AFNS Biology Chapter 03 — Biological Molecules
TOPIC 1: Introduction to Biological Molecules
What Are Biological Molecules?
All living things are made of chemical substances called biological molecules (also called biomolecules). These are the chemicals that make up the body and run all life processes.
Just like a house is built from bricks, cement, glass, and wood — the human body is built from four major types of biological molecules. Every cell, tissue, organ, and system in your body is made of these molecules.
| KEY FACT | 4 Major Biological Molecules: 1. Carbohydrates — energy source 2. Proteins — building blocks of body 3. Lipids (Fats) — energy storage and membranes 4. Nucleic Acids (DNA & RNA) — genetic information All 4 are made of Carbon (C), Hydrogen (H), Oxygen (O) — plus Nitrogen (N) and Phosphorus (P) in proteins and nucleic acids. |
Why Are They Important for AFNS?
As a nursing officer, you will encounter these molecules every day:
- Glucose (carbohydrate) — you will monitor blood glucose in diabetic patients
- Proteins — you will understand how haemoglobin carries oxygen, how enzymes work in digestion
- Lipids — you will understand cholesterol, cell membranes, fat-soluble vitamins
- DNA — you will understand genetics, inherited diseases, and genetic testing
TOPIC 2: Carbohydrates
What Are Carbohydrates?
Carbohydrates are organic molecules made of Carbon (C), Hydrogen (H), and Oxygen (O). The ratio of H to O is always 2:1 (same as water — H₂O). This is why they are called carbo-hydrates (carbon + water).
General formula: (CH₂O)n — where n is the number of units. For example, glucose is C₆H₁₂O₆.
| KEY FACT | Carbohydrates = C, H, O in ratio (CH₂O)n Glucose formula = C₆H₁₂O₆ — Most repeated formula in AFNS tests! Primary function = Energy source for all life processes |
Three Types of Carbohydrates
Carbohydrates are divided into 3 groups based on their size:
| Type | Description & Examples | Key Features |
| Monosaccharides (Simple sugars) | Glucose (C₆H₁₂O₆) — blood sugar, most important Fructose — fruit sugar (same formula as glucose) Galactose — milk sugar Ribose (C₅H₁₀O₅) — in RNA Deoxyribose — in DNA | Single sugar unit Smallest carbs Directly absorbed Sweet taste Soluble in water |
| Disaccharides (Double sugars) | Sucrose = Glucose + Fructose (table/cane sugar) Maltose = Glucose + Glucose (malt sugar) Lactose = Glucose + Galactose (milk sugar) | 2 monosaccharides joined Joined by glycosidic bond Must be digested first Sweet taste |
| Polysaccharides (Complex carbs) | Starch — energy storage in PLANTS Glycogen — energy storage in ANIMALS (liver/muscle) Cellulose — plant cell wall structure Chitin — fungal cell wall / insect exoskeleton | Many monosaccharides Not sweet Insoluble in water Storage or structural role |
| KEY FACT | Most Repeated Carbohydrate Facts for AFNS: • Glucose = C₆H₁₂O₆ = primary energy source • Glycogen = animal starch (stored in liver and muscles) • Starch = plant energy storage (made of glucose) • Cellulose = plant cell wall (structural, not digestible by humans) • Lactose = milk sugar (some people are lactose intolerant — their body lacks lactase enzyme) |
Glucose — The Most Important Monosaccharide
Glucose (C₆H₁₂O₆) is the most important carbohydrate. It is the main fuel for cellular respiration — every cell in your body burns glucose to make ATP energy.
- In blood: Normal fasting blood glucose = 70–100 mg/dL
- In diabetes: Blood glucose is abnormally HIGH (hyperglycemia)
- In hypoglycemia: Blood glucose is dangerously LOW
- Insulin (hormone from pancreas) lowers blood glucose
- Glucagon (hormone from pancreas) raises blood glucose
| 📌 EXAMPLE Q: Q: What is the molecular formula of glucose? What is its primary function? A: Glucose = C₆H₁₂O₆. Its primary function is to serve as the main energy source for cellular respiration. Every cell uses glucose + oxygen to produce ATP energy. |
Condensation and Hydrolysis Reactions
Two small carbohydrate units join together by losing a water molecule. This is called a condensation reaction. The bond formed is called a glycosidic bond.
To break them apart, a water molecule is added. This is called hydrolysis (hydro = water, lysis = breaking). Digestion uses hydrolysis to break down food.
| QUICK TIP | CONDENSATION = join molecules + LOSE water (dehydration synthesis) HYDROLYSIS = break molecules + ADD water BOTH reactions apply to ALL biological molecules (not just carbs). This is frequently asked in AFNS tests! |
TOPIC 3: Proteins
What Are Proteins?
Proteins are large, complex molecules made of Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), and often Sulfur (S). They are the most versatile and important biological molecules — they build, repair, and regulate almost everything in the body.
