Factors affecting photosynthesis Questions in English

(C) The correct answer is $C$.
In dry habitats (xerophytic conditions),plants possess a thick,waxy cuticle on the surface of their leaves.
This adaptation is essential to minimize the rate of transpiration and prevent excessive water loss,allowing the plant to survive in water-scarce environments.

(A) Stomata that open at night and close during the day are known as $scotoactive$ stomata.
This mechanism is an adaptation to reduce water loss through transpiration in arid environments.
These are typically found in succulent plants such as $Opuntia$,$Aloe$,and $Bryophyllum$,which are classified as $Xerophytes$.

(B) The transpiration ratio is defined as the amount of water lost through transpiration per unit of $CO_2$ fixed during photosynthesis.
Since plants need to open their stomata to take in $CO_2$ for photosynthesis,they inevitably lose water vapor through the same pores.
$A$ lower transpiration ratio indicates that the plant is highly efficient at fixing $CO_2$ while losing the minimum amount of water.
Therefore,this ratio serves as a measure of the effectiveness of stomata in balancing the trade-off between maximizing photosynthesis and minimizing water loss.

(B) Mansfield $(1965)$ reported that the removal of $CO_2$ maintains stomatal opening even in the dark.
Conversely,stomata tend to close in the presence of light if the $CO_2$ concentration is increased around the leaf surface.
Therefore,an increase in $CO_2$ concentration leads to the partial closing of stomata.

(A) Scotoactive stomata are those that open during the night and remain closed during the day. This phenomenon is typically observed in succulent plants ($CAM$ plants). The mechanism of scotoactive stomatal opening was explained by Nishida in $1963$.

(B) In plants like $Lotus$ (Nelumbo),the leaves are $epistomatic$,meaning stomata are present only on the upper surface of the leaves.
Coating the upper surface with wax will block these stomata,thereby inhibiting gaseous exchange ($CO_2$ uptake for photosynthesis and $O_2$ release) and stomatal transpiration.
Since $Hydrilla$,$Pistia$,and $Vallisneria$ are aquatic plants with different stomatal distributions or adaptations (often $hypostomatic$ or submerged),the impact of waxing the upper surface is most critical in $Lotus$ where the upper surface is the primary site for gas exchange.

(A) The law of limiting factors for photosynthesis 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.
$Blackman$ also proposed the existence of a light-independent (dark) phase in photosynthesis.

(C) The 'law of minimum' was proposed by Justus von Liebig in $1840$.
It states that the growth of an organism is limited by the scarcest or 'limiting' resource,even if all other factors are abundant.
This concept was later adapted into the 'law of limiting factors' in plant physiology,particularly in the context of photosynthesis by Blackman.

(D) The graph shows that at low light intensities,the rate of photosynthesis is the same for both untreated and treated $Chlorella$ samples,indicating that light is the limiting factor and potassium cyanide has no effect here.
As light intensity increases,the rate of photosynthesis in the untreated sample continues to rise,whereas in the treated sample,it plateaus.
Potassium cyanide is a known inhibitor of the dark reaction (Calvin cycle) by inhibiting enzymes like cytochrome oxidase. Since the dark reaction becomes the limiting factor at higher light intensities,the inhibition by potassium cyanide becomes apparent only at these higher intensities.
Therefore,it can be deduced that potassium cyanide has no inhibiting effect on photosynthesis at low light intensities,where the light reaction is the limiting factor.

(D) The correct answer is $D$.
Plants capture only a small fraction of the total solar radiation reaching the Earth's surface.
Out of the total photosynthetically active radiation $(PAR)$,higher plants utilize only about $1-4\%$ of the light energy for the process of photosynthesis.
Therefore,$1$ to $2\%$ is the most appropriate range among the given options.

(A) $Lux$ meter is an instrument specifically designed to measure the intensity of light in $lux$ units.
$Wilmott's$ bubbler is used to measure the rate of photosynthesis by counting oxygen bubbles.
$Ganong's$ potometer and $Farmer's$ potometer are used to measure the rate of transpiration in plants.
Therefore,the correct instrument for measuring light intensity is the $Lux$ meter.

