Sight (vision) - Passage 2
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- Sight (vision) - Passage 2
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Sight (vision) - Passage 2
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[post_date] => 2024-12-26 10:58:06
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[post_content] => Practice Passage (Question 1-4)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
The phototransduction cascade is responsible for converting light rays into neural impulses. This cascade occurs within the rods and cones of the retina, and a diagram of the cascade is shown below:
Retinal is essential to this cascade, and is synthesized from vitamin A, which is found in certain foods like carrots. The cascade begins with light converting 11-cis retinal to all-trans retinal:
This activates transducin, leading to the activation of cGMP phosphodiesterase, the hydrolyzation of cGMP, and the closing of cGMP dependent Ca++/Na+ channels.
[post_title] => Sight (vision) - Passage 2
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[question] => If the cGMP-dependent Na+ channels involved in the phototransduction cascade were open in the absence of cGMP, what would occur if light was shined into the retina?
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[answer] => 2
[description] => Reason for the Correct Answer:
In order for a rod or cone to fire, Na+ channels must close.
The Na+ channels are always open, since they are open when light is absent, and when it is present (since in this case, the channels remain open in the absence of cGMP).
Since the cGMP-dependent Na+ channels are open when light is absent (cGMP levels are high) and when it is present (cGMP levels are low but the channel remains open), the cell will not fire an action potential.
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[each_answer] => A. Rods would fire in the presence of light, while cones would fire in the absence of light.
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[each_answer] => B. Both rods and cones would not fire in the presence of light.
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[2] => Array
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[each_answer] => C. Rods would fire in the absence of light, while cones would fire in the presence of light.
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[each_answer] => D. Both rods and cones would fire in the presence of light.
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[quiz_unique_key] => 3873426850
[question] => What specialized cellular structure is primarily involved in the phototransduction cascade?
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[answer] => 3
[description] => Reason for the Correct Answer:
There are hundreds of these structures per cell.
This structure contains specialized proteins, known as rhodopsin (in rods) or photopsin (in cones) embedded within the membrane, which contain a molecule known as retinal. Retinal is responsible for absorbing light and causing a chain of events that eventually allows the photoreceptor to activate.
These specialized structures are known as photoreceptor disks, and are stacked upon one another inside the cell body of a photoreceptor.
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[each_answer] => A. Actin cytoskeleton
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[each_answer] => B. Vacuoles
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[each_answer] => C. Photoreceptor disks
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[each_answer] => D. Mitochondria
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[quiz_unique_key] => 83407773
[question] => What happens to the cytoplasmic concentrations of sodium and calcium ions in photoreceptors at night?
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[answer] => 3
[description] => Reason for the Correct Answer:
Use the diagram in the passage.
The phototransduction cascade eventually leads to a decrease in the amount of cGMP molecules present in the cell, and thus closure of ion channels.
cGMP-dependent Na+ channels are mostly open when light is absent. Therefore, ions such as calcium and sodium are able to move into the cell, leading to an increase in their concentration intracellularly.
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[0] => Array
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[each_answer] => A. Remains the same
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[1] => Array
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[each_answer] => B. Both decrease
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[each_answer] => C. Both increase
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[each_answer] => D. Sodium increases while calcium decreases
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[quiz_unique_key] => 2261298308
[question] => High-resolution dependent vision, such as reading, relies on light entering the eye to fall on the fovea. Which of the following choices does not account for the fovea’s visual acuity?
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[answer] => 2
[description] => Reason for the Correct Answer:
The fovea is responsible for obtaining detailed information from an object (such as form, color), and thus has excellent spatial resolution.
The fovea is rich in cones. Although cones are fast-acting, they have a very slow recovery period. Therefore, their ability to fire successive action potentials is inhibited.
Since cones are not as adept as rods at detecting light and changes in light, they result in poor temporal resolution, despite having excellent spatial resolution.
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[each_answer] => A. The fovea is rich in cones, resulting in color vision.
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[each_answer] => B. The fovea has excellent temporal resolution.
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[each_answer] => C. The fovea lacks a layer of neurons and axons that is present in the rest of the retina.
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[each_answer] => D. The fovea lies along the optical axis.
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