Std. 10
Biology

A chromosome is a thread-like structure made mainly of DNA and associated proteins. It carries hereditary information in the form of genes.

Chromatin is the DNA-protein material present in the nucleus. After replication, a chromosome consists of two sister chromatids joined at a centromere.

A gene is a functional unit of heredity located on DNA. DNA carries coded information that helps determine inherited characteristics.

The cell cycle is the sequence through which a cell grows, copies its DNA and divides. Interphase includes G₁ (growth), S (DNA synthesis) and G₂ (preparation for division).

Mitosis is nuclear division that produces genetically similar daughter nuclei. It supports growth, repair and replacement of cells.

Prophase: chromosomes condense; metaphase: chromosomes align at the equator; anaphase: sister chromatids separate; telophase: new nuclei form. Cytokinesis then divides the cell.

Animal cells generally divide by a cleavage furrow, while plant cells form a cell plate that develops into a new cell wall.

Meiosis is reduction division that produces cells with half the chromosome number. It is essential in sexual reproduction and contributes to variation; detailed stages are not required at this level.

Homologous chromosomes are a matching pair, one inherited from each parent, carrying genes for the same kinds of characteristics at corresponding positions.

A root hair is a long, thin extension of an epidermal cell. Its large surface area and thin wall make it well suited for absorbing water and mineral salts.

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration until equilibrium is approached.

Osmosis is movement of water through a selectively permeable membrane from a region of higher water potential/dilute solution toward lower water potential/concentrated solution.

Endosmosis is entry of water into a cell in a suitable hypotonic environment; exosmosis is movement of water out of a cell in a hypertonic environment.

A plant cell becomes turgid when it gains water and its contents press against the cell wall. It becomes flaccid when it loses turgor.

Plasmolysis occurs when a plant cell loses sufficient water in a hypertonic solution and the cell membrane pulls away from the cell wall. Deplasmolysis is reversal when water enters again.

Passive transport does not require metabolic energy and follows a concentration/water-potential gradient. Active transport uses cellular energy to move substances against a gradient.

Root pressure develops from activity in roots and can contribute to upward movement of water. Cohesion between water molecules, adhesion to xylem walls and transpiration pull are important forces in ascent of sap.

Transpiration is loss of water vapour from aerial parts of plants, mainly through stomata. It creates a pull that assists movement of water upward.

Stomatal transpiration occurs through stomata; cuticular transpiration through the cuticle; lenticular transpiration through lenticels.

A stoma is a pore controlled by guard cells. Opening and closing regulate gas exchange and water loss.

According to the prescribed explanation, movement of potassium ions alters the osmotic concentration of guard cells, causing water movement and changes in guard-cell turgidity, which open or close the stoma.

Light, temperature, humidity, wind and availability of water affect transpiration. Their effects must be understood rather than memorised as an isolated list.

Plants in dry environments may have thick cuticles, reduced leaves/spines, sunken stomata or other adaptations that reduce water loss.

Guttation is loss of liquid water droplets, usually through hydathodes. Bleeding refers to sap exuding from a cut or injured plant part due to internal pressure.

Photosynthesis is the process by which green plants manufacture organic food from carbon dioxide and water using light energy captured by chlorophyll.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of light and chlorophyll. This is a simplified overall representation of a complex process.

Chloroplasts contain chlorophyll and internal membrane systems. Light-dependent reactions are associated with the thylakoid membranes; carbon-fixation reactions occur in the stroma.

Stomata permit carbon dioxide to enter and oxygen to leave, while also controlling water loss through transpiration.

Experiments demonstrate the need for light, carbon dioxide and chlorophyll, and show starch formation and oxygen release. Each experiment has a control and an observation.

Photosynthesis removes carbon dioxide from the atmosphere and incorporates carbon into organic matter. Respiration, decomposition and combustion return carbon dioxide to the environment.

Plant growth and responses are coordinated by chemical substances called plant hormones or growth regulators.

Auxin promotes cell elongation and is involved in phototropism and other growth responses. Unequal distribution can cause bending toward or away from a stimulus.

Phototropism is growth in response to light; geotropism/gravitropism is response to gravity. Positive and negative describe growth toward or away from the stimulus.

Plant growth responses help roots and shoots position themselves for obtaining water, minerals and light. The response depends on the organ and hormone distribution.

Students should connect a named hormone or tropism with the stimulus, direction of growth and biological advantage rather than learning only the definition.

Blood transports oxygen, carbon dioxide, nutrients, hormones and wastes. It also helps regulate temperature and protect against disease.

Plasma is the liquid medium; red blood cells carry oxygen using haemoglobin; white blood cells defend the body; platelets help clotting.

The human heart has four chambers: right and left atria and right and left ventricles. Valves prevent backflow and maintain one-way movement of blood.

Pulmonary circulation carries blood between heart and lungs; systemic circulation carries blood between heart and body tissues. The two circuits work together as double circulation.

Arteries generally carry blood away from the heart and have thick elastic walls; veins return blood and have valves; capillaries have thin walls suited for exchange.

Blood pressure is the force exerted by blood on vessel walls. Pulse is the rhythmic expansion of arteries associated with heartbeats.

Lymph is tissue fluid collected into lymphatic vessels. It returns excess fluid to the blood and participates in immune defence.

Excretion removes metabolic waste products from the body. It is different from egestion, which removes undigested food.

Kidneys filter blood and regulate water, salts and waste. The functional unit is the nephron.

A nephron includes the glomerulus, Bowman's capsule and a tubular system. Filtration occurs at the renal corpuscle; selective reabsorption and secretion modify the filtrate.

