Chemistry Essentials — Periodic Table, Bonding & Reactions

Why This Chapter Matters

Open any SSC CGL Tier-1 paper from the last five years and you will find three to five questions straight from this chapter: which vitamin's deficiency causes night blindness, which organ produces insulin, how malaria spreads, what BMR stands for. Human body and diseases together are worth more marks in the General Awareness section than almost any other biology topic, and unlike history dates, this content rarely feels dry because it is about you. You already own a digestive system and a heart; you just haven't labelled the parts yet.

The single biggest mistake aspirants make here is treating vitamins and hormones as one giant list to cram the night before the exam. SSC loves to pair a vitamin with its deficiency disease and then swap in a similar-sounding wrong option (Vitamin B1 vs B2, rickets vs osteomalacia). Students who memorise vitamins as an isolated list get these wrong under pressure. Students who understand what each vitamin actually does in the body rarely do, because the deficiency disease follows logically from the function. That is how this chapter is built: function first, name second.

1. The Digestive System — Your Body's Kitchen

Think of your digestive tract as a long processing line, like a flour mill that takes in raw grain and outputs something the body can absorb. Food enters the mouth, where saliva (containing the enzyme salivary amylase) begins breaking down starch. From there it travels down the oesophagus by rhythmic muscular contractions called peristalsis — the same wave motion that lets you swallow while lying down or even upside down.

The stomach is a muscular bag that churns food and bathes it in gastric juice, which contains hydrochloric acid (HCl) and the enzyme pepsin. The acid does two jobs: it kills most bacteria that arrive with food, and it activates pepsin to start breaking down proteins. This is why an empty, acid-heavy stomach can cause that burning sensation you may have felt — the acid has nothing to digest.

Exam trap: Students often think the stomach digests and absorbs food. It only digests. Most absorption of nutrients happens in the small intestine, not the stomach.

The small intestine has three parts, moving in this order:

Memory hook: "Dear Jack, I love you" — Duodenum, Jejunum, Ileum.

The duodenum receives bile from the liver (stored in the gallbladder) and pancreatic juice from the pancreas. Bile doesn't digest fat chemically; it emulsifies fat, breaking large fat globules into smaller droplets, the way dish soap breaks oil into smaller beads when you wash a greasy plate. This gives enzymes more surface area to work on. Pancreatic juice supplies trypsin (for proteins), amylase (for starch), and lipase (for fats).

By the time food reaches the ileum, most nutrients are absorbed through tiny finger-like projections called villi, which increase surface area enormously — much like a bath towel absorbs more water than a flat sheet of the same material because of its looped fibres.

Whatever remains passes into the large intestine, where water is reabsorbed and the residue is compacted into faeces, stored in the rectum, and expelled through the anus.

Liver facts worth locking in: the liver is the largest gland in the human body. It produces bile, stores glucose as glycogen, detoxifies harmful substances, and regulates several blood proteins. The pancreas is unusual because it is both an exocrine gland (releasing digestive juice into the duodenum) and an endocrine gland (releasing insulin and glucagon into the blood).

2. The Respiratory System — Gas Exchange on Demand

Air enters through the nose, where hair and mucus filter dust, and travels through the pharynx, larynx (voice box, containing the vocal cords), trachea (windpipe), and into two bronchi, one for each lung. Inside the lungs, bronchi branch into smaller bronchioles, ending in tiny air sacs called alveoli.

An adult has roughly 300 million alveoli per lung, and their combined surface area is often compared to the size of a tennis court — this is the fact examiners like because it sounds surprising and sticks. That huge surface area exists for one reason: gas exchange needs space. Oxygen diffuses from alveoli into surrounding blood capillaries, and carbon dioxide diffuses the other way, out of the blood and into the alveoli to be exhaled.

Breathing itself is driven by the diaphragm, a dome-shaped muscle below the lungs. When it contracts and flattens, chest volume increases, pressure inside drops below atmospheric pressure, and air rushes in — inhalation. When it relaxes and domes upward again, volume decreases and air is pushed out — exhalation.

