Biology — Mixed Practice — Part 3: 40 MCQs with Explanations

Lower pH (higher H⁺/CO₂) and higher temperature decrease hemoglobin's O₂ affinity, enhancing O₂ unloading in metabolically active tissues.

Semicircular canals (along with utricle and saccule) in inner ear detect rotational and linear acceleration for balance.

Ciliary processes of ciliary body secrete aqueous humor, which maintains intraocular pressure and nourishes lens/cornea.

PKU is caused by phenylalanine hydroxylase deficiency, leading to phenylalanine accumulation and intellectual disability if untreated.

Tidal volume (air inhaled/exhaled per breath) and breathing rate increase during exercise to enhance alveolar ventilation.

Bile salts emulsify dietary fats into small droplets, increasing surface area for pancreatic lipase action.

Soft palate elevates to close nasopharynx during swallowing; epiglottis covers larynx to prevent tracheal entry.

Cerebellum compares intended movement (from cortex) with actual movement (via proprioception) to correct errors and ensure smooth coordination.

Countercurrent multiplier system in loop of Henle establishes hypertonic medulla essential for urine concentration in collecting duct.

Alcoholic fermentation: pyruvate → acetaldehyde + CO₂ → ethanol. Lactic acid fermentation and glycolysis do not produce CO₂.

The epiglottis is a flap of elastic cartilage that covers the glottis (opening of trachea) during swallowing to prevent aspiration.

The hypothalamus maintains homeostasis, including thermoregulation, by triggering sweating or shivering responses.

The loop of Henle creates a hypertonic medulla through countercurrent multiplication, allowing the collecting duct to concentrate urine under ADH influence.

At 25°C, [H⁺] = [OH⁻] = 10⁻⁷ M in pure water, so pH = -log(10⁻⁷) = 7. Note: pH of neutrality changes with temperature.

Fluorine (F) has the highest electronegativity value of 4.0 on the Pauling scale, making it the most electronegative element.

Let oxidation state of Cr be x. For K₂Cr₂O₇: 2(+1) + 2x + 7(-2) = 0 → 2 + 2x - 14 = 0 → 2x = 12 → x = +6.

BF₃ is a Lewis acid because boron has an incomplete octet and can accept an electron pair. NH₃, H₂O, and OH⁻ are Lewis bases (electron pair donors).

Water has a bent shape due to two lone pairs on oxygen. The ideal tetrahedral angle is 109.5°, but lone pair-bond pair repulsion reduces it to approximately 104.5°.

Charles's law: V ∝ T (at constant P and n). Boyle's law: P ∝ 1/V (constant T, n). Avogadro's law: V ∝ n (constant P, T). Gay-Lussac's law: P ∝ T (constant V, n).

Methane has tetrahedral geometry with bond angles of 109.5°, characteristic of sp³ hybridization (one s + three p orbitals).

Acid strength increases down the group: HF < HCl < HBr < HI. HI is the strongest acid due to weakest H-I bond and largest size of I⁻ ion stabilizing the conjugate base.

The Standard Hydrogen Electrode (SHE) is assigned a standard electrode potential of 0.00 V at all temperatures by convention.

NH₃ exhibits hydrogen bonding because N is highly electronegative and bonded to H. CH₄ lacks electronegative atoms. HCl has weak hydrogen bonding. PH₃ has low electronegativity of P, so no significant hydrogen bonding.

CH₃CH₂COCH₃ has 4 carbon atoms with a ketone group at carbon 2, so it's butanone (or butan-2-one).

Energy order in multi-electron atoms: 1s < 2s < 2p < 3s < 3p < 4s < 3d... So among the options, 3p has the highest energy.

Moles of NaCl = 5.85/58.5 = 0.1 mol. Volume = 500 mL = 0.5 L. Molarity = moles/volume = 0.1/0.5 = 0.2 M.

Fe (Z=26): [Ar] 4s² 3d⁶. Fe³⁺: [Ar] 3d⁵. Five electrons in d orbitals occupy all five orbitals singly (Hund's rule), so 5 unpaired electrons.

Surface tension is not a colligative property. Colligative properties (vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure) depend only on the number of solute particles, not their identity.

The standard enthalpy of neutralization for strong acid-strong base reactions is approximately -57.1 kJ/mol at 298 K, representing H⁺(aq) + OH⁻(aq) → H₂O(l).

Ethylene glycol (HO-CH₂-CH₂-OH) is commonly used as antifreeze because it lowers the freezing point of water significantly and has a high boiling point.

