Watch sunlight become sugar in the chloroplast, then sugar become ATP in the mitochondrion — eight steps that link photosynthesis and cellular respiration into one cycle.
Both organelles are cut open so you can see inside. The mitochondrion (left or top) releases energy from food as ATP; the chloroplast (right or bottom) captures light energy and stores it in sugar. Click any part, or use the colored legend.
Cells cannot use sunlight or a sugar molecule directly to power their work. They use ATP, a small molecule that hands out energy in convenient amounts. Two organelles do most of the energy converting. The chloroplast (in plants and algae) captures light energy and stores it as chemical energy in sugar — photosynthesis. The mitochondrion (in almost every eukaryotic cell, including plant cells) releases the energy in sugar and turns it into ATP — aerobic cellular respiration. Neither creates energy; each one only changes its form.
A chloroplast has a two-membrane envelope around a fluid called the stroma, and inside that a third membrane system: flattened sacs called thylakoids, stacked into grana and joined by stroma lamellae. The light-dependent reactions happen on the thylakoid membranes: chlorophyll absorbs light, water is split, oxygen is released, and ATP and NADPH are made. The Calvin cycle happens in the stroma: it uses that ATP and NADPH to fix CO₂ into sugar.
A mitochondrion has a smooth outer membrane and a folded inner membrane; the folds are the cristae, and the space they enclose is the matrix. Glucose is first split into two pyruvate by glycolysis in the cytoplasm. Pyruvate then enters the matrix, where the Krebs cycle releases CO₂ and loads electrons onto NADH and FADH₂. Finally the electron transport chain on the cristae uses those electrons to pump H⁺ ions, and ATP synthase uses the H⁺ flowing back to make most of the ATP — about 30–32 per glucose in total. Oxygen is the final electron acceptor and becomes water.
Both organelles make ATP the same way. An electron transport chain in a membrane pumps H⁺ ions to one side, building up a concentration gradient, and the H⁺ can only flow back through ATP synthase, which spins like a turbine and makes ATP. In a mitochondrion H⁺ is pumped from the matrix into the intermembrane space; in a chloroplast it is pumped from the stroma into the thylakoid space. Peter Mitchell proposed this chemiosmotic idea in 1961 and won the 1978 Nobel Prize in Chemistry. The heavy folding of cristae and thylakoids exists to pack in more of this membrane.
According to the endosymbiotic theory, championed by Lynn Margulis in 1967, mitochondria descend from aerobic bacteria (related to today’s alphaproteobacteria) that were engulfed by an ancestral cell more than 1.5 billion years ago, and chloroplasts descend from engulfed cyanobacteria. The evidence: both have two membranes, their own circular DNA, bacteria-like 70S-type ribosomes, and they divide by splitting in two, much like bacteria; their genes also sit closest to those bacterial groups on the tree of life.
Press Follow the energy and watch the ATP readout: glycolysis gives 2, the Krebs cycle 2 more, and the jump to about 30–32 happens only when electron transport and ATP synthase on the cristae kick in. Notice the oxygen released in step 2 is the oxygen used up in step 7, and the CO₂ released in step 6 is fixed again in step 4. Switch to the Mitochondrion view and click a crista and an ATP synthase; then turn off Cut open to see the organelles whole.
