The chloroplast, the site of photosynthesis

From the leaf to the chloroplast

Hold a leaf up to the light. It is thin and flat. A flat shape catches a lot of light, and the thin body lets carbon dioxide reach every cell. Inside the leaf, the layers under the epidermis are the mesophyll. Its cells hold the green chloroplasts.

A chloroplast is a small green body inside a cell. A good microscope shows it as a green oval. Let us look inside.

The parts of a chloroplast

• A chloroplast has a double membrane, the outer and the inner membrane.
• The space inside is filled with a colourless fluid, the stroma. The stroma holds the enzymes that build sugar.
• In the stroma lies a system of flat, membrane-bound sacs, the thylakoids.
• Thylakoids are piled up like a stack of coins. Each stack is a granum (plural grana).
• Flat membranes called stroma lamellae join the thylakoids of one granum to those of the next.
• The space inside a thylakoid is the lumen.
• The pigments, including chlorophyll, sit in the thylakoid membranes.

Who does which job?

A chloroplast has two sets of jobs, and each has its own place.

The first set is the light reactions, also called the photochemical phase. The second set is the biosynthetic phase, which you may also hear called the dark reactions. The old name does not mean that it works only in the dark. It means that these steps do not take in light themselves. They use the ATP and NADPH that the light reactions make. The two phases run side by side in a lit leaf.

Why the stacks help

A thylakoid is a thin disc. Putting many discs into a stack packs a large amount of membrane into a very small space. More membrane means room for more pigment and more of the enzymes that capture light. The chloroplast is only a few micrometres across, but its thylakoid membranes together are many times larger than its outer surface.

You should find that $40$ grana of $20$ thylakoids give $800$ thylakoids. Each disc of diameter $0.4$ micrometres has a membrane area of about a quarter of a square micrometre, counting both faces. The chloroplast holds about $200$ square micrometres of membrane.

Try it in Python

The program says in which part of the chloroplast each job happens, and which phase it belongs to.

You should see that trapping light, making ATP and NADPH, and holding chlorophyll belong to the thylakoid membrane and the light reactions. Fixing carbon dioxide and making sugar belong to the stroma and the biosynthetic phase. Water splitting also belongs to the thylakoid membrane, so the program calls it a light reaction.

Practise with new numbers

Common mistakes

• Putting chlorophyll in the stroma. Chlorophyll is in the thylakoid membranes.
• Putting the sugar-making enzymes in the grana. They are in the stroma.
• Calling the stroma the lumen, or the lumen the stroma. The stroma is the fluid around the grana. The lumen is the space inside a thylakoid.
• Thinking that dark reactions need darkness. They need only the ATP and NADPH from the light reactions, so in a leaf they run while it is lit.
• Confusing a granum with a thylakoid. A granum is a stack. A thylakoid is one disc of the stack.

Remember these

• The chloroplast has two membranes, the stroma, and thylakoids stacked as grana, joined by stroma lamellae.
• Chlorophyll and the light-trapping system are in the thylakoid membranes.
• The light reactions make ATP and NADPH in the thylakoids. The sugar-making enzymes work in the stroma.
• Chloroplasts are found mainly in the mesophyll cells of the leaf.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where do you find chloroplasts in a leaf?

2. Name the two membranes of a chloroplast and the fluid inside them.

3. What is the difference between a granum and a stroma lamella?

4. In which part of the chloroplast is carbon dioxide turned into sugar?

5. Why do the dark reactions stop soon after the light is switched off?