(C) The photosynthetic quotient $(PQ)$ is defined as the ratio of the volume of $O_2$ evolved to the volume of $CO_2$ consumed during photosynthesis.
For a typical green leaf performing photosynthesis,the chemical equation is $6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$.
Since the volume of $O_2$ released is equal to the volume of $CO_2$ absorbed,the ratio is $6/6 = 1$.
Therefore,the common value of $PQ$ for a leaf is $1$.

(B) Solarisation is a phenomenon where high-intensity light causes the photo-oxidation and subsequent destruction of chlorophyll pigments in plants.
This process typically occurs when plants are exposed to excessive solar radiation,leading to a reduction in photosynthetic efficiency.

(D) At very high light intensities,the rate of photosynthesis declines due to several factors:
$1$. Photo-oxidation of chlorophyll: Excessive light energy can damage the chlorophyll molecules,leading to their breakdown.
$2$. Increased respiration: High light intensity is often accompanied by high temperatures,which increase the rate of respiration,thereby consuming more of the produced carbohydrates.
$3$. Decreased hydration: High light intensity and heat lead to increased transpiration,which can cause water stress and stomatal closure,limiting $CO_2$ availability.
Therefore,all these factors contribute to the decline in the rate of photosynthesis.

(D) The correct option is $D$.
The proteinaceous photosynthetic unit $(PSU)$ size increases as light levels decline.
At lower light levels,chloroplasts have a larger $PSU$ to increase the probability that a photon will strike the chlorophyll antenna.
Therefore,plants adapted to low light intensity have a larger photosynthetic unit size compared to sun plants.
Plants adapted to low light intensity do not have a higher rate of $CO_2$ fixation,nor do they typically have more extended root systems or leaves modified into spines (which are adaptations for arid environments).
Thus,the correct answer is option $D$.

(D) The temperature coefficient $(Q_{10})$ is defined as the ratio of the rate of a reaction at a temperature $T + 10^{\circ}C$ to the rate of the reaction at temperature $T$.
For most biological processes,including photosynthesis,the $Q_{10}$ value is approximately $2$ within the physiological temperature range.
This means that for every $10^{\circ}C$ rise in temperature,the rate of photosynthesis approximately doubles,provided other factors are not limiting.

(D) $C_3$ plants are less water-efficient compared to $C_4$ plants.
To produce $1$ gm of dry matter,$C_3$ plants typically require approximately $600-650$ gm of water (average $610$ gm).
In contrast,$C_4$ plants are highly efficient and require only about $250-350$ gm of water (average $300$ gm) to produce the same amount of dry matter.
Therefore,the water requirement for $C_3$ and $C_4$ plants is $610$ gm and $300$ gm respectively.

(C) In cold and foggy areas,the intensity of light is significantly reduced due to the presence of fog.
Additionally,the ambient temperature is low,which reduces the rate of enzymatic reactions involved in photosynthesis.
According to Blackman's Law of Limiting Factors,when multiple factors affect a process,the rate is determined by the factor that is nearest to its minimal value.
In such environments,both light and temperature act as limiting factors,as they are both present at suboptimal levels for the photosynthetic process.

(B) The correct answer is $B$.
Certain species of algae that inhabit hot springs or high-temperature ponds have evolved to perform photosynthesis at significantly higher temperatures than typical plants.
Research indicates that while most plants cease photosynthetic activity at lower temperatures,these specialized algae can maintain photosynthetic processes at temperatures up to $75^\circ C$.

(A) The German scientist Warburg $(1920)$ reported in the alga $Chlorella$ that a high $O_2$ level inhibits the rate of photosynthesis.
This inhibition of photosynthesis caused by an increased $O_2$ concentration is known as the Warburg effect.

(B) When $NaHCO_3$ (sodium bicarbonate) is added to the experimental setup,it acts as a source of $CO_2$.
Since $CO_2$ is a limiting factor for photosynthesis,its increased availability enhances the rate of the photosynthetic process.
Therefore,the rate of photosynthesis will be increased.

