Factors Affecting Photosynthesis

Q: How do C3 and C4 plants respond differently to CO2?

At low light neither group responds to extra CO2. At high light both increase their rate, but C4 plants saturate at about 360 microlitres per litre while C3 plants keep responding until beyond 450. This is why greenhouse tomatoes and bell pepper are grown in CO2-enriched air.

BiologyPhotosynthesis in Higher PlantsFor NEET aspirants

The rate of photosynthesis depends on internal (plant) factors and on external factors such as light, carbon dioxide, temperature and water. At any moment the factor in shortest supply limits it. This page covers Blackman's law of limiting factors, the light intensity curve, light saturation, the different responses of C₃ and C₄ plants to carbon dioxide and temperature, and the indirect effect of water. NEET asks the factors affecting photosynthesis through the light curve, the 10 per cent saturation fact and the 360 and 450 microlitre per litre values.

On this page1Why factors matter2Limiting factors3Light4Carbon dioxide5Temperature6Water7Exam essentials8Quick revision9Solved examples10Practice

Key Points at a Glance

The rate of photosynthesis largely decides the yield of plants, including crops.
Internal factors: number, size, age and orientation of leaves, mesophyll cells and chloroplasts; internal $C O_{2}$ ; amount of chlorophyll.
External factors: sunlight, temperature, $C O_{2}$ concentration and water.
★ Must learn Blackman's Law of Limiting Factors (1905): the rate is set by the factor nearest to its minimal value.
★ Must learn At low light the rate rises linearly with light; light saturation occurs at 10 per cent of full sunlight.
Too much light breaks down chlorophyll and lowers photosynthesis.
★ Must learn $C O_{2}$ is the major limiting factor: 0.03-0.04 per cent in air; up to 0.05 per cent raises the rate.
★ Must learn C₄ plants saturate at about $360 μ L L^{- 1}$ ; C₃ plants only beyond $450 μ L L^{- 1}$ .
Dark reactions are enzymatic and strongly temperature controlled; light reactions much less so.
Water acts mainly through the plant: water stress closes stomata and makes leaves wilt.

1. Why the Factors Matter

The rate of photosynthesis is very important in deciding the yield of plants, including crop plants.
Photosynthesis is influenced by several factors, both internal (plant) and external.

1.1 Internal and external factors

Internal (plant) factors	External factors
Number, size, age and orientation of leaves	Availability of sunlight
Number, size, age and orientation of mesophyll cells and chloroplasts	Temperature
Internal $C O_{2}$ concentration	$C O_{2}$ concentration
Amount of chlorophyll	Water

Internal factors depend on the genetic predisposition and the growth of the plant.

Memory Trick

External factors, "Let The Crop Win": Light (sunlight), Temperature, Carbon dioxide and Water.

Key idea

Leaf and chloroplast features are internal factors; light,

C O_{2}

, temperature and water are external factors.

2. Law of Limiting Factors

All the factors act on the rate at the same time.
Even so, usually one factor is the major cause, or the one that limits the rate.
★ Exam imp At any point, the rate is decided by the factor available at sub-optimal levels.

★ Very important Blackman's (1905) Law of Limiting Factors: a chemical process may be affected by more than one factor. Its rate is then decided by the factor that is nearest to its minimal value. Changing the quantity of this factor directly changes the rate of the process.

Example: a green leaf with optimal light and $C O_{2}$ may still not photosynthesise if the temperature is very low.
The same leaf starts photosynthesising when it is given the optimal temperature.
Here temperature is the limiting factor.

Memory Trick

Blackman = Bottleneck: the factor in shortest supply is the bottleneck that sets the rate, just as the narrowest neck of a bottle sets how fast it pours.

Tips and Tricks How to find the limiting factor: ask which factor, if increased, would raise the rate. That factor is limiting. Raising any other factor has no effect until the limiting one is improved.

Key idea

The scarcest factor sets the pace, like the slowest worker on an assembly line.

3. Light

Light acts as a factor in three ways: light quality, light intensity and the duration of exposure to light.
★ Exam imp At low light intensities, there is a linear relationship between incident light and the rate of $C O_{2}$ fixation.
At higher light intensities the rate gradually stops increasing, because other factors become limiting.

Test yourself: hide labels

Figure 1: Light intensity and the rate of photosynthesis. The rate rises in a straight line at low light (A), bends (B) and levels off (C) at the light intensity D; E is the maximum rate.