For first-order reactions: rate = k[A], so k = rate/[A] has units of (mol L⁻¹ s⁻¹)/(mol L⁻¹) = s⁻¹.

Atomic radius increases down a group due to addition of new electron shells. Rb (period 5) has the largest atomic radius among these alkali metals.

The longest chain has 4 carbons (butane) with a methyl group at carbon 2, so 2-methylbutane.

CSF cushions brain/spinal cord, provides buoyancy, transports nutrients, and removes metabolic wastes.

Menstruation (days 1-5 of cycle) involves shedding of uterine lining when pregnancy does not occur.

Hypothalamus contains centers for hunger, thirst, temperature regulation, and links nervous and endocrine systems via pituitary gland.

Large intestine absorbs water and electrolytes, forming solid feces; most nutrient absorption occurs in small intestine.

Homeostasis involves regulatory mechanisms (e.g., thermoregulation, osmoregulation) to maintain stable internal conditions despite external changes.

Esophagus is part of digestive system; it transports food to stomach. Respiratory system includes nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.

Cerebellum coordinates voluntary movements, maintains posture and equilibrium, and ensures smooth motor activity.

The light microscope can make the object 1000 times times bigger.

Locomotion in diatoms is carried out with the help of Raphe .

Golden brown colour of diatoms is due to presence of Fucoxanthin, diatoxanthin, diadinoxanthin .

Butterfly is the correct answer to “Which of the following is not nocturnal”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Moth is the correct answer to “Which one of these is an insect”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

A moth is closely related to Butterfly .

Honeybee, lac insect, cochineal insect is the correct answer to “Which one of these is a set of useful insects”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Liver is the correct answer to “Which of the following organ is responsible for body metabolism”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

The part of the brain that connects the two hemispheres of the brain is called Corpus callosum .

Diatomaceous earth is used for making Fire proof bricks .

Diatoms stay afloat due to Stored lpids .

Unlike other algae, diatoms do not readily decay due to Siliceous wall .

Diatom frustule/ shell is made of Silica .

The chloroplasts of diatoms and chrysophycean algae do not possess Defined grana and stroma .

Oenocytes is the correct answer to “The wax present on the body of cockroach is secreted:”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Diatoms is the correct answer to “Which organisms are sometimes called the  pastures of the sea”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Pinnularia, Navicula, Cymbella, Melosira is the correct answer to “Which one of the following is representing the group of diatoms”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Diatoms are found in All of the above .

Centric discoid form of diatoms is Coscinodiscus .

Suinea worm is the correct answer to “In man which organisms cause abscesses on the skin of arms or legs”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Chitin in cockroach is absent in Epicuticle .

Melanin pigment in cockroach is found in Exocuticle .

The bristles on the body of the cockroach arise from Trichogen cell .

The nephridia of earthworm open into the coelom by Nephrostome .

Nephrostome in earthworm opens in Internal Body cavity .

Scolidon or dog fish belong to the group of fishes called Cartilaginous fishes .

Jawless fishes is the correct answer to “They are considered to be the most primitive and are surely the first of the vertebrates”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Snails belong to Gastropods .

In mouth cavity of many molluscs there is a rasping tongue like structure Radula provided with many horny teeth.

3 is the correct answer to “All insects have how many parts of legs”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

In all arthropods,the body is covered with a chittinous cuticle which is secreted by the Epidermis .

Malphighian tubules absorb waste materials and salts from Hemolymph .

Vasa recta is the correct answer to “Which of the vascular channels are related to Juxtramedullary nephrous only”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Extracorporeal Shock wave lithotripsy is the correct answer to “A stone measuring 0.4 cm, blocking the outlet of renal pelvis resulting in mild to moderate tract obstruction. Which of the following technique will be opted to remove this stone”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Bats & humming birds belong to Heterotherms .

Thick, waxy & leathery cuticle around leaves is present in Xerophytes .

Birds. is the correct answer to “Which of the following animal excrete nitrogenous waste mainly as uric acid”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

Nephrostome is the correct answer to “Which of the following openings have cilia”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.

In osmosis water molecules move from area of Lower solute concentration to higher solute concentration through semipermeable memberane.

Ammonia is the correct answer to “Which of the following is most toxic”. Items of this type are drawn from the Biology — Mixed Practice — Part 3 syllabus of Pakistani competitive exams, where the fact is tested verbatim.