Proteins are made of smaller units called amino acids. There are 20 different amino acids that combine in different sequences to make thousands of different proteins.
| KEY FACT | Protein = made of AMINO ACIDS joined by PEPTIDE BONDS 20 types of amino acids exist Elements in protein: C, H, O, N (and sometimes S) Proteins are the most DIVERSE biological molecules — they do almost everything in the body! |
Levels of Protein Structure
Proteins have 4 levels of structural organization — these are frequently tested in AFNS:
| Level of Structure | Description |
| 1. Primary Structure | The sequence (order) of amino acids in the chain. Like letters of a word — changing one amino acid changes the whole protein. |
| 2. Secondary Structure | The chain folds into either an Alpha-helix (spiral shape) or Beta-pleated sheet. Held by hydrogen bonds. |
| 3. Tertiary Structure | The further folding of the secondary structure into a complex 3D shape. Held by various bonds including disulfide bridges. This 3D shape determines protein function. |
| 4. Quaternary Structure | Two or more polypeptide chains come together. Example: Haemoglobin has 4 polypeptide chains. |
| KEY FACT | Most tested protein structure facts: • Amino acids join by PEPTIDE BONDS (condensation reaction) • Secondary structure = Alpha helix or Beta pleated sheet • Tertiary structure = final 3D shape = determines FUNCTION • Haemoglobin = quaternary protein (4 chains) — carries oxygen in red blood cells |
Functions of Proteins — Very Important for Nurses!
Proteins perform more functions than any other biological molecule:
| Function | Examples |
| Structural | Collagen (skin, bones, tendons), Keratin (hair, nails) |
| Enzymatic | Amylase (digests starch), Pepsin (digests protein in stomach) |
| Transport | Haemoglobin (carries O₂ in blood), Albumin (carries substances in blood) |
| Hormonal | Insulin (lowers blood glucose), Glucagon (raises blood glucose) |
| Defence (Immunity) | Antibodies/Immunoglobulins — fight infections |
| Contractile | Actin and Myosin — muscle contraction |
| Receptor | Cell membrane proteins that receive chemical signals |
| Storage | Ferritin (stores iron), Casein (stores amino acids in milk) |
| ⭐ REMEMBER FOR AFNS TEST Enzymes are biological catalysts — they are proteins that SPEED UP chemical reactions without being used up. Key facts: • All enzymes are proteins (but not all proteins are enzymes) • Each enzyme is specific to one reaction (lock and key model) • Enzymes work best at optimum temperature (37°C in humans) and pH • High temperature DENATURES enzymes (destroys their 3D shape) • Enzyme + Substrate → Product (substrate fits into active site like a key into lock) |
Denaturation of Proteins
When a protein loses its 3D shape (tertiary structure), it is said to be denatured. A denatured protein cannot function. This happens when:
- Temperature rises above optimum (e.g., above 40°C for most human enzymes)
- pH changes drastically (too acidic or too basic)
- Heavy metals (mercury, lead) are present
| 📌 EXAMPLE Q: Q: Why does high fever (very high body temperature) become dangerous? A: High fever denatures enzymes and proteins in the body. Since enzymes control all metabolic reactions, denatured enzymes cannot function, disrupting vital processes. At temperatures above 40–41°C, brain damage and death can occur. |
TOPIC 4: Lipids (Fats and Oils)
What Are Lipids?
Lipids are a diverse group of molecules that include fats, oils, waxes, and steroids. Like carbohydrates, they contain Carbon (C), Hydrogen (H), and Oxygen (O) — but the H:O ratio is much higher than 2:1 (they have much more hydrogen). They do NOT dissolve in water (they are hydrophobic).
