All living things developed from pre-existing living things of the same species. Life produces life and lifeless object cannot produce living things. Hence, life must have developed from the already existing life, which reproduced and multiplied. This idea was pioneered by Francesco Redi in 1688 basically to disapprove the theory of spontaneous generation and entitled Biogenesis by Thomas Henry about 200 years later (1870)


-All living things are composed of one or more cells.

-The cell is the structural and functional unit of all living organisms.

-New cells are formed from pre-existing cells.

-All cells carry hereditary materials

The theory called the Cell Theory was put forward chiefly by Mathias Schneider and Theodore Schwan between 1838 and 1839 and Redoff Virchow in 1855.


Genes, which are responsible for the transfer of characters from parents to offspring are located on the chromosomes (mostly paired), hence occurring in pairs. A pair of homologous chromosomes, one allele on one chromosome carries each pair of genes. Walter Sutton and Theodor Boveri put the theory called the Chromosome Theory of Inheritance forward in 1902, following Mendel’s discovery 36 years earlier.


Cohesion i.e. the force of attraction, within the molecules of the same substance, is one of the factors responsible for the upward movement of water from the root of a plant to the leaves. Transpiration makes water to be lost from the leaves, but the cohesion force within the water molecules creates enough tension, which pulls other water molecules to follow the lost ones hence creating upward movement of the water. The theory is generally referred to as Cohesion- Tension Theory of Water Movement



Darwing’s theory is also known as the theory of natural selection or the law of survival of the fittest. (See natural selection)


In a large population where mating occurs randomly, the frequency of dominant and recessive alleles will always remain constant from generation to generation, provided there are no mutation, migration, preferential selection, genetic overlapping and any other physical alteration

The theory put forward by Godfrey Hardy and Wilhelm Weinberg in 1908 is generally referred to as Hardy- Weinberg Principle. (See Hardy Winberg equation under Biological equations)


Before a disease can be attributed to a particular pathogen the following conditions must be met:

-The organism that shows the symptoms of the disease must carry the micro-organism

-The micro-organism must be able to be cultured.

-When the micro-organism is removed from its host and injected into healthy organism the new host           must show the symptom of the disease

-The micro-organism must be found in the body of the new host.

Robert Kosh put these postulates called Kosh’s Postulates forward in 1876.


The population of a community brings variation and competition. As organisms compete for resources like food, light and space, those organisms that possess adaptive features or structural features that make them fit will live and multiply while the unfit ones will die in large number and gradually get extinct. That is the nature selects the fit ones, while the unfit ones die. Charles Darwin and Alfred Wallace jointly put the theory commonly called Darwin’s Theory of Natural Selection or The Law of the Survival of the Fittest forward in 1858.


A non living thing can produce living thing, probably of low complexity provided the condition is ideal for such generation. The Theory of Spontaneous Generation was pioneered by Aristotle about 350 B.C. but was convincingly disproved by Louis Pasteur over 2000 years later.


If Biogenesis is true i.e. life must come from pre-existing life, then where did the first living thing come from? Biblical calculation based on the existence of man on earth had shown that the earth is less than 10,000 years. Evidence from the fossils proved that the earth is just over 500 million years. Radioactive decay proved that the earth is about 5 billion years old therefore an improved scientific research can still extrapolate the age far higher. Perhaps the best explanation for the origin of the earth is that the earth has no origin but has always existed. The species have no origin but have always existed. The only alteration that can happen to life is the increase or decrease in the number of species and probably extinction of some species. The theory is generally referred to as Steady State Theory.


Due to changes in the environment, an organism living in a particular environment may need to use part of its body organs more than how it will use it in another environment. An extensive use of an organ will lead to a better development of the organ (hence a giraffe feeding by stretching its neck developed a long neck), while disuse of an organ leads to degeneration of the organ (disuse of legs make snakes limbless). As an organism continue using some part of its body more extensively the structure gets modified to make it fit to its environment, hence gradually, it will acquire new characters and the acquired characters can be transferred to the offspring from one generation to generation. The Theory of Use and Disuse is one of the Lamarck’s Theory of Evolution put forward by Jean Baptiste de Lamarck in 1809, but no accepted as Lamarck cannot provide any biological explanation for his theory.


