Biology — Human Body, Nutrition, Diseases & Classification

Biology (term coined by Lamarck & Treviranus, 1802) is the science of living organisms. Aristotle — the Father of Biology — laid its foundations. For SSC exams, biology questions focus on human body systems, nutrition & deficiency diseases, genetics, diseases & their pathogens, and ecology. This note covers every high-yield fact the exam has ever asked.

THE CELL

The Cell Theory was proposed by Schleiden & Schwann (1838): every living organism is made of one or more cells, and cells arise only from pre-existing cells. Cells are prokaryotic (no true nucleus — bacteria, mycoplasma, blue-green algae) or eukaryotic (nucleus + membrane-bound organelles — plants, animals, fungi). Think of it like: prokaryote = budget phone (no cover), eukaryote = flagship phone (fully covered).

CLASSIFICATION OF LIVING ORGANISMS

Carolus Linnaeus (18th century) introduced binomial nomenclature — every organism gets a two-part Latin name (genus + species). R.H. Whittaker (1969) classified all life into five kingdoms. The hierarchy from broadest to narrowest is: Kingdom → Phylum → Class → Order → Family → Genus → Species (mnemonic: King Philip Came Over For Good Soup).

HUMAN BODY SYSTEMS

The human body has 11 organ systems. Each system has a core structure, a key function, and landmark SSC facts. SSC loves asking about the smallest/largest bones, blood groups, and specific enzymes — so memorize those numbers cold.

Skeletal System

The human skeleton has 206 bones in adults (infants have ~300; many fuse during growth). Bones provide structure, protect organs, store calcium/phosphorus, and produce blood cells in the red bone marrow. Joints connect bones — ball-and-socket (hip, shoulder), hinge (knee, elbow), pivot (neck), etc.

Muscular System

The body has over 600 muscles. Muscles are the only tissue that can contract. The largest muscle is the Gluteus Maximus (buttock); the smallest is the Stapedius (attached to stapes in ear).

Circulatory System

The heart is a myogenic organ (self-contracts) with 4 chambers. The SA node (sino-atrial node) in the right atrium acts as the pacemaker — it initiates each heartbeat. Normal blood pressure = 120/80 mmHg (systole/diastole). An adult has 5–6 litres of blood.

Respiratory System

Respiration converts food (glucose) into energy using oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy). Aerobic respiration (with O₂) happens in mitochondria. Anaerobic respiration (without O₂) produces lactic acid in muscles (causing cramps). The normal breathing rate is 16–20 breaths/minute; tidal volume = 500 mL per breath.

Digestive System

Digestion converts complex food molecules into simple absorbable units via hydrolysis (enzyme-driven). The journey: Mouth → Oesophagus → Stomach → Small Intestine (duodenum → jejunum → ileum) → Large Intestine → Rectum → Anus. The small intestine (6–7 m long) is the chief site of digestion and absorption. The large intestine (1.5 m) absorbs water and forms faeces.

Nervous System

The nervous system has two divisions: CNS (brain + spinal cord) and PNS (all other nerves). The basic unit is the neuron (nerve cell) — the longest cell in the body. The brain has three major regions. Reflexes are involuntary responses controlled by the spinal cord, not the brain — that's why you pull your hand from fire before 'feeling' the pain.

Endocrine System (Hormones)

Endocrine glands are ductless — they secrete hormones directly into the bloodstream. The pituitary gland (smallest endocrine gland, at base of brain) is called the master gland because it controls all other endocrine glands. Hormones are chemical messengers that regulate metabolism, growth, reproduction, and responses to stress.

Excretory System

The kidneys maintain the body's fluid and electrolyte balance by filtering blood and producing urine. The nephron is the structural and functional unit of the kidney — each kidney has ~1 million nephrons. Urine is pale yellow due to the pigment urochrome. Normal urine composition: ~95% water, 2.6% urea, 2% salts, 0.3% uric acid. Volume: 1–2 litres/day. pH: ~6.0 (mildly acidic).

NUTRITION & DEFICIENCY DISEASES

The human body requires macronutrients (carbohydrates, proteins, fats) for energy and building, and micronutrients (vitamins, minerals) for metabolic processes. Vitamins are either fat-soluble (A, D, E, K — stored in body fat, so excess can be toxic) or water-soluble (B-complex, C — not stored, must be consumed daily). SSC frequently asks: which vitamin deficiency causes which disease.

GENETICS

Genetics is the study of heredity and variation. Gregor Mendel (1866) — the 'Father of Genetics' — experimented on pea plants (Pisum sativum) and discovered the laws of inheritance. The physical carrier of hereditary information is DNA (Deoxyribonucleic Acid). The double-helix structure of DNA was discovered by Watson & Crick (1953). DNA is organised into chromosomes in the cell nucleus.

DISEASES & PATHOGENS

Diseases are caused by pathogens (infectious agents) or by non-infectious causes (deficiency, genetic, degenerative). Pathogens include viruses (smallest; need host cell — no cure, only vaccines/antivirals), bacteria (treated with antibiotics; Robert Koch discovered cholera & TB bacterium), fungi (treated with antifungals), and parasites (protozoa, helminths — treated with antiparasitics). SSC regularly tests the causal agent, vector, and vaccine for major diseases.

PLANT BIOLOGY

Plants are autotrophs — they produce their own food via photosynthesis. Chlorophyll (found in chloroplasts) captures light energy. Leaves are the primary site of photosynthesis, although stems and other green parts can also photosynthesize. Plants also respire (like animals), transpire (lose water vapour), and reproduce via pollination.

