Unit 4 – Biological Molecules

Learning Objectives

• Why is water so important to living things.• What are carbohydrates, fats and proteins

made of, and their properties.• The role of carbohydrates, fats and proteins in

living organisms.• How to test for the presence of biological

materials.• The structure of DNA

What are you made of?

• We are made of many different chemicals.• Mostly, we are made of water. Almost 80%.• We also contain:– Carbohydrates– Proteins– Fats

• These are the substances our cells are made of and they are vital for life.

Water

• Inside every living organism, chemical reactions are going on all the time.

• This is called metabolism.• Metabolic reactions only take place if the

chemicals which are reacting are dissolved in water.

• Water is a very important solvent.• If cells dry out, the reactions stop, and the

organism dies.

Water• Water is also needed for other reasons:• Plasma is the liquid part of blood.

– It contains water.– Substances like glucose are dissolved in the plasma so they can be

transported around the body.• Digestion:

– Water is needed to dissolve enzymes and nutrients in the alimentary canal (digestive system) so that digestion can take place.

• Excretion:– Helps to remove waste products.– The kidneys remove urea from the body, which is dissolved in water,

forming urine.

Carbohydrates

• Carbohydrates include starches and sugars.• Their molecules contain 3 types of atom:– Carbon– Hydrogen– Oxygen

Sugars (Monosaccharides)

• The simplest kinds of carbohydrates are called simple sugars or monosaccharides.

• Glucose is a simple sugar that is made of 6 carbon atoms joined in a ring, with the hydrogen and oxygen atoms around it.

C6H12O6

Sugars

• Although the contain many atoms, simple sugars are very small.

• They are soluble in water.• They taste sweet.

Sugars (Disaccharides)

• If 2 simple sugar molecules join together, a larger molecule called a complex sugar or dissaccharide is made.

• Examples:– Sucrose (the sugar we use in hot drinks)– Maltose (malt sugar)

• They are also soluble in water and taste sweet.

Sugars (Polysaccharides)

• If many simple sugars join together, a very large molecules is made.

• This is called a polysaccharide.• Examples:– In plants: cellulose and starch– In animals: glycogen

• Most polysaccharides are insoluble and do not taste sweet.

Function of carbohydrates

• Carbohydrates are needed for energy.• The energy is released by respiration.• Usually, the carbohydrate used is glucose.• Animals transport glucose around the body in the

blood system.• Plants also use glucose for respiration but they

don’t transport glucose around their bodies.• Instead, they transport sucrose and then change

the sucrose to glucose when they need it.

Functions of carbohydrates

• Plants store carbohydrates as starch.– They can store them in large quantities in their

seeds and we use these as food.• Animals store carbohydrates as glycogen.– Only small amounts can be stored.– They are stored in the cells in the liver and

muscles.• Cellulose is a strong polysaccharide used in the

cell wall.

Testing for carbohydrates

• We can test for the presence of sugars by adding Benedict’s solution to a food and heating it.

• If the mixture contains a reducing sugar (simple sugar) then it will turn red.

• If there is no sugar the mixture will remain blue.

• See page 42 for a diagram of the experiment.

Testing for carbohydrates

• We can test for starch using iodine solution:– You add a couple of drops to a sample of food.– If there is starch present, the iodine will turn

blue/black.– If there is no starch present, the iodine will stay

orange brown.

Fats

• Fats are also known as lipids.• They also contain only carbon, hydrogen and

oxygen.• They are made of 4 smaller molecules:– Glycerol– 3 fatty acids.

• They are insoluble in water.• At room temperature they are liquids called oils.

Fats molecule

Functions of fats

• Similar to carbohydrates:– They can be used to release energy.– They release twice as much energy as

carbohydrates.• Most cells use carbohydrates first when they

need energy.• The extra energy in fat makes them useful for

storing energy.

• Animals:– The cells that store fats are called adipose tissue.– Adipose tissue is also useful for keeping heat

inside the body.• Plants:– Store oils in their seeds for germination (plant

grow from a seed)

Testing for fats and oils

• To test for fats we use the ethanol emulsion test:– Cut up food and mix it with ethanol.– Pour the ethanol into water.– If there is fat present, the mixture breaks up in the

water to make an emulsion.– It looks white and cloudy, like milk.– If there is no fat present, the mixture stays

transparent.– Page 45 shows a picture of a positive test result.

Proteins

• Proteins contain carbon, hydrogen and oxygen.

• They also contain nitrogen and small amounts of sulphur.

Amino acids

• There are about 20 different kinds of amino acid.

• Any of them can be joined together to make a protein molecule.

Functions of proteins

• Some proteins are soluble in water:– Haemoglobin (the red pigment in blood)

• Some proteins are insoluble:– Keratin (hair and fingernails are made of this)

• Most proteins are used for making new cells.• Cell membranes and cytoplasm contain lots of

proteins.

Functions of proteins.

• Proteins are also needed to make antibodies.– These help to kill bacteria and viruses in the body.

• Enzymes are also proteins.

• The shape is determined by the order of amino acids. This will decide the specific function of the protein.

Testing for proteins

• The test for protein is called the Biuret test.– Mix the food with water.– Add dilute copper sulphate solution.– Then add dilute potassium hydroxide solution.– If the mixture turns purple, protein is present.– If the mixture stays blue, no protein is present.

Coursebook questions

• Page 44 – questions on carbohydrates & water

• Page 45 – questions on fats

• Page 46 – questions on proteins