Introduction to Respiration

BiologyRespiration in PlantsFor NEET aspirants

Respiration in plants is the step-by-step oxidation of food inside living cells to release energy and trap it as ATP. This page covers why every cell needs respiration, how plants exchange gases without breathing organs, respiratory substrates, ATP as the energy currency, and glycolysis (the EMP pathway) step by step with its ATP and NADH count. It follows the NCERT Class 11 chapter Respiration in Plants. NEET asks this topic as sequence, match-the-list and statement questions on glycolysis steps, enzymes and ATP yield.

On this page1Why respiration2Respiration and ATP3Do plants breathe?4Glycolysis5Steps of glycolysis6Fate of pyruvic acid7Exam essentials8Quick revision9Solved examples10Practice

Key Points at a Glance

★ Must learn Respiration: breaking of C-C bonds of complex compounds by oxidation within cells, releasing a considerable amount of energy.
Respiratory substrates: compounds oxidised in respiration; usually carbohydrates, sometimes proteins, fats and organic acids.
★ Must learn Energy is released in slow, enzyme-controlled steps and trapped as ATP, the energy currency of the cell.
Plants have no specialised respiratory organs; stomata and lenticels allow gaseous exchange.
★ Must learn Glycolysis (EMP pathway): glucose to 2 pyruvic acid in the cytoplasm of all living organisms; given by Embden, Meyerhof and Parnas.
Invertase splits sucrose into glucose and fructose; hexokinase phosphorylates glucose to glucose-6-phosphate.
★ Must learn ATP is used at 2 steps: glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose 1,6-bisphosphate.
★ Must learn $NADH + H^{+}$ forms at one step only: PGAL to BPGA.
ATP is formed at 2 steps: BPGA to PGA, and PEP to pyruvic acid. Per glucose: 4 ATP made, net gain 2 ATP.
Pyruvic acid has three fates: lactic acid fermentation, alcoholic fermentation and aerobic respiration.

1. Why Living Organisms Need Respiration

1.1 Energy for life processes

All living organisms need energy for daily activities such as absorption, transport, movement, reproduction and even breathing.
This energy comes from the oxidation of certain macromolecules that we call food.
So the process of breathing is closely connected to the release of energy from food.
Green plants and cyanobacteria prepare their own food by photosynthesis.
Photosynthesis traps light energy and stores it as chemical energy in the bonds of carbohydrates such as glucose, sucrose and starch.
★ Exam imp In green plants, only cells containing chloroplasts photosynthesise; these lie mostly in the superficial layers.
All other non-green organs, tissues and cells also need food for oxidation, so food is translocated to every non-green part.
Animals are heterotrophic: they obtain food from plants directly (herbivores) or indirectly (carnivores).
Saprophytes, such as fungi, depend on dead and decaying matter for food.
Ultimately, all the food that is respired for life processes comes from photosynthesis.

★ Very important Cellular respiration: the mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for the synthesis of ATP.

1.2 Where the two processes occur

In eukaryotes, photosynthesis takes place within the chloroplasts.
The breakdown of complex molecules to yield energy takes place in the cytoplasm and in the mitochondria.
Mitochondria, like chloroplasts, are found only in eukaryotes.

Feature	Photosynthesis	Respiration
Energy	Light energy trapped as chemical energy	Chemical energy of food released and trapped as ATP
Site in eukaryotes	Chloroplasts	Cytoplasm and mitochondria
Which cells	Only chloroplast-containing (green) cells	All living cells, green and non-green

Key idea

All food that cells respire is made by photosynthesis; respiration releases its energy in every living cell.

2. Respiration, Respiratory Substrates and ATP

★ Very important Respiration: the breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy.

★ Exam imp Respiratory substrates: the compounds that are oxidised during respiration.
★ Exam imp Usually carbohydrates are oxidised to release energy.
In some plants, under certain conditions, proteins, fats and even organic acids are used as respiratory substrates.
The energy in a respiratory substrate is not released free into the cell, and not in a single step.
It is released in a series of slow, step-wise reactions, each controlled by an enzyme.
The released energy is trapped as chemical energy in the form of ATP.
Energy released by oxidation cannot be used directly; it is first used to synthesise ATP.
ATP is broken down whenever and wherever energy is needed. Hence ATP acts as the energy currency of the cell.
The energy trapped in ATP drives the various energy-requiring processes of the organism.
★ Exam imp The carbon skeletons produced during respiration act as precursors for the biosynthesis of other molecules in the cell.