Exam trap: Do not confuse breathing (a mechanical, muscular process of moving air) with respiration (the cellular, chemical process of breaking down glucose to release energy, which happens inside every living cell, using oxygen). Breathing supplies the oxygen; respiration uses it.

Normal adult breathing rate at rest is about 12 to 18 breaths per minute. This slows in trained athletes and rises sharply during exercise, fever, or anxiety.

3. The Circulatory System — A 24-Hour Delivery Network

The heart is a four-chambered pump: two upper chambers called atria (singular: atrium) and two lower chambers called ventricles. The right side handles deoxygenated blood; the left side handles oxygenated blood, and the two never mix in a healthy heart because a muscular wall, the septum, separates them.

Picture the heart as a busy railway junction with two separate platforms that must never let trains cross onto the wrong track. Deoxygenated blood arrives at the right atrium, moves to the right ventricle, and is pumped to the lungs via the pulmonary artery (the only artery in the body that carries deoxygenated blood — examiners love this exception). In the lungs it picks up oxygen and returns via the pulmonary vein (the only vein carrying oxygenated blood) to the left atrium, then the left ventricle, which pumps it out through the aorta to the entire body.

Memory hook: "Arteries Away, Veins Home" — arteries carry blood away from the heart, veins carry it back home. This rule holds regardless of oxygen content, which is exactly why the pulmonary artery and pulmonary vein break the "arteries = oxygenated" myth many students carry from school.

Blood composition:

Component	Approx. share of blood	Main function
Plasma	55%	Carries nutrients, hormones, waste, proteins
Red blood cells (RBCs / erythrocytes)	~45%	Carry oxygen via haemoglobin
White blood cells (WBCs / leukocytes)	<1%	Fight infection, immunity
Platelets (thrombocytes)	<1%	Blood clotting

RBCs are made in bone marrow, live about 120 days, and unlike most human cells, mature RBCs have no nucleus — they sacrifice it to carry more haemoglobin, the way you might empty a suitcase of everything except the one item you actually need to carry more of it. Haemoglobin binds oxygen and gives blood its red colour; low haemoglobin causes anaemia.

Normal adult blood pressure is quoted as 120/80 mmHg: the higher number (systolic) measures pressure when the heart contracts, the lower number (diastolic) measures pressure when it relaxes between beats. A normal resting pulse is 72 beats per minute for an adult, a figure so standard that SSC treats it almost like a constant.

Blood groups follow the ABO system (A, B, AB, O) combined with the Rh factor (positive or negative). O-negative is called the universal donor because it lacks A, B, and Rh antigens that could trigger an immune reaction; AB-positive is the universal recipient because it already carries all the major antigens, so it won't reject incoming ones.

4. The Nervous System — Command and Control

The nervous system has two main divisions. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of nerves branching out to the rest of the body.

The brain has three major regions:

• Cerebrum — the largest part, responsible for thinking, memory, voluntary movement, and interpreting senses. Divided into left and right hemispheres.
• Cerebellum — sits below the cerebrum, controls balance, posture, and coordination of fine movements. Damage here produces a wobbly, uncoordinated gait, not memory loss.
• Medulla oblongata — the lowest part of the brainstem, controls involuntary, life-sustaining functions: heartbeat, breathing, blood pressure, swallowing.

Exam trap: Cerebrum controls thought and voluntary action; cerebellum controls balance and coordination. SSC frequently swaps these two in options because the names sound alike.

The basic working unit of the nervous system is the neuron, a cell with a cell body, branching dendrites that receive signals, and a long axon that transmits signals onward. Neurons don't physically touch; they communicate across tiny gaps called synapses using chemical messengers.

The reflex arc is a classic exam favourite: touch a hot object, and your hand pulls back before you consciously register pain. This happens because the signal travels to the spinal cord, not all the way to the brain, and a response is issued immediately — a shortcut built for survival, like a factory floor worker who is authorised to stop a machine on their own instead of waiting for the manager's approval every single time.

