Dark Reactions and Photorespiration

Q: What is the first stable product of CO2 fixation in C3 and C4 plants?

In C3 plants the first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon acid, discovered by Melvin Calvin using radioactive carbon-14. In C4 plants it is oxaloacetic acid (OAA), a 4-carbon acid formed in the mesophyll cells by PEP carboxylase.

Q: What are the three stages of the Calvin cycle?

Carboxylation fixes CO2 onto RuBP using RuBisCO, forming two molecules of 3-PGA. Reduction uses 2 ATP and 2 NADPH per CO2 to form sugar. Regeneration uses one ATP to re-form RuBP, so that the cycle can continue. The cycle runs in all photosynthetic plants, whether C3 or C4.

Q: How many ATP and NADPH are needed to make one glucose?

Each CO2 fixed needs 3 ATP and 2 NADPH. One glucose needs six CO2 molecules and six turns of the Calvin cycle, so it uses 18 ATP and 12 NADPH. Cyclic photophosphorylation probably supplies the extra ATP needed. Six CO2 molecules go in and one glucose molecule comes out.

Q: What is Kranz anatomy?

Kranz anatomy is the leaf structure of C4 plants such as maize and sorghum. Large bundle sheath cells form a wreath around each vascular bundle. These cells have many chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. Kranz means wreath.

Q: Why do C4 plants not show photorespiration?

In C4 plants, C4 acids made in the mesophyll are broken down in the bundle sheath, releasing CO2. This raises the CO2 concentration around RuBisCO, so it works as a carboxylase and its oxygenase activity is minimised. As a result, photorespiration does not occur.

Q: What happens in photorespiration?

In C3 plants some oxygen binds RuBisCO, and RuBP forms one phosphoglycerate and one 2-carbon phosphoglycolate instead of two PGA. No sugar, ATP or NADPH is made; instead CO2 is released and ATP is used. Its biological function is not yet known.

BiologyPhotosynthesis in Higher PlantsFor NEET aspirants

The dark reactions, or biosynthetic phase, of photosynthesis use the ATP and NADPH from the light reactions to fix carbon dioxide into sugar in the stroma. This page covers the discovery of PGA and RuBP, the three stages of the Calvin cycle with its ATP and NADPH count, the C₄ (Hatch and Slack) pathway and Kranz anatomy, photorespiration, and a full comparison of C₃ and C₄ plants. NEET often asks the first stable products, enzyme locations, the 18 ATP and 12 NADPH per glucose, and why C₄ plants lack photorespiration in the dark reactions.

On this page1Biosynthetic phase2Calvin cycle3C₄ pathway4Photorespiration5C₃ versus C₄6Exam essentials7Quick revision8Solved examples9Practice

Key Points at a Glance

Light reaction products: ATP, NADPH and $O_{2}$ ; $O_{2}$ diffuses out, ATP and NADPH drive sugar synthesis.
★ Must learn First stable product: PGA (3C) in C₃ plants; OAA (4C) in C₄ plants.
★ Must learn Primary $C O_{2}$ acceptor of the Calvin cycle: RuBP, a 5-carbon ketose sugar; RuBP + $C O_{2}$ → 2 PGA.
Calvin cycle stages: carboxylation, reduction and regeneration; it runs in all photosynthetic plants.
★ Must learn Per $C O_{2}$ fixed: 3 ATP + 2 NADPH. Per glucose (6 turns): 18 ATP + 12 NADPH.
RuBisCO (RuBP carboxylase-oxygenase) is the most abundant enzyme in the world.
★ Must learn C₄ pathway: PEP (3C) + $C O_{2}$ → OAA by PEPcase in mesophyll; Calvin cycle by RuBisCO in bundle sheath.
Kranz anatomy: large bundle sheath cells with many chloroplasts, thick gas-tight walls and no intercellular spaces.
★ Must learn Photorespiration (C₃ plants): RuBisCO binds $O_{2}$ ; no sugar, no ATP, no NADPH; $C O_{2}$ is released.
C₄ plants lack photorespiration, tolerate higher temperatures and have greater biomass productivity.

1. Where Are ATP and NADPH Used?

The products of the light reaction are ATP, NADPH and $O_{2}$ .
$O_{2}$ diffuses out of the chloroplast.
ATP and NADPH drive the processes that synthesise food, more accurately sugars. This is the biosynthetic phase of photosynthesis.
This phase does not directly depend on light. It depends on the products of the light reaction (ATP and NADPH), besides $C O_{2}$ and $H_{2} O$ .
Evidence: when light is removed, the biosynthetic process continues for some time and then stops. It starts again when light returns.

