Question 1

By looking at a plant externally, can you tell whether a plant is C3 or C4? Why and how?

Accepted Answer

No, by looking at a plant externally, it is generally not possible to tell whether it is a C3 or C4 plant because the external morphology of most plants does not show distinctive features that differentiate the two types. The difference lies mainly in their internal leaf anatomy and biochemical pathways. C4 plants have Kranz anatomy with bundle sheath cells surrounding the vascular bundles, whereas C3 plants lack this feature. — The distinction between C3 and C4 plants is based on their photosynthetic pathways and leaf anatomy, which are internal features. External features like leaf shape, size, or plant height do not reliably indicate the photosynthetic type. Therefore, internal examination or biochemical tests are required.

Question 2

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

Accepted Answer

You can tell whether a plant is C3 or C4 by looking at the internal leaf anatomy, specifically the arrangement of mesophyll and bundle sheath cells. C4 plants exhibit Kranz anatomy, where the vascular bundles are surrounded by a layer of large, chloroplast-rich bundle sheath cells, which in turn are surrounded by mesophyll cells. In contrast, C3 plants lack Kranz anatomy and have mesophyll cells arranged more uniformly without distinct bundle sheath cells. — The presence of Kranz anatomy is a hallmark of C4 plants and is associated with their specialized photosynthetic pathway that concentrates CO2 in bundle sheath cells to minimize photorespiration. This anatomical difference can be observed under a microscope by examining transverse sections of the leaf.

Question 3

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?

Accepted Answer

In C4 plants, the Calvin cycle occurs mainly in the bundle sheath cells, which are fewer in number compared to mesophyll cells. Despite this, C4 plants are highly productive because they efficiently concentrate CO2 in the bundle sheath cells, reducing photorespiration and increasing the efficiency of photosynthesis. The initial fixation of CO2 into a four-carbon compound occurs in mesophyll cells, which then transports it to bundle sheath cells for the Calvin cycle. This spatial separation enhances carbon fixation efficiency, especially under high light intensity, high temperatures, and low CO2 conditions. — The C4 pathway minimizes photorespiration by increasing CO2 concentration at the site of RuBisCO in bundle sheath cells, thus improving photosynthetic efficiency and productivity. This adaptation allows C4 plants to thrive in hot and dry environments where C3 plants would be less efficient.

Question 4

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

Accepted Answer

RuBisCO carries out more carboxylation in C4 plants because these plants have a mechanism to concentrate CO2 around RuBisCO in the bundle sheath cells. The C4 pathway initially fixes CO2 into a four-carbon compound in mesophyll cells, which is then transported to bundle sheath cells where CO2 is released in high concentration. This high CO2 concentration suppresses the oxygenase activity of RuBisCO, thus favoring carboxylation and reducing photorespiration. — In C3 plants, RuBisCO fixes both CO2 and O2, leading to photorespiration which wastes energy and reduces photosynthetic efficiency. C4 plants avoid this by spatially separating initial CO2 fixation and the Calvin cycle, ensuring RuBisCO operates in an environment rich in CO2, enhancing carboxylation.

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 plants have chlorophyll b and other accessory pigments?

Accepted Answer

Plants lacking chlorophyll a but having only chlorophyll b would not be able to carry out photosynthesis effectively because chlorophyll a is the primary pigment responsible for the photochemical reactions of photosynthesis. Chlorophyll b and other accessory pigments assist by capturing light energy and transferring it to chlorophyll a. Accessory pigments broaden the spectrum of light that can be absorbed, protecting the plant from photo-damage and increasing the efficiency of light harvesting. — Chlorophyll a is essential for converting light energy into chemical energy. Chlorophyll b and accessory pigments like carotenoids absorb light wavelengths that chlorophyll a cannot, thus complementing the light absorption spectrum and protecting the photosynthetic apparatus.

Question 6

Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?

Accepted Answer

Leaves kept in the dark often become yellow or pale green because chlorophyll degrades in the absence of light, and new chlorophyll synthesis stops. The yellow color is due to the presence of carotenoids, which are more stable pigments and do not degrade as quickly as chlorophyll. Therefore, carotenoids are more stable pigments in leaves. — Chlorophyll requires light for its synthesis and maintenance. In darkness, chlorophyll breaks down, revealing the carotenoids that are normally masked by the green chlorophyll. This results in yellow or pale green coloration of leaves kept in the dark.

Question 7

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?

Accepted Answer

Leaves on the shady side or plants kept in the shade generally have darker green leaves compared to those on the sunny side or plants kept in sunlight. This is because shade leaves have a higher concentration of chlorophyll to capture the limited light available, making them appear darker green. Sunlight-exposed leaves have less chlorophyll as they receive abundant light and do not need to maximize pigment concentration. — Plants adapt to light conditions by adjusting chlorophyll content. In low light, increased chlorophyll helps maximize light absorption, while in high light, excess chlorophyll is not necessary and may even cause photo-damage.