| KEY FACT | Lipids = C, H, O (but H:O ratio much higher than carbohydrates) Are HYDROPHOBIC (do not dissolve in water) Made of: GLYCEROL + FATTY ACIDS (triglycerides) Joined by: ESTER BONDS (condensation reaction) |
Types of Lipids
| Type of Lipid | Description and Examples |
| Triglycerides (Fats & Oils) | Made of 1 glycerol + 3 fatty acids joined by ester bonds. Fats = solid at room temperature (animal fats, butter). Oils = liquid at room temperature (vegetable oils). Used for long-term energy storage. |
| Phospholipids | Made of 1 glycerol + 2 fatty acids + 1 phosphate group. Have hydrophilic head (water-loving) and hydrophobic tails (water-fearing). Form the phospholipid bilayer of cell membranes. |
| Steroids | Ring-shaped lipids. Examples: Cholesterol (membrane component), sex hormones (estrogen, testosterone), corticosteroids. |
| Waxes | Long fatty acids + alcohols. Waterproof coating on plant leaves and insect exoskeletons. |
| KEY FACT | Most tested lipid facts for AFNS: • Triglyceride = glycerol + 3 fatty acids (ester bond) • Phospholipid = makes the cell membrane (bilayer) • Cholesterol = steroid lipid = important in cell membranes and hormone production • Saturated fatty acids = no double bonds = solid at room temp (butter, lard) • Unsaturated fatty acids = has double bonds = liquid at room temp (olive oil) |
Saturated vs Unsaturated Fatty Acids
| Type | Description |
| Saturated Fatty Acids | No double bonds between carbon atoms. Maximum hydrogen atoms. Solid at room temperature. Found in animal products (butter, ghee, meat fat). Associated with cardiovascular disease if eaten in excess. |
| Unsaturated Fatty Acids | Has one or more double bonds between carbons. Less hydrogen. Liquid at room temperature (oils). Monounsaturated = 1 double bond. Polyunsaturated (PUFA) = multiple double bonds. Healthier than saturated. |
| Trans Fats | Artificially produced unsaturated fats. Formed during hydrogenation of oils. Most harmful type of dietary fat. Raise bad cholesterol (LDL), lower good cholesterol (HDL). |
Functions of Lipids
- Energy storage — 1g fat gives 9 kcal energy (more than carbs at 4 kcal/g or proteins at 4 kcal/g)
- Cell membrane structure — phospholipid bilayer
- Insulation — subcutaneous fat insulates the body against cold
- Protection — fat cushions organs (kidneys, heart)
- Hormone production — steroid hormones (estrogen, testosterone, cortisol)
- Fat-soluble vitamins (A, D, E, K) are dissolved and transported in fats
| QUICK TIP | Calorie content: FAT = 9 kcal/g | CARBS = 4 kcal/g | PROTEIN = 4 kcal/g Fat provides more than DOUBLE the energy per gram compared to carbs or proteins. This fact is directly tested in AFNS nutrition questions! |
TOPIC 5: Nucleic Acids — DNA and RNA
What Are Nucleic Acids?
Nucleic acids are the molecules that store and transmit genetic information. They are called ‘nucleic’ because they are found in the nucleus of cells. There are two types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid).
Nucleic acids are made of smaller units called nucleotides. Each nucleotide has 3 parts: a phosphate group, a sugar (pentose sugar), and a nitrogenous base.
| KEY FACT | Nucleic Acid = polymer of NUCLEOTIDES Each nucleotide = Phosphate + Sugar + Nitrogenous Base DNA = Deoxyribose sugar | RNA = Ribose sugar DNA stores genetic information | RNA helps make proteins |
DNA — Deoxyribonucleic Acid
DNA is the genetic material of all living organisms (except some viruses that use RNA). It carries the instructions for making all proteins in the body.
- Structure: Double helix — two strands twisted around each other (Watson & Crick, 1953)
- Sugar: Deoxyribose (C₅H₁₀O₄) — missing one oxygen compared to ribose
- Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
- Base pairing rules: A pairs with T (2 hydrogen bonds) | G pairs with C (3 hydrogen bonds)
- Two strands held together by HYDROGEN BONDS between base pairs
- DNA is found mainly in the NUCLEUS (also in mitochondria and chloroplasts)
| KEY FACT | DNA Base Pairing — Most Repeated in AFNS: A (Adenine) — T (Thymine) — 2 hydrogen bonds G (Guanine) — C (Cytosine) — 3 hydrogen bonds Remember: AT = 2 bonds, GC = 3 bonds AT pairs are weaker (fewer bonds) — DNA denatures at A-T rich regions first |
RNA — Ribonucleic Acid
RNA is single-stranded (unlike DNA which is double-stranded). It carries the genetic instructions from DNA to the ribosomes to make proteins.
- Sugar: Ribose (C₅H₁₀O₅) — has oxygen that deoxyribose lacks
- Bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C) — Uracil replaces Thymine!
- Structure: Single-stranded
- Found in: Nucleus, cytoplasm, ribosomes
| Type of RNA | Function |
| mRNA (Messenger RNA) | Carries genetic code from DNA in nucleus to ribosome in cytoplasm. Each 3-base sequence = codon = codes for one amino acid. |
| tRNA (Transfer RNA) | Brings specific amino acids to ribosome during protein synthesis. Has anticodon that matches mRNA codon. |
| rRNA (Ribosomal RNA) | Makes up the structure of ribosomes. Made in the nucleolus. |
| Feature | DNA | RNA |
| Full name | Deoxyribonucleic acid | Ribonucleic acid |
| Structure | Double-stranded (double helix) | Single-stranded |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C (U instead of T) |
| Location | Mainly in nucleus | Nucleus + cytoplasm |
| Function | Stores genetic information | Protein synthesis |
| Stability | More stable (double strand) | Less stable (single strand) |
| ⭐ REMEMBER FOR AFNS TEST Key differences to remember for AFNS: • DNA has THYMINE — RNA has URACIL (no thymine in RNA) • DNA is DOUBLE stranded — RNA is SINGLE stranded • DNA has DEOXYRIBOSE sugar — RNA has RIBOSE sugar • These 3 differences are asked repeatedly in AFNS tests! |