★ Very important Calling the biosynthetic phase the dark reaction is a misnomer. It does not need darkness, and it stops soon after light is removed because ATP and NADPH run out.

1.1 Discovery of the first product

$C O_{2}$ combines with $H_{2} O$ to produce $(C H_{2} O)_{n}$ , that is, sugars.
Scientists wanted to find the first product formed when $C O_{2}$ is taken into a reaction, or fixed.
After World War II, radioisotopes were put to beneficial use. The work of Melvin Calvin is a fine example.
★ Exam imp Using radioactive $^{14} C$ in algal photosynthesis, Calvin found that the first $C O_{2}$ fixation product is a 3-carbon organic acid.
This acid is 3-phosphoglyceric acid (PGA), which has 3 carbon atoms.
Calvin also worked out the complete biosynthetic pathway, so it is called the Calvin cycle.

About Melvin Calvin

Born in Minnesota in April 1911; Ph.D. in Chemistry from the University of Minnesota.
Professor of Chemistry at the University of California, Berkeley.
After the Hiroshima-Nagasaki bombings, he and his co-workers put radioactivity to beneficial use.
With J.A. Bassham, he labelled $C O_{2}$ with $^{14} C$ to study how green plants form sugar from $C O_{2}$ , water and minerals.
He proposed that plants change light energy to chemical energy by transferring an electron in an organised array of pigment molecules.
He received the Nobel Prize in 1961 for mapping the pathway of carbon assimilation.
His principles are now used in research on renewable resources and solar energy.

Experiments on a wide range of plants found another group in which the first stable product was a 4-carbon organic acid, oxaloacetic acid (OAA).
So $C O_{2}$ assimilation is of two main types, named after the first product.

Pathway	First stable product	Carbon atoms
C₃ pathway	3-phosphoglyceric acid (PGA)	3
C₄ pathway	Oxaloacetic acid (OAA)	4

1.2 The primary acceptor of CO₂

★ Exam imp Unexpectedly, the acceptor molecule turned out to be a 5-carbon ketose sugar: ribulose bisphosphate (RuBP).
Because the first product had 3 carbons, scientists believed the acceptor would be a 2-carbon compound.
They spent many years searching for a 2-carbon compound before discovering the 5-carbon RuBP.

Tips and Tricks Carbon count check: RuBP (5C) +

C O_{2}

(1C) = 6C, which splits into 2 molecules of PGA (3C each). A 2-carbon acceptor would give only one 3C product.

Key idea

Calvin's 14C work showed that

C O_{2}

is fixed onto 5-carbon RuBP to give two molecules of 3-carbon PGA.

2. The Calvin Cycle

Calvin and his co-workers showed that the pathway runs in a cycle, because RuBP is regenerated.
★ Exam imp The Calvin pathway occurs in all photosynthetic plants, whether they have C₃, C₄ or any other pathway.
For ease of study it is described in three stages.

Carboxylation: the fixation of $C O_{2}$ into a stable organic intermediate. It is the most crucial step. $C O_{2}$ carboxylates RuBP, catalysed by RuBP carboxylase, giving two molecules of 3-PGA. The enzyme also has oxygenation activity, so it is better called RuBP carboxylase-oxygenase, or RuBisCO.
Reduction: a series of reactions that lead to glucose. Per $C O_{2}$ fixed, they use 2 ATP for phosphorylation and 2 NADPH for reduction. Six $C O_{2}$ molecules and 6 turns of the cycle form one glucose molecule.
Regeneration: re-forming the $C O_{2}$ acceptor RuBP, which is crucial for the cycle to continue. It uses one ATP for phosphorylation to form RuBP.

Test yourself: hide labels

Figure 1: The Calvin cycle in three stages: (1) carboxylation, in which CO₂ joins ribulose-1,5-bisphosphate; (2) reduction, in which ATP and NADPH form carbohydrate; (3) regeneration of ribulose-1,5-bisphosphate so that the cycle continues.

Quick Recall: name the parts of the Calvin cycle

Name the stage marked 1 in Figure 1 and the enzyme that catalyses it.

Carboxylation, catalysed by RuBisCO.