5. The Skeletal and Muscular System — The Body's Scaffolding

An adult human skeleton has 206 bones, though a newborn baby has around 270 to 300, because many bones fuse together as the child grows — the way separate building blocks eventually get cemented into one wall.

Key bone facts SSC tests repeatedly: the femur (thighbone) is the longest and strongest bone in the body. The stapes, located in the middle ear, is the smallest bone. Bones meet at joints, and joints are classified by how much movement they allow: the knee and elbow are hinge joints (movement in one plane, like a door), the shoulder and hip are ball-and-socket joints (movement in almost every direction), and the skull sutures are fixed, immovable joints.

Muscles come in three types: skeletal muscle (voluntary, attached to bones, striped in appearance), smooth muscle (involuntary, found in organs like the stomach and intestines), and cardiac muscle (involuntary but striped, found only in the heart, capable of contracting rhythmically on its own without a nerve signal telling it to start each beat).

6. The Endocrine System — Chemical Messengers

While the nervous system sends fast, wire-like electrical signals, the endocrine system sends slower chemical messages called hormones through the bloodstream, the difference between a phone call and a letter: one is instant, the other takes longer but can reach many recipients at once and its effect lingers.

Gland	Key hormone(s)	Main function
Pituitary	Growth hormone (GH)	Regulates body growth; called the "master gland" because it controls other glands
Thyroid	Thyroxine	Regulates metabolic rate
Pancreas	Insulin, glucagon	Regulates blood sugar
Adrenal	Adrenaline	Triggers "fight or flight" response
Pituitary (posterior)	ADH, oxytocin	Water balance; labour contractions

The pituitary gland, roughly the size of a pea and located at the base of the brain, is called the master gland because its hormones instruct other glands, like a head office that doesn't manufacture the final product itself but tells every branch office what to make.

The pancreas deserves special attention because its malfunction causes one of India's most tested diseases: diabetes mellitus. Insulin lowers blood sugar by helping cells absorb glucose; when the pancreas produces too little insulin, or the body stops responding to it properly, blood sugar rises, producing diabetes. Type 1 diabetes usually appears early in life and involves little or no insulin production; Type 2 diabetes, far more common, usually develops in adulthood and involves insulin resistance.

7. Vitamins and Deficiency Diseases

This is the single most exam-dense topic in the whole chapter, and the trick is to stop memorising a bare list and instead remember what each vitamin actually does.

Vitamin	Chemical name	Deficiency disease	Common source
A	Retinol	Night blindness	Carrots, milk, liver
B1	Thiamine	Beriberi	Whole grains, pulses
B2	Riboflavin	Cracked lips, sore tongue	Milk, eggs, green vegetables
B3	Niacin	Pellagra	Meat, groundnuts
B12	Cobalamin	Pernicious anaemia	Meat, eggs, dairy
C	Ascorbic acid	Scurvy	Citrus fruits, amla
D	Calciferol	Rickets (children), Osteomalacia (adults)	Sunlight, fish oil
E	Tocopherol	Infertility, muscle weakness	Vegetable oils, nuts
K	Phylloquinone	Delayed blood clotting	Green leafy vegetables

Exam trap: Rickets and osteomalacia are both caused by Vitamin D deficiency and both involve weak bones, but SSC distinguishes them by age group — rickets affects children whose bones are still growing, causing bowed legs, while osteomalacia affects adults, causing soft, brittle bones. Do not treat them as interchangeable answers.

Memory hook for water-soluble versus fat-soluble vitamins: "A, D, E, K live in fat" — think of a locker (fat storage) that keeps these four safe for later use, which is also why excess doses of these four can build up to toxic levels. Vitamin C and the B-group are water-soluble, flushed out daily through urine, which is why you need them more regularly in your diet.

Vitamin C deserves a special note because SSC frequently asks about it directly: humans, unlike most animals, cannot synthesise Vitamin C in the body, so it must come entirely from diet. Its deficiency, scurvy, causes bleeding gums and slow wound healing, historically a major problem for sailors on long voyages without fresh fruit — a fact that also explains why citrus fruits were nicknamed the sailor's medicine.

