CBSE Class 11 Biology

1. Diversity of Living Organisms

The Living World · Biological Classification · Plant Kingdom · Animal Kingdom

• The Living World – Earth has millions of kinds of living things; this variety is biodiversity. To study them we sort them into groups. Taxonomy is naming, describing and classifying; systematics also asks how groups are related. All life falls in three domains: Bacteria, Archaea and Eukarya. Organisms are placed in 7 main ranks, from kingdom down to species, and each species gets a two-part Latin name like Mangifera indica.
• Biological Classification – Whittaker (1969) sorted living things into five kingdoms, Monera, Protista, Fungi, Plantae and Animalia, using cell type, cell wall, body plan, mode of nutrition, reproduction and relationships. Monera are prokaryotes (bacteria, archaebacteria, cyanobacteria, mycoplasma). Protista are one-celled eukaryotes in five groups. Fungi are heterotrophs with chitin walls in four classes. Lichens are an alga and a fungus living together. Viruses, viroids and prions are not placed in any kingdom.
• Plant Kingdom – The plant kingdom climbs a ladder of new features. Algae are simple, mostly water plants with no roots or stems. Bryophytes (mosses) live on damp land but have no vascular tissue. Pteridophytes (ferns) are the first with xylem and phloem. Gymnosperms carry naked seeds. Angiosperms hide seeds inside fruits and have flowers. In all of them, a haploid gametophyte and a diploid sporophyte take turns: alternation of generations.
• Animal Kingdom – Animals are sorted using a few body-plan questions: level of organisation, symmetry, number of germ layers, type of body cavity (coelom), segmentation and presence of a notochord. These give 11 major phyla: Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. Chordates have a notochord, a dorsal hollow nerve cord and gill slits at some stage; vertebrates among them fall into seven classes from jawless fish to mammals.

2. Structural Organisation in Plants and Animals

Morphology of Flowering Plants · Anatomy of Flowering Plants · Structural Organisation in Animals

• Morphology of Flowering Plants – Morphology means the outer form of a plant. A flowering plant has a root system below the soil and a shoot system above it. The shoot carries stems, leaves, flowers and fruits. Each part has a normal form and many changed forms (modifications) for special jobs like storing food, climbing or protection. Flowers are described with a floral formula; family Solanaceae (potato, brinjal, tomato) is the model family.
• Anatomy of Flowering Plants – Anatomy is the study of the inside structure of a plant, seen by cutting thin slices (sections). Plant cells form tissues, and tissues form three tissue systems: epidermal (the skin), ground (the filling) and vascular (xylem and phloem pipes). Dicots and monocots differ in how these systems are arranged in their root, stem and leaf.
• Structural Organisation in Animals: the Frog – In animals, cells form tissues, tissues form organs and organs work together in organ systems. The common Indian frog (Rana tigrina, now Hoplobatrachus tigerinus) is a good model: its body has a head and trunk, and inside it has digestive, respiratory, circulatory, nervous, excretory, endocrine and reproductive systems. It breathes through both skin and lungs, has a 3-chambered heart and lays eggs in water.

3. Cell: Structure and Function

Cell: The Unit of Life · Biomolecules · Cell Cycle and Cell Division

• Cell: The Unit of Life – Every living thing is made of cells, and every new cell comes from an old cell. Prokaryotic cells (bacteria) have no nuclear envelope and no membrane-bound organelles; eukaryotic cells have a true nucleus and many organelles. The membrane is a fluid mosaic of lipids and proteins; plants add a cellulose wall. ER, Golgi, lysosomes and vacuoles form the endomembrane system. Mitochondria and plastids make energy and food, ribosomes make proteins, the cytoskeleton, cilia, flagella and centrioles give shape and movement, and the nucleus holds the DNA.
• Biomolecules – A cell is mostly water, plus four big families of carbon compounds: proteins, carbohydrates, lipids and nucleic acids. Grinding tissue in acid separates small molecules (acid-soluble pool) from big ones – proteins, polysaccharides and nucleic acids (acid-insoluble pool). Proteins are chains of amino acids folded into four levels of structure. Polysaccharides are chains of sugars; lipids are fatty acids on glycerol; nucleic acids are chains of nucleotides. Enzymes are protein catalysts that bind a substrate at the active site, lower the activation energy, and are affected by temperature, pH, substrate level and inhibitors.
• Cell Cycle and Cell Division – A cell grows, copies its DNA and splits in a fixed order called the cell cycle: interphase (G1, S, G2) and M phase. In S phase the DNA doubles (2C → 4C) but the chromosome number stays the same. Mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis give two cells identical to the parent – it is an equational division used for growth and repair. Meiosis has two divisions; in meiosis I homologous chromosomes pair, cross over and separate, halving the chromosome number (2n → n). It makes four haploid cells for gametes and creates variation.