Memory Trick

Respiration gives a cell two things: Energy and Building blocks. Energy is stored as ATP; building blocks are the carbon skeletons used to make other molecules.

Key idea

Respiration releases energy in small enzyme-controlled steps and stores it as ATP, the cell's energy currency.

3. Do Plants Breathe?

3.1 Gaseous exchange in plants

Plants need $O_{2}$ for respiration, and they give out $C O_{2}$ .
So plants have systems that ensure the availability of $O_{2}$ .
★ Exam imp Unlike animals, plants have no specialised organs for gaseous exchange.
★ Exam imp Instead, gases are exchanged through stomata and lenticels.

Why plants manage without respiratory organs

Each part looks after itself. Every plant part takes care of its own gas-exchange needs; very little gas is transported from one part to another.
Low demand. Roots, stems and leaves respire at rates far lower than animals do.
Large volumes of gases are exchanged only during photosynthesis, and each leaf is well adapted to meet its own needs then.
In photosynthesising cells, $O_{2}$ is released within the cell itself, so its availability is not a problem.
Short diffusion distance. Even in large, bulky plants, gases need not diffuse far, because every living cell lies quite close to the surface.

In thick, woody stems, the living cells are organised in thin layers inside and beneath the bark.
Stems also have openings called lenticels.
The cells in the interior of the stem are dead and provide only mechanical support.
Thus most plant cells have at least a part of their surface in contact with air.
★ Exam imp Loose packing of parenchyma cells in leaves, stems and roots provides an interconnected network of air spaces.

Memory Trick

Plants manage without lungs because of the three S's: Self-sufficient parts (each part handles its own gases), Slow respiration rate, and a Short diffusion distance.

Quick Recall: tap to check

Name the two openings through which plants exchange gases.

Stomata and lenticels.

Which cells of a woody stem are dead, and what is their role?

The cells in the interior; they provide only mechanical support.

Which tissue provides an interconnected network of air spaces?

Loosely packed parenchyma in leaves, stems and roots.

3.2 Combustion versus step-wise oxidation

Complete combustion of glucose produces $C O_{2}$ and $H_{2} O$ as end products.
In combustion, most of the energy is given out as heat. The combustion reaction requires oxygen.

C_{6} H_{12} O_{6} + 6 O_{2} ⟶ 6C O_{2} + 6 H_{2} O + Energy

For this energy to be useful, the cell must be able to use it to synthesise other molecules it needs.
So the plant cell catabolises (breaks down) glucose in a way that not all the energy goes out as heat.
★ Exam imp Glucose is oxidised not in one step but in several small steps. Some steps release energy just large enough to be coupled to ATP synthesis.
This step-wise, ATP-coupled oxidation is essentially the story of respiration.
During respiration, $O_{2}$ is used, and $C O_{2}$ , water and energy are released as products.

Feature	Respiration	Combustion
Number of steps	Many small steps	A single step
Control	Each step controlled by an enzyme	Not enzyme-controlled
Fate of energy	Much of it trapped as ATP	Most of it given out as heat
Where	Inside living cells	Outside cells, as in burning

3.3 Life without oxygen

Some cells live where oxygen may or may not be available.
The first cells on this planet probably lived in an atmosphere that lacked oxygen.
Even today, several organisms are adapted to anaerobic conditions (absence of oxygen).
Facultative anaerobes can live with or without oxygen; obligate anaerobes require anaerobic conditions.
★ Exam imp All living organisms retain the enzymes needed to partially oxidise glucose without the help of oxygen.
This breakdown of glucose to pyruvic acid is called glycolysis.

Key idea

Step-wise oxidation lets the cell catch energy as ATP; glycolysis, its oxygen-free first stage, is common to all life.

