Chapter 13 Photosynthesis in Higher Plants Solutions
Question - 1 : - By looking at a plant externally can you tell whether a plant is C3 orC4 ? Why and how?
Answer - 1 : -
Though plants whichare adapted to dry tropical climate have the C4 pathway, theydo not show any external morphologic characteristics. Hence it is not possibleto say whether the plant is C3 and C4 bylooking at its external appearance.
Question - 2 : - By looking at which internal structure of aplant can you tell whether a plant is C3 or C4 ?Explain.
Answer - 2 : -
Leaves of C4 plantshave kranz anatomy which makes them different than C3 plants. C4 plantsalso have specialised cells called a bundle sheath cell which surround thevascular bundles. Every cell of the bundle sheath has chloroplasts. Themesophyll cells of leaves do not differentiate into the spongy and palisadeparenchyma possessing less intercellular spaces while there is normaloccurrence of chloroplasts in the mesophyll cells. But in C3 plants,the bundle shealth does not exhibit chloroplast and the mesophyll cells of theleaves are differentiated into the spongy and palisade parenchyma. Hence byexamining the internal structure we can tell whether the plant is C3 orC4.
Question - 3 : - Even though a very few cells in a C4 plant carry out thebiosynthetic – Calvin pathway, yet they are highly productive. Can you discusswhy?
Answer - 3 : -
Plant productivity canbe estimated by the rate at which photosynthesis takes place. The amount ofcarbon dioxide in a plant is directly proportional to the rate ofphotosynthesis. In C4 plants, very few of the cells carry outthe biosynthetic pathway yet they are highly productive for the followingreasons.
- They have a mechanism that increases the CO2 concentration at the site of enzyme.
- Mesophyll cells are broken down in the bundle sheath cells that results in CO2 release which inturn increases the intracellular CO2 concentration .
- Rubisco functions as a carboxylase minimizing the oxygenase activity.
- Increase in photosynthesis make C4 plants more productive.
Question - 4 : - RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Whydo you think RuBisCO carries out more carboxylation in C4 plants?
Answer - 4 : -
The affinity ofRUBISCO is much higher than its affinity for Oxygen. It is the concentration ofOxygen and CO2
that determines thebinding of the enzyme. Mesophyll cells of C4 plants lack thisenzyme but are found in the bundle sheath cells that girdle the vascularbundles where the Calvin cycle occurs.
RuBisco functions asoxygenase when the concentration of Oxygen is higher and it acts as carboxylasewhen the concentration of CO2 is high. In the mesophyll cells,the primary carbon dioxide acceptor is a three carbon compound – phosphoenolpyruvate which is converted into a four-carbon compound, oxaloacetic acid orOAA. This is converted further into malic acid which is transported to thebundle-sheath cells where it undergoes decarboxylation and carbon fixationtakes place through the Calvin cycle which prevents RuBisCo to serve as anoxygenase.
Question - 5 : - Suppose there were plants that had a high concentration of Chlorophyll b,but lacked chlorophyll a, would it carry out photosynthesis? Then why do plantshave chlorophyll b and other accessory pigments?
Answer - 5 : -
In the absence of chlorophyll-aphotosynthesis will not take place because chlorophyll-a is a reaction centreresponsible for the conversion of solar energy into chemical energy. Althoughchlorophyll is the primary pigment that traps sunlight but accessory pigmentslike chlorophyll-b, xanthophylls and carotenoids also absorb sunlight andtransfer energy to chlorophyll-a.
Question - 6 : - Why is the colour of a leaf kept in the dark frequently yellow, or palegreen? Which pigment do you think is more stable?
Answer - 6 : -
Chlorophyll fails toabsorb the energy in the absence of light; hence it loses stability to give theleaf yellow colouration. This proves that carotenoids which imparts yellowcolor is more stable.
Question - 7 : - Look at leaves of the same plant on the shadyside and compare it with the leaves on the sunny side. Or, compare the pottedplants kept in the sunlight with those in the shade. Which of them has leavesthat are darker green ? Why?
Answer - 7 : -
Leaves in the shadeget less sunlight to carry out photosynthesis than the plants kept in sunlight.In leaves that get less sunlight, more chlorophyll is present to carry outphotosynthesis efficiently. In the presence of strong light, the orientation ofthe chloroplasts in the mesophyll cells is irregular and in vertical filesalong the walls. Also, in bright light, photooxidation of chloroplasts takesplace while non-oxidation takes place in shaded regions. Hence the plants keptin shade have dark green leaves due to high concentration of chlorophyll.
Question - 8 : - Figure shows the effect of light on the rate of photosynthesis. Based onthe 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 limiting factor/s in region A?
(c) What do C and D represent on the curve?
Answer - 8 : -
a) Light is a limitingfactor at A and 50% of B which is due to the increase in the photosyntheticrate with increase in the intensity of light
b) Light, CO2 andH2O could be the limiting factor/s in region A.
c) C indicates a stagebeyond which light is not a limiting factor and D is the line beyond which theintensity of light does not affect the photosynthetic rate.
Question - 9 : - Give a comparison between the following:
(a) C3 and C4 pathways
(b) Cyclic and non-cyclic photophosphorylation
(c) Anatomy of leaf in C3 and C4 plants
Answer - 9 : -
a) C3 and C4 pathways
| C3 Pathway | C4 Pathway |
| RUBP is the primary acceptor of CO2 | PEP is the primary acceptor of CO2 |
| 3- Phosphoglycerate is the first stable product | Oxalo-acetic acid is the first stable product |
| Occurs in mesophyll cell of the leaves | Occurs in mesophyll cells and bundle sheath |
| Process of Carbon fixation is slower | Process of Carbon fixation is faster |
b) Cyclic and non-cyclic photophosphorylation
| Cyclic photophosphorylation | Non-cyclic photophosphorylation |
| Occurs only in photosystem-I | Occurs both in Photosystem-I and II |
| ATP is produced | ATP and NADPH2 are produced |
| Photolysis of water does not occur hence Oxygen is not produced | Phtolysis of water occurs hence Oxygen is produced |
| Electrons move in a closed circle | Electrons do not move in a closed circle |
c) Anatomy of leaf in C3 and C4 plants
| Anatomy of leaf in C3 | Anatomy of leaf in C4 |
| Do not possess Kranz anatomy. | They have Kranz anatomy |
| Chloroplasts are not dimorphic | Chloroplasts are dimorphic, organized centripetally. Size of bundle sheaths are larger |
| Mesophyll cells possess intercellular space. | Mesophyll cells do not possess intercellular space. |