Class 11 Biology — Important Board Questions with Answers

Taxonomy is the branch of biology dealing with the identification, nomenclature and classification of organisms based on their characteristics. A taxon is a unit or group of organisms of any rank in classification (for example, mammals, insects, dogs). Taxonomic hierarchy is the arrangement of taxonomic categories in a definite descending order, from kingdom down to species.

Binomial nomenclature is the system of naming organisms with two words, given by Carolus Linnaeus, where the first word is the genus and the second is the species (for example, Mangifera indica for mango). Rules: (1) the generic name begins with a capital letter and the specific name with a small letter; (2) the biological name is printed in italics (or underlined separately when handwritten) and is usually in Latin.

The taxonomic categories in descending order are: Kingdom, Phylum (Division in plants), Class, Order, Family, Genus and Species. Species is the lowest category and represents a group of individual organisms with fundamental similarities that can interbreed. As we move up the hierarchy, the number of common characteristics decreases while the number of organisms in the group increases.

R. H. Whittaker (1969) proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. The main criteria used were: cell structure (prokaryotic or eukaryotic), body organisation (unicellular or multicellular), mode of nutrition (autotrophic or heterotrophic), reproduction and phylogenetic relationships.

Kingdom Monera includes bacteria, which are prokaryotic (they lack a true membrane-bound nucleus and membrane-bound organelles). They are microscopic, unicellular organisms with a cell wall (except mycoplasma). Their nutrition may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic). They are grouped as Archaebacteria (live in extreme habitats) and Eubacteria (true bacteria).

Kingdom Protista includes single-celled eukaryotes. They have a well-defined nucleus and membrane-bound organelles. Some have flagella or cilia for locomotion, and they may be autotrophic or heterotrophic. This kingdom forms a link between plants, animals and fungi. Examples: Chrysophytes (diatoms), dinoflagellates, Euglenoids (Euglena), slime moulds and protozoans (Amoeba, Paramecium).

Fungi are eukaryotic, mostly multicellular (except yeast), heterotrophic organisms with a cell wall made of chitin. Their body is made of thread-like hyphae forming a mycelium. Nutrition is saprophytic (on dead organic matter), parasitic or symbiotic; they store food as glycogen. The four classes are Phycomycetes (e.g. Rhizopus, Mucor), Ascomycetes (e.g. Penicillium, yeast), Basidiomycetes (e.g. mushroom, Puccinia) and Deuteromycetes (imperfect fungi, e.g. Alternaria).

Viruses are non-cellular infectious agents that are inert outside the host but reproduce inside a living host cell. A virus has a protein coat called a capsid enclosing genetic material of either DNA or RNA (never both). Viroids are the smallest infectious agents, made of free naked RNA without a protein coat (e.g. potato spindle tuber disease). Lichens are symbiotic associations between an alga (phycobiont) and a fungus (mycobiont) and are good indicators of air pollution.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms. They reproduce vegetatively, asexually (by spores) and sexually. The three classes are: Chlorophyceae (green algae, with chlorophyll a and b, e.g. Spirogyra, Chlamydomonas); Phaeophyceae (brown algae, with chlorophyll a, c and fucoxanthin, e.g. Laminaria, Fucus); and Rhodophyceae (red algae, with chlorophyll a, d and phycoerythrin, e.g. Polysiphonia, Gracilaria).

Bryophytes are called the amphibians of the plant kingdom because they can live in soil but need water for sexual reproduction, as the male gametes (antherozoids) require a film of water to swim to the female sex organ. They lack true vascular tissue and true roots (they have rhizoids). Examples: Funaria (moss), Marchantia and Riccia (liverworts).

Pteridophytes are the first terrestrial plants to have true vascular tissues (xylem and phloem). The plant body is a sporophyte differentiated into true root, stem and leaves. They reproduce by spores produced in sporangia, and water is required for fertilisation. Examples: Selaginella, Equisetum and ferns such as Pteris and Dryopteris.

Gymnosperms are plants that bear naked seeds, that is, the seeds are not enclosed inside a fruit because the ovules are not enclosed in an ovary. Characteristics: (1) they are woody perennials (trees or shrubs); (2) the ovules are exposed and develop into naked seeds; (3) they are heterosporous, producing haploid microspores and megaspores, and the pollen grains are usually dispersed by wind. Examples: Cycas, Pinus and Ginkgo.

Animals are classified on the basis of certain fundamental features: (1) levels of organisation (cellular, tissue, organ or organ-system); (2) symmetry (asymmetrical, radial or bilateral); (3) germ layers (diploblastic or triploblastic); (4) presence and nature of a body cavity or coelom (acoelomate, pseudocoelomate or coelomate); (5) segmentation of the body; and (6) presence of a notochord.

Diploblastic animals have cells arranged in two embryonic germ layers, an outer ectoderm and an inner endoderm (e.g. coelenterates). Triploblastic animals have three germ layers: ectoderm, mesoderm and endoderm (e.g. from flatworms upward). A coelomate animal is one that has a true body cavity (coelom) lined by mesoderm, for example annelids, arthropods and chordates.

