The human retina in sufficient detail to show how differences in…

The idea

Rods and cones differ in pigment and wiring, explaining sensitivity, colour vision and acuity. Rods contain rhodopsin, which is broken down by light and is sensitive at low light intensities; this pigment enables dim-light vision. Many rods share a single bipolar neurone, allowing spatial summation of generator potentials; this gives high sensitivity in dim light but low acuity. The brain cannot distinguish which rod was stimulated, so nearby points produce a single impulse. Rods provide monochrome vision. Cones contain iodopsin, requiring higher light intensity, so they function in bright light. Each cone usually connects to its own bipolar neurone. Thus, adjacent cones send separate impulses and are resolved as two points, giving high visual acuity. Three cone types with different optical pigments allow colour vision.

What earns the marks

• Cones are the photoreceptors involved in colour vision.
• Each type of cone contains a different optical pigment, absorbing a different range of wavelengths.
• Colour perception depends on the relative stimulation of the three cone types and the resulting impulses sent to the brain.
• Many rods connect to one bipolar neurone, providing spatial summation and high sensitivity but low visual acuity.
• One cone connects to one bipolar neurone, so two stimulated cones are resolved as two separate points, giving high acuity.
• Rhodopsin is broken down by light and is sensitive at low light intensities, enabling rods to function in dim light.

Common mistakes and how to put them right

• Writing 'red cones' and 'green cones' rather than 'red-sensitive' and 'green-sensitive' cones, which lacks precision.

• Writing signals or messages instead of impulses or action potentials.

• Saying rods have low acuity because there are fewer of them, rather than because many share one bipolar neurone.

• Claiming a single cone detects the colour orange, when colour perception arises from the relative stimulation of three cone types.

• Confusing sensitivity with acuity, so summation is incorrectly offered as the reason for sharp vision.

• Stating that rhodopsin is broken down by low light, when it is broken down by light and is sensitive at low light intensities.

Exam tips

• Use the term 'red-sensitive cone' rather than 'red cone' to accurately describe the receptor's function.
• When reading an absorption spectrum, quote approximate percentage values at the wavelength given to support comparisons.
• Answer acuity questions by discussing neuronal connections (wiring) and sensitivity questions by discussing pigments and spatial summation.