state that within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light-dependent stage and the stroma is the site of the light-independent stage

Photosynthesis – An Energy Transfer Process

Learning Objective

State that within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light‑dependent stage and the stroma is the site of the light‑independent stage.

1. Overview of Photosynthesis

Photosynthesis converts light energy into chemical energy stored in carbohydrate molecules. The overall simplified equation is:

\$\$

6\,\text{CO}2 + 6\,\text{H}2\text{O} \xrightarrow{\text{light}} \text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2

\$\$

2. Chloroplast Structure Relevant to Energy Transfer

• Outer membrane – permeable to small molecules.
• Inner membrane – encloses the stroma.
• Stroma – fluid matrix surrounding the thylakoid system; site of the light‑independent (Calvin‑Benson) reactions.
• Thylakoid system

  • Thylakoid membranes contain photosynthetic pigments (chlorophyll a, b) and the protein complexes of the electron transport chain.
  • Thylakoid spaces (lumen) become acidic during the light‑dependent reactions.
  • Stacks of thylakoids are called grana; inter‑granal thylakoids are termed lamellae.

3. Light‑Dependent Reactions (Thylakoid Membranes & Grana)

These reactions occur in the thylakoid membranes of the grana and require light to drive electron flow.

• Absorption of photons by chlorophyll excites electrons.
• Water is split (photolysis) producing \$O_2\$, \$H^+\$ and electrons:

  \$\text{2 H}2\text{O} \rightarrow 4\,\text{H}^+ + 4\,e^- + \text{O}2\$

• Electrons travel through Photosystem II → plastoquinone → cytochrome b₆f → plastocyanin → Photosystem I.
• ATP is synthesised by chemiosmosis as \$H^+\$ flow back through ATP synthase.
• NADP\$^+\$ is reduced to NADPH.

4. Light‑Independent Reactions (Calvin‑Benson Cycle – Stroma)

The stroma hosts the Calvin‑Benson cycle, which fixes carbon dioxide into carbohydrate without direct light input.

• Carbon fixation: \$3\,\text{CO}_2 + 3\,\text{RuBP} \rightarrow 6\,\text{3‑PGA}\$.
• Reduction phase uses ATP and NADPH from the light‑dependent reactions:

  \$6\,\text{3‑PGA} + 6\,\text{ATP} + 6\,\text{NADPH} \rightarrow 6\,\text{G3P} + 6\,\text{ADP} + 6\,\text{P}_i + 6\,\text{NADP}^+\$

• Regeneration of RuBP allows the cycle to continue.
• Two G3P molecules exit the cycle to form one molecule of glucose (\$\text{C}6\text{H}{12}\text{O}_6\$).

5. Summary Table – Location, Inputs & Outputs

6. Key Points to Remember

1. The thylakoid membranes of the grana are the only site where light energy is captured and converted into chemical energy (ATP, NADPH).
2. The stroma contains the enzymes of the Calvin‑Benson cycle, using the ATP and NADPH produced in the thylakoids to fix CO₂ into carbohydrate.
3. Energy transfer in photosynthesis proceeds from photons → excited electrons → proton gradient → ATP/NADPH → carbon fixation.