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 40 of 103
Question #781
Bile juice is secreted by the:
A. Gall bladder
B. Pancreas
C. Liver
D. Duodenum
Discuss
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Correct Answer: Option C

Explanation:
Bile juice is continuously secreted by the liver. It is stored and concentrated in the gall bladder and released into the duodenum (first part of the small intestine) when food enters. Bile contains bile salts (like bilirubin and biliverdin) which are crucial for the emulsification of fats, breaking them into smaller micelles for easier digestion by lipases.
This question belongs to: Science Biology
Question #782
Which enzyme in pancreatic juice is responsible for digesting proteins into peptides?
A. Trypsin
B. Nuclease
C. Pancreatic lipase
D. Pancreatic amylase
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Correct Answer: Option A

Explanation:
Pancreatic juice contains inactive enzymes like trypsinogen, chymotrypsinogen, and procarboxypeptidase. When trypsinogen enters the small intestine, it is activated into trypsin by the enzyme enterokinase (secreted by the intestinal mucosa). Trypsin then digests proteins into smaller peptides. Pancreatic amylase digests carbohydrates, and lipase digests fats.
This question belongs to: Science Biology
Question #783
The actual site of gas exchange in the mammalian respiratory system is:
A. Larynx
B. Trachea
C. Alveoli
D. Bronchi
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Correct Answer: Option C

Explanation:
The alveoli are the terminal, microscopic, thin-walled, highly vascularized air sacs in the lungs. They are the primary and actual sites of gas exchange, where oxygen diffuses from the inhaled air into the blood, and carbon dioxide diffuses from the blood into the alveolar air to be exhaled. Their large surface area and thin walls facilitate this diffusion.
This question belongs to: Science Biology
Question #784
During inspiration, the diaphragm:
A. Remains unchanged
B. Moves laterally
C. Contracts and flattens
D. Relaxes and becomes dome-shaped
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Correct Answer: Option C

Explanation:
Inspiration (inhalation) occurs when the intra-pulmonary pressure is less than the atmospheric pressure. This is achieved by the contraction of the diaphragm, which flattens and moves downwards, and the contraction of external intercostal muscles, which lift the ribs and sternum. This increases the thoracic volume and lung capacity, drawing air in. Expiration involves relaxation of these muscles.
This question belongs to: Science Biology
Question #785
The majority of carbon dioxide (CO2) in the blood is transported as:
A. Carbaminohemoglobin
B. Carbonic acid
C. Bicarbonate ions (HCO3-)
D. Dissolved CO2 in plasma
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Correct Answer: Option C

Explanation:
Approximately 70% of CO2 is transported in the blood as bicarbonate ions (HCO3-). CO2 diffuses into red blood cells, where it reacts with water to form carbonic acid, catalyzed by the enzyme carbonic anhydrase. Carbonic acid then dissociates into H+ and HCO3-, which moves into the plasma. About 20-25% is carried as carbaminohemoglobin.
This question belongs to: Science Biology
Question #786
Which chamber of the human heart has the thickest muscular wall?
A. Right ventricle
B. Right atrium
C. Left ventricle
D. Left atrium
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Correct Answer: Option C

Explanation:
The left ventricle has the thickest and most muscular wall of all the heart chambers. This is because it must pump oxygenated blood with high pressure through the aorta to supply the entire systemic circulation of the body. The right ventricle only pumps blood to the lungs, which requires much less pressure.
This question belongs to: Science Biology
Question #787
The 'P' wave in a standard ECG represents:
A. Atrial repolarization
B. Ventricular depolarization
C. Ventricular repolarization
D. Atrial depolarization
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Correct Answer: Option D

Explanation:
An Electrocardiogram (ECG or EKG) is a graphical record of the heart's electrical activity. The 'P' wave represents the depolarization of the atria, which leads to atrial contraction. The QRS complex represents ventricular depolarization, and the 'T' wave represents ventricular repolarization, which leads to ventricular relaxation.
This question belongs to: Science Biology
Question #788
Which of the following is the most abundant type of white blood cell (WBC) in human blood?
A. Neutrophils
B. Monocytes
C. Eosinophils
D. Lymphocytes
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Correct Answer: Option A

Explanation:
Neutrophils are the most abundant white blood cells, constituting about 60-65% of the total WBC count. They are polymorphonuclear and are highly phagocytic, acting as the first line of defense against bacterial infections. Lymphocytes are the second most abundant (20-25%), while monocytes and eosinophils are present in smaller percentages.
This question belongs to: Science Biology
Question #789
A person with blood group 'O' is considered a universal donor because:
A. Their RBCs lack A and B antigens
B. Their blood has no Rh factor
C. Their plasma lacks antibodies
D. Their RBCs have both A and B antigens
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Correct Answer: Option A

Explanation:
Individuals with blood group 'O' have red blood cells that lack both A and B antigens on their surface. Because there are no antigens to react with the recipient's antibodies, their blood can be safely transfused to individuals of any ABO blood group in emergencies. However, their plasma contains both anti-A and anti-B antibodies.
This question belongs to: Science Biology
Question #790
Which ion is essential for the blood clotting process?
A. Calcium
B. Magnesium
C. Potassium
D. Sodium
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Correct Answer: Option A

Explanation:
Calcium ions (Ca2+) play a crucial role in the blood clotting (coagulation) cascade. They are required for the conversion of prothrombin to thrombin, and fibrinogen to fibrin. Without calcium, the clotting process cannot proceed, which is why calcium salts are often added to blood collection tubes to prevent clotting during tests.
This question belongs to: Science Biology
Question #791
The structural and functional unit of the kidney is the:
A. Villus
B. Neuron
C. Nephron
D. Alveolus
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Correct Answer: Option C

