Question 1

The primary CO₂ acceptor in C4 plants (mesophyll cells) is ______, and the first product is ______:

Accepted Answer

PEP; oxaloacetic acid (OAA). C4 plants fix CO₂ first via PEP carboxylase into OAA (4-carbon) in mesophyll cells.

Question 2

C4 plants (e.g. maize, sugarcane) possess a special leaf anatomy called:

Accepted Answer

Kranz anatomy. Kranz ("wreath") anatomy features chloroplast-rich bundle sheath cells around the veins.

Question 3

In C4 plants the Calvin cycle operates in the ______ cells, which receive CO₂ released from a 4-carbon acid:

Accepted Answer

Bundle sheath. The 4-C acid moves to bundle sheath cells and releases CO₂ there, concentrating it for RuBisCO.

Question 4

Photorespiration occurs because RuBisCO also acts as an ______, especially at ______:

Accepted Answer

Oxygenase; high O₂/low CO₂. When O₂ is high and CO₂ low, RuBisCO oxygenates RuBP, starting photorespiration (loss of fixed carbon).

Question 5

C4 plants have negligible photorespiration because:

Accepted Answer

The bundle sheath maintains a high CO₂ concentration around RuBisCO. By pumping CO₂ into bundle sheath cells, C4 plants keep RuBisCO carboxylating, suppressing its oxygenase activity.

Question 6

Photorespiration is considered a wasteful process because it:

Accepted Answer

Consumes O₂ and releases CO₂ without producing ATP or NADPH (or sugar). Unlike normal respiration, photorespiration yields no ATP/NADPH and results in a net loss of previously fixed carbon.

Question 7

The C4 pathway is also known as the Hatch and Slack pathway. A defining anatomical feature that makes it work is:

Accepted Answer

Kranz anatomy, with large bundle-sheath cells wreathed around the vascular bundles. C4 leaves show Kranz ("wreath") anatomy: bundle-sheath cells with special chloroplasts encircle the veins, allowing CO₂ to be concentrated for RuBisCO.

Question 8

In a C4 plant the initial fixation of atmospheric CO₂ occurs in the mesophyll, where PEP carboxylase attaches CO₂ to phosphoenolpyruvate. The immediate 4-carbon product is:

Accepted Answer

oxaloacetic acid (OAA). PEP carboxylase fixes CO₂ onto the 3-C PEP to form the 4-C OAA in mesophyll cells; OAA is later converted to malate/aspartate that carry CO₂ to the bundle sheath.

Question 9

The reason C4 plants show little or no photorespiration is best explained as follows: in the bundle-sheath cells,

Accepted Answer

the CO₂ concentration is kept high enough to keep RuBisCO acting as a carboxylase. C4 plants pump CO₂ into bundle-sheath cells so that the CO₂:O₂ ratio around RuBisCO stays high, suppressing its oxygenase activity — RuBisCO is present and works as a carboxylase there.

Question 10

Assertion (A): RuBisCO in C3 plants can wastefully bind oxygen instead of carbon dioxide. Reason (R): RuBisCO has a much greater affinity for oxygen than for carbon dioxide.

Accepted Answer

A is true but R is false. RuBisCO does bind O₂ (starting photorespiration), but its affinity is actually greater for CO₂; oxygenation dominates only when the O₂:CO₂ ratio rises, so R is false.

Question 11

Photorespiration is regarded as a wasteful process in C3 plants because it:

Accepted Answer

consumes energy and releases CO₂ without producing any sugar, ATP or NADPH. In photorespiration RuBisCO oxygenates RuBP, giving one 3-C and one 2-C compound; the pathway releases CO₂ and yields no sugar, no ATP and no NADPH — a net loss.

Question 12

When RuBisCO acts as an oxygenase on RuBP, the 2-carbon compound produced is:

Accepted Answer

phosphoglycolate (glycolate). Oxygenation of RuBP yields one molecule of 3-C phosphoglycerate and one molecule of 2-C phosphoglycolate; the glycolate is then processed in the photorespiratory pathway.

Question 13

The cell organelle, besides the chloroplast, that is centrally involved in the photorespiratory (glycolate) pathway is the:

Accepted Answer

peroxisome. The photorespiratory pathway shuttles glycolate through the peroxisome (and mitochondrion); the chloroplast, peroxisome and mitochondrion cooperate in it.

Question 14

Between a C3 plant such as rice and a C4 plant such as maize, the C4 plant is generally the more efficient carbon fixer at high temperatures mainly because it:

Accepted Answer

avoids the loss of fixed carbon through photorespiration. By concentrating CO₂ in the bundle sheath, C4 plants virtually eliminate photorespiration, which otherwise wastes fixed carbon in C3 plants — especially in hot, bright conditions; C4 plants still run the Calvin cycle.

Question 15

In C4 pathway, RuBisCO is-

Accepted Answer

present in bundle sheath cell. In C4 plants, RuBisCO is present only in the bundle sheath cells and absent in the mesophyll cells.

Question 16

PEPcase enzyme is-

Accepted Answer

None of these. PEP carboxylase is present in mesophyll cells and absent in bundle sheath cells, so neither statement is true.

Question 17

Calvin cycle takes place in __(i)__ in all C3 plants and in __(ii)__ in all C4 plants.

Accepted Answer

mesophyll cells, bundle sheath cells. In C3 plants the Calvin cycle occurs in mesophyll cells, while in C4 plants it occurs in bundle sheath cells.

Question 18

C4 plants are adaptation of plants to

Accepted Answer

dry tropics. C4 photosynthesis is an adaptation to hot, dry tropical conditions.

Question 19

(i) C4 plants lack Calvin cycle (ii) C4 plants lack photorespiration (iii) C4 plants have more productivity than C3 plants (iv) C4 plants cannot tolerate higher temperature. How many of the above statements are incorrect?

Accepted Answer

2. Statements (i) and (iv) are incorrect: C4 plants do have the Calvin cycle and they tolerate high temperatures, so 2 statements are wrong.

Question 20

First CO2 fixation product in C4 cycle is

Accepted Answer

OAA. The first stable CO2 fixation product in the C4 pathway is the 4-carbon oxaloacetic acid (OAA).

C4 Pathway & Photorespiration — NEET Biology MCQs with Solutions

Questions with solutions