Biological Molecules: Carbohydrate Classification, Nutrition, and Food Testing Protocols

Carbohydrates function

Main source of energy for your body

Protein

Proteins are made out of amino acids. They repair, maintain, and grow body tissues.

Enzyme

Biological catalysts, typically proteins, that accelerate chemical reactions in the body by lowering the activation energy required, playing a vital role in metabolism and biochemical processes.

Lipids

Fats and oils that serve as energy stores in the body. Pure fats and oils, such as emulsified butter and cold-pressed extra virgin olive oil, are lipid-dense sources.

Carbohydrate-dense sources

Starchy staples and grains, including sliced bread, processed pasta, milled rice, and whole cereals.

Protein-dense sources

Animal muscle tissues such as beef steaks, chicken drumsticks, spare ribs, and sausages, fish such as salmon, and mammalian milk.

Fibre and micronutrient sources

Plant vegetative structures such as dark leafy greens (chard, bok choy, spinach), vine fruits (tomatoes), and whole fresh fruits (pineapples, melons).

Hierarchical classification of carbohydrates

Grouping by degree of polymerisation into three primary structural classes: monosaccharides, disaccharides, and polysaccharides.

Monosaccharide

The simplest form of carbohydrate, consisting of a single sugar molecule (monomer) that cannot be broken down into simpler sugars.

Examples of monosaccharides

Glucose, fructose, and galactose, which are all sweet and soluble in water.

Disaccharide

Formed when two monosaccharide molecules chemically bond together through glycosidic bonds.

Properties of disaccharides

They are sweet and soluble in water, and all disaccharides are reducing sugars except sucrose.

Reducing and non-reducing sugars

Reducing sugars include all monosaccharides and the disaccharides maltose and lactose, while sucrose is a non-reducing sugar.

Sucrose

A disaccharide formed from glucose bonded to fructose, abundant in sugar cane and sugar beet, and structurally distinct as a non-reducing sugar.

Maltose

A disaccharide formed from two linked glucose units, found predominantly in germinating grains such as barley.

Lactose

A disaccharide formed from glucose bonded to galactose, the characteristic carbohydrate found in mammalian milk.

Polysaccharide

Large polymers made of hundreds or thousands of monosaccharide units linked in long, straight, or branched chains.

Properties of polysaccharides

They are not sweet and are not soluble in water.

Cellulose

A polysaccharide found in leafy green vegetables that acts as a source of dietary fibre and prevents constipation.

Starch

The primary carbohydrate energy store in plants, found in potato, wheat grains, maize, and rice seeds. Excess glucose made during photosynthesis is converted into it.

Glycogen

The primary carbohydrate energy store in animals and fungi. Excess carbohydrates are converted into it in the liver.

Significance of food tests

Identifying and verifying the specific nutrients present in different food products, ensuring quality control and food safety, and helping people plan healthy, balanced diets.

Controlled experiment

A scientific procedure designed to test a hypothesis by isolating a single independent variable while keeping all other conditions strictly constant.

Control group

A baseline group identical to the experimental group in all ways except the experimental condition. In the bio-fertilizer example, another plant of the same type is sprayed with an identical volume of plain water.

Controlled variables

All other physical, environmental, and procedural factors kept strictly constant across all setups, such as temperature, reagent volume, and reaction duration, to prevent them from affecting the results.

Iodine test basis

Iodine dissolved in aqueous potassium iodide interacts with the helical coiled amylose core of starch, forming a charge-transfer complex that alters light absorption.

Iodine test procedure

Add a drop of starch solution to a well on a spotting tile and then a drop of iodine solution. Blue-black means starch is present. Repeat with distilled water as a control, which stays yellow-brown.

Benedict's test procedure

Mix $2\,cm^3$ of glucose solution with $2\,cm^3$ of Benedict's solution, then boil the mixture in a water bath for 5 minutes. A brick-red precipitate is positive, and a clear blue mixture is negative, with distilled water used as a control.

Benedict's test colour observations

Green or yellow precipitate indicates traces of reducing sugar, orange-red indicates a moderate concentration, and brick-red indicates a large, high concentration.