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 11 of 103
Question #201
The first stable product of CO₂ fixation in C4 plants is:
A. 3-phosphoglyceric acid (PGA)
B. Oxaloacetic acid (OAA)
C. Ribulose bisphosphate (RuBP)
D. Phosphoenolpyruvate (PEP)
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Correct Answer: Option B

Explanation:
In C4 plants like maize and sugarcane, CO₂ is initially fixed in mesophyll cells by PEP carboxylase, producing a four-carbon compound oxaloacetic acid (OAA), which is then converted to malate or aspartate and transported to bundle sheath cells. Here, CO₂ is released and enters the Calvin cycle. This adaptation minimizes photorespiration.
This question belongs to: Science Biology
Question #202
Which plant hormone is known as the 'stress hormone' and promotes seed dormancy?
A. Gibberellin
B. Ethylene
C. Abscisic acid (ABA)
D. Cytokinin
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Correct Answer: Option C

Explanation:
Abscisic acid (ABA) is called the stress hormone because it accumulates under adverse conditions (drought, cold) and triggers adaptive responses like stomatal closure. It also promotes seed dormancy and inhibits germination. Gibberellins break seed dormancy, cytokinins promote cell division, ethylene promotes fruit ripening. ABA is antagonistic to gibberellins.
This question belongs to: Science Biology
Question #203
The movement of water through the apoplast pathway does NOT cross which layer in the root?
A. Pericycle
B. Epidermis
C. Cortex
D. Endodermis
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Correct Answer: Option D

Explanation:
In the apoplast pathway, water moves through cell walls and intercellular spaces without crossing membranes. However, at the endodermis, the Casparian strip (suberin deposition) blocks apoplastic movement, forcing water to enter the symplast (via plasma membrane). Thus, apoplastic movement stops at the endodermis. This ensures selective uptake of minerals.
This question belongs to: Science Biology
Question #204
The food conducting tissue in plants, phloem, is composed of which type of cells?
A. Sieve tubes and companion cells
B. Guard cells and subsidiary cells
C. Fibers and sclereids
D. Tracheids and vessels
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Correct Answer: Option A

Explanation:
Phloem is composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibers. Sieve tubes are living cells that transport organic solutes; companion cells assist in loading and unloading. Tracheids and vessels are xylem elements. Fibers and sclereids are sclerenchyma cells. Guard cells regulate stomatal movement.
This question belongs to: Science Biology
Question #205
Which part of the human digestive system produces hydrochloric acid?
A. Liver
B. Goblet cells of intestine
C. Pancreas
D. Parietal cells of stomach
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Correct Answer: Option D

Explanation:
Parietal (oxyntic) cells in the gastric glands of the stomach secrete hydrochloric acid (HCl), which activates pepsinogen to pepsin, kills bacteria, and provides an acidic pH (1.5-2.0) for protein digestion. Liver produces bile, pancreas secretes digestive enzymes and bicarbonate, goblet cells secrete mucus. Intrinsic factor, required for vitamin B₁₂ absorption, is also secreted by parietal cells.
This question belongs to: Science Biology
Question #206
The finger-like projections in the small intestine that increase the surface area for absorption are:
A. Alveoli
B. Cilia
C. Villi
D. Nephrons
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Correct Answer: Option C

Explanation:
Villi are microscopic finger-like projections on the inner lining of the small intestine, covered with microvilli (brush border), greatly increasing surface area for absorption of digested nutrients. Each villus contains a lacteal (lymph capillary) and blood capillaries. Alveoli are in lungs, nephrons in kidneys, cilia are hair-like projections on cells.
This question belongs to: Science Biology
Question #207
Which digestive enzyme breaks down lactose into glucose and galactose?
A. Sucrase
B. Lactase
C. Maltase
D. Amylase
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Correct Answer: Option B

Explanation:
Lactase is a brush border enzyme of the small intestine that hydrolyzes the disaccharide lactose (milk sugar) into its constituent monosaccharides, glucose and galactose. Maltase acts on maltose, sucrase on sucrose, and amylase on starch. Lactose intolerance results from lactase deficiency, causing gastrointestinal symptoms after dairy consumption.
This question belongs to: Science Biology
Question #208
The pulmonary veins carry:
A. Deoxygenated blood from heart to lungs
B. Deoxygenated blood from body to heart
C. Oxygenated blood from heart to body
D. Oxygenated blood from lungs to heart
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Correct Answer: Option D

Explanation:
The pulmonary veins are the only veins in the body that carry oxygenated blood. They transport oxygen-rich blood from the lungs back to the left atrium of the heart. In contrast, the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. All other veins carry deoxygenated blood and arteries carry oxygenated blood.
This question belongs to: Science Biology
Question #209
The pacemaker of the human heart is the:
A. Bundle of His
B. Atrioventricular node
C. Sinoatrial node
D. Purkinje fibers
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Correct Answer: Option C

Explanation:
The sinoatrial (SA) node, located in the right atrium, generates electrical impulses spontaneously and initiates each heartbeat, setting the rhythm. It is called the natural pacemaker. The atrioventricular (AV) node delays the impulse before it travels to the ventricles. The bundle of His and Purkinje fibers conduct the impulse through the ventricles.
This question belongs to: Science Biology
Question #210
Which component of blood plays a crucial role in blood clotting?
A. Red blood cells
B. Plasma proteins (albumin)
C. Platelets
D. White blood cells
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Correct Answer: Option C

Explanation:
Platelets (thrombocytes) are small, anucleate cell fragments that adhere to the site of injury, release clotting factors, and aggregate to form a platelet plug, initiating the coagulation cascade. RBCs transport oxygen, WBCs defend against infection, and plasma proteins like albumin maintain osmotic balance. Clotting factors are also present in plasma, but platelets are the primary cellular component.
This question belongs to: Science Biology
Question #211
The blood vessel with the thinnest wall, allowing exchange of substances, is:
A. Vein
B. Capillary
C. Artery
D. Arteriole
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Correct Answer: Option B

