Photosynthesis: Capturing Energy Photosynthesis: Capturing Energy Chapter 9

Learning Objective 1 What are the physical properties of light? What is the relationship between a wavelength of light and its energy?

Electromagnetic Spectrum

Fig. 9-1, p. 192 One wavelength Longer wavelength TV and radio waves Red 700 nm Micro- waves Orange InfraredColor spectrum of visible light 600 nm Yellow Visible X-rays 500 nm Green Blue Gamma rays Violet 400 nm Electromagnetic spectrum Shorter wavelength 760 nm 380 nm UV

Light Consists of particles (photons) that move as waves Photons with shorter wavelengths have more energy than those with longer wavelengths

Sunlight

Fig. 9-2, p. 192 Sun Sunlight is a mixture of many wavelengths

Light and Energy

Fig. 9-3, p. 193 Photon Photon is absorbed by an excitable electron that moves into a higher energy level. Low energy level Electron High energy level Either Electron acceptor molecule The electron may return to ground level by emitting a less energetic photon. The electron may be accepted by an electron acceptor molecule. Or

KEY CONCEPTS Light energy powers photosynthesis, which is essential to plants and most life on Earth

Learning Objective 2 What is the internal structure of a chloroplast? How do its components interact and facilitate the process of photosynthesis?

Structures Photosynthesis in plants occurs in chloroplasts located in mesophyll cells inside the leaf

Leaf Structure

Fig. 9-4a, p. 194 (a) This leaf cross section reveals that the mesophyll is the photosynthetic tissue. CO 2 enters the leaf through tiny pores or stomata, and H 2 O is carried to the mesophyll in veins. Palisade mesophyll Vein Air space Stoma Spongy mesophyll

Fig. 9-4b, p. 194 (b) Notice the numerous chloroplasts in this LM of plant cells. Mesophyll cell 10 μm

Fig. 9-4c, p. 194 Outer membrane Inner membrane Stroma 1 μm Intermembrane space Thylakoid membrane (c) In the chloroplast, pigments necessary for the light-capturing reactions of photosynthesis are part of thylakoid membranes, whereas the enzymes for the synthesis of carbohydrate molecules are in the stroma. Granum (stack of thylakoids) Thylakoid lumen

Chloroplasts Enclosed by a double membrane inner membrane encloses stroma and thylakoids Thylakoids enclose thylakoid lumen arranged in stacks (grana)

Photosynthetic Pigments In thylakoid membranes chlorophyll a chlorophyll b Carotenoids

Fig. 9-5, p. 195 Hydrocarbon side chain Porphyrin ring (absorbs light) in chlorophyll b in chlorophyll a

Learning Objective 3 What happens to an electron in a biological molecule such as chlorophyll when a photon of light energy is absorbed?

Energizing Electrons Photons excite photosynthetic pigments chlorophyll Energized electrons move to electron acceptor compounds

Active Wavelengths Combined absorption spectra of chlorophylls a and b action spectrum for photosynthesis

Absorption Spectra

Fig. 9-6a, p. 195 Chlorophyll b Chlorophyll a Estimated absorption (%) Wavelength (nm) (a) Chlorophylls a and b absorb light mainly in the blue (422 to 492 nm) and red (647 to 760 nm) regions.

Fig. 9-6b, p. 195 Relative rate of photosynthesis Wavelength (nm) (b) The action spectrum of photosynthesis indicates the effectiveness of various wavelengths of light in powering photosynthesis. Many plant species have action spectra for photosynthesis that resemble the generalized action spectrum shown here.

Action Spectrum

Fig. 9-7a, p μm (a)

Fig. 9-7b, p. 196 Wavelength of light (nm)

Learning Objective 4 Describe photosynthesis as a redox process

Photosynthesis Light energy is converted to chemical energy (carbohydrates) Hydrogens from water reduce carbon Oxygen from water is oxidized, forming molecular oxygen

Learning Objective 5 What is the difference between light- dependent reactions and carbon fixation reactions of photosynthesis?

Energy for Reactions Light-dependent reactions light energizes electrons that generate ATP and NADPH Carbon fixation reactions use energy of ATP and NADPH to form carbohydrate

Photosynthesis

Fig. 9-8, p. 197 Light-dependent reactions (in thylakoids) Carbon fixation reactions (in stroma) Chloroplast ATP Light reactions ADP Calvin cycle NADPH NADP + H2OH2OO2O2 CO 2 Carbohydrates

Stepped Art Fig. 9-8, p. 197 Light-dependent reactions (in thylakoids) Chloroplast Carbon fixation reactions (in stroma) Light reactions ATP ADP Calvin cycle NADPH NADP + H2OH2OO2O2 CO 2 Carbohydrates

Learning Objective 6 How do electrons flow through photosystems I and II in the noncyclic electron transport pathway? What products are produced? Contrast this with cyclic electron transport

The Photosystems Photosystems I and II photosynthetic units include chlorophyll, accessory pigments organized with pigment-binding proteins into antenna complexes

A Photosystem

Fig. 9-10, p. 198 Primary electron acceptor e-e- Chloroplast Photon Thylakoid membrane Photosystem

Reaction Centers Reaction center of antenna complex special pair of chlorophyll a molecules release energized electrons to acceptor P700 reaction center for photosystem I P680 reaction center for photosystem II

Noncyclic Electron Transport Light-dependent reactions form ATP and NADPH

Noncyclic Systems Electrons in photosystem I energized by light pass through electron transport chain convert NADP + to NADPH Redox reactions pass energized electrons along ETC from photosystem II to photosystem I

