How are photosynthesis and cellular respiration related, and why are they often described as opposites?
Compare the reactants, products, and energy flow of photosynthesis and cellular respiration, and explain how they form a connected cycle of energy and matter (TEKS Biology, Reporting Category 4; energy and matter; systems and system models).
A TEKS-level answer comparing photosynthesis and cellular respiration for the Texas STAAR Biology EOC: how their reactants and products mirror each other, the contrast in energy flow, and how together they cycle energy and matter.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The Biology TEKS ask you to compare photosynthesis and cellular respiration, their reactants, products, and energy flow, and to see how they connect. For STAAR Reporting Category 4 you need to recognize that the two processes mirror each other and together cycle energy and matter. This is an energy and matter and systems and system models topic, and the comparison is one of the most frequently tested ideas on the exam.
The two processes side by side
| Feature | Photosynthesis | Cellular respiration |
|---|---|---|
| Reactants | Carbon dioxide and water | Glucose and oxygen |
| Products | Glucose and oxygen | Carbon dioxide and water |
| Energy | Stores energy (light to chemical) | Releases energy (chemical to ATP) |
| Location | Chloroplasts | Mainly mitochondria |
| Occurs in | Plants, algae, some bacteria | Nearly all cells (including plants) |
| When | In light | At all times |
The two equations make the mirror clear:
Energy flows, matter cycles
The deepest idea here is the difference between energy and matter:
- Energy flows. Light energy is captured by photosynthesis into glucose, then released by respiration and ultimately lost as heat. Energy moves one way through the system; it is not recycled, which is why life needs a continuous input of sunlight.
- Matter cycles. The carbon and oxygen atoms are passed back and forth: carbon dioxide and water made by respiration are used by photosynthesis, and the glucose and oxygen made by photosynthesis are used by respiration. The same atoms are reused over and over.
This contrast, energy flowing one way while matter cycles, is the same principle that governs whole ecosystems (see energy flow and food webs and the cycling of matter).
A common misconception to clear up
A frequent error is thinking plants only photosynthesize. In fact, plants both photosynthesize and respire. In the light, a plant may photosynthesize faster than it respires, so it gives off oxygen overall; in the dark, it only respires, taking in oxygen and giving off carbon dioxide. STAAR likes to test this with a sealed-container or day-night scenario.
Try this
Q1. State how the reactants and products of photosynthesis compare with those of respiration. [2]
- Cue. They are reversed: the products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the products of respiration (carbon dioxide and water) are the reactants of photosynthesis.
Q2. Explain why energy must keep entering this system from the Sun, while matter does not. [2]
- Cue. Energy flows one way and is lost as heat, so it must be replaced; matter (carbon and oxygen) cycles between the two processes and is reused.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksWhich statement best describes the relationship between photosynthesis and cellular respiration? (A) They are identical processes. (B) The products of one are the reactants of the other. (C) Both release oxygen. (D) Neither involves glucose.Show worked answer →
A 1-point multiple-choice item on the relationship between the two processes.
The correct answer is B. Photosynthesis makes glucose and oxygen (using carbon dioxide and water); respiration uses glucose and oxygen and makes carbon dioxide and water. The products of one are the reactants of the other. A is false (they differ), C is false (only photosynthesis releases oxygen), and D is false (both involve glucose).
The two processes mirror each other, forming a cycle.
STAAR Biology (2024 SCR style)2 marksA sealed, lit terrarium contains a plant. Explain how photosynthesis and cellular respiration together allow carbon and oxygen to cycle within the terrarium. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response on cycling matter.
Full credit (2 points): the plant photosynthesizes, taking in carbon dioxide and releasing oxygen and making glucose; the plant (and other organisms) respire, taking in oxygen and glucose and releasing carbon dioxide. Because the products of each process are the reactants of the other, carbon and oxygen are recycled within the terrarium rather than used up.
Partial credit (1 point): describes one process or states they are opposites without explaining the recycling of matter. The science is scored.
Related dot points
- Describe the reactants, products, and energy transformation of photosynthesis, and explain its role in capturing light energy as chemical energy in glucose (TEKS Biology, Reporting Category 4; energy and matter; cause and effect).
A TEKS-level answer on photosynthesis for the Texas STAAR Biology EOC: the reactants and products, the role of light and chlorophyll in chloroplasts, the energy transformation from light to chemical energy, and the overall word and balanced equation.
- Identify cellular respiration as the process that releases energy from glucose, describe its reactants and products, and distinguish aerobic respiration from fermentation (TEKS Biology, Reporting Category 4; energy and matter; cause and effect).
A TEKS-level answer on cellular respiration for the Texas STAAR Biology EOC: the reactants and products, the role of mitochondria and ATP, the overall equation, and the difference between aerobic respiration and fermentation.
- Identify the four major classes of biological macromolecules and their functions, and explain how enzymes act as biological catalysts affected by temperature and pH (TEKS Biology, Reporting Category 4; structure and function; cause and effect).
A TEKS-level answer on biomolecules and enzymes for the Texas STAAR Biology EOC: carbohydrates, lipids, proteins, and nucleic acids and their functions, and how enzymes catalyze reactions and are affected by temperature and pH.
- Describe how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and explain the role of decomposers in returning nutrients (TEKS Biology, Reporting Category 5; energy and matter; systems and system models).
A TEKS-level answer on biogeochemical cycles for the Texas STAAR Biology EOC: how carbon, nitrogen, and water cycle through ecosystems, the role of decomposers, and why matter cycles while energy flows one way.
- Interpret food chains, food webs, and energy pyramids to explain how energy flows through an ecosystem and is lost at each trophic level (TEKS Biology, Reporting Category 5; energy and matter; using mathematics).
A TEKS-level answer on energy flow for the Texas STAAR Biology EOC: food chains and webs, producers and consumers, trophic levels, the energy pyramid and the 10 percent rule, and why energy is lost at each level.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)