RESPIRATION – Complete Notes for Competitive Exams
Respiration is one of the most important topics of Biology for competitive examinations. Questions are commonly asked from types of respiration, aerobic and anaerobic respiration, respiratory organs, cellular respiration, ATP, respiratory quotient and differences between aerobic and anaerobic respiration.
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1. What is Respiration?
Respiration is the biochemical process by which food, mainly glucose, is broken down to release energy.
The energy released during respiration is stored mainly in the form of ATP (Adenosine Triphosphate).
Simplified equation of aerobic respiration:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Important:
Respiration is an energy-releasing (catabolic) process.
It occurs in all living organisms.
In eukaryotic cells, different stages occur in different parts of the cell.
Glycolysis occurs in the cytoplasm.
Krebs cycle and electron transport chain are associated with the mitochondria in aerobic respiration.
2. Respiration vs Breathing
These two terms are often confused.
Breathing
Breathing is the physical/mechanical process of taking in oxygen and giving out carbon dioxide.
Respiration
Respiration is the chemical/biochemical process in which food molecules are broken down to release energy.
Quick Difference
| Breathing | Respiration |
|---|---|
| Physical process | Biochemical process |
| Involves inhalation and exhalation | Involves breakdown of food |
| Gas exchange occurs | Energy is released |
| Does not directly produce ATP | Produces ATP |
| Occurs through respiratory organs | Occurs at cellular level |
Exam Trap: Breathing and respiration are not synonymous.
3. Main Types of Respiration
Respiration can broadly be classified into:
Aerobic respiration
Anaerobic respiration
4. Aerobic Respiration
Aerobic respiration is respiration that takes place in the presence of oxygen.
In this process, glucose is completely oxidised into carbon dioxide and water, releasing a relatively large amount of energy.
Equation:
Glucose + O₂ → CO₂ + H₂O + Energy
Examples:
Humans
Most animals
Plants
Many bacteria and fungi
Energy Yield
In school-level biology, aerobic respiration is commonly stated to produce about 36–38 ATP per glucose molecule, although modern biochemical accounting often gives approximately 30–32 ATP in eukaryotic cells.
JKSSB Tip: If an exam follows traditional textbook convention, 38 ATP may appear as the expected answer. Be alert to the wording/source.
5. Stages of Aerobic Respiration
Aerobic respiration can be understood through three major stages:
1. Glycolysis
Occurs in the cytoplasm/cytosol.
One molecule of glucose (6-carbon) is converted into two molecules of pyruvate (3-carbon).
Oxygen is not directly required for glycolysis.
A small amount of ATP is produced.
2. Krebs Cycle
Also called:
Citric Acid Cycle
TCA Cycle (Tricarboxylic Acid Cycle)
It occurs mainly in the mitochondrial matrix in eukaryotic cells.
3. Electron Transport Chain (ETC)
Located in the inner mitochondrial membrane in eukaryotes.
Produces most of the ATP during aerobic respiration.
Oxygen acts as the final electron acceptor and is ultimately reduced to water.
6. Anaerobic Respiration
Anaerobic respiration occurs without the use of oxygen.
Food is only partially broken down, so less energy is released compared with aerobic respiration.
Examples:
Yeast
Some bacteria
Muscle cells during intense exercise, in terms of anaerobic glycolysis/lactate formation
7. Fermentation
Fermentation is an anaerobic pathway in which glucose is incompletely broken down.
A. Alcoholic Fermentation
Occurs commonly in yeast.
Glucose → Ethanol + CO₂ + Energy
Uses:
Bread making
Brewing
Production of alcoholic beverages
Important: The CO₂ produced by yeast helps bread dough rise.
B. Lactic Acid Fermentation
Occurs in certain microorganisms and can occur in human skeletal muscles during vigorous exercise when oxygen availability is insufficient.
Glucose → Lactic acid + Energy
Accumulation of lactate is associated with the burning/fatigue sensation during intense exercise.
8. Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen | Required | Not required |
| Breakdown of glucose | Generally complete | Incomplete |
| Main products | CO₂ + H₂O | Ethanol + CO₂ OR lactic acid, depending on organism/pathway |
| Energy release | High | Low |
| Glycolysis | Occurs | Occurs |
| Main site after glycolysis | Mitochondria in eukaryotes | Cytoplasm |
| ATP production | Much higher | Low |
| Example | Humans under normal conditions | Yeast; muscle cells during intense exercise |
One-line revision:
Aerobic = Oxygen + Complete oxidation + More ATP
Anaerobic = No oxygen + Incomplete breakdown + Less ATP
9. Cellular Respiration
Cellular respiration refers to the series of metabolic reactions through which cells extract usable energy from nutrients.
The principal energy currency produced is:
ATP – Adenosine Triphosphate
ATP is often called the “energy currency of the cell.”
ATP provides energy for:
Muscle contraction
Active transport
Biosynthesis
Cell division
Other cellular activities
10. Respiratory Substrates
The substances that are oxidised during respiration are called respiratory substrates.
Major respiratory substrates include:
1. Carbohydrates
Usually the primary respiratory substrate.
Example: Glucose
2. Fats
Can be broken down into:
Fatty acids
Glycerol
3. Proteins
Can be broken down into amino acids and subsequently enter metabolic pathways after appropriate processing.
Important:
Glucose is a commonly used respiratory substrate.
11. Respiration in Plants
Plants also respire continuously.
Plants obtain oxygen mainly through:
Stomata – leaves
Lenticels – woody stems
Root surfaces – roots
Important:
Plants perform:
Photosynthesis
Respiration
But these are different processes.
Photosynthesis:
Stores energy in glucose.
Respiration:
Releases usable energy from food.
12. Respiration in Human Beings
The human respiratory system includes:
Nostrils
Nasal cavity
Pharynx
Larynx
Trachea
Bronchi
Bronchioles
Alveoli
Lungs
Alveoli
Alveoli are tiny air sacs present in the lungs.
They are the primary sites of exchange of oxygen and carbon dioxide between air and blood.
Features of alveoli:
Very large surface area
Thin walls
Rich blood supply
Suitable for rapid gaseous exchange
13. Transport of Oxygen
Oxygen is transported in blood mainly by haemoglobin present in red blood cells.
Haemoglobin + Oxygen → Oxyhaemoglobin
Most oxygen is transported in this form.
14. Transport of Carbon Dioxide
Carbon dioxide is transported in blood mainly in the form of:
Bicarbonate ions (HCO₃⁻)
Smaller amounts are transported:
Dissolved in plasma
Bound to haemoglobin as carbamino compounds
Exam Point:
Most CO₂ is transported as bicarbonate ions.
15. Respiratory Quotient (RQ)
Respiratory Quotient is:
RQ = Volume of CO₂ evolved / Volume of O₂ consumed
For carbohydrates:
RQ = 1
For fats:
RQ ≈ 0.7
For proteins:
RQ ≈ 0.8–0.9
Quick Memory Trick:
Carbohydrate → 1
Fat → 0.7
Protein → about 0.8
16. Important Terms for JKSSB
Oxidation
Loss of electrons or increase in oxidation state; biologically, often associated with removal of hydrogen or addition of oxygen.
Reduction
Gain of electrons or decrease in oxidation state.
Glycolysis
Breakdown of glucose into pyruvate.
Pyruvate
Three-carbon end product of glycolysis.
Krebs Cycle
Also called TCA cycle/Citric Acid Cycle.
ATP
Energy currency of the cell.
Fermentation
Anaerobic breakdown of organic substrates producing products such as ethanol or lactate.
