Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Energy is stored in the chemical bonds of biological molecules: mainly carbohydrates and lipids. Respiration breaks down glucose to release this energy for cell processes. Lipids store the most energy per gram.
All living organisms need a constant supply of energy. This energy is locked in the chemical bonds of food molecules and released through respiration. This guide explains how different molecules store energy and why organisms use specific molecules for short-term and long-term energy storage.
Energy Content of Different Molecules
Molecule type
Energy per gram (approx.)
Main use
Carbohydrates
17 kJ/g
Quick-release energy source; short-term storage (glycogen)
Lipids (fats/oils)
37 kJ/g
Long-term energy storage; insulation
Proteins
17 kJ/g
Not usually used for energy; used for growth and repair. Only respired as a last resort.
Lipids contain more than twice the energy per gram of carbohydrates or proteins. This is why they are used for long-term energy storage: they are the most efficient way to store energy in a compact space.
How Energy Is Released
Energy stored in glucose is released by respiration:
Active transport (moving substances against a concentration gradient)
Protein synthesis (building new proteins from amino acids)
Cell division (mitosis)
Maintaining body temperature (in endotherms)
Nerve impulse transmission
Measuring Energy in Food (Calorimetry)
A simple calorimetry experiment can estimate the energy content of food:
Weigh a food sample (e.g. 1 g of crisp/nut).
Measure a known volume of water in a boiling tube and record its starting temperature.
Ignite the food sample and hold it under the boiling tube until it stops burning.
Record the final water temperature.
Calculate energy released: Energy (J) = mass of water (g) × specific heat capacity (4.2 J/g/°C) × temperature rise (°C).
Limitations: Heat is lost to the surroundings, not all the food may burn, and heat may not be transferred efficiently to the water. The calculated value will always be lower than the actual energy content.
Try a focused question (3 marks)
Burning 0.5 g of a nut raises the temperature of 20 cm³ of water by 35°C. Calculate the energy released per gram. (3 marks)
Show the answer and marking guidance
Energy = mass × SHC × temp rise = 20 × 4.2 × 35 = 2940 J.
Energy per gram = 2940 ÷ 0.5 = 5880 J/g.
= 5.88 kJ/g.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying "energy is created": energy is released or transferred, never created (conservation of energy).
Forgetting that proteins can be used for energy: they can be, but usually they are used for growth and repair first.
Not accounting for heat loss in calorimetry experiments: always state that the actual value is higher than the calculated value.