Photosynthesis and limiting factors
If increasing one input no longer raises the rate, which other factor may be limiting the process?
Use a normalized qualitative model to compare relative availability of light, carbon dioxide, and water and identify the limiting input.
Learning goals
- Identify the main inputs and outputs of photosynthesis.
- Explain a limiting factor as the least available input.
- Distinguish a qualitative model output from a measured experimental rate.
Before you begin
- Know that plants need energy and matter to grow.
- Compare percentage values.
Interactive science experiment
- 1Identify inputs
- 2Vary light
- 3Vary CO₂
- 4Find the limiting factor
Inputs: Light · CO₂ · H₂O Outputs: Sugar · O₂
A normalized qualitative model, not a measured rate. Plants still respire in darkness.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
-
Photosynthesis uses light energy to form sugars from carbon dioxide and water, releasing oxygen. Matter in sugar comes from reactants; light supplies energy.
Your task: Follow the arrows to distinguish inputs entering the leaf from outputs leaving it.
-
When other inputs are sufficient, low light can limit the qualitative rate. If another input is lower, more light does not raise this minimum-input model output.
Your task: Choose Light-limited, then increase light and watch when the limiting factor changes.
-
Low carbon dioxide availability can become limiting. The model assigns zero when water is unavailable, representing a missing required input stopping photosynthesis in this model.
Your task: Try CO₂-limited and Water-limited; compare the rate and limiting factor.
-
When two inputs share the lowest value, the model calls them co-limiting. Raising only one leaves the other as the ceiling. Real plants are also affected by temperature, leaf condition, and more.
Your task: Choose Co-limited, raise only light, and explain why the rate initially stays unchanged.
Build an explanation
Do not confuse matter and energy
CO₂ and H₂O provide atoms used in sugars. Light is not matter packed into sugar; it is the energy source driving the process.
Darkness does not stop respiration
Without light, photosynthesis is limited, but plant cells can still carry out cellular respiration. This model shows only normalized photosynthesis output, not net gas exchange.
Where oxygen comes from
In oxygenic photosynthesis, released O₂ comes from splitting water. The introductory overall equation is useful for matter accounting but does not show every light-reaction step.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Biology
Photosynthesis and limiting factors
If increasing one input no longer raises the rate, which other factor may be limiting the process?
- What are the main matter inputs to photosynthesis?
A. CO₂ and water B. Sugar and oxygen C. Light only
- With 80% light, 20% CO₂, and water available, what limits this model?
A. Light B. CO₂ C. Water
- If the model shows zero photosynthesis at zero light, can we conclude that respiration stops?
A. Yes; they are the same process B. No; plants can still respire in darkness C. Only leaves stop respiring
- If light and CO₂ are both 50%, what happens when only light rises to 80%?
A. It remains 50% B. It immediately becomes 80% C. It falls to 0%
Answer page
Photosynthesis and limiting factors
- CO₂ and water
Carbon dioxide and water provide atoms for sugar; light supplies energy rather than matter.
- CO₂
The model uses the least available input, and 20% CO₂ is lowest.
- No; plants can still respire in darkness
Photosynthesis and cellular respiration are different processes; this model does not simulate respiration or net gas exchange.
- It remains 50%
CO₂ remains the lowest input at 50%, so the minimum-input normalized rate stays unchanged.