Urine formation involves ultrafiltration, selective reabsorption and tubular secretion. The final urine contains wastes and excess substances that the body needs to eliminate.

Kidneys help maintain the body's water and salt balance. Hormonal control alters the amount of water reabsorbed according to the body's needs.

Lungs remove carbon dioxide and water vapour; skin removes sweat containing water, salts and small amounts of nitrogenous waste. The liver plays major roles in processing substances and detoxification.

A neuron is a specialised cell that receives and transmits nerve impulses. Dendrites receive signals, the cell body contains the nucleus, and the axon carries impulses away.

The central nervous system consists of brain and spinal cord. The peripheral nervous system connects the CNS with receptors, muscles and glands.

A reflex action is a rapid, automatic response to a stimulus. A reflex arc provides a pathway through which the impulse travels from receptor to effector.

The cerebrum is associated with conscious activities, thinking and voluntary actions; the cerebellum coordinates movement and balance; the medulla controls important involuntary functions.

The cornea and lens refract light; the iris controls pupil size; the retina contains light-sensitive receptors; the optic nerve carries signals to the brain.

The outer and middle ear conduct sound vibrations. The cochlea converts vibrations into nerve signals; the semicircular canals contribute to balance.

Receptors detect specific stimuli such as light, sound, touch, temperature and chemicals. The nervous system processes the information and coordinates responses.

Endocrine glands release hormones directly into the blood. Hormones are chemical messengers that act on target tissues.

The pituitary is often called the master gland because its hormones regulate several other endocrine glands and physiological processes.

Thyroxine helps regulate metabolic activity. Iodine is required for normal thyroid hormone production.

The endocrine part of the pancreas produces hormones including insulin, which helps regulate blood glucose concentration.

Adrenal hormones help the body respond to stress and regulate several metabolic and physiological processes.

Hormonal systems are often controlled by feedback mechanisms. This prevents excessive or insufficient hormone action and helps maintain internal balance.

The testes produce sperm and male sex hormones. Sperm pass through ducts and are mixed with secretions from accessory glands to form semen.

The ovaries produce ova and female sex hormones. The oviducts, uterus, cervix and vagina have specialised roles in reproduction.

The menstrual cycle involves coordinated changes in the ovaries and uterus. Hormones regulate follicle development, ovulation and preparation of the uterine lining.

Fertilisation is fusion of male and female gametes to form a zygote. In humans it normally occurs in the oviduct.

After early development, the embryo implants in the uterine lining. The placenta develops a vital exchange interface between maternal and embryonic circulations.

The placenta allows exchange of nutrients, oxygen and wastes while maintaining separate maternal and foetal blood supplies under normal conditions. It also has endocrine functions.

Puberty is the period when hormonal changes lead to reproductive maturity and secondary sexual characteristics.

Heredity is transmission of characteristics from parents to offspring. Variation produces differences among individuals and provides the basis for natural selection.

A gene is a hereditary unit; an allele is an alternative form of a gene. Alleles occur at corresponding loci on homologous chromosomes.

A dominant allele expresses its effect in a heterozygous condition; a recessive allele is expressed when the relevant dominant allele is absent, under the simplified Mendelian model.

Genotype is the genetic constitution for a characteristic; phenotype is the observable expression resulting from genotype and environmental influences.

Mendel's principles include segregation of alleles during gamete formation and independent assortment of pairs of alleles when applicable. His work explains inheritance patterns in crosses.

A monohybrid cross follows inheritance of one characteristic. Students should understand genotype and phenotype ratios and how they arise from gamete combinations.

A dihybrid cross follows two characteristics. For the prescribed level, focus on the phenotypic outcome and the logic of independent assortment.

In humans, females are XX and males are XY. The ovum carries X, while sperm carry either X or Y; therefore the sperm determines the chromosomal sex of the offspring.

X-linked conditions such as haemophilia and colour blindness show characteristic inheritance because the relevant genes are located on the X chromosome.

A population is a group of individuals of the same species living in a particular area. Population size changes through births, deaths, immigration and emigration.

Rapid increase can occur when birth rate exceeds death rate. Limited resources, disease, competition and environmental resistance can restrict growth.

Air, water, soil, forests and minerals support life and human activity. Sustainable use aims to meet present needs without destroying the resource base.

Pollution is harmful alteration of the environment. Air, water and soil pollution can affect organisms, ecosystems and human health.

Certain gases absorb outgoing infrared radiation and warm the lower atmosphere. Human activities can enhance this effect, contributing to global climate change.

Stratospheric ozone absorbs much harmful ultraviolet radiation. Certain chemicals can damage ozone, increasing UV exposure at Earth's surface.

Conservation protects biodiversity and natural resources through measures such as habitat protection, sustainable use, afforestation, pollution control and responsible waste management.

Sustainable development balances environmental protection with social and economic needs, so resources remain available for future generations.

Osmosis experiments demonstrate movement of water through a selectively permeable membrane. Water absorption and xylem-conduction experiments connect structures with functions.

Loss in mass of a potted plant/leafy shoot and cobalt chloride paper can demonstrate water loss and unequal transpiration from different leaf surfaces.

Experiments demonstrate the necessity of light, carbon dioxide and chlorophyll, formation of starch and release of oxygen. Always state the setup, control, observation and conclusion.

Biology diagrams should be large, clear and accurately labelled. Use clean single lines, avoid unnecessary shading and place labels neatly.

For a specimen or model, identify it, draw a labelled outline when required and write the characteristic features that justify the identification.

Write: Aim → Materials/Apparatus → Procedure → Observation → Inference/Conclusion → Diagram/Equation where applicable. Never invent an observation.