Factors affecting photosynthesis, Practice Problems and FAQs

At home your parents will be preparing various types of dishes for you. Sometimes they may make special items like biryani for you. While preparing food at home, they have to think about all the essential items needed for cooking. You know before starting cooking they will arrange all the required items right. But what things are essential for making your favourite dish at home? 

First they have to check, do they have enough gas or other mode of energy to cook. Then they need to arrange utensils to cook, and the oil to make the food. What else? All the other ingredients will be depending upon the type of food they are making. If they are making vegetarian food, they have to look for good vegetables and if they are cooking some non- vegetarian food then they have to buy some kind of meat or fish. After setting up all these they will be able to cook our favourite dishes right. 

Now think about the unavailability of any of the essential items. They really cannot prepare the food without the basic ingredients. Right? For example, salt. Just like this there are many factors that affect the synthesis of food by plants too. Yes, we are going to discuss the factors that can affect the process of photosynthesis. From the simple stoichiometric equation of photosynthesis given below, we will get all the essential ingredients for the process. 

Fig:  Stoichiometric equation of photosynthesis

But do you think, only these factors are enough to carry out an effective photosynthesis? The answer will be no. There are many other factors which can affect the process and rate of photosynthesis. What are those factors then? We will check out how the absence of the various factors will affect photosynthesis in this article. After understanding this, we will be able to conclude the significance of photosynthesis. 

Table of contents

• Factors affecting photosynthesis
• Internal factors
• External factors 
• Significance of photosynthesis
• Practice Problems
• FAQs

Factors affecting photosynthesis

Photosynthesis is a process which needs many factors to occur. We know that the rate of photosynthesis determines the yield of a plant. Both internal and external factors can affect the rate of photosynthesis and the absence of any one factor can affect the life of a plant. 

Cardinal points

The rate of photosynthesis is affected by various factors. The minimum, optimum and maximum values of each factor affecting photosynthesis is called cardinal points. The minimum value is a point below which the physiological process cannot occur. Maximum value is a point beyond which the process stops. Optimum value is the point where the particular process continues indefinitely with maximum velocity.

Law of minimum

Liebig in 1943 proposed the law of minimum. This law states that “when a process is governed by a number of separate factors, the rate of the process is limited by pace of the slowest factor”.

Blackman’s law of limiting factors

Blackman extended the law of minimum to photosynthesis and proposed the law of limiting factor. The law of limiting factors is observed when several factors affect any chemical reaction. F.F. Blackman is a British plant physiologist. Blackman gave the law in the context of photosynthesis as it is a chemical reaction which is affected by several factors. Balckman’s law of limiting factors states that “if a chemical process is affected by more than one factor, then its rate will be determined by the factor that is nearest to its minimal value. It is the factor that directly affects the process if its quantity is changed.’

Fig: F.F. Balckman

Experiments by Blackman

Blackman studied the effect of CO₂ concentration, light intensity and temperature on the rate of photosynthesis. All other components were kept at their ideal concentration for this experiment. Initially he kept the plant in low light intensity and provided a temperature of 20 ⁰C and the CO₂ concentration was 1 ppm. When he started to increase the intensity of the light, the rate of photosynthesis also started to increase. But then it became constant. Now the photosynthesis will continue when there is an increase in the concentration of CO₂. 

So before in this experiment, the intensity of light was the limiting factor, but when it got sufficient light, now the limiting factor became the CO₂ concentration. 

Suppose we have provided enough light and CO₂ to the plant, the photosynthesis will occur and will reach a peak, which cannot increase further. So now the increase in the temperature will make the photosynthetic rate go high. So what is the limiting factor now? Yes, now the temperature is the limiting factor.

Now it is clear how a single factor can affect the process of photosynthesis and the rate of photosynthesis is limited only by the factor which is deficient. Both the internal and external factors have an equal role in this. So let’s check out the internal and external factors that affect the rate of photosynthesis. First we will see what the internal factors are.

Internal factors 

Most of the internal factors related to the plant characteristics which affect photosynthesis are genetically determined. It include the following:

• Age of leaves
• Orientation of leaves
• Size of leaves
• Orientation of chloroplast
• Orientation and number of mesophyll cells
• Amount of chlorophyll
• Number of leaves
• Accumulation of carbohydrates

Fig: Internal factors affecting photosynthesis

Now let’s discuss each of these factors in detail.

