Where It Starts: Photosynthesis Chapter 6

Introduction Before photosynthesis evolved, Earth’s atmosphere had little free oxygen Oxygen released during photosynthesis changed the atmosphere Favored evolution of new metabolic pathways, including aerobic respiration

Then and Now

6.1 Sunlight as an Energy Source Visible light A small part of a spectrum of electromagnetic energy radiating from the sun Electromagnetic energy Travels in waves Is organized as photons

Electromagnetic Spectrum

Wavelengths of visible light (in nanometers) 400 500 600 700 Wavelengths of visible light (in nanometers) Fig. 6.2b, p.94

Photosynthetic Pigments Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules Pigment molecules absorb only light of particular wavelengths Photons not captured are reflected as color

Pigments Reflect Color

Major Photosynthetic Pigments Chlorophyll a Main photosynthetic pigment Absorbs violet and red light (appears green) Chlorophyll b, carotenoids, phycobilins Absorb additional wavelengths Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light

Chlorophyll a

6.2 Exploring the Rainbow

Engelmann’s Experiment

Outcome of T. Engelmann’s experiment. alga a Outcome of T. Engelmann’s experiment. Fig. 6.4a, p.96

Absorption Spectra

Wavelength (nanometers) 80 60 Light absorption (%) 40 20 400 500 600 700 Wavelength (nanometers) b Absorption spectra for chlorophyll a (solid graph line) and chlorophyll b (dashed line). Compare these graphs with the clustering of bacteria shown in (a). Fig. 6.4b, p.96

Wavelength (nanometers) 80 60 Light absorption (%) 40 20 400 500 600 700 Wavelength (nanometers) c Absorption spectra for beta-carotene (solid line) and one of the phycobilins (dashed line). Fig. 6.4c, p.96

Key Concepts: THE RAINBOW CATCHERS A great one-way flow of energy through the world of life starts after chlorophylls and other pigments absorb the energy of visible light from the sun’s rays In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates

6.3 Overview of Photosynthesis Photosynthesis proceeds in two stages Light-dependent reactions Light-independent reactions Summary equation: 6H2O + 6CO2 6O2 + C6H12O6

Visual Summary of Photosynthesis

end products (e.g., sucrose, starch, cellulose) Light- Dependent Reactions sunlight H2O O2 ADP + Pi ATP NADP+ NADPH Light- Independent Reactions Calvin-Benson cycle CO2 H2O phosphorylated glucose end products (e.g., sucrose, starch, cellulose) Fig. 6.13, p.104

Sites of Photosynthesis: Chloroplasts Light-dependent reactions occur at a much-folded thylakoid membrane Forms a single, continuous compartment inside the stroma (chloroplast’s semifluid interior) Light-independent reactions occur in the stroma

Sites of Photosynthesis

upper epidermis photosynthetic cells leaf vein lower epidermis a Zooming in on a photosynthetic cell. lower epidermis Fig. 6.6a, p.97

Sites of Photosynthesis

two outer membranes of chloroplasts stroma part of thylakoid membrane system bathed in stroma: thylakoid compartment, cutaway view b Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma. Fig. 6.6b, p.97

Sites of Photosynthesis

c In chloroplasts, ATP and NADPH form in the light-dependent stage of sunlight O2 H2O CO2 CHLOROPLAST NADPH, ATP light- dependent reactions light- independent reactions NADP+, ADP sugars CYTOPLASM c In chloroplasts, ATP and NADPH form in the light-dependent stage of photosynthesis, which occurs at the thylakoid membrane. The second stage, which produces sugars and other carbohydrates, proceeds in the stroma. Fig. 6.6c, p.97

Products of Light-Dependent Reactions Typically, sunlight energy drives the formation of ATP and NADPH Oxygen is released from the chloroplast (and the cell)

Key Concepts: OVERVIEW OF PHOTOSYNTHESIS Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy Second, chemical energy drives synthesis of carbohydrates

6.4 Light-Dependent Reactions Two types of photosystems In thylakoid membrane Light-harvesting complexes Absorb light energy and pass it to photosystems which then release electrons Electrons enter light-dependent reactions

Noncyclic Photophosphorylation Electrons released from photosystem II flow through an electron transfer chain At end of chain, they enter photosystem I Photon energy causes photosystem I to release electrons, which end up in NADPH Photosystem II replaces lost electrons by pulling them from water (photolysis)

Noncyclic Photophosphorylation

electron transfer chain light energy light energy electron transfer chain NADPH Photosystem II Photosystem I THYLAKOID COMPARTMENT THYLAKOID MEMBRANE oxygen (diffuses away) STROMA Fig. 6.8b, p.99

Cyclic Photophosphorylation Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I NADPH does not form, oxygen is not released

ATP Formation In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment A hydrogen ion gradient builds up across the thylakoid membrane H+ flows back across the membrane through ATP synthases Results in formation of ATP in the stroma

6.5 Energy Flow in Light-Dependent Reactions

PHOTOPHOSPHORYLATION excited P700 energy light energy P700 Photosystem I CYCLIC PHOTOPHOSPHORYLATION a Energy from light-harvesting complexes causes photosystem I to lose electrons. b Electrons give up energy as they pass through an electron transfer chain. The energy drives H+ across the thylakoid membrane, against its gradient. The electrons reenter photosystem I. Fig. 6.9ab, p.100