Porifera (sponges): (1) they are primitive multicellular animals with a cellular level of organisation and body with pores (ostia) and a canal system; (2) the body is supported by spicules and they show intracellular digestion (e.g. Sycon, Spongilla). Coelenterata (Cnidaria): (1) they are diploblastic with tissue-level organisation and radial symmetry; (2) they possess stinging cells called cnidoblasts and show two body forms, polyp and medusa (e.g. Hydra, jellyfish).

Arthropoda is the largest phylum of the animal kingdom. Characteristics: (1) they are triploblastic, bilaterally symmetrical, segmented and coelomate; (2) they have a chitinous exoskeleton and jointed appendages (the name means jointed legs); (3) the body is divided into head, thorax and abdomen, with an open circulatory system and respiration by gills, book gills, book lungs or tracheae. Examples: cockroach, prawn, butterfly, honeybee and spider.

The three fundamental (defining) characteristics of chordates are: (1) presence of a notochord, a flexible rod-like structure on the dorsal side that supports the body; (2) a dorsal, hollow, tubular nerve cord; and (3) paired pharyngeal gill slits at some stage of life. In addition they are triploblastic, coelomate, bilaterally symmetrical and have a post-anal tail. Examples: fishes, amphibians, reptiles, birds and mammals.

A tap root system has one main primary root growing vertically downward from which lateral roots (secondary and tertiary) arise; it develops from the radicle and is characteristic of dicots (e.g. mustard, mango). A fibrous root system has a cluster of thin, thread-like roots of similar size arising from the base of the stem; the primary root is short-lived and it is characteristic of monocots (e.g. wheat, grass).

Modifications of the stem include: (1) underground stems for storage such as the tuber of potato and the rhizome of ginger; (2) stem tendrils for climbing, as in gourds and grapevine; (3) thorns for protection, as in Citrus and Bougainvillea; and (4) runners or stolons for vegetative propagation, as in grass and strawberry. These modifications help in storage, support, protection or propagation.

Placentation is the arrangement of ovules within the ovary of a flower. Types: (1) Marginal - the placenta forms a ridge along the ventral suture and ovules are borne on it, as in pea. (2) Axile - the placenta is axial and ovules are attached to it in a multilocular ovary, as in China rose, tomato and lemon. (3) Parietal - ovules develop on the inner wall of the ovary, as in mustard. (4) Free central - ovules borne on a central axis with no septa, as in Dianthus. (5) Basal - a single ovule is attached at the base, as in sunflower.

A typical flower has four whorls: (1) Calyx (sepals) - the outermost green whorl that protects the flower in the bud stage; (2) Corolla (petals) - usually coloured, it attracts pollinators; (3) Androecium (stamens) - the male reproductive whorl producing pollen; and (4) Gynoecium (carpels/pistil) - the female reproductive whorl containing ovules that develop into seeds after fertilisation.

An inflorescence is the arrangement of flowers on the floral axis. In a racemose inflorescence, the main axis continues to grow (it has unlimited growth) and the flowers are borne laterally in an acropetal order (older flowers at the base, younger towards the tip), e.g. radish. In a cymose inflorescence, the main axis ends in a flower and so has limited growth, and the flowers are borne in a basipetal order (older flowers at the top or centre), e.g. Jasmine.

Venation is the arrangement of veins and veinlets in the lamina of a leaf. In reticulate venation the veinlets form a network, typical of dicot leaves (e.g. mango); in parallel venation the veins run parallel to one another, typical of monocot leaves (e.g. grass). Phyllotaxy is the pattern of arrangement of leaves on the stem or branch; it may be alternate, opposite or whorled.

Meristematic tissue consists of actively dividing cells that are responsible for the growth of the plant. The cells are small, thin-walled, with dense cytoplasm and prominent nuclei, and little or no vacuole. Based on position the three types are: apical meristem (at the tips of root and shoot, causing increase in length), intercalary meristem (at the base of internodes or leaves) and lateral meristem (along the sides, e.g. cambium, causing increase in girth).

Simple permanent tissues are made of one type of cell. (1) Parenchyma - thin-walled living cells with intercellular spaces; they perform storage, photosynthesis and secretion. (2) Collenchyma - living cells with corners thickened by cellulose and pectin; they provide mechanical support and flexibility to young stems and leaves. (3) Sclerenchyma - dead cells with thick, lignified walls (fibres and sclereids); they provide mechanical strength and rigidity.

Xylem conducts water and dissolved minerals from the roots upward to other parts of the plant and gives mechanical support. Its components are tracheids, vessels, xylem fibres and xylem parenchyma. Phloem transports food (mainly sucrose) prepared in the leaves to the rest of the plant (translocation). Its components are sieve tube elements, companion cells, phloem fibres and phloem parenchyma. Both are complex (conducting) tissues.