Age of leaves

The rate of photosynthesis in too young or too old leaves is lower than that of the mature leaves. This is due to the fact that as the leaves develop, the rate of photosynthesis increases gradually reaching a maximum at its fully expanded stage.

Fig: Leaves with different ages

Orientation of leaves

Turgid and upright leaves have more surface area to capture the maximum amount of light, as compared to dry and drooping leaves. So there is an increase in the rate of photosynthesis in the turgid and upright leaves.

Fig: Orientation of leaves and photosynthesis

Size of leaves

Photosynthesis is higher in bigger leaves due to more surface area for capturing sunlight as compared to smaller leaves.

Fig: Leaves with different sizes

Orientation of chloroplast

Chloroplasts align themselves within the mesophyll cells usually in three distinct ways so as to get the optimum quantity of incident light. The arrangement of the chloroplasts with respect to the incident light intensities also affects the rate of photosynthesis. There are three types of alignment of chloroplast. They are as follows:

• Parastophe
• Epistrophe
• Apostrophe

Fig: Types of chlorophyll alignments in leaves

Parastrophe 

Parastrophe alignment of chloroplast occurs under high light conditions. The chloroplasts align on the lateral walls of the mesophyll cells in a parallel fashion to the direction of the incident light here. This condition is known as parastrophe. This alignment protects the chloroplasts from being damaged by the high light intensities.

Fig: Parastrophe alignment

Apostrophe

Apostrophe arrangement occurs under moderate light conditions. The chloroplasts are arranged in a random pattern in the mesophyll cells here. This alignment is known as apostrophe arrangement.

Fig: Apostrophe alignment

Epistrophe

Epistrophe alignment occurs under low light intensities. The chloroplasts align themselves perpendicular to the incident light. This alignment is known as epistrophe. It allows the chloroplasts to capture the maximum amount of incident sunlight.

Fig: Epistrophe 

Orientation and number of mesophyll cells

Mesophyll cells contain chlorophylls that help in photosynthesis. Hence higher the number of mesophyll cells, the greater will be the rate of photosynthesis. In the following figure, the second arrangement of mesophyll cells will result in the higher rate of photosynthesis. So it is clear that the mesophyll cells in leaves should be more in number and they should be closely packed along with air spaces of spongy mesophylls for more photosynthesis. 

Fig: Comparison of arrangement of mesophyll cells in two different leaves

Amount of chlorophyll

Chlorophyll is a green pigment located within the thylakoid membrane of the chloroplast in plants. The number of chlorophyll molecules in the cell directly affects the rate of photosynthesis. Photosynthesis happens in the thylakoids and stroma of the chloroplast. Hence the more the number of chloroplasts the more will be the rate of photosynthesis.

Fig: Chlorophyll and rate of photosynthesis 

Number of leaves

Plants with more leaves would have more chlorophyll. Hence, they will have a higher rate of photosynthesis.

Fig: Plant with more leaves and less leaves

Accumulation of carbohydrates

In the leaves, an increased accumulation of carbohydrates tends to saturate and reduce the rate of photosynthesis. This happens because of the feedback inhibition. The accumulation of the products of photosynthesis prevents further photosynthetic reactions.

Fig: Feedback inhibition of photosynthesis

Now we have understood what are the internal factors that affect the rate of photosynthesis and its effects. Right? So next we are going to discuss the various external factors that affect the rate of photosynthesis.

External factors

The external factors that affect photosynthesis include oxygen, carbon dioxide, temperature, sunlight and water. 

Fig: List of external factors affecting photosynthesis

Sunlight

Plants trap light energy from the Sun to carry out photosynthesis. Three main characteristics of sunlight that influence the rate of photosynthesis. They are as follows:

• The intensity of light
• The quality of light 
• The duration of exposure to light

Fig: Characteristics of Sunlight

The intensity of light 

As the intensity of light increases, the rate of photosynthesis also increases in a gradual manner (represented by ‘A’ in the graph). After a certain point, the increase in the intensity of light does not cause a proportional increase in the rate of photosynthesis. This is because, at this stage, though sunlight is available, the plant does not have enough other resources to perform photosynthesis (represented by ‘B’ in the graph). When the intensity of sunlight falls on leaves increases beyond a point, chlorophylls are destroyed. It occurs in the presence of oxygen and is called photooxidation. This also reduces photosynthesis. This saturation occurs at 10% of the total incident sunlight (represented by ‘C’ in the graph). Plants cannot use more than 10% of the sunlight falling on them. Light is therefore rarely a limiting factor in nature, with the exception of plants in shadow or in dense forests. 

