Nutrition & Biomolecules – NDA Biology Notes

Every cell in your body needs raw materials. It needs energy to run its processes. It needs building blocks to grow and repair itself. It needs regulators to keep everything working correctly. All of these come from food.

Nutrition is the process by which living organisms obtain and use food for energy, growth, and the regulation of body processes. The substances in food that serve these purposes are called biomolecules, biological molecules that life is built from.

The four major classes of biomolecules are: carbohydrates, proteins, lipids (fats), and nucleic acids. For NDA, this chapter is a precision chapter. Questions concentrate on the enzyme-protein relationship (tested three times across papers), carbohydrate classification, lactose composition, fat energy density, kwashiorkor, and rancidity. Every section below marks exactly where exam questions originate.

1. What is Nutrition?

Nutrition is the process by which living organisms obtain and use food for energy, growth, and the regulation of body processes. The substances in food that serve these purposes are biomolecules.

Nutrition can be classified into two broad types based on the source:

• Autotrophic nutrition: organisms produce their own food from inorganic materials using energy from sunlight (photosynthesis in plants) or chemical reactions (chemosynthesis in some bacteria).
• Heterotrophic nutrition: organisms obtain food by consuming other organisms or organic matter (animals, fungi, most bacteria).

2. Balanced Diet: The Complete Picture

A balanced diet is a specific combination of all necessary nutritional components in the right proportions. A complete balanced diet has seven components: carbohydrates, proteins, fats, vitamins, minerals, dietary fibre, and water. No single component is optional.  [NDA 2010-III | NDA 2024-II]

Macronutrient Proportions

The three macronutrients are not equal in proportion in a balanced diet: [NDA 2012-I]

Macronutrient	Proportion in Balanced Diet	Primary Role
Carbohydrates	3/5 (three-fifths) : the dominant component	Primary and most rapidly available energy source
Proteins	1/5 (one-fifth)	Growth, repair, enzymes, hormones, immune defence
Fats	1/5 (one-fifth)	Energy storage, cell membranes, fat-soluble vitamin absorption

A diet consisting only of rice and potato causes protein deficiency; both are carbohydrate-rich foods with inadequate protein.

Milk is NOT a rich source of Vitamin A. Milk provides calcium and protein. For Vitamin A, the correct sources are animal liver, egg yolks, and β-carotene-rich vegetables such as carrots and spinach.

Pulses (lentils, beans, chickpeas) are the richest plant-based protein source. Cereals are rich in carbohydrates. The statement “cereals are a poor source of carbohydrates” is factually wrong and appears as a deliberate exam confusion.

3. Carbohydrates: Classification and Properties

Carbohydrates are the body’s primary and most accessible energy source. They are made of carbon, hydrogen, and oxygen. They are classified by the number of sugar units they contain.

Tier	Class	Examples	Source / Function
Single unit	Monosaccharides	Glucose, Fructose, Galactose, Ribose, Xylose	Direct energy; basic building blocks
Two units	Disaccharides	Maltose, Lactose, Sucrose	Intermediate sugars; broken down to monosaccharides
Many units	Polysaccharides	Starch, Glycogen, Cellulose	Storage and structural roles

4. Monosaccharides and Disaccharides

Monosaccharides: Single Sugar Units

Glucose, the most important monosaccharide. All cells use glucose directly for energy. Glucose is a carbohydrate, not a protein, not a fat, not a nucleic acid.

Fructose, found in fruit and honey. It is the sweetest common sugar.

Galactose, found in milk (as part of lactose).

Ribose and Xylose, pentose monosaccharides (5-carbon sugars). Ribose is a component of RNA; Xylose is found in plant cell walls.

Disaccharides: Two Units Joined

Disaccharide	Components	Source / Notes
Maltose	Glucose + Glucose	Produced when starch is broken down by amylase; found in germinating seeds
Lactose	Glucose + Galactose	The sugar in milk; digested by the enzyme lactase
Sucrose	Glucose + Fructose	Table sugar; extracted from sugarcane

★ IMPORTANT: Lactose = glucose + galactose. NOT glucose alone. NOT glucose + fructose (that is sucrose). Lactose is specifically glucose + galactose. Starch and glycogen are polysaccharides: NOT disaccharides.  [NDA 2012-II] Glucose and galactose are monosaccharides: NOT disaccharides. Maltose and lactose are the two disaccharides most directly tested.

