Cell Biology
Cell Structure
The cell is the basic unit of life. All living organisms are made of cells.
Prokaryotic cells (bacteria, archaea): No membrane-bound nucleus or organelles. DNA is in a region called the nucleoid. Smaller, simpler.
Eukaryotic cells (plants, animals, fungi): Have a membrane-bound nucleus and organelles.
| Organelle | Function |
|---|---|
| Nucleus | Control centre — contains DNA, directs cell activities |
| Mitochondria | Energy production (ATP) via cellular respiration — "powerhouse of the cell" |
| Chloroplast | Photosynthesis in plant cells — converts light to glucose |
| Ribosome | Protein synthesis |
| Endoplasmic reticulum | Transport network; rough ER has ribosomes |
| Golgi apparatus | Packages and ships proteins |
| Vacuole | Storage; central vacuole in plants maintains turgor pressure |
| Cell wall | Rigid outer layer in plants (cellulose) and bacteria — absent in animal cells |
| Cell membrane | Controls what enters and leaves the cell (selectively permeable) |
Cell Division
Mitosis: 4 phases — Prophase, Metaphase, Anaphase, Telophase. Produces 2 identical daughter cells. Used for growth, repair, and asexual reproduction.
Meiosis: 2 rounds of division. Produces 4 cells with half the chromosome number (haploid). Used for sexual reproduction. Introduces genetic variation through crossing over.
Human Body Systems
Digestive System
Path of food: Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus
- Mouth: Saliva contains amylase (digests starch).
- Stomach: HCl kills bacteria; pepsin digests proteins.
- Small intestine: Main absorption site. Bile (from liver) emulsifies fats. Pancreatic juice digests proteins, fats, and carbohydrates.
- Large intestine: Absorbs water; forms solid faeces.
- Liver: Produces bile, detoxifies blood, stores glucose as glycogen.
Circulatory System
Heart: 4 chambers — 2 atria (receive blood), 2 ventricles (pump blood). Right side handles deoxygenated blood; left side handles oxygenated blood.
Blood vessels:
- Arteries: carry blood away from the heart (mostly oxygenated)
- Veins: carry blood to the heart (mostly deoxygenated)
- Capillaries: exchange nutrients and gases with tissues
Blood components:
- Red blood cells (RBCs): carry oxygen via haemoglobin; no nucleus in mature human RBCs
- White blood cells (WBCs): immunity — fight infection
- Platelets (thrombocytes): blood clotting
- Plasma: liquid component; carries nutrients, hormones, waste
Blood groups: ABO system (A, B, AB, O) + Rh factor (+/−). O− = universal donor. AB+ = universal recipient.
Respiratory System
Path of air: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Alveoli are tiny air sacs where gas exchange occurs — oxygen enters blood, carbon dioxide exits. Large surface area and thin walls facilitate diffusion.
Breathing muscles: Diaphragm contracts and flattens during inhalation (lungs expand). Intercostal muscles lift the ribcage.
Nervous System
Central nervous system (CNS): Brain + Spinal cord.
- Cerebrum: thinking, voluntary movement, sensory perception
- Cerebellum: balance and coordination
- Medulla oblongata: controls involuntary functions — breathing, heartbeat, blood pressure
Peripheral nervous system (PNS): All nerves outside the CNS.
- Somatic: voluntary control of skeletal muscles
- Autonomic: involuntary — sympathetic (fight or flight) and parasympathetic (rest and digest)
Reflex arc: Stimulus → Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector → Response. Reflex actions bypass the brain for speed.
