Which one of the following depicts the correct circuit of a reflex arc?
- A.Effector—sensory neuron—spinal cord—motor neuron—receptor
- B.Receptor—sensory neuron—spinal cord—motor neuron—effector
- C.Receptor—sensory neuron—brain—motor neuron—effector
- D.Sensory neuron—receptor—brain—effector—motor neuron
Correct answer
Explanation
The correct answer is B, Receptor—sensory neuron—spinal cord—motor neuron—effector. In a reflex arc the message runs from the sense organ to the spinal cord and straight back to a muscle or gland.
A reflex is a quick, automatic response, such as pulling the hand away from a hot object. The receptor in the skin picks up the stimulus, a sensory neuron carries the impulse to the spinal cord, a relay neuron there passes it to a motor neuron, and the motor neuron carries the command to the effector, usually a muscle, which acts. Because the spinal cord handles the response without waiting for the brain, the action is very fast; the brain learns of it only afterwards.
A is wrong because it runs the path backwards, starting at the effector and ending at the receptor.
C is wrong because a simple spinal reflex is processed in the spinal cord, not the brain.
D is wrong because it puts the sensory neuron before the receptor and the effector before the motor neuron.
Exam tip: reflex arc order: receptor → sensory neuron → spinal cord → motor neuron → effector.
Practice Questions
View allWhich one among the following statements is not correct?
- A.The reaction of methane and chlorine in sunlight produces hydrogen gas.
- B.Palladium catalyzes the addition of hydrogen gas on an alkene.
- C.Ethanol burns in oxygen to give carbon dioxide, water and releases heat and light.
- D.Unsaturated hydrocarbons burn with a sooty flame.
Show answer
Methane and chlorine react in sunlight to give chloromethane and hydrogen chloride, not hydrogen gas. This is a substitution reaction: CH4 + Cl2 → CH3Cl + HCl in the presence of sunlight.
Read the full explanation →Which of the following statements relating to carbon and its compounds are correct? 1. 1-Propyne has three double bonds. 2. The boiling point of acetic acid is higher than methane. 3. Ethanol reacts with sodium metal and produces hydrogen gas. 4. Esters give effervescence on reaction with sodium bicarbonate. Select the answer using the code given below.
- A.1 and 3
- B.2 and 3 only
- C.1 and 2
- D.2, 3 and 4
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Acetic acid boils far above methane, and ethanol gives hydrogen with sodium, while statements 1 and 4 are false. Propyne (CH3–C≡CH) is an alkyne: it has one carbon–carbon triple bond and no double bonds, so statement 1 is wrong.
Read the full explanation →Which one among the following statements regarding gypsum is not correct?
- A.On heating gypsum at 373 K, it loses water molecules and becomes CaSO4 · H2O.
- B.On mixing with water, plaster of Paris changes to gypsum.
- C.Plaster of Paris is used for supporting fractured bones.
- D.Although gypsum possesses water of crystallization, it is not a liquid at room temperature.
Show answer
Gypsum heated at 373 K becomes calcium sulphate hemihydrate, CaSO4 · ½H2O, which is plaster of Paris, not CaSO4 · H2O as the statement says.
Read the full explanation →Which of the following is/are the basic step(s) in genetically modifying an organism? 1. Identification of DNA with desirable genes 2. Introduction of the identified DNA into the host 3. Maintenance of introduced DNA in the host and transfer of the DNA to its progeny Select the correct answer using the code given below.
- A.1 only
- B.1, 2 and 3
- C.2 and 3 only
- D.3 only
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Making a genetically modified organism needs all three steps: find the useful DNA, put it into the host, and make sure the host keeps it and passes it on.
Read the full explanation →If one set of chromosomes for a given plant is represented as N; in case of double fertilization, the zygote and the endosperm nucleus of a diploid plant would have how many sets of chromosomes respectively?
- A.N and 2N
- B.2N and 2N
- C.N and 3N
- D.2N and 3N
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The zygote is diploid (2N) and the primary endosperm nucleus is triploid (3N). Double fertilisation is a feature of flowering plants.
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