THE LAW OF BIOGENESIS: The stages of the embryonic development of many organisms bear many similarities, for organisms inherit mechanism of development from their ancestors, hence organisms of a common ancestor have common stages of development.


Whenever two individuals of different characters are crossed, the offspring inherits the two characters from both parents but only a character is expressed physically in the offspring. The expressed character is controlled by the dominant gene while the hidden character is controlled by the recessive gene.


The factors controlling the rate of photosynthesis are interdependent such that when one factor causes a high increase in the rate of photosynthesis, its effect will make another factor to cause a decrease in the rate. At any point in time, the rate is determined by the factor that causes the minimum rate.


THE FIRST LAW (The law of segregation of genes)

THE SECOND LAW (The law of independent assortment):

Each character behaves as a separate unit and independent of one another, hence the selection of an allele for a particular character does not affect the selection of allele for another character.




ABATTOIR: A place where animals are killed for human consumption.

ABDOMEN: 1. Soft part of vertebrates unto which the pelvic girdles are attached. 2. One of the body divisions of some invertebrates most commonly the arthropods.

ABDOMINAL CAVITY: The cavity of the abdominal region, usually covered with soft delicate tissues.

ABIOTIC FACTOR: Ecological term that is used to describe the non living components of an ecosystem.

ABO BLOOD GROUP: See Blood Group.

ABOMASUM: The last chamber of the stomach of ruminant animals. It corresponds to the stomach of non-ruminants. Compare Rumen, Reticulum, Omasum.

ABORTION: The destruction of the foetus before gestation period is completed.

ABYSSAL ZONE: See Aquatic Biome.

ACCOMODATION: Ability of the eye to focus objects at varying distances.

ACEBUTALUM: A cavity at the junction of the tight bone, fits to form a ball and socket joint.

ACHENE: A dry indehiscent, simple, one seeded fruit, which the pericarp and seed coat are not fused. It is usually formed from a superior ovary of a monocarpous pistil.

ACID: A substance with pH less than seven and that ionizes in water to produce hydrogen ion as the only positive ion. It neutralizes bases to produce salt and water.

ACID RAIN: Rain which the water precipitated in the atmosphere has reacted with the acidic gas accumulated in the atmosphere as a result of air pollution, hence making the rain water to be acidic.



ACROMEGALY: A defect caused by oversecretion of growth hormone after adult stage has been reached. It causes sudden development of large nose, thick lips, huge hands and feet and a coarse skin. Compare Giantism

ACTH: See Adrenocorticotrophic Hormone.

ACTINOMORPHIC FLOWER: Also Regular Flower. Flower that is radially symmetrical. See Radial Symmetry.

ADAPTATION: Ability of an organism to modify its structure or its way of live so as to live successfully in its environment.

ADAPTIVE FEATURES: Structural features possessed by an organism that enable it to live successful in its environment e.g. wings in birds.

ADENOSINE DIPHOSPHATE: A.D.P.A compound containing ribose, adenine and two phosphate groups. It is converted to A.T.P. during respiration for energy storage in the cells according to the equation.

ADENOSINE MONOPHOSPHATE: A.M.P. A nucleotide constituent of the coenzyme NADP. It contains the sugar adenine, a base ribose and a phosphate group.

ADENOSINE TRIPHOSPHATE: A.T.P. A compound in form of which energy is stored in the cells. It is manufactured in the mitochondrion during respiration. It composes of ribose, adenine and three phosphate groups. It releases energy to the body by being hydrolysed to form A.D.P.

ADIPOSE TISSUE: Fat tissue found in animals. It is used for storing fats and oils,and it is usually located beneath the skin or around some organs like digestive reproductive, excretory, transport and heart.

