BIOLOGY · Stage 8 · Chapter 6: Nutrition, Health and Movement

Biology 6.2 Carbohydrates, fats and energy

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Lesson summary

Biology 6.2 Carbohydrates, fats and energy — Alhussan International Academy — Biology (Stages 7–9) notes: Compare sugars, starch, glycogen and lipids as energy foods and energy stores.Respiration — aerobic respiration releases energy from food in cells using oxygen.Main energy fo…

Main energy foods
Energy content
Attraction
Other roles
Excess energy
Simple sugars

Section 1 of 6Foods that supply energy

Respiration
- aerobic respiration releases energy from food in cells using oxygen.
Main energy foods
- carbohydrates and lipids (fats and oils) provide animals, including humans, with the energy they need.

Section 2 of 6Carbohydrates: sugars and starch

1Sugars3 points

small, soluble carbohydrate molecules such as glucose, which often taste sweet.
They move quickly by diffusion from the digestive system into the blood, giving an energy boost.
Dates and bananas contain lots of sugar.

2Starch4 points

a carbohydrate made by plants from lots of sugar molecules.
Bread, rice and pasta contain lots of starch.
It must be broken down by the body, so energy from starchy foods is released slowly.
Potatoes, sweet potatoes, carrots and beets are starch-filled plant roots that make energy-rich food.

Section 3 of 6Lipids

Energy content
- fats and oils contain much more energy per gram than carbohydrates, about twice as much.
Structure
- all lipid molecules have the same basic structure, a glycerol molecule with fatty acids attached.
- Each fat or oil has a different set of fatty acids.
Plants
- they store energy-rich lipids in seeds to feed the new plant when it grows.
- Many animals eat seeds and nuts to get the fats and oils they need.
Attraction
- people and animals like fat- and oil-rich foods because they are good energy sources,
  - and fats combined with sugars are very high in energy but rare in nature.
  - (cakes, ice cream)
Other roles
- fat insulates the body and protects organs.

Section 4 of 6How energy is stored

Reason
- animals need energy all the time but do not eat all the time, so energy is stored for when food is not available.
Glycogen
- a short-term carbohydrate energy store in the muscles and liver, made of many sugar molecules joined together like starch.
- It gives instant energy if the blood sugar level gets low.
Fat
- a long-term store of energy.
- Excess food molecules are converted into fat and stored in fat cells under the skin and around organs.
- If not enough food is eaten, the body breaks down fat stores to give cells energy.

Section 5 of 6Comparison of energy foods

Food type	Speed of energy release	Energy per gram (approx.)	Examples
Sugars	Fast	17 kJ	Dates, bananas
Starch	Slow, after digestion	17 kJ	Bread, rice, potatoes
Glycogen	Very fast, an instant store	17 kJ	Muscle, liver
Lipids	Slowest, long-term store	37 kJ	Oils, nuts, butter

Choice
- sugary fruit gives a quick boost, starchy food a steady supply, and fatty food packs the most energy per gram.
Athletes
- they eat starchy meals before long events so glycogen stores are full.
Excess energy
- any excess energy is stored as fat, so energy eaten and used must balance.

Section 6 of 6Testing for energy foods

Starch
- iodine solution turns blue-black.
Simple sugars
- Benedict's solution turns orange-red on heating.
Lipids
- a smear makes translucent marks on paper, or a cloudy emulsion forms in ethanol and water.

Last updated: 13 October 2026