8. Balanced Diet Basics

A balanced diet supplies carbohydrates (primary energy source), proteins (growth and repair, built from amino acids), fats (concentrated energy and insulation), vitamins, minerals, fibre, and water, in proportions matched to a person's age, activity level, and health.

Proteins are called the body's "building blocks" because they form muscle, enzymes, and antibodies. Carbohydrates are the quickest energy source, broken down into glucose. Fats provide more than double the energy per gram compared to carbohydrates or proteins, which is why fat-rich diets pack more calories into less volume.

Basal Metabolic Rate (BMR) is the energy the body uses at complete rest just to keep vital functions running — heartbeat, breathing, cell repair. It is like the electricity bill of a house with every appliance switched off except the refrigerator and the security system: some consumption is unavoidable even when nothing else is running.

Malnutrition covers both undernutrition (too little food or nutrients) and overnutrition (too much, leading to obesity). Kwashiorkor, caused by severe protein deficiency, typically affects children and causes a swollen belly despite thin limbs. Marasmus, caused by an overall calorie and protein deficiency, causes severe wasting of both muscle and fat.

9. Common Diseases: Communicable and Non-Communicable

Diseases fall into two broad exam categories, and SSC often tests whether you can correctly classify a given disease into one or the other.

Communicable diseases spread from person to person or via a vector (carrier), caused by pathogens: bacteria, viruses, fungi, or protozoa.

Disease	Pathogen type	Causative agent	Mode of transmission
Malaria	Protozoa	Plasmodium	Female Anopheles mosquito bite
Dengue	Virus	Dengue virus	Aedes aegypti mosquito bite
Typhoid	Bacteria	Salmonella typhi	Contaminated food/water
Cholera	Bacteria	Vibrio cholerae	Contaminated water
Tuberculosis (TB)	Bacteria	Mycobacterium tuberculosis	Airborne droplets
Hepatitis A	Virus	Hepatitis A virus	Contaminated food/water
Chickenpox	Virus	Varicella-zoster virus	Airborne/direct contact
Rabies	Virus	Rabies virus	Bite of infected animal

Exam trap: Malaria and dengue are both mosquito-borne, but different mosquitoes carry them, and SSC often swaps these in options. Anopheles (only the female bites, since she needs blood protein to produce eggs) transmits malaria; Aedes transmits dengue, and Aedes typically bites during the daytime, unlike most mosquitoes, which is itself a testable fact.

Non-communicable diseases do not spread from person to person. They arise from lifestyle, genetics, environment, or organ malfunction over time.

• Diabetes mellitus — chronic high blood sugar due to insulin problems.
• Hypertension — chronically high blood pressure, often called the "silent" disease because it frequently shows no symptoms until damage is advanced.
• Cancer — uncontrolled, abnormal cell growth that can invade surrounding tissue.
• Cardiovascular diseases — including heart attacks and stroke, often linked to blocked or narrowed blood vessels.
• Asthma — a chronic respiratory condition causing narrowed airways and breathing difficulty.

Think of communicable diseases as a fire that jumps from house to house, while non-communicable diseases are more like slow structural damage from years of wear inside a single house; the first spreads outward, the second builds up within.

10. Vaccination and Immunity Basics

Immunity is the body's defence system against pathogens, and it comes in two broad types. Innate immunity is the general, non-specific defence you are born with — skin as a barrier, stomach acid, white blood cells that attack any invader. Acquired (adaptive) immunity develops over your lifetime and is specific: your body "remembers" a particular pathogen after exposure and responds faster the next time.

Vaccination works by training acquired immunity without making you suffer the actual disease. A vaccine introduces a weakened, inactivated, or partial version of a pathogen (or, in newer technology, genetic instructions to build a harmless piece of it). The immune system responds by producing antibodies and creating memory cells. If the real pathogen ever attacks later, the immune system recognises it instantly and mounts a faster, stronger response, the way a fire drill trains staff to evacuate quickly even though no actual fire occurred during the drill.

