Photosynthesis

Question 1

Connect redox reactions to energy transformations that occur throughout photosynthesis

Answer 1

Process: Water splitting (PSII)
Redox event: H2O oxidized to O2
Energy transformation: Light -> ATP

Process: NADPH Formation (PSI)
Redox event: NADP+ reduced -> NADPH
Energy transformation: Light -> Stored reducing power

Process: Calvin cycle
Redox event: CO2 reduced -> Glucose
Energy transformation: NADPH/ATP -> Sugar

Question 2

What are the two stages of photosynthesis? And where do they take place?

Answer 2

1. Light-dependent reactions (in thylakoid membranes)
2. Calvin cycle (in stroma)

Question 3

How does the Calvin Cycle transform carbon dioxide into sugar, and where did the energy for this transformation come from?

Answer 3

1. Carbon Fixation (“Catching CO₂”)
   What happens: CO₂ gets attached to a 5-carbon molecule (RuBP).

Result: Makes two 3-carbon pieces (3-PGA).

Helper enzyme: Rubisco (the most abundant enzyme on Earth!).

2. Reduction (“Adding Energy”)
   What happens: The 3-carbon pieces get energy from ATP and electrons from NADPH.

Result: They turn into a sugar (G3P), which can build glucose.

3. Regeneration (“Recycling the Helper”)
   What happens: Most G3P gets recycled back into RuBP (using more ATP).

Why? So the cycle can keep running!

Key Concept: The cycle uses chemical energy (ATP/NADPH) from light reactions to turn CO₂ into sugar.

Question 4

Describe the role of Rubisco in the process of photosynthesis

Answer 4

Rubisco is the enzyme that grabs CO₂ from the air and attaches it to a 5-carbon sugar (RuBP) to start making sugar in the Calvin Cycle.

Question 5

How does light energy reach photosystem reaction centers, and what happens when it gets there?

Answer 5

Light is absorbed by pigment molecules (antenna pigments) and then transferred to a chlorophyll molecule in the reaction center

At the reaction center: 1. Light excites an electron → jumps to the electron acceptor.

2. Starts a chain reaction:

PSII: Splits H₂O → makes ATP + O₂.

PSI: Makes NADPH.

3. ATP + NADPH power sugar-making (Calvin Cycle).

Energy Conversion:
Light → excited e⁻ → chemical energy (ATP + NADPH)

Question 6

How would introducing changes in one stage of photosynthesis affect the production of NADPH, ATP, and sugar?

Answer 6

1. Light-Dependent Reactions Disrupted

Reduced light/water:
→ Less NADPH/ATP made
→ Calvin Cycle slows → less sugar

ETC blocked:
→ No proton gradient → no ATP
→ NADPH production stops

2. Calvin Cycle Disrupted

Rubisco inhibited:
→ CO₂ not fixed → sugar production halts
→ NADPH/ATP accumulate (no outlet)

ATP/NADPH shortage:
→ 3-PGA can’t be reduced → no G3P/sugar

Key Concept:
Light reactions supply energy (ATP/NADPH); Calvin Cycle uses it. Disrupting either stage creates bottlenecks.

Question 7

What can a plant cell do with the products of photosynthesis once they are produced?

Answer 7

1. G3P →
   * Sugar: Glucose → starch (storage) or sucrose (transport)
   * Structures: Cellulose (walls), lipids, amino acids
2. ATP/NADPH →
   * Reused in Calvin Cycle
   * Powers other cell processes
3. O₂ →
   * Released via stomata
   * Used in respiration

Question 8

How is the process of photosynthesis related to the process of cellular respiration?

Answer 8

• Photosynthesis is the process where plants create glucose and oxygen out of sunlight, carbon dioxide, and water.
• Cellular respiration is the process that breaks down glucose into usable energy for the cell.