Lesson 5: Cellular Respiration

Definition of Cellular Respiration

Cellular respiration is the process by which living organisms get energy from food — especially from glucose, a type of sugar.

Think of it like this:
Just as a car burns petrol to get energy and move, our cells “burn” glucose to get energy for working, growing, repairing, and staying alive. But instead of fire, this “burning” happens through controlled chemical reactions.

Overall Chemical Equation

Here’s the chemical formula for cellular respiration:

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

Let’s break this down:

  • C₆H₁₂O₆ is glucose (sugar from the food you eat).
  • O₂ is oxygen (from the air you breathe).
  • These two go into the cell.
  • They are broken down into:
    • CO₂ (carbon dioxide) – which you breathe out.
    • H₂O (water).
    • Energy – in the form of ATP (this is what your body uses to do everything).

Why Energy is Released

To understand where the energy comes from, you need to understand chemical bonds.

  • Glucose has many C–H bonds (carbon-hydrogen bonds), and these bonds store a lot of energy.
  • When glucose is broken down, these high-energy bonds are broken.
  • The final products, CO₂ and H₂O, do not have C–H bonds – meaning the energy that was stored is now released.

That energy is captured and stored in ATP molecules, which your body uses like batteries.

Why Glucose is Called the Respiratory Fuel

Glucose is the main source of energy for nearly all cells. It’s like the petrol for our biological engines.

No matter what you eat (rice, bread, fruits), your body usually converts the nutrients into glucose, because it’s simple, efficient, and easy to use for producing energy.

That’s why glucose is known as a respiratory fuel – it’s the fuel used in cellular respiration.

Types of Cellular Respiration

There are two main types:

1. Anaerobic Respiration (Also Called Fermentation)

  • “Anaerobic” means “without oxygen”.
  • Some organisms or cells can break down glucose without using oxygen.
  • However, this process releases only a little energy.

2. Aerobic Respiration

  • “Aerobic” means “with oxygen”.
  • This is the main method used by most organisms (including humans).
  • It gives the maximum energy from glucose.

First Common Step: Glycolysis

Both types of respiration start the same way — with a process called glycolysis.

  • “Glyco” means sugar, and “lysis” means to break.
  • So, glycolysis = breaking of glucose.

In this process:

  • One molecule of glucose is broken down into two molecules of pyruvic acid (a smaller 3-carbon compound).
  • This takes place in the cytoplasm of the cell.
  • It doesn’t need oxygen, so it happens in both types of respiration.

Anaerobic Respiration Explained

After glycolysis, if no oxygen is available, the process takes a different path.

Where Does Anaerobic Respiration Happen?

  • In microorganisms like bacteria.
  • In yeast (used in bread-making).
  • In muscle cells during hard exercise when oxygen is low.

Why It Gives Less Energy?

In anaerobic respiration:

  • Glucose is not completely broken down.
  • It only gives 2 ATPs (energy units) per glucose molecule.
  • That’s just 2% of the total energy glucose contains.

🧠 Imagine you had Rs.100 but only got Rs.2 from it. That’s what anaerobic respiration is like – quick but not efficient.

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