Biology Fundamentals for Competitive Exams

A red blood cell has no nucleus at all. It gave that up on purpose, to carry more oxygen. This article covers the body’s core fundamentals.

📑 Contents

1Cells and the Circulatory System
2Vaccines and Vitamins
3Plant Structure and Nutrition
4How Plants Reproduce
5How Animals Reproduce
6What Makes Something Alive
7Disease Types, Genes, and Digestion
8How Vaccines and Deficiencies Actually Work
9Exam Point of View
10Related Topics

✊ Must Know

Cells and the Circulatory System

The CellThe cell is the basic structural and functional unit of life. Every living organism is made of one or more cells.
Circulatory SystemThe heart pumps blood through the body, delivering oxygen and nutrients while carrying waste products away.
Haemoglobin and WBCsRed blood cells owe their colour to haemoglobin, the pigment that binds and carries oxygen. White blood cells instead defend the body by fighting germs.
Arteries vs VeinsArteries carry blood away from the heart under high pressure, through thick elastic walls. Veins carry blood back to the heart at lower pressure, using one-way valves to stop backflow.
The Pulmonary ExceptionThe pulmonary artery is the one artery that carries carbon-dioxide-rich blood, since “artery” just means it leaves the heart. The pulmonary vein, in turn, carries oxygen-rich blood back to the heart.
Pulse RateA resting adult’s pulse rate normally runs between 72 and 80 beats per minute, felt wherever an artery passes close to the skin, like the wrist.
CapillariesArteries branch into ever-thinner capillaries at the tissues, where the actual exchange of gases and nutrients happens. Capillaries then rejoin into veins heading back to the heart.
Four Heart ChambersThe heart has two upper atria and two lower ventricles, kept separate by a central wall so oxygen-rich and carbon-dioxide-rich blood never mix.
Double CirculationHuman blood passes through the heart twice in one complete circuit: once to the lungs and back, then out to the rest of the body and back. This double circulation keeps oxygenated and deoxygenated blood fully separate.
StethoscopeA stethoscope amplifies the sound of the heartbeat for a doctor to hear clearly. Each heartbeat generates one pulse felt out in the arteries.
Blood GroupsHuman blood sorts into four ABO types: A, B, AB, and O, based on which antigens sit on the red blood cell surface.
A TwistMature red blood cells and platelets have no nucleus at all. Red blood cells eject theirs before entering the bloodstream, freeing up space to carry more oxygen. Platelets never had one either, since they are cell fragments, not full cells.

📘 Good to Know

Vaccines and Vitamins

VaccinesA vaccine trains the immune system to recognise a specific disease-causing organism, without causing the disease itself.
VitaminsVitamins are essential nutrients the body usually cannot make on its own. A missing vitamin causes a specific, identifiable illness.
Vitamin C ExampleBleeding gums, loose teeth, fragile bones, and slow-healing wounds together point to vitamin C deficiency, known as scurvy. Vitamin C helps build collagen, which holds gums, bones, and healing wounds together.
Other Named DeficienciesVitamin A deficiency causes night blindness. Vitamin D deficiency causes rickets in children, softening their bones. Vitamin B1 deficiency causes beriberi, and Vitamin K deficiency impairs blood clotting.