4. Glycolysis: An Overview

The term glycolysis comes from the Greek words glycos (sugar) and lysis (splitting).
The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J. Parnas.
★ Exam imp Hence glycolysis is often called the EMP pathway.
★ Exam imp In anaerobic organisms, glycolysis is the only process in respiration.
★ Exam imp Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms.

★ Very important Glycolysis: the partial oxidation of one molecule of glucose into two molecules of pyruvic acid in the cytoplasm, without the need for oxygen.

In plants, the glucose for glycolysis comes from sucrose (the end product of photosynthesis) or from storage carbohydrates.
★ Exam imp The enzyme invertase converts sucrose into glucose and fructose.
Both these monosaccharides readily enter the glycolytic pathway.

Memory Trick

E-M-P = Embden, Meyerhof, Parnas, in the same order as the letters of the pathway's name.

Key idea

Glycolysis splits one glucose (6C) into two pyruvic acid (3C) in the cytoplasm of every organism.

5. Steps of Glycolysis

Glycolysis is a chain of ten reactions, each controlled by a different enzyme, that turns glucose into pyruvate.
While learning the steps, note where ATP is used, where ATP is made and where $NADH + H^{+}$ is formed.

Phosphorylation of glucose. Glucose is phosphorylated to glucose-6-phosphate by the enzyme hexokinase. One ATP is used. Fructose is also phosphorylated and joins the same pathway.
Isomerisation. Glucose-6-phosphate isomerises to fructose-6-phosphate. From here on, the steps for glucose and fructose are the same.
Second phosphorylation. Fructose-6-phosphate is converted to fructose 1,6-bisphosphate. A second ATP is used.
Splitting. Fructose 1,6-bisphosphate (6C) splits into two triose phosphates (3C): dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL).
Interconversion. DHAP and PGAL (glyceraldehyde-3-phosphate) interconvert, so both trioses continue as PGAL. Every later step therefore runs twice per glucose.
Oxidation of PGAL. PGAL is converted to 1,3-bisphosphoglycerate (BPGA). Two redox equivalents (two hydrogen atoms) are removed from PGAL and passed to $NA D^{+}$ , forming $NADH + H^{+}$ . PGAL is oxidised and combines with inorganic phosphate.
First ATP formation. BPGA is converted to 3-phosphoglyceric acid (PGA). This step yields energy, which is trapped by forming ATP.
PGA is converted to 2-phosphoglycerate.
2-phosphoglycerate loses water to form phosphoenolpyruvate (PEP).
Second ATP formation. PEP is converted to pyruvic acid, and another ATP is synthesised.

Test yourself: hide labels

Figure 1: Steps of glycolysis. The 6C half uses 2 ATP; after the split, each step runs twice per glucose, giving 2

NADH + H^{+}

and 4 ATP. Net gain: 2 ATP per glucose.

NEET Focus

ATP is used at glucose to glucose-6-phosphate and at fructose-6-phosphate to fructose 1,6-bisphosphate. ATP is made at BPGA to PGA and at PEP to pyruvic acid. $NADH + H^{+}$ forms only at PGAL to BPGA. Statement questions often swap the NADH step with the BPGA to PGA step.

5.1 ATP and NADH count per glucose

Step	What happens	Per glucose
Glucose to glucose-6-phosphate	ATP used	1 ATP used
Fructose-6-phosphate to fructose 1,6-bisphosphate	ATP used	1 ATP used
PGAL to BPGA (twice)	$NA D^{+}$ reduced	2 $NADH + H^{+}$ formed
BPGA to PGA (twice)	ATP formed	2 ATP formed
PEP to pyruvic acid (twice)	ATP formed	2 ATP formed
Total	4 ATP formed, 2 ATP used	Net gain: 2 ATP and 2 $NADH + H^{+}$

Memory Trick

The 6C half spends, the 3C half earns. Before the split, glycolysis spends 2 ATP. After the split, each step runs twice and earns 4 ATP and 2 $NADH + H^{+}$ . Spend 2, earn 4, keep 2.

Tips and Tricks

To place a compound, count its carbons. Every compound up to fructose 1,6-bisphosphate is 6C; every compound after the split is 3C, and from 1,3-bisphosphoglycerate onwards each forms twice per glucose (written "2 ×" in Figure 1).