(1) In a dicot root the number of xylem bundles (vascular bundles) is few, usually two to four (tetrarch or less), whereas in a monocot root they are many (polyarch, often more than six). (2) A dicot root has secondary growth due to the presence of cambium, whereas a monocot root usually lacks secondary growth as cambium is absent. Also, the pith is small or absent in dicot roots but large and well developed in monocot roots.

Stomata are tiny pores present mainly in the epidermis of leaves. Each stoma is bounded by two bean-shaped (kidney-shaped) guard cells that contain chloroplasts; in monocots the guard cells are dumb-bell shaped. Functions: stomata allow the exchange of gases (carbon dioxide and oxygen) between the plant and the atmosphere and control transpiration (loss of water vapour). The opening and closing of the stoma is regulated by the turgidity of the guard cells.

Epithelial tissue is a covering or protective tissue that lines the free surfaces of the body and its organs; it rests on a basement membrane and has little intercellular material. Simple epithelium is a single layer of cells and its types are: (1) squamous epithelium - flat cells, found in the walls of alveoli and blood vessels, involved in diffusion; (2) cuboidal epithelium - cube-shaped cells, found in kidney tubules and glandular ducts, for secretion and absorption; and (3) columnar epithelium - tall cells, found in the lining of the stomach and intestine, for secretion and absorption.

Connective tissue links, supports and binds together other tissues and organs of the body; its cells are loosely spaced in an abundant matrix. Main types: (1) loose connective tissue, e.g. areolar tissue and adipose (fat-storing) tissue; (2) dense connective tissue, e.g. tendons (connect muscle to bone) and ligaments (connect bone to bone); and (3) specialised connective tissue, e.g. cartilage, bone and blood (a fluid connective tissue that transports materials).

The three types of muscular tissue are: (1) striated (skeletal) muscle - cylindrical, unbranched, multinucleate and voluntary, attached to bones for movement; (2) smooth (unstriated) muscle - spindle-shaped, uninucleate and involuntary, found in the walls of internal organs such as the stomach, intestine and blood vessels; and (3) cardiac muscle - branched, striated and involuntary, found only in the wall of the heart.

A neuron is the structural and functional unit of nervous tissue. It has three main parts: (1) the cell body (cyton), which contains the nucleus and cytoplasm with Nissl granules; (2) dendrites, which are short branched processes arising from the cell body that receive impulses and carry them towards the cell body; and (3) the axon, a single long process that carries impulses away from the cell body to the next neuron or effector. The axon may be covered by an insulating myelin sheath.

The cell theory was proposed by M. Schleiden (a botanist) and T. Schwann (a zoologist) in 1838-39. Its postulates are: (1) all living organisms are composed of cells and products of cells; (2) the cell is the basic structural and functional unit of life. Later Rudolf Virchow (1855) added that all cells arise from pre-existing cells (omnis cellula-e-cellula).

(1) Prokaryotic cells lack a true membrane-bound nucleus (the genetic material lies free in the cytoplasm as a nucleoid), whereas eukaryotic cells have a well-defined nucleus enclosed by a nuclear membrane. (2) Prokaryotes lack membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi bodies, while eukaryotes possess them. (3) The ribosomes are 70S in prokaryotes and 80S in the cytoplasm of eukaryotes. Examples: bacteria are prokaryotic; plant and animal cells are eukaryotic.

A mitochondrion is a double-membrane organelle. The outer membrane is smooth, while the inner membrane is folded inward to form finger-like cristae that increase the surface area; the inner space is filled with a matrix. It contains its own DNA and 70S ribosomes. Function: it is the site of aerobic respiration, where the Krebs cycle (in the matrix) and the electron transport chain (on the inner membrane) produce ATP, the energy currency of the cell. Because it produces most of the cell's ATP, it is called the powerhouse of the cell.

A chloroplast is a double-membrane organelle found in green plant cells. Its inner space, the stroma, contains stacks of disc-shaped membranes called thylakoids; a stack of thylakoids is a granum (plural grana), and grana are connected by stroma lamellae. The thylakoid membranes contain the green pigment chlorophyll. It has its own DNA and 70S ribosomes. Function: it is the site of photosynthesis - the light reactions occur on the thylakoid membranes and the dark reactions (Calvin cycle) occur in the stroma.

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs in the cytoplasm. Rough ER (RER) has ribosomes attached to its surface and is mainly involved in the synthesis and transport of proteins. Smooth ER (SER) lacks ribosomes and is involved in the synthesis of lipids and steroids and in detoxification. Both help in intracellular transport of materials.

The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae arranged parallel to one another, along with associated vesicles. Functions: it packages, modifies and sorts materials (such as proteins and lipids) received from the endoplasmic reticulum and secretes them in vesicles. It is also the site of formation of lysosomes and the synthesis of certain complex carbohydrates and cell wall materials.

