NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis
These Solutions are part of NCERT Solutions for Class 11 Biology. Here we have given NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis.
By looking at a plant externally can yon tell whether a plant is C3 or C4 ? Why and how?
C4 plants are adopted to the xerophytic climatic conditions they can grow well in high temperature. It cannot be said conclusively that the plant is a C3 or C4 by looking at external appearance, some guess can be made by looking at fleshy leaf structure of C4 plants.
By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain.
In C4 plant internal structure of leaf possess special type of anatomy called ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ and is a reflection of arrangement of cells.
The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
While in C3 plants, there is no special type of leaf anatomy. There is only a single type of chloroplast inC3 i.e. granal, while in C4 chloroplasts are dimorphic, i.e, granal in the mesophyll cells and agranal in the bundle sheath cells.
Even though a very few cell in a C4 plant carry out the biosynthetic-Calvin pathway, yet they are highly productive, can you discuss why?
(1) In C4 plants, biosynthetic calvin cycle occurs only in bundle sheaths. Despite few number of cells performing calvin cycle in C4 plants, they are highly productive due to minimum photorespiration losses.
(2) They are adopted to diverse climatic conditions as C4 plants can synthesize at very low C02 concentration while for C3 plants C02 concentration is the limiting factor.
(3) C4 plants can synthesize at high temperature while C3 plants cannot.
(4) Rapid withdrawl of photosynthates from the bundle sheath cells as they lie over the vascular bundles.
(5) Photosynthesis continues even when stomata are closed due to fixation of C02 released through respiration.
RuBisCO is an enzyme that acts both as carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants.
RuBisCO or Ribulose bisphosphate carboxylase – oxygenase enzyme can bind to both C02 and O2. This binding is competitive. The relative concentration of C02 and 02 determines which one of the two will bind to the enzyme.
In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of C02 at the enzyme site.
This takes place when oxaloacetic acid is broken down in the bundle sheath cells to release C02.
It results in increasing intracellular concentration of C02. This ensures that the RuBisCO functions as carboxylase and minimising the oxygenase activity.
Suppose there were plants that had a high concentration of chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments.
No, photosynthesis occurs in plants having high concentration of chlorophyll ‘b’ but lacks chlorophyll ‘a’ because chlorophyll ‘a’ molecule forms reaction center in both photosystem I and II which converts light energy into electrical energy and excites the electrons for photolysis of water.
Maximum photosynthesis occurs at the wavelengths which is absorbed by chlorophyll ‘a’ molecule i.e. blue and red regions.
Though chlorophyll a is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoid, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a.
Indeed they not only enable a wider range of wavelength of incoming light to be utilized for photosynthesis but also protect chlorophyll a from photo oxidation.
Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?
Chlorophyll is unable to absorb energy in the absence of light and loses its stability, giving the leaf a yellowish colour. This shows that xanthophyll is more stable.
Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why?
In sunny plant colour of leaves is darker green because in sunny plant photosynthesis takes place while in shady plant rate of photosynthesis is low.
Figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions.
(a) At which point/s (A, B or C) in the curve is light a limiting factor?
(b) What could be the Jimiting factor/s in region A?
(c) What do C and D represent on the curve?
(a) In the region ‘A’ and half of ‘BTight is limiting factor because rate of photosynthesis is increasing with the intensity of light.
(b) All the other factors except light.
(c) C represents a region where factor other than light is limiting, e.g., COu. D represents the light intensity at which rate of photosynthesis is maximum under existing conditions (e.g., C02).
Give comparison between the following:
(a) C3 and C4 pathways
(b) Cyclic and non-cyclic potophasphorylation
(c) Anatomy of leaf in C3 and C4.
(a) Differences between C3 and C4 pathway
(b) Differences between cyclic and non-cyclic photophosphorylation are
(c) Differences between anatomy of leaf in C3 plants and anatomy of leaf in C4 plants are
VERY SHORT ANSWER QUESTIONS
Write one anatomical feature of C4 plants.