Plant Structure and Nutrition

Herbs, Shrubs, TreesHerbs have short, soft, green stems. Shrubs branch near the base on a harder stem. Trees are tall, with a hard, thick stem that branches higher up.
Root TypesA taproot has one main root with smaller lateral roots branching off it. Fibrous roots instead form a dense cluster, with no single root standing out.
Root Type Meets Leaf VenationPlants with taproots usually have reticulate, net-like leaf venation. Plants with fibrous roots usually have parallel venation, as in grass.
Leaf PartsThe petiole attaches a leaf to the stem. The lamina is the leaf’s broad, green blade, and the midrib is its most prominent central vein.
PhotosynthesisLeaves make food using sunlight, water, and carbon dioxide, in a process called photosynthesis. Oxygen is released as a by-product.
TranspirationPlants release water vapour into the air through their leaves. This process is called transpiration.
Xylem vs PhloemXylem carries water and minerals upward from the roots, in one direction only. Phloem carries food made in the leaves to every other part of the plant, in either direction.
Meristematic vs Permanent TissueMeristematic tissue is made of actively dividing cells, found only at growing regions like root and shoot tips. Permanent tissue forms once these cells stop dividing and take on a fixed shape and function.
Three Simple Permanent TissuesParenchyma is soft and living, filling most of a plant’s ground tissue. Collenchyma has thickened corners for flexible support. Sclerenchyma has hard, dead cells that give plants their rigidity.
Flower PartsA flower’s outer sepals protect the bud. Inside them sit the petals, then the pollen-bearing stamens, and finally the pistil, the innermost part that contains the ovary and ovules.
Plant HormonesAuxin promotes cell growth and bending towards light. Gibberellin promotes stem growth. Cytokinin promotes cell division. Abscisic acid inhibits growth and helps plants respond to stress.
TropismsA tropism is directional plant growth in response to a stimulus. Phototropism bends a shoot towards light; geotropism grows roots downward with gravity, both driven by uneven auxin distribution.
Autotrophs vs HeterotrophsGreen plants make their own food and are called autotrophs. Animals and non-green plants depend on others for food and are called heterotrophs.
Parasites and SaprotrophsA parasite, like the leafless Cuscuta vine, steals ready-made food from a living host plant. A saprotroph, like most fungi, instead feeds on dead and decaying matter.
Insectivorous PlantsThe pitcher plant traps and digests insects with a modified, lidded leaf. It still photosynthesises, but insects supply nutrients the soil around it lacks.
Symbiosis and RhizobiumSymbiosis is two organisms sharing shelter and nutrients, as in lichens, where a fungus and an alga live as one. The bacterium Rhizobium lives in the root nodules of legumes and converts atmospheric nitrogen into a usable form, in exchange for food and shelter.

How Plants Reproduce

Asexual vs SexualAsexual reproduction produces new plants without seeds, using a single parent. Sexual reproduction fuses a male and female gamete to produce seeds.
Vegetative PropagationNew plants can grow directly from a parent plant’s parts: stem cuttings in rose, eyes in potato, and leaf-margin buds in Bryophyllum. These copies are genetically identical to the parent and mature faster than seed-grown plants.
Budding and FragmentationYeast reproduces by budding, growing a small outgrowth that detaches as a new cell. Algae like Spirogyra reproduce by fragmentation, breaking into pieces that each grow into a new organism.
Spore FormationFungi, moss, and ferns reproduce using spores, tiny protected reproductive bodies light enough to travel long distances on air currents.
Unisexual vs Bisexual FlowersA unisexual flower carries only stamens or only a pistil, as in corn and papaya. A bisexual flower carries both, as in mustard and rose.
Self vs Cross-PollinationSelf-pollination moves pollen to a stigma on the same plant. Cross-pollination moves it to a flower on a different plant of the same species, often carried by insects, wind, or water.
FertilisationFertilisation is the fusion of a male and female gamete into a zygote, which then develops into an embryo. After fertilisation, the ovary matures into a fruit, and ovules become seeds.
Seed Dispersal MethodsWind disperses light, winged, or hairy seeds, like maple and madar. Water disperses buoyant seeds like coconut. Animals carry hooked seeds like Xanthium on their fur. Some fruits, like castor, burst open and fling their seeds out.

How Animals Reproduce

Male and Female OrgansThe testes produce sperm and are part of the male reproductive system. The ovaries produce eggs, or ova, and are part of the female system, alongside the oviducts and uterus.
Internal vs External FertilisationInternal fertilisation happens inside the female’s body, as in humans, cows, and hens. External fertilisation happens outside it, usually in water, as in frogs and fish.
Zygote to FoetusA fertilised egg is a zygote, which develops into an embryo. Once all its body parts become identifiable, the embryo is called a foetus.
Oviparous vs ViviparousOviparous animals, like hens and lizards, lay eggs that develop outside the body. Viviparous animals, like humans, cows, and dogs, give birth directly to live young.
MetamorphosisMetamorphosis is a drastic transformation from larva to adult, as in a frog’s egg-tadpole-adult life cycle or a moth’s caterpillar-pupa-adult cycle.
Budding and Binary FissionHydra reproduces asexually by budding, growing a new individual as an outgrowth. Amoeba reproduces by binary fission, splitting its nucleus and then its body into two daughter cells.
Dolly the SheepDolly, born in 1996, was the first mammal cloned from an adult cell, created by Ian Wilmut’s team at Scotland’s Roslin Institute. Her genetic material came entirely from the donor sheep whose cell nucleus was used, not from the sheep that carried and birthed her.
RegenerationRegeneration is regrowing a whole organism from a body fragment, a form of asexual reproduction seen in simple animals like Planaria and Hydra.
Contraception and STDsContraceptive methods, like barrier methods, oral pills, and IUDs, prevent pregnancy. Barrier methods like condoms also protect against sexually transmitted diseases such as HIV-AIDS and syphilis, which other contraceptive methods do not.

