or F • Leaves are green because that is the color

§ 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 © 2014 Pearson Education, Inc.

$Figure 8. 8 Technique Refracting prism White light Chlorophyll solution 2 1 Slit moves$

Figure 8. 8 Technique Refracting prism White light Chlorophyll solution 2 1 Slit moves to pass light of selected wavelength. Galvanometer 3 4 Green light Blue light © 2014 Pearson Education, Inc. Photoelectric tube

$Figure 8. 8 Technique Refracting prism White light Chlorophyll solution 2 1 Slit moves$

Figure 8. 8 Technique Refracting prism White light Chlorophyll solution 2 1 Slit moves to pass light of selected wavelength. Galvanometer 3 4 Green light Blue light © 2014 Pearson Education, Inc. Photoelectric tube The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.

§ 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 § Accessory pigments include chlorophyll b and a group of pigments called carotenoids § An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process © 2014 Pearson Education, Inc.

Results Absorption of light by chloroplast pigments Figure 8. 9 Chlorophyll a Chlorophyll b Carotenoids 500 600 Wavelength of light (nm) 400 700 Rate of photosynthesis (measured by O 2 release) (a) Absorption spectra 400 (b) Action spectrum 500 600 700 Aerobic bacteria Filament of alga 500 400 (c) Engelmann’s experiment © 2014 Pearson Education, Inc. 600 700

Absorption of light by chloroplast pigments Figure 8. 9 a Chlorophyll b Carotenoids 500

Analyze the diagram • Which two wavelengths of light has the highest absorbance for chlorophyll a? • Which two wavelengths of light has the highest absorbance for chlorophyll b? • Which pigment absorbs light at wavelength 550 nm? • Which pigment absorbs light at wavelength 480 nm? © 2014 Pearson Education, Inc.

Rate of photosynthesis (measured by O 2 release) Figure 8. 9 b 400 (b)

Analyze the diagram • What is the action spectrum measuring? How does it measure this? • What is the relationship between absorption of light and rate of photosynthesis? Explain whether the action spectrum effectively shows this relationship © 2014 Pearson Education, Inc.

Figure 8. 9 c Aerobic bacteria Filament of alga 500 400 600 (c) Engelmann’s

§ The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. 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 the growth of aerobic bacteria clustered along the alga as a measure of O 2 production © 2014 Pearson Education, Inc.

§ Chlorophyll a is the main photosynthetic pigment § Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis § A slight structural difference between chlorophyll a and chlorophyll b causes them to absorb slightly different wavelengths § Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll Video: Chlorophyll Model © 2014 Pearson Education, Inc.

Figure 8. 10 CH 3 in chlorophyll a CHO in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown © 2014 Pearson Education, Inc.

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 © 2014 Pearson Education, Inc.

Figure 8. 11 e − Excited state Energy of electron Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule © 2014 Pearson Education, Inc. (b) Fluorescence

Figure 8. 11 a (b) Fluorescence © 2014 Pearson Education, Inc.

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 © 2014 Pearson Education, Inc.

Figure 8. 12 Photosystem Lightharvesting complexes Reactioncenter complex Thylakoid membrane Photon STROMA Primary electron acceptor Chlorophyll Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light © 2014 Pearson Education, Inc. STROMA Thylakoid membrane e Protein subunits (b) Structure of a photosystem THYLAKOID SPACE

Figure 8. 12 a Thylakoid membrane Photon Photosystem Reactioncenter complex Lightharvesting complexes STROMA Primary electron acceptor e− Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light © 2014 Pearson Education, Inc.

Figure 8. 12 b Thylakoid membrane Chlorophyll Protein subunits (b) Structure of a photosystem

§ A primary electron acceptor in the reaction center accepts excited electrons 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 © 2014 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 P 680 © 2014 Pearson Education, Inc.

§ Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

Linear Electron Flow § Linear electron flow involves the flow of electrons through both

§ Linear electron flow can be broken down into a series of steps 1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P 680 2. An excited electron from P 680 is transferred to the primary electron acceptor (we now call it P 680 ) 3. H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P 680 , thus reducing it to P 680; O 2 is released as a by-product © 2014 Pearson Education, Inc.

Figure 8. UN 02 H 2 O CO 2 Light NADP Calvin Cycle Light

Figure 8. 13 -1 Primary acceptor e− 2 P 680 1 Light Pigment molecules

Figure 8. 13 -2 Primary acceptor 2 H H 2 O 1 2 O

Figure 8. 13 -3 Primary acceptor 2 H H 2 O 1 2 O 2 3 − e 2 4 Electron transport chain Pq Cytochrome complex Pc e− 1 e− P 680 5 Light ATP Pigment molecules Photosystem II (PS II) © 2014 Pearson Education, Inc.