Kranz anatomy in leaf.
Which of the following is not a useful function of the light reaction in photosynthesis?
(a) splitting water
(b) synthesis of NADPH
(c) converting light energy into chemical energy
(d) releasing oxygen for photorespiration
(d) Releasing oxygen for photorespiration.
Which one is the most important limiting factor in photosynthesis?
Where is PS II located in a chloroplast?
PS II is located in the appressed regions of grana thylakoid
Name the reaction centre of PS I and PS II.
P700 & P680‘
Mention the two ways in which Ca++ is involved in cell division in plants.Where are the photosynthetic pigments located in a chloroplast?
In the thylakoid membrane.
How many molecules of ATP and how many molecules of NADPH are spent to fix three molecules of C02 in Calvin cycle?
9 ATP and 6 NADPH
Why do the stomata of CAM plants open during night?
As these plants grow in dry area , they keep stomata close during day to conserve water and open their stomata during night for the diffusion of gases.
Mention one useful role of photorespiration in plants.
It protects the plants from photooxidative damage.
Cyanobacteria and some other photosynthetic bacteria don’t have chloroplasts. How do they conduct photosynthesis?
Cyanobacteria have bluish pigment phycocyanin, which they use to capture light for photosynthesis. Some green bacteria (cyanobacteria) are red or pink due to pigment phycoerythrin. Whatever the colour of cyanobacteria, they are photosynthetic and so can manufacture food.
What does the variegated leaf experiment of photosynthesis prove?
It proves that chlorophyll is necessary for photosynthesis.
Name the organism Englemann used in his experiment.
Write the currently accepted equation of photosynthesis in plants.
What is a pigment?
A pigment is a substance that absorbs light of certain wavelength(s).
Write the full form of NADP
NADP – Nicotinamide adenine dinucleotide Phosphate.
Ribulose 1, 5 bisphosphate.
Who discovered C4 cycle?
Hatch and Slack.
Mention two conditions where light can become a limiting factor.
Conditions in which light can become a limiting factor:
(i) Plants in the shade.
(ii) Plants growing under the canopy in a dense forest.
What are antenna molecules?
Antenna molecules are light harvesting pigment molecules that occur on the outer side of a photosynthetic unit.
What is a quantasome? Where is it present?
Quantasome means photosynthetic units. It is equivalent is 230 chlorophyll molecules. These are present in the grana lamellae.
SHORT ANSWER QUESTIONS
Specify how C4 photosynthetic pathway increases carbondioxide concentration in bundle sheath cells of sugarcane?
In C4 pathway of sugarcane, C02 from atmosphere enters through the stomata in the mesophyll cell and combines with phosphoenol pyruvate to form a 4-C compound oxaloacetic acid. The OAA is then transported to the bundle sheath where it is decarboxylatedto release C02 in bundle sheath.
Differentiate between absorption spectrum and action spectrum.
The main differences between absorption spectrum and action spectrum are as followings.
Why does chlorophyll appear red in reflected light and green in transmitted light? Explain the significance of these phenomena in terms of photosynthesis.
In reflected light, the chlorophyll appears red because of fluorescence. The light absorbed by chlorophyll molecules loses its energy and emits light of wavelengths corresponds to red colour. In transmitted light, chlorophyll appears green because it absorbs only light of wavelengths correspond to green colour.
How does temperature influence the biosynthetic phase of photosynthesis ?
Influences of temperature on the biosynthetic phase of photosynthesis :
(i) At higher temperature enzymes become inactive as it gets denatured.
(ii) At low temperature also enzyme become ‘ inactive.
(iii) Affinity of the enzymes for C02 decreases with increasing temperature.
What is Blackman’s law of limiting factors ?
F.F. Blackman (1905) extended a law to formulate the principle of limiting factors. “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of slowest factors.”
In the condition of water stress why the rate of photosynthesis declines?
Due to water stress stomata remain closed and so there is decrease in C02concentration and the leaf water potential is also reduced, decline the rate of photosynthesis.
