Photoreceptors 1

What are the main sensory modalities discussed in this lecture?

Vision, taste, smell, hearing, touch, chemoreception, mechanoreception, magnetoreception, and electroreception.

How is “light” defined in the context of sensory ecology?

Light is a form of electromagnetic radiation, consisting of energy waves that travel at approximately 299,792,458 m/s in a vacuum.

What are the three primary properties of a beam of light?

Intensity, wavelength (or frequency), and polarization.

How does light intensity relate to photoreceptors?

Intensity refers to the number of photons striking a surface, which is critical since photoreceptors detect light by counting photons.

How are wavelength and frequency related in the electromagnetic spectrum?

They are inversely related; a shorter wavelength corresponds to a higher frequency and vice versa.

What equation is used to calculate the energy of a photon?

The Planck-Einstein equation: E = hf, where h is Planck’s constant and f is the frequency.

What is the significance of ultraviolet (UV) light in biological contexts?

UV light has a short wavelength and high energy, capable of breaking covalent bonds and potentially damaging DNA and other cellular structures.

How does infrared (IR) light interact with matter?

Infrared light increases molecular movement, which is important for temperature regulation.

What is polarization of light?

Polarization refers to the orientation of the light wave’s oscillating electric field; polarized light oscillates in only one plane.

How can polarized light be generated in nature?

Light reflecting off surfaces, like water, becomes at least partially polarized, and skylight polarization patterns arise from sunlight’s interaction with the atmosphere and ozone.

Why is polarization important for certain animals?

Many insects, crustaceans, and some arachnids can detect polarized light, which they use for navigation, prey detection, and other behaviors.

What is phototransduction?

Phototransduction is the process by which light energy is converted into neural signals in the retina.

What are photopigments, and what do they consist of?

Photopigments are molecules in photoreceptor cells made up of an opsin protein bound to a chromophore (usually retinal) that confer light sensitivity.

What happens when a photopigment absorbs a photon?

Absorption of a photon causes a conformational change in the chromophore, triggering a chemical cascade that converts the light signal into an electrical signal.

How are opsins structurally characterized?

Opsins are G-protein-coupled receptors (GPCRs) with seven transmembrane domains forming a binding pocket for the chromophore.

What are the two major types of photoreceptor cells in vertebrates?

Rods and cones.

What are the primary functions of rod cells?

Rods are highly sensitive to light, enabling vision in low-light conditions, but they do not contribute to color vision and provide lower spatial resolution.

What role do cone cells play in vision?

Cones are responsible for high-resolution and color vision, though they require more light to function effectively.

What are the different opsin gene classes associated with vertebrate cone cells?

Cone opsins include SWS1 (UV/very short wavelengths), SWS2 (short wavelengths), RhB/Rh2 (medium wavelengths), and LWS/MWS (longer wavelengths).

Why do many mammals have a reduced color palette compared to other vertebrates?

Early mammals experienced a “nocturnal bottleneck,” where adaptations for low-light vision led to a reduction in the diversity of cone opsins.

How did trichromacy re-evolve in Old-World primates?

Trichromacy was achieved when the ancestral long-wavelength opsin split into two genes (M and L opsins) via gene duplication or unequal crossing-over, enabling sensitivity to three distinct wavelengths.

What is the “Young Leaf Hypothesis” in relation to trichromacy?

It suggests that trichromatic vision evolved to help primates detect nutrient-rich, young, red leaves during food shortages, thereby enhancing survival.

What are the two main types of photoreceptor cells in invertebrates?

Invertebrates possess microvillar (rhabdomeric) and ciliary photoreceptors, each with distinct strategies to maximize membrane surface area and optimize light detection.

How do optical specializations in photoreceptor cells enhance visual performance?

Specializations such as colored oil droplets in vertebrates or modified cellular structures (like chromatin organization in nocturnal mammals) improve spectral filtering, focus light, or enhance sensitivity.

How does refraction affect light in the context of vision?

Refraction occurs when light changes speed as it passes through different media (e.g., air to glass), bending the light rays and aiding in the focusing mechanism of eye