Figure 8. 13 -4 Primary acceptor 2 H H 2 O 1 2 O 2 3 e− 2 4 Electron transport chain Pq e− Cytochrome complex Pc e− 1 e− Primary acceptor P 680 P 700 5 Light 6 ATP Pigment molecules Photosystem II (PS II) © 2014 Pearson Education, Inc. Photosystem I (PS I)

Figure 8. 13 -5 Primary acceptor 2 H H 2 O 1 2 O 2 3 − e 2 4 Electron transport chain Pq 1 e− e− Cytochrome complex e− P 700 5 Light 6 ATP Pigment molecules Photosystem II (PS II) © 2014 Pearson Education, Inc. 8 NADP reductase Pc P 680 Fd e− e− 7 Electron transport chain Primary acceptor Photosystem I (PS I) NADP H NADPH

4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I 5. 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 © 2014 Pearson Education, Inc.

6. In PS I (like PS II), transferred light energy excites P 700, causing it to lose an electron to an electron acceptor (we now call it P 700 ) § P 700 accepts an electron passed down from PS II via the electron transport chain © 2014 Pearson Education, Inc.

7. Excited electrons “fall” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) 8. The electrons are transferred to NADP , reducing it to NADPH, and become available for the reactions of the Calvin cycle § This process also removes an H from the stroma © 2014 Pearson Education, Inc.

§ The energy changes of electrons during linear flow can be represented in a

Figure 8. 14 Mill makes ATP Photon Photo n NADPH © 2014 Pearson Education,

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria § Chloroplasts and mitochondria generate ATP by chemiosmosis but use different sources of energy § Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP § Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities © 2014 Pearson Education, Inc.

§ In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix § In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma © 2014 Pearson Education, Inc.

Figure 8. 15 CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane H Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Matrix Key Stroma ADP P i Higher [H ] Lower [H ] © 2014 Pearson Education, Inc. H ATP

Figure 8. 15 a CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane H Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Matrix Key Higher [H ] Lower [H ] © 2014 Pearson Education, Inc. Stroma ADP P i H ATP

§ 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 H 2 O to NADPH © 2014 Pearson Education, Inc.

Figure 8. 16 Photosystem II 4 H Light Cytochrome complex Light NADP reductase Photosystem I 3 Fd Pq H 2 O THYLAKOID SPACE (high H concentration) e− 1 e− NADPH Pc 2 1 2 O 2 2 H NADP H 4 H To Calvin Cycle STROMA (low H concentration) © 2014 Pearson Education, Inc. Thylakoid membrane ATP synthase ADP P i H ATP

Figure 8. 16 a Cytochrome complex Photosystem II Light 4 H Light Photosystem I Fd Pq H 2 O THYLAKOID SPACE (high H concentration) e − 1 e− 2 O 2 2 H 1 Thylakoid membrane STROMA (low H concentration) © 2014 Pearson Education, Inc. Pc 2 4 H ATP synthase ADP Pi ATP H

Figure 8. 16 b Cytochrome complex Light NADP reductase Photosystem I 3 Fd NADP H NADPH Pc 2 4 H THYLAKOID SPACE (high H concentration) To Calvin Cycle ATP synthase © 2014 Pearson Education, Inc. ADP Pi STROMA (low H concentration) ATP H

• trace the path of an electron from water to NADPH. Use the terms split, photosystem, reduction and oxygen in your explanation. • if the creation of ATP is endergonic, where is the energy for this process coming from? Use the terms light, H+, and ATP synthase in your explanation. © 2014 Pearson Education, Inc.

Concept 8. 3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO 2 to sugar § The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle § Unlike the citric acid cycle, the Calvin cycle is anabolic § It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH © 2014 Pearson Education, Inc.

§ Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde 3 -phospate (G 3 P) § For net synthesis of one G 3 P, the cycle must take place three times, fixing three molecules of CO 2 § The Calvin cycle has three phases § Carbon fixation § Reduction § Regeneration of the CO 2 acceptor © 2014 Pearson Education, Inc.