Ribosomes are non-membrane-bound organelles made of RNA and protein; they are the site of protein synthesis. Lysosomes are single-membrane-bound vesicles containing powerful digestive (hydrolytic) enzymes that digest worn-out organelles, food and foreign material. They are called suicidal bags because on rupture their enzymes can digest the cell's own contents, causing the cell to die (autolysis).

The nucleus is a spherical, membrane-bound organelle that controls the activities of the cell. It is bounded by a double membrane called the nuclear envelope, which has nuclear pores for exchange of materials. Inside is the nucleoplasm containing chromatin (DNA plus proteins) and one or more nucleoli. Functions: it stores the genetic material (DNA), controls cell metabolism and heredity, and the nucleolus is the site of ribosome (rRNA) synthesis. During cell division the chromatin condenses into chromosomes.

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis) and act as the main source of energy. They are classified as: (1) monosaccharides - simple single-unit sugars such as glucose and fructose; (2) oligosaccharides (mainly disaccharides) - such as sucrose, maltose and lactose, which yield 2 to 10 monosaccharides on hydrolysis; and (3) polysaccharides - large polymers such as starch, glycogen and cellulose.

Proteins are polymers of amino acids joined by peptide bonds. Their four levels of structure are: (1) Primary structure - the linear sequence of amino acids in a polypeptide chain; (2) Secondary structure - local folding into an alpha-helix or beta-pleated sheet held by hydrogen bonds; (3) Tertiary structure - the overall three-dimensional folding of the whole chain, giving the functional shape; and (4) Quaternary structure - the association of two or more polypeptide chains, as in haemoglobin.

Lipids are organic compounds that are insoluble in water but soluble in organic solvents; they include fats, oils, phospholipids and steroids and are generally esters of fatty acids and glycerol. Functions: (1) they are a concentrated store of energy (fats give about twice the energy of carbohydrates); (2) phospholipids form the structural basis of cell membranes; and they also act as thermal insulation and protect internal organs.

Nucleic acids are polymers of nucleotides and are of two types, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid); they store and transmit genetic information. Each nucleotide has three components: (1) a nitrogenous base (adenine, guanine, cytosine, and thymine in DNA or uracil in RNA); (2) a pentose sugar (deoxyribose in DNA, ribose in RNA); and (3) a phosphate group.

Enzymes are biological catalysts, mostly proteins, that speed up biochemical reactions without being consumed. Properties: (1) they are highly specific, each acting on a particular substrate; (2) they work best at an optimum temperature and pH and are sensitive to changes in these. Mechanism: the substrate binds to a specific region of the enzyme called the active site, forming an enzyme-substrate complex (the lock-and-key model); the enzyme lowers the activation energy, converts the substrate into product, and is then released unchanged.

Essential amino acids are those that the body cannot synthesise and must be obtained from the diet (for example lysine and valine). Non-essential amino acids are those that the body can synthesise itself (for example glycine and alanine). A peptide bond is the covalent amide bond formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of the next, with the removal of a water molecule.

The cell cycle is the sequence of events by which a cell duplicates its contents and divides into two daughter cells. It has two main phases: interphase (the preparatory, non-dividing phase) and the M phase (mitosis or division). Interphase is divided into three sub-phases: G1 phase (cell growth and normal metabolism), S phase (synthesis, in which DNA is replicated) and G2 phase (further growth and preparation for division).

Mitosis (M phase) has four stages: (1) Prophase - chromosomes condense and become visible as two sister chromatids, the spindle begins to form and the nuclear membrane and nucleolus start to disappear. (2) Metaphase - chromosomes align at the equator (metaphase plate) and spindle fibres attach to their centromeres. (3) Anaphase - the centromeres split and sister chromatids move to opposite poles. (4) Telophase - chromosomes reach the poles, decondense, and the nuclear membrane and nucleolus reappear, forming two nuclei.

Mitosis produces two daughter cells that are genetically identical to the parent cell, each with the same (diploid) number of chromosomes. Its significance: (1) it is responsible for growth of the body by increasing the number of cells; (2) it repairs and replaces old, worn-out or damaged cells and tissues; and (3) it maintains the constant chromosome number and enables asexual reproduction in some organisms.

Meiosis is a type of cell division that occurs in the reproductive cells and consists of two successive divisions, meiosis I and meiosis II, but only one round of DNA replication. It produces four haploid daughter cells from one diploid parent cell, so it is also called reductional division. Significance: (1) it halves the chromosome number so that fertilisation restores the diploid number in the next generation; and (2) crossing over during meiosis I produces genetic variation, which is important for evolution.

(1) Mitosis involves a single division producing two daughter cells, whereas meiosis involves two successive divisions producing four daughter cells. (2) In mitosis the daughter cells have the same (diploid) chromosome number as the parent and are genetically identical, whereas in meiosis the chromosome number is halved (haploid daughter cells) and genetic variation is introduced through crossing over. Mitosis occurs in body (somatic) cells while meiosis occurs in reproductive cells.