What two molecules enter at the reduction stage?

ATP and NADPH.

What is formed from triose phosphate, and what is regenerated?

Sucrose and starch are formed; ribulose-1,5-bisphosphate is regenerated.

Memory Trick

"Calvin Rarely Rests" = Carboxylation, Reduction, Regeneration.

2.1 ATP and NADPH used

★ Exam imp For every $C O_{2}$ entering the cycle: 3 ATP (2 in reduction + 1 in regeneration) and 2 NADPH.
Cyclic photophosphorylation probably takes place to meet this difference between ATP and NADPH needs.
One glucose needs 6 turns: 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH.

In	Out
Six $C O_{2}$	One glucose
18 ATP	18 ADP
12 NADPH	12 $NAD P^{+}$

★ Very important 3 ATP and 2 NADPH per $C O_{2}$ ; 18 ATP and 12 NADPH per glucose. Multiply by 6 because one glucose needs six turns of the cycle.

Memory Trick

"3-2, then times 6": 3 ATP and 2 NADPH per turn; times 6 gives 18 and 12.

Key idea

The Calvin cycle fixes

C O_{2}

onto RuBP, reduces PGA using ATP and NADPH, and regenerates RuBP with more ATP.

3. The C₄ Pathway

Plants adapted to dry tropical regions have the C₄ pathway.
Their first $C O_{2}$ fixation product is the C₄ acid OAA, but they use the Calvin cycle as the main biosynthetic pathway.

Why C₄ plants are special

They have a special type of leaf anatomy.
They tolerate higher temperatures.
They respond to high light intensities.
They lack photorespiration.
They have greater productivity of biomass.

Memory Trick

Five C₄ features, "A Hot, High-Light, No-Loss, Big-Yield plant": special Anatomy, High temperature tolerance, High light response, No photorespiration, Biomass productivity.

3.1 Kranz anatomy

Vertical sections of C₃ and C₄ leaves differ in their mesophyll and in the cells around the vascular bundles.
The particularly large cells around the vascular bundles of C₄ plants are called bundle sheath cells.
★ Exam imp Leaves with this anatomy have Kranz anatomy. Kranz means wreath, which describes the ring-like arrangement of cells.
Bundle sheath cells may form several layers around the vascular bundles.
★ Exam imp They have a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
Kranz anatomy can be seen in sections of maize or sorghum leaves.
The presence of the bundle sheath helps to identify C₄ plants.

Memory Trick

Bundle sheath cells, "Many, Thick, None": many chloroplasts, thick gas-tight walls, no intercellular spaces.

3.2 The Hatch and Slack pathway

The C₄ pathway is called the Hatch and Slack pathway. It is also a cyclic process.

In the mesophyll cells, the primary $C O_{2}$ acceptor is a 3-carbon molecule, phosphoenol pyruvate (PEP).
The enzyme for this fixation is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO. The C₄ acid OAA forms in the mesophyll.
In the mesophyll itself, OAA forms other 4-carbon compounds such as malic acid or aspartic acid.
These C₄ acids are transported to the bundle sheath cells.
In the bundle sheath cells, the C₄ acids are broken down to release $C O_{2}$ and a 3-carbon molecule.
The released $C O_{2}$ enters the C₃ (Calvin) pathway. Bundle sheath cells are rich in RuBisCO but lack PEPcase.
The 3-carbon molecule is transported back to the mesophyll, where it is converted to PEP again, completing the cycle.

Test yourself: hide labels

Figure 2: The Hatch and Slack pathway. CO₂ is first fixed into a C₄ acid in the mesophyll cell, then released in the bundle sheath cell for the Calvin cycle; the C₃ acid returns to regenerate phosphoenolpyruvate.

Feature	Mesophyll cell (C₄)	Bundle sheath cell (C₄)
$C O_{2}$ acceptor	PEP (3C)	RuBP (5C), in the Calvin cycle
Enzyme present	PEPcase	RuBisCO
Enzyme absent	RuBisCO	PEPcase
Main event	Fixation to OAA (C₄ acid)	Decarboxylation and Calvin cycle

Quick Recall: name the parts of the Hatch and Slack pathway

In Figure 2, which molecule accepts HCO₃⁻ in the mesophyll cell?

Phosphoenolpyruvate (PEP).

What passes through the plasmodesmata from the mesophyll to the bundle sheath?

The C₄ acid (malic acid or aspartic acid).

