Chloroplast
A chloroplast is a photosynthetic organelle in plants and algae. It captures light energy, uses it to make energy-rich molecules, and supports the carbon-fixing chemistry that helps build sugars.
What a chloroplast is
A chloroplast is a membrane-bound organelle that carries out photosynthesis in plants and algae. It is a kind of plastid, a broader family of plant and algal organelles involved in storage, pigmentation, and metabolism. Chloroplasts are the green plastids most closely associated with light capture and sugar production.
Structure inside a chloroplast
A chloroplast has an outer membrane and an inner membrane surrounding a fluid-filled stroma. Inside the stroma is a thylakoid membrane system, often arranged in stacks called grana. Chlorophyll and other photosynthetic pigments sit in the thylakoid membranes, where they absorb light and begin the energy-conversion work of photosynthesis.
Light reactions
The light-dependent reactions occur in the thylakoid membranes. Pigments absorb light, energized electrons move through protein complexes, and the chloroplast builds a proton gradient across the thylakoid membrane. ATP synthase uses that gradient to make ATP, while electron flow also helps produce NADPH.
Carbon fixation
The Calvin cycle takes place in the stroma. It uses ATP and NADPH from the light reactions to help convert carbon dioxide into carbohydrate building blocks. Those products can support sucrose, starch, cellulose, amino acids, and many other molecules that plants and food webs depend on.
Chloroplast DNA and proteins
Chloroplasts contain their own circular DNA and ribosomes, a clue to their evolutionary history. Even so, the chloroplast genome encodes only a fraction of the proteins the organelle needs. Many chloroplast proteins are encoded in the nucleus, made in the cytoplasm, and imported into the chloroplast through membrane transport systems.
Endosymbiotic origin
The leading explanation for chloroplast origins is endosymbiosis. An early eukaryotic cell formed a lasting partnership with a photosynthetic cyanobacterium-like cell. Over time, that partner became an organelle, transferred many genes to the host nucleus, and made oxygen-producing photosynthesis central to plants and algae.
Not just green color
Chloroplasts give many leaves their green color because chlorophyll reflects green wavelengths, but their importance goes far beyond color. They are chemical factories that connect sunlight, water, carbon dioxide, minerals, and plant growth. Other plastids can specialize for storage or pigments, and chloroplasts can change as tissues develop or age.
Why it matters
Chloroplasts matter because they help turn solar energy into biological matter. Their work supports crops, forests, algae, oxygen production, the carbon cycle, and most food webs on Earth. Understanding chloroplasts also helps explain plant productivity, climate feedbacks, crop breeding, and the deep evolutionary link between cells and microbes.