4. Plant Physiology

Photosynthesis in Higher Plants · Respiration in Plants · Plant Growth and Development

• Photosynthesis in Higher Plants – Green plants make sugar from carbon dioxide and water using light. It happens in chloroplasts. The light reaction in the thylakoids splits water, gives out oxygen and makes ATP and NADPH. The Calvin cycle in the stroma uses that ATP and NADPH to fix CO₂ into sugar. C₄ plants add a CO₂ pump that stops the wasteful photorespiration. The slowest factor, such as light, CO₂ or temperature, sets the rate.
• Respiration in Plants – Respiration breaks food like glucose to release energy stored as ATP. Plants take in O₂ through stomata and lenticels. Glycolysis in the cytoplasm splits glucose into two pyruvates. Without O₂, pyruvate is fermented to ethanol or lactic acid. With O₂, pyruvate enters the mitochondrion, runs the Krebs cycle, and the electron transport system uses O₂ to make most of the ATP: up to 38 per glucose. The same path also builds molecules (amphibolic). RQ = CO₂ given out ÷ O₂ taken in.
• Plant Growth and Development – Growth is a lasting increase in size. A seed germinates when it gets water, oxygen and the right temperature. At root and shoot tips cells divide, then get longer, then mature. Growth can be arithmetic (same amount each day) or geometric (a share of what is there), which gives an S-shaped curve. Cells then specialise (differentiate), can go back to dividing (dedifferentiate) and specialise again (redifferentiate). Five hormone groups, auxin, gibberellin, cytokinin, ethylene and ABA, control it all.

5. Human Physiology

Breathing and Exchange of Gases · Body Fluids and Circulation · Excretory Products and their Elimination · Locomotion and Movement · Neural Control and Coordination · Chemical Coordination and Integration

• Breathing and Exchange of Gases – Breathing moves air in and out of the lungs. Air travels nose → trachea → bronchi → bronchioles → alveoli. We breathe in when the diaphragm and outer rib muscles contract and make the chest bigger, so pressure inside falls. In the alveoli, oxygen moves into blood and carbon dioxide moves out, always from higher partial pressure to lower. Blood carries O2 mostly on haemoglobin and CO2 mostly as bicarbonate. A centre in the brain sets the rhythm. Lung volumes like tidal volume and vital capacity can be added and measured.
• Body Fluids and Circulation – Blood and lymph are the body's transport fluids. Blood is plasma (about 55%) plus formed elements: RBCs, WBCs and platelets. Blood groups (ABO, Rh) depend on antigens on RBCs. A clot forms when fibrinogen turns into fibrin threads. The human heart has four chambers and pumps blood through two loops: to the lungs and to the body (double circulation). The SA node starts each beat; one beat is a cardiac cycle of about 0.8 s, recorded as an ECG with P, QRS and T waves. Cardiac output = stroke volume × heart rate ≈ 5 L/min.
• Excretory Products and their Elimination – Animals must remove nitrogen waste. Ammonia is most toxic and needs lots of water; urea is less toxic; uric acid needs least water. Humans are ureotelic. Each kidney has about a million nephrons. Urine forms in three steps: glomerular filtration (about 125 mL/min, 180 L/day), reabsorption (about 99% taken back) and secretion. The loop of Henle and vasa recta make the medulla salty by a counter-current mechanism, so urine can be concentrated. ADH, the renin-angiotensin-aldosterone system and ANF control water and salt. Lungs, liver, skin and sweat also help. Failed kidneys need dialysis or a transplant.
• Locomotion and Movement – Movement is a change in position of a body part; locomotion is moving the whole body from place to place. Cells move in three ways: amoeboid, ciliary and flagellar. Humans use muscular movement. Muscles are skeletal, visceral or cardiac. A skeletal muscle is made of fibres, each packed with myofibrils made of repeating sarcomeres of actin and myosin. A nerve signal releases Ca2+, myosin heads pull actin using ATP, and the sarcomere shortens (sliding filament theory). The skeleton has 206 bones. Joints can be fibrous, cartilaginous or synovial (ball and socket, hinge, pivot, gliding, saddle). Disorders include myasthenia gravis, tetany, muscular dystrophy, arthritis, osteoporosis and gout.
• Neural Control and Coordination – Coordination means organs working together. The nervous system does this fast, using neurons. A neuron has a cell body, dendrites and an axon; many axons have a myelin sheath with nodes of Ranvier. The central nervous system (brain and spinal cord) processes information; the peripheral nervous system (nerves) carries it. The PNS has a somatic part (skeletal muscles) and an autonomic part (sympathetic and parasympathetic) for internal organs, the visceral system. At rest the axon is polarised: inside −, outside +, about −70 mV. A stimulus lets Na+ rush in, creating an action potential that travels along the axon. At a synapse, neurotransmitters carry the signal to the next neuron.
• Chemical Coordination and Integration – Hormones are chemical messengers made in tiny amounts by ductless (endocrine) glands. They travel in blood and act on target cells that have the right receptor. The hypothalamus controls the pituitary, which controls many other glands; negative feedback keeps levels steady. Main glands: pineal, pituitary, thyroid, parathyroid, thymus, adrenal, pancreas (islets), testis and ovary. The heart, kidney and gut also make hormones. Protein hormones act through membrane receptors and second messengers like cAMP; steroid and thyroid hormones enter the cell and change gene expression. Too little (hypo-secretion) or too much (hyper-secretion) of a hormone causes disorders like dwarfism, gigantism, goitre, diabetes, Addison's disease and Cushing's syndrome.

6. Formative-only topic

Digestion and Absorption

Coming soon