Chapter 10 Photosynthesis

Rubisco video- photorespiration

https://www.pbs.org/wgbh/nova/video/enzyme-driving-photosynthesis/

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RU

Solving the problem

https://youtu.be/beROF-QJwoI?si=bL_Qz8-yqi7pnkNz

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RU

Warm-Up

1. Compare and contrast heterotrophs to autotrophs.

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• Write the balanced chemical equation for photosynthesis.

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• Why is the leaf shaped and structured as it is? (think structure → function)

Warm-Up

1. A photon of which color of light would contain more energy:
• Orange (620 nm) or Blue (480 nm)?
• Why?
2. How did Engelmann determine the absorption spectrum for algae? What were his results?
3. What is happening in the leaves during autumn?

Warm-Up

1. What pigments are found in spinach leaves?
2. Based on the color of the pigments, what wavelengths of visible light are absorbed by each of these pigments? (see Figure 10.7 in textbook for help)

Warm-Up

(Refer to notes / Campbell)

1. What is the main function of the Light Reactions?
2. What are the reactants of the Light Reactions? What are the products?
3. Where does the Light Reactions occur?

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Warm-Up

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1. LDR: What is its function? Where does it occur?
2. (See Fig. 10.5) What products of the Light Reaction are used for the Calvin Cycle?

Warm-Up

1. (See Figure 10.17) What are the 3 locations that H+ is used to create the proton gradient?
2. What purpose does cyclic e^- flow serve?
3. What is the main function of the Calvin Cycle? Where does it occur?
4. What are the reactants of the Calvin cycle? What are the products?
5. Which enzyme is responsible for carbon fixation?

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Warm-Up

1. Draw the chloroplast and label it. Where does the light reaction, Calvin cycle, chemiosmosis occur?

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• What is RuBP, rubisco and G3P?

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• Compare Respiration to Photosynthesis.

Warm-Up

1. Why do C₄ plants photosynthesize without photorespiration?

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• What is the purpose of the proton gradient?

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• State the differences and similarities between C₄ and CAM plants.

Warm-Up

1. Draw a T-Chart. Compare/contrast Light Reactions vs. Calvin Cycle.
2. What is photorespiration? How does it affect C3 plants?

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Chapter 10�Photosynthesis

Chemosynthesis

What you need to know:

• The summary equation of photosynthesis including the source and fate of the reactants and products.
• How leaf and chloroplast anatomy relates to photosynthesis.
• How photosystems convert solar energy to chemical energy.
• How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O₂.
• How chemiosmosis generates ATP in the light reactions.
• How the Calvin Cycle uses the energy molecules of the light reactions to produce G3P.
• The metabolic adaptations of C₄ and CAM plants to arid, dry regions.

Photosynthesis

• Plants and other autotrophs are producers

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• Photoautotrophs: use light E to make organic molecules

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• Heterotrophs: consume organic molecules from other organisms for E and carbon

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What is a photoautotroph?

Where does photosynthesis happen?

Photosynthesis: Converts light energy to chemical energy of food

• Chloroplasts: site of photosynthesis in plants

Remember me from the endosymbiont model?

How do plants get what they need for Photosynthesis?

Leaves are Photosynthetic Organs

• Mesophyll: chloroplasts are mainly found in these cells of leaf (palisade layer)
• Chloroplasts: contain chlorophyll in thylakoid membranes
• Xylem: brings water to the leaf
• Stomata: pores in leaf where CO₂ enters (O and H₂O exit)

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How are Stomata Regulated?