17. Respiration in Different Organisms
| Organism | Respiratory process/example |
|---|---|
| Human | Mainly aerobic |
| Yeast | Anaerobic fermentation can produce ethanol + CO₂ |
| Certain bacteria | May perform anaerobic respiration or fermentation |
| Plants | Mainly aerobic respiration; fermentation can occur under oxygen deficiency |
| Muscle cells | Anaerobic glycolysis can occur during intense exercise |
18. Important Differences for Competitive Exams
Respiration vs Photosynthesis
| Respiration | Photosynthesis |
|---|---|
| Releases energy from food | Stores light energy in chemical form |
| Occurs in living cells continuously | Occurs in photosynthetic organisms under suitable light conditions |
| Glucose is broken down | Glucose is synthesised |
| Usually consumes O₂ in aerobic respiration | Uses CO₂ |
| Produces CO₂ in aerobic respiration | Produces O₂ |
| Mainly associated with mitochondria and cytoplasm | Photosynthesis occurs in chloroplasts |
Aerobic vs Anaerobic
Aerobic
Oxygen present
Complete oxidation
More ATP
CO₂ and H₂O are typical end products
Anaerobic
Oxygen not used
Incomplete breakdown
Less ATP
Ethanol + CO₂ or lactate depending on organism/pathway
19. Quick Revision – Most Important Facts
⭐ Respiration releases energy from food.
⭐ ATP is the energy currency of the cell.
⭐ Glycolysis occurs in cytoplasm/cytosol.
⭐ Glycolysis converts glucose into pyruvate.
⭐ Krebs cycle occurs in the mitochondrial matrix in eukaryotic cells.
⭐ ETC is associated with the inner mitochondrial membrane.
⭐ Oxygen is the final electron acceptor in aerobic respiration.
⭐ Yeast performs alcoholic fermentation.
⭐ Alcoholic fermentation produces ethanol + CO₂.
⭐ Lactic acid fermentation produces lactate.
⭐ Alveoli are the major sites of gaseous exchange in human lungs.
⭐ Haemoglobin transports most oxygen.
⭐ Most CO₂ is transported as bicarbonate ions.
⭐ RQ of carbohydrate = 1.
⭐ RQ of fat ≈ 0.7.
⭐ Aerobic respiration releases much more ATP than anaerobic pathways.
⭐ Breathing is a physical process, while respiration is a biochemical process.
20. MCQs for JKSSB Exam
Q1. Respiration is primarily a process of:
A. Food synthesis
B. Energy release from food
C. Water absorption
D. Protein synthesis
Answer: B. Energy release from food
Q2. The energy currency of the cell is:
A. DNA
B. RNA
C. ATP
D. ADP
Answer: C. ATP
Q3. Glycolysis occurs in:
A. Nucleus
B. Cytoplasm
C. Mitochondrial matrix
D. Ribosome
Answer: B. Cytoplasm
Q4. The end product of glycolysis is:
A. Glucose
B. Acetyl-CoA
C. Pyruvate
D. Lactic acid
Answer: C. Pyruvate
Q5. Krebs cycle is also known as:
A. Calvin cycle
B. TCA cycle
C. Urea cycle
D. C4 cycle
Answer: B. TCA cycle
Q6. Krebs cycle mainly occurs in the:
A. Cytoplasm
B. Nucleus
C. Mitochondrial matrix
D. Ribosome
Answer: C. Mitochondrial matrix
Q7. Which gas acts as the final electron acceptor in aerobic respiration?
A. Nitrogen
B. Carbon dioxide
C. Oxygen
D. Hydrogen
Answer: C. Oxygen
Q8. Anaerobic respiration occurs in the absence of:
A. Carbon dioxide
B. Oxygen
C. Nitrogen
D. Hydrogen
Answer: B. Oxygen
Q9. Alcoholic fermentation in yeast produces:
A. Lactic acid only
B. Ethanol and CO₂
C. Oxygen and water
D. Glucose and oxygen
Answer: B. Ethanol and CO₂
Q10. During vigorous exercise, human muscles may produce:
A. Ethanol
B. Lactic acid/lactate
C. Methane
D. Acetic acid
Answer: B. Lactic acid/lactate
Q11. The respiratory quotient for carbohydrates is:
A. 0.7
B. 0.8
C. 1
D. 2
Answer: C. 1
Q12. The respiratory quotient is calculated as:
A. O₂ consumed / CO₂ produced
B. CO₂ produced / O₂ consumed
C. ATP produced / O₂ consumed
D. Glucose consumed / ATP produced
Answer: B. CO₂ produced / O₂ consumed
Q13. Exchange of gases in human lungs primarily occurs in:
A. Trachea
B. Bronchi
C. Alveoli
D. Larynx
Answer: C. Alveoli
Q14. Most oxygen in human blood is transported by:
A. Plasma proteins
B. Haemoglobin
C. Platelets
D. White blood cells
Answer: B. Haemoglobin
Q15. Most carbon dioxide is transported in blood as:
A. Oxygen
B. Carbon monoxide
C. Bicarbonate ions
D. Oxyhaemoglobin
Answer: C. Bicarbonate ions
21. Statement-Type MCQs – JKSSB Pattern
Q16. Consider the following statements:
Glycolysis occurs in the cytoplasm.