What Makes Something Alive

The Core ListLiving things share a set of traits: they take in food, grow, respire, excrete waste, respond to stimuli, and reproduce.
RespirationRespiration releases energy from food, usually using oxygen and releasing carbon dioxide. It happens in every living cell, not just through breathing.
ExcretionExcretion is how an organism removes the waste products its own body generates. Plants excrete too, often by storing waste in their tissues instead of expelling it.
Response to StimuliA stimulus is any change in the surroundings that triggers a reaction, like light, touch, or a threat. The touch-me-not plant folding its leaves when touched is a classic example.
The TrapGrowth and movement alone don’t prove something is alive, since clouds grow and vehicles move. It’s the full combination of traits together that distinguishes living things from non-living ones.
Five KingdomsWhittaker’s five-kingdom system sorts all life into Monera (bacteria, no true nucleus), Protista (single-celled, with a nucleus), Fungi, Plantae, and Animalia.
Binomial NomenclatureCarl Linnaeus devised binomial nomenclature, naming every species with two parts: its genus, then its species, as in Homo sapiens.
Taxonomic HierarchyClassification runs from broadest to narrowest: kingdom, phylum, class, order, family, genus, and species.

🌟 Great to Know

Disease Types, Genes, and Digestion

Communicable vs. Non-CommunicableDiseases split into communicable (spreading person to person, like influenza) and non-communicable (not directly spreading, like diabetes). This split matters for public-health strategy: communicable diseases call for quarantine and vaccination drives, while non-communicable ones need individual lifestyle and long-term care instead.
GenesGenes, made of DNA, carry hereditary instructions passed from parents to children, shaping many inherited traits.
Digestive SystemThe digestive system breaks food down into nutrients the body can absorb, through a coordinated chain of mechanical and chemical steps.

How Vaccines and Deficiencies Actually Work

Immune MemoryA vaccine works by exploiting the immune system’s memory. Once exposed to a weakened or harmless version of a germ, immune cells “remember” it. A much faster, stronger response kicks in if the real germ shows up later.
Why Antibiotics Can’t HelpAntibiotics work by disrupting living bacterial processes, like building a cell wall or copying DNA. A virus has no such processes of its own to block, so an antibiotic simply has nothing to target. A vaccine instead works by boosting the body’s own immune response against the virus.
Discovering Vitamins HistoricallyVitamin deficiency diseases were often how scientists first found a nutrient’s role. Remove it from the diet, watch what goes wrong, and work backward from there.

📝 Exam Point of View

Exam Point of View

CDS-I 2026, Q3Penicillin targets the bacterial cell wall, not the plasma membrane, ribosome, or chromosome — the same “antibiotics disrupt a bacteria-only structure” logic as the CDS-I 2021 entry below. View this question on the full CDS-I 2026 paper →
CDS-I 2017, Q7Bleeding gums, loose teeth, and slow wound healing together name one specific vitamin: C, not K, D, or B. View this question on the full CDS-I 2017 paper →
CDS-I 2021, Q109The correct reasoning for “antibiotics don’t work on viruses” is about missing biochemical targets, not about RNA vs. DNA or the antibiotic’s chemical class. View this question on the full CDS-I 2021 paper →
CDS-II 2016, Q23Of root hair cells, red blood cells, platelets, and monocytes, only red blood cells and platelets lack a nucleus — a common multi-item trap testing which of several cell types is the exception. View this question on the full CDS-II 2016 paper →
CDS-2 2025, Q43A direct recall check: the four ABO blood types are A, B, AB, and O — not variations like “AO” or “BO,” which aren’t real blood types. View this question on the full CDS-2 2025 paper →
The PatternBiology fundamentals questions often test the “exception in a list” (which cell type breaks the pattern) or the “correct mechanism” (why something works the way it does), rather than plain definitions.