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Photosynthesis = Light Reactions + Calvin Cycle

“photo” “synthesis”

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6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

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Redox Reaction:

water is split → e^- transferred with H⁺ to CO₂ → sugar

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Remember: OILRIG

Oxidation: lose e^-

Reduction: gain e^-

PUT A CODE IN EACH BOX ABOVE:

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L or D for…. Light Rx or Dark Rx (Calvin Cycle/Light-Independent Rx)

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AND

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R or P for… Reactant or Product

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For example: LR would mean used in light reaction as a reactant

LR

Tracking atoms through photosynthesis

• Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis (C.B. van Niel)

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Light Dependent Reactions: Convert solar E to chemical E of ATP and NADPH

Nature of sunlight Nature of Light - H excitation

• Light = Energy = electromagnetic radiation
• Shorter wavelength (λ): higher E
• Visible light - detected by human eye
• Light: reflected, transmitted or absorbed

Electromagnetic Spectrum

Which wavelength in the visible portion of the EMS, has the greatest amount of energy?

Which arrow represents the light used in photosynthesis?

Interaction of light with chloroplasts

Photosynthetic pigments

• Pigments absorb different λ of light
• chlorophyll – absorb violet-blue/red light, reflect green
• chlorophyll a (blue-green): light reaction, converts solar to chemical E
• chlorophyll b (yellow-green): conveys E to chlorophyll a
• carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis
• Types: xanthophyll (yellow) & carotenes (orange)
• anthocyanin (red, purple, blue): photoprotection, antioxidants

Absorption Spectrum: determines effectiveness of different wavelengths for photosynthesis

Action Spectrum: plots rate of photosynthesis vs. wavelength

(absorption of chlorophylls a, b, & carotenoids combined)

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Engelmann: used bacteria to measure rate of photosynthesis in algae; established action spectrum

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Which wavelengths of light are most effective in driving photosynthesis?

Effects of Light Intensity and Wavelength on Photosynthesis- Simulation

Light Reactions

Light Reactions

Summary:

1. Light energy splits H₂O to O₂ releasing high energy electrons (e^-)
2. Movement of e^- used to generate ATP
3. Electrons end up on NADP⁺, reducing it to NADPH

Electrons in chlorophyll molecules are excited by absorption of light

Photosystem: reaction center & light-harvesting complexes (pigment + protein)

Electron Flow

Two routes for electron flow:

A. Linear (noncyclic) electron flow

B. Cyclic electron flow

Light Reaction (Linear electron flow)

1. Chlorophyll excited by light absorption
2. E passed to reaction center of Photosystem II (protein + chlorophyll a)
3. e^- captured by primary electron acceptor
• Redox reaction → e^- transfer
• e- prevented from losing E (drop to ground state)
4. H₂O is split to replace e^- → O₂ formed

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5. e^- passed to Photosystem I via ETC
6. E transfer pumps H⁺ to thylakoid space
7. ATP produced by photophosphorylation
8. e^- moves from PS I’s primary electron acceptor to 2^nd ETC
9. NADP⁺ reduced to NADPH

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MAIN IDEA: Use solar E to generate ATP & NADPH to provide E for Calvin cycle

Photosynthesis Animation Forest Biology

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Light Dependent Reactions Animation Smith

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Mechanical analogy for the light reactions

Cyclic Electron Flow: uses PS I only; produces ATP for Calvin Cycle (no O₂ or NADPH produced)

What another name for this?

Both respiration and photosynthesis use chemiosmosis to generate ATP

Proton motive force generated by:

• H⁺ from water
• H⁺ pumped across by cytochrome
• Removal of H⁺ from stroma when NADP⁺ is reduced

Calvin Cycle

Calvin Cycle: Uses ATP and NADPH to convert CO₂ to sugar

• Occurs in the stroma
• Uses ATP, NADPH, CO₂
• Produces 3-C sugar G3P (glyceraldehyde-3-phosphate)

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Three phases:

• Carbon fixation
• Reduction
• Regeneration of RuBP (CO₂ acceptor)

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Phase 1: 3 CO₂ + RuBP (5-C sugar ribulose bisphosphate)

• Catalyzed by enzyme rubisco (RuBP carboxylase)

Phase 2: Use 6 ATP and 6 NADPH to produce 1 net G3P

Phase 3: Use 3 ATP to regenerate RuBP

G3P basis for organic molecules

Photorespiration!