What happens to the CO₂ released by decarboxylation?

It is fixed by the Calvin cycle in the bundle sheath cell.

Thus the Calvin pathway, which forms sugars, is common to C₃ and C₄ plants.
★ Exam imp In C₃ plants the Calvin pathway runs in all the mesophyll cells. In C₄ plants it does not run in the mesophyll; it occurs only in the bundle sheath cells.

NEET Focus

PEPcase is in the mesophyll and RuBisCO is in the bundle sheath of C₄ plants. Each cell type lacks the other enzyme.
The primary acceptor in C₄ plants is PEP (3C), but the first stable product is OAA (4C). Do not confuse acceptor and product.
Malic acid and aspartic acid are the C₄ acids carried to the bundle sheath.

Key idea

C₄ plants fix

C O_{2}

twice: first by PEPcase in the mesophyll, then by RuBisCO in the bundle sheath.

4. Photorespiration

Photorespiration creates an important difference between C₃ and C₄ plants.
To understand it, recall the first step of the Calvin pathway: RuBP combines with $C O_{2}$ to form two molecules of 3-PGA, catalysed by RuBisCO.

RuBP + C O_{2} RuBisCO 2 \times 3 PGA

★ Exam imp RuBisCO is the most abundant enzyme in the world.
Its active site can bind both $C O_{2}$ and $O_{2}$ , hence the name carboxylase-oxygenase.
RuBisCO has a much greater affinity for $C O_{2}$ when the $C O_{2}$ : $O_{2}$ ratio is nearly equal.
★ Exam imp The binding is competitive. The relative concentration of $O_{2}$ and $C O_{2}$ decides which one binds.

4.1 Photorespiration in C₃ plants

In C₃ plants some $O_{2}$ does bind to RuBisCO, so $C O_{2}$ fixation decreases.
RuBP then binds $O_{2}$ instead of forming 2 PGA. It forms one phosphoglycerate and one phosphoglycolate (2 carbons). This pathway is photorespiration.
★ Exam imp In photorespiration there is no synthesis of sugars, ATP or NADPH.
Instead, it releases $C O_{2}$ and uses ATP.
The biological function of photorespiration is not known yet.

Carboxylation by RuBisCO

RuBP + $C O_{2}$
Gives 2 molecules of PGA (3C)
Leads to sugar
Favoured by high $C O_{2}$

Oxygenation by RuBisCO (photorespiration)

RuBP + $O_{2}$
Gives 1 phosphoglycerate + 1 phosphoglycolate (2C)
No sugar, ATP or NADPH; $C O_{2}$ released, ATP used
Favoured by high $O_{2}$ and low $C O_{2}$

4.2 Why C₄ plants lack photorespiration

In C₄ plants photorespiration does not occur.
They have a mechanism that increases the concentration of $C O_{2}$ at the enzyme site.
The C₄ acid from the mesophyll is broken down in the bundle sheath, releasing $C O_{2}$ and raising the intracellular $C O_{2}$ concentration.
This makes RuBisCO work as a carboxylase and minimises its oxygenase activity.
Lacking photorespiration, C₄ plants have better productivity and yields. They also tolerate higher temperatures.

★ Very important Photorespiration is wasteful: RuBisCO fixes $O_{2}$ instead of $C O_{2}$ , no sugar, ATP or NADPH is made, and $C O_{2}$ is lost. C₄ plants avoid it by concentrating $C O_{2}$ around RuBisCO in the bundle sheath.

Extra Depth: The photorespiratory pathway involves three organelles: the chloroplast, the peroxisome and the mitochondrion.

Key idea

The more

C O_{2}

there is around RuBisCO, the more it acts as a carboxylase; C₄ plants exploit this.

5. C₃ and C₄ Plants Compared

Characteristics	C₃ plants	C₄ plants
Cell type in which the Calvin cycle takes place	Mesophyll	Bundle sheath
Cell type in which the initial carboxylation occurs	Mesophyll	Mesophyll
Number of cell types that fix $C O_{2}$	One: mesophyll	Two: bundle sheath and mesophyll
Primary $C O_{2}$ acceptor	RuBP	PEP
Carbons in the primary acceptor	5	3
Primary $C O_{2}$ fixation product	PGA	OAA
Carbons in the primary fixation product	3	4
Does the plant have RuBisCO?	Yes	Yes
Does the plant have PEPcase (for photosynthetic $C O_{2}$ fixation)?	No	Yes
Cells that have RuBisCO	Mesophyll	Bundle sheath
$C O_{2}$ fixation rate under high light	Low	High
Photorespiration at low light intensities	Negligible	Negligible
Photorespiration at high light intensities	High	Negligible
Photorespiration at low $C O_{2}$ concentrations	High	Negligible
Photorespiration at high $C O_{2}$ concentrations	Negligible	Negligible
Temperature optimum	20-25°C	30-40°C
Examples	Tomato, bell pepper	Maize, sorghum