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Found in | Almost all eukaryotic cells (animals, plants, fungi, protists) | Plants and algae only |
| Process | Aerobic cellular respiration (Krebs cycle, electron transport) | Photosynthesis (light reactions, Calvin cycle) |
| Energy change | Chemical energy in food → ATP | Light energy → chemical energy in sugar |
| Takes in | Pyruvate (from glucose) and O₂ | CO₂, H₂O and light |
| Gives out | CO₂, H₂O and ATP | Sugar and O₂ |
| Membranes | 2; inner one folded into cristae | 2 in the envelope + an inner thylakoid system |
| Fluid inside | Matrix | Stroma |
| Where ATP is made | ATP synthase in the inner membrane (cristae) | ATP synthase in the thylakoid membranes |
| H⁺ pumped into | Intermembrane space | Thylakoid space (lumen) |
| Electron carriers | NADH and FADH₂ | NADPH |
| Pigment | None | Chlorophyll (green), carotenoids |
| Own DNA & ribosomes | Yes — circular DNA, 70S-type ribosomes | Yes — circular DNA, 70S ribosomes |
| Ancestor | Aerobic bacterium (alphaproteobacterium) | Cyanobacterium |
| Part | What it is | Job |
|---|---|---|
| Outer membrane | Smooth membrane with porin channels | Boundary; lets small molecules in |
| Intermembrane space | Gap between the membranes | Holds the H⁺ build-up |
| Inner membrane & cristae | Folded, H⁺-tight membrane | Electron transport chain; huge surface |
| ATP synthase | Rotary enzyme in the inner membrane | Makes most of the cell’s ATP |
| Matrix | Dense fluid inside | Pyruvate breakdown and Krebs cycle |
| mtDNA & ribosomes | Circular DNA, small ribosomes | Make some of its own proteins |
| Part | What it is | Job |
|---|---|---|
| Envelope | Outer and inner membrane | Controls what enters and leaves |
| Thylakoids | Flattened membrane sacs with chlorophyll | Light reactions: split water, make ATP and NADPH |
| Grana | Stacks of thylakoids | Pack in a lot of light-harvesting membrane |
| Stroma lamellae | Thylakoids linking the grana | Connect the grana into one system |
| Stroma | Fluid around the thylakoids | Calvin cycle: fixes CO₂ into sugar |
| Starch grains | Stored carbohydrate | Store extra sugar for later |
| cpDNA & ribosomes | Circular DNA, 70S ribosomes | Make some of its own proteins |
Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (about 30–32 ATP)
The products of one are the reactants of the other, so the matter cycles between chloroplasts and mitochondria (and between plants and animals). Energy does not cycle: it flows in as light, is stored in sugar, is passed to ATP, and is finally lost as heat. Plant cells carry out both processes — in daylight their chloroplasts usually fix more CO₂ than their mitochondria release.
Chloroplasts carry out photosynthesis: they capture light energy and store it in sugar, taking in CO₂ and water and giving out oxygen. Mitochondria carry out aerobic respiration: they release the energy stored in sugar as ATP, taking in oxygen and giving out CO₂ and water. Chloroplasts are found only in plants and algae; mitochondria are in almost all eukaryotic cells, including plant cells.
Key takeaway: chloroplasts store energy in sugar; mitochondria release it as ATP.Yes. Plant cells have both chloroplasts and mitochondria. Chloroplasts make sugar, but the plant still needs mitochondria to turn that sugar into ATP — at night, in roots, in seeds and in every other part that cannot photosynthesise, and in green cells during the day as well.
Key takeaway: plants photosynthesise and respire; they have both organelles.Mitochondria are often called the powerhouse of the cell because they make most of its ATP. Pyruvate from glucose is broken down in the matrix by the Krebs cycle, and the electron transport chain and ATP synthase on the inner membrane use the energy of its electrons to make about 30–32 ATP per glucose, using oxygen and releasing CO₂ and water. They also help control cell death, calcium levels and heat production.
Key takeaway: mitochondria convert the energy in food into ATP using oxygen.Chloroplasts carry out photosynthesis. In the thylakoid membranes, chlorophyll absorbs light, water is split and oxygen released, and ATP and NADPH are made. In the stroma, the Calvin cycle uses that ATP and NADPH to fix CO₂ into sugar, which the plant uses for energy and to build cellulose, starch and other molecules.
Key takeaway: thylakoids capture light energy; the stroma uses it to make sugar.Thylakoids are flattened membrane sacs inside a chloroplast that hold chlorophyll and carry out the light-dependent reactions. A granum (plural grana) is a stack of thylakoids, like a pile of coins, and the stacks are joined by single thylakoids called stroma lamellae. The stroma is the fluid around them, where the Calvin cycle makes sugar.
Key takeaway: thylakoids stacked into grana, floating in the stroma.The electron transport chain and ATP synthase sit in the inner membrane, so the more membrane there is, the more ATP can be made. Folding it into cristae packs a large surface into a small organelle — cells with high energy needs, such as heart muscle, have mitochondria with especially dense cristae.
Key takeaway: more folds = more membrane = more ATP.The endosymbiotic theory says mitochondria and chloroplasts were once free-living bacteria that were engulfed by an ancestral cell and became permanent partners. Mitochondria came from aerobic bacteria related to alphaproteobacteria, and chloroplasts from cyanobacteria. Evidence: both have two membranes, their own circular DNA, bacteria-like 70S-type ribosomes, they divide by binary fission, and their DNA is most similar to those bacteria.
Key takeaway: double membranes, own circular DNA and ribosomes, and division by fission point to bacterial ancestors.They are near-opposites. Photosynthesis uses CO₂, water and light to make glucose and oxygen; aerobic respiration uses glucose and oxygen to make CO₂, water and ATP. The products of one are the raw materials of the other, so carbon and oxygen cycle between chloroplasts and mitochondria — but energy flows one way, from sunlight to ATP to heat.
Key takeaway: matter cycles between them; energy flows through them.