Quick Recall: tap to check

Name the enzyme that phosphorylates glucose to glucose-6-phosphate.

Hexokinase.

At which step is

NADH + H^{+}

formed in glycolysis?

When PGAL (3-phosphoglyceraldehyde) is converted to BPGA (1,3-bisphosphoglycerate).

In Figure 1, which molecule is removed when 2-phosphoglycerate becomes phosphoenolpyruvate?

Water (

H_{2} O

How many ATP molecules are directly synthesised in glycolysis from one glucose?

Four; since two are used, the net gain is two.

Key idea

Glycolysis uses 2 ATP, makes 4 ATP and 2 NADH + H⁺: a net gain of 2 ATP per glucose.

6. Fate of Pyruvic Acid

★ Exam imp Pyruvic acid is the key product of glycolysis.
Its metabolic fate depends on the cellular need: on the availability of oxygen and on the organism.
Cells handle pyruvic acid in three major ways.

Pathway	Oxygen	Product of pyruvic acid
Lactic acid fermentation	Absent (anaerobic)	Lactic acid
Alcoholic fermentation	Absent (anaerobic)	Ethanol and $C O_{2}$
Aerobic respiration	Needs $O_{2}$ supply	Complete oxidation to $C O_{2}$ and $H_{2} O$

★ Exam imp Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
For the complete oxidation of glucose to $C O_{2}$ and $H_{2} O$ , organisms use Krebs' cycle, the central part of aerobic respiration, which needs $O_{2}$ .

Tips and Tricks

If an option places glycolysis in the mitochondria, or says it needs oxygen, eliminate it at once. Glycolysis is cytoplasmic and oxygen-independent in every organism.

Key idea

Glycolysis is the shared first stage; the oxygen supply decides whether pyruvic acid is fermented or fully oxidised.

7. Exam Essentials

Pairs to Match

List I	List II
Glycolysis	Cytoplasm of all living organisms
EMP pathway	Embden, Meyerhof and Parnas
Invertase	Sucrose to glucose and fructose
Hexokinase	Glucose to glucose-6-phosphate
Glucose-6-phosphate to fructose-6-phosphate	Isomerisation
Fructose 1,6-bisphosphate	Splits into DHAP and PGAL
PGAL to BPGA	$NA D^{+}$ reduced to $NADH + H^{+}$
BPGA to PGA	ATP formed
2-phosphoglycerate to PEP	Water removed
PEP to pyruvic acid	ATP formed
Stomata and lenticels	Gaseous exchange in plants
Loosely packed parenchyma	Interconnected network of air spaces
Green plants and cyanobacteria	Make their own food by photosynthesis
Saprophytes such as fungi	Food from dead and decaying matter
ATP	Energy currency of the cell

Exceptions

Not all cells of a green plant photosynthesise; only chloroplast-containing cells do, so non-green parts must receive food.
Plants, unlike animals, have no specialised organs for gaseous exchange.
The interior cells of woody stems are dead; they give only mechanical support.
Energy is not released in a single step in respiration, unlike combustion.
Carbohydrates are the usual substrates, but proteins, fats and organic acids are used only in some plants, under certain conditions.
Glycolysis needs no oxygen; it is the only respiratory process in anaerobic organisms.
Glycolysis is only a partial oxidation; glucose is not broken down to $C O_{2}$ .
$NADH + H^{+}$ is formed at only one step of glycolysis.

Numbers to Remember

10 reactions in glycolysis, each under a different enzyme.
2 molecules of pyruvic acid (3C) from 1 glucose (6C).
2 ATP used, 4 ATP formed directly, net 2 ATP per glucose.
2 $NADH + H^{+}$ per glucose, one from each PGAL.
2 ATP-using steps and 2 ATP-forming steps.
3 fates of pyruvic acid; 3 reasons plants manage without respiratory organs.
Combustion of 1 glucose uses 6 $O_{2}$ and gives 6 $C O_{2}$ and 6 $H_{2} O$ .