Leaf anatomy

Feature	C₃ leaf	C₄ leaf (Kranz anatomy)
Bundle sheath cells	Not prominent; few chloroplasts	Large, in one or several layers, with many chloroplasts
Walls of bundle sheath	Thin	Thick, impervious to gaseous exchange
Intercellular spaces around the bundle	Present	Absent
Where the Calvin cycle runs	All mesophyll cells	Only bundle sheath cells

Tips and Tricks Elimination trick for C₃/C₄ questions: if a statement puts RuBisCO in the C₄ mesophyll, PEPcase in the bundle sheath, or photorespiration in C₄ plants, it is false.

Key idea

C₄ plants split the work between two cell types; C₃ plants do everything in the mesophyll and suffer photorespiration.

6. Exam Essentials

Pairs to Match

List I	List II
Melvin Calvin	$^{14} C$ in algae; first product PGA; Nobel Prize 1961
Hatch and Slack	C₄ pathway
PGA	First stable product in C₃ plants (3C)
OAA	First stable product in C₄ plants (4C)
RuBP	Primary $C O_{2}$ acceptor of the Calvin cycle (5C ketose)
PEP	Primary $C O_{2}$ acceptor in C₄ mesophyll (3C)
RuBisCO	Carboxylation of RuBP; bundle sheath in C₄
PEPcase	Fixation of $C O_{2}$ in C₄ mesophyll
Phosphoglycolate	2-carbon product of photorespiration
Malic acid, aspartic acid	C₄ acids transported to the bundle sheath
Kranz	Wreath
Carboxylation	Most crucial step of the Calvin cycle
Regeneration	One ATP per $C O_{2}$ to re-form RuBP
Reduction	2 ATP and 2 NADPH per $C O_{2}$
Maize, sorghum	C₄ plants with Kranz anatomy

Exceptions

The dark reaction does not need darkness; the name is a misnomer.
The primary acceptor of the Calvin cycle is not a 2-carbon compound but 5-carbon RuBP.
C₄ mesophyll cells lack RuBisCO; C₄ bundle sheath cells lack PEPcase.
In C₄ plants the Calvin cycle does not run in the mesophyll.
Photorespiration makes no sugar, ATP or NADPH; it releases $C O_{2}$ and uses ATP.
C₄ plants show no photorespiration.
The Calvin cycle is not limited to C₃ plants; it runs in all photosynthetic plants.

Numbers to Remember

First stable product: PGA (3C) in C₃; OAA (4C) in C₄.
Acceptors: RuBP (5C) in the Calvin cycle; PEP (3C) in the C₄ mesophyll.
RuBP + $C O_{2}$ → 2 PGA. RuBP + $O_{2}$ → 1 phosphoglycerate + 1 phosphoglycolate (2C).
Per $C O_{2}$ : 3 ATP and 2 NADPH. Per glucose: 6 $C O_{2}$ , 6 turns, 18 ATP, 12 NADPH.
Temperature optimum: C₃ 20-25°C; C₄ 30-40°C.
Calvin: born April 1911; Nobel Prize 1961.

Examples to Remember

Group	Examples
C₄ plants (Kranz anatomy)	Maize, sorghum
C₃ plants grown in $C O_{2}$ -enriched greenhouses	Tomato, bell pepper
Organism in Calvin's $^{14} C$ studies	Algae