Fig: Graph on intensity of light and its effect on the rate of photosynthesis

Types of plants on the basis of requirement of the light

Plants can be divided into two types depending on the requirement of the light. They are as follows:

Sciophytes

They are the plants that require low light intensity for optimum photosynthesis. They are also known as shade plants. They always grow under the shade of other plants. Examples include Oxalis and Ipomea. 

Fig: Sciophytes

Heliophytes

They are those plants that require high intensity of light for optimum photosynthesis. They are also known as sun plants. They grow in open habitats. Examples include Banyan and Dalbergia.

Fig: Banyan

The quality of light

The wavelength of the incident light has an impact on the rate of photosynthesis.The experiments conducted by Engelmann prove that blue and red wavelengths of light are ideal for photosynthesis. The light between the wavelengths 400 nm to 700 nm is most effective for photosynthesis. This light is called PAR (Photosynthetically Active Radiation). Minimum photosynthesis occurs in green light. 

Fig: Engelmann’s experimental observation

The duration of exposure to light

The longer the plant is exposed to light, the longer will be the process of photosynthesis. Exposure of plants to 10 -12 hours of light per day is sufficient for normal photosynthesis. As per the duration of light exposure needed for flowering there are two types of plants. They are as follows:

• Short day plants
• Long day plants

Short day plants

They are those plants which flower and photosynthesise at their optimum at low light durations. Examples include rice and cotton.

Fig: Short day plants

Long day plants

These include those plants which thrive at higher durations of light exposure. Examples include spinach and lettuce.

Fig: Long day plants

Carbon dioxide

CO₂ is an important precursor of dark reaction in photosynthesis. Hence it is a major limiting factor of photosynthesis. The atmospheric levels of carbon dioxide are quite low and are about 0.03 - 0.0.4%.

Fig: Amount of gases in the atmosphere 

Effect of carbon dioxide on photosynthesis

An increase in the concentration of carbon dioxide (up to 0.05%), increases the rate of carbon dioxide fixation in plants. The increase in the levels of CO₂ beyond 0.05% damage the plants over longer periods. This is because at higher rates of photosynthesis, it is seen that there is a reduction in the assimilation of nitrates, calcium, magnesium and other nutrients. This reduces the overall plant health.

Fig: Increasing levels of CO₂ 

Effect of intensity of light and carbon dioxide concentration on photosynthesis

The response of plants to an elevation of carbon dioxide levels is dependent on the amount and intensity of sunlight available to the plants. Under low light conditions, an increase in CO₂ levels does not lead to an increase in the rate of photosynthesis. However, under higher light conditions, an increase in CO₂ levels leads to an increase in the rate of photosynthesis. So both the light intensity and carbon dioxide concentration should be more for an effective photosynthesis.

Fig: Response to elevation of CO₂ concentration under various light conditions

Responses of C₃ and C₄ plants to increasing CO₂ concentrations

An increase in the levels of CO₂, increases the rate of uptake of CO₂ in both C₃ and C₄ plants. Beyond a certain level, increase in CO₂ concentration does not increase the rate of assimilation or uptake of CO₂. This concentration of atmospheric CO₂ is said to be the saturation level.

Saturation level

It is the concentration of carbon dioxide beyond which an increase in the concentration would not lead to an increase in the rate of photosynthesis. The saturation levels of CO₂ for C₃ and C₄ plants are different. For C₄ plants it is approximately 360 μL/L and for C₃ plants it is approximately 450 μL/L.

Fig: CO₂ saturation of C₃ and C₄ plants

Since the C₄ plants can efficiently assimilate CO₂ using the CO₂ concentration mechanisms, the atmospheric levels of CO₂ reach saturation quite fast. But for C₃ plants, more the CO₂, the less RuBisCo would bind to O₂, making photosynthesis more efficient. Hence, they achieve saturation only at higher levels of CO₂.

