Short & Long Question Answers with Solution: Photosynthesis in Higher Plants | Biology for Grade 12 PDF Download

Short & Long Type Questions

Q.1. Define accessory pigments. State their significance in photosynthesis.
Ans: Chlorophyll-b, xanthophylls, and carotenoids, often referred to as photosynthetic pigments, play a crucial role in capturing sunlight and transferring it to chlorophyll-a. These pigments are not directly responsible for releasing excited electrons. In the process, these pigments themselves primarily absorb sunlight in the blue and green regions. Consequently, chlorophyll-a is considered the principal pigment, while the other pigments, namely chlorophyll-b, xanthophylls, and carotenoids, function as accessory pigments.

Q2. What is the relationship between photosynthesis and respiration?
Ans: These processes are interconnected. In both photosynthesis and respiration, plants acquire energy: photosynthesis harnesses energy from solar radiation, while respiration breaks down glucose molecules to generate ATP energy. These two processes rely on each other. Respiration utilizes the final product of photosynthesis, glucose in the form of food, to produce ATP. As a result of this process, carbon dioxide and water are released, and photosynthesis then utilizes these compounds to generate additional sugars.

Q3. How are short plants growing below thick canopies and receiving filtered light able to carry out photosynthesis in tropical rain forests?
Ans: Photosynthesis occurs in plants when they are exposed to light. The quality and intensity of light are the sole factors that influence the rate of photosynthesis. Consequently, the rate of photosynthesis may vary among plants growing in different canopy regions.

Q4. In a C3 plant, a light dependent cyclic process is occurring that requires oxygen. Instead of producing, it consumes energy.
a) Name the process
b) Is it necessary for survival?
c) Write the end products of this process.
d) Where does it take place?
Ans:
a) Photorespiration
b) Not necessary for the survival of a C3 plant.
c) The end product – H₂O₂
d) Three cell organelles are involved – chloroplast, peroxisome, mitochondria

Q5. Explain why is the colour of a leaf in the dark changes?  Which pigment do you think is more stable?
Ans: When a leaf is kept in darkness, its color transitions from green to a pale green or yellow hue. This change primarily occurs due to the absence of sunlight and the chlorophyll pigment. In the absence of light, such as in darkness, photosynthesis doesn't occur, resulting in no synthesis of starch or chlorophyll pigment. Among the pigments, chlorophyll a is the most stable.

Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?
Ans: It is essential to have light to carry out photosynthesis in leaves. When a dark leaf is given more exposure to sunlight, the colour of the leaf changes into a lighter shade through a gradual process. This is due to a green pigment called chlorophyll present in the leaf. The production of chlorophyll pigment is directly proportional to the amount of sunlight available. When light is absent, the amount of chlorophyll and its production stops; as a result, they degenerate. Due to this, the colour of the leaf changes from bright green to a lighter shade of green. 

While this process occurs, the xanthophyll and carotenoid pigments become predominant, which causes the leaf to become yellow. The xanthophyll and carotenoid pigments are more stable and remain unaffected in the absence of light.

Q7. Does photosynthesis occur in leaves only? If no, what are the other parts that are capable of carrying out photosynthesis? Justify.
Ans: The process of photosynthesis primarily occurs in leaves of all green plants. The leaves are designed in such a way that they capture sunlight and convert solar energy into chemical energy effectively. However, in some plants, modified parts other than leaves carry out photosynthesis. In some plants, roots develop chlorophyll and initiate photosynthesis, they are referred to as assimilatory roots. Example – Tinospora, Trapa. The stem in some plants such as the Opuntia gets modified and adapts to perform the functions of leaves, be turning thick, succulent and flattened, performing photosynthesis, these structures are referred to as phylloclade. In Australian Acacia, the petiole takes the shape and function of photosynthesis when stamina of leaf fails.

Q8. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?
Ans: The full form of RuBisCO is ribulose-1,5-bisphosphate carboxylase/oxygenase. RuBisCO is formed from Ribulose bisphosphate (RuBP), a five-carbon ketose sugar. RuBisCO is termed the most abundant enzyme on earth. It has a unique feature where it has two active sites and can bind with CO₂ and O₂. The relative concentration of CO₂ and O₂ determines which of the two will bind to this enzyme. This enzyme is present in bundle-sheath cells that are around the vascular bundles. However, they are absent in mesophyll cells of C4 plants. The Calvin cycle occurs in the bundle sheath of C4 plants. 

Phosphoenolpyruvate is a three-carbon compound that is a primary CO₂ acceptor in these mesophyll cells. Phosphoenolpyruvate is converted into a four-carbon compound called oxaloacetic acid (OAA), which is converted into malic acid. Bundle-sheath cells receive the malic acid where further steps of the Calvin cycle are completed. The malic acid undergoes decarboxylation and CO₂ fixation. 

