Factors affecting photosynthesis Questions in English

(N/A) Photosynthesis is a process catalyzed by enzymes. Each enzyme functions optimally within a specific temperature range. When the temperature increases beyond this optimal level (typically $25-35^{\circ}C$),the enzymes undergo denaturation or lose their functional efficiency. Consequently,the rate of photosynthesis decreases.

(C) The temperature optimum for photosynthesis in different plants depends on the habitat to which they are adapted.
- $C_{4}$ plants generally respond to higher temperatures and exhibit a higher rate of photosynthesis compared to $C_{3}$ plants.
- $C_{3}$ plants typically have a much lower temperature optimum for photosynthesis.
- For example,plants native to tropical regions have a higher temperature optimum compared to plants adapted to temperate climates,as their enzymes and metabolic processes are evolved to function efficiently at those specific environmental temperatures.

(C) Water stress causes the stomata to close.
- This reduces the $CO_{2}$ availability for photosynthesis.
- Besides,water stress also causes leaves to wilt.
- This reduces the surface area of the leaves and their metabolic activity as well.

(N/A) Plants cannot perform photosynthesis in moonlight because moonlight does not have sufficient intensity or energy to stimulate chlorophyll molecules.
Photosynthesis requires light of a specific intensity to initiate the light-dependent reactions.
Since moonlight is reflected sunlight with very low intensity,it fails to trigger the electron transport chain in $PS-I$ and $PS-II$.
Therefore,photosynthesis does not occur under moonlight conditions.

(N/A) Plants adapt to limited water supply by reducing the rate of transpiration through the closure of stomata.
Despite this reduction in water loss,they continue to perform essential metabolic activities such as photosynthesis,respiration,and the translocation of food materials.
By regulating stomatal aperture,plants maintain internal water potential to sustain these vital physiological processes even under water-stressed conditions.

(D) Plants living in arid or water-scarce environments have evolved several adaptations to minimize water loss through transpiration:
$1$. Shedding leaves: Many desert plants shed their leaves during dry seasons to reduce the surface area available for transpiration.
$2$. $CAM$ (Crassulacean Acid Metabolism) pathway: This is a specialized photosynthetic adaptation where plants open their stomata at night to take in $CO_2$ and keep them closed during the hot,dry day to prevent water loss.
$3$. Stomatal regulation: Keeping stomata closed during the day is a direct mechanism to reduce transpiration rates in high-temperature environments.
Therefore,all the mentioned options are correct adaptations.

(B) $C_{3}$-plants exhibit optimum rates of photosynthesis under conditions of high $CO_{2}$ concentration. This is because $C_{3}$-plants are prone to photorespiration,a process where the enzyme $RuBisCO$ binds with $O_{2}$ instead of $CO_{2}$. By increasing the concentration of $CO_{2}$,the probability of $RuBisCO$ binding with $CO_{2}$ increases,thereby minimizing photorespiration and maximizing the efficiency of the Calvin cycle.

(D) Carbon dioxide is a major limiting factor for photosynthesis.
The concentration of $CO_{2}$ in the atmosphere is very low,ranging between $0.03-0.04 \%$ or $300-400 \;ppm$.
An increase in concentration up to $0.05 \%$ can cause an increase in $CO_{2}$ fixation rates.
Beyond this level,high concentrations can become damaging to plants over longer periods.

(B) Photosynthesis is influenced by several external factors,including light,temperature,$CO_2$ concentration,and water.
$1$. Light: Light is an essential factor for photosynthesis. It affects the rate directly through its intensity,quality,and duration.
$2$. Carbon Dioxide $(CO_2)$: $CO_2$ is a major limiting factor. It is directly involved as a substrate in the Calvin cycle (dark reactions) for the synthesis of carbohydrates.
$3$. Temperature: The dark reactions of photosynthesis are enzymatic and are directly controlled by temperature. The light reactions are also temperature-sensitive,though to a lesser extent.
$4$. Water: Water is a reactant in photosynthesis,but its effect on the process is primarily indirect. Water stress causes the stomata to close,which reduces $CO_2$ availability for gaseous exchange. Additionally,water stress causes leaves to wilt,reducing the surface area available for photosynthesis and decreasing metabolic activity. Therefore,water acts more through its physiological effect on the plant rather than directly on the photosynthetic machinery.

(B) The Law of Limiting Factors was proposed by Blackman in $1905$.
He stated that when a process is conditioned as to its rate by a number of separate factors,the rate of the process is determined by the pace of the slowest factor.
Carbon dioxide is usually the most important limiting factor in photosynthesis under normal field conditions,particularly on clear summer days when light and water are available in adequate amounts.

