Detecting and Amplifying an
Environmental Stimulus
CfE Advanced Higher Biology
Cells and Proteins
Unit 1: Subtopic 2 d
Detection of Light
• Light detectors in the animal kingdom range
from a simple group of cell that detect only
the direction and the intensity of light to
complex organs that can form images.
• Photoreceptor proteins are light-sensitive
proteins involved in the sensing and response
to light in a variety of organisms.
• They are found across 3 domains
The 3 domains
• Organisms have been evolving into 3
independent lineages for over 1.5 billion years
• 3 distinct groups
– Bacteria
– Archaea
– Eukarya (Eukaryotes)
• In archaea, bacteriorhodopsin molecules
generate potential differences by absorbing
light to pump protons across the membrane.
• Bacteriorhodopsin is a protein molecule that
acts as a proton pump. It captures light energy
and uses it to move protons across the
membrane out of the cell.
• The resulting proton gradient is subsequently
converted into chemical energy.
Archaea
Plants
• In plants the light absorbed by photosynthetic pigments within
protein systems drives an electron flow that pumps hydrogen ions
across the thylakoid membrane of the chloroplast.
• A thylakoid is a membrane-bound compartment inside
chloroplasts and cyanobacteria. They are the site of the lightdependent reactions of photosynthesis.
• Thylakoids consist of a thylakoid membrane surrounding a
thylakoid lumen. Chloroplast thylakoids frequently form stacks of
disks referred to as grana (singular: granum). Grana are connected
by intergranal or stroma thylakoids, which join granum stacks
together as a single functional compartment.. In both cases the
resulting diffusion of hydrogen ions back across the membrane
drives ATP synthase.
•
In plants the light absorbed by photosynthetic pigments
within protein systems drives an electron flow that pumps
hydrogen ions across the thylakoid membrane of the
chloroplast.
In both cases the resulting diffusion of hydrogen ions back
across the membrane drives ATP synthase.
Structure of the Chloroplast
Inner Membrane
Granum – stack
of thykaloids
Thykaloid
Lamellae
Lumen
(inside of
Animals
•
In animals there are two types of photoreceptors, rods and cones. Each rod or
cone cell has an outer segment with stacked folded membrane with or disc in
which visual pigments are embedded.
•
Cones are used for colour vision and function in bright light during the day. They
contain the light-sensitive molecule retinal which is combined with a membrane
protein called opsin. In cone cells, different forms of opsin give sensitivity to
specific wavelengths (red, green, blue or UV light).
•
•
Rod cells are very sensitive to light and function in black and white vision at night.
Visual pigments are embedded in folded membranes comprising of a stack of discs
in the outer segment of each rod and cone.
•
•
In cone cells, different forms of opsin give sensitivity to specific wavelengths.
In rod cells, the rhodopsin gives a wider range of wavelengths and a greater
degree of amplification by the protein cascade results in sensitivity at low light
intensities.
Retinal – a light absorbing pigment molecule (derivative of
vitamin A) bonded to a membrane protein called an opsin.
Rods contain their own type of opsin which combined with
retinal makes up the visual pigment rhodopsin.
When rhodopsin absorbs light, its retinal component
changes shape, triggering a signal transduction pathway that
ultimately results in a receptor potential in the rod cell
membrane.
• Intially, retinal’s shape change causes a
conformational change in its opsin partner. The
altered opsin molecule then activates a relay
molecule in the signal transduction pathway, a G
protein called transducin, which is also in the disc
membrane.
• In turn transducin activates an effector enzyme
that chemically alters the second messenger in
the rod cell, a nucleotide called cyclic guanosine
monophosphate (cGMP)
• Bacteriorhodopsin is a
protein used by Archaea,
the most notable one being
Halobacteria. It is built into
the plasma membrane, so
it is an intergal membrane
protein.
• Structurally related to
visual pigments in the
retina of the eye
Mechanism
• Bacteriorhodopsin absorbs light and uses the
energy to pump hydrogen ions (H+) out of the
cell. The H+ gradient the drives the synthesis of
ATP.
• This is the simplest known mechanism of
photophosphorylation.
• The resulting proton gradient is subsequently
converted into chemical energy.[2]
• Researchers are studying halophiles as model
systems for solar energy conversion