Explanation:
The nephron is the fundamental structural and functional unit of the kidney. Each kidney contains about one million nephrons. A nephron consists of two main parts: the renal corpuscle (Malpighian body, containing the glomerulus and Bowman's capsule) and the renal tubule (PCT, Loop of Henle, DCT). It is responsible for urine formation.
This question belongs to: Science Biology
Question #792
The normal Glomerular Filtration Rate (GFR) in a healthy adult is approximately:
A. 250 ml/min
B. 125 ml/min
C. 75 ml/min
D. 500 ml/min
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Correct Answer: Option B

Explanation:
The Glomerular Filtration Rate (GFR) is the volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time. In a healthy adult, the normal GFR is approximately 125 ml per minute, which amounts to about 180 liters per day. Almost all of this filtrate is reabsorbed, leaving only 1.5 liters of urine.
This question belongs to: Science Biology
Question #793
The counter-current mechanism in the kidney is primarily associated with which structure?
A. Glomerulus
B. Collecting Duct
C. Loop of Henle and Vasa Recta
D. Proximal Convoluted Tubule
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Correct Answer: Option C

Explanation:
The counter-current mechanism involves the Loop of Henle and the vasa recta (blood capillaries surrounding the loop). The flow of filtrate in the two limbs of the Loop of Henle is in opposite directions, and the flow of blood in the two limbs of vasa recta is also counter-current. This mechanism helps maintain a high osmolarity gradient in the medullary interstitium, essential for concentrating urine.
This question belongs to: Science Biology
Question #794
Antidiuretic Hormone (ADH) primarily acts on the kidney to:
A. Decrease potassium reabsorption
B. Increase sodium excretion
C. Increase glucose excretion
D. Increase water reabsorption
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Correct Answer: Option D

Explanation:
Antidiuretic Hormone (ADH), also known as vasopressin, is released by the posterior pituitary in response to osmotic pressure or fluid loss. It acts primarily on the collecting ducts of the nephrons to increase water reabsorption by making them more permeable to water. This prevents diuresis (excessive urine production) and helps conserve body water.
This question belongs to: Science Biology
Question #795
The long, cylindrical, highly specialized part of a neuron that conducts nerve impulses away from the cell body is the:
A. Dendrite
B. Axon
C. Myelin sheath
D. Soma
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Correct Answer: Option B

Explanation:
A neuron consists of a cell body (soma), dendrites, and an axon. Dendrites are short, branched processes that receive impulses and conduct them towards the cell body. The axon is a long, single, cylindrical fiber that conducts nerve impulses away from the cell body to other neurons, muscles, or glands. The myelin sheath is an insulating layer around the axon.
This question belongs to: Science Biology
Question #796
During the depolarization phase of an action potential, there is a rapid influx of:
A. Sodium ions (Na+)
B. Chloride ions (Cl-)
C. Potassium ions (K+)
D. Calcium ions (Ca2+)
Discuss
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Correct Answer: Option A

Explanation:
An action potential is initiated when a stimulus causes the membrane potential to reach a threshold. This opens voltage-gated sodium channels, leading to a rapid influx of sodium ions (Na+) into the neuron. This makes the inside of the cell positively charged relative to the outside, a process called depolarization. Repolarization follows with the efflux of K+ ions.
This question belongs to: Science Biology
Question #797
The gap between the axon terminal of one neuron and the dendrite of another neuron is called the:
A. Node of Ranvier
B. Synaptic cleft
C. Axolemma
D. Myelin sheath
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Correct Answer: Option B

Explanation:
A synapse is the junction where a nerve impulse is transmitted from one neuron to another. The synaptic cleft is the microscopic, fluid-filled gap between the presynaptic neuron (axon terminal) and the postsynaptic neuron (dendrite or cell body). Neurotransmitters are released into this cleft to carry the signal across to the next cell.
This question belongs to: Science Biology
Question #798
Which part of the human brain is primarily responsible for maintaining balance and posture?
A. Medulla oblongata
B. Hypothalamus
C. Cerebrum
D. Cerebellum
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Correct Answer: Option D

Explanation:
The cerebellum is a highly folded structure located at the back of the brain, below the cerebrum. It is primarily responsible for coordinating voluntary movements, maintaining balance, posture, and equilibrium. Damage to the cerebellum results in ataxia (lack of muscle coordination). The medulla controls vital involuntary functions like breathing and heart rate.
This question belongs to: Science Biology
Question #799
In a reflex arc, the component that carries the response signal to the effector organ is the:
A. Receptor
B. Motor neuron
C. Interneuron
D. Sensory neuron
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Correct Answer: Option B

Explanation:
A reflex arc is the neural pathway that controls a reflex action. It typically involves: a receptor (detects stimulus), a sensory neuron (carries impulse to CNS), an interneuron (in the spinal cord), a motor neuron (carries impulse away from CNS), and an effector organ (muscle or gland that responds). The motor neuron is responsible for the final output.
This question belongs to: Science Biology
Question #800
Which hormone is secreted by the anterior pituitary and stimulates growth of bones and tissues?
A. Thyroid Stimulating Hormone (TSH)
B. Growth Hormone (GH)
C. Follicle Stimulating Hormone (FSH)
D. Adrenocorticotropic Hormone (ACTH)
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Correct Answer: Option B

Explanation:
Growth Hormone (GH), also known as somatotropin, is secreted by the anterior pituitary gland. It stimulates growth, cell reproduction, and cell regeneration, particularly in bones and skeletal muscle. Overproduction in childhood leads to gigantism, while underproduction leads to pituitary dwarfism. TSH, ACTH, and FSH are other anterior pituitary hormones with different target organs.
This question belongs to: Science Biology

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