  1. See Adenosine Diphosphate

ADRENAL GLANDS: Endocrine glands location above each kidney. They secrete adrenalin and corticosterones.

ADRENALIN: Also emergency hormone. Hormone that prepares the body for emergency action by stimulating increase in the rate of heartbeat, muscular control and conversion of glycogen to glucose. See Hormone.

ADRENOCORTICOTROPHIC HORMONE: ACTH. Hormone secreted by the anterior pituitary gland due to the stimulation by the cortex. See Hormone.

ADULT: A fully matured organism. 2. imago organism in the last stage of metamorphosis. Adult stage.

AEROBIC RESPIRATION: Respiration in the presence of oxygen. See Respiration.

AESTIVATION: Act of being inactive, shown by some organisms living in hot areas, when the temperature is too hot for them to carry out their normal activities until the environmental condition becomes favourable again. Compare Hibernation.

AFFORESTATION: The process of planting forest trees.

AFTERBIRTH: Placenta sent out by an animal after giving birth to young one together with the emboryonic membrane and the cut-off part of the umbilical cord.

AGAMA LIZARD: Agama agama. The common species of lizard. A reptile with two pairs of limbs. Males usually possess brightly coloured heads.

AGGLUTINATION: Clumping of red blood cells when blood of contrasting antigens are mixed.

AGGLUTINS: Antibodies that make bacteria to clump together.

AGGREGATE FRUIT: Fruit formed from a flower that has many free carpels i.e. apocarpous pistils.

AIDS: Acquired Immune Deficiency Syndrome. A viral disease cause by HIV (Human Immuno Deficiency Virus). The viruses attack the antibodies and make the body unable to fight off pathogens. It leads to death.

AIR: Mixture of gases containing about 78% of Nitrogen, 20.8% of oxygen,1% of noble gases and 0.03% of carbondioxide by volume together with some water vapour.

AIR BLADDER: Thin walled sac found in body of fish to keep it afloat (also swim bladder ) 2. Leaflike structure found in some algae to maintain buoyancy.

AIR SAC: Respiratory organ found commonly in birds into which the bronchus from the lungs leads to

ALBUMEN: Protein substances surrounding the yolk in the egg of birds.

ALBUMINOUS: See Endospermous

ALCOHOL: Organic compound made up of carbon hydrogen and oxygen with general molecular formula CnH2n+1OH commonly C2H5OH. It is produced by fermentation and has intoxicating effect on the body if consumed.

ALCOHOLIC FERMENTATION: The process by which alcohol is produced by the effect of the anaerobic respiration of the yeast on carbohydrates. Carbondioxide is also produced at the same time.

ALDOSTERONE: A liquid soluble hormone (steroid) secreted by the adrenal cortex. It stimulates the kidney to retain sodium and potassium. See Hormone.

ALGAE: (Sing. Alga) Unicellular or simple multicellular plant with chlorophyll usually classified as thallophyta. They lack definite stems, leaves and roots, but only possess a body called thallus. Most of them are aquatic. Common examples are euglena viridis and chlamydomonas rivalis.

ALIMENTARY CANAL: A tube in animals in which digestion and egestion of food take place during nutrition. It includes the mouth, the oesophagus, the stomach, the small intestine and the large intestine.

ALLANTOIS: Embryonic membrane that forms the planceta alongside the chorion.

ALLELIC PAIR: Pair of genes located on the same locus of hormologous chromosome that control a characters.

ALLELE: One of the allelic pair. See Allelic Pair.

ALBINISM: An inheritable disease carried by recessive gene which lacks the enzyme tyrosinase, hence making the skin lacks the colour pigment melanin. An albino (person suffering from albinism) has a light skin and pink eyes.

ALLELOPATHY: Territoriallity found in plants in which chemicals are secreted by a plant to inhibit the growth of other plants.

ALLELOMETRIC GROWTH: Growth which has a faster or slower rate compared with the growth rate of other parts of the body.