7. Quick Revision

ATP and NADPH from light reactions drive the biosynthetic phase; $O_{2}$ diffuses out.
The biosynthetic phase stops soon after light is removed: "dark reaction" is a misnomer.
Calvin used $^{14} C$ in algae: first product PGA (3C) → Calvin cycle (C₃ pathway).
Another group forms OAA (4C) first → C₄ pathway.
Primary acceptor: RuBP, a 5-carbon ketose sugar, not the expected 2-carbon compound.
Stages: carboxylation (RuBisCO, most crucial), reduction (2 ATP + 2 NADPH), regeneration (1 ATP).
Per glucose: 6 $C O_{2}$ , 6 turns, 18 ATP, 12 NADPH; cyclic photophosphorylation makes up the extra ATP.
C₄ plants: dry tropics, Kranz anatomy, heat tolerance, response to high light, no photorespiration, high biomass.
Bundle sheath: several layers, many chloroplasts, thick gas-tight walls, no intercellular spaces.
Hatch and Slack: PEP + $C O_{2}$ → OAA (PEPcase, mesophyll) → malic/aspartic acid → bundle sheath → $C O_{2}$ + 3C molecule.
Calvin cycle: all mesophyll cells in C₃; only bundle sheath in C₄.
RuBisCO: most abundant enzyme; binds $C O_{2}$ and $O_{2}$ competitively.
Photorespiration: RuBP + $O_{2}$ → phosphoglycerate + phosphoglycolate; no sugar, ATP or NADPH.
C₄ plants concentrate $C O_{2}$ at RuBisCO, so they lack photorespiration.

8. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. RuBP; B. PEP; C. OAA; D. Phosphoglycolate
List II: I. First stable product of C₄ plants; II. Primary acceptor in C₄ mesophyll; III. 2-carbon product of photorespiration; IV. Primary acceptor of the Calvin cycle
Choose the correct answer.
(A) A-IV, B-II, C-I, D-III
(B) A-II, B-IV, C-I, D-III
(C) A-IV, B-I, C-II, D-III
(D) A-II, B-IV, C-III, D-I

Solution:

Answer: (A). RuBP accepts $C O_{2}$ in the Calvin cycle (IV); PEP is the C₄ mesophyll acceptor (II); OAA is the first C₄ product (I); phosphoglycolate is the 2C product of photorespiration (III).

Solved Example 2

Read the statements about C₄ plants.
A. Their mesophyll cells lack RuBisCO.
B. Their bundle sheath cells are rich in PEPcase.
C. The Calvin cycle occurs only in the bundle sheath cells.
D. They show high photorespiration at high light intensities.
E. Their bundle sheath cells have thick walls impervious to gaseous exchange.
Choose the correct answer.
(A) A, C and E only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only

Solution:

Answer: (A). B is false: bundle sheath cells lack PEPcase and are rich in RuBisCO. D is false: C₄ plants lack photorespiration. A, C and E are true.

Solved Example 3

Arrange the steps of the Hatch and Slack pathway in the correct order.
A. C₄ acid is transported to the bundle sheath cell
B. PEP accepts

C O_{2}

in the mesophyll cell
C. The 3-carbon molecule returns and PEP is regenerated
D. C₄ acid is broken down, releasing

C O_{2}

for the Calvin cycle
Choose the correct order.
(A) B, A, D, C
(B) B, D, A, C
(C) A, B, D, C
(D) B, A, C, D

Solution:

Answer: (A). Fixation by PEP in the mesophyll (B), transport of the C₄ acid (A), decarboxylation in the bundle sheath (D), and return of the 3C molecule to regenerate PEP (C).

Solved Example 4

How many ATP and NADPH molecules are needed to make one molecule of glucose through the Calvin cycle?
(A) 12 ATP and 18 NADPH
(B) 18 ATP and 12 NADPH
(C) 6 ATP and 6 NADPH
(D) 3 ATP and 2 NADPH

Solution:

Answer: (B). Each $C O_{2}$ needs 3 ATP and 2 NADPH. One glucose needs 6 $C O_{2}$ and 6 turns, so 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH.

Solved Example 5

Which statement about photorespiration is NOT correct?
(A) It occurs when RuBisCO binds

O_{2}

(B) It forms phosphoglycolate
(C) It produces ATP and NADPH
(D) Its biological function is not known yet

Solution:

Answer: (C). Photorespiration makes no sugar, ATP or NADPH. It uses ATP and releases $C O_{2}$ .

Solved Example 6

Statement I: The primary

C O_{2}

acceptor of the Calvin cycle is a 5-carbon ketose sugar.
Statement II: Because PGA has 3 carbons, scientists first searched for a 2-carbon acceptor.
Choose the correct answer.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (A). Both are correct. A 2-carbon acceptor was expected because 2C + $C O_{2}$ would give one 3C PGA. In fact 5C RuBP + $C O_{2}$ gives two molecules of PGA.