Why do scientist expect faster growth and more yield by C3 plants, if the atmospheric C02 increases?
If the concentration of C02 in the atmosphere increases, the rate of photosynthesis by C3 plants will increase for following two reasons.
(a) Fligh availability of substrate (C02) for carboxylation.
(b) Photorespiration is reduced due to more availability of C02 as enzyme will function only as carboxylase.
Why is photorespiration considered a wasteful process?
Photorespiration considered a wasteful process because
(i) 25% of photosynthetically fixed carbon is lost in the form of C02.
(ii) There is no energy rich useful compound produced during this process.
Give two reasons as to why photosynthesis is important for sustaining life on earth.
Photosynthesis is the most important process because;
(i) it is the only natural process by which oxygen is liberated into the atmosphere.
(ii) it is the process by which food is manufactured for all living organisms.
Why does the rate of photosynthesis decrease at higher light intensities? What plays a protective role in such situations?
Rate of photosynthesis decreases for two reasons :
(i) Other factors required for photosynthesis become limiting.
(ii) Destruction of chlorophyll by photo oxidation.
Carotenoids play a protective role by:
(i) absorbing the excess light and
(ii) acting as antioxidant to detoxify the effect of activated oxygen species.
Expand PEP. Where is it produced in C4 plants? What is its role in the biosynthetic process?
PEP – phosphoenolpyruvate. It is produced in the mesophyll cells of leaves of C4 plants. It is the primary acceptor of carbon dioxide and is converted into oxaloacetic acid (OAA). Thus it helps in carbon fixation in these plants. By this pathway the carbon dioxide concentration in the bundle sheath increases and photorespiration is prevented from occurring.
What is kranz anatomy in plants?
In Kranz Anatomy vascular bundles are surrounded by a layer of bundle sheath that contains large number of chloroplasts in mesophyl cells and it is present in C4 plants e.g, Maize, Sugarcane, etc.
LONG ANSWER QUESTIONS
How is photosystem I different from photosystem II?
The main differences between photosystem I and photosystem II are
Give the importance of C4 plants.
Importance of C4 plants are :
(1) C4 plants have little photorespiration,.
(2) C4 plants are more efficient in picking up C02 even when it is found in low concentration because of high affinity of phosphoenolpyruvate (i.e. PEP).
(3) Concentric arrangement of mesophyll cells produces a smaller area in relation to volume for better utilization of available water and rdduce the intensity of solar radiation.
(4) They are adapted to high temperature and intense radiation.
(5) It prevents photorespiration
Explain the process of biosynthetic phase of photosynthesis occurring in the chloroplasts.
Biosynthetic phase of photosynthesis :
• It occurs in the stroma of chloroplasts.
• These reactions reduce the carbon dioxide into carbohydrates, making use of the ATP and NADPH2 produced in the photochemical reactions.
• The reactions are also called as Calvin cycle.
• The three phases of Calvin cycle are as follows:
Six molecules of Ribulose 1,5 bisphosphate
react with six molecules of carbon dioxide to form six molecules of a short-lived 6C- compound.
The reaction is catalysed by RuBP carboxylase (RuBisCo).
The six molecules of the 6C-intermediate break into 12 molecules of 3- phosphoglyceric acid (3-PGA), a SC- compound.
It is through this step that carbon dioxide is fixed in the plant.
12 molecules of 3-phosphoglyceric acid are converted into 12 molecules of 1, 3 diphospho-glyceric acid, utilising 12 molecules of ATP and then reduced to 3- phosphoglyceraldehyde making use of 12 molecules of NADPH. Two molecules of phosphoglyceraldhyde react to form one molecule of glucose. It is in this step that there is actual reduction of carbon dioxide leading to sugar formation.
(iii) Regeneration of RuBP
10 molecules of phosphoglyceraldehyde, by a series of complex enzyme-catalysed reactions, are converted into six molecules of ribulose 1,5-bisphosphate; six molecules of ATP are needed for this step. This step of ‘ regeneration of RuBP is important for the cycle to continue
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