Biology — HSSC-I Practice Quiz

(i) Microfilaments — thinnest, made of actin; give cell shape and enable movement (cytoplasmic streaming, muscle contraction). (ii) Microtubules — thickest, made of tubulin; form the spindle, cilia, flagella and act as intracellular "tracks". (iii) Intermediate filaments — rope-like; provide mechanical strength and anchor organelles.

Membrane-bound sacs of hydrolytic enzymes. (i) Intracellular digestion of food taken in by phagocytosis. (ii) Autophagy — digestion of worn-out organelles. (iii) Autolysis ("suicidal bags") and destruction of invading bacteria.

Prokaryotes: no true (membrane-bound) nucleus or organelles, 70S ribosomes, small, DNA naked in cytoplasm. Eukaryotes: true nucleus and membrane-bound organelles, 80S ribosomes, larger, DNA wound on histone proteins.

A glycosidic bond joins two monosaccharides (between their –OH groups) to form carbohydrates, releasing water. A peptide bond joins two amino acids (between –COOH of one and –NH₂ of the next) to form proteins, also releasing water (condensation).

(i) High specific heat and high heat of vaporization → temperature regulation. (ii) Universal solvent and medium for metabolic reactions. (iii) Cohesion & adhesion (H-bonding) → ascent of sap and surface tension.

DNA: deoxyribose sugar, double-stranded, has thymine, stores hereditary information. RNA: ribose sugar, usually single-stranded, has uracil instead of thymine, takes part in protein synthesis.

Pepsin ≈ 2.0 (strongly acidic), Sucrase ≈ 6.2, Enterokinase ≈ 8.0, Arginase ≈ 9.7 (alkaline). Each enzyme works fastest at its own optimum pH and is denatured far from it.

It lowers the activation energy. The substrate binds the active site forming an enzyme–substrate complex (lock-and-key / induced fit), which brings reactants close, orients and strains their bonds; products are released and the enzyme is reused unchanged.

The active site has a rigid, specific shape complementary to one substrate. Only that substrate fits, like a key in a lock, forming the enzyme–substrate complex — explaining enzyme specificity.

A graph of the rate of photosynthesis against the wavelength of light. The rate is highest in the blue (~430 nm) and red (~660 nm) regions and lowest in green light (which is reflected by chlorophyll).

Glycolysis splits glucose into 2 pyruvate in the cytoplasm. Pyruvate enters the mitochondrion and is oxidatively decarboxylated to acetyl-CoA (link reaction). Acetyl-CoA combines with oxaloacetate to enter the Krebs cycle.

Light-driven electron flow from PSII → PSI through an electron transport chain. Water is photolysed (releasing O₂); electrons are not returned to PSII (hence "non-cyclic") and the process yields both ATP and NADPH.

An antiseptic is a chemical applied to living tissue/skin to inhibit microbes (e.g. Dettol). An antibiotic is a substance produced by one microbe that kills or inhibits other microbes inside the body (e.g. penicillin).

The capsid is the complete protein coat enclosing the viral nucleic acid. Capsomeres are the individual protein subunits that assemble together to build the capsid.

(i) Attachment to host CD4 cell & penetration. (ii) Uncoating; reverse transcriptase makes DNA from viral RNA. (iii) Integration into host DNA, transcription & translation of viral parts. (iv) Assembly of new virions and release by budding.

Made of peptidoglycan (murein). Gram-positive: thick peptidoglycan layer with teichoic acids → stains purple. Gram-negative: thin peptidoglycan plus an outer lipopolysaccharide membrane → stains pink.

Special thick-walled cells called heterocysts contain the enzyme nitrogenase, which reduces atmospheric N₂ into ammonia/usable nitrogen compounds for the cell.

Autotrophic — photosynthetic and chemosynthetic; and Heterotrophic — saprophytic, parasitic and symbiotic (mutualistic).

Its members do not descend from a single common ancestor; they arose from several different ancestral lines and are grouped together only because they are eukaryotes that do not fit the plant, animal or fungal kingdoms.

