The World of the Living — Plants, Animals & Life Processes | CTET Science P2

Cell — The Basic Unit of Life

The cell is the structural and functional unit of all living organisms. Robert Hooke first observed cells in 1665 when he examined a thin slice of cork under a microscope and saw tiny box-like compartments. The word cell comes from the Latin cellula, meaning a small room.

Cells are of two broad types. Prokaryotic cells (found in bacteria and blue-green algae) lack a membrane-bound nucleus; their genetic material floats freely in the cytoplasm. Eukaryotic cells (found in plants, animals, fungi and protists) have a well-defined nucleus enclosed by a nuclear membrane.

Key organelles found in a typical animal cell include: the nucleus (controls cell activities and carries DNA), mitochondria (sites of aerobic respiration, called the powerhouse of the cell), ribosomes (sites of protein synthesis), endoplasmic reticulum (transport network), and Golgi apparatus (packaging and secretion of substances).

Plant cells have three additional features not found in animal cells: a rigid cell wall made of cellulose (gives shape and protection), a large central vacuole (maintains turgor pressure), and chloroplasts containing the green pigment chlorophyll (sites of photosynthesis).

Organisms made of a single cell — such as Amoeba, Paramecium, and Euglena — are called unicellular organisms. They perform all life functions within that one cell. Organisms with many cells organised into tissues and organs are called multicellular organisms (e.g., plants, animals, fungi).

• Cell membrane: selectively permeable; controls entry and exit of substances.
• Cytoplasm: jelly-like fluid that fills the cell and suspends the organelles.
• Nucleus: contains chromosomes made of DNA; directs all metabolic activities.

For CTET Paper 2, students should be able to distinguish plant cells from animal cells, identify organelles from diagrams, and understand why the cell is called the basic unit of life.

Nutrition in Plants and Animals

Nutrition is the process by which organisms obtain and use food to build body materials and provide energy. Organisms are classified into two nutritional types based on how they obtain food.

Autotrophs (self-feeders) synthesise their own food using simple inorganic raw materials. Green plants are the chief autotrophs; they perform photosynthesis — the process by which chlorophyll-containing cells use sunlight, carbon dioxide, and water to produce glucose and oxygen. The overall equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Stomata on leaves allow gas exchange, while roots absorb water and dissolved minerals from the soil.

Some plants such as Cuscuta (dodder) are parasitic — they lack chlorophyll and draw nutrients from a host plant. Pitcher plant and Venus flytrap are insectivorous plants that obtain nitrogen by digesting insects. Fungi and certain bacteria are saprophytes, breaking down dead organic matter.

Heterotrophs cannot make their own food and depend on other organisms. Animals are classified by feeding strategy:

• Herbivores (e.g., cow, rabbit) eat only plants.
• Carnivores (e.g., lion, eagle) eat other animals.
• Omnivores (e.g., humans, crow) eat both plants and animals.

Human digestion begins in the mouth where saliva (containing amylase) breaks down starch. Food passes through the oesophagus to the stomach, where gastric juice (HCl + pepsin) digests proteins. In the small intestine, bile (from liver) emulsifies fats and pancreatic juice completes digestion. Nutrients are absorbed through villi — finger-like projections that greatly increase the absorptive surface area. Undigested matter is expelled through the large intestine and rectum.

Understanding the difference between autotrophic and heterotrophic nutrition, and the role of each digestive organ, is a recurring focus in CTET Science questions.

Respiration in Organisms

Respiration is the biochemical process by which living cells break down food (primarily glucose) to release energy in the form of ATP (adenosine triphosphate). Respiration is different from breathing: breathing is the mechanical process of moving air in and out of the lungs, while respiration occurs at the cellular level.

Aerobic respiration occurs in the presence of oxygen and takes place mainly in the mitochondria. The overall reaction is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP). This is an exothermic reaction that releases a large amount of energy — about 38 ATP molecules per glucose molecule.

Anaerobic respiration occurs without oxygen. In yeast and some bacteria, glucose is converted to ethanol and CO₂ (fermentation). In muscle cells during intense exercise, glucose is converted to lactic acid, causing fatigue and cramps. Anaerobic respiration yields only 2 ATP molecules per glucose — far less efficient than aerobic respiration.

Different organisms have evolved different respiratory organs:

• Fish: gills extract dissolved oxygen from water. Fish must process large volumes of water because water contains far less dissolved oxygen than air, so their breathing rate is faster, not slower, compared to land animals.
• Insects: a system of tubes called tracheae carries air directly to cells.
• Earthworms: breathe through the moist skin by diffusion.
• Amphibians (frogs): breathe through both lungs and moist skin.
• Mammals and birds: breathe through lungs with highly efficient gas exchange surfaces (alveoli).

