Nutrition – The Chemistry of Food

Nutrition – The Chemistry of Food

Food is material that is a good source of one or more of the following: protein, carbohydrate or lipid.

Living organisms need food for energy, growth, repair, defence and reproduction.

Food often contains vitamins and minerals.

Metabolism

Metabolism is the full set of chemical processes carried out by a living organism (anabolism + catabolism).

Anabolism: the formation of large complex organic molecules by linking smaller simpler organic molecules.

Catabolism: the breakdown of large complex molecules into smaller simpler biomolecules.

Anabolic reactions require energy input and catabolic reactions release energy.

Protein

Elements: C, H, O and N in all proteins. Some proteins also contain P and/or S.

Subunits: Amino acids are the subunits that are linked by peptide bonds in chains, folds and branches.

Twenty different amino acids — each different sequence of amino acids produces a different protein.

Each protein has a specific functional shape.

Proteins synthesis takes place at the ribosomes.

Meat, fish, eggs, milk, beans, peas and nuts are good sources of dietary protein.

Structural Role of Protein

Keratin: in hair and outer layer of the skin.

Myosin: major protein in skeletal and cardiac muscle.

Metabolic Role of Protein

Many proteins function as enzymes (specific biological catalysts).

Some proteins function as hormones.

Carbohydrate

Elements: CHO. General Formula: (CH2O)n or CX(H2O)Y

Monosaccharides: single sugar unit

Pentoses: C5H10O5 Deoxyribose of DNA and Ribose of RNA

Hexoses: C6H12O6 Glucose, Fructose, Galactose — use for respiration

Disaccharides: double sugars — two sugar units linked together

Maltose: glucose + glucose — intermediate between glucose and starch

Sucrose: glucose + fructose — food transported in the phloem of plants is a sucrose solution

Lactose: glucose + galactose — the sugar present in milk

Polysaccharides: multisugars — the three examples are multiglucoses

Starch: plant glucose reserve

Glycogen: glucose reserve of animals and fungi. Glycogen stored in skeletal muscle and liver

Cellulose: plant cells walls and fibre in our diet

Dietary Sources of Carbohydrates

Monosaccharides: fruit, honey and jam.

Disaccharides: Sucrose - fruit, table sugar. Lactose - milk. Maltose - germinating seeds.

Polysaccharides: Starch: bread, rice, pasta, potatoes, seeds.

Cellulose: fruit, vegetables, wholegrain cereals, nuts.

Structural Role of Carbohydrate

Cellulose walls of plant cells.

Chitin in the cell walls of fungi.

Metabolic Role of Carbohydrate

Energy Source: energy released by the respiration of glucose is used to make ATP.

Energy Storage: starch in plants, glycogen in animals and fungi.

Lipid

Elements: CHO – with more H but less O than carbohydrates.

Composed of glycerol with three fatty acids linked to the glycerol.

Fat – solid lipid at room temperature. Oil – lipid that is liquid at room temperature.

Phospholipid: two fatty acids and a phosphate group linked to the glycerol.

Good Dietary Sources: meat, milk, butter, cheese, plant oils, margarine.

Structural Role of Lipid

Lipids and Phospholipids are very important in cell membrane structure.

The protective wax cuticle on the outside of leaves.

Metabolic Role of Lipids

Energy storage: more than twice the energy of carbohydrate or protein.

Energy source: released during respiration.

Storage of fat-soluble vitamins.

Some lipids function as hormones.

Hormones as Regulators of Metabolic Activity

Hormones are chemical messengers that cause their target cells or tissues to adjust or alter their activity.

Hormones stimulate or inhibit specific metabolic reactions.

The level of stimulation or inhibition depends on the concentration of hormone in the blood.

Hormones play an important role in homeostasis.

Vitamins

A vitamin is an organic compound needed in small quantities in the diet for health.

Water-soluble Vitamin: Vitamin C (ascorbic acid)

Obtained in fresh fruit and vegetables.

Needed to make and maintain connective tissue and the absorption of iron by the gut.

Long term deficiency of vitamin C causes a disease called scurvy.

Scurvy symptoms: internal bleeding, bruising, bleeding gums, poor healing.

Fat-soluble Vitamin: Vitamin D (calciferol)

Obtained from milk, eggs, liver, fish liver oils and produced in skin exposed to UV light.

It is needed for bone and tooth formation, bone maintenance and the absorption of calcium from the gut.

Long term deficiency cause diseases known as rickets and osteomalacia.

Major deficiency symptoms: late teething and walking, deformed legs and arms, weak bones.

Minerals

Minerals or mineral nutrients are soluble inorganic salts that contain elements essential for metabolism.

Minerals are only needed in small quantities in comparison to protein, carbohydrate and lipid.

Plants obtain their minerals by absorbing them from external ‘water’ — soil water, freshwater and seawater.

Animals receive most of their minerals in the food they eat; some from the ‘water’ they drink.

Plant Mineral Requirement (any two)

Calcium: for the middle lamella that ‘glues’ neighbouring plant cell walls.

Magnesium: for the production of chlorophyll so vital for photosynthesis.

Animal Mineral Requirement (any two)

Calcium: formation of teeth and bones.

Iron: formation of haemoglobin so vital for oxygen transport in our blood.

General Role of Minerals in Living Organisms

Construction of Hard Parts: calcium for teeth and bone; nitrogen for chitin in the cell walls of fungi.

Formation of Soft Tissue: nitrogen and sulphur in the protein of muscle tissue.

Maintain Correct Fluid Concentration: sodium chloride role in blood plasma concentration.

Water: H2O

Importance of Water for Organisms

Fluid Component: 90% of cell cytoplasm, 92% of blood plasma, 97% of tissue fluid and lymph.

Multipurpose Solvent: medium for metabolism and transport.

Take Part in Metabolic Reactions

Photosynthesis: water is a raw material in the light stage.

Respiration: aerobic respiration produces water.

Anabolism: water in produced when the subunits of macromolecules link together.

Catabolism: water is used to break the bonds that hold together the subunits of macromolecules.

Movement of Materials through Cell Membranes: diffusion, osmosis and active transport.

Control Cell Shape

Immature plant cell enlarge to mature size and shape as a result of their absorption of water by osmosis.

Opening and closing of the stoma by change in shape of the guard cells by change in their turgor.

Turgor plays an important role in the support of soft plant tissue.

Good Absorber of Heat Energy

A lot of heat energy has to be absorbed to bring about an increase in temperature or vaporisation.

Water as a medium is a temperature-stable which is so important for homeostasis.

Vaporisation of water is an excellent cooling mechanism.

Mandatory Food Tests

Starch

Yellow-brown iodine solution is placed on the food sample.

A blue-black colour indicates that starch is present.

A yellow-brown colour indicates that starch is not present.

Reducing Sugar

E.g., glucose, fructose, maltose, lactose. (Sucrose is a non-reducing sugar).

Add an equal volume of blue Benedict’s Reagent to the food solution.

Heat but do not boil.

A brick-red colour indicates that reducing sugar is present.

A blue colour indicates that reducing sugar is not present.

Control: water – blue colour result.

Lipid

Rub the food onto brown paper.

A translucent stain that does not ‘dry out’ indicates fat is present.

Control: water – stain dries out and the brown paper remains opaque.

Protein

Biuret Test: Add sodium hydroxide solution to the food solution.

Then add a few drops of blue copper sulphate solution.

Shake the contents vigorously.

A purple-violet colour indicates protein is present.