ECOLOGY & ENVIRONMENT

Ecology (term coined by Reiter) studies the relationships between organisms and their environment. The term was popularized by Ernst Haeckel. Ecological organization: individual → population → community → ecosystem → biome → biosphere. Energy flows through ecosystems in one direction (sun → producers → consumers → decomposers) — only ~10% is transferred to the next trophic level (10% law by Lindemann).

A simple food chain: Grass → Grasshopper → Frog → Snake → Eagle. Food webs are interconnected food chains. The trophic level that has the most energy is always the producers (plants). Ecological pyramids (of number, biomass, energy) show relationships between trophic levels. Pyramid of energy is always upright (energy decreases up the chain).

VITAMINS — COMPLETE GUIDE

Vitamins are organic compounds our body needs in tiny amounts but cannot make on its own (except Vitamin D from sunlight and Vitamin K from gut bacteria). They're split into two groups: fat-soluble (A, D, E, K — stored in body fat; excess is dangerous) and water-soluble (B-complex and C — excess flushed out in urine; need regular intake). Deficiency of each vitamin causes a specific disease — this is a direct SSC question type almost every paper.

Fat-Soluble Vitamins (A, D, E, K)

Fat-soluble vitamins are absorbed along with fats in food and stored in the liver and fatty tissues. Since the body stores them, you don't need them daily — but too much can build up to toxic levels. Memory hook: A-D-E-K (think of "ADEK the fat kid who stores everything").

Water-Soluble Vitamins (B-complex & C)

Water-soluble vitamins dissolve in water and aren't stored — excess is excreted in urine. This means you need them regularly from food. The B-complex group has many members (B1, B2, B3, B5, B6, B9, B12) — each with a different role and deficiency disease. Vitamin C (ascorbic acid) is famous for scurvy prevention (the disease that killed sailors who had no fresh fruit for months).

BLOOD GROUPS — DONATION & TRANSFUSION

Human blood comes in 4 main blood groups (A, B, AB, O) based on the antigens (protein markers) on red blood cells and the antibodies in blood plasma. If you receive blood with the wrong antigens, your antibodies attack the donor red cells — a life-threatening reaction. This is why blood group matching matters in transfusions. For SSC, memorise who can give blood to whom — especially O (universal donor) and AB (universal recipient).

BIOTECHNOLOGY — CLONING, IVF & GENETIC ENGINEERING

Biotechnology is biology put to practical use — using living organisms (bacteria, plants, animals) to make products or solve problems. It covers three big areas that SSC tests: Genetic Engineering (editing DNA), Test Tube Baby / IVF (fertilisation outside the body), and Cloning (creating a genetic copy). These sound sci-fi but each has real applications in medicine and agriculture.

BRANCHES OF BIOLOGY & MEDICAL DISCOVERIES

SSC often asks "What is the study of X called?" or "Who discovered Y?" These are pure memory questions — 1 mark each, easily scored. Learn the -ology words and the key discoverers.

BIOLOGICAL RECORDS — SMALLEST & LARGEST

SSC loves asking "what is the largest/smallest X?" questions from biology. These are pure fact-recall — spend 10 minutes learning this table and you'll always get these marks.

QUICK-FIRE FACTS (SSC Favourites)

COMMON EXAM TRAPS

1. 'Dengue spreads via Anopheles mosquito' — WRONG. Dengue = Aedes aegypti. Malaria = Anopheles (female only).
2. 'Digestion starts in the stomach' — WRONG. It starts in the mouth (salivary amylase breaks down starch).
3. 'AB blood group is the universal donor' — WRONG. AB is the universal RECIPIENT. O is the universal donor.
4. 'Viruses are treated with antibiotics' — WRONG. Antibiotics kill bacteria only. Viral infections need antivirals or vaccines.
5. 'The pituitary is the largest endocrine gland' — WRONG. Pituitary is the smallest endocrine gland; thyroid is the largest.
6. 'Infants have 206 bones' — WRONG. Infants have ~300 bones; adults have 206 (many bones fuse during growth).
7. 'Robert Koch invented the rabies vaccine' — WRONG. Koch discovered the cholera/TB bacterium. Rabies vaccine = Louis Pasteur (1885).
8. 'Mendel experimented on fruit flies' — WRONG. Mendel used pea plants (Pisum sativum). Fruit flies (Drosophila) were used by T.H. Morgan for chromosome mapping.
9. 'Photosynthesis occurs only in leaves' — WRONG. It occurs in all green parts of plants (stems, unripe fruits, etc.).
10. 'DNA was discovered by Watson & Crick' — WRONG. DNA was discovered earlier by Friedrich Miescher (1869). Watson & Crick discovered the double-helix structure (1953).
11. 'Sex of a child is determined by the mother' — WRONG. Sex is determined by the father's sperm chromosome (X → daughter; Y → son).
12. 'Insulin is secreted by the thyroid gland' — WRONG. Insulin is secreted by beta cells of the Islets of Langerhans in the pancreas.
13. 'Vitamin D is obtained only from food' — WRONG. Vitamin D is also synthesised by skin under sunlight (UV-B radiation converts 7-dehydrocholesterol → Vitamin D₃).
14. 'All mosquitoes spread malaria' — WRONG. Only the female Anopheles mosquito spreads malaria (males feed on nectar, not blood).
15. 'Pyramid of biomass is always upright' — WRONG. Pyramid of biomass is inverted in a pond ecosystem (phytoplankton have less biomass but support larger zooplankton). Pyramid of energy is always upright.