ALTERNATION OF GENERATION: A life cycle that has a haploid phase (sporophyte) and a diploid phase (Gametophyte) in an orderly sequence. The sporophyte and the gametophyte may look alike and they may look different, it is formed in plants.

ALVEOLI: (Sing. Alveolus). Part of the respiratory organ through which gaseous exchange takes place in the lungs. They are microscopic in nature and are attached in large number to each bronchus in the lungs.

AMINO GROUP: A functional group of some organic compounds. It has the formula –NH2.

AMONIFICATION: The process by which the amino acid present in dead organism or animal waste are converted to ammonium ions.

AMNION: Embryonic membrane that protects the embryo from shock.

AMNIOTIC FLUID: Fluid present in the amnion that serves as shock absorber for the embryo. It also lubricates the point of contact between the embryo and the uterus.

AMOEBA: (Pl. Amoebae). A shapeless unicellular organism usually found in ponds. It is a free living organism and moves by protoplasmic streaming by extending its cell membrane called plasmallema to form false legs known as pseudopodia. It excretes through its contractile vacuole, respires by diffusion and reproduces by binary fission.

AMOEBOCYTE: A shapeless cell found in sponges. It has no specialised function and can change to another type of cell.

AMPHIBIANS: Class of vertebrates that are adapted to life both on land and in water. Their lives started in water having tails and breathing with gills. They later develop to the adult, loosing their tails but with well developed limbs (few are limbless) and they breath with lungs. They mostly return to the water for reproduction. Common examples are toad and frogs.

AMPULA: See Ear.

AMYLASES: Group of enzymes that act on carbohydrates and carbohydrate products and break them down to glucose.

ANABOLISM: Metabolic reaction that involves building up or synthesizing complex materials from simpler ones. ANABOLIC PROCESS

ANAEMIA: A diseases due to shortage of haemoglobin in the red blood cell.

ANAEROBIC RESPIRATION: Respiration in absence of oxygen. See Respiration

ANAL FIN: An unpaired fin in the bony fish, located near the anus at the ventral edge.

ANAL PORE: See Paramecium.

ANAL STYLE: A pair of structures in the last segment of a male cockroach that distinguish it from female.

ANAPHASE: The fourth stage of cell division during mitosis or meiosis. In mitosis, the chromatids separate and migrate to the opposite poles of the cell while in meiosis, the first anaphase occur when the bivalent separate and the members move to the opposite poles of the spindle while the chromosome divides during the second meiotic division and each move to the opposite poles of the cell.See Mitosis, Meiosis. Compare Prophase, Metaphase, Anaphase, Telophase, Interphase.

ANAPOPHYSES: A pair of processes in lumber vertebrate that are used for attachment of the vertebrate with the abdominal muscle.

ANATOMY: Branch of natural science that deals with the study of the structure of animals’ bodies’ (can also be used for the study of plants’ structure).

ANDROECIUM: Floral whorl of a flower that serves as the male reproductive organ. It is made up of stamens, which consist of anther and filament.

ANEMOME TER: Instrument used for measuring t he speed of wind.

ANEMOPHILOUS FLOWER: Wind pollinating flower. Compare Entomophilous Flower.

ANGIOSPERM: Flowering plant. Green vascular plant that bears flower. It produces seeds which are enclosed in fruits.

ANIMAL: A living organism that shows active movement and possesses cells that are covered with thin and delicate cell membrane.

ANNELIDA: Also segmented worm. A phylum of bilaterally symmetrical invertberates with segmented bodies both internally and externally . A common example is earthworm.

ANNUAL PLANT: Plant that completes its life cycle within one growing season.

ANOPHELES MOSQUITO: A kind of mosquito which lays its eggs singly in water and which larva lies parallel to the surface of the water. Its female carries the malaria parasite, plasmodium.

ANOSMA: Odour blindness. Inability to detect odour.