Figure 8. UN 03 H 2 O CO 2 Light NADP Calvin Cycle Light

Figure 8. 17 -1 as 3 CO 2 Input 3 Phase 1: Carbon fixation

Fill in the blanks: § Phase 1, carbon fixation, involves the incorporation of the

Figure 8. 17 -2 as 3 CO 2 Input 3 Phase 1: Carbon fixation Rubisco 3 P P P 6 P 3 -Phosphoglycerate Ru. BP 6 ATP 6 ADP Calvin Cycle 6 P P 1, 3 -Bisphoglycerate 6 NADPH 6 NADP 6 Pi 6 P G 3 P 1 P G 3 P Output © 2014 Pearson Education, Inc. Phase 2: Reduction Glucose and other organic compounds

§ Phase 2, reduction, involves the _______ of ATP into ADP + energy and the _____ of NADPH into NADP+, H+ and e-. § Where does the energy come from? Which molecules does it go into? © 2014 Pearson Education, Inc.

Figure 8. 17 -3 as 3 CO 2 Input 3 Phase 1: Carbon fixation Rubisco 3 P P P 6 P 3 -Phosphoglycerate Ru. BP 6 ATP 6 ADP Calvin Cycle 3 ADP 3 ATP 6 P P 1, 3 -Bisphoglycerate 6 NADPH Phase 3: Regeneration of Ru. BP 6 NADP 6 Pi P 5 G 3 P 6 G 3 P 1 P G 3 P Output © 2014 Pearson Education, Inc. P Phase 2: Reduction Glucose and other organic compounds

§ Phase 3, regeneration, involves the rearrangement of G 3 P to regenerate the initial CO 2 acceptor, ______. § Which molecule undergoes hydrolysis? Which molecule gets phosphorylated? © 2014 Pearson Education, Inc.

Exit slip check: §Write out the equation for photosynthesis, and label which part of photosynthesis (LR or DR) each reactant/product gets used/produced. §Can the dark reactions happen without light? Explain.

Do now: §Using the words ATP, NADPH, glucose and carbon dioxide explain what occurs in the calvin cycle. §What is photorespiration? Is this process beneficial or detrimental for the plant? Explain. § how do c 3, c 4 plants differ?

Evolution of Alternative Mechanisms of Carbon Fixation in Hot, Arid Climates § Adaptation to dehydration is a problem for land plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis § On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis § The closing of stomata reduces access to CO 2 and causes O 2 to build up § These conditions favor an apparently wasteful process called photorespiration © 2014 Pearson Education, Inc.

§ In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound (3 phosphoglycerate) § In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle, producing a two-carbon compound § Photorespiration decreases photosynthetic output by consuming ATP, O 2, and organic fuel and releasing CO 2 without producing any ATP or sugar © 2014 Pearson Education, Inc.

§ Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 § Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle © 2014 Pearson Education, Inc.

C 4 Plants § C 4 plants minimize the cost of photorespiration by incorporating CO 2 into a four-carbon compound § An enzyme in the mesophyll cells has a high affinity for CO 2 and can fix carbon even when CO 2 concentrations are low § These four-carbon compounds are exported to bundlesheath cells, where they release CO 2 that is then used in the Calvin cycle © 2014 Pearson Education, Inc.

Figure 8. 18 Sugarcane CO 2 C 4 Mesophyll cell Bundlesheath cell 2 CO 2 Calvin Cycle Sugar (a) Spatial separation of steps © 2014 Pearson Education, Inc. CO 2 1 CAM Night Organic acid CO 2 1 Pineapple 2 Day (b) Temporal separation of steps

Figure 8. 18 a Sugarcane © 2014 Pearson Education, Inc.

Figure 8. 18 b Pineapple © 2014 Pearson Education, Inc.

Figure 8. 18 c CO 2 C 4 Mesophyll cell Bundlesheath cell 2 CO 2 Calvin Cycle Sugar (a) Spatial separation of steps © 2014 Pearson Education, Inc. CO 2 1 CAM Night Organic acid CO 2 1 2 Day (b) Temporal separation of steps

CAM Plants § Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon § CAM plants open their stomata at night, incorporating CO 2 into organic acids § Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle © 2014 Pearson Education, Inc.

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 § Plants store excess sugar as starch in the chloroplasts and in structures such as roots, tubers, seeds, and fruits § In addition to food production, photosynthesis produces the O 2 in our atmosphere © 2014 Pearson Education, Inc.

Figure 8. 19 H 2 O CO 2 Light NADP Pi Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain Ru. BP 3 -Phosphpglycerate Calvin Cycle ATP NADPH G 3 P Starch (storage) Chloroplast O 2 © 2014 Pearson Education, Inc. Sucrose (export)

Figure 8. UN 04 © 2014 Pearson Education, Inc.

Figure 8. UN 05 El Pq t or sp an tr in on a tr ch O 2 ec H 2 O El Primary acceptor Fd Pc Photosystem II © 2014 Pearson Education, Inc. tr on ch tr ai an n sp or NADP reductase Cytochrome complex ATP ec Photosystem I t NADP H NADPH