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Food
Chemical composition of food
Food contains 14 chemical elements. They combine in different ways to form four main food biomolecules in
living things: carbohydrates, lipids (fats), proteins and vitamins.
Biomolecule: molecules found in living things
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6 most common elements in food: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorous and Sulfur
(CHONPS)
5 elements found dissolved in salts are: sodium, magnesium, chlorine, potassium, calcium
Three elements found in trace amounts: iron, copper zinc
Trace element: Chemical element required by living organisms in tiny amounts
Carbohydrates
Contain Carbon, Hydrogen and Oxygen in the ratio CX(H2O)Y where x and y are the same number i.e. Glucose
is C6H12O6. There are there main types of carbohydrates:
1. Monosaccharides:
 One sugar unit i.e. glucose (found in fruit, sweets), fructose (fruits)
 Taste sweet, soluble in water
 Soluble and taste sweet
2. Disaccharides:
 Two monosaccharides joined together i.e. maltose (glucose + glucose), sucrose (glucose + fructose)
 Soluble and sweet
3. Polysaccharides:
 Many monosaccharides joined together
 Starch is a storage polysaccharide found in plants
 Cellulose is a structural polysaccharide in plants
 Glycogen is a polysaccharide stored by animals
 Polysaccharides are insoluble
Sources of Carbohydrates: Bread, potatoes, pasta, rice, fruits
Lipids (Fats & Oils)
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Lipids contain carbon, hydrogen and oxygen
Difference between oil and fat: Fat is solid at room temperature, oil is
liquid.
Triglyceride: The smallest lipid it has one molecule of glycerol linked
to three fatty acid
Phospholipid: a lipid where one of the fatty acids have been replaced with a
phosphate group or has a phosphate group added to it
Sources of lipids: butter, oils, margarine, cream
Proteins
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Contain carbon, hydrogen, oxygen and nitrogen. Sometimes also contain small amounts of sulphur &
phosphorus.
Proteins are made up of amino acids. There are 20 different amino acids. Amino acids are joined
together by peptide bonds.
A peptide is made up of a small number of amino acids (less than 20)
A polypeptide has more than 20 amino acids
A protein is a long polypeptide ( at least 200 amino acids)
Protein function depends on the sequence of amino acids AND how the protein is folded to give its
3D shape
Fibrous Proteins: little/no folding, long fibres that are tough & strong – keratin in nails and hair
Globular Proteins: lots of folding, rounded shape – egg white (albumen)
Sources of proteins: fish, egg, nuts, peas & beans
Deamination: If we eat too much protein excess amino acids are brought to the liver and broken down to
urea. Following deamination, urea is taken to the kidney where it is used to make urine.
Vitamins
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Can’t be made by the body, needed in small amounts
Water soluble vitamin: Vitamin C/Ascorbic Acid, found in vegetables and fresh fruits (especially
oranges & lemons)
Fat soluble vitamin: Vitamin D2/Calciferol, found in liver, fish oils, milk. Vitamin D can be made by
the action of uv rays on chemicals in the skin.
Energy Transfer Reactions
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Metabolism: all the chemical reactions that take place in an organism. Metabolic reactions can be
anabolic reactions or catabolic reactions.
Anabolic reactions: convert smaller molecules into larger ones. Anabolic reactions need enzymes and
energy. Example: Photosynthesis, formation of muscle from amino acids.
Catabolic reactions: Larger molecules broken down into smaller ones. Catabolic reactions need
enzymes and release energy. Example: Respiration, digestion of food
Structural Roles of Biomolecules
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Carbohydrates: Cellulose helps form plant cell walls
Protein: Keratin is found in skin and hair, myosin is found in muscle
Lipids:
1. Lipids are food/energy stores in plants and animals, one gram of lipid contains twice as
much energy as one gram of carbohydrate.
2. In animals stored lipids provide heat insulation and protect organs
3. Lipids combine with phosphorous to give phospholipids. Lipids also combine with
proteins to give lipoproteins. Phospholipids and lipoproteins are important in the
structure of cell membranes.
Metabolic Role of Biomolecules
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Carbohydrates and lipids: broken down in respiration to give energy
Proteins:
1. Act as enzymes to control metabolic reactions
2. Act as antibodies to fight infection.
3. Some proteins are hormones that can regulate body reactions.
Vitamin C:
1. Helps form connective tissue (surrounds body structures and holds them together) such
as skin, gums, ligaments
2. Growth and maintenance of bones and teeth
3. Necessary for immune system to work properly
4. Deficiency of vitamin C: scurvy (bleeding gums with loose teeth, bleeding under skin,
poor healing of skin).
Vitamin D:
1. Helps absorb calcium into blood from intestine. Needed for healthy bones & teeth
2.Deficiency of vitamin D: rickets in children, osteomalacia in adults. Symptoms: weak,
deformed, easily broken bones.
Minerals
Minerals are needed by animals and plants in small amounts:
1. Calcium helps make rigid body structures such as bone and the cement between plant cell walls
2. Soft body parts like muscle need nitrogen and sulphur
3. They help form cell and body fluids (sodium in tears, saliva)
4. To help form biomolecules (haemoglobin is based on iron, chlorophyll is based on magnesium)
Importance of water for living things.
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Component of cytoplasm and body fluids (70 – 90% mass of cell)
Good solvent: it is able to dissolve a range of molecules. This allows many reactions to take place in
water in the cell cytoplasm and it allows transport of molecules in plants and animals.
Participates in chemical reactions:
1. Condensation reactions: small molecules join to
form a larger molecules with the loss of a water
molecule.
2. Hydrolysis reactions: a molecule gains water and is broken
down to give smaller molecules.
3. Photosynthesis
4. Respiration
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Movement through membranes: water and substances dissolved in it can easily pass into and out
of cells & organelles
Good absorber of heat: water is slow to heat up and slow to cool down. This means organisms living
in water or mainly made up of water tend to have stable temperatures.