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Respiration - important molecules
Respiration - important molecules
Why is it Important?
Respiration is one of the most important processes of life. Every living cell, from tiny bacteria to the cells in your body, needs energy. This energy comes mainly from glucose, and it is released through a series of chemical reactions called respiration.
Thanks to modern tools, in this chapter you won’t just read about respiration, you will also be able to visualize the essential molecules in 3D (O₂, CO₂, glucose, water, lactic acid, ethanol, etc.) and see how they participate in this fundamental process.
Why do we need energy?
Our cells use energy to:
- Contract muscles so we can move.
- Build proteins from amino acids.
- Divide to grow and repair tissues.
- Transport substances across membranes (active transport).
- Send nerve impulses.
- Keep the body warm.
All of this energy comes from breaking down glucose molecules. But glucose by itself is not enough, cells must process it using respiration.
Storage of energy
Glucose in the body is often stored as chains of glucose molecules:
- In animals, glucose is stored as glycogen (mainly in the liver and muscles).
- In plants, it is stored as starch.
When cells need energy, enzymes break these chains into individual glucose molecules. Those glucose molecules then enter respiration, where their bonds are broken down step by step, releasing energy.
So the important clarification is:
- Breaking glycogen/starch releases glucose units (not directly energy).
- Respiration of glucose is what actually releases the energy.
Types of respiration
1. Aerobic Respiration (with oxygen)
Most of the time, cells combine glucose with oxygen inside the mitochondria to release energy.
Word equation:
glucose + oxygen → carbon dioxide + water (+ energy)
Balanced equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
This is a highly efficient way to release energy.
Visualization:
Glucose
Oxygen
Carbon dioxide
Water
2. Anaerobic Respiration (without oxygen)
When oxygen is not available, cells can still release some energy from glucose — but much less.
In yeast and some plants:
glucose → ethanol + carbon dioxide (+ little energy)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂In human muscle cells (during intense exercise):
glucose → lactic acid (+ little energy)
C₆H₁₂O₆ → 2C₃H₆O₃
Anaerobic respiration in humans allows short bursts of energy when oxygen is limited, such as during sprinting. However, lactic acid accumulates in the muscles, leading to fatigue. Later, when oxygen becomes available again, lactic acid is transported to the liver and converted back into glucose (the Cori cycle).
Visualization:
Ethanol
Lactic acid
Gas Exchange: Oxygen In, Carbon Dioxide Out
Respiration depends on gas exchange in the lungs. The tiny air sacs in the lungs are called alveoli. Their thin walls, together with surrounding capillaries, allow oxygen to diffuse into the blood and carbon dioxide to diffuse out.
Oxygen enters red blood cells and binds with hemoglobin, traveling to all body cells.
Carbon dioxide, a waste product of respiration, diffuses back into the alveoli and is breathed out.
Visualization:
Hemoglobin
Heme b
Hemoglobin and Heme B – Oxygen Carriers
Oxygen does not travel through the blood on its own, it is carried by a special protein called hemoglobin in red blood cells.
Each hemoglobin molecule is made of four protein chains, and in each chain there is a heme group.
The heme group contains an iron (Fe²⁺) atom at its center. This iron atom can bind oxygen molecules (O₂) reversibly.
When you breathe in, oxygen from the alveoli binds to hemoglobin in red blood cells. When blood reaches body tissues, hemoglobin releases oxygen for respiration.
The waste product, carbon dioxide (CO₂), travels back to the lungs, either dissolved in plasma, bound to hemoglobin, or as bicarbonate ions.