Cytokinesis is the division of the cytoplasm of a cell that follows nuclear division (karyokinesis) to form two separate daughter cells. In animal cells it occurs by the formation of a cleavage furrow that deepens and constricts the cell from the outside inward. In plant cells, because of the rigid cell wall, it occurs by the formation of a cell plate that grows from the centre outward to divide the cell.

Diffusion is the net movement of molecules of a substance from a region of higher concentration to a region of lower concentration until they are evenly distributed; it does not require a membrane or energy. Osmosis is the movement of solvent (water) molecules from a region of higher water potential to a region of lower water potential across a semipermeable membrane. Its two types are endosmosis (water entering the cell) and exosmosis (water leaving the cell).

Plasmolysis is the shrinkage of the protoplasm (cell contents) away from the cell wall when a plant cell is placed in a hypertonic solution, due to loss of water by exosmosis. Turgor pressure is the pressure exerted by the cell contents (the water-filled protoplast) against the cell wall when the cell is fully turgid (swollen with water); it keeps herbaceous plant parts firm and erect.

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of leaves. Significance: it creates a transpiration pull that helps in the upward transport (ascent) of water and minerals, cools the plant surface, and helps in the absorption of water by roots. Factors affecting it include light, temperature, humidity of the air, and wind speed - transpiration increases with light, higher temperature and wind, but decreases with high humidity.

Macronutrients are mineral elements required by plants in large amounts (more than 10 millimole per kg of dry matter); they include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Micronutrients (trace elements) are required in very small amounts (less than 10 millimole per kg of dry matter); they include iron, manganese, copper, zinc, boron, molybdenum and chlorine.

Biological nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (a usable form) by living organisms using the enzyme nitrogenase. Rhizobium is a symbiotic bacterium that lives in the root nodules of leguminous plants (such as pea and gram); it fixes atmospheric nitrogen into ammonia, which the plant uses to make amino acids, while the plant provides the bacterium with food and shelter.

Photosynthesis is the process by which green plants and some other organisms synthesise their own food (glucose) from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen as a by-product. Overall equation: 6CO2 + 12H2O -> (in the presence of light and chlorophyll) C6H12O6 + 6H2O + 6O2. It takes place in the chloroplast.

6CO2 + 12H2O -> C6H12O6 + 6H2O + 6O2

The light reaction takes place on the thylakoid membranes of the grana in the chloroplast. Chlorophyll absorbs light energy and gets excited, and this energy is used to split water molecules (photolysis of water) into hydrogen ions, electrons and oxygen; the oxygen is released. The energy is used to synthesise the energy-rich molecules ATP (by photophosphorylation) and NADPH. Thus the products of the light reaction are ATP, NADPH and O2, which are then used in the dark reaction.

In non-cyclic photophosphorylation both photosystems (PS I and PS II) take part, the electron travels in a one-way path, water is split (releasing oxygen), and both ATP and NADPH are produced. In cyclic photophosphorylation only PS I is involved, the electron is recycled back to the same photosystem, no water is split (so no oxygen is released) and only ATP is produced (no NADPH).

The Calvin cycle (C3 pathway) occurs in the stroma of the chloroplast and does not directly need light. Its three phases are: (1) Carboxylation - carbon dioxide combines with a 5-carbon acceptor RuBP, catalysed by the enzyme RuBisCO, to form two molecules of 3-carbon 3-phosphoglyceric acid (3-PGA). (2) Reduction - 3-PGA is reduced to G3P (a sugar) using ATP and NADPH from the light reaction. (3) Regeneration - some G3P is used to regenerate the acceptor RuBP so the cycle can continue, using ATP. Glucose is formed from the G3P produced.

(1) In C3 plants the first stable product of carbon fixation is the 3-carbon compound 3-PGA, whereas in C4 plants it is the 4-carbon compound oxaloacetic acid (OAA). (2) C3 plants use only the Calvin cycle, while C4 plants use the Hatch-Slack pathway in addition to the Calvin cycle. (3) C4 plants have a special leaf anatomy called Kranz anatomy with bundle-sheath cells and are more efficient photosynthetically (e.g. maize, sugarcane), while C3 plants lack Kranz anatomy (e.g. rice, wheat).

The main photosynthetic pigments are chlorophyll a (the primary pigment, bluish-green), chlorophyll b (yellowish-green) and the carotenoids (carotenes and xanthophylls, which are yellow to orange). Chlorophyll a is the chief pigment that traps light and drives the light reaction. Chlorophyll b and the carotenoids are accessory pigments: they absorb light at wavelengths that chlorophyll a cannot and pass the energy to chlorophyll a, and carotenoids also protect chlorophyll from harmful photo-oxidation.

Blackman's law of limiting factors states that when a process is affected by more than one factor, its rate is limited by the factor that is nearest to its minimum (the limiting factor); it is that factor which directly determines the rate at that moment. The main factors affecting photosynthesis are external factors - light intensity, concentration of carbon dioxide, temperature and water - and internal factors such as the amount of chlorophyll.

