iGCSE Notes: iGCSE Biology - Food and Digestion

Five main food groups: carbohydrates, lipids, proteins, minerals, and vitamins.
We also need fibre and water.
Carbohydrates:

Make up 5% of mass of human body.

Main “fuel” for supplying cells with energy.

Cells release this energy by oxidising glucose, which is a carbohydrate.

Glucose is found in many sweet-tasting foods.

Fructose is found in fruit.

Lactose is found in milk.

Sucrose is ordinary table sugar.

Sugars can cause tooth decay.

Most of the carbohydrates we use come from starch, which is insoluble.

Starch is a polymer of glucose.

Starch is found only in plants, and the equivalent of it in animals is glycogen.

Another carbohydrate is cellulose, found in plant cell walls. Humans cannot digest this, and therefore forms roughage in the gut.

Lipids:

Make up 10% of human body.

If solid at room temperature called fats, if liquid at room temperature called oils.

Fats come from animals, oils come from plants.

Fat is used to insulate the human body as well as to protect organs by surrounding them.

Lipids are made up of glycerol and fatty acids.

Excess lipids, especially saturated fats such as cholesterol, are unhealthy and have been linked to heart disease.

Proteins:

Make up 18% of human body.

Needed for growth and repair.

Meat, fish, cheese, and eggs are rich in protein.

Protein deficiency disease is kwashiorkor.

Proteins are polymers, made up of amino acids.

Shape is crucial to function of protein.

Minerals:

Calcium deficiency is rickets.

Iron deficiency is anaemia.

Mineral	Amount in an adult body (g)	Location or role in body	Examples of foods rich in minerals
Calcium	1000	Making teeth and bones	Dairy products, fish, bread, vegetables
Phosphorus	650	Making teeth and bones; part of many chemicals, e.g. DNA	Most foods
Sodium	100	In body fluids, e.g. blood	Common salt, most foods
Chlorine	100	In body fluids, e.g. blood	Common salt, most foods
Magnesium	30	Making bones, found inside cells	Green vegetables
Iron	3	Part of haemoglobin in red blood cells, helps carry oxygen	Red meat, liver, eggs, some vegetables e.g. spinach

Vitamins:

Vitamin	RDA	Use in body	Deficiency	Source in food
A	0.8mg	Making chemical in retina, protecting surface of the eye.	Night blindness, damaged cornea	Fish liver oils, liver butter, carrots
B1	1.4mg	Helps with cell respiration	Beriberi	Yeast extract, cereals
B2	1.6mg	Helps with cell respiration	Poor growth, dry skin	Green vegetables, eggs, fish
B3	18mg	Helps with cell respiration	pellagra	Liver, meat, fish
C	60mg	Sticks together cell linings	scurvy	Fresh fruit and vegetables
D	5ug	Helps bones absorb calcium and phosphate	Rickets, poor teeth	Fish liver oils

Food tests:

Starch test:

1. Add dilute iodine solution.

2. If it goes blue-black, starch is present.

Glucose test:

1. Place in test-tube, and add 2cm of water.

2. Add several drops of Benedict’s solution.

3. Boil the test-tube in a water bath.

4. If it goes brick-red, glucose is present.

Protein test:

1. Add to 2cm of water in test-tube and mix.

2. Add potassium hydroxide solution.

3. Add copper sulphate solution.

4. If it goes mauve, protein is present.

Lipid test:

1. Add ethanol and shake.

2. Add to cold water.

3. If an emulsion forms, lipids are present.

· Energy content of food is measured in kilojoules (kJ).

· To find the energy content of food do the following:

1. Find mass of food.

2. Place 20cm³ of water into a boiling tube.

3. Measure the temperature of the water.

4. Light the food in a Bunsen.

5. When it lights, hold under the boiling tube until it no longer burns.

6. Measure final temperature of water.

7. (final temperature – temperature at start) x 20 x 4.2

Mass of food (g)

· Food is broken down through digestion so that it can be absorbed.

· Digestion is speeded up by enzymes.

· Digestion can be chemical or mechanical.

· Muscles move food along the gut.

There are two muscle layers in wall of the intestine.

Circular muscles and longitudinal muscles contract and relax in succession, causing narrowing and widening of the gut.

Waves of muscle contraction push food along the gut.

This is called peristalsis.

The mouth, stomach and first part of intestine (duodenum) break down food using enzymes.
Digested food is absorbed in the last part of the small intestine (ileum).
The large intestine absorbs water from the remains and stores faeces.
Digestive Enzymes:

Class of Enzyme	Examples	Digestive Action	Source of Enzyme	Where it acts in the gut
Carbohydrase	Amylase	Starch à maltose	Salivary glands	Mouth
		Starch à maltose	Pancreas	Small intestine
		Maltose à glucose	Small intestine wall	Small intestine
Protease	Pepsin	Proteins à peptides	Stomach wall	Stomach
	Trypsin	Proteins à peptides	pancreas	Small intestine
	Peptidases	Peptides à amino acids	Small intestine wall	Small intestine
Lipase	Lipase	Lipids à glycerol and fatty acids	Pancreas	Small intestine

Process of digestion:

Saliva, containing amylase, moistens the food in the mouth.
The chewed food passes through oesophagus to the stomach.
Stomach secretes hydrochloric acid, killing bacteria on food.
Pepsin breaks down protein in the stomach.
Food is held in stomach by a sphincter muscle, and when this relaxes food is released into the duodenum.
Amylase, trypsin, and lipase (all made by pancreas) are added to the food.
Bile, produced in the liver and stored in the gall bladder, passes down the bile duct onto the food, emulsifying it and increasing its surface area. This means the lipase can act faster on it.
Once the food has been broken down, it is absorbed in the ileum.
Ileum has very large surface area: very long, folded upon itself, and contains villi, each with their own microvilli. Villi are small extrusions from wall of ileum.
Most food products enter capillaries in villi.
Products of fat digestion enter a lacteal, part of the lymphatic system, and form lymph. This is later drained into blood
Blood vessels in ileum form hepatic portal vein which leads to liver, which “processes” food molecules.
The food molecules are assimilated: absorbed into cells from the blood.