12.16.2 Photoreceptors and the Vertebrate Eye

Question 1

Photoreceptors and the Vertebrate Eye

Answer 1

• Eyes are antennas for light. The most primitive eyes are eyespots, which are photosensitive pigments embedded in cell membranes.
• Arthropods have compound eyes composed of thousands of ommatidia.
• The structure of the vertebrate eye reflects its function—to focus light on rhodopsin.
• The major light-absorbing structure within the vertebrate eye is the retina. Retinal cells are modified neurons. They include highly light-sensitive rod cells and cone cells specialized for color vision and sharpness.

Question 2

The Euglena

Answer 2

• a one-celled flagellated organism. These organisms each have an eyespot, a structure that contains a
  group of photosensitive pigments embedded in the
  membrane.
• The flatworm has photosensitive eye cups that relay
  information to the worm’s primitive nervous system.

Question 3

eyes

Answer 3

• Arthropods have compound eyes. A compound eye is made up of many smaller units called ommatidia. A single ommatidium is made up of a lens and associated receptor proteins.
• The ommatidium is made up of seven retinula cells. Retinula cells are elongated and surround the crystalline cone. The crystalline cone focuses light on the center of the ring of retinula cells.
• The inner edges of the retinula cells are covered in microvilli. These microvilli contain rhodopsin. The microvilli from different cells overlap forming the rhabdom. The light signals are channeled to nerve axons at the base of the ommatidia and then to the arthropod’s brain.

Question 4

note

Answer 4

• Light passes through the cornea and the lens. The lens, convex in shape, projects the image upside down. The image passes through the vitreous humor and is focused on the back of the eye, the retina (labeled retinal layer). The message goes from the retina to the brain via the optic nerve.
• Embedded in the retina are retinal cells. Retinal cells are modified neurons (top left diagram). There are two types of retinal cells, rods and cones.
• In the human, there are about 3 million cones in the center of the fovea. Cones function in perceiving color and providing sharpness of the image. Rods are found outside of the fovea and do not perceive color and are very light sensitive.
• Light travels through the first four layers of the retina where it is absorbed by rods and cones. The sensory signal is then processed back from the rods and cones, through the four cell layers and after reaching the ganglion cells, it is finally sent to the optic nerve.

Question 5

Compound eyes are found on

Answer 5

• arthropods

Question 6

True or false?

There are more rod cells than cone cells in the human retina.

Answer 6

• true

Question 7

How many retinula cells make up an ommatidium?

Answer 7

• 7

Question 8

Which of the following statements regarding the human retina is false?

Answer 8

• The sensory signal is passed directly from the rod cells to the ganglion cells.

Question 9

True or false?

Rods are molecules that sense light but do not distinguish color.

Answer 9

• false