Respiration is the process of breaking down food (glucose) to release energy stored as ATP. Aerobic respiration occurs in the presence of oxygen, completely oxidises glucose to carbon dioxide and water, and releases a large amount of energy (about 38 ATP per glucose). Anaerobic respiration occurs in the absence of oxygen, incompletely breaks down glucose into products such as ethanol or lactic acid, and releases only a small amount of energy (2 ATP per glucose).

C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

Glycolysis (the EMP pathway) is the first step of respiration in which one molecule of glucose (6 carbon) is partially broken down into two molecules of pyruvic acid (3 carbon). It occurs in the cytoplasm of the cell and does not require oxygen. Its net products from one glucose molecule are 2 molecules of pyruvic acid, a net gain of 2 ATP and 2 molecules of NADH (reduced coenzyme).

glucose -> 2 pyruvic acid + 2 ATP + 2 NADH

The Krebs cycle (also called the citric acid cycle or TCA cycle) is the stage of aerobic respiration in which acetyl CoA (formed from pyruvic acid) is completely oxidised. It takes place in the matrix of the mitochondria. Each turn releases carbon dioxide and transfers energy to the reduced coenzymes NADH and FADH2, along with a small amount of ATP (GTP); the NADH and FADH2 then pass electrons to the electron transport chain to make more ATP.

Fermentation is the incomplete breakdown of glucose (anaerobic respiration) that occurs without oxygen. In alcoholic fermentation, pyruvic acid is converted into ethanol and carbon dioxide (for example by yeast), which is used in brewing and baking. In lactic acid fermentation, pyruvic acid is converted into lactic acid (for example in our muscles during vigorous exercise and by Lactobacillus in curd formation). Both release only a small amount of energy.

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration. RQ = volume of CO2 evolved / volume of O2 consumed. For carbohydrates, which are completely oxidised, the value of RQ is 1. For fats the RQ is less than 1 (about 0.7) and for organic acids it is greater than 1.

RQ = CO2 evolved / O2 consumed

Growth is an irreversible and permanent increase in the size, mass or number of cells of an organism or its parts. In plants, growth has three phases along the growing region: (1) the phase of meristematic (cell division) activity at the tips of roots and shoots; (2) the phase of elongation, in which cells just behind the meristem enlarge; and (3) the phase of maturation, in which the enlarged cells differentiate and attain their final form and function.

Auxin promotes cell elongation, apical dominance (suppression of lateral buds by the apical bud), and is used in rooting of stem cuttings and preventing premature fruit drop. Gibberellin promotes stem elongation (increases the length of the internodes), breaks seed and bud dormancy, and can induce bolting in rosette plants. Cytokinin promotes cell division (cytokinesis), delays the ageing (senescence) of leaves and helps in the growth of lateral buds.

Abscisic acid (ABA) is a growth inhibitor: it promotes dormancy of seeds and buds, causes closure of stomata during water stress, and promotes abscission (shedding) of leaves and fruits; it is often called the stress hormone. Ethylene is a gaseous hormone that promotes the ripening of fruits, causes ageing and abscission of leaves and flowers, and breaks the dormancy of seeds and buds.

Photoperiodism is the response of plants to the relative lengths of day and night (the photoperiod), especially with regard to flowering; on this basis plants are grouped as short-day, long-day and day-neutral plants. Vernalisation is the promotion of flowering in certain plants by exposing them to a period of low temperature (cold treatment); it prevents premature flowering and enables the plant to flower at the proper time.

The human alimentary canal is a continuous tube from the mouth to the anus. In sequence it consists of: the mouth (buccal cavity), pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum) and large intestine (caecum, colon and rectum), ending at the anus. Associated digestive glands are the salivary glands, liver and pancreas.

Salivary amylase (ptyalin) is present in saliva in the mouth and acts on starch, breaking it down into maltose. Pepsin is secreted by the stomach (active in acidic medium) and acts on proteins, breaking them into peptones and proteoses. Trypsin is secreted by the pancreas and acts in the small intestine on proteins and peptones, breaking them into smaller peptides and amino acids.

The dental formula of an adult human is 2123/2123, which represents, for one half of each jaw, 2 incisors, 1 canine, 2 premolars and 3 molars. Since this is for one half, the total number of teeth is 32. The four types of teeth are incisors (for cutting/biting), canines (for tearing), premolars and molars (for grinding and chewing). Human teeth are thecodont (fixed in sockets), diphyodont (two sets in life) and heterodont (different types).

2123/2123 (total 32 teeth)

Most of the absorption of digested food takes place in the small intestine, mainly the ileum. Its inner surface is highly folded and bears numerous finger-like projections called villi, and each villus has microvilli on its cells. These greatly increase the surface area for absorption. Each villus contains a network of blood capillaries and a lymph vessel (lacteal); simple sugars and amino acids pass into the blood capillaries while fatty acids and glycerol enter the lacteals.

