Biology MCQs

Question #241

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Which of the following organelles is surrounded by a single membrane?

A. Mitochondria

B. Chloroplast

C. Lysosome

D. Nucleus

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Correct Answer: Option C

Explanation:
Lysosomes are single-membrane-bound organelles containing hydrolytic enzymes. Mitochondria, chloroplasts, and the nucleus are double-membrane-bound. Peroxisomes, vacuoles, and the endoplasmic reticulum are also single-membrane structures. The single membrane of lysosomes helps maintain an acidic pH (5.0) for enzyme activity and protects the cell from autolysis.

This question belongs to: Science Biology

Question #242

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The organelle that detoxifies drugs and poisons in liver cells is the:

A. Lysosome

B. Rough endoplasmic reticulum

C. Smooth endoplasmic reticulum

D. Golgi apparatus

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Correct Answer: Option C

Explanation:
Smooth endoplasmic reticulum (SER) contains enzymes like cytochrome P450 that detoxify lipid-soluble drugs, metabolic wastes, and poisons by making them water-soluble for excretion. It is abundant in hepatocytes (liver cells). Rough ER synthesizes proteins, lysosomes digest cellular material, Golgi packages. SER also synthesizes lipids and steroids.

This question belongs to: Science Biology

Question #243

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In a plant cell, the turgor pressure is maintained mainly by the:

A. Cell wall

B. Central vacuole

C. Chloroplast

D. Plasma membrane

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Correct Answer: Option B

Explanation:
The central vacuole occupies a large volume of a mature plant cell and stores water, creating turgor pressure against the cell wall, which maintains cell rigidity and plant structure. The cell wall provides mechanical support but turgor pressure generated by the vacuole is essential for maintaining turgidity, especially in herbaceous plants.

This question belongs to: Science Biology

Question #244

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The subunits of 80S ribosomes in eukaryotes are:

A. 30S + 50S

B. 40S + 60S

C. 60S + 40S

D. 50S + 30S

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Correct Answer: Option B

Explanation:
Eukaryotic ribosomes are 80S and composed of a small 40S subunit and a large 60S subunit. Prokaryotic ribosomes are 70S with 30S and 50S subunits. The S (Svedberg) value is a measure of sedimentation rate, not simply additive. Both ribosome types synthesize proteins, but the structural difference allows selective antibiotic targeting (e.g., streptomycin acts on 70S).

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Question #245

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Which of the following is a characteristic of mitosis but NOT meiosis?

A. Pairing of homologous chromosomes

B. Formation of haploid gametes

C. Production of genetically identical daughter cells

D. Crossing over

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Correct Answer: Option C

Explanation:
Mitosis produces two genetically identical diploid daughter cells (clones) essential for growth, repair, and asexual reproduction. Meiosis involves pairing (synapsis) and crossing over during prophase I, and produces four genetically diverse haploid cells (gametes). The conservation of chromosome number and genetic identity is a hallmark of mitosis.

This question belongs to: Science Biology

Question #246

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The term 'phylum' in animal taxonomy is equivalent to which category in plant taxonomy?

A. Order

B. Class

C. Division

D. Genus

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Correct Answer: Option C

Explanation:
In botanical classification, the term 'Division' corresponds to 'Phylum' in zoological classification. For example, flowering plants are in the Division Magnoliophyta (or Angiosperms), while animals are in Phyla like Chordata. The hierarchy is Kingdom → Division/Phylum → Class → Order → Family → Genus → Species. This terminological difference is a key point in taxonomy.

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Question #247

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Which of the following is an example of an amphibian?

A. Snake

B. Turtle

C. Frog

D. Lizard

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Correct Answer: Option C

Explanation:
Frogs belong to the class Amphibia, characterized by a life cycle that includes an aquatic larval stage (tadpole) with gills and an adult stage with lungs, moist skin, and a three-chambered heart. Turtle, snake, and lizard are reptiles, with dry scaly skin and amniotic eggs. Amphibians are the first terrestrial vertebrates but depend on water for reproduction.

This question belongs to: Science Biology

Question #248

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The botanical name of neem is:

A. Ocimum sanctum

B. Azadirachta indica

C. Ficus religiosa

D. Mangifera indica

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Correct Answer: Option B

Explanation:
Azadirachta indica is the scientific name of neem, known for its medicinal and insecticidal properties. Ocimum sanctum is holy basil (tulsi), Ficus religiosa is the peepal tree, and Mangifera indica is mango. Neem belongs to the family Meliaceae. Its leaves, seeds, and oil are used in various traditional remedies and biopesticides.

This question belongs to: Science Biology

Question #249

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The meristematic tissue responsible for the increase in the length of the plant is:

A. Apical meristem

B. Both B and C

C. Lateral meristem

D. Intercalary meristem

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Correct Answer: Option B

Explanation:
Apical meristems at root and shoot tips contribute to primary growth (increase in length). Intercalary meristems, located at nodes and leaf bases, also aid in elongation, especially in grasses. Lateral meristems (vascular cambium, cork cambium) cause secondary growth (increase in girth). So both apical and intercalary meristems are responsible for length increase.

This question belongs to: Science Biology

Question #250

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In leaves, the small openings surrounded by guard cells that allow gas exchange are known as:

A. Sclereids

B. Lenticels

C. Hydathodes

D. Stomata

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Correct Answer: Option D

Explanation:
Stomata are microscopic pores typically on the lower epidermis of leaves, bounded by two guard cells that regulate opening and closing for gas exchange (CO₂ in, O₂ out) and transpiration. Lenticels are on bark, hydathodes release water droplets (guttation), sclereids are thick-walled sclerenchyma cells. Stomatal density varies with environmental conditions.