India's Universal Immunization Programme (UIP) covers vaccine-preventable diseases in children, including tuberculosis (BCG), polio, diphtheria, pertussis, tetanus (DPT), measles, and hepatitis B, among others. India was declared polio-free by the World Health Organization in 2014, a landmark public health achievement built on decades of the oral polio vaccine campaign.

Exam trap: "Immunity" and "immunisation" are related but not identical — immunity is the state of protection; immunisation is the process (usually via vaccination) of building that protection artificially.

Quick Revision — One-Line Facts

• The liver is the largest gland in the human body.
• Bile is produced by the liver and stored in the gallbladder; it emulsifies fat.
• Villi in the small intestine increase surface area for nutrient absorption.
• Breathing rate at rest: 12–18 breaths per minute for a healthy adult.
• The pulmonary artery is the only artery carrying deoxygenated blood.
• The pulmonary vein is the only vein carrying oxygenated blood.
• Normal blood pressure is quoted as 120/80 mmHg.
• Normal adult resting pulse rate is about 72 beats per minute.
• RBCs have no nucleus and live for about 120 days.
• O-negative blood is the universal donor; AB-positive is the universal recipient.
• The cerebrum controls thought and voluntary movement; the cerebellum controls balance and coordination.
• The medulla oblongata controls involuntary functions like heartbeat and breathing.
• The femur is the longest bone; the stapes is the smallest.
• An adult skeleton has 206 bones; a newborn has more, which later fuse.
• Cardiac muscle is involuntary but striped, unlike smooth muscle.
• The pituitary gland is called the master gland.
• Insulin and glucagon, both from the pancreas, regulate blood sugar.
• Vitamin A deficiency causes night blindness.
• Vitamin C deficiency causes scurvy; humans cannot synthesise Vitamin C themselves.
• Vitamin D deficiency causes rickets in children and osteomalacia in adults.
• Vitamins A, D, E, and K are fat-soluble; B and C are water-soluble.
• Kwashiorkor results from protein deficiency; marasmus from overall calorie deficiency.
• Malaria is caused by Plasmodium, spread by female Anopheles mosquitoes.
• Dengue is caused by a virus, spread by Aedes aegypti mosquitoes, which bite during the day.
• Typhoid and cholera both spread through contaminated food or water.
• Tuberculosis spreads through airborne droplets, caused by Mycobacterium tuberculosis.
• Non-communicable diseases like diabetes and hypertension do not spread person to person.
• Innate immunity is general and inborn; acquired immunity is specific and develops after exposure.
• Vaccines create immune memory without causing the full disease.
• India was declared polio-free by the WHO in 2014.

Memory Tables

Table A: Body systems at a glance

System	Main organs	Core function
Digestive	Mouth, stomach, small/large intestine, liver, pancreas	Breaks food into absorbable nutrients
Respiratory	Nose, trachea, bronchi, lungs, alveoli	Exchanges oxygen and carbon dioxide
Circulatory	Heart, arteries, veins, blood	Transports nutrients, gases, waste
Nervous	Brain, spinal cord, nerves	Sends and processes control signals
Skeletal	Bones, joints	Support, protection, movement
Endocrine	Pituitary, thyroid, pancreas, adrenal	Chemical signalling via hormones

Table B: Disease type and one key example

Category	Example disease	Cause type	Spreads person to person?
Communicable — vector-borne	Malaria	Protozoa	Via mosquito, not directly
Communicable — waterborne	Cholera	Bacteria	Via contaminated water
Communicable — airborne	Tuberculosis	Bacteria	Yes
Non-communicable — metabolic	Diabetes	Insulin dysfunction	No
Non-communicable — lifestyle	Hypertension	Circulatory strain	No

Practice MCQs

Q1. Which part of the digestive system absorbs most of the nutrients from digested food? (a) Stomach (b) Small intestine (c) Large intestine (d) Oesophagus