5. Polysaccharides: Starch, Glycogen, Cellulose

All three major polysaccharides are made entirely of glucose units:

Polysaccharide	Made Of	Storage / Structure	Where Found
Starch	Glucose units	Storage form in plants [NDA 2012-II]	Grains, roots, tubers (rice, potato, wheat)
Glycogen	Glucose units	Storage form in animals [NDA 2012-II]	Liver and skeletal muscle
Cellulose	Glucose units	Structural: NOT storage; humans cannot digest it	Plant cell walls; natural source is cotton wool

Rubber is NOT a carbohydrate. Rubber is a polymer of isoprene (2-methylbutadiene). It contains no glucose units whatsoever. Starch, glycogen, and cellulose are all made of glucose units. Rubber is made of isoprene. This distinction is directly tested.

6. Honey: Fructose Predominates

The predominant sugar in honey is fructose, not sucrose, not galactose, not maltose.  [NDA 2014-I] Honey contains both fructose and glucose, but fructose is present in the greater proportion. This gives honey its characteristic sweetness.

7. Why Carbohydrates Dissolve in Water

Carbohydrates are covalent compounds, yet they dissolve in water. The reason: their multiple hydroxyl (−OH) groups form hydrogen bonds with water molecules. This hydrogen bonding makes them water-soluble despite being covalent.  [NDA 2006-II]

8. Carbohydrates Are NOT the Only Energy Source

A common misconception states that carbohydrates are the only source of energy for the human body. This is false.  [NDA 2014-II]

Fats and proteins also provide energy. Fats in particular yield more energy per gram than carbohydrates (~9 kcal/g vs ~4 kcal/g for carbohydrates). Carbohydrates are the preferred and most rapidly available energy source, but they are not the only one.

9. Proteins: Structure, Enzymes, and Deficiency

Proteins are large, complex molecules made of amino acids. They perform the widest range of functions of any biomolecule: structural support, enzymatic catalysis, hormonal signalling, oxygen transport (haemoglobin), immune defence (antibodies), and more.

Primary Structure

The primary structure of a protein is the most basic level: a linear chain of amino acids linked sequentially by peptide bonds. It describes the sequence of amino acids only, not the folding or three-dimensional shape.  [NDA 2025-I]

Higher levels of structure (secondary, tertiary, quaternary) arise from progressively more complex folding and inter-chain interactions. Primary structure is always the foundation.

10. Enzymes : Biological Catalysts

Enzymes are biological catalysts, protein molecules that speed up chemical reactions in living organisms without being consumed in the process.  [NDA 2010-I]

Enzymes ARE	Enzymes are NOT
Proteins: all enzymes are protein molecules	Hormones: hormones are chemical messengers (different function)
Biological catalysts: speed up reactions without being consumed [NDA 2010-I]	Vitamins: vitamins are organic micronutrients (different category)
Specific: each enzyme catalyses one type of reaction	Steroids: steroids are lipid-based molecules (different chemistry)
Functional at ~37°C: optimal at human body temperature	Co-enzymes: co-enzymes are non-protein cofactors that assist enzymes

11. The One-Way Relationship: All Enzymes Are Proteins

This is the single most-askedconceptual question in this chapter, appearing across separate papers.

★ IMPORTANT: All enzymes are proteins: This is CORRECT.  [NDA 2008-II | NDA 2017-II] All proteins are enzymes: This is WRONG; it is the most-tested incorrect statement in this chapter.   The relationship is ONE-DIRECTIONAL: → Every enzyme is a protein (enzymes belong to the protein category). → NOT every protein is an enzyme (proteins serve many other roles).   Counter-examples: Haemoglobin = protein (NOT an enzyme) | Insulin = protein (NOT an enzyme) | Keratin = protein (NOT an enzyme)   Every exam that tests this concept has one option that reverses the direction. That option is always wrong.

12. Isoenzymes

Isoenzymes (also called isozymes) are enzymes that catalyse the same chemical reaction but differ in their amino acid composition. They may have different kinetic properties or be found in different tissues.

Confused Term	What It Actually Means	Is It the Same as Isoenzyme?
Isotopes	Atoms of the same element with different numbers of neutrons	No: entirely different (physics/chemistry concept)
Isomers	Molecules with the same molecular formula but different structural arrangement	No: entirely different (chemistry concept)
Co-enzymes	Non-protein cofactors that assist enzymes in catalysis	No: co-enzymes assist enzymes; isoenzymes ARE enzymes

13. Protein Denaturation

When egg white (albumin) is heated, the transparent liquid turns solid and opaque white. This happens because heat disrupts the bonds maintaining the protein’s three-dimensional shape. The protein unfolds and aggregates, a process called thermal denaturation.

It is the protein (albumin) that is denatured, not fat, not ribose sugar, not carbohydrates. Egg white is almost entirely protein. Only proteins denature in this way, changing from soluble to insoluble when their three-dimensional structure is disrupted.