In humans, air enters through the nose, passes through the trachea and bronchi into alveoli — tiny air sacs surrounded by capillaries. Oxygen diffuses into blood and CO₂ diffuses out. The diaphragm and intercostal muscles control inhalation and exhalation.

Photosynthesis absorbs energy (endothermic), while respiration releases energy (exothermic) — an important distinction tested in CTET.

Transportation in Plants and Animals

Living organisms need efficient transport systems to carry nutrients, gases, and wastes to and from every cell. In plants this is achieved by a vascular system, while in animals a circulatory system serves this role.

Transport in plants involves two types of conducting tissues:

• Xylem carries water and dissolved minerals from roots to leaves in a one-way, upward flow. The driving force is transpiration pull — water evaporated from leaf surfaces creates tension that pulls water upward. Xylem vessels are dead cells.
• Phloem carries food (sucrose and amino acids) manufactured in leaves to all parts of the plant — both upward and downward. Phloem cells are living cells. This process is called translocation.

Transpiration is the loss of water vapour through stomata. It helps in the absorption and upward movement of water and minerals, cools the leaf surface, and maintains turgidity. Stomata open during the day (in sunlight) and close at night in most plants.

Transport in animals — the circulatory system: In humans, the circulatory system consists of the heart, blood vessels (arteries, veins and capillaries), and blood.

• Arteries carry oxygenated blood away from the heart (except the pulmonary artery). They have thick, elastic walls.
• Veins carry deoxygenated blood towards the heart (except the pulmonary vein). They have thinner walls and valves to prevent backflow.
• Capillaries are microscopic vessels where actual exchange of gases, nutrients and wastes occurs between blood and cells.

The human heart has four chambers: right atrium, right ventricle, left atrium, left ventricle. It has a closed, double circulatory system: pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → body → heart). Blood never mixes between the two circuits in a four-chambered heart.

Blood has four components: plasma (liquid portion), red blood cells or RBCs (carry haemoglobin for oxygen transport), white blood cells or WBCs (immune defence), and platelets (blood clotting). Lymph is a colourless fluid that returns excess tissue fluid to the bloodstream.

Reproduction

Reproduction is the biological process by which organisms produce offspring of the same species, ensuring continuity of life. There are two main modes: asexual and sexual reproduction.

Asexual reproduction involves a single parent and produces genetically identical offspring (clones). Methods include:

• Binary fission: the parent organism splits into two (e.g., Amoeba, bacteria).
• Budding: a small outgrowth (bud) develops on the parent and separates (e.g., Hydra, yeast).
• Spore formation: bread mould (Rhizopus) releases spores that germinate in suitable conditions.
• Vegetative propagation: new plants grow from vegetative parts — stem (potato tubers, ginger rhizomes, runners in strawberry), roots (sweet potato), or leaves (Bryophyllum).
• Fragmentation: the body breaks into pieces, each growing into a new organism (e.g., Spirogyra).

Sexual reproduction involves fusion of male and female gametes (fertilisation) to produce a zygote. It introduces genetic variation, increasing adaptability.

In flowering plants, sexual reproduction involves pollination — the transfer of pollen from anther to stigma. Self-pollination occurs only in bisexual (hermaphrodite) flowers, where pollen from the same flower (or plant) reaches the stigma. Cross-pollination transfers pollen between different plants via wind, water, insects or animals. Bisexual flowers can undergo both self and cross-pollination.

After fertilisation, the ovule becomes a seed and the ovary becomes a fruit. Seeds are dispersed by wind (maple), water (coconut), animals (burr seeds), or self-dispersal (touch-me-not/Impatiens).

In human reproduction, sperm from the male fuse with eggs from the female. When two sperm fertilise two separate eggs, fraternal (dizygotic) twins are formed — they are genetically distinct and may differ in sex. When one fertilised egg splits into two, identical (monozygotic) twins form — they are genetically identical. The CTET question on twins tests this concept directly.

Puberty brings physical and hormonal changes that prepare the body for reproduction. Menstruation is a monthly cycle in females that prepares the uterus for a fertilised egg.

Classification of Living Organisms

Classification is the systematic grouping of organisms into categories based on shared characteristics. It simplifies the study of the vast diversity of life and reveals evolutionary relationships.

The two-kingdom classification (animals and plants) by Linnaeus was later replaced by the five-kingdom classification proposed by R. H. Whittaker (1969): Monera, Protista, Fungi, Plantae, and Animalia.

The standard hierarchy of classification (from broadest to most specific) is: Kingdom → Phylum (or Division) → Class → Order → Family → Genus → Species. The biological name of an organism consists of its genus and species (binomial nomenclature).

