Coupon Accepted Successfully!



By looking at a plant externally can you tell whether a plant if C3 of C4? Why and how?

As compared to C3 plants, C4 plants are adapted to dry tropical regions. Such plants are able to tolerate high temperatures and show marked responses towards high light intensities. Also C4 plants have a greater productivity of biomass.


By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.

By studying the anatomical details of the leaf. We can tell whether a plant is C3 or C4.

C4 plants show typical Kranz anatomy.

The vein of the leaf is surrounded by bundle sheath which contains numerous chloroplast of mesophyll cells have well developed grana, while chloroplasts of bundle sheath have peripheral reticulum.


Even though a very few cells in a C4 plant carry out the biosynthetic Calvin pathway, yet they are highly productive. Can you discuss why?

Even though very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. This is because of the following reasons:

(a) C4 plants have a high value of net assimilation at high temperatures.

(b) Their rate of carbon assimilation is quite rapid.

(c) C4 plants perform photosynthesis even when stomata are closed.

(d) Their rate of net photosynthesis at full sunlight is as high as 40-80 mg CO2/ dm2/hr.


Rubisco is an enzyme that acts both as a carboxylase and oxygenase. Why do you think Rubisco carries out more carboxylation in C4 plants?

Rubisco, i.e., ribulose biphophate carboxylase and oxygenase. This is because its active site can bind to both CO2 and O2. But actually rubisco has much greater affinity for CO2, than O2. The relative concentration of CO2 and O2 determines which of the two will bind to the enzyme. In case of C4 plants, since there is a high concentration of CO2 plants, at the enzyme site, photorespiration does not occur. Therefore rubisco carries out more carboxylation in C4 plants.


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 and other accessory pigments?

If there were complete absence of chlorophyll a in a plant, it would not carry out photosynthetic activity at all because chlorophyll a is the chief pigment associated with photosynthesis as it traps light. Other accessory pigments like chlorophyll b, santhophylls and carotenoids are equally essential as they also absorb light and transfer energy to chlorophyll a. They also enable a wider range of wavelength of incoming light to be utilised for photosynthesis and 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?

The colour of leaf kept in dark frequently changes because of absence of chlorophyll in dark- Carotenoids and Xanthophyll pigments are 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?

The plants placed in light will have darker leaves as compared to leaves of a plant placed in shade. It is due to the fact that chlorophyll pigments will be more in light.


Compare the following:

(a) C3 and C4 pathways

(b) Cyclic and non- cyclic photophophorylation


(c) Anatomy of leaf in C3 and C4 plants


(a) C3 and C4 pathways.


C3 plants

C4 plants

1. CO2 acceptor 

The CO2 acceptor is ribulose 1.5 diphosphate.

The CO2 acceptor is phosphoenol-pyruvate. 

2. First stable product The fist stable product is phosphoglyceric acid. Oxaloacetate is the first stable product.
3. Type of chloroplast All cells participating in photosynthesis have one type not chloroplast. The chloroplast of parenchymatous bundle sheath is different from that of mesophyll cells. Leaves have β€˜Kranz’ type of anatomy. The bundle sheath chloroplasts lack grana. Mesophyll cells have normal chloroplasts.
4. Cycles Only reductive pentose phosphate cycle is found. Both C4- dicarbodylic acid and reductive pentose phosphate cycles are found.
5. Optimum temperature The optimum temperature for the process is 10-250C. In C4 plants it is 30-450C.
6. Oxygen inhibition Oxygen present in air (= 21% O2). markedly inhibit the photosynthetic process as compared to an external atmosphere containing no oxygen. The process of photosynthesis is not inhibited in air as compared to an external atmosphere containing no oxygen.
7. PS I and PS II In each chloroplast, Photosystems I and II are present. Thus the Calvin cycle occurs. In the chloroplasts of bundle sheath cells, the photosystem II is absent. Therefore, these are dependent to mesophyll chloroplast of the supply of NADPH + H+.
8. Enzymes The Calvin cycle enzymes are present in mesophyll chloroplast The calvin cycle enzymes are absent in mesophyll chloroplasts. The cycle occurs only in the chloroplasts of sheath cells.
9. Compensation point The CO2 compensation point is 50-150 ppm. CO2 compensation point is 0-10 ppm.
10. Photorespiration Photorespiration is present and easily detectable. Photorespiration is present only to a slight degree and difficult to detect.
11. Net rate Net rate of photosynthesis in full sunlight (10,000- 12,000 ft.c) is 15-35 mg of CO2 per dm2 of leaf area per h. It is 40-80 mg. Of CO2 per dm2 of leaf area per h that is photosynthetic rate is quite high. The plants are efficient.
12. Saturation intensity The saturation intensity reached is in the range of 100-4000 ft.c. It is difficult to reach saturation even in full sunlight.

(b) Differences between cyclic and non- cyclic photophosphorylation.


Cyclic- Photophosphorylation

Non- cyclic photophosphorylation

1. Synthesis

It synthesizes only ATP.

It forms ATP as well as NADPH2.

2. Oxygen Oxygen is not evolved. Oxygen is liberated due to photolysis of water.
3. Requirement It requires only PS-I. It is performed with collaboration of PS-I and PS-II.
4. Cycling Electron emitted by P700 is recycled back and no external source of electron is needed. It needs external electron donor.
5. Performance This process is operative at low light intensity. Less availability of CO2 and under anaerobic conditions. It performs best under optimum light presence of CO2 and under aerobic conditions.
6. Operation It does not usually operate for photosynthesis except bacteria. It is functional for normal CO2 fixation.

(c) Differences in Anatomy of leaf in C3 and C4 plants.

C4 plants

C3 plants

1. Chloroplasts are present in mmesophyll cells as well as in bundle sheath cells.

1. Chloroplasts are present in mesophyll cells only.

2. Kranz type of anatomy present. 2. Kranz type of anatomy absent.
3. Intercellular spaces between mesophyll cells are smaller. 3. Intercellular spaces are larger.
4. Dimorphic chloroplasts are present. 4. Only one type of normal chloroplasts is present.


Test Your Skills Now!
Take a Quiz now
Reviewer Name