Cambridge O Level Biology · Syllabus 5090 · Cells
Chloroplast
What is Chloroplast?
A green organelle found in the photosynthetic cells of a plant. It contains the pigment chlorophyll, which absorbs light energy, and it is the site of photosynthesis. Chloroplasts are absent from plant cells that receive no light, such as root hair cells.
This definition is part of the Cells chapter in Cambridge O Level Biology.
Chloroplast in context
Animal, plant and bacterial cells all share a cell membrane, cytoplasm and ribosomes, but they differ in what else they contain. An animal cell also has a nucleus and mitochondria. A plant cell adds a cellulose cell wall, a permanent sap vacuole and, in its photosynthetic cells only, chloroplasts. A bacterial cell has a cell wall that is not made of cellulose, circular DNA lying free in the cytoplasm and sometimes plasmids, but no nucleus, no mitochondria and no chloroplasts. In every cell the membrane controls what enters and leaves; a cell wall only supports, because it is fully permeable.
Common mistakes with Chloroplast
- “All plant cells have chloroplasts.” Repair Only photosynthetic cells do. A root hair cell has none, because no light reaches it. An onion epidermal cell has none either.
Examiner tips on Chloroplast
- Read the “Plant” column carefully. Two of its entries are conditional, not absolute. Chloroplasts occur in photosynthetic cells, and the entry for the animal column is no large permanent vacuole rather than a flat “no vacuole”. Precision in these two cells of the table is worth real marks.
Questions students ask about Chloroplast
Do bacterial cells have DNA if they have no nucleus?
Yes. A bacterial cell has a circular molecule of DNA lying free in the cytoplasm, and sometimes small extra circles of DNA called plasmids. What it lacks is a nucleus to enclose that DNA, together with mitochondria and chloroplasts. It still has a cell membrane, cytoplasm, ribosomes and a cell wall, although that wall is not made of cellulose.
Why do only some plant cells have chloroplasts?
Chloroplasts contain chlorophyll and are the site of photosynthesis, so they are found only in plant cells that receive light, such as the palisade mesophyll cells packed just below the upper surface of a leaf. A root hair cell has none because no light reaches it underground, and an onion epidermal cell has none either. “All plant cells have chloroplasts” is a common error.