# Mitochondria and Chloroplasts > How cells make ATP and capture sunlight, and why these two organelles look like bacteria that moved in. Level: High school | Reading time: 13 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/energy-organelles **Quick answer:** Mitochondria release usable energy by breaking down food molecules to make ATP, and chloroplasts capture sunlight to build sugar by photosynthesis. Both have double membranes, their own DNA and ribosomes, and divide on their own, which supports the endosymbiotic theory that they descend from engulfed bacteria. ## What you will learn - Explain the role of ATP in the cell. - Describe the structure of a mitochondrion and what it produces. - Describe the structure of a chloroplast and what it produces. - List the evidence for the endosymbiotic origin of both organelles. ## Key takeaways - ATP is the cell's energy currency: cells spend it to power work and rebuild it from food or light. - A mitochondrion's inner membrane is folded into cristae, which increases the surface available for making ATP. - Chloroplasts stack thylakoids into grana and turn light energy into chemical energy stored in sugar. - Evidence for endosymbiosis: their own circular DNA, bacteria-like ribosomes, double membranes and division by splitting. ## ATP: the energy currency Cells constantly need energy to move molecules, build proteins, and contract muscles. They pay for most of this with a single molecule, ATP. When a phosphate group is cut off from ATP, a small packet of energy is released and used to drive a reaction. The cell then rebuilds ATP from the leftover pieces. A working human cell recycles its whole stock of ATP in a minute or so, and an adult turns over an amount of ATP close to their own body weight in a day. Two organelles are responsible for most of the ATP supply in eukaryotic cells: mitochondria, which release energy from food, and chloroplasts, which capture it from sunlight. ## Mitochondria: powerhouses with a double wall A mitochondrion (plural: mitochondria) is a bean-shaped organelle 1 to 10 µm long, with two membranes. The smooth outer membrane encloses the organelle. The inner membrane is folded into shelves called **cristae**, which multiply the surface area on which ATP is made. The space inside is the **matrix**. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/mitochondrion) - A mitochondrion in an animal cell. Mitochondria carry out most of cellular respiration. In summary: > glucose + oxygen → carbon dioxide + water + ATP First, glucose is split into smaller pieces in the cytoplasm. These pieces are then broken down completely inside the mitochondrion, and the energy released is used to pump protons across the inner membrane. The protons flow back through a molecular turbine called ATP synthase, and as they do, the turbine's rotation joins phosphate onto ADP to make ATP. One glucose molecule can yield about 30 ATP. Cells with high energy demands have many mitochondria. A muscle fiber can hold thousands, packed between its contractile filaments. Endurance training increases their number. Interactive 3D model: [Muscle Cell](https://cellnaut.com/studio/muscle/mitochondria) - Muscle cells need many mitochondria for a steady ATP supply. **Check your understanding:** Which organelle makes most of the ATP in an animal cell? - A. Golgi apparatus - B. Lysosome - C. Mitochondrion - D. Nucleus **Answer:** C. Mitochondrion Mitochondria carry out most of cellular respiration and produce the bulk of a cell's ATP. ## Chloroplasts: solar panels with stacks A chloroplast is a lens-shaped organelle, typically 3 to 10 µm long, found in plant and algal cells. Like a mitochondrion it has a double membrane, but it also contains a third membrane system: flattened sacs called thylakoids, stacked into piles called **grana**. The thylakoid membranes contain the green pigment chlorophyll. The fluid around them is the **stroma**. Photosynthesis happens in two stages: 1. **Light reactions** (thylakoid membranes): chlorophyll absorbs light, splits water, releases oxygen, and produces ATP and an energy-carrying molecule. 2. **Calvin cycle** (stroma): the ATP and energy carriers are used to build sugar from carbon dioxide. > carbon dioxide + water + light → glucose + oxygen ![Stylized 3D illustration of a chloroplast showing stacked thylakoids, with numbered markers.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/chloroplast-photosynthesis-teaching.jpg) *A stylized teaching illustration of a chloroplast. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/chloroplast-photosynthesis) Interactive 3D model: [Plant Cell](https://cellnaut.com/studio/plant/chloroplast) - A plant cell with chloroplasts, a central vacuole, and a cell wall. Interactive 3D model: [Chlamydomonas](https://cellnaut.com/studio/chlamydomonas/chloroplast) - Chlamydomonas is a single-celled alga with one large, cup-shaped chloroplast. > **Common misconception** > > **"Plants do photosynthesis, and animals do respiration."** Plant cells have mitochondria too, and they carry out cellular respiration all the time. Photosynthesis makes the sugar; respiration is how plants release its energy as ATP, day and night. ## Two organelles that were once bacteria Mitochondria and chloroplasts are strange organelles. They have their own DNA, their own ribosomes, and a double membrane, and they divide on their own by splitting in two. These are bacterial traits. The endosymbiotic theory explains why: more than a billion years ago, an ancestral cell engulfed an oxygen-using bacterium, but instead of digesting it, the two began to live together. The bacterium became the mitochondrion. Later, a similar partnership with a photosynthetic bacterium gave rise to the chloroplast. The evidence: - Both organelles have a **circular DNA genome**, like bacteria. - Their **ribosomes** are more similar to bacterial ribosomes than to the ones in the cytoplasm. - They are **surrounded by two membranes**, consistent with an engulfed cell. - They reproduce by a process like **binary fission**. - Gene sequences place them close to certain groups of living bacteria. Interactive 3D model: [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria/thylakoids) - A cyanobacterium photosynthesizes with thylakoid membranes but has no chloroplast at all. **Check your understanding:** Which observation supports the idea that mitochondria were once free-living bacteria? - A. They have their own circular DNA - B. They are found in every prokaryote - C. They contain chlorophyll - D. They have no membranes **Answer:** A. They have their own circular DNA Having its own circular DNA and bacteria-like ribosomes is strong evidence that mitochondria descend from engulfed bacteria. **Check your understanding:** Which statement about plant cells is correct? - A. They have chloroplasts but no mitochondria - B. They have both chloroplasts and mitochondria - C. They have mitochondria but no chloroplasts - D. They have neither **Answer:** B. They have both chloroplasts and mitochondria Most plant cells that photosynthesize have both organelles. Chloroplasts make sugar; mitochondria use it to make ATP. ## Sources - [Biology 2e, Chapter 7: Cellular Respiration (OpenStax)](https://openstax.org/books/biology-2e/pages/7-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 8: Photosynthesis (OpenStax)](https://openstax.org/books/biology-2e/pages/8-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Project teaching illustration: chloroplast photosynthesis (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset)