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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Aerobic Respiration Equations
Aerobic respiration is one of the most important processes that happens in your body right now - even as you're reading this! It's how your cells get energy from the food you eat and it happens in nearly every living cell on Earth.
Key Definitions:
Your body performs aerobic respiration constantly - you'll use aerobic respiration to process about 30 trillion glucose molecules while reading this sentence!
Aerobic respiration can be summarised in a single chemical equation. This equation shows what goes in (reactants) and what comes out (products):
Glucose + Oxygen → Carbon Dioxide + Water + Energy
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP)
Let's break this down:
Glucose (C6H12O6) - This comes from the food you eat. Your digestive system breaks down complex carbohydrates into glucose.
Oxygen (O2) - This is why you breathe! Your lungs take in oxygen which is transported in your blood to all your cells.
Carbon dioxide (CO2) - This waste product is carried back to your lungs and breathed out.
Water (H2O) - This is either used by the body or excreted.
Energy - Released in the form of ATP, which powers cellular activities.
The main purpose of respiration is to release energy from glucose. This energy is stored in ATP molecules, which are like rechargeable batteries for your cells.
Aerobic respiration is a complex process that happens in specific parts of the cell:
The first stage of respiration (glycolysis) happens here. Glucose is partially broken down into a substance called pyruvate.
Often called the "powerhouse of the cell," this is where most of the ATP is produced. The pyruvate enters the mitochondria to complete respiration.
These are folded membranes inside the mitochondria that increase the surface area for more reactions to happen simultaneously.
Aerobic respiration happens in several stages. Each stage is a series of chemical reactions that gradually break down glucose and capture its energy.
This first stage happens in the cytoplasm and doesn't need oxygen. Glucose (6 carbon atoms) is split into two molecules of pyruvate (3 carbon atoms each).
Energy yield: 2 ATP molecules and 2 NADH molecules (which carry energy).
Pyruvate enters the mitochondria and is converted to a 2-carbon molecule called acetyl coenzyme A (acetyl CoA). During this process, carbon dioxide is released.
Energy yield: 2 NADH molecules.
Also called the citric acid cycle, this is a series of reactions that completely breaks down the remaining carbon atoms, releasing more carbon dioxide.
Energy yield: 2 ATP, 6 NADH and 2 FADH₂ molecules per glucose.
This is where oxygen comes in! The energy carriers (NADH and FADH₂) from earlier stages donate electrons to a chain of proteins. Oxygen accepts these electrons at the end of the chain, forming water.
Energy yield: About 34 ATP molecules.
Without oxygen, the electron transport chain can't function. This is why we need to breathe continuously - our cells need a constant supply of oxygen to keep producing ATP efficiently through aerobic respiration.
The balanced equation for aerobic respiration shows that for every glucose molecule, we need 6 oxygen molecules. This produces 6 carbon dioxide molecules and 6 water molecules:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP)
This balance is important to understand because:
When you exercise, your muscles need more energy, so respiration increases. This is why you breathe faster during exercise - your body is trying to get more oxygen to your cells for aerobic respiration.
If you can't get enough oxygen, your muscles switch to anaerobic respiration temporarily, which produces lactic acid and causes muscle fatigue.
Plants perform aerobic respiration too! While they produce oxygen during photosynthesis, they use oxygen for respiration just like animals do.
At night, when plants can't photosynthesize, they only perform respiration - taking in oxygen and releasing carbon dioxide.
Many elite athletes train at high altitudes where there's less oxygen in the air. This forces the body to produce more red blood cells to carry oxygen more efficiently. When they return to lower altitudes, their bodies can deliver more oxygen to muscles, enhancing aerobic respiration and improving performance.
This demonstrates how understanding the oxygen requirements of aerobic respiration has practical applications in sports science.
Aerobic respiration is essential for life because:
Understanding the equation for aerobic respiration helps explain why we eat, why we breathe and how our bodies convert food into usable energy. This knowledge forms the foundation for understanding many biological processes and health conditions.
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