Photosynthesis

Question 1

Overall reaction for photosynthesis

Answer 1

nCO₂ + nH₂O + photons (sunlight) → (CH₂O)_n + nO₂

Carbohydrates

n > 3

• Light drives the reduction of carbon
• Takes place in plants, algae, & cyanobacteria
• Carbon is “fixed” as carbohydrates
• Photosynthesis and cellular respiration are near opposite processes. They differ only in the form of energy absorbed or released.

Question 2

Light reactions

Answer 2

Overall Rxn for Light (light-dependent) Rxns:

2 H₂O + 2 NADP⁺+ 8 photons → O₂+ 2 NADPH + 2 H+

• Requires photons and water

2 H₂O → O2 + 4H⁺ + 4e-

• H2O is oxidized to O2
• & H2O is the source of electrons
• Produces O₂, ATP, and NADPH

Question 3

Dark Reactions

Answer 3

Overall Rxn for Dark (light-independent) Rxns:

Dark rxns = Calvin cycle

3 CO₂ + 9 ATP + 6 NADPH → GAP + 9 ADP + 6 NADP⁺ +6 Pi

• ATP & NADPH are used to reduce CO2 to carbohydrates
  
  • 4H• + CO₂ → (CH₂O)n + H2O
• CO₂ is the source of electrons
• Still takes place in the light but doesn’t require light to take place
• In the end, the energy that started out as light (from light reactions) winds up trapped in the bonds of both sugars

Question 4

What are the site of photosynthesis in eukaryotes?

Answer 4

Chloroplasts = are the site of photosynthesis in eukaryotes

Question 5

Where do the light reactions take place?

Answer 5

Light reactions take place in the thylakoid membrane and require a continuous supply of light energy

Question 6

Where do the dark reactions take place?

Answer 6

Dark reactions take place in the stroma and do not directly require light

Question 7

The chloroplast ______________ is the site of light absorption

Answer 7

the chloroplast thylakoid membrane is the site of light absorption

Question 8

Stroma

Answer 8

Stroma = fluid inside the chloroplasts

• The stroma contains the enzymes for carbohydrate biosynthesis (so dark rxns take place in the stroma)
• DNA, RNA, ribosomes to synthesize chloroplasts

Question 9

Granum

Answer 9

Granum = stack of thylakoids

Question 10

Each mesophyll cell contains organelles called ___________, which are specialized to carry out the reactions of photosynthesis.

Answer 10

Each mesophyll cell contains organelles called chloroplasts, which are specialized to carry out the reactions of photosynthesis.

Question 11

The membrane of each thylakoid contains _____________ that absorb light

Answer 11

The membrane of each thylakoid contains chlorophylls that absorb light

Question 12

Why do photosynthetic organisms contain several types of pigment molecules?

Answer 12

• Chlorophyll is the principle photoreceptor in photosynthesis
• The various R groups alter the pigment’s maximum absorption wavelength
• Other pigments “fill in” absorption holes of chlorophyll ← called accessory pigments

Question 13

Light Harvesting Complexes (LHC)

Answer 13

Light Harvesting Complex (LHC) = complex of antenna pigments that collects light energy and transfers it to a rxn center

• Most chlorophyll molecules function as light harvesting complexes (LHC)

Question 14

Photosynthetic reaction center (RC)

Answer 14

Photosynthetic Reaction Center (RC) = chlorophyll molecule where photosynthesis takes place

Question 15

Citation energy trapping by the photosynthetic reaction center

Answer 15

Question 16

Photosynthesis is a _____ processs in photosynthetic bacteria

&

Photosynthesis is a _________ process in plants & cyanobacteria

Answer 16

Photosynthesis is a cyclic process in photosynthetic bacteria

&

Photosynthesis is a noncyclic process in plants and plants & cyanobacteria

Question 17

Electron transport in photosynthesis is __________________

b/c ____________________________________________

____________________________________________

Answer 17

Electron transport in photosynthesis is essentially irreversible b/c electrons are transferred to progressively lower energy states (more + positive standard reduction potentials)

Question 18

PSII Complex

Answer 18

PSII complex:

When light is absorbed by one of the many pigments in PS II, energy is passed inward from pigment to pigment until it reaches the RC.

There, energy is transferred to P680, boosting an electron to a high energy level.

The high- energy electron is passed to an acceptor molecule and replaced with an electron from water. (This splitting of water releases the O2 we breathe.)

Question 19

Cytochrome b6f

Answer 19

Cytochrome b6f:

Electrons pass from QH2 to plastocyanin (a mobile electron carrier)

A proton gradient is also generated that drives the synthesis of ATP in chloroplasts

Question 20

Plastocyanin (PC)

Answer 20

Plastocyanin (PC):

Transports Electrons From Cytochrome b6f to PSI

Question 21

PS I

Answer 21

PS I:

PSI uses light energy to generate reduced ferrodoxin, a powerful reductant.

Question 22

Noncyclic Pathway electron transfer in PSI

Answer 22

Non-cyclic pathway: Ferredoxin (Fd) reduction

The reaction occurs in the stroma

Question 23

Cyclic Pathway electron transfer in PSI

Answer 23

Cyclic pathway: Return to PSI via Cytochrome b6

• The electrons are transferred to the Q_pool → translocates protons across the thylakoid membrane
• No O₂ or NADPH generation

Question 24

Why two pathways for electron transport in PSI?