Endocrine System — Key Hormones
| Gland | Hormone | Function |
|---|---|---|
| Pituitary | Growth hormone, TSH, ACTH | Growth, controls other glands |
| Thyroid | Thyroxine | Metabolic rate, growth |
| Parathyroid | PTH | Calcium regulation |
| Adrenal | Adrenaline, cortisol | Stress response, inflammation |
| Pancreas | Insulin, glucagon | Blood glucose regulation |
| Ovaries | Oestrogen, progesterone | Female reproduction |
| Testes | Testosterone | Male reproduction |
Nutrition and Deficiency Diseases
Vitamins — Deficiency Diseases
| Vitamin | Deficiency Disease | Source |
|---|---|---|
| A (Retinol) | Night blindness, Xerophthalmia | Carrots, leafy greens, liver |
| B₁ (Thiamine) | Beriberi (nerve and heart damage) | Whole grains, legumes |
| B₂ (Riboflavin) | Ariboflavinosis (mouth sores, eye inflammation) | Milk, eggs, meat |
| B₃ (Niacin) | Pellagra (3 Ds: Dermatitis, Diarrhoea, Dementia) | Meat, fish, peanuts |
| B₉ (Folic acid) | Neural tube defects in foetus, anaemia | Green vegetables, legumes |
| B₁₂ | Pernicious anaemia | Meat, dairy, eggs (absent in plant foods) |
| C (Ascorbic acid) | Scurvy (bleeding gums, joint pain) | Citrus fruits, amla, guava |
| D (Calciferol) | Rickets (children), Osteomalacia (adults) | Sunlight, fish oil, dairy |
| E (Tocopherol) | Muscle weakness, nerve damage | Nuts, seeds, vegetable oils |
| K | Poor blood clotting | Green leafy vegetables |
Minerals — Deficiency Diseases
| Mineral | Deficiency | Source |
|---|---|---|
| Iron | Anaemia (low haemoglobin) | Spinach, lentils, meat |
| Calcium | Weak bones, teeth, muscle cramps | Dairy, ragi, sesame |
| Iodine | Goitre (swollen thyroid) | Iodised salt, seafood |
| Zinc | Stunted growth, impaired immunity | Meat, legumes, nuts |
| Fluoride | Dental cavities (deficiency); fluorosis (excess) | Fluoridated water, fish |
Genetics Basics
DNA (Deoxyribonucleic Acid): Double-helix molecule made of nucleotides. Carries genetic information.
Gene: A segment of DNA that codes for a specific protein. Humans have approximately 20,000–25,000 genes.
Chromosome: Condensed DNA + proteins (histones). Humans have 46 chromosomes (23 pairs). Sex chromosomes: XX = female, XY = male.
Mendel's Laws:
- Law of Segregation: alleles separate during gamete formation; offspring inherit one allele from each parent.
- Law of Independent Assortment: alleles of different genes segregate independently.
Dominant vs Recessive: A dominant allele (A) masks a recessive allele (a). Trait appears if at least one dominant allele is present (AA or Aa). Recessive trait only appears when both alleles are recessive (aa).
Common Diseases
| Disease | Cause | Transmission |
|---|---|---|
| Malaria | Plasmodium parasite | Female Anopheles mosquito bite |
| Dengue | Dengue virus (DENV) | Aedes aegypti mosquito bite |
| Typhoid | Salmonella typhi bacteria | Contaminated food and water |
| Cholera | Vibrio cholerae bacteria | Contaminated water |
| Tuberculosis (TB) | Mycobacterium tuberculosis | Airborne droplets |
| Polio | Poliovirus | Contaminated food and water |
| Rabies | Rabies virus | Animal bite (dogs, bats) |
| AIDS | HIV virus | Blood, sexual contact, mother to child |
| COVID-19 | SARS-CoV-2 virus | Respiratory droplets |
| Diabetes (Type 2) | Lifestyle, insulin resistance | Non-communicable |
Board Exam Tips
- Vitamin and mineral deficiency tables are directly asked in 1–2 questions every Group I prelims. Memorise the table — especially Vitamin A, C, D, B₁, B₃, and Iron.
- Blood group O− and AB+ (universal donor/recipient) appear almost every year.
- Mitosis vs meiosis is a standard 2-choice MCQ — know the difference in purpose and output.
- Hormones and their glands (especially insulin, adrenaline, thyroxine) are frequently tested.
- Disease vectors: Anopheles for malaria, Aedes for dengue — commonly confused, always tested.