(B) The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point,the amount of $CO_2$ produced during respiration is exactly equal to the amount of $CO_2$ consumed during photosynthesis. Consequently,there is no net exchange of $CO_2$ or $O_2$ with the atmosphere. This typically occurs during the early morning and late evening hours when light intensity is low.

(A) According to Blackman's Law of Limiting Factors,when several factors affect any biochemical process,the rate of the process is limited by the factor which is nearest to its minimal value.
Factors such as $CO_2$ concentration,light intensity,temperature,and water are external limiting factors.
Chlorophyll is an internal factor that influences the rate of photosynthesis.
Oxygen is a byproduct of photosynthesis and is not a limiting factor for the process itself under normal physiological conditions.

(D) Photosynthesis produces carbohydrates (food) in the leaves.
These carbohydrates are translocated to other parts of the plant through the phloem.
If the rate of translocation is slow,the synthesized food accumulates in the leaves.
This accumulation of end products (feedback inhibition) inhibits the process of photosynthesis,leading to a decrease in its rate.

(C) The atmospheric concentration of $CO_2$ is approximately $0.03\%$ to $0.04\%$,which is equivalent to $300-400 \, ppm$. Since $300 \, ppm$ is the ambient level of $CO_2$ in the atmosphere,the plant will perform photosynthesis at its normal,baseline rate. Therefore,the correct option is $(c)$.

(A) The light reaction is significantly faster than the dark reaction. In continuous light,there is an accumulation of $ATP$ and $NADPH_2$,which can lead to a reduction in the overall rate of photosynthesis due to feedback inhibition. However,in intermittent (discontinuous) light,the $ATP$ and $NADPH_2$ produced during the light phase are efficiently consumed during the dark phase for the reduction of $CO_2$ into carbohydrates. This prevents the accumulation of products and allows the photosynthetic process to proceed more efficiently,thereby increasing the overall rate.

(C) When a plant is exposed to very high light intensity,it leads to a phenomenon known as solarization or photo-oxidation.
In this process,the chlorophyll pigments are damaged,and the photosynthetic apparatus is destroyed.
Consequently,the rate of photosynthesis decreases significantly and may eventually stop.

(B) Water is a vital reactant in the process of photosynthesis. When water supply is extremely low,it leads to the wilting of leaves.
Wilting causes the stomata to close to prevent further water loss through transpiration.
Closing of stomata restricts the entry of $CO_2$ into the leaves,which is essential for the dark reaction of photosynthesis.
Additionally,water stress affects the metabolic activities of the plant and the structural integrity of the chloroplasts.
Therefore,under conditions of severe water stress,the rate of photosynthesis decreases significantly.

(D) The rate of photosynthesis is influenced by light quality (wavelength) and temperature.
$1$. Light Quality: Photosynthesis is most efficient in red and blue light and least efficient in green light,as chlorophyll reflects green light.
$2$. Temperature: According to the $Q_{10}$ effect (often related to Van't Hoff's law),the rate of enzymatic reactions like photosynthesis generally increases with temperature up to an optimum point (around $30^{\circ}C$ to $35^{\circ}C$).
- Plant $X$ is at $10^{\circ}C$ in daylight (white light,which contains red and blue wavelengths).
- Plant $Y$ is at $20^{\circ}C$ in red light (highly effective for photosynthesis).
- Plant $Z$ is at $30^{\circ}C$ in green light (least effective for photosynthesis).
Comparing these,plant $Z$ will photosynthesize the slowest because green light is poorly absorbed by chlorophyll,despite the higher temperature. Plant $Y$ will photosynthesize the fastest because it is exposed to red light at a favorable temperature $(20^{\circ}C)$. Therefore,$Z$ is slowest and $Y$ is fastest.

(C) According to Blackman's Law of Limiting Factors,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: it is the factor which directly affects the process if its quantity is changed.
In nature,$CO_2$ concentration is usually the major limiting factor for photosynthesis because its atmospheric concentration is very low $(0.03-0.04\%)$ compared to the requirement for optimal photosynthetic activity. While light and temperature also affect the rate,$CO_2$ is frequently the limiting factor under natural conditions.