Question 8

Figure 11.10 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 light is a limiting factor?

(b) What could be the limiting factor/s in region A?

(c) What do C and D represent on the curve?

Accepted Answer

(a) Light is a limiting factor at point A on the curve because the rate of photosynthesis increases with increasing light intensity in this region.

(b) In region A, the limiting factor is light intensity as photosynthesis rate is directly dependent on the amount of light available.

(c) Points C and D represent the saturation point and the maximum rate of photosynthesis respectively. Beyond point C, increasing light intensity does not increase the rate of photosynthesis, indicating other factors have become limiting. — At low light intensities (region A), photosynthesis rate increases linearly with light because light energy drives the process. At saturation (point C), all photosynthetic pigments and enzymes are working at full capacity. Beyond this, other factors like CO2 concentration or temperature limit the rate, so the curve plateaus at point D.

Question 9

Give comparison between the following:

(a) C3 and C4 pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C3 and C4 plants

Accepted Answer

(a) Comparison between C3 and C4 pathways:
- C3 pathway involves direct fixation of CO2 by RuBisCO into a 3-carbon compound (3-phosphoglycerate).
- C4 pathway involves initial fixation of CO2 into a 4-carbon compound (oxaloacetate) by PEP carboxylase in mesophyll cells, followed by transport to bundle sheath cells where CO2 is released for the Calvin cycle.
- C4 pathway minimizes photorespiration and is more efficient under high temperature and light.

(b) Comparison between cyclic and non-cyclic photophosphorylation:
- Cyclic photophosphorylation involves only Photosystem I, produces ATP but no NADPH or oxygen.
- Non-cyclic photophosphorylation involves both Photosystem I and II, produces ATP, NADPH, and oxygen.

(c) Anatomy of leaf in C3 and C4 plants:
- C3 leaves have mesophyll cells arranged uniformly without distinct bundle sheath cells.
- C4 leaves show Kranz anatomy with bundle sheath cells surrounding vascular bundles, surrounded by mesophyll cells.
- Bundle sheath cells in C4 plants have more chloroplasts and are the site of Calvin cycle. — These comparisons highlight the biochemical and anatomical adaptations that distinguish C3 and C4 plants, as well as the differences in light reactions of photosynthesis. The anatomical differences support the biochemical pathways and efficiency of photosynthesis in different environments.

Question 10

Which of the following are essential for photosynthesis to occur in green plants?

Accepted Answer

Chlorophyll, light, and carbon dioxide — Photosynthesis requires chlorophyll to capture light energy, light as the energy source, and carbon dioxide as the carbon source to synthesize organic compounds.

Question 11

In an experiment, a part of a leaf is enclosed in a test tube containing KOH-soaked cotton while the other part is exposed to air and then exposed to light. Which part will test positive for starch and why?

Accepted Answer

The part of the leaf exposed to air will test positive for starch because KOH absorbs carbon dioxide, which is essential for photosynthesis. Without CO2, photosynthesis cannot occur, so starch is not formed in the enclosed part. — Photosynthesis requires carbon dioxide, light, and chlorophyll. The KOH-soaked cotton absorbs CO2, preventing photosynthesis in the enclosed part. The exposed part receives CO2 and light, enabling starch formation as an indicator of photosynthesis.

Question 12

Joseph Priestley's experiment demonstrated that plants restore the air quality by releasing which gas essential for combustion and respiration?

Accepted Answer

Oxygen — Priestley's experiment showed that plants release oxygen, which supports combustion and respiration, thus restoring the air quality.

Question 13

Jan Ingenhousz showed that oxygen bubbles are produced only when the green parts of aquatic plants are exposed to sunlight. What conclusion can be drawn from this experiment?

Accepted Answer

Oxygen is released by plants only in the presence of sunlight and chlorophyll. This shows that photosynthesis requires light and occurs in green parts of plants. — Ingenhousz's experiment demonstrated that sunlight is essential for photosynthesis and oxygen release, and that only green parts containing chlorophyll produce oxygen.

Question 14

Which scientist used a prism and aerobic bacteria to demonstrate that blue and red light are most effective for photosynthesis?

Accepted Answer

T.W. Engelmann — Engelmann used a prism to split light and aerobic bacteria to detect oxygen, showing bacteria accumulated near blue and red light, indicating these wavelengths are most effective for photosynthesis.

Question 15

Write the balanced overall chemical equation for photosynthesis in green plants.

Accepted Answer

6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2 — The balanced photosynthesis equation shows that six molecules of carbon dioxide and twelve molecules of water, in presence of light, produce one molecule of glucose, six molecules of oxygen, and six molecules of water. Oxygen released comes from water molecules.

PLANT PHYSIOLOGY

PLANT PHYSIOLOGY — Study Notes

11.1 What do we Know?

11.2 Early Experiments

11.3 Where does Photosynthesis take place?

Practice Questions — PLANT PHYSIOLOGY

All 19 Chapters in Biology