Respiration (Edexcel GCSE Biology Combined Science): Revision Notes

Respiration

What is Cellular Respiration?

The energy produced is used for various metabolic processes, including:

• Building complex molecules (e.g., carbohydrates, proteins, and lipids).

• Movement (muscle contraction in animals and transport in plants).

• Cell division (for growth and repair).

• Active transport (moving molecules against their concentration gradients).

• Homeostasis (maintaining stable internal conditions).

• Transmission of nerve impulses (to coordinate body functions). Important Notes:

• Respiration is not the same as breathing (ventilation); respiration takes place in the cells to release energy.

• Photosynthesis and respiration are different processes. Photosynthesis is endothermic and requires energy (light) to build glucose, while respiration is exothermic and breaks down glucose to release energy.

Aerobic Respiration

Word equation:

$$\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy (ATP)}$$

Balanced chemical equation:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$$

• Location: Aerobic respiration mainly occurs in the mitochondria of cells.
• Energy release: Aerobic respiration releases a large amount of energy, which is stored in the form of ATP (adenosine triphosphate), the energy currency of the cell.
• By-products: The process produces carbon dioxide (which is exhaled) and water.

Anaerobic Respiration

In animals (such as during intense exercise when oxygen is low): Word equation:

$$\text{Glucose} \rightarrow \text{Lactic Acid} + \text{Energy (ATP)}$$

• Location: Anaerobic respiration takes place in the cytoplasm of cells.
• Energy release: This process produces much less energy than aerobic respiration.
• By-product: Lactic acid builds up in muscles, leading to muscle fatigue. The lactic acid needs to be broken down later, which requires oxygen (this is why you breathe heavily after intense exercise – to repay the oxygen debt). In plants and yeast (fermentation): Word equation:

$$\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide} + \text{Energy (ATP)}$$

Comparison of Aerobic and Anaerobic Respiration:

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Requirement	Requires oxygen	Does not require oxygen
Location	Occurs in the mitochondria	Occurs in the cytoplasm
Energy Released	Large amount of energy (around 36-38 ATP per glucose)	Small amount of energy (around 2 ATP per glucose)
Glucose Breakdown	Complete breakdown of glucose	Incomplete breakdown of glucose
By-products	Carbon dioxide and water	Lactic acid (in animals) or ethanol and carbon dioxide (in plants/yeast)
Efficiency	More efficient, releasing more energy	Less efficient, releasing less energy
Example	Most of the time in animals and plants	Short bursts of intense exercise (in animals); yeast fermentation (in plants/yeast)

Why Organisms Need Energy

Energy produced during respiration is essential for:

• Metabolic reactions: Building proteins, lipids, and carbohydrates from simpler molecules.
• Movement: Muscle contractions in animals, and transport processes in plants.
• Cell division: For growth, repair, and reproduction.
• Active transport: Moving substances against their concentration gradient across cell membranes.
• Homeostasis: Maintaining stable internal conditions like temperature.
• Transmission of nerve impulses: To allow communication within the body.

Oxygen Debt and Anaerobic Respiration in Animals

• During intense exercise, when the muscles require more oxygen than the lungs and blood can supply, cells switch to anaerobic respiration.
• This leads to the build up of lactic acid in muscles, which causes muscle fatigue.
• After exercise, the body needs to break down the lactic acid, which requires oxygen. The amount of oxygen needed to break down lactic acid and restore normal conditions is known as the oxygen debt.

Summary:

• Cellular respiration is a continuous exothermic process that releases energy for metabolic processes. It occurs either as aerobic respiration (with oxygen) or anaerobic respiration (without oxygen).
• Aerobic respiration is more efficient and produces more energy, while anaerobic respiration produces less energy and lactic acid (or ethanol in plants/yeast).
• Respiration provides energy for all life processes, such as movement, cell division, and active transport, making it essential for the survival of all living organisms.