This question belongs to: Science Biology

Question #251

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The green pigment that traps light energy in plants is located in the:

A. Cytoplasm

B. Nucleus

C. Thylakoid membranes of chloroplasts

D. Cell wall

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Correct Answer: Option C

Explanation:
Chlorophyll, the primary photosynthetic pigment, is embedded in the thylakoid membranes within chloroplasts. These membranes contain photosystems I and II, where light-dependent reactions occur. The stroma of the chloroplast houses the Calvin cycle enzymes. Chlorophyll absorbs mainly blue and red light, reflecting green, giving leaves their color.

This question belongs to: Science Biology

Question #252

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The waxy layer that covers the outer surface of leaves to reduce water loss is the:

A. Spongy mesophyll

B. Cuticle

C. Epidermis

D. Palisade mesophyll

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Correct Answer: Option B

Explanation:
The cuticle is a waxy, water-repellent layer secreted by epidermal cells on the outer surface of leaves and young stems. It minimizes water loss through transpiration. Its thickness varies with plant species and habitat; xerophytes often have a thick cuticle. The epidermis is the tissue layer, palisade and spongy mesophyll are photosynthetic tissues.

This question belongs to: Science Biology

Question #253

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Which of the following plant hormones delays leaf senescence (aging)?

A. Cytokinin

B. Auxin

C. Abscisic acid

D. Ethylene

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Correct Answer: Option A

Explanation:
Cytokinins are plant hormones that promote cell division, delay senescence by preventing chlorophyll degradation, and stimulate nutrient mobilization. Ethylene promotes ripening and senescence, abscisic acid promotes abscission and dormancy, auxin is involved in cell elongation. Cytokinins are synthesized in roots and transported upward. They are used to prolong the shelf life of leafy vegetables.

This question belongs to: Science Biology

Question #254

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Double fertilization in angiosperms was discovered by:

A. Strasburger

B. Nawaschin

C. Hofmeister

D. Amici

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Correct Answer: Option B

Explanation:
Double fertilization in angiosperms was discovered by the Russian botanist Sergei Nawaschin in 1898 in Lilium and Fritillaria. One male gamete fuses with the egg to form the zygote, the other with the secondary nucleus to form the triploid endosperm. Strasburger discovered fertilization in flowering plants, Amici observed pollen tube, Hofmeister studied alternation of generations.

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Question #255

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Which of the following is an accessory digestive gland?

A. Esophagus

B. Stomach

C. Small intestine

D. Pancreas

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Correct Answer: Option D

Explanation:
The pancreas, liver, and salivary glands are accessory digestive glands that secrete enzymes or substances aiding digestion into the alimentary canal. The pancreas produces pancreatic juice (enzymes and bicarbonate) released into the duodenum. The stomach and small intestine are part of the main digestive tract, and the esophagus is a conduit for food.

This question belongs to: Science Biology

Question #256

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The intestinal enzyme that converts trypsinogen to trypsin is:

A. Enterokinase

B. Lipase

C. Amylase

D. Nucleotidase

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Correct Answer: Option A

Explanation:
Enterokinase (enteropeptidase), secreted by the duodenal mucosa, is the specific activator of pancreatic trypsinogen, converting it to active trypsin. Trypsin then activates other pancreatic zymogens (chymotrypsinogen, procarboxypeptidase). This cascade ensures that potent digestive enzymes are activated only in the intestinal lumen, preventing autodigestion of the pancreas.

This question belongs to: Science Biology

Question #257

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The cartilage that prevents food from entering the larynx during swallowing is the:

A. Thyroid cartilage

B. Arytenoid cartilage

C. Epiglottis

D. Cricoid cartilage

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Correct Answer: Option C

Explanation:
The epiglottis is a leaf-shaped elastic cartilage that covers the glottis (opening of the larynx) during swallowing, directing food into the esophagus and preventing it from entering the trachea. The thyroid cartilage forms the Adam's apple, the cricoid is ring-shaped below, and arytenoids are involved in vocal cord movement.

This question belongs to: Science Biology

Question #258

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The volume of air that remains in the lungs after a maximum forced expiration is called:

A. Tidal volume

B. Vital capacity

C. Inspiratory reserve volume

D. Residual volume

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Correct Answer: Option D

Explanation:
Residual volume (RV) is the air left in the lungs (about 1.2 L) after a forceful expiration. This prevents lung collapse and ensures continuous gas exchange between breaths. Tidal volume is normal breath volume, vital capacity is the maximum exhaled after maximum inhalation, and inspiratory reserve volume is the extra air that can be inhaled beyond tidal volume.

This question belongs to: Science Biology

Question #259

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Which of the following blood vessels has the highest total cross-sectional area in the body?

A. Aorta

B. Arteries

C. Capillaries

D. Vena cava

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Correct Answer: Option C

Explanation:
Although individual capillaries are tiny, there are billions of them, giving the capillary bed the largest total cross-sectional area. This reduces blood flow velocity, allowing sufficient time for exchange of gases, nutrients, and wastes. The aorta has the smallest total cross-sectional area and the highest blood flow velocity.

This question belongs to: Science Biology

Question #260

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The first heart sound 'lub' is produced by the closure of:

A. Semilunar valves

B. Pulmonary valve only

C. Both AV and semilunar valves

D. Atrioventricular (AV) valves

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Correct Answer: Option D

Explanation:
The 'lub' sound occurs at the beginning of ventricular systole when the atrioventricular valves (mitral and tricuspid) close to prevent backflow of blood into the atria. The 'dub' sound occurs at the beginning of diastole due to closure of the semilunar valves (aortic and pulmonary). Valve closure is passive, resulting from pressure gradients.

This question belongs to: Science Biology

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