(A) The rate of photosynthesis is dependent on the absorption spectrum of chlorophyll pigments.
Chlorophyll $a$ and $b$ show maximum absorption in the red and blue regions of the visible spectrum.
Experimental data indicates that the rate of photosynthesis is highest in red light $(660 \; nm)$,followed by blue light $(440 \; nm)$.
Green light is largely reflected or transmitted by leaves,resulting in the minimum rate of photosynthesis.
Therefore,the correct sequence is Red $>$ Blue $>$ Green.

(B) The rate of a chemical process is determined by the factor that is closest to its minimal value. This is known as the $Law$ $of$ $Limiting$ $Factors$,proposed by $F. F.$ $Blackman$ in $1905$. It states that when a process is conditioned by a number of separate factors,the rate of the process is limited by the pace of the slowest factor,i.e.,the factor present in the minimum amount relative to its requirement.

(C) The correct answer is $C$.
Water stress occurs when the plant lacks sufficient water.
To prevent excessive water loss through transpiration,the guard cells lose turgidity,causing the stomata to close ($A$-close).
Since the stomata are the primary sites for gas exchange,their closure prevents the entry of atmospheric carbon dioxide into the leaf.
Therefore,this reduces the availability of $CO_{2}$ $(B-CO_{2})$ for the process of photosynthesis.

(D) Beyond the saturation point (the light intensity at which the plant achieves maximum photosynthesis),the rate of photosynthesis begins to decline due to the following two reasons:
$1$. Photo-inhibition: This occurs due to the reduction in hydration and the closing of stomata.
$2$. Photo-oxidation: This occurs due to the oxidation of photosynthetic pigments and enzymes.
Therefore,both $I$ and $II$ are correct.

(B) The light compensation point is the specific light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point,the amount of $CO_2$ fixed by photosynthesis is equal to the amount of $CO_2$ released by respiration,and the amount of $O_2$ consumed by respiration is equal to the amount of $O_2$ produced by photosynthesis. Consequently,there is no net gaseous exchange with the external environment.

$A$ refers to Tropical plants and $B$ refers to Temperate climate.
Tropical plants are adapted to warmer environments and therefore possess a higher temperature optimum for photosynthesis compared to plants adapted to temperate climates, which are adapted to cooler conditions.

(A) Compensation point refers to the light intensity at which the rate of respiration by a photosynthetic cell or organ is equal to the rate of photosynthesis.
At this point,there is no net gain or loss of $O_{2}$ or $CO_{2}$ from the structure.

(B) The Law of Limiting Factors was proposed by $F$.$F$. Blackman in $1905$.
According to this law,if a chemical process is affected by more than one factor,then its rate will be determined by the factor which is nearest to its minimal value,as it is the factor which directly affects the process if its quantity is changed.

(A) According to Blackman's Law of Limiting Factors,the rate of a physiological process is limited by the factor that is present in the least amount relative to its optimum requirement.
$CO_2$ is usually a limiting factor in nature because its atmospheric concentration is very low $(0.03-0.04\%)$.
Light is a limiting factor at low intensities.
Temperature affects the enzymatic activity of the process.
Chlorophyll is present in the leaves in sufficient quantities to capture light energy and is rarely a limiting factor under normal physiological conditions.

(B) The rate of photosynthesis is directly proportional to the intensity of incident light at low light intensities.
As light intensity increases,the rate of $CO_2$ fixation increases linearly.
At higher light intensities,the rate of photosynthesis eventually reaches a saturation point where other factors become limiting,and the relationship is no longer linear.

(A) In the given graph showing the effect of light intensity on the rate of photosynthesis:
$D$ represents the light saturation point,which is the intensity at which the rate of photosynthesis becomes maximum and does not increase further with an increase in light intensity.
$E$ represents the maximum rate of photosynthesis achieved at the saturation point.
Therefore,$D$-Saturation point and $E$-Maximum photosynthesis is the correct labelling.

(A) The dark reactions of photosynthesis (also known as the Calvin cycle) involve a series of biochemical reactions that are catalyzed by specific enzymes,such as $RuBisCO$.
Since these reactions are enzymatic in nature,they are highly sensitive to temperature changes.
Therefore,the rate of dark reactions is significantly influenced by temperature,making them temperature-controlled processes.
Thus,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the Assertion.

