Investigation of Limiting Factors

What Are Limiting Factors?

A limiting factor is something that is in the shortest supply and stops a process from going any faster. Think of it like this: if you're baking cookies and you have plenty of flour and sugar but only one egg, the egg is your limiting factor — you can't make more cookies until you get more eggs.

For photosynthesis to happen at its maximum rate, plants need several things:

• Photosynthetic pigments (like chlorophyll)
• Carbon dioxide
• Water
• Light energy
• The right temperature

If any one of these is in short supply or not at the right level, photosynthesis will slow down — even if everything else is perfect.

The three main external limiting factors you need to know are:

1. Light intensity — how bright the light is
2. Carbon dioxide concentration — how much CO₂ is available in the air
3. Temperature — how warm or cold it is

These are called "external" factors because they come from the plant's surroundings (the environment), not from inside the plant itself.

Effect of Light Intensity on Photosynthesis

How Light Intensity Affects the Rate

When light intensity increases, the rate of photosynthesis increases — but only up to a certain point.

At low light intensities:

• Light is the limiting factor
• More light means more energy is available for photosynthesis
• The greater the light intensity, the more energy reaches the chloroplasts
• This speeds up the light-dependent reactions (the stage that happens in the thylakoid membranes)
• More ATP and reduced NADP are produced
• These products fuel the Calvin cycle (the light-independent stage), which can then also work faster
• The overall rate of photosynthesis increases

Think of light like fuel for the chloroplasts — more fuel means they can work faster.

At high light intensities:

• Eventually, the rate stops increasing and levels off (reaches a plateau)
• This point is called the light saturation point
• Beyond this point, light is no longer the limiting factor
• Something else is now holding back photosynthesis, such as:
  • Not enough carbon dioxide
  • Temperature too low or too high
  • Enzymes working at their maximum speed already

Graph pattern:

• At lower light intensities, the relationship is linear (a straight line going up) — the rate increases steadily with light
• Then the line curves and becomes horizontal (flat) — the rate stays constant even if you add more light

Why Temperature Matters With Light

Increasing the light intensity also raises the temperature slightly. If the light intensity is very low, increasing the temperature won't help much because light is still the main problem. But at higher light intensities, temperature can become more important.

At very high light intensities and high oxygen levels in the leaf cells, a wasteful process called photorespiration can occur. This actually reduces photosynthesis efficiency because the enzyme rubisco uses oxygen instead of carbon dioxide, creating toxic waste products that the plant has to deal with.