Answer 24

Why two pathways for electron transport in PSI?

• Allows for increased ATP production relative to NADPH
• Cellular control of ATP and NADPH amounts

Question 25

Explain how the thylakoid membrane protein complexes are distributed and why this distribution is ideal

Answer 25

• Thylakoid Membrane Surfaces
  
  • Stromal lamellae – unstacked membranes
  • Grana – stacked membranes
• PSI – mostly in unstacked stromal lamellae membranes
  
  • Allows contact with stromal NADP+
• PSII – located between the stacked grana
  
  • Segregation from PSI prevents PSII from simply acting as an antenna for PSI (since PSI has a lower excitation energy)
  • Permits the chloroplast to respond to changes in illumination
• Cytochrome b6f - uniformly distributed

Question 26

Explain how the metabolic pathway by which plants incorporate CO2 into carbohydrates was elucidated

Answer 26

Calvin: Used ¹⁴C-labelled CO₂ to study how CO₂ is incorporated into sugars by plants

• Observed ¹⁴C-labelled 3-phosphoglycerate at the carboxylic acid carbon
• Followed the “flow” of 14C through various compounds involved in the cycle
• The ¹⁴CO₂ is added to a 5-C unit (Ru5P) to create RuBP
• RuBP then splits into two 3-C units (3- phosphoglycerate or 3PG)

Question 27

Explain why the Calvin cycle is also called the reductive pentose phosphate cycle

Answer 27

• Reductive: CO₂ is reduced to carbohydrates
• Pentose phosphate: a pentose phosphate (Ru5P) is carboxylated and regenerated

Question 28

Two stages of Calvin Cycle

Answer 28

Stage 1: Production of GAP (glyceraldehyde-3-phosphate)

• 3 Ru5P+3 CO₂ +9 ATP+6 NADPH→ 6 GAP + 9 ADP + 6 NADP+ + 6 Pi
• Carbon rearrangements:
  
  • 3 C5 (C5=Ru5P) + 3 C1 (C1=CO₂) → 6 C3 (C3=GAP)

Stage 2: Recovery of Ru5P (ribulose 1,5-bisphosphate)

• Carbon shuffling reactions, starting from GAP, are used to regenerate Ru5P
• Net product: 5 C3 (C3=GAP) → 3 C5 (C5=Ru5P)

Question 29

What is the main reaction that this enzyme catalyzes (CO2 fixation) and explain why this enzyme has been called the world’s most important enzyme

Answer 29

• RuBP carboxylase is (RuBisCO) the most abundant catalyst.
• Ribulose bisphosphate carboxylase (RuBP carboxylase or RuBisCO) catalyzes carboxylation of RuBP
  
  • Unique step of the dark reactions
  • CO₂ fixation (converts RuBP to 3PG using CO₂ )
• It’s so important because it recycles CO₂ to make carbohydrates

Question 30

Describe how GAP, the product of the Calvin Cycle, is converted to carbohydrates

Answer 30

• GAP is converted to FBP (fructose- 1,6-bisphosphate) by Calvin cycle enzymes
  
  • FBP can be converted to G1P (glucose-1-phosphate) by gluconeogenic enzymes and phosphoglucomutase
  • G1P is the key precursor for plant oligosaccharides (starch, sucrose, & cellulose)

Question 31

How is the Calvin Cycle is controlled through light, including the relevant enzymes and how these are deactivated in the dark

Answer 31

• RuBisCO:
  
  • Activated by increased pH and levels of Mg2+ (which occur in the presence of light)
  • SO plants synthesize a RuBisCO inhibitor that only works in the dark
• FBPase and SBPase:
  
  • Also activated by increased pH and levels of Mg2+
  • When there is light, reactions are moving forwards and levels of Fdred and NADPH are increased
    
    • Increased levels of Fdred allow it to reduce thioredoxin
  • Thioredoxin can then reduce FBPase and SBPase, activating them
  • In the dark, FBPase and SBPase will not be reduced and will therefore remain inactive

Question 32

Explain how photorespiration competes with photosynthesis and relate this to the RuBisCO mechanism and products

Answer 32

• Photosynthesis: make O₂, consume CO₂
• Photorespiration: consume O₂, make CO₂
  
  • Occurs under conditions of low CO₂ and high O₂
• RuBisCO has oxygenase activity→ causes O₂ to compete with CO₂ as a substrate
  
  • The oxygenase reaction of RuBisCO produces one 3PG and one 2-phosphoglycolate (instead of two 3PG)
  • The 2-phosphoglycolate leads to CO₂ production

Question 33

Explain why photorespiration is considered “bad” and what apparent advantage has been proposed for photorespiration

Answer 33

• Photorespiration is considered “bad” because it produced CO₂
  
  • Undoes the work of photosynthesis
  • ATP and NADPH is wasted
• Possible advantages:
  
  • May protect plants from oxidative damage when O₂ levels are high

Question 34

How do C₄ plants minimize photorespiration

Answer 34

• C₃ plants: produce 3-phosphoglycerate (3PG)
  
  • Photosynthesis occurs in the mesophyll cells
• C₄ plants: photosynthesis occurs in the bundle-sheath cells
  
  • C₄ cycle accelerates photosynthesis
  • Strategy: concentrates CO₂ to increase RuBP carboxylase’s activity
    
    • Do this by moving CO₂ from mesophyll cells to bundle-sheath cells