Photosynthesis Overview and Chloroplast Structure

Photosynthesis is the single most important metabolic process on Earth. It converts light energy into chemical energy stored in organic molecules, and releases the oxygen that almost every other organism depends on. OCR A-Level Biology A specification module 5.2.1(a)–(d) requires you to understand the role of photosynthesis in trapping energy, the overall equation, the structure of chloroplasts in relation to their function, and how that structure is adapted for the different stages of the reaction. This opening lesson sets the scene for everything that follows: if you cannot draw and label a chloroplast confidently, the light-dependent and light-independent stages will not make sense.

Key Definitions:

• Autotroph — an organism that synthesises its own complex organic molecules from simple inorganic molecules (e.g. CO₂, H₂O) using an external energy source.
• Photoautotroph — an autotroph that uses light as its energy source (e.g. plants, algae, cyanobacteria).
• Heterotroph — an organism that obtains complex organic molecules by consuming other organisms (e.g. animals, fungi, most bacteria).
• Photosynthesis — the process by which light energy is used to synthesise carbohydrates from carbon dioxide and water.
• Chloroplast — the double-membraned organelle in plant and algal cells where photosynthesis takes place.

---

Autotrophs vs Heterotrophs

All organisms on Earth fall into one of two broad nutritional categories based on how they obtain carbon and energy.

• Autotrophs fix inorganic carbon (CO₂) into organic molecules. Photoautotrophs use light energy; chemoautotrophs (e.g. nitrifying bacteria) use energy from inorganic chemical reactions.
• Heterotrophs cannot fix carbon and must consume ready-made organic molecules synthesised by autotrophs.

Photosynthesis is therefore the gateway process into the biosphere: virtually all the chemical energy used by heterotrophs originates from sunlight, captured by photoautotrophs and passed along food chains. An understanding of photosynthesis is essential for understanding ecology, energy transfer, climate change and crop productivity.

---

The Overall Equation

At GCSE you learned the summary equation for photosynthesis:

$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$6CO2​+6H2​Olight, chlorophyll​C6​H12​O6​+6O2​

At A-Level you need to realise that this single equation actually represents two separate stages, each with its own location, inputs and outputs:

1. Light-dependent stage — takes place on the thylakoid membranes. Splits water to release O₂, and produces the short-lived energy carriers ATP and reduced NADP.
2. Light-independent stage (Calvin cycle) — takes place in the stroma. Uses ATP and reduced NADP from the first stage to reduce CO₂ to carbohydrate.

Neither stage alone produces glucose directly. The Calvin cycle actually produces triose phosphate (TP), a three-carbon sugar that is later converted into glucose, starch, sucrose, amino acids, lipids and nucleotides.

---

Structure of the Chloroplast

Chloroplasts are typically 2–10 µm long, biconvex-shaped organelles found mostly in the palisade mesophyll cells of leaves. Each cell contains dozens of them, and they can move within the cytoplasm to optimise light capture.

flowchart TB
    subgraph CP[Chloroplast]
        OM[Outer membrane]
        IM[Inner membrane]
        IMS[Intermembrane space]
        ST[Stroma: site of Calvin cycle]
        subgraph G[Granum - stack of thylakoids]
            TH1[Thylakoid]
            TH2[Thylakoid]
            TH3[Thylakoid]
        end
        LM[Lamella: thylakoid connecting grana]
        SG[Starch grain]
        LD[Lipid droplet]
        DNA[Circular DNA]
        RB[70S ribosomes]
    end

Key Structures

Structure	Description	Function
Outer membrane	Smooth, permeable to small molecules and ions	Forms the boundary with the cytoplasm
Inner membrane	Less permeable; contains transport proteins	Controls movement of substances into/out of stroma
Intermembrane space	Thin gap between the two membranes	Small region of separation
Stroma	Fluid-filled matrix enclosed by inner membrane	Site of light-independent reactions (Calvin cycle); contains enzymes, DNA, ribosomes, starch grains, lipid droplets
Thylakoid	Flattened disc-like sac with its own membrane	Site of light-dependent reactions; membrane contains photosystems, ETC and ATP synthase
Granum (plural: grana)	Stack of thylakoids resembling a pile of coins	Maximises surface area for light absorption
Lamella (intergranal thylakoid)	Thin thylakoid membrane connecting adjacent grana	Maintains continuity of the thylakoid space
Starch grain	Insoluble carbohydrate store in the stroma	Stores excess photosynthetic product
Lipid droplet (plastoglobulus)	Lipid-rich inclusion	Reservoir for lipid-soluble molecules
Circular DNA and 70S ribosomes	Bacteria-like genetic machinery	Chloroplasts make some of their own proteins — evidence for endosymbiotic theory

---

How Chloroplast Structure Matches Function

The chloroplast is a textbook example of structure–function relationship. Every feature exists because it makes one or other stage of photosynthesis more efficient.