Examples to Remember

Example	What it shows
Green plants, cyanobacteria	Autotrophs that make food by photosynthesis
Herbivores	Obtain food from plants directly
Carnivores	Obtain food from plants indirectly
Fungi	Saprophytes on dead and decaying matter
Facultative and obligate anaerobes	Organisms adapted to anaerobic conditions

8. Quick Revision

Respiration breaks C-C bonds of complex compounds by oxidation within cells and releases considerable energy.
Cellular respiration releases energy from food inside the cell and traps it to synthesise ATP.
All food respired ultimately comes from photosynthesis; non-green parts receive food by translocation.
Respiratory substrates: usually carbohydrates; proteins, fats and organic acids in some plants under certain conditions.
Energy is released in slow, enzyme-controlled steps; ATP is the energy currency, and carbon skeletons serve as precursors.
Plants have no respiratory organs; stomata and lenticels allow gas exchange.
Three reasons: each part handles its own gases, low respiration rate, short diffusion distance.
Living cells of woody stems lie in thin layers beneath the bark; loose parenchyma forms a network of air spaces.
Combustion releases most energy as heat in one step; respiration releases it in small ATP-coupled steps.
The first cells probably lived without oxygen; facultative and obligate anaerobes exist today.
Glycolysis (EMP pathway: Embden, Meyerhof, Parnas) occurs in the cytoplasm of all organisms; it is the only respiratory process in anaerobes.
Invertase: sucrose to glucose + fructose. Hexokinase: glucose to glucose-6-phosphate. Then isomerisation to fructose-6-phosphate.
ATP used: glucose phosphorylation and fructose-6-phosphate to fructose 1,6-bisphosphate. Split gives DHAP and PGAL.
PGAL to BPGA forms $NADH + H^{+}$ ; BPGA to PGA and PEP to pyruvic acid form ATP. Net: 2 ATP and 2 $NADH + H^{+}$ per glucose.
Pyruvic acid goes to lactic acid fermentation, alcoholic fermentation or aerobic respiration.

9. Solved Examples

Solved Example 1

How many ATP molecules are directly synthesised in glycolysis from one molecule of glucose, and what is the net gain of ATP?
(A) 2 synthesised; net gain 2
(B) 4 synthesised; net gain 2
(C) 4 synthesised; net gain 4
(D) 2 synthesised; net gain 0

Solution:

Answer: (B). ATP forms at BPGA to PGA and at PEP to pyruvic acid. Each step runs twice per glucose, so 4 ATP are made. Two ATP were used earlier (glucose to glucose-6-phosphate, fructose-6-phosphate to fructose 1,6-bisphosphate). Net gain = 4 - 2 = 2 ATP.

Solved Example 2

Match List I with List II.
List I: A. Invertase, B. Hexokinase, C. PGAL to BPGA, D. PEP to pyruvic acid
List II: I. ATP is synthesised, II. Sucrose is split into glucose and fructose, III.

NA D^{+}

is reduced, IV. Glucose is phosphorylated
Choose the correct answer:
(A) A-II, B-IV, C-III, D-I
(B) A-IV, B-II, C-III, D-I
(C) A-II, B-IV, C-I, D-III
(D) A-III, B-IV, C-II, D-I

Solution:

Answer: (A). Invertase splits sucrose (II). Hexokinase phosphorylates glucose (IV). PGAL to BPGA reduces $NA D^{+}$ to $NADH + H^{+}$ (III). PEP to pyruvic acid forms ATP (I).

Solved Example 3

Read the statements about glycolysis.
A. It occurs in the cytoplasm of all living organisms.
B. It needs oxygen to proceed.
C. Its scheme was given by Embden, Meyerhof and Parnas.
D. Fructose 1,6-bisphosphate splits into dihydroxyacetone phosphate and PGAL.
E. ATP is used in the conversion of BPGA to PGA.
Choose the correct answer:
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) A, D and E only

Solution:

Answer: (A). B is wrong: glycolysis needs no oxygen and is the only respiratory process in anaerobes. E is wrong: BPGA to PGA forms ATP; ATP is used only before the split.