Bile is a greenish-yellow fluid secreted by the liver and stored in the gall bladder; it is released into the duodenum. Although it contains no digestive enzymes, bile salts emulsify fats, that is, they break large fat globules into tiny droplets, thereby increasing their surface area for the action of the enzyme lipase. Bile also makes the medium alkaline (neutralising the acidic food from the stomach) so that intestinal enzymes can work.

Breathing is the physical process of taking in oxygen-rich air (inspiration) and giving out carbon dioxide-rich air (expiration). The human respiratory system consists, in sequence, of the external nostrils, nasal cavity, pharynx, larynx, trachea (windpipe), two bronchi, bronchioles and finally the alveoli of the lungs, where the exchange of gases takes place.

During inspiration, the diaphragm contracts and flattens and the external intercostal muscles contract, raising the ribs and sternum. This increases the volume of the thoracic cavity and lowers the pressure inside the lungs below atmospheric pressure, so air rushes in. During expiration, the diaphragm and intercostal muscles relax, so the diaphragm becomes dome-shaped and the ribs move down and in. This decreases the volume of the thoracic cavity and raises the pressure in the lungs above atmospheric pressure, so air is pushed out.

Oxygen is transported mainly (about 97 percent) by binding to the haemoglobin of red blood cells, forming oxyhaemoglobin; a small amount is dissolved in the plasma. Carbon dioxide is transported in three ways: mostly (about 70 percent) as bicarbonate ions in the plasma, about 20 to 25 percent bound to haemoglobin as carbaminohaemoglobin, and a small amount dissolved in the plasma.

Tidal volume (TV) is the volume of air breathed in or out during a normal (quiet) breath, about 500 mL. Residual volume (RV) is the volume of air that remains in the lungs even after a forceful expiration, about 1100 to 1200 mL. Vital capacity (VC) is the maximum volume of air that a person can breathe out after the deepest possible inspiration (it equals tidal volume plus inspiratory reserve volume plus expiratory reserve volume).

VC = TV + IRV + ERV

Blood is a fluid connective tissue made of a liquid plasma (about 55 percent) and formed elements (about 45 percent). Plasma carries dissolved nutrients, wastes, hormones and proteins. The formed elements are: red blood cells (erythrocytes), which carry oxygen using haemoglobin; white blood cells (leucocytes), which defend the body against infection (immunity); and platelets (thrombocytes), which help in the clotting of blood.

The human heart is a muscular organ with four chambers: two upper thin-walled atria (right and left) and two lower thick-walled ventricles (right and left). The right side handles deoxygenated blood and the left side handles oxygenated blood, kept separate by a septum. Double circulation means the blood passes through the heart twice in one complete cycle: in pulmonary circulation deoxygenated blood is pumped from the right ventricle to the lungs and returns oxygenated to the left atrium; in systemic circulation oxygenated blood is pumped from the left ventricle to the body and returns deoxygenated to the right atrium.

The cardiac cycle is the sequence of events that occurs during one complete heartbeat, lasting about 0.8 second. It has two main phases: systole, the contraction phase, in which the heart muscle contracts and pumps blood out (atrial systole pushes blood into the ventricles, and ventricular systole pumps blood into the aorta and pulmonary artery); and diastole, the relaxation phase, in which the heart muscle relaxes and the chambers fill with blood. The rhythmic contraction and relaxation ensure continuous circulation of blood.

The ABO blood group system classifies human blood into four groups - A, B, AB and O - based on the presence or absence of two antigens (A and B) on the surface of red blood cells and the corresponding antibodies in the plasma. The Rh factor is another antigen (Rh antigen or D antigen) first found in rhesus monkeys; people who have it on their red cells are Rh-positive and those who lack it are Rh-negative. Blood groups must be matched before transfusion to avoid clumping (agglutination).

(1) Arteries carry blood away from the heart (usually oxygenated, except the pulmonary artery), whereas veins carry blood towards the heart (usually deoxygenated, except the pulmonary vein). (2) Arteries have thick, elastic muscular walls and a narrow lumen and lack valves, whereas veins have thinner walls and a wider lumen and possess valves that prevent the backflow of blood.

These terms describe the main nitrogenous waste excreted by animals. Ammonotelic animals excrete ammonia, which is highly toxic and needs a lot of water (e.g. many bony fishes and aquatic amphibians). Ureotelic animals excrete urea, which is less toxic and needs less water (e.g. mammals including humans, and adult amphibians). Uricotelic animals excrete uric acid, which is least toxic and almost insoluble, needing very little water (e.g. birds, reptiles and insects).

The nephron is the structural and functional unit of the kidney; each kidney has about one million nephrons. Each nephron has two main parts: (1) the Malpighian body (renal corpuscle), consisting of a cup-shaped Bowman's capsule enclosing a bunch of capillaries called the glomerulus; and (2) the renal tubule, which continues from Bowman's capsule as the proximal convoluted tubule (PCT), then the U-shaped loop of Henle, then the distal convoluted tubule (DCT), which opens into a collecting duct. Blood is filtered in the glomerulus and the filtrate is processed along the tubule to form urine.

