Chapter 3 – Biological Molecules (Carbohydrates, Fats, Proteins, Water & Food Tests)

Question-and-answer flashcards covering definitions, structures, functions and laboratory tests for carbohydrates, fats, proteins, water, and related nutrition concepts from Chapter 3 ‘Biological Molecules’. They support the stated learning outcomes and laboratory skills for GCE ‘O’/Normal (A) Level Science.

What are the three major classes of biological nutrient molecules found in living organisms?

Carbohydrates, fats (lipids) and proteins.

Why do marathon runners ‘carbo-load’ two days before a race?

To maximise their muscle and liver glycogen stores so that plenty of readily-available energy is on hand during the long race, despite possible weight gain.

What percentage of average human body mass is water?

About 70 % of body weight.

Give four key roles of water in living organisms.

(Any four) Solvent for chemical reactions, medium for transport of dissolved substances, maintains cell turgidity, raw material for photosynthesis, regulates body temperature.

How are carbohydrates classified?

Into single sugars (monosaccharides), double sugars (disaccharides) and complex carbohydrates (polysaccharides).

Name two common single sugars and state their molecular formula.

Glucose and fructose; both have the formula C₆H₁₂O₆.

What two single sugars join to form maltose?

Two glucose molecules.

What enzyme hydrolyses maltose into glucose?

Maltase.

Define a reducing sugar and give three examples.

A sugar that can reduce Benedict’s reagent; examples: glucose, fructose and maltose.

Describe the Benedict’s test procedure for reducing sugars.

Add 2 cm³ Benedict’s solution to 2 cm³ food sample, shake, heat in boiling water bath 2–3 min.

What colour precipitate indicates a large amount of reducing sugar in Benedict’s test?

Brick-red precipitate.

What colour indicates only a trace amount of reducing sugar in Benedict’s test?

Green precipitate.

What are complex carbohydrates?

Polysaccharides made of many monosaccharide (glucose) units bonded together.

State the storage form of carbohydrate in plants.

Starch.

State the storage form of carbohydrate in animals.

Glycogen (stored mainly in liver and muscles).

What structural carbohydrate builds plant cell walls?

Cellulose.

List three polysaccharides that are synthesised from glucose.

Cellulose, glycogen and starch.

Explain the iodine test for starch and its positive result.

Add a few drops of iodine solution; a blue-black colour indicates starch is present.

Give four biological functions of carbohydrates.

Immediate energy source, form supporting structures (e.g. cell wall), converted to other organic compounds (fats, amino acids), form part of nucleic acids (DNA/RNA) and lubricants/nectar.

What is the general definition of a fat (lipid)?

A biological molecule composed of glycerol joined to three fatty acid chains; serves as an essential nutrient.

Name the two types of smaller molecules that combine to form a fat molecule.

Glycerol and fatty acids.

Which enzyme catalyses the breakdown of fats?

Lipase.

State three common dietary sources of fats.

Butter or other oils, nuts/avocado/olives, meat or fish such as salmon.

Describe four biological functions of fats.

Long-term energy store, thermal insulation, solvent for fat-soluble vitamins/hormones, major component of cell membranes, reduces water loss from skin.

Summarise the steps of the ethanol emulsion test for fats.

Add 2 cm³ ethanol to 2 cm³ food sample, shake vigorously, add 2 cm³ water and shake; a cloudy white emulsion shows fats are present.

What observable result indicates a positive ethanol emulsion test?

A cloudy-white emulsion appears.

Explain why Eskimos traditionally rely on fats as their main energy source.

Their Arctic environment limits plant (carbohydrate) foods, so high-fat animal foods provide the necessary caloric energy and insulation.

What elements chiefly make up fats?

Carbon, hydrogen and oxygen.

Define a protein.

A large, complex biological molecule made of one or more polypeptide chains of amino acids; essential nutrient.

What three parts make up a typical amino acid?

An amino group (-NH₂), an acid/carboxyl group (-COOH) and a variable side chain (R-group).

Describe the structural hierarchy from amino acids to a protein.

Amino acids join by peptide bonds to form a polypeptide; one or more folded polypeptides constitute a protein.

List three major functions of proteins in the body.

Growth and repair (new protoplasm), formation of enzymes and some hormones, production of antibodies.

What are common food sources rich in protein?

Eggs, milk/cheese, beans/lentils, red meat, fish.

Outline the biuret test procedure for proteins.

Add 2 cm³ biuret solution to 2 cm³ food sample, shake and stand five minutes.

What colour change indicates a positive biuret test?

A violet or purple solution.

Which colour indicates absence of protein in the biuret test?

The solution remains blue.

State the main immediate role of carbohydrates in living organisms.

To provide a ready source of energy for cellular activities.

Which simple lab test detects starch?

Iodine test (iodine in potassium iodide solution).

Which test detects reducing sugars?

Benedict’s test.

Which nutrient is shown by a cloudy-white emulsion after adding ethanol and water?

Fats (lipids).

Which food test uses biuret solution, and what does it detect?

The biuret test detects proteins.

Which will generally have a higher glycaemic index: fresh apple or apple juice, and why?

Apple juice, because fibre has been removed and sugars are more readily absorbed, raising blood glucose faster.

Explain why whole fruits are nutritionally better than fruit juices.

Whole fruits contain fibre that slows sugar absorption, aids digestion, and provides satiety, whereas juices deliver rapid sugar spikes with little fibre.

Where in mammals is glycogen stored?

Primarily in the liver and skeletal muscles.

What simple preliminary test can reveal fats in potato chips at home?

The translucent spot test (grease spot on paper).