Glycolysis directly requires oxygen.
Glycolysis converts glucose into pyruvate.
Which of the statements given above is/are correct?
A. 1 only
B. 1 and 3 only
C. 2 and 3 only
D. 1, 2 and 3
Answer: B. 1 and 3 onl
Q17. Consider the following statements about aerobic respiration:
It uses oxygen.
Glucose is completely oxidised under aerobic conditions.
It produces more ATP than anaerobic fermentation.
Which is correct?
A. 1 only
B. 1 and 2 only
C. 2 and 3 only
D. 1, 2 and 3
Answer: D. 1, 2 and 3
Q18. Consider the following statements:
Yeast can perform alcoholic fermentation.
Alcoholic fermentation produces ethanol and carbon dioxide.
Alcoholic fermentation requires oxygen.
Which statement(s) is/are correct?
A. 1 and 2 only
B. 2 only
C. 1 and 3 only
D. 1, 2 and 3
Answer: A. 1 and 2 only
Q19. Consider the following statements about alveoli:
They provide a large surface area for gas exchange.
Their walls are thin.
They are richly supplied with blood capillaries.
Which is correct?
A. 1 only
B. 1 and 2 only
C. 2 and 3 only
D. 1, 2 and 3
Answer: D. 1, 2 and 3
Q20. Consider the following statements:
Aerobic respiration produces more energy than anaerobic fermentation.
Anaerobic pathways can operate without oxygen.
Glycolysis is common to both aerobic and anaerobic pathways.
Which is correct?
A. 1 only
B. 1 and 2 only
C. 1, 2 and 3
D. 2 only
Answer: C. 1, 2 and 3
Q21. Consider the following statements regarding RQ:
RQ is the ratio of CO₂ evolved to O₂ consumed.
RQ of carbohydrate is approximately 1.
RQ of fat is approximately 0.7.
Which is correct?
A. 1 only
B. 1 and 2 only
C. 2 and 3 only
D. 1, 2 and 3
Answer: D. 1, 2 and 3
Q22. Match the following:
| Column I | Column II |
|---|---|
| 1. Glycolysis | A. Ethanol + CO₂ |
| 2. Alcoholic fermentation | B. Pyruvate |
| 3. Lactic fermentation | C. Lactate |
Codes:
A. 1-B, 2-A, 3-C
B. 1-A, 2-B, 3-C
C. 1-C, 2-A, 3-B
D. 1-B, 2-C, 3-A
Answer: A. 1-B, 2-A, 3-C
🔥 JKSSB Last-Minute Revision
Respiration → Energy release
ATP → Energy currency
Glycolysis → Cytoplasm
Glucose → Pyruvate
Krebs Cycle → Mitochondrial matrix
ETC → Inner mitochondrial membrane
Aerobic → Oxygen + More ATP
Anaerobic → No oxygen + Less ATP
Yeast → Ethanol + CO₂
Muscle under intense exercise → Lactate
Alveoli → Gas exchange
Haemoglobin → Mainly O₂ transport
Bicarbonate → Main form of CO₂ transport
RQ = CO₂ evolved / O₂ consumed
Carbohydrate RQ = 1
Fat RQ ≈ 0.7
Protein RQ ≈ 0.8
⭐ Most Expected JKSSB Questions
Site of glycolysis?
End product of glycolysis?
Site of Krebs cycle?
Final electron acceptor?
ATP meaning?
Aerobic vs anaerobic respiration?
Products of alcoholic fermentation?
RQ of carbohydrate?
Site of gaseous exchange?
Main form of CO₂ transport in blood?