Urine formation involves three steps: (1) Glomerular filtration - blood is filtered under pressure in the glomerulus, and water, salts, glucose, urea and other small molecules pass into Bowman's capsule as the glomerular filtrate (blood cells and proteins are not filtered). (2) Tubular reabsorption - as the filtrate passes along the tubule, useful substances such as glucose, amino acids, most water and essential salts are reabsorbed back into the blood. (3) Tubular secretion - additional waste substances such as extra hydrogen ions, potassium ions and ammonia are actively secreted from the blood into the filtrate. The remaining fluid is urine.

Haemodialysis is an artificial method of removing nitrogenous wastes such as urea from the blood using a machine (an artificial kidney) when a person's kidneys have failed. The patient's blood is passed through tubes made of a semipermeable membrane that are surrounded by a dialysing fluid; wastes diffuse out of the blood into the fluid while useful substances are retained, and the cleaned blood is returned to the body. It is used to keep patients with kidney failure alive.

The two main contractile proteins of a muscle are actin (the thin filament) and myosin (the thick filament). A muscle fibre contains many myofibrils, each made of repeating units called sarcomeres. A sarcomere is the region of a myofibril between two adjacent Z-lines and is the basic functional (contractile) unit of a striated muscle; it contains an arrangement of actin and myosin filaments whose sliding produces contraction.

According to the sliding filament theory, muscle contraction occurs when the thin actin filaments slide over the thick myosin filaments, so the length of the filaments does not change but the sarcomere shortens. When a nerve impulse arrives, calcium ions are released and expose binding sites on actin; the myosin heads attach to actin forming cross-bridges, and using energy from ATP they pull the actin filaments towards the centre of the sarcomere. As a result the Z-lines come closer and the muscle shortens (contracts). When the impulse stops, calcium is pumped back and the muscle relaxes.

A joint is the point of contact between two or more bones (or between bone and cartilage) that allows movement or gives support. Three types of freely movable (synovial) joints are: (1) ball-and-socket joint, allowing movement in all directions, e.g. the shoulder and hip joint; (2) hinge joint, allowing movement in one plane, e.g. the elbow and knee; and (3) pivot joint, allowing rotational movement, e.g. between the atlas and axis vertebrae of the neck.

The human brain has three main parts: the forebrain, midbrain and hindbrain. The cerebrum (part of the forebrain) is the centre for thinking, memory, intelligence and voluntary actions. The cerebellum (part of the hindbrain) maintains balance and coordinates precise voluntary movements. The medulla oblongata (part of the hindbrain) controls involuntary vital activities such as heartbeat, breathing, blood pressure, swallowing and vomiting.

A reflex action is a sudden, rapid, automatic (involuntary) response to a stimulus that is controlled by the spinal cord without involving conscious thought by the brain (for example, quickly withdrawing the hand from a hot object). The pathway is called the reflex arc, whose components are: a receptor (detects the stimulus), a sensory (afferent) neuron (carries the impulse to the spinal cord), an interneuron in the spinal cord (relays the impulse), a motor (efferent) neuron (carries the impulse to the effector), and an effector such as a muscle or gland that produces the response.

The resting potential is the electrical potential difference across the membrane of a neuron when it is not conducting an impulse; the inside is negatively charged relative to the outside (polarised), maintained by the sodium-potassium pump keeping more sodium ions outside and potassium ions inside. The action potential (nerve impulse) is the rapid reversal of this charge when a stimulus is applied: sodium ions rush in and the membrane becomes depolarised (positive inside), and this change travels along the axon as the nerve impulse before the resting state is restored.

A synapse is the junction between the axon terminal of one neuron and the dendrite (or cell body) of the next neuron, with a tiny gap called the synaptic cleft between them. When a nerve impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters (such as acetylcholine) into the synaptic cleft. These diffuse across the gap and bind to receptors on the next neuron, generating a new impulse in it. Thus transmission across a synapse is chemical and one-directional.

Endocrine glands are ductless glands that pour their secretions (hormones) directly into the blood. Four important ones are: (1) the pituitary gland, which secretes growth hormone (and is called the master gland); (2) the thyroid gland, which secretes thyroxine; (3) the pancreas (islets of Langerhans), which secretes insulin and glucagon; and (4) the adrenal gland, which secretes adrenaline (epinephrine).

Insulin is a hormone secreted by the beta cells of the pancreas; it lowers the level of glucose in the blood by promoting the uptake of glucose by cells and its conversion into glycogen in the liver. A deficiency of insulin causes the disease diabetes mellitus. Adrenaline (epinephrine) is secreted by the adrenal medulla; it is the emergency or fight-or-flight hormone that prepares the body for stress by increasing heart rate, blood pressure, breathing rate and blood glucose level.