3.1 Reading the light curve

Point or region	What it shows
A	Linear rise: light is the limiting factor
B	The bend: light starts to stop being limiting as other factors take over
C	Plateau: light is no longer limiting; another factor (such as $C O_{2}$ or temperature) limits the rate
D	The light intensity at which saturation begins (light saturation)
E	The maximum rate of photosynthesis under the existing levels of other factors

★ Very important Light saturation occurs at 10 per cent of full sunlight. So, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.

Increasing incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

Quick Recall: read the light curve

In Figure 1, in which region is light the limiting factor?

Region A, where the rate rises in a straight line.

What does E on the rate axis show?

The maximum rate of photosynthesis under the existing levels of the other factors.

At what fraction of full sunlight does light saturation occur?

10 per cent.

Memory Trick

"Ten per cent is enough": leaves are light-saturated at one-tenth of full sunlight, so light limits only shade and forest-floor plants.

Key idea

Light limits the rate only in dim conditions; above saturation, other factors take control and very strong light even harms chlorophyll.

4. Carbon Dioxide Concentration

★ Exam imp Carbon dioxide is the major limiting factor for photosynthesis.
Its concentration in the atmosphere is very low, between 0.03 and 0.04 per cent.
★ Exam imp Raising it up to 0.05 per cent can increase the rate of $C O_{2}$ fixation.
Beyond this, the levels can become damaging over longer periods.

4.1 Response of C₃ and C₄ plants

C₃ and C₄ plants respond differently to $C O_{2}$ .
★ Exam imp At low light, neither group responds to high $C O_{2}$ .
At high light, both C₃ and C₄ plants show an increase in the rate of photosynthesis.
C₄ plants saturate early, but C₃ plants keep responding. Thus the present $C O_{2}$ level is limiting for C₃ plants.

Feature	C₃ plants	C₄ plants
Response to rising $C O_{2}$ (high light)	Keep responding	Saturate early
$C O_{2}$ saturation	Only beyond $450 μ L L^{- 1}$	At about $360 μ L L^{- 1}$

NEET Focus

C₄ saturates at about 360, C₃ only beyond 450 microlitres per litre. So the current $C O_{2}$ level is limiting for C₃ plants.
At low light, extra $C O_{2}$ helps neither group, because light is then the limiting factor.
Greenhouse $C O_{2}$ enrichment is used for C₃ crops such as tomato and bell pepper.

4.2 Use in agriculture

C₃ plants respond to higher $C O_{2}$ with higher rates of photosynthesis and higher productivity.
This is used for greenhouse crops such as tomatoes and bell pepper.
They are grown in a $C O_{2}$ -enriched atmosphere, which gives higher yields.

Memory Trick

"C₄ is full at 360, C₃ wants more than 450": the smaller number goes with C₄, which concentrates $C O_{2}$ on its own.

Key idea

Because C₃ plants are still

C O_{2}

-limited at today's levels, extra

C O_{2}

in greenhouses raises their yield.

5. Temperature

The dark reactions are enzymatic, so they are temperature controlled.
The light reactions are also temperature sensitive, but they are affected to a much lesser extent.
★ Exam imp C₄ plants respond to higher temperatures with a higher rate of photosynthesis. C₃ plants have a much lower temperature optimum.
The temperature optimum also depends on the habitat a plant is adapted to.
Tropical plants have a higher temperature optimum than plants adapted to temperate climates.

Plant group	Temperature optimum
C₃ plants	20-25°C
C₄ plants	30-40°C

Memory Trick

Bigger subscript, bigger optimum: C₃ works best at 20-25°C and C₄ at 30-40°C.

Key idea

Temperature acts mainly on the enzyme-driven dark reactions; C₄ and tropical plants work best when it is warmer.

6. Water

Water is one of the reactants in the light reaction.
Yet its effect as a factor is mainly through its effect on the plant, rather than directly on photosynthesis.

★ Exam imp Water stress makes the stomata close, which reduces the availability of $C O_{2}$ .
Water stress also makes leaves wilt, which reduces leaf surface area and metabolic activity.

Tips and Tricks Trap: a statement that water limits photosynthesis mainly as a raw material is false. Its main effect is indirect, through stomatal closure and wilting.

