Photosynthesis depends on chloroplast compartmentalisation. Understanding the stroma, thylakoids, and grana clarifies how membranes and internal spaces organise enzymes, pigments, and reactants so different stages occur in distinct locations.

Big Picture: Chloroplast Compartments

Labeled diagram of chloroplast internal architecture, highlighting how the envelope encloses the stroma and the thylakoid system. The figure explicitly identifies grana (stacks of thylakoids) and the thylakoid lumen, reinforcing the idea that membranes create distinct chemical microenvironments for different stages of photosynthesis. Source

Chloroplasts are membrane-bound organelles in plants and many algae. Their internal architecture creates specialised reaction spaces that support efficient energy capture and carbon fixation.

• The light reactions occur on internal membranes (thylakoids).

• The Calvin cycle reactions occur in the fluid interior (stroma).

• Physical separation helps maintain different local conditions (especially solute concentrations) in each compartment.

Structural Overview of a Chloroplast

Chloroplast Envelope

The chloroplast is bounded by a double membrane called the chloroplast envelope.

• Outer membrane: relatively permeable to small molecules and ions.

• Inner membrane: more selective; contains transport proteins controlling exchange with the cytosol.

• Intermembrane space: narrow region between the two membranes.

This envelope isolates chloroplast chemistry while still allowing regulated import of substrates and export of products.

Stroma: The Internal Fluid Matrix

The stroma is the aqueous interior enclosed by the inner envelope membrane and surrounding the thylakoid system.

Stromal contents support carbohydrate-building reactions and general organelle function.

• Contains many enzymes needed for carbon-fixation steps (Calvin cycle reactions).

• Houses chloroplast DNA and ribosomes, allowing synthesis of some chloroplast-encoded proteins.

• Provides a medium where substrates can diffuse between enzyme active sites and where metabolic intermediates can be channelled efficiently.

Thylakoids: Internal Membrane Sacs

Within the stroma is a third membrane system: the thylakoid membrane, arranged as flattened sacs.

The thylakoid system creates two distinct regions:

• Thylakoid membrane: a protein-rich surface that embeds pigment–protein complexes and other membrane components required for capturing light energy.

• Thylakoid lumen: the internal aqueous space of each thylakoid, physically separated from the stroma by the thylakoid membrane.

This membrane partitioning is essential because membranes can maintain different solute concentrations on each side, enabling specialised microenvironments.

Schematic of the thylakoid membrane showing the major protein complexes that drive the light reactions and establish a proton gradient across the membrane. By explicitly separating the stroma side from the lumen side, the diagram helps connect “compartmentalisation” to ATP production via chemiosmosis during the light reactions. Source

Grana: Stacks of Thylakoids

What a Granum Is

A granum (plural grana) is a stack of thylakoid discs, like a pile of coins. Grana increase the amount of thylakoid membrane that fits into a chloroplast.

Key structural consequences:

• High membrane surface area in a compact volume, allowing many membrane-associated components to be housed.

• Short diffusion distances within and between membrane regions due to tight packing.

• Organisation of thylakoids into stacked and unstacked regions, creating distinct membrane domains.

Connections Between Stacks

Grana are not isolated piles; they are connected by unstacked thylakoid membrane regions often called stroma lamellae (intergranal lamellae). These connections form a continuous internal membrane network.

• Supports distribution of lipids and proteins across the thylakoid system.

• Provides physical continuity for movement of small molecules within the chloroplast’s internal spaces.

Functional Separation: Where Each Stage Happens

The syllabus emphasis is that chloroplasts contain stroma and thylakoids organized into grana, separating light reactions from Calvin cycle reactions.

Overview diagram showing photosynthesis split into two stages: light-dependent reactions associated with thylakoid membranes and the Calvin cycle occurring in the stroma. The figure emphasizes the flow of ATP and NADPH from the thylakoids to the stroma, illustrating how chloroplast compartments coordinate energy capture with carbon fixation. Source

Structurally, this separation is achieved as follows:

• Light reactions: occur on the thylakoid membranes (especially abundant in grana), where pigment-containing complexes are embedded.

• Calvin cycle reactions: occur in the stroma, where soluble enzymes and substrates are located.

This arrangement supports efficiency by:

• Keeping membrane-based processes on thylakoids and solution-based enzyme reactions in the stroma.

• Allowing thylakoid membranes to establish distinct conditions between lumen and stroma, while stromal enzymes operate in a separate, relatively stable aqueous environment.

• Enabling coordinated exchange of molecules between compartments through controlled proximity (thylakoids are immersed in the stroma).