• Thylakoid membranes provide a vast surface area for the embedding of photosystems, electron carriers and ATP synthase. Without this surface area, very few photons could be captured.
• Grana stacks hold photosystems close together and maintain the thylakoid space as a sealed compartment, which is essential for the proton gradient that drives ATP synthesis (chemiosmosis).
• The thylakoid space is a small, enclosed compartment, which allows H⁺ ions pumped in during the light-dependent reaction to accumulate to high concentrations, producing a steep proton gradient across the thylakoid membrane.
• The stroma contains all the enzymes of the Calvin cycle in solution, including the most abundant enzyme on Earth: RuBisCO (ribulose bisphosphate carboxylase/oxygenase).
• The inner membrane is selectively permeable, controlling which molecules enter and leave the stroma.
• Proximity of thylakoids and stroma means that ATP and reduced NADP made in the thylakoids have only a very short distance to diffuse to reach the enzymes of the Calvin cycle.
• Starch grains act as a short-term store of fixed carbon, preventing a build-up of sugars that would disrupt osmotic balance.
• 70S ribosomes and circular DNA allow chloroplasts to make some of their own proteins, including parts of the photosystems — an important point for the endosymbiotic theory, which proposes that chloroplasts evolved from free-living cyanobacteria engulfed by a larger cell.

---

Comparing Chloroplasts with Mitochondria

Students often confuse chloroplast and mitochondrial structures. They share several features because both use chemiosmosis to make ATP — but they do the opposite jobs.

Feature	Chloroplast	Mitochondrion
Main function	Photosynthesis (builds glucose)	Respiration (breaks down glucose)
Number of membranes	2	2
Inner compartment	Stroma	Matrix
Folded membrane system	Thylakoids → grana	Cristae
Site of ETC	Thylakoid membrane	Inner mitochondrial membrane
Site of ATP synthase	Thylakoid membrane	Inner mitochondrial membrane
Proton gradient direction	H⁺ into thylakoid space	H⁺ into intermembrane space
Source of ATP	Photophosphorylation	Oxidative phosphorylation
DNA and ribosomes	Yes (70S)	Yes (70S)

---

Exam Tip

When asked to describe chloroplast adaptations, always tie the structure to a specific function. "Has a large surface area" is not enough — you must say "the thylakoid membranes provide a large surface area for the many photosystems and ATP synthase enzymes embedded in them, which maximises the rate of light absorption and ATP production." OCR specifically rewards answers that link structure to the light-dependent or light-independent stage explicitly.

---

Common Exam Mistakes

• Confusing thylakoid, granum and lamella. A thylakoid is a single flattened sac; a granum is a stack of thylakoids; a lamella (intergranal thylakoid) connects grana together.
• Saying glucose is the direct product of photosynthesis. The direct product is triose phosphate — glucose is assembled from two TP molecules later.
• Forgetting that chloroplasts have their own DNA. This is a common OCR extended-answer sting for the endosymbiotic theory.
• Confusing stroma (fluid of the chloroplast) with stomata (pores in the leaf epidermis). They are completely different and in different places.
• Claiming the inner and outer membranes of the chloroplast are the same. The outer is permeable and smooth; the inner is selectively permeable.
• Writing that oxygen comes from CO₂. Oxygen comes from the splitting of water in the light-dependent reaction, not from CO₂.

---

Quick Recap

• Autotrophs make their own organic molecules from inorganic carbon; photoautotrophs use light as the energy source.
• Photosynthesis occurs in chloroplasts and has two stages: light-dependent (thylakoids) and light-independent (stroma).
• The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Chloroplasts have a double membrane, a fluid stroma containing enzymes and starch grains, and internal thylakoid membranes stacked into grana and connected by lamellae.
• Thylakoids provide a large surface area for photosystems and ATP synthase; the thylakoid space is the site of proton accumulation.
• Stroma contains the Calvin cycle enzymes including RuBisCO.
• Chloroplasts contain 70S ribosomes and circular DNA, supporting the endosymbiotic theory.
• Chloroplast structure closely matches its function by compartmentalising the two stages and maximising surface area.

Reference: OCR A-Level Biology A (H420) specification 5.2.1(a)–(d).