IGCSE Science/Section 2: Structures and functions in living organisms/Biological molecules

Carbon is the building block for life, as there are an infinite number of possible carbon-based molecules.

Carbohydrates

Carbohydrates are types of organic compounds found in food and are the main energy source for humans. For example, sugars (glucose, fructose, sucrose, lactose), glycogen, cellulose or starch.

Simple sugars, called monosaccharides, such as glucose are the building blocks for longer carbohydrates. Starch and glycogen are large molecules (disaccharides or polysaccharides) made from these smaller, simpler units. At the atomic level, all carbohydrates are made out of carbon, hydrogen and oxygen.

A positive test for glucose

To test for glucose:

1. Heat a sample with Benedict's reagent
2. If it turns from orange to blue, glucose is present

To test for starch:

1. Apply iodine to the sample
2. If it turns from orange to blue/black, starch is present

Proteins

Proteins are large, very complex molecules. Whole proteins are made from blocks called amino acids. Amino acids are small, so can be absorbed by the intestine. They are made from carbon, hydrogen, oxygen, sulphur, phosphorus and nitrogen.

Foods rich in protein include nuts, meats and milk.

Lipids (fats and oils)

Fats and oils are types of lipids. Lipids are made from fatty acids and glycerol. These are made out of carbon, hydrogen and oxygen, like carbohydrates.

Enzymes

The lock-and-key mechanism

The effect of temperature on enzymes

The effect of pH on enzymes

Enzymes are biological catalysts. That means they increase the rate of a chemical reaction by lowering the activation energy, providing an alternate pathway for the reaction to take place, and while they do this they are not used up.

Enzymes have an active site, which is where substrate molecules (the reactants) bind and undergo a chemical reaction. The lock-and-key mechanism is an explanation for how enzymes function:

1. Substrate binds active site of enzyme
2. Reaction takes place
3. Products formed
4. Products ejected from the active site

Enzymes can be affected by temperature and pH.

The higher the temperature, the faster the rate of reaction. However at too high temperatures, enzymes denature. This happens as the active site's shape is changed, so substrate molecules can no longer bind to it. The point at which the rate is fastest is called the optimal temperature. Most enzymes in the human body have an optimal temperature of 37.5°C.

At pH values which are too extreme, enzymes denature. This happens as the active site's shape is changed, so substrate molecules can no longer bind to it. The point at which the rate is fastest is called the optimal pH. Most enzymes in the human body have an optimal pH of 7 (neutral).

This is why maintaining a constant body temperature and pH is very important (homeostasis).