Thursday 25 November 2021

9th Class Biology Important Definitions Notes A to Z

9th Class Biology Important Definitions Notes A to Z

Here are all the 9th Class Biology Important Definitions Notes from A to Z and from all the chapters of class 9 bio.


Active transport: The movement of molecules across a cell membrane from the area of lower to the area of higher concentration

Aerobic respiration: A type of cellular respiration in which oxygen is utilized and glucose is completely oxidized into carbon dioxide and water

Agriculture: The profession that deals with the food crops and animals which are the source of food

Amino acids: The organic molecules which are the units of proteins

Anaerobic respiration: A type of cellular respiration in which oxygen is not utilized and glucose is incompletely oxidized

Anatomy: The study of the structure of internal organs

Antibodies: Proteins produced by lymphocytes that attack antigens

Antigens: Foreign invaders in the body that trigger an immune response

Arteries: The thick-walled blood vessels which carry blood away from the heart

Asexual reproduction: Reproduction which does not involve the fusing of gametes

Assimilation: The incorporation of the products of digestion into the body, where they are used to provide energy or for growth and repair of tissues

ATP: A high-energy chemical used in cells as an energy store


Bile: A secretion produced in the liver which emulsifies fats to prepare them for digestion

Biodiversity: Variety in and between organisms, species and ecosystems

Bioelements: Elements that make the body of living organisms

Biogeography: The study of the occurrence and distribution of species in different geographical regions of the world

Biophysics: The interdisciplinary science that deals with the study of the principles of physics which are applicable to biological phenomena

Biosphere: The earth's landmasses, oceans and atmosphere, and all the organisms living in them

Bivalent: Having two chromosomes - one from the male gamete and one from the female gamete.


Capillaries: Extremely narrow tubes which carry blood around our tissues

Carbohydrate: Food belonging to the group consisting of sugars, starch and

cellulose. They are vital for energy in humans.

Carnivore: An animal which feeds only on other animals

Cellulose: A carbohydrate which forms the cell wall in plant cells

Chlorophyll: The green pigment inside the chloroplasts of plant cells. It enables photosynthesis to take place.

Chloroplasts: Cell organelles containing chlorophyll; found in green plant cells where photosynthesis takes place

Chromatid: One of two daughter strands of a replicated chromosome

Chromosome: One of the rod-shaped bodies found in the nucleus of ceils that contain genetic information (DNA)

Cloning: Producing organisms which are genetically identical to one another

Community: Populations of different organisms living together in a habitat

Cytokinesis: The division of the cytoplasm, after nuclear division (karyokinesis)


Decomposer: An organism which eats dead materials and breaks them down into simple materials

Defecation: Getting rid of undigested material from the body

Deforestation: The process of cutting down a large number of trees in a forest and not replacing them

Diaphragm: A large sheet of muscle that separates the lungs from the abdominal cavity

Dietary fibre: Non-digestible carbohydrates

Diffusion: The movement of molecules or ions from an area of higher concentration to an area of lower concentration

Digestive system: System of organs are involved with the ingestion, digestion, absorption of food

DNA: Deoxyribonucleic Acid: The material inside the nucleus of cells which carries genetic information.

Duodenum: The first part of the small intestine where most of the digestion takes



Ecology: The study of living things within their environment

Embryo: The developing human offspring inside the womb

Enzyme: Proteins which speed up chemical reactions inside living organisms, by minimizing the activation energy

Epiglottis: Small flap at the back of the tongue, it closes the glottis when we swallow so that food does not go down into trachea.

Extinct: Species of organism that is no longer living


Fatty acids: Important components of lipids. Long-chain molecules made up of hydrogen and carbon atoms

Fertilizers: Nutrients applied to crops to speed up growth

Flaccid: Lacking turgor; lacking in stiffness

Fossil: Remains or impression of an organism that existed in the geological past


Genetics: The study of inheritance. Inheritance means the transmission of characters from one generation to the other.

