Biology & life sciences · 02 / 12
Photosynthesis and respiration: matter and energy
Middle school · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
Photosynthesis lets chlorophyll-containing organisms make organic matter from CO₂ and water using light energy, releasing oxygen in the school model. Cellular respiration uses organic matter and often oxygen to provide usable energy, producing CO₂ and water. Plants also respire.
- 01Mineral matter
- 02Light energy
- 03Organic matter
- 04Respiration and balance
WORKED EXAMPLE
From principle to reasoning
An illuminated plant can photosynthesise and respire simultaneously. Net gas exchange depends on both rates. Darkness stopping light-dependent photosynthesis does not stop respiration.
Solve a problem in this subject
- Frame a question
State the relationship to explain and its scale: molecule, cell, organism, population or geological time.
- Describe evidence
Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.
- Connect a mechanism
Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.
- Answer with a limit
Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.
YOUR TURN
01 · Try before checking
Why is respiration only at night a false statement about plants?
One hint
The processes are not mutually exclusive.
Reveal and understand the solution
Plants respire during daytime too. Light enables photosynthesis; net CO₂ balance does not reveal each gross flux.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
Does a green plant build organic matter only from mineral salts?
A hint to get started
Identify the carbon source.
Step-by-step solution
- Carbon in organic matter comes notably from CO₂.
- Photosynthesis uses light energy to build organic matter from inorganic inputs.
- Mineral nutrients matter but cannot replace CO₂.
Which step did you miss? Retry without help, then explain why the method works.
An illuminated aquatic plant raises O₂ concentration. Interpret and qualify.
A hint to get started
The measurement is a net balance, not necessarily one process.
Step-by-step solution
- The increase is consistent with net O₂ production by photosynthesis.
- The plant also respires, so measurement combines fluxes.
- A control and controlled conditions strengthen interpretation.
Which step did you miss? Retry without help, then explain why the method works.
Why may extra light eventually stop increasing photosynthesis?
A hint to get started
Other factors can become limiting.
Step-by-step solution
- At low light, extra energy can raise activity.
- CO₂, temperature or enzymatic capacity can then limit production.
- The response need not stay proportional indefinitely.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Light supplies energy rather than atoms incorporated into matter.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
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