(C) According to Blackman's Law of Limiting Factors,when several factors affect any biochemical process,the rate of the process is limited by the factor which is present in the lowest concentration relative to its optimum requirement.
On a clear,sunny day,light is abundant and water is usually sufficient. However,the atmospheric concentration of $CO_2$ is very low (approximately $0.03-0.04\%$) compared to the amount required for optimal photosynthesis.
Therefore,$CO_2$ acts as the primary limiting factor for photosynthesis under these conditions.

(C) The rate of photosynthesis is influenced by several external and internal factors.
$1$. Light quality (wavelength) affects the absorption spectra of pigments like chlorophyll $a$ and $b$.
$2$. Light intensity directly affects the rate of the light-dependent reactions.
$3$. Temperature affects the activity of enzymes like $RuBisCO$ involved in the Calvin cycle.
$4$. Duration of light (photoperiod) generally affects flowering and plant growth,but the instantaneous rate of photosynthesis is not directly dependent on the duration of light exposure.

(D) The law of limiting factor was proposed by Blackman in $1905$.
He stated 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.
This law is specifically applied to the process of photosynthesis,where factors like light intensity,$CO_2$ concentration,and temperature act as limiting factors.

(C) According to Blackman's Law of Limiting Factors,photosynthesis is limited by the factor that is present in the lowest concentration relative to its optimum requirement.
Under normal environmental conditions,$CO_2$ and light are often limiting factors.
Water is rarely a limiting factor for photosynthesis because even a slight decrease in water content leads to the closure of stomata,which primarily affects $CO_2$ availability rather than acting as a direct limiting factor for the photosynthetic process itself.
However,among the given options,water is the factor that is least likely to be limiting under normal conditions compared to the others.

(D) Photosynthesis is a physiological process that is governed by Blackman's Law of Limiting Factors.
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.
Light,wind,and moisture are environmental factors,but photosynthesis does not simply stop at a 'limit' of these in a linear sense; rather,the rate of the process is limited by the factor in the shortest supply.
Therefore,none of the options provided correctly describe the physiological limit of photosynthesis in a scientific context.

(C) Photosynthesis is the process by which green plants synthesize food using chlorophyll,sunlight,water,and $CO_2$.
Leaves are the primary sites of photosynthesis.
When half of the leaves are removed,the total surface area available for light absorption and gas exchange is significantly reduced.
Consequently,the overall rate of photosynthesis for the entire plant decreases.
Since the rate of photosynthesis is lower,the total amount of food (photosynthetic yield) produced by the plant will also be less.

(B) The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration.
At this point,the net gas exchange is zero,meaning the $CO_2$ produced during respiration is entirely consumed by photosynthesis,and the $O_2$ produced during photosynthesis is entirely consumed by respiration.
This condition typically occurs during $Twilight$ (dawn or dusk),when the light intensity is low and gradually changing,allowing the photosynthetic rate to match the respiratory rate.

(D) The correct option is $D$.
Atmospheric carbon dioxide $(CO_2)$ is a limiting factor for photosynthesis.
At a concentration of $300 \, ppm$ (which is approximately the current ambient level),plants can perform photosynthesis efficiently.
Since $CO_2$ is a primary substrate for the Calvin cycle,this concentration supports optimal carbon fixation,allowing plants to thrive and grow well.

(D) The 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 $CO_2$ produced during respiration is exactly consumed by photosynthesis,and the amount of $O_2$ produced during photosynthesis is exactly consumed by respiration.
Therefore,there is no net exchange of gases with the external environment.

(C) Plants perform photosynthesis during the day (releasing $O_2$) and respiration throughout the $24$-hour cycle (releasing $CO_2$). At twilight (dawn or dusk),the light intensity is such that the rate of photosynthesis exactly equals the rate of respiration. This is known as the compensation point. At this specific intensity,the $O_2$ produced in photosynthesis is entirely consumed by respiration,and the $CO_2$ produced in respiration is entirely consumed by photosynthesis. Consequently,there is no net exchange of gases with the atmosphere.

