Question

What is the role in vision of the cis-trans isomerization of retinal?

Short answer

Cis-trans isomerization of retinal converts light into a biochemical signal, which is then transmitted to the brain as vision.

Step-by-step solution

Understand Retinal

Retinal is a molecule derived from vitamin A, and it's a vital component of the photoreceptor cells in the retina. It's attached to a protein called opsin to form rhodopsin.

Cis-Trans Isomerization

The isomerization process involves a change in the retinal molecule from the 11-cis form to the all-trans form. This change is triggered by the absorption of light.

Light Absorption

When light photons hit the retinal within rhodopsin, it absorbs the energy and converts from the 11-cis form to the all-trans form. This isomerization changes the shape of retinal.

Signal Transduction

The change in shape of retinal activates opsin. This stimulates a cascade of biochemical reactions in the photoreceptor cell, ultimately leading to changes in the cell's membrane potential.

Visual Signal Transmission

The change in membrane potential generates an electrical signal that is transmitted through the optic nerve to the brain, where it is interpreted as vision.

Key concepts

cis-trans isomerization

The concept of cis-trans isomerization is crucial to understanding vision. Retinal, derived from vitamin A, exists in two forms: 11-cis and all-trans. In the dark, retinal is in the 11-cis form. When light hits it, the molecule absorbs energy, isomerizing to the all-trans form. This process changes the shape of the retinal molecule, playing a pivotal role in signal transduction. This shape change is a mechanical switch that sets off a series of reactions necessary for vision.

rhodopsin

Rhodopsin, also known as visual purple, is a pigment found in photoreceptor cells in the retina. It consists of the protein opsin bound to retinal. In darkness, rhodopsin remains inactive, with retinal in its 11-cis form. When light strikes, retinal changes to its all-trans form, causing rhodopsin to activate. This activation kicks off a chain reaction necessary for the phototransduction process.

photoreceptor cells

Photoreceptor cells are specialized neurons found in the retina. They are responsible for absorbing light and converting it into electrical signals.
There are two main types: rods and cones.
Rods are more sensitive to light and are crucial for night vision, while cones are responsible for color vision and function best in bright light.
Each photoreceptor contains rhodopsin, which initiates the visual signaling process upon light absorption.

signal transduction

In the context of vision, signal transduction refers to the series of biochemical events triggered when light hits the retinal molecule. The energy from light causes retinal to change from 11-cis to all-trans, activating the opsin protein.
The activated opsin then interacts with a G-protein called transducin. This sets off a cascade of intracellular events, including the activation of enzymes that ultimately change the ion permeability of the photoreceptor cell membrane.
This series of events transforms the light signal into an electrical signal.

visual signaling process

The visual signaling process transforms the light information into a form the brain can understand. Once the retinal changes shape and activates opsin, it creates a cascade of reactions in photoreceptor cells, leading to changes in membrane potential. These changes initiate electrical signals that travel through the optic nerve to the brain.
The brain then interprets these signals into the images we see. This entire process happens almost instantaneously, allowing us to respond quickly to visual stimuli.