Cell Biology
Chloroplast
Site of photosynthesis — green organelle from ancient cyanobacterial ancestor
Chloroplasts are organelles in plant and algal cells where photosynthesis occurs. Bounded by double membrane. Internal: thylakoids (flat sacs containing chlorophyll; site of light reactions); grana (stacks of thylakoids); stroma (fluid surrounding thylakoids; site of Calvin cycle). Origin: endosymbiotic — derived from cyanobacterium engulfed by eukaryotic cell ~1.5 Gyr ago. Have own DNA (smaller than original; many genes transferred to nucleus). Reproduce by division. Number per cell: 30-100 typical. Size: ~5-10 µm. Most green color of plants from chlorophyll in chloroplasts.
- FunctionPhotosynthesis (light + dark reactions)
- StructureDouble membrane; thylakoids; grana; stroma
- OriginEndosymbiotic (cyanobacterium ancestor)
- Number per cell30-100 typical
- Size5-10 µm
- DNAOwn genome (~120 genes); reduced from ancestor
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Why chloroplasts matter
- Photosynthesis. Site of energy capture.
- Plant biology. Defining feature of plants.
- Endosymbiosis. Major evolutionary event.
- Crop science. Improving photosynthesis efficiency.
- Evolution. Plant phylogeny via chloroplast DNA.
- Synthetic biology. Engineering chloroplasts.
- Climate. Major CO₂ sink.
Common misconceptions
- Chloroplasts only in green parts. Some non-green plant parts have them too.
- Chloroplasts only photosynthesize. Also: amino acid synthesis, fatty acid synthesis, isoprenoid biosynthesis.
- Chloroplasts permanent. Can develop into other plastids.
- Chloroplast DNA same as nuclear. Different (circular, fewer genes).
- One chloroplast per cell. 30-100 typical.
- Chloroplasts always green. Other pigments give different colors (autumn leaves).
Frequently asked questions
What's the structure of a chloroplast?
Outer membrane: porous, allows small molecules. Inner membrane: selective; encloses stroma. Stroma: fluid; site of Calvin cycle; contains chloroplast DNA, ribosomes, enzymes. Thylakoids: flat membrane sacs in stroma; site of light reactions; contain chlorophyll. Grana: stacks of thylakoids (interconnected). Lumen: space inside thylakoid.
How are chloroplasts inherited?
Mostly maternal in flowering plants. Pollen contributes little to no plastids; ovule's plastids passed to embryo. Result: chloroplast DNA inherited from mother. Some species inherit paternally or biparentally. Useful for tracing maternal lineages, plant evolution. Different from animals (mitochondrial DNA also maternal).
How many genes are in chloroplast DNA?
~120 genes typically. Encodes mostly: photosynthesis machinery (some ETC components), ribosome components, tRNAs. Most chloroplast proteins encoded in nuclear DNA, made in cytoplasm, imported. Reduction from cyanobacterial ancestor (~3000 genes) — most genes transferred to nucleus over 1.5 Gyr. Endosymbiotic gene transfer common process.
What's the endosymbiotic origin?
~1.5 Gyr ago, eukaryotic cell engulfed cyanobacterium. Instead of digestion: established symbiosis. Cyanobacterium provided photosynthesis; host provided protection. Over time: integrated; became chloroplast. Evidence: double membrane, own DNA (circular like bacteria), bacterial-like ribosomes, divides by binary fission. Mitochondria: similar story (different bacterial ancestor). Lynn Margulis (1967) championed.
Do all photosynthetic eukaryotes have chloroplasts?
All have plastids (chloroplasts are one type). Plants and green algae: chloroplasts (from primary endosymbiosis). Other algae (diatoms, dinoflagellates): plastids from secondary endosymbiosis (engulfing photosynthetic eukaryote). Some dinoflagellates: tertiary. Result: complex evolutionary history of plastid acquisition.
How are proteins imported?
Most chloroplast proteins encoded in nuclear DNA. Synthesized in cytoplasm; transported to chloroplast. Transit peptide (signal at N-terminus) directs to outer membrane; TOC complex translocates; TIC complex on inner membrane completes import. Transit peptide cleaved; protein folds. Similar systems for thylakoid, stroma destinations.
How does chloroplast division work?
Binary fission (like bacteria). FtsZ protein (homolog of bacterial cell division protein) forms ring; constricts. Plus eukaryotic-derived dynamin-related protein (DRP) outside. Together: divide chloroplast into two. Coordinated with cell cycle: chloroplasts divided when cell divides. Each daughter cell gets approximately equal chloroplasts.