Alternative mechanisms of carbon fixation have evolved in hot, arid climates

Photorespiration

• Metabolic pathway which:
• Uses O₂ & produces CO₂
• Uses ATP
• No sugar production (rubisco binds O₂ → breakdown of RuBP)
• Occurs on hot, dry bright days when stomata close (conserve H₂O)
• Why? Early atmosphere: low O₂, high CO₂?

Evolutionary Baggage

1. Problem with C₃ Plants:
• CO₂ fixed to 3-C compound in Calvin cycle
• Ex. Rice, wheat, soybeans
• Hot, dry days:
• partially close stomata, ↓CO₂
• Photorespiration, uses ATP
• ↓ photosynthetic output (no sugars made)

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C₄ Plants (Coevolution-45x)

CO₂ fixed to 4-C compound

• Ex. Corn, Sugarcane, Grass
• Hot, dry days → partially stomata close
• 2 cell types = mesophyll & bundle sheath cells
• mesophyll : PEP carboxylase fixes CO₂ even at low concentrations (4-C), pump CO₂ to bundle sheath
• bundle sheath: CO₂ used in Calvin cycle
• ↓photorespiration, ↑sugar production
• WHY? Advantage in hot, sunny areas

C₄ Leaf Anatomy

Notice that the oxygen producing light dependent reactions occur in the mesophyll cells to insulate the rubisco from the oxygen and prevent photorespiration.

3. CAM Plants:
• Crassulacean acid metabolism (CAM)
• NIGHT: stomata open → CO₂ enters → converts to organic acid, stored in mesophyll cells (vacuole!)
• DAY: stomata closed → light reactions supply ATP, NADPH; CO₂ released from organic acids for Calvin cycle
• Ex. cacti, pineapples, succulent (H₂O-storing) plants
• WHY? Advantage in arid conditions

Comparison

Importance of Photosynthesis

Kleptoplasty

solar-powered sea slug

Elysia chlorotica

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Costasiella kuroshiCostasiella kuroshimae (or ‘Leaf Sheep’mae (or ‘Leaf Sheep’

Costasiella kuroshimae ‘Leaf Sheep’

Questions?

Questions?

Why do the chloroplasts remain active?

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Hypothesis:

https://phys.org/news/2021-07-chloroplast-acquisition-gene-photosynthetic-sea.html

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Questions?

Why do the chloroplasts remain active?

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Hypothesis: Horizontal Gene Transfer

https://phys.org/news/2021-07-chloroplast-acquisition-gene-photosynthetic-sea.html

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The Photosynthesis Fix- Great article

Around minute 5, describes how the liquid nanoclay help cultivate mycelium to increase growth

Review of Photosynthesis

Photosynthesis

Light Reaction

Light ENERGY

H₂O split

organic molecules

O₂ evolved

ETC

regenerate RuBP

photophosphorylation

ATP

chemiosmosis

energized electrons

Calvin Cycle

NADPH

CO₂ fixed to RuBP

C₃ phosphorylated and reduced

G3P

glucose & other carbs

stored in

in which

pass down

involves both

Reduce NADP+ to

by mechanism of

using

in process called

to form

using

LIGHT REACTIONS

CALVIN CYCLE

MITOCHONDRIA

CHLOROPLAST

Comparison

RESPIRATION

PHOTOSYNTHESIS

• Plants + Animals
• Needs O₂ and food
• Produces CO₂, H₂O and ATP, NADH
• Occurs in mitochondria membrane & matrix
• Oxidative phosphorylation
• Proton gradient across membrane

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• Plants
• Needs CO₂, H₂O, sunlight
• Produces glucose, O₂ and ATP, NADPH
• Occurs in chloroplast thylakoid membrane & stroma
• Photorespiration
• Proton gradient across membrane

Cellular Energetics in Prokaryotes vs. Eukaryotes