RespirationAQA GCSE Biology: Revision notes

Aerobic respiration is the process of breaking down glucose using oxygen to release energy. It occurs in the mitochondria of cells.

Word equation: Glucose + Oxygen → Carbon dioxide + Water (+ energy)

Balanced symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

This is an exothermic reaction, meaning it releases energy. The energy released is used for:

• Movement (muscle contraction)
• Synthesis of larger molecules (such as proteins and other complex compounds)
• Maintaining body temperature (keeping core temperature stable in mammals and birds)

Aerobic respiration is the most efficient way to release energy from glucose, as it completely oxidises the glucose molecule.

Anaerobic respiration occurs without oxygen and releases much less energy than aerobic respiration. It happens when oxygen is unavailable or in short supply.

In animals (including humans):

Word equation: Glucose → Lactic acid (+ small amount of energy)

Lactic acid builds up in muscles during intense exercise, causing muscle fatigue.

In plants and microorganisms (such as yeast):

Word equation: Glucose → Ethanol + Carbon dioxide (+ small amount of energy)

This process is used industrially in brewing and bread-making.

Key differences:

Anaerobic respiration is only used as an emergency energy source when oxygen becomes limiting.

Oxygen debt is the amount of extra oxygen needed after intense exercise to break down the lactic acid that accumulated during anaerobic respiration.

How oxygen debt occurs:

1. During intense exercise, muscles work so hard that oxygen demand exceeds supply
2. Muscles switch to anaerobic respiration
3. Lactic acid accumulates in muscles and blood
4. This causes the 'burning' sensation in muscles during and after exercise

How oxygen debt is repaid:

1. After exercise stops, breathing rate and heart rate remain elevated
2. Extra oxygen is delivered to muscles via increased blood flow
3. The liver takes up lactic acid from the blood
4. Lactic acid is converted back to glucose (or oxidised completely to CO₂ and H₂O)
5. When lactic acid levels return to normal, breathing and heart rate return to resting levels

This process can take several minutes depending on the intensity of exercise. The amount of oxygen debt is proportional to the intensity and duration of the exercise performed.

During physical exercise, the body makes several physiological changes to increase oxygen delivery to muscles and remove carbon dioxide:

Heart rate increases:

• Increases from a typical resting rate of 60-80 beats per minute
• Can reach 150-200+ beats per minute during intense exercise
• This pumps blood faster, delivering more oxygen to muscles

Breathing rate increases:

• Increases from a typical resting rate of 12-20 breaths per minute
• Can increase to 40-60+ breaths per minute during intense exercise
• This increases the rate of gas exchange in the lungs

Tidal volume increases:

• Tidal volume is the volume of air breathed in with each breath
• Increases significantly during exercise
• Deeper breaths bring more oxygen into the lungs

Why these changes occur:

• Muscles contract more frequently and require more energy
• More glucose must be broken down through respiration
• This produces more carbon dioxide that must be removed
• The body detects increased CO₂ levels and decreased O₂ levels
• The respiratory and cardiovascular centres in the brain respond by increasing both rate and depth of breathing
• The sympathetic nervous system increases heart rate
• Together, these changes ensure sufficient oxygen delivery and CO₂ removal

After exercise stops, these rates gradually return to resting levels as oxygen demand decreases.

Respiration rate can be measured by monitoring changes in gas volume, oxygen consumption, or carbon dioxide production.

Investigation using a respirometer (measuring oxygen consumption):

1. Place living organisms (such as seeds or small invertebrates) in a sealed container
2. Include an absorbent (such as soda lime or potassium hydroxide) to remove CO₂ produced
3. Connect a tube with a manometer to measure pressure changes
4. As organisms respire and consume oxygen, the pressure inside decreases
5. The manometer reading changes in proportion to oxygen consumed
6. Record measurements at regular intervals (e.g., every minute for 10 minutes)
7. Calculate the rate of oxygen consumption

Variables to consider:

• Temperature: Higher temperatures increase enzyme activity and respiration rate
• Organism type: Different organisms respire at different rates
• Organism mass: Larger organisms consume more oxygen overall
• Availability of substrate: More glucose allows faster respiration
• Activity level: Active organisms respire faster than inactive ones

Safety and ethical considerations:

• Ensure organisms are not harmed during the investigation
• Do not subject organisms to extreme temperatures
• Provide appropriate living conditions
• Use appropriate organisms (seeds germinate; invertebrates are used humanely)

Alternative method: Measure the uptake of oxygen or production of carbon dioxide using gas sensors connected to a data logger for continuous monitoring.