Biology MCQs

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Biology MCQs

Practice complete Biology MCQs covering Introduction to Biology, Cell Biology, Cell Structure, Cell Division, Biomolecules, Plant Physiology, Human Physiology, Genetics, Evolution, Biotechnology, Ecology, Classification of Living Organisms, Plant Kingdom, Animal Kingdom, Microbiology, Human Diseases, Nutrition, Reproduction, Environmental Biology, and all other important topics. Includes chapter-wise and exam-oriented multiple-choice questions with detailed answers and explanations for JKSSB, SSC, Banking, Railway, UPSC, CUET, NEET, State PSCs, and other competitive exams.

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Page 36 of 103
Question #701
The process of conversion of atmospheric nitrogen into ammonia by microorganisms is:
A. Nitrification
B. Ammonification
C. Denitrification
D. Nitrogen fixation
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Correct Answer: Option D

Explanation:
Biological nitrogen fixation, performed by bacteria (e.g., Rhizobium, Azotobacter) and cyanobacteria, reduces atmospheric N₂ to ammonia (NH₃) using the nitrogenase enzyme complex. Nitrification converts ammonia to nitrates, denitrification returns N₂ to atmosphere, ammonification releases ammonia from organic matter.
This question belongs to: Science Biology
Question #702
The main function of the large intestine is:
A. Digestion of proteins
B. Secretion of digestive enzymes
C. Absorption of amino acids
D. Absorption of water and electrolytes
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Correct Answer: Option D

Explanation:
The colon (large intestine) primarily absorbs water, sodium, and some vitamins (produced by gut flora) from the indigestible food residue, converting it into semisolid feces. It does not secrete digestive enzymes; that is the small intestine's role. Protein digestion and amino acid absorption occur in the small intestine.
This question belongs to: Science Biology
Question #703
The molecule that acts as a second messenger in many hormone signaling pathways is:
A. ATP
B. cAMP
C. DNA
D. Glucose
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Correct Answer: Option B

Explanation:
Cyclic adenosine monophosphate (cAMP) is a common intracellular second messenger produced by adenylyl cyclase upon hormone receptor activation (e.g., glucagon, adrenaline). It activates protein kinase A, triggering a cascade of cellular responses. Other second messengers include IP₃, DAG, Ca²⁺.
This question belongs to: Science Biology
Question #704
The protein that polymerizes to form blood clots is:
A. Fibrinogen
B. Prothrombin
C. Thrombin
D. Fibrin
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Correct Answer: Option D

Explanation:
Thrombin converts soluble fibrinogen into insoluble fibrin monomers, which polymerize and cross-link to form a stable blood clot. Fibrinogen is the precursor, prothrombin is the precursor of thrombin. The clotting cascade amplifies the signal.
This question belongs to: Science Biology
Question #705
The structure that produces cerebrospinal fluid is the:
A. Arachnoid villi
B. Choroid plexus
C. Dura mater
D. Subarachnoid space
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Correct Answer: Option B

Explanation:
The choroid plexus, a network of ependymal cells and capillaries in the ventricles of the brain, continuously produces CSF by filtration and secretion. CSF circulates through ventricles and the subarachnoid space and is reabsorbed by arachnoid villi into the venous system. Dura mater is the outer protective membrane.
This question belongs to: Science Biology
Question #706
The type of immunity resulting from the injection of ready-made antibodies is:
A. Cell-mediated immunity
B. Active immunity
C. Passive immunity
D. Innate immunity
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Correct Answer: Option C

Explanation:
Passive immunity is acquired by receiving pre-formed antibodies from another source, providing immediate but temporary protection. Examples: antivenom, immune globulin injections, and maternal antibodies. Active immunity is generated by the body's own immune system (infection or vaccination). Innate is non-specific, cell-mediated is T-cell based.
This question belongs to: Science Biology
Question #707
The pigment that absorbs light energy and gives green color to plants is:
A. Carotenoid
B. Phytochrome
C. Chlorophyll
D. Anthocyanin
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Correct Answer: Option C

Explanation:
Chlorophyll, mainly chlorophyll a and b, absorbs red and blue wavelengths and reflects green light, giving plants their characteristic color. It is the primary pigment of photosynthesis, located in the thylakoid membranes. Carotenoids reflect yellow-orange, phytochrome is a light-sensing pigment, anthocyanins are red/purple.
This question belongs to: Science Biology
Question #708
The scientist who proposed the 'lock and key' model of enzyme action was:
A. Emil Fischer
B. Leonor Michaelis
C. Daniel Koshland
D. Maud Menten
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Correct Answer: Option A

Explanation:
Emil Fischer proposed the lock-and-key model in 1894, suggesting that the enzyme's active site is exactly complementary to the substrate, like a key fits a lock. Koshland later proposed the induced fit model, which is more widely accepted. Michaelis and Menten developed enzyme kinetics equations.
This question belongs to: Science Biology
Question #709
The chromosomal abnormality in Down syndrome is:
A. Monosomy 21
B. Monosomy X
C. Trisomy 18
D. Trisomy 21
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Correct Answer: Option D

Explanation:
Down syndrome, the most common chromosomal aneuploidy, is caused by an extra copy of chromosome 21 (trisomy 21), usually from nondisjunction during meiosis. It is characterized by intellectual disability, distinct facial features, and increased risk of certain medical conditions. Monosomy X is Turner syndrome.
This question belongs to: Science Biology
Question #710
The element that is a constituent of the amino acids methionine and cysteine is:
A. Sulfur
B. Nitrogen
C. Iron
D. Phosphorus
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Correct Answer: Option A

