As light intensity increases, the rate of photosynthesis rises until another factor becomes limiting. The slowest or least available factor determines the overall rate.

---

Figure 2. Increasing light intensity increases photosynthesis until another factor becomes limiting.

---

Carbon Dioxide Concentration

Carbon dioxide is one of the raw materials needed for photosynthesis.

Low Carbon Dioxide

When carbon dioxide levels are low:

• Photosynthesis is limited.
• Less glucose is produced.

---

Increasing Carbon Dioxide

As carbon dioxide concentration increases:

• Photosynthesis becomes faster.
• More glucose is produced.

---

High Carbon Dioxide

Eventually the graph levels off because another factor becomes limiting.

For example:

• Light intensity
• Temperature

Many commercial greenhouses increase carbon dioxide levels to improve plant growth.

---

Figure 3. Increasing carbon dioxide concentration raises the rate of photosynthesis until another factor becomes limiting.

---

Temperature

Photosynthesis depends on many enzymes.

Enzymes work best within a certain temperature range.

Low Temperature

When temperatures are low:

• Molecules move slowly.
• Enzymes work slowly.
• Photosynthesis is slow.

---

Increasing Temperature

As temperature rises:

• Enzyme activity increases.
• Photosynthesis becomes faster.

Most plants photosynthesise most efficiently between about 20°C and 35°C, although the exact optimum varies between species.

---

Very High Temperature

If temperatures become too high:

• Enzymes begin to lose their shape (denature).
• Photosynthesis slows rapidly.
• Extremely high temperatures may damage plant tissues.

---

Figure 4. Photosynthesis increases with temperature until the optimum is reached, then decreases as enzymes become damaged.

---

Water Availability

Although not always considered one of the three main limiting factors, water is also essential.

Plants require water for:

• Photosynthesis.
• Maintaining cell pressure.
• Transporting minerals.

During drought:

• Stomata may close.
• Less carbon dioxide enters the leaf.
• Photosynthesis slows.

---

Interpreting Photosynthesis Graphs

Scientists often use graphs to study limiting factors.

Light Intensity Graph

• Starts low.
• Rises quickly.
• Levels off.

---

Carbon Dioxide Graph

• Starts low.
• Rises steadily.
• Levels off.

---

Temperature Graph

• Starts low.
• Increases to an optimum.
• Falls at higher temperatures.

Recognising these patterns is an important scientific skill.

---

Figure 5. Different environmental factors produce characteristic photosynthesis graphs.

---

Limiting Factors Work Together

Several factors influence photosynthesis at the same time.

For example:

A greenhouse plant may have:

• Plenty of sunlight.
• Plenty of water.

However, if carbon dioxide concentration is low, photosynthesis will still remain slow.

Similarly:

• High carbon dioxide cannot compensate for insufficient light.
• Bright light cannot compensate for extremely low temperatures.

The limiting factor is always the one preventing the rate from increasing further.

---

Improving Photosynthesis in Agriculture

Farmers and greenhouse growers can increase crop growth by controlling limiting factors.

Methods include:

• Providing artificial lighting.
• Heating greenhouses.
• Enriching air with carbon dioxide.
• Watering plants regularly.

These techniques increase photosynthesis, leading to faster growth and higher crop yields.

---

Worked Example

Question

A plant is growing under very bright light but the rate of photosynthesis remains low.

Suggest two possible limiting factors.

Solution

Possible limiting factors include:

• Low carbon dioxide concentration.
• Low temperature.
• Water shortage.

Increasing only the light intensity further would not increase the rate because another factor is limiting.

---

Real-World Connection

Modern commercial greenhouses carefully monitor temperature, carbon dioxide concentration, humidity, and lighting to maximise photosynthesis. Automated systems adjust these conditions throughout the day, allowing crops such as tomatoes, cucumbers, peppers, and lettuce to grow faster and produce higher yields than they would under natural conditions alone.

---

Did You Know?

Plants growing beneath the dense canopy of a rainforest often receive less than 2% of the available sunlight. Many of these shade-tolerant plants have evolved larger, thinner leaves with more chlorophyll, allowing them to photosynthesise efficiently even under very low light conditions.

---

Key Terms

Carbon dioxide concentration – The amount of carbon dioxide available in the air for photosynthesis.

Enzyme – A biological catalyst that speeds up chemical reactions in living organisms.

Light intensity – The amount of light reaching a surface.

Limiting factor – The factor that restricts the rate of a biological process because it is in the shortest supply.

Optimum temperature – The temperature at which an enzyme or biological process works most efficiently.

Photosynthesis – The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.

Rate of photosynthesis – The speed at which photosynthesis occurs.

Temperature – A measure of how hot or cold something is, affecting the activity of enzymes involved in photosynthesis.

---

Key Takeaways

• The rate of photosynthesis depends on environmental conditions.
• The main limiting factors are light intensity, carbon dioxide concentration, and temperature.
• Increasing light intensity or carbon dioxide concentration increases photosynthesis until another factor becomes limiting.
• Temperature increases the rate of photosynthesis up to an optimum, after which the rate decreases because enzymes become damaged.
• Scientists use graphs to identify limiting factors and understand how environmental conditions affect photosynthesis.
• Farmers use knowledge of limiting factors to improve crop growth and increase agricultural productivity.