Respiration

Energy from respiration

Respiration is the chemical process in cells that releases energy from glucose. Cells use this energy for:

• muscle contraction;
• protein synthesis;
• cell division;
• active transport;
• growth;
• nerve impulses; and
• maintaining body temperature.

Respiration is not the same as breathing. Breathing moves air into and out of the lungs; respiration releases energy in cells.

Aerobic respiration

Aerobic respiration uses oxygen to break down glucose and releases a large amount of energy.

Word equation:

glucose + oxygen → carbon dioxide + water

Balanced equation (Supplement):

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

Carbon dioxide and water are products. Aerobic respiration occurs in cells, including in mitochondria.

Anaerobic respiration

Anaerobic respiration breaks down glucose without oxygen. It releases less energy than aerobic respiration because glucose is only partially broken down.

Yeast

Word equation:

glucose → alcohol + carbon dioxide

The alcohol is ethanol.

Balanced equation (Supplement):

$$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$$

Applications — Enrichment

Yeast anaerobic respiration is used in bread-making and alcoholic-drink production. Carbon dioxide makes bread dough rise and ethanol is the alcohol product.

Human muscle cells

During vigorous exercise, oxygen delivery may not meet the muscles’ energy demand. Muscle cells then respire anaerobically.

Word equation (Supplement):

glucose → lactic acid

The balanced equation is not required. Anaerobic respiration releases less energy and lactic acid builds up in muscles.

Aerobic and anaerobic respiration

Investigating temperature and yeast respiration

Temperature affects the enzymes involved in yeast respiration.

Method

1. Prepare equal yeast suspensions with the same glucose concentration and volume.
2. Place flasks or test tubes in water baths at different temperatures.
3. Connect each vessel to a gas syringe or another suitable carbon-dioxide collection system.
4. Allow the mixtures to reach their test temperatures, then start timing.
5. Measure carbon dioxide produced during a fixed time, or measure the time needed to produce a fixed volume.
6. Repeat each temperature and calculate a mean rate.

The independent variable is temperature. The dependent variable is respiration rate, measured from carbon-dioxide production. Keep yeast concentration, glucose concentration, volume, pH, apparatus, time, and mixing conditions constant. Use a bung and delivery system that do not leak.

Expected results

• At low temperature, particles have less kinetic energy and there are fewer successful enzyme-substrate collisions, so respiration is slow.
• As temperature increases, respiration rate increases to an optimum.
• Above the optimum, enzymes lose their functional shape and the rate decreases.

Plot respiration rate against temperature. A rate calculated as carbon-dioxide volume divided by time makes comparisons clearer. Repeat trials improve reliability; a control without yeast can show that measured gas is produced by yeast rather than the apparatus or solution.

Oxygen debt

During vigorous exercise:

1. Muscles need energy faster than aerobic respiration alone can supply it.
2. Anaerobic respiration produces lactic acid.
3. Lactic acid accumulates, contributing to muscle fatigue.
4. This creates an oxygen debt: extra oxygen is needed after exercise to restore normal conditions.

After exercise:

• heart rate remains high to transport lactic acid in the blood to the liver;
• breathing remains deeper to supply extra oxygen; and
• the liver uses aerobic respiration to break down the lactic acid.

Heart rate and breathing rate stay above resting levels until the oxygen debt has been repaid.

Exam checks

• Aerobic respiration requires oxygen; anaerobic respiration does not.
• Anaerobic respiration releases less energy because glucose is only partly broken down.
• Use ethanol, not only “alcohol,” when naming the yeast product.
• Do not confuse the glucose equations for yeast and muscle cells.
• In the yeast investigation, state the measured variable, controlled variables, repeats, and expected temperature pattern.
• Oxygen debt involves transport of lactic acid to the liver, extra oxygen, and aerobic breakdown in the liver.