Explanation:
Sulfur is present in the amino acids methionine and cysteine (which forms disulfide bonds in proteins). Nitrogen is in all amino acids, phosphorus in nucleic acids and ATP, iron in heme. Sulfur is also in vitamins biotin and thiamine. Disulfide bonds are crucial for protein tertiary structure.
This question belongs to: Science Biology
Question #711
The human organ that detoxifies ammonia by converting it to urea is the:
A. Kidney
B. Liver
C. Pancreas
D. Intestine
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Correct Answer: Option B

Explanation:
The liver uses the urea cycle (ornithine cycle) to convert highly toxic ammonia, produced from amino acid deamination, into less toxic urea, which is excreted by the kidneys. The kidneys filter urea but do not synthesize it. Pancreas is digestive/endocrine, intestine absorbs nutrients.
This question belongs to: Science Biology
Question #712
The pore through which pollen tube enters the ovule is the:
A. Hilum
B. Chalaza
C. Funicle
D. Micropyle
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Correct Answer: Option D

Explanation:
The micropyle is a small opening in the integuments of the ovule through which the pollen tube usually enters to deliver sperm cells for fertilization (porogamy). The chalaza is the opposite end, funicle is the stalk, hilum is the point of attachment. The micropyle also allows water entry during germination.
This question belongs to: Science Biology
Question #713
The animal that can regenerate its lost body parts to a remarkable extent is:
A. Frog
B. Earthworm
C. Hydra
D. Cockroach
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Correct Answer: Option C

Explanation:
Hydra, a freshwater cnidarian, has extraordinary regenerative ability; even small fragments can regenerate a complete organism. This is due to its high proportion of stem cells. Planarians also show remarkable regeneration. Earthworms can regenerate some posterior segments, but not as extensively as Hydra or Planaria.
This question belongs to: Science Biology
Question #714
The enzyme that seals nicks in the sugar-phosphate backbone of DNA is:
A. Ligase
B. Primase
C. DNA polymerase
D. Helicase
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Correct Answer: Option A

Explanation:
DNA ligase catalyzes the formation of phosphodiester bonds between adjacent nucleotides, sealing breaks in the DNA backbone. It is essential for joining Okazaki fragments on the lagging strand during replication and in DNA repair. Polymerase adds nucleotides, helicase unwinds, primase synthesizes RNA primer.
This question belongs to: Science Biology
Question #715
The type of plant that has adapted to survive in very dry environments is called:
A. Halophyte
B. Mesophyte
C. Hydrophyte
D. Xerophyte
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Correct Answer: Option D

Explanation:
Xerophytes (e.g., cactus, acacia) have adaptations like thick cuticle, sunken stomata, reduced leaves, and water-storing tissues to conserve water in arid habitats. Hydrophytes live in water, mesophytes in moderate conditions, halophytes in saline soils. Xerophytes minimize transpiration and maximize water uptake.
This question belongs to: Science Biology
Question #716
The medical condition where the lens of the eye becomes opaque is:
A. Glaucoma
B. Myopia
C. Astigmatism
D. Cataract
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Correct Answer: Option D

Explanation:
Cataract is clouding of the crystalline lens, causing blurred vision and eventual blindness if untreated. It is often age-related or due to UV exposure. Myopia is nearsightedness, glaucoma optic nerve damage, astigmatism irregular curvature. Cataract is treated by surgical removal and lens replacement.
This question belongs to: Science Biology
Question #717
The molecule that is the final electron acceptor in the mitochondrial electron transport chain is:
A. FAD
B. Cytochrome c
C. NAD⁺
D. Oxygen
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Correct Answer: Option D

Explanation:
Molecular oxygen (O₂) accepts electrons at the end of the electron transport chain, combining with protons to form water (metabolic water). Without oxygen, the chain backs up, stopping ATP production. This is why oxygen is vital for aerobic respiration. NAD⁺ and FAD are electron carriers, cytochrome c a component.
This question belongs to: Science Biology
Question #718
The hormone that stimulates the conversion of glycogen to glucose in the liver is:
A. Glucagon
B. Insulin
C. Somatostatin
D. Thyroxine
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Correct Answer: Option A

Explanation:
Glucagon, secreted by alpha cells of the pancreas when blood glucose is low, binds to liver receptors activating glycogenolysis (glycogen → glucose) and gluconeogenesis. Insulin promotes glycogenesis (glucose → glycogen). Thyroxine affects metabolism, somatostatin inhibits both insulin and glucagon.
This question belongs to: Science Biology
Question #719
The type of WBC that is the first to respond to bacterial infection and performs phagocytosis is:
A. Eosinophil
B. Lymphocyte
C. Basophil
D. Neutrophil
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Correct Answer: Option D

Explanation:
Neutrophils are the most abundant white blood cells and are part of the innate immune system. They quickly migrate to infection sites, where they phagocytose bacteria and release antimicrobial substances. Lymphocytes are for adaptive immunity, eosinophils combat parasites, basophils release histamine.
This question belongs to: Science Biology
Question #720
The outermost layer of the skin is called the:
A. Dermis
B. Hypodermis
C. Endodermis
D. Epidermis
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Correct Answer: Option D

Explanation:
The epidermis is the avascular, outermost layer of skin, composed mainly of keratinized stratified squamous epithelium. It provides a waterproof barrier and creates skin tone via melanocytes. The dermis is below, containing vessels and nerves, hypodermis is subcutaneous fat, endodermis is a root layer in plants.
This question belongs to: Science Biology

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