The Products of Digestion

Section 1 of 6From absorbed molecule to body molecule

Assimilation

• Assimilation Once digested food has been absorbed into the blood, the cells of the body take it up and use it to build or fuel their own structures.
• This incorporation of absorbed substances into the body is called assimilation.

Glucose

• Glucose Glucose is used in respiration to release energy.
• Extra glucose can be joined into the large carbohydrate glycogen, which is stored in the liver and muscles.

Amino acids

• Amino acids Amino acids are joined in a chosen order to make new proteins, such as keratin in skin and hair, enzymes and the proteins of muscle.

Fatty acids and glycerol

• Fatty acids and glycerol These are rebuilt into fats (lipids) for storage and insulation, and they are also used to make cell membranes.

Section 2 of 6Functions and sources of the main food groups

Food group	Main functions	Good sources
Carbohydrate	The main source of energy; glucose is a reactant in respiration	Bread, cereals, pasta, rice, potatoes
Protein	Growth and repair of tissues; making enzymes, hormones and structural proteins	Meat, fish, eggs, pulses, nuts
Fat (lipid)	Energy storage, insulation under the skin and protection of organs; a very concentrated energy store	Butter, oils, nuts, oily fish

Section 3 of 6How digestion products are used

1Food is digested

• Food is digested Enzymes break large insoluble molecules into small soluble ones.

2Products are absorbed

• Products are absorbed Small molecules cross the villi into the blood or lymph.

3Blood delivers them

• Blood delivers them The circulatory system carries them to every cell in the body.

4Cells assimilate them

• Cells assimilate them Cells use them for energy, to grow, to repair damage or to store.

Section 4 of 6Linking function to structure

Using the products of digestion

Energy

• Energy Carbohydrates release energy most readily because glucose is respired quickly.

Growth and repair

• Growth and repair Proteins are needed in larger amounts by growing children, since every new cell needs new protein.

Storage

• Storage Fats hold more energy per gram than carbohydrates, but their energy is harder to release, so they are a long-term store.

Section 5 of 6Key terms

Term	Meaning
Assimilation	The use of absorbed food molecules by cells to build new substances or release energy.
Glycogen	A large carbohydrate made from many glucose units, used for short-term storage in the liver and muscles.
Amino acid	A small molecule that is the building block of protein.
Keratin	A structural protein found in skin, hair and nails.
Lipid	The scientific name for fats and oils.
Insulation	A layer that reduces heat loss from the body; fat under the skin acts as insulation.

Section 6 of 6Exam technique

1

Saying that digestion products are 'used up' — they are reassembled into the body's own carbohydrates, proteins and fats.

2

Writing that protein is the main source of energy — carbohydrates are the main energy source; protein is for growth and repair.

3

Mixing up glucose and glycogenglucose is the small soluble sugar, glycogen is the large storage carbohydrate.

4

Saying fats release energy faster than carbohydrates — fat stores more energy per gram but releases it more slowly.

5

Forgetting that the products reach the cells via the blood, so absorption must come before assimilation.