Quick Recall: tap to check

Name two crops grown in CO₂-enriched greenhouses.

Tomato and bell pepper.

Which are more sensitive to temperature, light reactions or dark reactions?

Dark reactions, because they are enzymatic.

Give two ways water stress lowers photosynthesis.

Stomata close, cutting

C O_{2}

supply; leaves wilt, reducing surface area and metabolic activity.

Key idea

A thirsty plant closes its stomata, so it is short of

C O_{2}

long before it is short of water for the light reaction.

7. Exam Essentials

Pairs to Match

List I	List II
Blackman (1905)	Law of Limiting Factors
Region A of the light curve	Light is limiting
Region C of the light curve	Light not limiting; other factors limit
10 per cent of full sunlight	Light saturation
Very high light intensity	Breakdown of chlorophyll
0.03-0.04 per cent	Atmospheric $C O_{2}$
0.05 per cent $C O_{2}$	Upper level that raises the rate
About $360 μ L L^{- 1}$	$C O_{2}$ saturation of C₄ plants
Beyond $450 μ L L^{- 1}$	$C O_{2}$ saturation of C₃ plants
Tomato, bell pepper	Greenhouse crops grown in $C O_{2}$ -enriched air
Dark reactions	Strongly temperature controlled (enzymatic)
Water stress	Stomatal closure and wilting

Exceptions

Light is rarely limiting in nature, except for plants in shade or in dense forests.
At low light, neither C₃ nor C₄ plants respond to high $C O_{2}$ .
$C O_{2}$ levels beyond about 0.05 per cent can become damaging over longer periods.
Light reactions are much less sensitive to temperature than dark reactions.
Water affects photosynthesis mainly indirectly, not as a reactant.
Increasing light beyond a point decreases photosynthesis by breaking down chlorophyll.

Numbers to Remember

Blackman's law: 1905.
Light saturation: 10 per cent of full sunlight.
Atmospheric $C O_{2}$ : 0.03 to 0.04 per cent; useful increase up to 0.05 per cent.
$C O_{2}$ saturation: C₄ about $360 μ L L^{- 1}$ ; C₃ beyond $450 μ L L^{- 1}$ .
Temperature optimum: C₃ 20-25°C; C₄ 30-40°C.
External factors: four (sunlight, temperature, $C O_{2}$ , water).

8. Quick Revision

Photosynthetic rate largely decides plant and crop yield.
Internal factors: leaves, mesophyll cells and chloroplasts (number, size, age, orientation), internal $C O_{2}$ , chlorophyll.
Internal factors depend on genetic predisposition and growth.
External factors: sunlight, temperature, $C O_{2}$ and water.
The factor at a sub-optimal level limits the rate.
Blackman (1905): the factor nearest to its minimal value decides the rate.
Light: quality, intensity and duration; rate rises linearly at low intensity.
Light saturation at 10 per cent of full sunlight; light limits only shade and forest plants.
Excess light breaks down chlorophyll and lowers photosynthesis.
$C O_{2}$ is the major limiting factor: 0.03-0.04 per cent in air; up to 0.05 per cent helps.
C₄ saturate near 360, C₃ beyond 450 microlitres per litre; present $C O_{2}$ limits C₃.
Tomato and bell pepper give higher yields in $C O_{2}$ -enriched greenhouses.
Dark reactions are temperature controlled; C₄ and tropical plants have higher optima.
Water stress closes stomata (less $C O_{2}$ ) and wilts leaves (less area and activity).

9. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Light saturation; B. Atmospheric

C O_{2}

; C.

C O_{2}

saturation in C₄ plants; D.

C O_{2}

saturation in C₃ plants
List II: I. Beyond

450 μ L L^{- 1}

; II. About

360 μ L L^{- 1}

; III. 10 per cent of full sunlight; IV. 0.03-0.04 per cent
Choose the correct answer.
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-IV, C-I, D-II
(D) A-I, B-IV, C-II, D-III

Solution:

Answer: (A). Light saturates at 10 per cent of full sunlight (III); air has 0.03-0.04 per cent $C O_{2}$ (IV); C₄ plants saturate near 360 (II) and C₃ plants beyond 450 microlitres per litre (I).

Solved Example 2

Read the statements about factors affecting photosynthesis.
A.