Classification of Plants (Kingdom Plantae):

• Thallophyta: simplest plants; no roots, stems or leaves; include algae (Spirogyra, Ulothrix) and fungi.
• Bryophyta: amphibians of the plant kingdom; live in moist, shaded places; no vascular tissue; e.g., mosses and liverworts.
• Pteridophyta: first vascular land plants; reproduce by spores; e.g., ferns and horsetails.
• Gymnosperms: bear naked seeds (not enclosed in fruit); e.g., pine, fir, cycas.
• Angiosperms: flowering plants with seeds enclosed in fruits. Divided into monocots (one cotyledon, parallel leaf venation, e.g., wheat, maize) and dicots (two cotyledons, reticulate venation, e.g., peas, mango).

Classification of Animals (Kingdom Animalia):

• Porifera: pore-bearing animals; simplest animals; e.g., sponges.
• Coelenterata: hollow-bodied; e.g., Hydra, jellyfish, corals.
• Platyhelminthes: flatworms; e.g., tapeworm, planaria.
• Nematoda: roundworms; e.g., Ascaris.
• Annelida: segmented worms; e.g., earthworm, leech.
• Arthropoda: largest phylum; jointed legs; e.g., insects, spiders, crabs.
• Mollusca: soft-bodied, often with shell; e.g., snail, octopus.
• Echinodermata: spiny-skinned marine animals; e.g., starfish, sea urchin.
• Chordata: animals with a notochord; includes fish, amphibians, reptiles, birds and mammals.

Microorganisms: Friend and Foe

Microorganisms (microbes) are living organisms too small to be seen with the naked eye; they can be observed only under a microscope. Major groups include bacteria, viruses, fungi, algae, and protozoa.

Beneficial roles of microorganisms:

• Nitrogen fixation: Rhizobium bacteria (in root nodules of legumes) and free-living Azotobacter fix atmospheric nitrogen into ammonia, enriching soil fertility.
• Decomposition: bacteria and fungi break down dead organisms and recycle nutrients back into the soil — essential for ecosystem functioning.
• Food industry: Lactobacillus converts milk to curd (yoghurt). Yeast (Saccharomyces) ferments sugar to produce alcohol and CO₂ — used in baking (bread rises due to CO₂) and brewing.
• Medicine: Penicillin (the first antibiotic) was discovered by Alexander Fleming from the fungus Penicillium notatum. Many vaccines are produced using weakened or killed microorganisms.
• Biogas production: methane-producing bacteria (methanogens) decompose organic waste in biogas plants, producing renewable fuel.

Harmful roles of microorganisms:

• Disease in humans: typhoid and cholera (bacteria), common cold and influenza (viruses), malaria (protozoan Plasmodium, spread by female Anopheles mosquito), ringworm (fungus).
• Food spoilage: bacteria and fungi cause food to rot. Preservatives, refrigeration, pasteurisation, and salting prevent spoilage.
• Plant diseases: rust of wheat, citrus canker are caused by microorganisms.
• Antibiotic resistance: overuse of antibiotics creates resistant strains of bacteria, a growing public health concern.

Important public health measures include proper sanitation, safe drinking water, vaccination, and personal hygiene — all topics that connect microorganism biology to social responsibilities, relevant to both CTET Science content and pedagogy questions.

Adaptations in Organisms

Adaptation refers to the structural, physiological, or behavioural features of an organism that improve its chances of survival and reproduction in a particular habitat. Adaptations arise over many generations through natural selection.

Aquatic adaptations: Fish have a streamlined body to reduce water resistance, gills for extracting dissolved oxygen, and fins for steering and propulsion. Whales and dolphins (mammals) have flippers and breathe through a blowhole. Aquatic plants such as lotus have long, flexible stems and air spaces (aerenchyma) to keep leaves afloat and allow gas exchange.

Desert adaptations: Cacti have thick, fleshy stems to store water, spines (modified leaves) to reduce water loss and deter herbivores, and deep or wide root systems to maximise water absorption. Desert animals such as camels store fat in their hump, produce concentrated urine, and have long eyelashes to protect against blowing sand.

Polar/cold-climate adaptations: Polar bears have thick fur and a layer of fat (blubber) for insulation. Penguins huddle together to conserve heat and have counter-current heat exchange in their flippers. Many arctic plants are low-growing to avoid freezing winds.

Forest/rainforest adaptations: Trees in dense forests grow tall to reach sunlight; many have buttress roots for support. Climbing plants and epiphytes (orchids, mosses) have adapted to low-light conditions on the forest floor.

Seasonal adaptations: Many animals migrate to warmer regions in winter (e.g., Siberian cranes fly to India). Some mammals hibernate, reducing their metabolic rate to survive food scarcity. Trees in temperate zones shed leaves (deciduous trees) to reduce water loss.

Behavioural adaptations include nocturnal behaviour (active at night to avoid heat and predators), camouflage (stick insects, chameleons), and mimicry (some harmless snakes mimic venomous species).

Understanding adaptation connects to ecological concepts and helps teachers design activities where students observe local plants and animals and identify adaptive features — a core expectation of inquiry-based upper-primary science teaching.