Noncyclic Systems Electrons given up by P700 (photosystem I) replaced by electrons from P680 (photosystem II) Electrons given up by P680 (photosystem II) replaced by electrons from photolysis of H 2 O (releasing oxygen)

Cyclic Electron Transport Electrons from photosystem I return to photosystem I ATP produced by chemiosmosis No NADPH or oxygen generated

Table 9-1, p. 200

KEY CONCEPTS Photosynthesis, which occurs in chloroplasts, is a redox process

Learning Objective 7 Explain how a proton (H + ) gradient is established across the thylakoid membrane and how this gradient functions in ATP synthesis

Proton Gradient

Fig. 9-12, p. 200 Stroma Thylakoid lumen Thylakoid membrane Protons (H + )

Photophosphorylation Photophosphorylation synthesis of ATP coupled to transport of electrons energized by photons Electron energy pumps protons across thylakoid membrane energy gradient generates ATP by chemiosmosis

ATP Synthesis ATP synthase enzyme complex in thylakoid membrane protons diffuse through enzyme phosphorylate ADP to ATP

Electron Transport and Chemiosmosis

Fig. 9-13, p. 201 Thylakoid lumen Thylakoid membrane Photosystem II Photon Thylakoid membrane Plastocyanin Plastoquinone Ferredoxin Cytochrome complex Photosystem I Photon Ferredoxin- NADP+ reductase NADPH NADP+ ADP PiPi ATP synthase

KEY CONCEPTS Light-dependent reactions convert light energy to the chemical energy of NADPH and ATP

Learning Objective 8 Summarize the three phases of the Calvin cycle, and the roles of ATP and NADPH

Calvin Cycle (C 3 pathway) Carbon fixation reactions 3 phases CO 2 uptake phase Carbon reduction phase RuBP regeneration phase

CO 2 Uptake Phase Enzyme rubisco (ribulose bisphosphate carboxylase/ oxygenase) combines CO 2 with ribulose bisphosphate (RuBP), a five-carbon sugar forms 3-carbon phosphoglycerate (PGA)

Carbon Reduction Phase Energy of ATP and NADPH convert PGA molecules to glyceraldehyde- 3-phosphate (G3P) For each 6 CO 2 fixed 12 G3P are produced 2 G3P leave cycle to produce 1 glucose

RuBP Regeneration Phase Remaining G3P molecules are modified to regenerate RuBP

The Calvin Cycle

KEY CONCEPTS Carbon fixation reactions incorporate CO 2 into organic molecules

Learning Objective 9 How does photorespiration reduce photosynthetic efficiency?

Photorespiration C 3 plants use O 2 and generate CO 2 by degrading Calvin cycle intermediates but do not produce ATP On bright, hot, dry days plants close stomata, conserving water prevents passage of CO 2 into leaf

Learning Objective 10 Compare the C 4 and CAM pathways

C 4 Pathway Takes place in mesophyll cells Enzyme PEP carboxylase binds CO 2 CO 2 fixed in oxaloacetate converted to malate Malate moves into bundle sheath cell CO 2 is removed Released CO 2 enters Calvin cycle

C 4 and C 3 Plants

Fig. 9-15a, p. 205 Upper epidermis Palisade mesophyll Bundle sheath cells of veins Spongy mesophyll Chloroplasts (a) In C 3 plants, the Calvin cycle takes place in the mesophyll cells and the bundle sheath cells are nonphotosynthetic.

Fig. 9-15b, p. 205 Upper epidermis Bundle sheath cells of veins Mesophyll Chloroplasts (b) In C 4 plants, reactions that fix CO 2 into four-carbon compounds take place in the mesophyll cells. The four-carbon compounds are transferred from the mesophyll cells to the photosynthetic bundle sheath cells, where the Calvin cycle takes place.

C 4 Pathway

Fig. 9-16, p. 206 CO 2 Mesophyll cell Phosphoenol- pyruvate Oxaloacetate ADP NADPH ATP NADP + Pyruvate Bundle sheath cell NADP + CO 2 Glucose NADPH Vein (4C)(3C) Malate (4C) Malate (3C)(4C)

(CAM) Pathway Crassulacean acid metabolism (CAM) similar to C 4 pathway PEP carboxylase fixes carbon at night in mesophyll cells Calvin cycle occurs during the day

A CAM Plant

Learning Objective 11 How do photoautotrophs and chemoheterotrophs differ with respect to their energy and carbon sources?

Energy Sources Photoautotrophs use light as energy source incorporate atmospheric CO 2 into pre- existing carbon skeletons Chemoheterotrophs obtain energy by oxidizing chemicals obtain carbon from other organisms

KEY CONCEPTS Most photosynthetic organisms are photoautotrophs

Learning Objective 12 What is the importance of photosynthesis to plants and other organisms?

Photosynthesis Ultimate source of all chemical energy and organic molecules available to plants and other organisms Replenishes oxygen in the atmosphere vital to all aerobic organisms

Summary Reactions Light-dependent reactions (noncyclic electron transport) 12 H 2 O + 12 NADP ADP + 18 P i → (light energy, chlorophyll) → 6 O NADPH + 18 ATP

Summary Reactions Carbon fixation reactions (Calvin cycle) 12 NADPH + 18 ATP + 6 CO 2 → C 6 H 12 O NADP ADP + 18 P i + 6 H 2 O

Summary Reactions Overall equation for photosynthesis 6 CO H 2 O → (light energy, chlorophyll) → C 6 H 12 O O H 2 O

CLICK TO PLAY Calvin-Benson Cycle

Sites of Photosynthesis CLICK TO PLAY

Wavelengths of Light CLICK TO PLAY

Photosynthesis Overview CLICK TO PLAY