ANTAGONISTIC MUSCLES: Two muscles working together to produce effect through contraction of one and simultaneous relaxation of the other .e.g. tricept and bicept.

ANTENNA: (Pl. Antennae). A jointed flexible sensory appendage extending from the head of some arthropoda (All athropoda possess antennae except the arachnida).

ANTHER: Bean-shaped part of the androecium of a flower where pollen grains are produced.

ANTHERIDIUM: A cell that produce sperms in some sexually reproducing plants.

ANTHEROZOID: Also Spermatozoid. A flagellated, mobile male gamete that produces antheridium. See Antheridium.

ANTHOCYANINS: A glucoside present in the cell vacuole of coloured plants, which is responsible for blue, red and purple colours in plants.

ANTIBIOTICS: Protein produced by the blood cells to fight against toxins (poisonous substances) in the body.

ANTIGENS: Substances that cause antibodies to be produced.

ANTI-PERISTALTIC MOVEMENT: See Peristaltic Movement.

ANTIPODAL CELLS: Three cells located at one end of the micropyle of a flower, which later disappear after fertilization.

ANTISEPTICS: Substance used for preventing infections (mostly of bacteria).

ANTITOSIN: Antibody that makes toxins (poisonous substances in the body) harmless.

ANUS: Opening of the alimentary canal located at the posterior end of an animal. It is used for egestion and excretion.

ANVIL: See Ear.

AORTA: A large artery that originates from the left ventricle in the heart and carries blood to the other arteries in the body.

AORTIC SEMILUNAR VALVE: Any of the three valves at the base of the inner wall of the aorta that control the direction of blood flow

APICAL DORMINANCY: The production of auxin by terminal bud to facilitate the growth of lateral bud.

APICAL MERISTEM: Cells that produce new cells, located at the tips of the stems of roots.

APOCARPOUS PISTILL: Pistill of a flower with free carpels.

APPENDAGES: A structure that grows out from the main part of the body. e.g legs and antennae.

APPENDICITIS: A situation whereby the appendix swells and becomes painful. It is usually treated by surgical operation.

APPENDICULAR SKELETON: All parts of the mammalian skeleton except the skull, the vertebral column and the rib cage i.e the limbs and the limb girdles.

APPENDIX: A structure at the junction of the small and large intestines. It aids digestion of cellulose in some animals, but serves no function in humans. See Vestigian Organ.

AQUATIC: Of water. Aquatic animal, aquatic plant, aquatic habitat.

AQUATIC BIOME: Biome that involves water. See Biome.

Unlike other biomes which are named according to the characteristic plant communities and climatic conditions, aquatic biomes are named based on their salt contents, hence they are divided to fresh water, marine and estuarine biomes. The type of organisms living in these biomes depends greatly on the salinity of the water. Freshwater biomes have less than 0.005 percent salt. They are divided into running water and standing water, explained as the names implied. The kind of organisms living in these biomes also depends on the speed of the water. The marine biomes have salt of higher percentage between 3 and 3.7 percent. Each habitat is divided into smaller habitat called micro-habitat, so we have the splash zone which is a zone above the tide mark, hence almost always wetted by the spray from the waves, the littoral zone or the intertidal zone is also exposed sometimes to the air and sometimes being underwater depending on the tide. Sublittoral or subtidal zone is more stable than the intertidal, it lies between the littoral zone and the edge of the continental shelf, and it is not affected by the tides. The next zone beyond the sublittoral zone is the pelagic zone which is stratified vertically to photic zone, the zone which light can penetrate easily which is about 200m deep from the surface of the water, bathyal zone, zone which light can penetrate to a little extent ranges from the depth of 200m and finally the abyssal zone which the light cannot penetrate at all follows. Estuaries are the points where marine and freshwater meet. They are shallow and allow light to penetrate easily. They are the most productive aquatic habitat.

AQUATIC HABITAT: Habitat in water. It may be freshwater, marine or estuarine.


