The Need for Food
Why we Need Food
All living things need food as a source of raw materials for growth and as a source of energy. There are two ways in which organisms obtain their food:
a. Plants make their own food by the process of photosynthesis.
b. Animals rely on ready-made food, by eating either plants or other animals.
All foods contain a mixture of chemicals. The main chemicals are:
- Carbohydrates e.g sugar and starch (which provide energy).
- Proteins (which are needed for growth and repair).
- Fats (store energy, insulate, and make cell membranes).
These foods contain a lot of carbohydrate:
These foods contain a lot of protein:
These foods contain a lot of fat:
Chemical Test for Glucose
- Add a few drops of Benedict's solution to food solution.
- Heat in a water bath until it boils.
- If glucose is present, an orange precipitate will form
Chemical Test for Starch
- Add two drops of brown iodine solution to food solution.
- Solution will turn blue/black if starch is present.
Chemical Structure of Food Types
Carbohydrates, fats and proteins are mostly large molecules formed from many similar, smaller molecules linked together.
- Elements present: Carbon (C), Hydrogen (H) and Oxygen (O).
- The basic units they are built from are glucose molecules.
- Diagram of structure (Glucose molecules in a chain to form e.g starch or cellulose).
- Elements Present: Carbon, Hydrogen, Oxygen.
- The basic units fat is made form is fatty acids and glycerol.
- Diagram of the Structure:
- Elements Present: Carbon, Oxygen, Hydrogen and Nitrogen (N).
- The basic units protein is made from is amino acids.
- Diagram of Structure (amino acids in a chain to form a protein):
Mechanical Breakdown of Food
How Food is Broken Down
Much of the food we eat must be changed before the body can use it. This involves the breakdown of large insoluble food particles into smaller soluble particles that can pass from the small intestine into the bloodstream.
The breakdown of food takes place in two steps:
a. The mechanical breakdown by teeth.
b. The chemical breakdown by enzymes.
e.g Starch (insoluble) -e-n-z-y-m-e-s-> Glucose (soluble).
First we will look at the mechanical breakdown of food - the first step of digestion.
Human Teeth - Humans have four kinds of teeth; each has a role in breaking up food.
You need to look after your teeth.
- Tooth decay is caused by bacteria in your mouth mixing with saliva to form plague.
- Bacteria change the sugar in your food to acid, which attacks the enamel.
- Fluorides in toothpaste strengthen the enamel and make it more resistant to acid.
Humans are omnivores - they can eat both meat and plants. Omnivores have teeth that are suitable for chewing both meat and plants.
Carnivores (meat eaters) and herbivores (plant eaters) have teeth to suit the food they eat.
Chemical Breakdown of Food