Chloroplasts: structure and function

What is a chloroplast?

A chloroplast is an organelle found in plant cells and algae, where photosynthesis happens. It contains the green pigment chlorophyll, which absorbs light energy, which is why leaves are green. In a leaf, most chloroplasts are in the mesophyll cells, according to OpenStax Biology. Animal cells, fungi and bacteria don't have chloroplasts, and neither do plant root cells, which get no light.

Try it: this question asks which kinds of cell have chloroplasts and other features.

Chloroplast structure

Seen in an electron micrograph, a chloroplast is a disc or lens shape with stacks of membranes inside. These are the parts to label on a chloroplast diagram, as described in OpenStax Biology:

How to draw a chloroplast diagram

1. Draw a long oval for the chloroplast, then a second line just inside it: the two lines are the outer and inner membranes of the envelope.
2. Inside, draw several stacks of thin flat discs, like piles of coins seen from the side. Each stack is a granum, and each disc is a thylakoid.
3. Join neighbouring stacks with single thin lines: the lamellae.
4. Add a few dark ovals in the space between for starch grains, and a small loop for the DNA and dots for ribosomes.
5. Label the fluid between everything as the stroma, using ruled label lines that don't cross.

Explore a labelled plant cell, chloroplasts included, in our Cell Simulator.

What is the function of a chloroplast?

Its job is photosynthesis: turning carbon dioxide and water into glucose, powered by light, with oxygen given off as a by-product.

Photosynthesis happens in two stages, each in a different part of the chloroplast. OpenStax states that the light-dependent reactions take place in the thylakoid membrane and the Calvin cycle takes place in the stroma.

At A-level, the AQA specification adds the detail: light causes photoionisation of chlorophyll, photolysis of water produces protons, electrons and oxygen, and in the Calvin cycle carbon dioxide reacts with ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco, to form glycerate 3-phosphate (GP). Try one: this question asks you to build the photosynthesis equation.

How chloroplast structure is adapted to its function

"Explain how the structure of the chloroplast is adapted" is a favourite exam question. Link each feature to what it does:

• Many thylakoids stacked into grana give a large membrane surface area for chlorophyll, so more light is absorbed, and room for the proteins and ATP synthase of the light-dependent reactions.
• The stroma contains the enzymes of the Calvin cycle, such as rubisco, right next to the thylakoids that supply ATP and reduced NADP.
• The double membrane keeps the reactions and their enzymes in one compartment, separate from the rest of the cell.
• Its own DNA and ribosomes let it make some of its own proteins, such as enzymes for photosynthesis.
• Starch grains store the sugar made, as insoluble starch that doesn't affect the cell's water balance.

Worked example: the real size of a chloroplast

In an electron micrograph at a magnification of ×6,000, a chloroplast measures 30 mm long. Real size = image size ÷ magnification = 30 ÷ 6,000 = 0.005 mm. Converting to micrometres (1 mm = 1,000 µm) gives 5 µm. Our Magnification Calculator handles the units for you.

Chloroplasts vs mitochondria

The similarities aren't a coincidence. Both are thought to come from bacteria that were engulfed by early cells and lived on inside them, an idea called endosymbiosis. OpenStax describes chloroplasts as descended from photosynthetic bacteria (cyanobacteria), which is why they have their own DNA and ribosomes. Plant cells contain both: chloroplasts make sugar, and mitochondria release its energy. The Cell Biology course covers both organelles.

Common mistakes

• Saying all plant cells have chloroplasts. Root cells don't, because they get no light.
• Mixing up grana and thylakoids. A thylakoid is one sac; a granum is a stack of them.
• Putting the Calvin cycle in the thylakoids. It happens in the stroma.
• Saying chloroplasts make energy. Energy can't be made; chloroplasts transfer light energy into chemical energy in glucose.
• Saying plants don't respire. Plant cells have mitochondria and respire all the time, as well as photosynthesising in the light.
• Forgetting units in magnification sums. Convert image and real size to the same unit.

Read next

Biology · CellsMitochondria diagram: structure and function, labelledA labelled mitochondria diagram, the structure of a mitochondrion, what each part does in aerobic respiration, and how to draw and label one in the exam.