All Free Biology MCQs with Answers

121. Visual acuity is greatest at the fovea because it contains

• A. only rods, packed very closely
• B. no photoreceptors at all
• C. densely packed cones, each with its own connection to the optic nerve
• D. the point where the optic nerve leaves the eye

Explanation: At the fovea the cones are crowded together and each has a private line to the brain, so two nearby points of light stimulate two separate pathways and are seen as separate. Rods share connections, which improves sensitivity but destroys detail. The place with no photoreceptors is the blind spot, where the optic nerve leaves, and nothing falling there is seen at all.

122. A receptor is best described as a structure that

• A. detects a change in the environment and converts it into a nerve impulse
• B. carries impulses from the spinal cord to a muscle
• C. produces a hormone in response to a stimulus
• D. connects two sensory neurones together

Explanation: Receptors are transducers: each responds to one kind of stimulus, such as light, sound, pressure or a chemical, and converts that energy into the electrical signal the nervous system can carry. A structure that carries impulses to a muscle is a motor neurone, and a structure that produces a hormone is a gland. The receptor is therefore always the first component of any reflex pathway.

123. In a neurone, impulses are carried away from the cell body by the

• A. dendrites
• B. dendrons
• C. nucleus
• D. axon

Explanation: Each neurone has one long axon that conducts the impulse away towards the next cell, and many short branched dendrites that collect signals and carry them towards the cell body. The direction of travel is what defines the two, not the length. The axon ends in swellings that release neurotransmitter into a synapse or onto a muscle fibre.

124. A myelinated neurone conducts impulses faster than an unmyelinated one because

• A. the impulse jumps from one node of Ranvier to the next
• B. myelin conducts electricity better than cytoplasm
• C. myelinated axons are always much thinner
• D. myelin increases the number of sodium channels along the whole axon

Explanation: Myelin is an insulator, so ions can cross the membrane only at the gaps between Schwann cells, and depolarisation therefore leaps from node to node in what is called saltatory conduction. This can raise conduction speed above 100 metres per second, against a few metres per second in an unmyelinated fibre of the same diameter. Loss of myelin in multiple sclerosis slows or blocks conduction.

125. The resting potential of about minus 70 millivolts across the membrane of a neurone is maintained mainly by

• A. the free diffusion of sodium into the cell
• B. the sodium potassium pump, which moves three sodium ions out for every two potassium ions in
• C. the opening of voltage gated sodium channels
• D. the release of neurotransmitter at the synapse

Explanation: The pump uses ATP to move more positive charge out than it brings in, and since the membrane is far more permeable to potassium, which leaks back out, the inside is left negative relative to the outside. Voltage gated sodium channels are shut at rest and open only when the threshold is reached, which is what produces the action potential rather than the resting state.

126. The rapid depolarisation phase of an action potential is caused by

• A. potassium ions rushing out of the axon
• B. chloride ions entering the axon
• C. sodium ions rushing into the axon through voltage gated channels
• D. the sodium potassium pump reversing direction

Explanation: When the membrane is depolarised to threshold, voltage gated sodium channels open and sodium floods in down both its concentration and its electrical gradient, driving the potential to about plus 40 millivolts. Those channels then close and potassium channels open, so potassium leaves and the membrane repolarises. The pump restores the original ion distribution afterwards but is far too slow to create the spike itself.

127. The all or none law of nerve impulse conduction states that

• A. a stronger stimulus produces a larger action potential
• B. an impulse can travel in either direction along the axon
• C. every stimulus produces an impulse regardless of its strength
• D. any stimulus at or above threshold produces an action potential of the same size

Explanation: Once threshold is reached the sodium channels open fully, so the size of the spike is fixed by the ion gradients and not by how strong the stimulus was. Stimulus intensity is coded instead by the frequency of impulses and by how many neurones are firing. A stimulus below threshold produces no impulse at all, so it is not true that every stimulus produces one.

128. During the refractory period a neurone

• A. cannot be stimulated again, which keeps the impulse travelling in one direction
• B. conducts impulses at double speed
• C. releases neurotransmitter continuously
• D. reverses its resting potential permanently

Explanation: For a brief interval after an action potential the sodium channels are inactivated and cannot reopen, so the region just passed cannot fire again and the impulse can only move forwards. The same period also sets an upper limit on how many impulses a second an axon can carry, which limits how strong a sensation can be coded. The membrane returns fully to its resting state afterwards.

129. Transmission across a chemical synapse is one way because

• A. the postsynaptic membrane is thicker
• B. the neurotransmitter is released only from the presynaptic knob and receptors are only on the postsynaptic membrane
• C. the synaptic cleft is too wide for diffusion in the reverse direction
• D. the impulse loses all its energy at the synapse

Explanation: Vesicles of transmitter are stored on one side only and the matching receptors are on the other, so the signal cannot cross backwards even though diffusion itself has no preferred direction. This one way valve is what gives the nervous system its organised pathways. The cleft is only about 20 nanometres wide, so diffusion across it takes well under a millisecond.

130. The correct sequence of components in a spinal reflex arc is

• A. effector, motor neurone, spinal cord, sensory neurone, receptor
• B. receptor, motor neurone, spinal cord, sensory neurone, effector
• C. receptor, sensory neurone, interneurone in the spinal cord, motor neurone, effector
• D. sensory neurone, receptor, effector, motor neurone, spinal cord

Explanation: The stimulus is detected by the receptor, carried into the spinal cord by the sensory neurone through the dorsal root, passed to a relay neurone and then out along the motor neurone through the ventral root to the muscle or gland that responds. Sensory always means towards the central nervous system and motor always means away from it, which is enough to rule out the reversed options.