Photosynthesis and Cellular Respiration
All living things need energy, and almost all of that energy traces back to two connected processes: photosynthesis and cellular respiration. Plants capture the Sun’s energy and store it in food; then living cells release that energy to do their work. Understanding how these two fit together answers a surprising number of life-science questions.
This lesson shows what each process does and, more importantly, how they connect.
Photosynthesis: Storing the Sun’s Energy
Photosynthesis happens in the chloroplasts of plant cells. The plant takes in carbon dioxide and water, uses light energy from the Sun, and produces glucose (a sugar) and oxygen. In plain words: the plant builds food and releases the oxygen we breathe. The energy from sunlight ends up stored in the chemical bonds of glucose.
Cellular Respiration: Releasing the Energy
Cellular respiration happens in the mitochondria of cells — in plants and animals alike. It does the reverse: a cell takes in glucose and oxygen and releases energy, giving off carbon dioxide and water. That released energy powers everything a cell does. Notice the pattern: respiration uses exactly what photosynthesis makes, and makes exactly what photosynthesis uses.
The Cycle That Connects Them
Put the two together and you get a loop. Photosynthesis takes in carbon dioxide and water and produces glucose and oxygen. Respiration takes in glucose and oxygen and produces carbon dioxide and water. The output of each process is the input of the other, which keeps carbon and oxygen cycling between plants, animals, and the air. A common test question asks you to recognize this relationship — that plants and animals depend on each other through these two processes.
One point that trips people up: plants do both. They photosynthesize to make food and also respire to use it. Animals only respire; they get their glucose by eating.
Watch: A Short Video Lesson
Point Source Science walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A Routine for These Questions
- Photosynthesis: carbon dioxide + water + light → glucose + oxygen (in chloroplasts).
- Respiration: glucose + oxygen → carbon dioxide + water + energy (in mitochondria).
- The two are opposites: each one’s products are the other’s raw materials.
- Plants do both processes; animals only respire.
Practice
- What does photosynthesis produce?
- Where in the cell does cellular respiration take place?
- What gas do animals give off during respiration?
- Do plants carry out respiration, photosynthesis, or both?
- What is the source of energy for photosynthesis?
- How are the two processes related?
Answers
- Glucose (sugar) and oxygen.
- In the mitochondria.
- Carbon dioxide.
- Both.
- Light energy from the Sun.
- The products of one are the raw materials of the other, forming a cycle.
Where This Fits in Your Science Prep
This energy cycle builds on the cell and its parts and connects to energy flow and the cycles of matter in ecosystems. See all topics on the Science Topics Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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