Photosynthesis and cellular respiration are near-opposite processes that power almost all life on Earth. Plants use photosynthesis to convert carbon dioxide, water and light energy into glucose and oxygen (6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂). Cellular respiration then reverses this: cells (in plants and animals) break down glucose using oxygen to release usable energy, producing carbon dioxide and water as by-products (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy).
Example
A plant photosynthesises during the day using sunlight to build glucose and release oxygen, but at night — with no light available — it relies on cellular respiration alone, consuming oxygen and releasing carbon dioxide just like an animal does.
Key terms
Photosynthesis:
The process plants use to convert light energy into chemical energy (glucose).
Cellular respiration:
The process cells use to break down glucose and release usable energy.
Glucose:
A sugar molecule that stores chemical energy for use by cells.
Questions
1. Photosynthesis converts:
Carbon dioxide, water and light into glucose and oxygen
Glucose into carbon dioxide only
Oxygen into carbon dioxide directly
Nothing; it produces no output
2. Cellular respiration breaks down:
Glucose, using oxygen, to release energy
Only water, with no other input
Only sunlight, with no glucose involved
Carbon dioxide into oxygen directly
3. Photosynthesis requires:
Light energy
Only darkness, with no light
Only oxygen, with no light needed
No inputs of any kind
4. A by-product of cellular respiration is:
Carbon dioxide and water
Only sunlight
Only glucose
Nothing at all
5. Glucose is best described as:
A sugar molecule storing chemical energy
A type of gas only
A type of light only
A mineral found only in rocks
6. Photosynthesis mainly occurs in:
Plants
Only animals, never plants
Only fungi, never plants
No living organism at all
7. Cellular respiration occurs in:
Both plants and animals
Only animals, never plants
Only plants, never animals
No living organism at all
8. At night, without sunlight, a plant relies on:
Cellular respiration alone
Photosynthesis alone
Neither process at all
A completely different, unrelated process
9. Why are photosynthesis and cellular respiration often described as near-opposite processes?
One builds glucose using light energy, while the other breaks glucose down to release energy
They both produce exactly the same outputs from exactly the same inputs
Photosynthesis and cellular respiration are actually identical processes
Neither process has any connection to energy at all
10. The oxygen released during photosynthesis is later used by organisms (including the plant itself) during:
Cellular respiration
A separate, unrelated process
Nothing; oxygen has no further use
Only during additional photosynthesis
11. Why do plant cells contain mitochondria (the site of cellular respiration) as well as chloroplasts (the site of photosynthesis)?
Plants need to carry out both processes — building energy stores via photosynthesis and releasing energy via respiration
Plants only ever carry out photosynthesis, never respiration
Mitochondria and chloroplasts perform the exact same function
Plant cells contain neither of these structures
12. The carbon dioxide a plant releases during cellular respiration at night is:
A normal by-product of breaking down glucose for energy
Evidence the plant has stopped functioning entirely
Never actually produced by plants
Only ever produced by animals, not plants
13. Why is sunlight considered the ultimate energy source for almost all life, even for animals that never photosynthesise?
Animals rely on eating plants (or other animals that ate plants), which captured that solar energy through photosynthesis
Animals generate all their own energy with no connection to sunlight at all
Sunlight has no connection to any energy used by animals
Only plants require any form of energy to survive
14. Why might increased carbon dioxide in the atmosphere affect the balance between photosynthesis and respiration on a global scale?
Photosynthesis rates can be influenced by available carbon dioxide, affecting how much is absorbed versus released globally
Carbon dioxide levels have no effect on photosynthesis rates at all
Only cellular respiration is ever affected by atmospheric carbon dioxide
Global carbon dioxide levels are entirely unrelated to living organisms
15. Why is the relationship between photosynthesis and respiration often described as a "cycle" at the ecosystem level?
The gases and energy exchanged between the two processes are continuously cycled between organisms and the atmosphere
These two processes never interact with each other in any way
A cycle implies the processes never actually exchange any materials
Photosynthesis and respiration only ever occur in a single, isolated organism
16. Why might a scientist measure a plant's oxygen output to estimate its rate of photosynthesis?
Oxygen is a direct product of photosynthesis, so its rate of release reflects how much photosynthesis is occurring
Oxygen output has no connection to the rate of photosynthesis
Plants never release oxygen as part of any process
Oxygen is only ever produced during cellular respiration, not photosynthesis
17. Why might deforestation on a large scale disrupt the global balance between carbon dioxide production and absorption?
Fewer plants means less carbon dioxide is being converted into glucose and oxygen via photosynthesis
Removing plants always increases the rate of global photosynthesis
Deforestation has no measurable effect on atmospheric gas balance
Plants play no role in absorbing or converting atmospheric carbon dioxide
18. Why do scientists consider both photosynthesis and cellular respiration essential for maintaining atmospheric oxygen and carbon dioxide levels?
Together, they form a continuous exchange that helps regulate the balance of these gases over time
Neither process has any measurable effect on atmospheric gas levels
Only cellular respiration affects atmospheric gases, not photosynthesis
Atmospheric gas levels remain completely fixed regardless of biological processes
19. Why do algae and phytoplankton in the ocean play a major role in global photosynthesis, not just land plants?
These aquatic organisms carry out a substantial share of the world's total photosynthesis and oxygen production
Photosynthesis can only ever occur in land-based plants
Ocean organisms play no role in global oxygen or carbon cycles
Algae and phytoplankton only ever perform cellular respiration, never photosynthesis
20. Why might a plant kept in complete darkness for an extended period eventually die, despite having water available?
Without light, it cannot photosynthesise to produce the glucose needed to fuel its own cellular respiration
Plants can photosynthesise perfectly well without any light at all
Water alone is always sufficient to keep any plant alive indefinitely
Darkness has no effect on a plant's ability to produce energy
21. Why is the equation for cellular respiration often described as the reverse of the equation for photosynthesis?
The reactants and products are essentially swapped — what photosynthesis builds, respiration breaks back down
The two equations share no mathematical or chemical relationship at all
Cellular respiration produces glucose, exactly like photosynthesis does
Photosynthesis and respiration always use completely unrelated chemical elements
Answer key (parent copy)
1. Carbon dioxide, water and light into glucose and oxygen
2. Glucose, using oxygen, to release energy
3. Light energy
4. Carbon dioxide and water
5. A sugar molecule storing chemical energy
6. Plants
7. Both plants and animals
8. Cellular respiration alone
9. One builds glucose using light energy, while the other breaks glucose down to release energy
10. Cellular respiration
11. Plants need to carry out both processes — building energy stores via photosynthesis and releasing energy via respiration
12. A normal by-product of breaking down glucose for energy
13. Animals rely on eating plants (or other animals that ate plants), which captured that solar energy through photosynthesis
14. Photosynthesis rates can be influenced by available carbon dioxide, affecting how much is absorbed versus released globally
15. The gases and energy exchanged between the two processes are continuously cycled between organisms and the atmosphere
16. Oxygen is a direct product of photosynthesis, so its rate of release reflects how much photosynthesis is occurring
17. Fewer plants means less carbon dioxide is being converted into glucose and oxygen via photosynthesis
18. Together, they form a continuous exchange that helps regulate the balance of these gases over time
19. These aquatic organisms carry out a substantial share of the world's total photosynthesis and oxygen production
20. Without light, it cannot photosynthesise to produce the glucose needed to fuel its own cellular respiration
21. The reactants and products are essentially swapped — what photosynthesis builds, respiration breaks back down