Explanation:
Capillaries are the smallest blood vessels, with walls only one cell thick (endothelium), facilitating diffusion of oxygen, carbon dioxide, nutrients, and waste products between blood and tissues. Arteries and veins have thicker walls with smooth muscle and connective tissue. Capillaries form extensive networks (capillary beds) throughout the body.
This question belongs to: Science Biology
Question #212
The functional unit of the human kidney where filtration occurs is the:
A. Renal corpuscle
B. Bowman's capsule
C. Loop of Henle
D. Distal tubule
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Correct Answer: Option A

Explanation:
The renal corpuscle consists of the glomerulus (capillary tuft) and Bowman's capsule. This is where ultrafiltration of blood occurs. The filtrate then passes through the proximal tubule, loop of Henle, distal tubule, and collecting duct. The loop of Henle concentrates urine, the tubules reabsorb and secrete. The nephron is the complete functional unit.
This question belongs to: Science Biology
Question #213
Which hormone regulates water reabsorption in the kidney collecting ducts?
A. Aldosterone
B. Parathyroid hormone
C. Antidiuretic hormone (ADH)
D. Calcitonin
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Correct Answer: Option C

Explanation:
ADH (vasopressin), released from the posterior pituitary, increases water permeability of the collecting duct and distal tubule, promoting water reabsorption and producing concentrated urine. Its secretion is stimulated by increased plasma osmolarity or decreased blood volume. Aldosterone regulates sodium reabsorption. Parathyroid hormone and calcitonin regulate calcium.
This question belongs to: Science Biology
Question #214
The gap between two neurons across which nerve impulses pass is called:
A. Dendrite
B. Synapse
C. Axon
D. Soma
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Correct Answer: Option B

Explanation:
A synapse is the junction between two neurons (or neuron and effector) where the nerve impulse is transmitted from the presynaptic neuron to the postsynaptic neuron, usually via neurotransmitters. Axon carries impulses away from the cell body, dendrites receive signals, soma is the cell body. Synaptic transmission can be chemical or electrical.
This question belongs to: Science Biology
Question #215
Which part of the human brain controls involuntary functions like breathing and heartbeat?
A. Cerebrum
B. Hypothalamus
C. Cerebellum
D. Medulla oblongata
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Correct Answer: Option D

Explanation:
The medulla oblongata, part of the brainstem, contains vital centers that control respiration, heart rate, and blood pressure. It also regulates reflex actions like coughing, swallowing, and vomiting. The cerebrum is for voluntary actions and thinking, cerebellum for coordination, and hypothalamus for homeostasis and endocrine control.
This question belongs to: Science Biology
Question #216
Which hormone, produced by the alpha cells of the pancreas, raises blood glucose levels?
A. Pancreatic polypeptide
B. Glucagon
C. Somatostatin
D. Insulin
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Correct Answer: Option B

Explanation:
Glucagon is secreted by alpha cells of the islets of Langerhans when blood glucose is low. It acts on the liver to stimulate glycogenolysis (glycogen to glucose) and gluconeogenesis (synthesis of glucose from non-carbohydrates), thereby increasing blood glucose. Insulin, from beta cells, lowers blood glucose. Somatostatin inhibits both.
This question belongs to: Science Biology
Question #217
The hormone responsible for the development of secondary sexual characteristics in females is:
A. Testosterone
B. Prolactin
C. Estrogen
D. Progesterone
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Correct Answer: Option C

Explanation:
Estrogen, secreted by the developing ovarian follicles, promotes the development of female secondary sexual characteristics such as breast development, widening of hips, and distribution of body fat. Testosterone is the male sex hormone. Progesterone prepares the uterus for pregnancy, and prolactin stimulates milk production.
This question belongs to: Science Biology
Question #218
The process of spermatogenesis in the male testes is stimulated by:
A. Estrogen
B. Oxytocin
C. Prolactin
D. Follicle-stimulating hormone (FSH)
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Correct Answer: Option D

Explanation:
FSH, released from the anterior pituitary, stimulates Sertoli cells in the seminiferous tubules to support spermatogenesis (production of sperm). Luteinizing hormone (LH) stimulates Leydig cells to produce testosterone, which is also necessary for spermatogenesis. Estrogen is female hormone, prolactin for milk, oxytocin for uterine contraction.
This question belongs to: Science Biology
Question #219
The human ovum completes meiosis II at which stage?
A. After fertilization, upon sperm entry
B. Before ovulation
C. During embryonic development
D. At fertilization
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Correct Answer: Option A

Explanation:
The secondary oocyte is arrested in metaphase II of meiosis and is ovulated. Meiosis II is completed only when a sperm penetrates the oocyte, producing a mature ovum and a second polar body. If fertilization does not occur, the secondary oocyte degenerates. This ensures that the egg is only fully mature upon fertilization.
This question belongs to: Science Biology
Question #220
Which sexually transmitted disease is caused by the bacterium Treponema pallidum?
A. Syphilis
B. Genital herpes
C. Chlamydia
D. Gonorrhea
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Correct Answer: Option A

Explanation:
Syphilis is caused by the spirochete bacterium Treponema pallidum, typically transmitted through sexual contact. It progresses through primary, secondary, latent, and tertiary stages, potentially causing severe neurological and cardiovascular damage if untreated. Gonorrhea is by Neisseria gonorrhoeae, herpes by HSV, chlamydia by Chlamydia trachomatis. Penicillin is an effective treatment.
This question belongs to: Science Biology

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