(i) Chitinous cell wall reducing water loss. (ii) Extensive mycelium giving a large absorptive surface. (iii) Resistant spores for dispersal & survival. (iv) Saprophytic/parasitic nutrition by extracellular digestion.

Foraminiferans: marine protozoans with porous calcium-carbonate (chalky) shells. Actinopods (radiolarians/heliozoans): have silica skeletons and stiff radiating axopods.

(i) Heterospory (separate micro- and megaspores). (ii) Retention of the megaspore inside the megasporangium. (iii) Reduction to one functional megaspore & development of the female gametophyte within it. (iv) Formation of integuments → ovule → seed.

The dominant sporophyte bears sori with sporangia → meiosis → spores → germinate into a heart-shaped prothallus (gametophyte) with antheridia & archegonia → fertilization (needs water) → zygote → new sporophyte.

Bryophytes: non-vascular, gametophyte dominant, no true roots (rhizoids). Pteridophytes: vascular (xylem & phloem), sporophyte dominant, with true roots, stem and leaves.

Cohesion–tension (transpiration-pull) theory: transpiration from leaves creates a negative pressure; cohesion of water molecules (H-bonds) and adhesion to xylem walls keep an unbroken column that is pulled upward, aided by root pressure.

At the source, sugar is loaded into sieve tubes; water enters by osmosis raising the hydrostatic pressure; bulk flow then pushes the sap to the sink, where sugar is unloaded and water leaves.

Transpiration: loss of water as vapour, mainly through stomata. Guttation: loss of liquid water as droplets through hydathodes under positive root pressure.

RBCs (erythrocytes) carry O₂ via haemoglobin. WBCs (leucocytes — neutrophils, lymphocytes, monocytes, eosinophils, basophils) provide defence/immunity. Platelets (thrombocytes) help in blood clotting.

A tracheal system: air enters through paired spiracles → branching tracheae → fine tracheoles that deliver O₂ directly to tissues. Gas exchange is independent of the blood.

A Y-shaped immunoglobulin of four polypeptide chains — two heavy and two light — held by disulfide bonds. The variable regions at the tips form two antigen-binding sites; the rest is the constant region.

e.g. Obelia: the asexual polyp colony buds off medusae; the sexual medusa produces gametes → zygote → ciliated planula larva → settles to form a new polyp colony.

Asexual: one parent, no gametes, offspring genetically identical (e.g. budding, binary fission). Sexual: two parents, fusion of gametes (fertilization), offspring show genetic variation.

The two alleles of a gene separate (segregate) from each other during gamete formation, so each gamete carries only one allele; the alleles reunite at fertilization.

Genotype: the genetic make-up / allele combination of an organism (e.g. Tt). Phenotype: the observable expressed characteristic (e.g. tall).

Somatic cell nuclear transfer: a nucleus from a donor body cell is inserted into an enucleated egg; the egg is stimulated to divide into an embryo, implanted in a surrogate, producing a genetically identical individual (e.g. Dolly the sheep).

Replication: DNA → DNA, the whole genome is copied by DNA polymerase. Transcription: DNA → RNA, a single gene is copied into mRNA by RNA polymerase.

Darwin's mechanism: organisms vary; individuals with favourable heritable traits survive and reproduce more ("survival of the fittest") and pass those traits on, so over generations the population changes.

Fossil record, comparative anatomy (homologous, analogous & vestigial organs), embryology, molecular/biochemical similarities (DNA & proteins) and biogeography.

Glycosidic bond → joins monosaccharides (carbohydrates); peptide bond → joins amino acids (proteins). Both form by condensation (loss of water).

By lowering activation energy through formation of an enzyme–substrate complex at the active site, then releasing the products and being reused.

Because its members evolved from several different ancestral lineages rather than from one common ancestor.

Cohesion–tension (transpiration-pull) theory, supported by root pressure.

RBCs (O₂ transport), WBCs (defence) and platelets (clotting).

Pyruvate from glycolysis is converted to acetyl-CoA, which enters the Krebs cycle.