Guard cells: Bean-shaped cells which control the opening and closing of stomata in a green plant's leaves


Habitat: A place where plants, animals and micro-organisms live

Haemoglobin: The red protein found in red blood cells that transports oxygen

Herbivore: An animal which feeds only on plants

Horticulture: The growing of garden plants: the cultivation of fruit and vegetables


Ileum: The longest part of the small intestine, where food is completely digested and absorbed

Immune system: The body's defence system against diseases and infections


Limiting factor: Something which, when not sufficient, prevents or slows down a process

Lymphocytes: A type of white blood cells which attack pathogens


Meiosis: Type of cell division which forms gametes in animals and spores in higher plants, each with half the usual number of chromosomes

Mesophyll: The inner tissue of a leaf, whose cells are green because they

contain chloroplasts

Metabolism: The sum of all chemical processes taking place in our cells, and the rate at which these proceed

Microbiology: The division of biology that studies the life of microorganisms

Mitochondria: Structures in the cytoplasm of all cells where respiration takes place. Singular is mitochondrion

Mitosis: A type of cell division which produces daughter cells identical to the parent

Morphology: The study of the structures of living organisms

Mutation: A change in DNA which results in a change in an organism's genome.


Nucleus: The organelle of eukaryotic cells where genetic information is stored


Omnivore: An animal which feeds on both plants and animals

Organ: Group of tissues doing a particular job e.g. the heart

Organ system: Group of organs doing a particular job, e.g. the circulatory system

Organelle: Microscopic structure which does a particular job in a cell, e.g. the nucleus

Osmosis: The movement of water molecules across a semi permeable membrane, from a solution of lower solute concentration to a solution of higher solute concentration

Oxygenated blood: The blood containing more oxygen


Palisade mesophyll: The upper layer of mesophyll where photosynthesis mostly takes place

Pancreas: Large gland located in the abdomen near the stomach which produces Pancreatic juice for digestion and the hormone insulin.

Parasite: An organism that lives in or on another organism and causes harm to it Partially permeable or semi-permeable: Allowing some particles to pass through but not others

Peristalsis: Wave-like muscular contractions in the walls of the gut which move food through the alimentary canal

Phloem: Compound tissue through which food is transported in a plant

Photosynthesis: The chemical change in autotrophs that uses light energy to convert carbon dioxide and water into glucose

Physiology: The study of the functions of living organisms and their parts

Plasma: Liquid, non-cellular part of the blood

Platelets: White blood cell fragments which circulate in the blood and assist in clotting

Population: Members of a single species living in a habitat

Predator: Animal that hunts, kills and eats other animals for food

Producer: An organism which can make its own food - i.e. a green plant

Proteins: Organic compounds made up of amino acid molecules, one of the three main food groups


Red blood cells: Cells which contain haemoglobin and carry oxygen

Renal artery: Related to kidneys e.g. renal artery, renal vein

RNA: (Ribonucleic acid) - it converts the genetic information stored in DNA into proteins

Root hairs: Tiny hairs covering the ends of the smallest roots. They give the root a very large surface area to absorb water and mineral salts from the soil


Saliva: Fluid secreted by glands in the mouth which moistens, softens and semi- digests food

Sexual reproduction: Reproduction which involves the fusing of male and female gametes

Spindle fibres: Fibres produced during cell division. They contract to drag  chromatids to the poles of the cell.

Spongy mesophyll: The lower layer of mesophyll which contains numerous air spaces where gas exchange takes place

Starch: A type of carbohydrate. Plants can turn the glucose into starch for storage

Stomata (singular 'stoma'): Tiny pores in the epidermis of a leaf. They control water loss and gas exchange by opening and closing.


Thorax: The chest, containing the heart and lungs and encased by the ribs

Tissue: Group of cells of the same type doing a particular job, e.g. the glandular tissue, muscular tissue, nervous tissue etc

Trachea: The windpipe or tube from the back of the mouth to the top of the lungs

Transpiration: The loss of water from plant surface by evaporation

Turgor: Swelling and stiffness in plant cell due to endosmosis of water


Vaccines: Substances containing disabled antigens of a particular disease; Vaccines stimulate the body to produce antibodies to provide immunity against that disease

Valves: Structures containing a flap or flaps to ensure one-way flow of liquid

Variegated leaf: Leaf with alternating green and white patches

Veins: Thin-walled, valved tubes which carry blood back to the heart

Villi: Minute hair-like projections which cover the lining of the small intestine provide surface area for absorption

Virus: Ultramicroscopic non-cellular organism that replicates itself inside the cells of living hosts

Vitamins: Organic substances which are essential in small amounts to regulate the metabolism and maintain the immune system


Xylem: Compound tissue which transports water and minerals up a plant


Zygote: diploid cell from fertilization, fusion of sex cell.

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