14. Protein Food Sources and Deficiency Diseases

Protein Food Sources

Source	Nutritional Significance
Egg albumin (egg white)	Most easily digestible protein source: the reference standard for protein digestibility; more digestible than soyabean, fish, or red meat
Soybean oil	Highest protein content among commonly available plant oils; castor oil, sunflower oil, and safflower oil have lower protein
Pulses (lentils, beans, chickpeas)	Richest plant-based protein source

Protein Deficiency Diseases

Disease	Cause	Who Affected	Key Symptoms
Kwashiorkor	Protein deficiency: NOT mineral, NOT vitamin, NOT fat deficiency	Young children who receive enough calories from carbohydrates but insufficient protein	Oedema (swelling), growth retardation, muscle wasting
Marasmus	Total calorie deficiency: both protein AND energy are insufficient	Young children with severely inadequate total food intake	Severe underweight, wasting: “skin and bones” appearance

Kwashiorkor and marasmus are both childhood malnutrition conditions, but their causes differ. Kwashiorkor = protein deficit. Marasmus = total calorie deficit. These cannot be swapped.

15. Fats and Lipids: Energy, Structure, and Rancidity

Energy Density of Fats

Fats are the most energy-dense of all macronutrients. Fats yield approximately 9 kcal per gram on oxidation, more than double the energy from carbohydrates or proteins (each ~4 kcal/g).  [NDA 2014-II]

Fats are the richest energy source among all macronutrients. Vitamins yield no energy at all.

Functions of Fats

Function	Detail	PYQ
Cell membrane structure	Phospholipids (a type of fat) form the bilayer that makes up every cell membrane. Without phospholipids, no cell membrane can exist.
Slow-release energy	The body releases energy from fats more slowly and sustainably than from carbohydrates. Carbohydrates = quick energy. Fats = longer-lasting energy.
Fat-soluble vitamin transport	Vitamins A, D, E, K dissolve in fat and require dietary fat for absorption	—

What fats do NOT do: Fats do NOT help the body absorb calcium. Vitamin D does that. Fats are NOT involved in tissue repair. Protein does that.

Rancidity

When fat- and oil-containing foods are left in air for extended periods, the fats undergo oxidation, a chemical reaction with atmospheric oxygen. This produces malodorous compounds, the rancid smell and off-flavour.  [NDA 2025-I]

Fats are not reduced. They do not freeze. They do not remain unreacted. They are oxidised. That is the specific chemical process of rancidity.

Phytosterols: Plant Sterols

Plant cells contain sterols that closely resemble animal cholesterol. These include stigmasterol, ergosterol, and sitosterol, collectively called phytosterols (phytols). They are not calciferols (Vitamin D forms), not ergocalciferols, not lumisterols.

16. Cell Membrane: Phospholipid Bilayer

The cell membrane consists of a bilayer of phospholipid molecules in which proteins and cholesterol are embedded.

It is not a bilayer of proteins alone. It is not a bilayer of neutral lipids. Cholesterol is an integral structural component. It is present, not absent. The phospholipid bilayer + embedded proteins + cholesterol is the universally accepted membrane structure.

This connects directly to the B1 (Cell Biology) content on cell membrane composition. Phospholipids, proteins, and cholesterol are all present. Cellulose is in the plant cell wall, not in the cell membrane.

17. Lactose and Lactase

Lactose is the sugar found in milk. It is a disaccharide made of glucose + galactose.

The enzyme that digests lactose is lactase. Lactase breaks lactose into its two component monosaccharides, glucose and galactose, which can then be absorbed.  

Most infants produce lactase. Most adults worldwide gradually lose lactase production after childhood. Without lactase, lactose cannot be digested, causing bloating, gas, and diarrhoea (lactose intolerance). An elderly person who can still digest milk is still producing adequate lactase.

Enzyme	Substrate It Digests	What It Does NOT Digest
Lactase	Lactose (milk sugar → glucose + galactose)	Does NOT digest cellulose, starch, or pectin
Cellulase	Cellulose (in cattle, produced by gut bacteria)	Does NOT digest lactose
Amylase	Starch (→ maltose)	Does NOT digest lactose
Pectase	Pectin (plant cell wall component)	Does NOT digest lactose

18. Onion and Sulphur: Why Eyes Water

When onions are cut or peeled, they release volatile compounds that irritate the eyes and cause tearing (lacrimation). The cause is sulphur. Onion cells contain sulphur-based compounds (specifically cysteine sulphoxide derivatives). When cells are broken, these compounds convert into volatile sulphenic acids and propanethial S-oxide, a tear-inducing gas.

The cause is sulphur compounds, not carbon, not fat, not amino acids alone. Sulphur is the specific and correct answer.

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