9. Practice Questions

Practice Questions

By looking at a plant externally, can you tell whether it is C₃ or C₄? Why and how?Answer: No. C₃ and C₄ plants can look alike from outside. The difference lies in the leaf's internal anatomy (Kranz anatomy) and in the first product of $C O_{2}$ fixation, which need a section or a biochemical test.
By looking at which internal structure of a plant can you tell whether it is C₃ or C₄? Explain.Answer: A vertical section of the leaf. C₄ leaves show Kranz anatomy: large bundle sheath cells around the vascular bundles, in one or more layers, with many chloroplasts, thick walls impervious to gases and no intercellular spaces.
Even though very few cells in a C₄ plant carry out the Calvin pathway, the plants are highly productive. Why?Answer: PEPcase fixes $C O_{2}$ in the mesophyll and the C₄ acids release it in the bundle sheath, keeping $C O_{2}$ high around RuBisCO. RuBisCO then works only as a carboxylase, so there is no photorespiration. The plants also tolerate high temperature and respond to high light. This gives greater biomass.
RuBisCO acts both as a carboxylase and an oxygenase. Why does it carry out more carboxylation in C₄ plants?Answer: In C₄ plants the breakdown of C₄ acids raises the $C O_{2}$ concentration in the bundle sheath cells, whose thick walls stop gases from leaking. Binding to RuBisCO is competitive, so the high $C O_{2}$ makes it act mostly as a carboxylase.
Give a comparison between (a) C₃ and C₄ pathways and (b) the anatomy of leaves in C₃ and C₄ plants.Answer: (a) C₃: acceptor RuBP (5C), first product PGA (3C), RuBisCO in mesophyll, photorespiration present, optimum 20-25°C. C₄: acceptor PEP (3C), first product OAA (4C), PEPcase in mesophyll and RuBisCO in bundle sheath, no photorespiration, optimum 30-40°C. (b) C₄ leaves have Kranz anatomy with large, chloroplast-rich, thick-walled bundle sheath cells and no intercellular spaces; C₃ leaves lack it.
Match List I with List II.
List I: A. Carboxylation; B. Reduction; C. Regeneration; D. Photorespiration
List II: I. One ATP per $C O_{2}$ to form RuBP; II. RuBP binds $O_{2}$ ; III. RuBP + $C O_{2}$ → 2 PGA; IV. 2 ATP and 2 NADPH per $C O_{2}$
Choose the correct answer.
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-I, C-IV, D-II
(D) A-II, B-IV, C-I, D-IIIAnswer: (A). Carboxylation fixes $C O_{2}$ on RuBP, reduction uses 2 ATP and 2 NADPH, regeneration uses 1 ATP, and photorespiration is RuBP binding $O_{2}$ .
Read the statements about the Calvin cycle.
A. It occurs in all photosynthetic plants.
B. Carboxylation is its most crucial step.
C. It needs 2 ATP and 3 NADPH per $C O_{2}$ fixed.
D. RuBP is regenerated so the cycle continues.
Choose the correct answer.
(A) A, B and D only
(B) A and C only
(C) B, C and D only
(D) A, B, C and DAnswer: (A). C is false: it needs 3 ATP and 2 NADPH per $C O_{2}$ .
Arrange the stages of the Calvin cycle starting from the entry of $C O_{2}$ .
A. Regeneration of RuBP
B. Reduction of PGA to triose phosphate
C. Carboxylation of RuBP
(A) C, B, A
(B) C, A, B
(C) B, C, A
(D) A, C, BAnswer: (A). Carboxylation, then reduction, then regeneration.

Common Mistakes to Avoid

Watch out

Calling PEP the first stable product of C₄ plants. PEP is the acceptor; OAA is the first stable product.
Placing RuBisCO in C₄ mesophyll cells. Correct: mesophyll has PEPcase; bundle sheath has RuBisCO.
Writing 12 ATP and 18 NADPH per glucose. Correct: 18 ATP and 12 NADPH.
Saying the Calvin cycle occurs only in C₃ plants. It runs in all photosynthetic plants.
Thinking photorespiration produces ATP. It produces no ATP or NADPH and actually uses ATP.
Writing that the primary acceptor of $C O_{2}$ is a 2-carbon compound. It is RuBP, a 5-carbon ketose sugar.
Believing RuBisCO always prefers $C O_{2}$ . Binding is competitive and depends on the relative $O_{2}$ and $C O_{2}$ concentrations.
Translating Kranz as "crown" or "ring of vessels". Correct: Kranz means wreath, for the arrangement of bundle sheath cells.

