What Are Chloroplasts?
Chloroplasts are organelles found in the cells of plants and algae. They are the sites of photosynthesis — the process by which plants capture sunlight and use it to make their own food from carbon dioxide and water.
Chloroplasts give leaves and stems their green colour because they contain a green pigment called chlorophyll. Cells in roots rarely have chloroplasts because they grow underground away from light.
Structure of a Chloroplast
Like mitochondria, chloroplasts have a double membrane and their own DNA. Their internal structure is specialised for capturing light:
Outer membrane — A smooth boundary layer surrounding the whole organelle.
Inner membrane — Encloses the fluid-filled interior.
Stroma — The fluid inside the inner membrane. It contains enzymes used in the second stage of photosynthesis (the Calvin cycle), as well as the chloroplast's own DNA and ribosomes.
Thylakoids — Flattened, disc-like membrane sacs stacked in piles called grana (singular: granum). Thylakoid membranes contain chlorophyll and other pigments that absorb light. The light-dependent reactions of photosynthesis happen here.
Chloroplasts contain their own DNA and can replicate (make copies of themselves). Like mitochondria, they are believed to have evolved from free-living photosynthetic bacteria that were engulfed by early eukaryotic cells — an idea called the endosymbiotic theory.
How Chloroplasts Perform Photosynthesis
Photosynthesis takes place in two main stages:
Stage 1 — Light-dependent reactions (in the thylakoid membranes): Chlorophyll absorbs sunlight and uses the energy to split water molecules. This releases oxygen as a by-product and produces energy-carrying molecules (ATP and NADPH).
Stage 2 — Light-independent reactions / Calvin cycle (in the stroma): The energy from Stage 1 is used to convert carbon dioxide from the air into glucose. The plant then uses this glucose for energy, growth, and repair.
In simple terms: Carbon dioxide + Water + Light energy → Glucose + Oxygen
Chloroplasts and Mitochondria: Partners in the Cell
Chloroplasts and mitochondria work as a team. Chloroplasts make glucose using sunlight; mitochondria break down that glucose to release energy as ATP. The oxygen released by chloroplasts is the same oxygen that mitochondria use in respiration. Plants use both organelles simultaneously — they photosynthesise and respire at the same time.
During the day, photosynthesis produces more oxygen than respiration consumes — so plants release oxygen into the air. At night, when there is no light for photosynthesis, plants only respire and take in oxygen like animals do.
Why Chloroplasts Matter for Life on Earth
Without chloroplasts, there would be no photosynthesis. Without photosynthesis, there would be no oxygen in the atmosphere and no food for almost all other organisms. Chloroplasts are, quite literally, the engines that power life on Earth.
Real world: Photosynthesis in houseplants
When you put a houseplant near a sunny window, the plant's leaf cells contain chloroplasts that capture sunlight and convert it into glucose. If you place the same plant in a dark corner, the leaves eventually yellow as the chlorophyll breaks down — the plant cannot photosynthesise and begins to starve. This is why plants need light to grow.
Real world: Why leaves change colour in autumn
In autumn, trees stop producing chlorophyll. As the green chlorophyll breaks down, the yellow and orange pigments that were always present in the chloroplasts become visible — giving us the famous autumn colours. This is chloroplast chemistry visible to the naked eye every year.
Real world: Oxygen from plants
Chloroplasts in aquatic plants like pondweed release oxygen as a by-product of photosynthesis. You can see tiny oxygen bubbles forming on the leaves. This is the same oxygen that fish and other aquatic animals breathe and the same process that produces most of Earth's atmospheric oxygen.
Where do you see this in real life? Observe a sunny garden: the bright green colour of leaves is chlorophyll inside chloroplasts. Put a leaf in a glass of water in strong sunlight and you may see tiny oxygen bubbles forming on it — proof of photosynthesis happening right in front of you.
Practice Problems
Where exactly inside a chloroplast do the light-dependent reactions take place?
Show answer
Why do root cells not contain chloroplasts?