Study Notes
Overview
Respiration is the chemical process that releases energy from glucose in every living cell. It is a fundamental topic in biology, connecting to everything from cell biology and enzymes to ecology and the carbon cycle. For your WJEC GCSE exam, you will be expected to understand the two types of respiration: aerobic (with oxygen) and anaerobic (without oxygen). A common pitfall for candidates is confusing respiration with breathing (ventilation); this guide will make the distinction crystal clear. Expect questions that require you to recall word and symbol equations, compare the efficiency of the two respiration types, and interpret data from practical investigations. This guide will equip you with the knowledge and exam technique to tackle any respiration question with confidence.
Key Concepts
Concept 1: Aerobic Respiration
Aerobic respiration is the most efficient way for cells to release energy from glucose. It occurs in the presence of oxygen and takes place primarily in the mitochondria. This process breaks down glucose completely into carbon dioxide and water, releasing a large amount of energy in the form of ATP (adenosine triphosphate). All living organisms, including plants and animals, carry out aerobic respiration continuously to fuel their metabolic processes.
Example: During a long-distance run, your muscle cells are performing aerobic respiration. They use the oxygen you breathe in and the glucose from your food to generate the large amounts of ATP needed to sustain your movement over a prolonged period.
Concept 2: Anaerobic Respiration
Anaerobic respiration occurs when there is insufficient oxygen for aerobic respiration. It is the incomplete breakdown of glucose and releases much less energy than aerobic respiration. This process occurs in the cytoplasm of the cell.
- In Animals: During intense exercise, such as sprinting, your muscles may not receive oxygen fast enough. They switch to anaerobic respiration, converting glucose into lactic acid. This build-up of lactic acid causes muscle fatigue and creates an 'oxygen debt'.
- In Yeast and Plants: In low-oxygen environments, yeast and plant cells carry out a type of anaerobic respiration called fermentation. They break down glucose into ethanol and carbon dioxide. This process is vital in the baking and brewing industries.
Mathematical/Scientific Relationships
Aerobic Respiration:
- Word Equation (Must memorise): Glucose + Oxygen → Carbon Dioxide + Water (+ Energy released)
- Balanced Symbol Equation (Higher Tier - Must memorise): C6H12O6 + 6O2 → 6CO2 + 6H2O
Anaerobic Respiration (Animals):
- Word Equation (Must memorise): Glucose → Lactic Acid (+ Energy released)
Anaerobic Respiration (Yeast/Plants):
- Word Equation (Must memorise): Glucose → Ethanol + Carbon Dioxide (+ Energy released)
Practical Applications
Required Practical 1: Investigating CO2 production
This practical uses hydrogencarbonate indicator to show that living organisms produce carbon dioxide during respiration. The indicator is sensitive to CO2 levels:
- High CO2: Turns Yellow
- Normal atmospheric CO2: Stays Orange/Red
- Low CO2: Turns Purple
Candidates are often asked to set up an experiment with small invertebrates (like woodlice) or germinating seeds on a gauze platform in a sealed test tube, with the indicator at the bottom. The indicator will turn yellow, demonstrating that respiration has occurred.
Required Practical 2: Investigating Heat Production
Respiration is an exothermic reaction, meaning it releases heat. This can be demonstrated using two vacuum flasks. One contains germinating seeds (which are respiring rapidly) and the other contains boiled, dead seeds as a control. A thermometer is placed in each flask. The temperature in the flask with the germinating seeds will rise significantly more than the control, providing evidence that respiration releases heat energy.