Introduction to Respiration

Respiration is a vital biochemical process in which organic food molecules are oxidised inside living cells to release energy. This energy is essential for sustaining life and is stored in the form of ATP (adenosine triphosphate).

In plants, respiration occurs continuously in all living cells, both day and night. Unlike animals, plants do not possess specialised respiratory organs. Instead, each cell independently exchanges gases and carries out respiration to meet its energy demands.

What is Cellular Respiration?

Cellular respiration is the stepwise, enzyme-controlled oxidation of glucose (or other organic substrates) within cells. The energy released is not liberated all at once but in small, manageable steps, allowing efficient capture as ATP.

The overall equation for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Key Point: Respiration converts chemical energy of food into usable energy (ATP).

Types of Cellular Respiration

Based on the availability of oxygen, cellular respiration is of two main types:

1. Aerobic Respiration

Aerobic respiration occurs in the presence of oxygen and involves the complete oxidation of glucose.

  • Oxygen: Required
  • End products: Carbon dioxide (CO₂), water (H₂O), and ATP
  • Energy yield: High (about 36–38 ATP per glucose)
  • Major stages:
  • Glycolysis (cytoplasm)
  • Link reaction (mitochondria)
  • Krebs cycle (mitochondrial matrix)
  • Electron Transport System (ETS) (inner mitochondrial membrane)

Key Point: Aerobic respiration is the most efficient way of releasing energy from glucose.

2. Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen and involves partial oxidation of glucose. It releases much less energy compared to aerobic respiration.

  • Oxygen: Not required
  • Energy yield: Low (2 ATP per glucose)
(a) Alcoholic Fermentation (Plants & Yeast)
  • End products: Ethanol + CO₂
  • Occurs in yeast and plant cells under anaerobic conditions
(b) Lactic Acid Fermentation (Animal Muscles)
  • End product: Lactic acid
  • Occurs in muscle cells during strenuous exercise when oxygen supply is insufficient

Key Point: Anaerobic respiration helps organisms survive when oxygen is limited.

Comparison: Aerobic vs Anaerobic Respiration

  • Aerobic: Complete oxidation, high ATP yield, oxygen required
  • Anaerobic: Partial oxidation, low ATP yield, no oxygen required

Importance of Respiration in Plants

Respiration is essential because it:

  • Provides energy for biosynthesis of proteins, nucleic acids, and other macromolecules
  • Supplies energy for active transport across membranes
  • Supports cell division, growth, and repair
  • Produces metabolic intermediates used in other biochemical pathways
  • Maintains overall cellular organisation and survival

Key Point: Without respiration, plants cannot grow, repair, or even survive.

Respiration vs Combustion

Although both involve oxidation and energy release, respiration and combustion differ fundamentally:

Respiration Combustion
Slow, stepwise process Rapid, uncontrolled process
Enzyme-controlled Not enzyme-controlled
Occurs in living cells Occurs outside living systems
Energy conserved as ATP Energy released as heat and light

Key Point: Respiration is a controlled energy-release process, unlike combustion.

Sites of Respiration in the Cell

Respiration occurs in different parts of the cell:

  • Glycolysis: Takes place in the cytoplasm
  • Krebs Cycle: Occurs in the mitochondrial matrix
  • Electron Transport System (ETS): Occurs on the inner mitochondrial membrane

Key Point: Mitochondria are the main sites of energy production.

Energy Currency of the Cell – ATP

ATP is called the energy currency of the cell because:

  • It stores energy in its high-energy phosphate bonds
  • Energy is released when ATP is hydrolysed to ADP + Pi
  • This released energy directly powers cellular processes such as movement, synthesis, and transport

Key Point: ATP links energy release to energy utilisation in the cell.

Memory Capsules – Introduction to Respiration

  • Respiration = Oxidation of food + ATP production
  • Occurs in all living cells, day & night
  • ATP = Energy currency
  • Respiration is stepwise & enzyme-controlled
  • Glycolysis → Cytoplasm
  • Krebs + ETS → Mitochondria

One-liner for Exams: Respiration is the controlled release of energy from food inside living cells.

💡 Questions and Answers

Q1. Why is respiration essential for plants even during the night?

A1: Plants require energy continuously for growth, repair, and maintenance. Since these activities occur day and night, respiration must continue even in the absence of light.

Q2. Why is respiration described as a stepwise process?

A2: Respiration occurs through many enzyme-controlled steps. This prevents sudden release of energy and allows efficient storage of energy as ATP.

Q3. Why is ATP called the energy currency of the cell?

A3: ATP stores energy in its phosphate bonds and releases it when required to perform cellular activities, making it the immediate source of energy.

Q4. Mention two differences between respiration and combustion.

A4: Respiration is a slow, controlled process occurring in living cells, whereas combustion is a rapid, uncontrolled process occurring outside cells.

Q5. Name the cellular organelle mainly involved in respiration and justify your answer.

A5: Mitochondria are mainly involved in respiration because the Krebs cycle and electron transport system occur there, producing most of the ATP.