C₃ plants respond to higher CO₂ concentrations by showing an increased rate of photosynthesis, leading to higher productivity. Due to this, C₃ plants like tomatoes and bell peppers are grown in greenhouses having a carbon dioxide enriched atmosphere to increase the yield.

 Fig: Green house

Temperature

Photosynthesis is an enzyme controlled reaction. Enzyme activity is highly dependent on temperature. Hence, the rate of photosynthesis is affected by the temperature. As temperature rises, photosynthesis proceeds more quickly. However, after a certain temperature maximum, the rate of photosynthesis declines as the enzymes are made of proteins, they begin to denature.

Fig: Rate of enzyme activity and temperature

Temperature tolerance level of C₄ and C₃ plants

Temperature tolerance level of C₄ and C₃ plants varies in the following way:

C₄ plants

They have a higher temperature maximum. This is because these plants are adapted to the dry and high temperature conditions with the help of the C₄ pathway. 

C₃ plants

They have a much lower temperature optimum as they possess only the Calvin cycle.

Fig: Relationship between the temperature and rate of photosynthesis of C₃ and C₄ plants

Temperature tolerance level of tropical plants and temperate plants

Temperature tolerance level of tropical plants and temperate plants varies in the following way:

Tropical plants

They live in tropical regions. Temperature optimum depends mainly on the habitat. They are adapted to higher temperatures and have a higher optimum temperature. Examples include orchids.

Temperate plants 

They live in moderate temperatures. They grow in places where the weather is neither very hot nor very cold. They have a low optimum temperature compared to tropical plants. Examples include maple trees.

Fig: Adaptations based on habitat

Water

The availability of water mainly affects the plant more than photosynthesis. The loss of water from the plant as a result of transpiration causes the stomata to close, thereby reducing the availability of CO₂. Carbon dioxide enters inside the plants through stomata from outside. 

Fig: Effect of closing of stomata on photosynthesis

The loss of water also causes leaves to wilt, thereby reducing the surface area of the leaves. This reduces the metabolic activities of the plant and thus leads to a reduction in the rate of photosynthesis.

 Fig: Wilting of leaves and its effect on photosynthesis

Oxygen

Under high concentrations of oxygen, RuBisCo (Ribulose-1, 5 - bisphosphate carboxylase - oxygenase) competitively binds to oxygen and acts as an oxygenase. This is known as the Warburg effect. This prevents RuBisCO from binding to CO₂ and catalysing the CO₂ fixation. Thus, it reduces the rate of photosynthesis.

Fig: The Warburg effect

So this is the way the internal and external factors affect the rate of photosynthesis. Any change from the required amount of any one factor could affect the rate of photosynthesis drastically. Since photosynthesis is the process which helps to retain life on the planet, deviations in the rate of photosynthesis will affect the whole ecosystem.

Significance of photosynthesis

The following are the significances of photosynthesis:

• Photosynthesis is essential for the existence of all life on Earth, as this is the mechanism producers use for the fixation of carbon dioxide and energy.
• Sunlight is the ultimate source of energy for almost all ecosystems and photosynthesis facilitates the conversion of solar energy into chemical energy. 
• Producers occupy the first trophic level of the food chains and photosynthesis serves a crucial role in the food chain. 

Fig: Plants as part of food webs

• The plants create their food using this process, thereby, act as the primary producers.
• The production of oxygen, which is essential for the life of the majority of creatures on the Earth, is a byproduct of photosynthesis.

Fig: Release of oxygen during photosynthesis

• Carbohydrates are stored in various forms like seeds, vegetables, fruits, rhizome etc. which are useful to animals as food and for storage. It also forms the basis of agriculture. 
• It removes carbon dioxide from the atmosphere and maintains the carbon dioxide: oxygen level in the atmosphere.

Practice Problems

Q1. Which of the following has a low temperature photosynthetic capacity?

A. Bacteria
B. Lichen
C. Virus
D. Fungi

Solution: Lichen is a symbiotic relationship between algae and fungi in which the algal partner can generate its own food through photosynthesis even at temperatures as low as - 24 °C despite lacking roots to collect water and nutrients. Bacteria do not typically show photosynthesis. A few exceptions include the photosynthetic bacterium like cyanobacteria. Viruses are not showing photosynthesis and are obligate intracellular parasites. Chlorophyll is absent in fungi and they are heterotrophic organisms, that means they depend on other organisms for food. Hence the correct option is b.