C O_{2}

is the major limiting factor.
B. At low light, C₃ plants respond strongly to high

C O_{2}

.
C. Light reactions are much less affected by temperature than dark reactions.
D. Water stress reduces

C O_{2}

availability by closing stomata.
E. Light is the most common limiting factor for plants in open fields.
Choose the correct answer.
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only

Solution:

Answer: (A). B is false: at low light neither group responds to high $C O_{2}$ . E is false: light saturates at 10 per cent of full sunlight, so it rarely limits plants in the open. A, C and D are true.

Solved Example 3

Arrange the events by which water stress lowers photosynthesis.
A. Less

C O_{2}

enters the leaf
B. Water supply to the plant falls
C. Stomata close
D. Rate of

C O_{2}

fixation falls
Choose the correct order.
(A) B, C, A, D
(B) B, A, C, D
(C) C, B, A, D
(D) B, C, D, A

Solution:

Answer: (A). Water stress (B) closes stomata (C), so less $C O_{2}$ enters (A) and $C O_{2}$ fixation falls (D).

Solved Example 4

A leaf has optimal light and

C O_{2}

but does not photosynthesise because the temperature is very low. According to Blackman's law, the limiting factor is:
(A) Light
(B)

C O_{2}

Solution:

Answer: (C). Temperature is nearest to its minimal value. The leaf photosynthesises once given the optimal temperature.

Solved Example 5

Which statement about light as a factor is NOT correct?
(A) The rate rises linearly with light at low intensities
(B) Light saturation occurs at 10 per cent of full sunlight
(C) Light is usually the limiting factor in nature
(D) Very high light intensity breaks down chlorophyll

Solution:

Answer: (C). Light is rarely limiting in nature, except for plants in shade or in dense forests.

10. Practice Questions

Practice Questions

Figure 1 shows the effect of light on the rate of photosynthesis. (a) At which point or points (A, B or C) is light a limiting factor? (b) What could be the limiting factor in region A? (c) What do C and D represent on the curve?Answer: (a) At A, and partly at B, where the rate still rises with light. (b) Light intensity. (c) C is the plateau where light is no longer limiting and another factor, such as $C O_{2}$ or temperature, limits the rate; D is the light intensity at which saturation begins.
Match List I with List II.
List I: A. Blackman; B. Tomato and bell pepper; C. Water stress; D. Dark reactions
List II: I. Stomatal closure; II. Temperature controlled; III. Law of Limiting Factors; IV. $C O_{2}$ -enriched greenhouses
Choose the correct answer.
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-I, C-IV, D-II
(D) A-III, B-IV, C-II, D-IAnswer: (A). Blackman: limiting factors; tomato and bell pepper: $C O_{2}$ enrichment; water stress: stomatal closure; dark reactions: temperature controlled.
Read the statements about $C O_{2}$ as a factor.
A. Atmospheric $C O_{2}$ is between 0.03 and 0.04 per cent.
B. Raising $C O_{2}$ up to 0.05 per cent can increase the rate.
C. C₄ plants saturate only beyond $450 μ L L^{- 1}$ .
D. Current $C O_{2}$ levels are limiting for C₃ plants.
Choose the correct answer.
(A) A, B and D only
(B) A and C only
(C) B, C and D only
(D) A, B, C and DAnswer: (A). C is false: C₄ plants saturate at about 360; it is C₃ plants that saturate beyond 450 microlitres per litre.
Statement I: Temperature affects the dark reactions more than the light reactions.
Statement II: The dark reactions are enzymatic.
Choose the correct answer.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (A). Both are correct; enzyme-driven reactions are temperature controlled.
Which of these is an internal factor affecting photosynthesis?
(A) Light intensity
(B) Atmospheric $C O_{2}$
(C) Amount of chlorophyll
(D) TemperatureAnswer: (C). The amount of chlorophyll belongs to the plant; the others are external.
Which is NOT an effect of water stress on photosynthesis?
(A) Closure of stomata
(B) Wilting of leaves
(C) Reduced metabolic activity of leaves
(D) Increased $C O_{2}$ uptakeAnswer: (D). Water stress reduces $C O_{2}$ uptake because the stomata close.
Arrange the parts of the light curve in the order of increasing light intensity.
A. Plateau, where another factor limits
B. Linear rise
C. Bend in the curve
(A) B, C, A
(B) C, B, A
(C) A, B, C
(D) B, A, CAnswer: (A). The rate first rises linearly, then bends and finally levels off.