Q2. Bile, which helps in the digestion of fats, is produced by which organ? (a) Pancreas (b) Gallbladder (c) Liver (d) Stomach

Q3. Which of the following is the only artery in the human body that carries deoxygenated blood? (a) Aorta (b) Pulmonary artery (c) Renal artery (d) Coronary artery

Q4. A normal healthy adult's resting blood pressure is generally quoted as: (a) 80/120 mmHg (b) 120/80 mmHg (c) 140/90 mmHg (d) 100/60 mmHg

Q5. Deficiency of which vitamin causes night blindness? (a) Vitamin B1 (b) Vitamin C (c) Vitamin A (d) Vitamin D

Q6. Malaria is caused by which type of pathogen? (a) Bacteria (b) Virus (c) Protozoa (d) Fungus

Q7. Which part of the human brain is primarily responsible for maintaining balance and coordination? (a) Cerebrum (b) Cerebellum (c) Medulla oblongata (d) Hypothalamus

Q8. Which of the following is the longest bone in the human body? (a) Humerus (b) Tibia (c) Femur (d) Fibula

Q9. Which gland is referred to as the "master gland" of the endocrine system? (a) Thyroid (b) Adrenal (c) Pancreas (d) Pituitary

Q10. Rickets, a bone-weakening disease seen in children, results from deficiency of which vitamin? (a) Vitamin A (b) Vitamin B12 (c) Vitamin D (d) Vitamin K

Q11. Which blood group is known as the universal donor? (a) AB positive (b) O negative (c) A positive (d) B negative

Q12. Dengue fever is transmitted by which of the following? (a) Female Anopheles mosquito (b) Aedes aegypti mosquito (c) Housefly (d) Tsetse fly

Q13. Which of the following diseases is classified as non-communicable? (a) Tuberculosis (b) Cholera (c) Diabetes mellitus (d) Chickenpox

Q14. Kwashiorkor, a disease commonly seen in children with a swollen belly and thin limbs, results primarily from deficiency of: (a) Carbohydrates (b) Protein (c) Vitamin C (d) Iron

Q15. Vaccination works mainly by: (a) Directly killing pathogens already present in the body (b) Training the immune system to recognise and respond faster to a pathogen (c) Replacing red blood cells with immune cells (d) Increasing the basal metabolic rate

Answer Key

Q	Answer	Reason
1	(b)	Villi in the small intestine give it the surface area needed for most nutrient absorption; the stomach only digests.
2	(c)	The liver produces bile; the gallbladder only stores and releases it, a distinction SSC tests often.
3	(b)	The pulmonary artery carries deoxygenated blood to the lungs, the one exception to "arteries carry oxygenated blood."
4	(b)	Systolic (contraction) pressure is written first, diastolic (relaxation) second, standardised as 120/80 mmHg.
5	(c)	Vitamin A supports the retina's low-light vision cells; its lack causes night blindness first.
6	(c)	Plasmodium, a protozoan parasite, causes malaria and is transmitted by the female Anopheles mosquito.
7	(b)	The cerebellum governs balance and coordination; the cerebrum handles thought and voluntary action, a frequently swapped pair.
8	(c)	The femur (thighbone) is both the longest and strongest bone in the human body.
9	(d)	The pituitary gland controls the activity of several other endocrine glands, earning its "master gland" title.
10	(c)	Vitamin D deficiency in growing children weakens bone mineralisation, producing rickets; in adults, the same deficiency causes osteomalacia.
11	(b)	O-negative blood lacks A, B, and Rh antigens, so it rarely triggers an immune rejection in recipients.
12	(b)	Aedes aegypti, a daytime-biting mosquito, transmits dengue, unlike the Anopheles mosquito that carries malaria.
13	(c)	Diabetes mellitus arises from insulin dysfunction, not an infectious pathogen, so it does not spread person to person.
14	(b)	Kwashiorkor results from severe protein deficiency despite sufficient calorie intake, distinguishing it from marasmus.
15	(b)	Vaccines introduce a harmless form or piece of a pathogen so the immune system builds memory cells for a faster future response.