Photosynthesis as an Energy Transfer Process

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe the relationship between the structure of chloroplasts and their function
  2. Explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the Calvin cycle to produce complex organic molecules
  3. State that thylakoids (membranes and spaces) in stacks called grana are the site of the light-dependent stage, and the stroma is the site of the light-independent stage
  4. Describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption
  5. Interpret absorption spectra and action spectra for photosynthesis
  6. Describe and use chromatography to separate and identify chloroplast pigments using Rf values
  7. State that cyclic and non-cyclic photophosphorylation occur during the light-dependent stage
  8. Explain cyclic photophosphorylation
  9. Explain non-cyclic photophosphorylation
  10. Explain the mechanism of photophosphorylation (electron transport chain, proton pumping, ATP synthase)
  11. Outline the three main stages of the Calvin cycle
  12. State that Calvin cycle intermediates are used to produce other molecules

1. Chloroplast Structure and Function

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What is a chloroplast?

A chloroplast is the organelle (small structure inside a cell) where photosynthesis takes place in plant cells. Chloroplasts are typically 3-10 micrometers in diameter and have a disc-like shape.

The envelope

Each chloroplast is surrounded by a double membrane called the envelope. This consists of:

  • An outer membrane (smooth and permeable to small molecules)
  • An inner membrane (less permeable and more selective)
  • A small space between them called the intermembrane space

The stroma

Inside the envelope is a fluid-filled region called the stroma. Think of this as the "background liquid" that fills the chloroplast. The stroma contains:

  • Enzymes needed for the light-independent reactions
  • A loop of DNA (similar to bacterial DNA)
  • 70S ribosomes (small structures that make proteins)
  • Starch grains (where glucose is stored as starch)

The thylakoid system

Floating within the stroma is a separate system of membranes arranged as flattened sacs called thylakoids. These thylakoids stack up like pancakes to form structures called grana (singular: granum).

The thylakoids have:

  • Thylakoid membranes - where the light-dependent reactions occur
  • Thylakoid spaces (also called thylakoid lumen) - the hollow interior of each thylakoid

Adjacent stacks of grana are connected by unstacked membrane sections called intergranal lamellae (or stroma lamellae).

Structure-function relationships

StructureHow it helps photosynthesis
Double membrane envelopeControls what enters and leaves the chloroplast; outer membrane lets through O₂, CO₂, ATP, and ADP
Large surface area of thylakoid membranesProvides space for many pigment molecules, photosystems, and electron transport chains - increases the number of light-dependent reactions
Fluid-filled stromaContains enzymes for the Calvin cycle; provides a medium for the reactions to occur
Stacks of thylakoids (grana)Increase surface area for maximum light absorption
Thylakoid spaceAllows accumulation of protons (H⁺ ions) to create a concentration gradient used to make ATP
Small stalked particles on thylakoid membranesContain ATP synthase enzyme, which makes ATP

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