6.5 Energy Flow in Light-Dependent Reactions

PHOTOPHOSPHORYLATION Photosystem II excited P700 excited P680 NADPH light energy energy light energy P700 P680 Photosystem I water O2 + H+ NONCYCLIC PHOTOPHOSPHORYLATION c. Energy from a light- harvesting complex drives electrons out of photosystem II. Then, the photosystem pulls replacement electrons d. Electrons from photosystem II pass through an electron transfer chain. Energy lost at each step moves H+ across the thylakoid membrane. At the end of the chain, the electrons enter photosystem e. NADP+ combines with hydrogen and with electrons driven from photosystem II by energy from a light-harvesting complex. Fig. 6.9cde, p.100

Key Concepts: MAKING ATP AND NADPH In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP The coenzyme NADPH forms in a pathway that also releases oxygen

6.6 Light Independent Reactions: The Sugar Factory Light-independent reactions proceed in the stroma Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle

Calvin–Benson Cycle Cyclic pathway makes phosphorylated glucose Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose) Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2

Light-Independent Reactions

stroma Fig. 6.10b, p.101

a CO2 in air spaces inside a leaf diffuses into a photosynthetic cell. Six times, rubisco attaches a carbon atom from CO2 to the RuBP that is the starting compound for the Calvin–Benson cycle. f It takes six turns of the Calvin–Benson cycle (six carbon atoms) to produce one glucose molecule and regenerate six RuBP. 6CO2 b Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. The resulting intermediate is called PGAL. e Ten of the PGAL get phosphate groups from ATP. In terms of energy, this primes them for an uphill run—for the endergonic synthesis reactions that regenerate RuBP. 6 RuBP 12 PGA 12 ATP 6 ADP Calvin-Benson cycle 12 ADP + 12 Pi 6 ATP 12 NADPH 4 Pi 12 NADP+ d The phosphorylated glucose enters reactions that form carbohydrate products—mainly sucrose, starch, and cellulose. c Two of the twelve PGAL molecules combine to form a molecule of glucose with an attached phosphate group. 10 PGAL 12 PGAL 1 Pi phosphorylated glucose Fig. 6.10, p.101

6.7 Adaptations: Different Carbon-Fixing Pathways Environments differ Plants have different details of sugar production in light-independent reactions On dry days, plants conserve water by closing their stomata O2 from photosynthesis cannot escape

Plant Adaptations to Environment C3 plants High O2 level; Rubisco attaches to O2 instead of CO2 to RuBP; Photorespiration reduces efficiency of sugar production

a C3 plants. On dry days, stomata close CO2 glycolate RuBP Calvin-Benson cycle PGA sugar ATP NADPH a C3 plants. On dry days, stomata close and oxygen accumulates in air spaces inside leaves. The high concentration of oxygen makes rubisco attach oxygen instead of carbon to RuBP. Cells lose carbon and energy as they make sugars. Fig. 6.11a2, p.102

Plant Adaptations to Environment C4 plants Carbon fixation occurs twice First reactions release CO2 near rubisco, limit photorespiration when stomata are closed

C4 cycle Calvin-Benson cycle CO2 from inside plant C4 cycle oxaloacetate CO2 RuBP Calvin-Benson cycle PGA sugar b C4 plants. Oxygen also builds up in the air spaces inside the leaves when stomata close. An additional pathway in these plants keeps the CO2 concentration high enough to prevent rubisco from using oxygen. Fig. 6.11b2, p.102

Plant Adaptations to Environment CAM plants Open stomata and fix carbon at night

C4 cycle Calvin-Benson cycle CO2 from outside plant C4 cycle oxaloacetate night day CO2 RuBP Calvin-Benson cycle PGA sugar c CAM plants open stomata and fix carbon with a C4 pathway at night. When stomata are closed during the day, organic compounds made during the night are converted to CO2 that enters the Calvin–Benson cycle. Fig. 6.11c2, p.102

Key Concepts: MAKING SUGARS Second stage is the “synthesis” part of photosynthesis Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide Reactions use ATP and NADPH that form in the first stage of photosynthesis

Key Concepts: MAKING SUGARS (cont.) ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites Details of the reactions vary among organisms

6.8 A Burning Concern Photoautotrophs remove CO2 from atmosphere; metabolic activity of organisms puts it back Human activities disrupt the carbon cycle Add more CO2 to the atmosphere than photoautotrophs can remove Imbalance contributes to global warming

Fossil Fuel Emissions

Animation: C3-C4 comparison CLICK HERE TO PLAY

Animation: Calvin-Benson cycle CLICK HERE TO PLAY

Animation: Energy changes in photosynthesis CLICK HERE TO PLAY

Animation: Noncyclic pathway of electron flow CLICK HERE TO PLAY

Animation: Photosynthesis overview CLICK HERE TO PLAY

Animation: Sites of photosynthesis CLICK HERE TO PLAY

Animation: Wavelengths of light CLICK HERE TO PLAY