(C) Sciophytes,also known as shade plants,are those that thrive in environments with moderate or low light intensity.
These plants achieve their optimum photosynthetic rate and growth when exposed to light intensity that is approximately $10-30 \%$ of full sunlight.

(D) Abiotic factors are non-living components of the environment.
$1$. Water,nutrients,and oxygen are essential for the germination of seeds and the early stages of plant growth.
$2$. Light is not strictly required for the initial germination or early growth of many plants,as they can utilize stored food reserves in the seed.
$3$. However,once the plant exhausts its stored reserves,it must perform photosynthesis to continue growing,which requires light. Therefore,sustained growth is dependent on the presence of light.

(A) Photosynthetic yield is directly dependent on the availability of sunlight and favorable climatic conditions.
In tropical regions near the equator,the intensity and duration of sunlight are significantly higher throughout the year compared to other regions.
This abundance of solar energy,combined with optimal temperature and moisture,leads to maximum photosynthetic productivity in these areas.

(N/A) Three physiological processes in plants that are affected by light are:
$1$. Photosynthesis: Light is the primary energy source for the synthesis of organic compounds.
$2$. Transpiration: Light influences the opening and closing of stomata,which regulates the rate of water loss.
$3$. Flowering (Photoperiodism): The duration of light exposure (photoperiod) is critical for the induction of flowering in many plant species.

(C) Photosynthesis occurs primarily due to the absorption of light by chlorophyll pigments.
Chlorophyll $a$ and $b$ show maximum absorption in the blue and red regions of the visible spectrum.
Therefore, the rate of photosynthesis is highest in the blue and red regions of light $(P = \text{Blue and Red})$.
Green light is mostly reflected or transmitted by the leaves, which is why plants appear green.
Consequently, the rate of photosynthesis is minimum in the green region of light $(Q = \text{Green})$.

(A) The "Law of Limiting Factors" was proposed by $F.F. Blackman$ in $1905$.
This law states that if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value, as it is the factor which directly affects the process the most.

(B) Photosynthesis is influenced by various factors,which are broadly classified into internal (plant) factors and external (environmental) factors.
$1.$ Internal factors: These are related to the plant itself,such as the number,size,age,and orientation of leaves,mesophyll cells and chloroplasts,internal $CO_2$ concentration,and the amount of chlorophyll. These correspond to $III, IV, V, VIII,$ and $IX$.
$2.$ External factors: These are environmental conditions such as sunlight (light),temperature,$CO_2$ concentration in the atmosphere,and water availability. These correspond to $I, II, VI,$ and $VII$.
Therefore,the correct sequence for internal factors and external factors is $III, IV, V, VIII, IX$ and $I, II, VI, VII$.

(D) According to Blackman's Law of Limiting Factors,if a physiological process is controlled by several factors,the rate of the process is limited by the factor present in the lowest concentration relative to its optimum requirement.
In this scenario,temperature,light,and $CO_2$ are provided at optimal levels,meaning they are not limiting the process.
Since water is supplied in limited amounts,it becomes the limiting factor.
Therefore,the rate of photosynthesis will be determined by the availability of water.

(B) Light is a crucial factor for photosynthesis.
As the intensity of light increases,the rate of photosynthesis also increases up to a certain point.
Beyond this point,the rate of photosynthesis does not increase further even if light intensity is increased,because other factors become limiting.
This point is known as the light saturation point.
For most plants,light saturation occurs at about $10\%$ of the intensity of full sunlight.
Therefore,except for plants in shade or in dense forests,light is rarely a limiting factor in nature.

(D) The concentration of $CO_2$ in the atmosphere is very low,ranging between $0.03 \%$ and $0.04 \%$.
$CO_2$ is a major limiting factor for photosynthesis.
If the concentration of $CO_2$ is increased up to $0.05 \%$,it can cause an increase in the rate of $CO_2$ fixation in many plants.
Beyond this level,it can become damaging over longer periods.

(A) The concentration of $CO_2$ is a major factor affecting the rate of photosynthesis.
$C_3$ plants show saturation at about $450 \mu L L^{-1}$ of $CO_2$ concentration.
$C_4$ plants show saturation at about $360 \mu L L^{-1}$ of $CO_2$ concentration.
Therefore,$P = 360 \mu L L^{-1}$ and $Q = 450 \mu L L^{-1}$.
Thus,the correct option is $A$.