Solved Example 4

Arrange these intermediates of glycolysis in the order in which they are formed.
A. Fructose-6-phosphate
B. Phosphoenolpyruvate
C. Glucose-6-phosphate
D. 1,3-bisphosphoglycerate
E. 3-phosphoglyceric acid
Choose the correct answer:
(A) C, A, D, E, B
(B) A, C, D, E, B
(C) C, A, E, D, B
(D) C, D, A, E, B

Solution:

Answer: (A). Glucose-6-phosphate (C) isomerises to fructose-6-phosphate (A). After the split, PGAL is oxidised to BPGA (D), which gives PGA (E). PGA becomes 2-phosphoglycerate and then phosphoenolpyruvate (B).

Solved Example 5

Which of the following is NOT a reason why plants can manage without specialised respiratory organs?
(A) Each plant part takes care of its own gas-exchange needs
(B) Roots, stems and leaves respire at rates far lower than animals
(C) Living cells lie close to the plant surface
(D) Large volumes of oxygen are transported from leaves to roots

Solution:

Answer: (D). There is very little transport of gases from one plant part to another. Options A, B and C are the three actual reasons.

Solved Example 6

Statement I: The energy released by oxidation in respiration is used directly by the cell for its activities.
Statement II: ATP acts as the energy currency of the cell.
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: (D). The energy released by oxidation cannot be used directly; it is used to synthesise ATP. ATP is then broken down wherever energy is needed, so it is the energy currency.

10. Practice Questions

Practice Questions

Match List I with List II. List I: A. Stomata and lenticels, B. Saprophytes, C. Cyanobacteria, D. Interior cells of woody stems. List II: I. Dead cells giving mechanical support, II. Gaseous exchange, III. Depend on dead and decaying matter, IV. Prepare their own food. Options:
(A) A-II, B-III, C-IV, D-I
(B) A-II, B-IV, C-III, D-I
(C) A-I, B-III, C-IV, D-II
(D) A-III, B-II, C-IV, D-IAnswer: (A). Stomata and lenticels allow gas exchange; saprophytes use dead matter; cyanobacteria photosynthesise; interior stem cells are dead.
Which statements about respiration are correct? A. It breaks C-C bonds by oxidation. B. All the energy is released in one step. C. Carbon skeletons from respiration are precursors for biosynthesis. D. Proteins and fats can never be respiratory substrates. Options:
(A) A and C only
(B) A, B and C only
(C) B and D only
(D) A, C and D onlyAnswer: (A). B is wrong: energy is released in slow, step-wise reactions. D is wrong: proteins and fats can be used in some plants under certain conditions.
Arrange in the order of glycolysis: A. Pyruvic acid, B. 2-phosphoglycerate, C. Fructose 1,6-bisphosphate, D. Glyceraldehyde-3-phosphate, E. Glucose. Options:
(A) E, C, D, B, A
(B) E, D, C, B, A
(C) C, E, D, B, A
(D) E, C, B, D, AAnswer: (A). Glucose, fructose 1,6-bisphosphate, glyceraldehyde-3-phosphate, 2-phosphoglycerate, pyruvic acid.
Which of the following is NOT correct about glycolysis?
(A) It is a partial oxidation of glucose
(B) It occurs in the mitochondrial matrix
(C) It forms two molecules of pyruvic acid per glucose
(D) It is the only respiratory process in anaerobic organismsAnswer: (B). Glycolysis occurs in the cytoplasm.
Statement I: Hexokinase phosphorylates glucose to glucose-6-phosphate. Statement II: Both ATP-using steps of glycolysis occur before fructose 1,6-bisphosphate splits. Options:
(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 correctAnswer: (A). ATP is used at glucose to glucose-6-phosphate and at fructose-6-phosphate to fructose 1,6-bisphosphate, both before the split.
In which step of glycolysis are two redox equivalents removed and passed to $NA D^{+}$ ?
(A) Glucose to glucose-6-phosphate
(B) PGAL to BPGA
(C) BPGA to PGA
(D) PEP to pyruvic acidAnswer: (B). Two hydrogen atoms are removed from PGAL and passed to $NA D^{+}$ , forming $NADH + H^{+}$ .
Differentiate between respiration and combustion.Answer: Respiration occurs inside living cells in many small, enzyme-controlled steps, and much of the energy is trapped as ATP. Combustion is a single, uncontrolled step in which most of the energy is lost as heat.
What are respiratory substrates? Name the most common respiratory substrate.Answer: Compounds oxidised during respiration to release energy. Carbohydrates are the usual substrates, and glucose is the most common (favoured) one.
Give the schematic representation of glycolysis.Answer: Glucose to glucose-6-phosphate (ATP used) to fructose-6-phosphate to fructose 1,6-bisphosphate (ATP used), which splits into DHAP and PGAL. PGAL to BPGA ( $NADH + H^{+}$ formed) to PGA (ATP formed) to 2-phosphoglycerate to PEP (water lost) to pyruvic acid (ATP formed). See Figure 1.
What is the significance of the step-wise release of energy in respiration?Answer: Releasing energy in small steps lets each step be coupled to ATP synthesis, so the energy is trapped as ATP instead of being lost as heat. The intermediates also supply carbon skeletons for biosynthesis.