Q2. Which one of the following statements is untrue in terms of the components influencing the rate of photosynthesis?

A. The rate at which atmospheric CO₂ is fixed can be accelerated by raising the concentration of carbon dioxide by up to 0.05 percent
B. While C₄ plants have a significantly lower temperature optimum, C₃ plants adapt to higher temperatures by increasing photosynthesis.
C. To increase output, tomatoes can be grown in greenhouses with enriched CO2 atmospheres
D. For CO₂ fixation, light saturation occurs at 10% of maximum illumination

Solution: CO₂ is present in the atmosphere in concentrations of between 0.03 and 0.04 percent. An increase in CO₂ fixation rates can result from increases in CO₂ concentration up to 0.05 percent in the atmosphere. In addition to this, CO₂ builds up in the plants, which slows down the process. When exposed to high CO₂ levels at high light intensities, C₃ plants exhibit an increase in photosynthetic rates. Some C₃ plants, such as tomatoes, have been found to be more productive when cultivated in a CO₂ enriched atmosphere. When the amount of light is low, the rate of photosynthesis varies linearly; when the amount of light is more, the rate of photosynthesis becomes constant. For CO₂ fixation, light saturation occurs at 10% of maximum illumination. Hence the correct option is b. 

Q3. Due to ________ being the limiting factor, the rice crops in the eastern states of India produce less during the monsoon.

A. CO₂
B. Light
C. Temperature
D. Water

Solution: A change in the quantity of the limiting factor directly affects the rate of the process of photosynthesis. Light is necessary for photosynthesis, which affects how much yield a crop will produce. When the light intensity is more photosynthesis proceeds at the fastest rate. However, during the monsoon, the amount of light intensity decreases, which in turn slows down photosynthesis and affects yields negatively. Hence the correct option is b.

Q4. The concentration of CO₂ is 420 ppm and there is high light intensity. Which of the following statements is accurate in this case?

A. Due to an excessive concentration of CO₂, C3 plants will die
B. C3 plants will become more productive
C. Plants that are C₃ will exhibit a decline in the rate of photosynthesis
D. C₃ plants will become less productive

Solution: Between 0.03 and 0.04 percent of CO2 is present in the atmosphere. If the concentration rose above 0.05 percent for an extended length of time, it would harm the plants.. C₃ plants are those that display the C3 pathway (Calvin cycle) during the dark reaction of photosynthesis, where the first observable product is 3-phosphoglycerate, which has three carbon atoms. The rate of photosynthesis in these plants rises as the rate of CO₂ fixing rises. Beyond 450 ppm, however, saturation is observed in such plants. Therefore, when the CO₂ content rises to 420 ppm, productivity increases. Hence the correct option is b.

FAQs

Question 1. What is the compensation point?
Answer: The point on the light curve when photosynthesis and respiration occur at the exact same rate is known as the compensation point. Therefore, this is the point at which photosynthesis and respiration in a plant are equal. As a result, the carbon dioxide that is released during respiration is equal to the amount that is absorbed during photosynthesis. As light intensity rises, the compensation point is reached. After the point of compensation, increasing the light intensity causes a proportional rise in the rate of photosynthesis, which continues until the point of light saturation, beyond which the rate of photosynthesis is unaffected by light intensity.

Question 2. Do plants respond when people communicate?
Answer: Yes, plants do respond to your voice. The results of research by the Royal Horticultural Society showed that plants indeed react to human voices. There were 10 tomato plants in this study, and 8 of them had headphones placed around their pots.

Question 3. Why are most plants green?
Answer: A colour wheel is created using the colours of visible light. An object seems to be the complementary colour to the colour it absorbs most strongly within that wheel. As a result, plants seem green because they efficiently absorb red light while reflecting green light.

Question 4. How do you estimate the gas that photosynthesis releases?
Answer: With the aid of an infrared gas analyzer, the CO₂ entering and leaving the leaf chamber can be measured. The amount of CO₂ that results from the difference allows us to determine the rate of photosynthesis.

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