Frequently Asked Questions

What is the first stable product of CO2 fixation in C3 and C4 plants?

In C₃ plants the first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon acid, discovered by Melvin Calvin using radioactive carbon-14. In C₄ plants it is oxaloacetic acid (OAA), a 4-carbon acid formed in the mesophyll cells by PEP carboxylase.

What are the three stages of the Calvin cycle?

Carboxylation fixes $C O_{2}$ onto RuBP using RuBisCO, forming two molecules of 3-PGA. Reduction uses 2 ATP and 2 NADPH per $C O_{2}$ to form sugar. Regeneration uses one ATP to re-form RuBP, so that the cycle can continue. The cycle runs in all photosynthetic plants, whether C₃ or C₄.

How many ATP and NADPH are needed to make one glucose?

Each $C O_{2}$ fixed needs 3 ATP and 2 NADPH. One glucose needs six $C O_{2}$ molecules and six turns of the Calvin cycle, so it uses 18 ATP and 12 NADPH. Cyclic photophosphorylation probably supplies the extra ATP needed. Six $C O_{2}$ molecules go in and one glucose molecule comes out.

Why is RuBisCO called carboxylase-oxygenase?

The active site of RuBisCO can bind both carbon dioxide and oxygen. With $C O_{2}$ it carboxylates RuBP to form two PGA molecules; with O2 it oxygenates RuBP in photorespiration. The binding is competitive, so the relative concentrations of the two gases decide which reaction occurs.

What is Kranz anatomy?

Kranz anatomy is the leaf structure of C₄ plants such as maize and sorghum. Large bundle sheath cells form a wreath around each vascular bundle. These cells have many chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. Kranz means wreath.

Why do C4 plants not show photorespiration?

In C₄ plants, C₄ acids made in the mesophyll are broken down in the bundle sheath, releasing $C O_{2}$ . This raises the $C O_{2}$ concentration around RuBisCO, so it works as a carboxylase and its oxygenase activity is minimised. As a result, photorespiration does not occur.

What happens in photorespiration?

In C₃ plants some oxygen binds RuBisCO, and RuBP forms one phosphoglycerate and one 2-carbon phosphoglycolate instead of two PGA. No sugar, ATP or NADPH is made; instead $C O_{2}$ is released and ATP is used. Its biological function is not yet known.

How should I revise C3 and C4 plants for NEET from NCERT?

Learn the comparison table row by row. It covers the Calvin cycle site, initial carboxylation site, acceptors (RuBP, PEP), first products (PGA, OAA) and enzyme locations. It also covers photorespiration, temperature optima (20-25 and 30-40 degrees Celsius) and examples. NEET builds statement and match-list questions directly from these pairs.

Previous year questions on Dark Reactions and Photorespiration

10 questions from past papers, each with a step-by-step solution.

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1. Where Are ATP and NADPH Used?

1.1 Discovery of the first product

1.2 The primary acceptor of CO2

2. The Calvin Cycle

2.1 ATP and NADPH used

3. The C4 Pathway

3.1 Kranz anatomy

3.2 The Hatch and Slack pathway

4. Photorespiration

4.1 Photorespiration in C3 plants

4.2 Why C4 plants lack photorespiration

5. C3 and C4 Plants Compared

6. Exam Essentials

7. Quick Revision

8. Solved Examples

9. Practice Questions

Common Mistakes to Avoid

Frequently Asked Questions

What is the first stable product of CO2 fixation in C3 and C4 plants?

What are the three stages of the Calvin cycle?

How many ATP and NADPH are needed to make one glucose?

Why is RuBisCO called carboxylase-oxygenase?

What is Kranz anatomy?

Why do C4 plants not show photorespiration?

What happens in photorespiration?

How should I revise C3 and C4 plants for NEET from NCERT?

Previous year questions on Dark Reactions and Photorespiration

Ready to master Photosynthesis in Higher Plants?

1.2 The primary acceptor of CO₂

3. The C₄ Pathway

4.1 Photorespiration in C₃ plants

4.2 Why C₄ plants lack photorespiration

5. C₃ and C₄ Plants Compared