ARACHNIDA: A class of arthropoda with four pairs of walking legs and with body divided into cephalothorax and abdomen. Common examples are spider, tick and scorpion. See Arthropoda.

ARACHNOID MATTER: The middle layer of the meninges. See Meninges.

ARBOREAL HABITAT: Terrestrial habitat in or on trees

ARCHAEZOIC ERA: Evolutionary era. The period between the past two billion years and one billion years ago. The era left no fossil hence showing that there is no organism or organisms with skeleton on the earth then.

ARTERIOLE: Small branch of the artery that links the artery and the capillaries.

ARTERY: Narrow muscular blood vessels that carries blood away from the heart. Except pulmonary artery, other arteries carry oxygenated blood.

ARTHROPODA: A phylum of invertebrates with jointed and segmented bodies mostly with jointed appendages. They are covered with exoskeletons, they are divided into four main classes, insect, arachnida, crustanceans and myriapoda (diplopoda and chilopoda). They are the largest phylum in the animal kingdom.

ARTIFICIAL INSEMINATION: Method of reproduction by which semen from the male animal is collected and artificially injected into the reproductive system of the female.

ARTIFICIAL RESPIRATION: An aid to respiration giving to patient with lungs or breathing problem. It includes mouth to mouth respiration and the use of breathing machine.

ASCORBIC ACID: See Vitamins (Vitamin C).

ASEXUAL REPRODUCTION: Reproduction that does not involve fussion of two gametes.

ASSIMILATION: Process by which the absorbed nutrients are utilized by the body cells.

ASSOCIATION: Ecological term for describing a long time relationship between organisms of different species.

ASTIGMATISM: See colour blindness.

ASYMMETRICAL ANIMAL: Animal with irregular shape.

ATLAS VERTEBRA: First cervical vertebra in vertebrates. It is characterized by large neural canal, reduced neural spine, absence of centrum and presence of facets and a pair of zygophyses. See Cervical vertebra.

ATMOSPHERE: Gaseous part of the earth.

  1. See Adenosine Triphosphate.

ATRIUM: (pl. Atria) Also auricle. See Auricle.




AURICLE: Also atrium. The upper chamber of the heart.

AUTECOLOGY: Ecological study of a single organism and its relationship with its environment.

AUTONOMIC NERVOUS SYSTEM: Part of the pheripheral nervous system that is concerned with regulating internal activities. It is divided into sympathetic and parasympathetic nervous system.

AUTOSOMES: Paired chromosomes i.e. all chromosome except X and Y chromosomes (sex chromosomes).

AUTOTROPH: Organism that can manufacture its food itself e.g. green plants.

AUTOTROPHIC NUTRITION: Mode of nutrition in which the organism depends on itself for its food, it can either be through photosynthesis or through chemosynthesis.

AUXANOMETER: A lever system that magnifies an image’s length. It is used for measuring the rate of growth of a shoot of a plant.

AUXIN: See Hormone. (Plant hormone)

AVES: Birds. Class of oviparous homeiothemic vertebrate animals that are well adapted to the life in air by the possession of a pair of muscular wings. They have light bodies, hollow bones and a pair of walking legs, their mouth parts are modified to beaks and their fore-limbs to wings, but some are flightless.

AXIAL SKELETON: Part of the mammalian skeleton consisting the skull, the vertebral column and the rib cage.

AXIL: The angle between the petiole of a leaf and the stem of a plant

AXILLARY BUD: Bud grown from the axil of a leaf.

AXILLARY FLOWER: Flower borne out from the axil of a leaf.

AXIS VERTEBRA: Second cervical vertebra. It is characterized by presence of odontoid process, large neural spine and presence of a pair of postzygapophyses. See Cervical vertebra.

AXON: See Neurone.


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Biology Dictionary concisely defines over two thousand Biological terms that are constantly used in Biology, given examples and explanation. It gives the plural forms of many terms and made comparison between similar and opposing terms. It also explains many Biological laws, hypotheses and theories.




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