(B) The concentration of $CO_2$ is a major factor affecting the rate of photosynthesis.
$C_3$ plants show an increase in the rate of photosynthesis at higher $CO_2$ concentrations because their enzyme $RuBisCO$ has a lower affinity for $CO_2$ and also acts as an oxygenase.
In contrast,$C_4$ plants show saturation at about $360 \mu L L^{-1}$,meaning they are already efficient at low $CO_2$ concentrations.
Therefore,the current atmospheric level of $CO_2$ is limiting for $C_3$ plants.

(A) The dark reactions of photosynthesis are enzymatic processes (Calvin cycle),which are highly sensitive to temperature. In contrast,the light reactions are primarily photochemical and are less affected by temperature. Therefore,the statement that light reactions are more sensitive to temperature than dark reactions is incorrect. $C_4$ plants are adapted to warmer climates and have a higher temperature optimum compared to $C_3$ plants. Tropical plants,living in warmer regions,naturally have a higher temperature optimum than plants from temperate regions. Temperature significantly influences the activity of enzymes involved in metabolic pathways.

(D) Water is a critical factor affecting photosynthesis in the following ways:
$1$. Water acts as a reactant in the light-dependent reactions (photolysis of water),so its deficiency directly limits the rate of photosynthesis.
$2$. Water stress leads to the closure of stomata,which restricts the entry of $CO_2$ into the leaves,thereby reducing the rate of photosynthesis.
$3$. Water stress causes wilting of leaves,which reduces the effective surface area of the leaves and their metabolic activity,further decreasing the rate of photosynthesis.
Therefore,all the given statements are correct.

(B) The rate of photosynthesis increases with an increase in light intensity at low light intensities.
As light intensity increases further,the rate of photosynthesis continues to increase until it reaches a saturation point,where other factors become limiting.
Beyond this saturation point,further increases in light intensity do not increase the rate of photosynthesis.
Graph $B$ correctly represents this relationship,showing a steady increase followed by a plateau (saturation) as light intensity increases.

(B) The rate of photosynthesis and productivity in $C_3$ plants increases with an increase in $CO_2$ concentration because $C_3$ plants show $CO_2$ saturation at levels much lower than current atmospheric concentrations.
$C_4$ plants,such as sugarcane,sorghum,and maize,do not show an increase in photosynthetic rate at high $CO_2$ concentrations because they have a mechanism to concentrate $CO_2$ around the enzyme $RuBisCO$.
Bell pepper and tomatoes are $C_3$ plants,which respond positively to increased $CO_2$ levels (often used in greenhouse crops to increase yield).
Therefore,the correct answer is Bell pepper and tomatoes.

(B) The Assertion is true because,in natural ecosystems,light is usually available in abundance for most plants,except for those growing in deep shade or dense forest canopies where light becomes a limiting factor.
The Reason is also true because $C_4$ plants have evolved to efficiently utilize high light intensities for photosynthesis without showing light saturation at lower levels compared to $C_3$ plants.
However,the Reason does not explain why light is rarely a limiting factor in nature for most plants. The availability of light in the environment is an ecological condition,whereas the response of $C_4$ plants to high light is a physiological adaptation. Therefore,the Reason is not the correct explanation for the Assertion.

(A) The correct answer is $A$.
Desert plants,such as succulents,utilize the Crassulacean Acid Metabolism $(CAM)$ pathway.
This specialized photosynthetic pathway allows these plants to open their stomata at night to take in $CO_2$ and store it as organic acids,which helps in minimizing water loss through transpiration during the hot,dry day.

(D) is the correct answer.
In the experiment with $Hydrilla$,adding sodium bicarbonate $(NaHCO_3)$ increases the availability of carbon dioxide $(CO_2)$ in the water.
$CO_2$ is a key substrate for photosynthesis.
This enhanced availability of carbon dioxide improves the rate of photosynthesis,which,in turn,increases the amount of oxygen $(O_2)$ evolved as a byproduct.

(A) is the correct answer.
Greenhouse crops such as tomatoes and bell pepper produce higher yields because they are grown in a $CO_2$ enriched atmosphere.
These plants are $C_3$ plants,which respond to higher concentrations of $CO_2$ by increasing their rate of photosynthesis,as $CO_2$ is often a limiting factor for them in normal atmospheric conditions.

(C) Blackmann.
The law of limiting factors was proposed by $F.F. Blackmann$ in $1905$ with specific reference to the rate of photosynthesis.
According to this law,if a chemical process is affected by more than one factor,then its rate will be determined by the factor which is nearest to its minimal value,known as the limiting factor.