Sciences - Co-ordinated (Double) (0654) | Respiration

Welcome to Chapter B12: Respiration!

Hello future biologists! Respiration is one of the most fundamental processes in all living things. It's often confused with simply breathing, but cellular respiration is much more exciting—it’s how your cells literally make the energy needed to keep you alive!

In this chapter, we will learn about two types of respiration and see exactly why you sometimes feel breathless after a hard sprint. Let's power up your knowledge!

B12.1 Uses of Energy (Why Respire?)

Respiration is the process that unlocks the energy stored in nutrient molecules (like glucose). This released energy is used for every single activity a living organism performs.

Think of energy as the currency of the cell, usually stored as a molecule called ATP. Without respiration, there is no ATP, and without ATP, the cell shuts down.

Core Concept: Essential Uses of Energy

Energy released during respiration is vital for many life processes, including:

• Muscle Contraction: Allowing movement, like lifting a heavy book or even just blinking.
• Protein Synthesis: Building new proteins needed for growth and repair.
• Cell Division: Making new cells (mitosis) for growth or to replace damaged tissues.
• Growth: The overall permanent increase in size and dry mass of the organism.
• Passage of Nerve Impulses: Sending electrical signals quickly along neurones (B13).
• Maintenance of a Constant Body Temperature: Keeping mammals and birds warm, especially in cold conditions.

Quick Review: Respiration provides the fuel (ATP) for all metabolism (all the chemical reactions of life).

B12.2 Aerobic Respiration (The Efficient Way)

Aerobic respiration is the main way organisms produce energy because it is highly efficient.

Core Definition

Aerobic Respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules (like glucose) to release a large amount of energy for metabolism.

The Ingredients and Products

Aerobic respiration happens in the mitochondria (the powerhouses of the cell). It requires glucose (from food) and oxygen (from breathing/gas exchange).

Core: Word Equation for Aerobic Respiration:

glucose + oxygen → carbon dioxide + water (+ energy)

Did you know? The carbon dioxide produced is what we exhale, and the water produced is actually used by your body!

Extended Content: The Chemical Equation

For those taking the Extended paper, you must know the balanced symbol equation.

Supplement: Balanced Symbol Equation:
\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O\]

Memory Aid: Notice how the equation is the exact reverse of the overall equation for photosynthesis, but instead of needing light energy, it releases chemical energy!

Key Takeaway for Aerobic Respiration: It uses oxygen, produces lots of energy, and its waste products are carbon dioxide and water.

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B12.3 Anaerobic Respiration (The Emergency Option)

What happens if your cells need energy right now, but there isn't enough oxygen available? This happens most often during vigorous exercise when muscles use oxygen faster than the blood can deliver it.

Supplement: Definition and Energy Yield

Anaerobic Respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.

This process is fast, but inefficient.

• Less Energy Released: Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.

Supplement: Anaerobic Respiration in Muscles

When you are exercising intensely (like sprinting), your muscle cells switch to anaerobic respiration to get extra energy quickly.

Word Equation (Muscles during vigorous exercise):

glucose → lactic acid (+ much less energy)

The problem with this pathway is the build-up of the waste product: lactic acid.

Analogy: Aerobic respiration is like using a big power station (efficient, sustainable, lots of power). Anaerobic respiration is like using a tiny battery (quick burst of energy, but quickly runs out and creates messy waste).

B12.4 Oxygen Debt and Recovery (Extended)

Supplement: The Problem with Lactic Acid

During vigorous exercise, lactic acid builds up in the muscles and blood. This build-up causes muscle fatigue and pain, which signals the body to stop or slow down.

The body must then break down the toxic lactic acid. The amount of oxygen needed to break down this accumulated lactic acid is called the oxygen debt.

Supplement: Removing the Oxygen Debt

After vigorous exercise, you continue to breathe heavily for a while—this is your body literally "repaying" the oxygen debt.

The removal of oxygen debt involves three crucial steps:

Step 1: Transport Lactic Acid

• A continuation of fast heart rate (B9.2, Supp 8) ensures rapid transport of the lactic acid in the blood from the muscles to the liver.

Step 2: Supply Oxygen

• A continuation of deeper and faster breathing (B11.1, Core 4) supplies the increased oxygen needed.

Step 3: Lactic Acid Breakdown in the Liver

• In the liver, the lactic acid is broken down using aerobic respiration (B12.1, Supp 9c), converting it back into carbon dioxide and water, or sometimes converting it back into glucose or glycogen.

Don't worry if this seems tricky at first! Remember: Lactic acid is the problem. Oxygen is the cure. Repaying the debt means getting oxygen to the liver to finish the job!

Common Mistake Alert: Students often think that during vigorous exercise, anaerobic respiration produces carbon dioxide. It does not! Only aerobic respiration produces carbon dioxide and water. Anaerobic respiration in muscles produces only lactic acid.