(D) . Stomatal opening is indeed regulated by various environmental factors and the influx of $K^+$ ions.
$B$. While most enzymes are proteins (ribozymes being the exception),not all proteins function as enzymes.
$C$. All angiosperms are seed-bearing,but gymnosperms are also seed-bearing plants,making the statement correct.
$D$. This statement is incorrect. Factors that affect respiration (like temperature,$CO_2$ concentration,and $O_2$ availability) also significantly influence photosynthesis,and vice versa. Both processes are metabolically linked and influenced by similar environmental parameters.

(B) Photosynthesis is a process that is highly sensitive to temperature because it involves enzymes like RuBisCO,which are easily denatured at higher temperatures.
Respiration,on the other hand,is a catabolic process involving a series of enzymatic reactions that are generally more heat-stable than those involved in photosynthesis.
Therefore,when the temperature exceeds $35^{\circ}C$,the rate of photosynthesis begins to decline due to the thermal inactivation of enzymes and the closure of stomata,while the rate of respiration continues to increase or remains stable for a longer period before it eventually declines.
Thus,the rate of decline of photosynthesis occurs earlier than the decline of respiration.

(B) At a light intensity of $0$ units,photosynthesis cannot occur because light is an essential requirement for the light-dependent reactions of photosynthesis.
However,plant cells continue to perform cellular respiration $24$ hours a day to generate energy $(ATP)$ for survival.
During respiration,plants consume oxygen and release carbon dioxide as a byproduct.
Since there is no photosynthesis to consume the carbon dioxide produced by respiration,the net result is the release of carbon dioxide into the environment.

(C) $DCMU$ (Dichlorophenyl dimethyl urea) is a potent herbicide.
It acts by inhibiting the electron transport chain in photosynthesis.
Specifically,$DCMU$ binds to the $Q_B$ binding site of the $D1$ protein in $PS-II$ (Photosystem $II$),thereby blocking electron flow from $PS-II$ to plastoquinone.
This inhibition stops the light-dependent reactions,preventing the formation of $ATP$ and $NADPH$,which are essential for carbon dioxide fixation in the Calvin cycle.
Therefore,it effectively kills the plant by inhibiting carbon dioxide fixation.

(A) The concentration of $CO_2$ is a critical factor for photosynthesis. While $CO_2$ is essential for plant growth,an excess amount of $CO_2$ beyond the optimal level can lead to physiological stress and toxicity in plants. High concentrations of $CO_2$ can cause stomatal closure,reduce transpiration,and inhibit metabolic processes,which ultimately retards the overall growth of the plant.

(A) Visible light (specifically the photosynthetically active radiation or $PAR$) is the primary source of energy for plants to perform photosynthesis. Since photosynthesis is the fundamental process for biomass production and growth,providing plants with visible light allows them to synthesize carbohydrates,which directly supports their normal growth and development.

(B) Plants primarily depend on $CO_2$ for the process of photosynthesis,which is essential for their growth and survival.
While plants also require $O_2$ for cellular respiration,the primary gas that defines their autotrophic nature and biomass production is $CO_2$.
Therefore,the availability of $CO_2$ is the most critical factor for plant life among the given options.

(B) The accumulation of dust on the leaf surface acts as a physical barrier.
This layer of dust reduces the intensity of sunlight reaching the leaf surface.
Since the opening of stomata is light-dependent,a reduction in light intensity leads to the closure of stomata.

(C) The correct answer is $C$.
Algae are primary producers in aquatic ecosystems.
In large water bodies,the surface area available for photosynthetic activity is vast,and the density of phytoplankton (like algae) is extremely high.
These organisms efficiently capture solar energy to synthesize organic food material through photosynthesis.
Compared to terrestrial plants,the rapid growth rate and high biomass production of algae in large water bodies allow for a higher total capture of solar energy per unit area.

(B) The compensation point is the specific light intensity at which the rate of photosynthesis exactly equals the rate of respiration in a plant.
At this point,the amount of $CO_2$ produced during respiration is exactly consumed by photosynthesis,and the amount of $O_2$ produced during photosynthesis is exactly consumed by respiration.
Consequently,there is no net exchange of gases between the plant and the atmosphere.
Therefore,the correct option is $B$.