Common Mistakes to Avoid

Watch out

Writing that light saturation occurs at full sunlight. Correct: at 10 per cent of full sunlight.
Swapping the $C O_{2}$ saturation values. Correct: C₄ about 360 and C₃ beyond 450 microlitres per litre.
Assuming more $C O_{2}$ always helps. Above about 0.05 per cent it can damage plants over long periods, and at low light it gives no gain.
Saying the light reactions are more temperature sensitive than the dark reactions. It is the reverse.
Treating water mainly as a raw material that limits photosynthesis. Its main effect is through stomatal closure and wilting.
Listing temperature or light as internal factors. They are external; leaf and chloroplast features are internal.
Calling light the major limiting factor. $C O_{2}$ is the major limiting factor; light limits mainly in shade.
Giving C₃ plants the higher temperature optimum. C₄ plants (30-40°C) have the higher optimum.

Frequently Asked Questions

What is Blackman's Law of Limiting Factors?

Proposed by Blackman in 1905, the law states that when a chemical process depends on several factors, its rate is decided by the factor nearest to its minimal value. Changing that factor directly changes the rate. For example, very low temperature can stop photosynthesis even in good light.

What are the internal and external factors affecting photosynthesis?

Internal factors are the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, the internal $C O_{2}$ concentration and the amount of chlorophyll. External factors are sunlight, temperature, $C O_{2}$ concentration and water. Internal factors depend on the plant's genes and growth.

At what light intensity does photosynthesis saturate?

Light saturation occurs at about 10 per cent of full sunlight. Below this the rate rises linearly with light; above it other factors limit the rate. So light is rarely limiting in nature except for plants in shade or in dense forests.

Why is carbon dioxide called the major limiting factor?

Carbon dioxide is present in the air at only 0.03 to 0.04 per cent. Raising it up to 0.05 per cent can increase the rate of fixation, which shows that normal levels hold photosynthesis back. C₃ plants are limited by present levels because they saturate only beyond 450 microlitres per litre.

How do C3 and C4 plants respond differently to CO2?

At low light neither group responds to extra $C O_{2}$ . At high light both increase their rate, but C₄ plants saturate at about 360 microlitres per litre while C₃ plants keep responding until beyond 450. This is why greenhouse tomatoes and bell pepper are grown in $C O_{2}$ -enriched air.

Why does temperature affect the dark reactions more?

The dark reactions are catalysed by enzymes, so their rate is controlled by temperature. The light reactions are temperature sensitive too, but much less. C₄ and tropical plants have higher temperature optima than C₃ and temperate plants. The optima are about 20-25 degrees Celsius for C₃ and 30-40 degrees Celsius for C₄ plants.

How does water stress reduce photosynthesis?

Water is a reactant in the light reaction, but its main effect is on the plant. Water stress closes the stomata, which cuts the supply of $C O_{2}$ , and makes leaves wilt, which reduces their surface area and metabolic activity. So water acts more through the plant than as a raw material.

Which numbers on factors affecting photosynthesis should I learn for NEET from NCERT?

Learn Blackman's law (1905) and light saturation at 10 per cent of full sunlight. Atmospheric $C O_{2}$ is 0.03 to 0.04 per cent, and a rise up to 0.05 per cent helps. $C O_{2}$ saturation comes near 360 microlitres per litre in C₄ plants and only beyond 450 in C₃ plants.

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1. Why the Factors Matter

1.1 Internal and external factors

2. Law of Limiting Factors

3. Light

3.1 Reading the light curve

4. Carbon Dioxide Concentration

4.1 Response of C3 and C4 plants

4.2 Use in agriculture

5. Temperature

6. Water

7. Exam Essentials

8. Quick Revision

9. Solved Examples

10. Practice Questions

Common Mistakes to Avoid

Frequently Asked Questions

What is Blackman's Law of Limiting Factors?

What are the internal and external factors affecting photosynthesis?

At what light intensity does photosynthesis saturate?

Why is carbon dioxide called the major limiting factor?

How do C3 and C4 plants respond differently to CO2?

Why does temperature affect the dark reactions more?

How does water stress reduce photosynthesis?

Which numbers on factors affecting photosynthesis should I learn for NEET from NCERT?

Ready to master Photosynthesis in Higher Plants?

4.1 Response of C₃ and C₄ plants