During the C4 pathway, the malic acid from mesophyll cells gets broken down in the bundle sheath. In this process, CO₂ gas is released, and the intracellular concentration of CO₂ increases. This prevents RuBisCO from acting as an oxygenase and functions like carboxylase. While functioning as carboxylase, it binds with the plants and carries out carboxylation in them.

Q9. List the location in the cell where the following reactions take place during the process of photosynthesis.
a) Synthesis of NADPH and ATP
b) Photolysis of water
c) CO₂ fixation
d) Synthesis of sugar molecule
e) Synthesis of starch
Ans: 
a) Outer side of the thylakoid membrane
b) The inner side of the thylakoid membrane
c) Stroma of chloroplast
d) Chloroplast
e) Cytoplasm

Q10. Which property of the pigment is responsible for its ability to initiate the process of photosynthesis? Why is the rate of photosynthesis higher in the red and blue regions of the spectrum of light?
Ans: Substances that absorb light at specific wavelengths are called pigments. This implies that different pigments absorb light of different colours. Pigments have the property of excitability. Pigments get excited by absorbing light and providing energy. They send electrons in an excited state. This light can be utilised for the next steps of photosynthesis. 

The most abundant pigment in plants is chlorophyll A. It shows optimum efficiency between red and blue wavelengths. This property is also shown by other pigments but is primarily shown by chlorophyll A. By exhibiting the property of maximum absorption and excitation during the emission of red and blue wavelengths; chlorophyll A initiates the process of photosynthesis.

Q11. The rate of photosynthesis decreases at higher temperatures. Why?
Ans: The process of photosynthesis is enzyme-specific. Enzymes work efficiently when they are exposed to an optimum range of temperatures. The optimum temperature range of enzymes lies between 25 to 35∘C. When exposed to higher temperatures above optimum value, the enzyme gets denatured. This hampers the photosynthetic process. As a result, under high-temperature conditions, the rate of photosynthesis decreases.

Q12. Name the pigment that is responsible for its ability to initiate the process of photosynthesis. The rate of photosynthesis is higher in the red and blue regions of the spectrum of light, why?
Ans: The thylakoid membranes contain the chlorophyll pigments which have the property of excitability and emits e– in the excited stage, though it is replaced and transferred by the e– produced from splitting of water molecules. Chlorophyll pigments absorb the maximum energy possessed by the red and blue light, thereby getting excited and initiating photosynthesis. Its wavelength lies between the PAR (Photosynthetic Active Radiation). Hence the rate is higher in the red and blue regions.

Q13. In which cells of the leaf, is pyruvate converted to PEP in the C4 pathway?
Ans: A plant that fixes Co2 in a molecule consisting of four carbon atoms before the initiation of the Calvin cycle during the process of photosynthesis is called a C4 plant. 

In plants following the C4 pathway, initial fixation takes place in mesophyll cells. Phosphoenolpyruvate is the primary acceptor of CO₂. Co2 gets fixed in the bundle sheath during the Calvin cycle. The pyruvate is sent back where it gets converted to phosphoenolpyruvate by mesophyll cells. Hence in mesophyll cells of the leaf, pyruvate is converted to PEP in the C4 pathway.

Q14. How do photosynthetic bacteria such as cyanobacteria conduct photosynthesis in the absence of chloroplasts?
Ans: Photosynthesis takes place within the chloroplasts, small organelles found inside plant and algal cells. Prokaryotes like cyanobacteria and photosynthetic bacteria lack distinct organelles enclosed by membranes. However, they possess photosynthetic pigments in primitive membranous forms.

Due to their primitive nature, these pigments are capable of capturing solar energy and using it for subsequent steps. Oxygenic cyanobacteria, in particular, possess pigments that are more primitive and include light-harvesting pigments. They absorb carbon dioxide and release oxygen, facilitating the process of photosynthesis.

Q6. What are the important events and end products of the light reaction?
Ans: The important events occurring in the light reaction and its end products are as follows:

• Absorption of light: 
  The process of light absorption is carried out by the light-harvesting complex (LHC). Different photosystems from the light-harvesting system have different pigments except for chlorophyll A. The pigments are also called antennae. These pigments increase the system’s efficiency by absorbing light of different wavelengths. The chlorophyll A molecule works as a reaction centre. By absorbing light, these chlorophyll molecules emit a pair of electrons and utilise their energy to form ATP from ADP + Pi. The process is called photophosphorylation.
• Splitting water: 
  Energy absorbed from the sunlight is utilised to split water. The water molecules are converted into hydrogen ions and oxygen. At the same time, the extra electrons released after the splitting process are utilised in the conversion of NADP+ to NADPH.
  The splitting reaction of the water molecule is given as:
  H₂O⟶4H+4e−+O₂↑ (b) NADP+2H+⟶NADPH
• Release of oxygen
  Oxygen is released after the reaction in splitting the water molecule. This oxygen goes out through the stomata.
• Formation of high-energy intermediates like ATP and NADH:
  This is the final step of the light reaction. At the end of this reaction, the two energy-rich products formed are ATP and NADH. These products are utilised for further reactions.