Common Mistakes to Avoid

Watch out

Writing that glycolysis occurs in the mitochondria. It occurs in the cytoplasm.
Saying glycolysis needs oxygen. It is oxygen-independent and is the only respiratory process in anaerobes.
Taking 4 ATP as the net gain of glycolysis. Four are made, but two are used, so the net gain is 2 ATP.
Placing $NADH + H^{+}$ formation at BPGA to PGA. It forms at PGAL to BPGA; BPGA to PGA forms ATP.
Calling invertase the enzyme that phosphorylates glucose. Invertase splits sucrose; hexokinase phosphorylates glucose.
Assuming every cell of a green plant photosynthesises. Only chloroplast-containing cells do; the rest depend on translocated food.
Thinking plants move large amounts of oxygen from leaves to roots. There is very little transport of gases between plant parts.
Writing that respiration, like combustion, releases its energy as heat in one step. Respiration releases energy in small steps and traps much of it as ATP.

Frequently Asked Questions

What is respiration in plants?

Respiration is the breaking of C-C bonds of complex compounds through oxidation within cells, which releases a considerable amount of energy. The energy is released in slow, enzyme-controlled steps and trapped as ATP. Usually carbohydrates are oxidised, but proteins, fats and organic acids can be used in some plants under certain conditions.

Do plants breathe like animals?

Plants need oxygen for respiration and give out carbon dioxide, but they have no specialised organs for gaseous exchange. Gases enter and leave through stomata and lenticels, and diffuse through a network of air spaces formed by loosely packed parenchyma cells in leaves, stems and roots.

Why can plants survive without respiratory organs?

There are three reasons. Each plant part takes care of its own gas exchange. Plant parts respire far more slowly than animals, and photosynthesising cells release oxygen within themselves. Finally, every living cell lies close to the surface, so gases need to diffuse only a short distance.

What is glycolysis and where does it occur?

Glycolysis is the partial oxidation of one glucose molecule into two molecules of pyruvic acid through a chain of ten enzyme-controlled reactions. It occurs in the cytoplasm of all living organisms and needs no oxygen. In anaerobic organisms it is the only process of respiration.

Why is glycolysis called the EMP pathway?

The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J. Parnas, so it is named the EMP pathway after the initials of these three scientists. The word glycolysis itself comes from the Greek glycos, meaning sugar, and lysis, meaning splitting.

How many ATP are produced in glycolysis?

Four ATP molecules are synthesised directly per glucose: two at BPGA to PGA and two at PEP to pyruvic acid. Two ATP are used earlier, at glucose to glucose-6-phosphate and at fructose-6-phosphate to fructose 1,6-bisphosphate. So the net gain is 2 ATP, along with 2 NADH + H+.

Why is ATP called the energy currency of the cell?

The energy released by oxidation in respiration cannot be used directly. It is first used to synthesise ATP, which is broken down whenever and wherever energy is needed. Like money, ATP stores energy in one place and spends it in another, for all energy-requiring processes.

Which parts of Introduction to Respiration are most important for NEET?

Every line of the NCERT chapter can be asked, but the most common points are the definition of respiration, why plants lack respiratory organs, the site of glycolysis, the EMP scientists, the enzymes invertase and hexokinase, the ATP-using and ATP-forming steps, the single NADH step and the net gain of 2 ATP.

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