1. AH Biology: Unit 1 Proteins Topic 4
Detecting and amplifying an environmental stimulus

2. Detecting and amplifying an environmental stimulus LOs
Photoreceptor protein systems are found across the three domains.
In archaea, bacteriorhodopsin molecules generate potential differences by absorbing light to pump protons across the membrane.
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.

3. Photoreceptor proteins
Light is an abundant and readily available source of energy and as such it is not surprising to find systems to harness that energy across all three domains of life.

4. Photoreceptor systems
Photoreceptor cells can absorb photons (light) and convert it into another signal. For example photoreceptor systems are the basis of our vision.
Photoreceptor systems are found across the three domains: Archaea, Prokaryota and Eukaryota.
We will consider the way light is detected and used in:
Archaea (a group of prokaryotic organisms)
Eukaryotes
Animals
Plants.

5. 1. Photoreceptors in archaea

6. Archaea revision
The Archaea are a group of single-celled organisms with no membrane-bound organelles or defined nucleus. They share these features with the Prokaryotes, although modern evolutionary phylogeny regards them as having a separate evolutionary path.
Many Archaea were thought to be extremophiles found only in high-saltwater regions or at very high temperature. They are now known to occupy many niches.
Some groups, eg the Haloarchae, can photosynthesise.
The photosynthetic nature of the Haloarchea relies on bacterial rhodopsin causing activation of ATP synthase.

7. Archaea
In archaea, bacteriorhodosin molecules absorb light to actively pump H+ ions (protons) across the membrane. This creates a potential difference across the cell membrane that can be used to drive ATP synthase.

9. Bacterial rhodopsin
Bacterial rhodopsin consists of retinal, a light-sensitive chromophore (the part of a molecule responsible for its colour), sitting within a transmembrane protein bacterial opsin. (a protein which forms part of the visual pigment rhodopsin and is released by the action of light)
The retinal–opsin complex is called rhodopsin.
The photoisomerisation (change driven by the absorption of light) of retinal results in a conformational change in the opsin, causing it to pump protons(positively charged particles) from the intracellular compartment to the outside.

10. Bacterial rhodopsin
When sunlight strikes a bacterial rhodopsin molecule the bound retinal undergoes photoisomerisation. The resultant shape change causes the rhodopsin molecule to be activated.
The activated rhodopsin pumps protons (H+) out of the cell.
The electrochemical gradient causes the proton to flow back into the cell, driving ATP synthase.
ATP synthase complexes with Pi and ADP.
ATP is generated.
GFDL image by Kirsten Carlson, MBARI (© 2001).

11. 2. Photoreceptors in plants

12. Photosynthesis in plants
Photosynthesis in plants relies on light energy being converted into chemical energy.
Chloroplasts contain photosynthetic pigments, mainly chlorophyll a and accessory pigments.
These pigments are packed into thylakoid membrane-bound stacks called grana.
The light energy trapped by these pigments is used to split water and to generate ATP and NADPH.
By Kristian Peters -- Fabelfroh (photographed by myself) [GFDL ( or CC-BY-SA-3.0 (
via Wikimedia Commons

13. Photoreceptors in plants
In plants this takes place specifically in the Thylakoid membranes of the chloroplast using photosynthetic pigments to absorb the light energy.

14. Light energy absorbed in grana
Photolysis
Water is split and oxygen is lost as oxygen gas. Hydrogen and electrons are produced and used to reduce NADP to form NADPH. This reduced carrier delivers hydrogen and electrons to the light-independent reactions involved in carbon fixation (Calvin cycle).
Photophosphorylation
Light energy is also used to generate two molecules of ATP from ADP. This process is called photophosphorylation as the trapped light energy is used to pump H + across into the thylakoid space from the stroma. The diffusion of H+ back into the stroma drives ATP synthase.
This adds Pi to ADP to make ATP.
The ATP produced is used in carbon fixation.
Light energy absorbed in grana
2H2O H+ + 4e- + O2
Photophosphorylation
ADP + Pi
ATP
Photolysis
(water split)

15. Photosynthesis
Photolysis and photophosphorylation take place using two Photosystems.
The splitting of water releases oxygen and H+ are pumped into the Thylakoid.
The back flow of H+ drives ATP synthase to produce ATP. ATP and H+ are then used in the Calvin cycle.

16. Detecting and amplifying an environmental stimulus LOs
Photoreceptor protein systems are found across the three ____________.
In archaea, ___________ molecules generate potential differences by absorbing ______ to pump protons across the membrane.
In plants the light absorbed by ____________ pigments within protein systems drives an ____________ flow that pumps hydrogen ions across the __________ membrane of the chloroplast.
In both cases the resulting diffusion of ________ ______ back across the membrane drives _____ ___________.

17. Detecting and amplifying an environmental stimulus in Achaea and plants Key Concepts
Photoreceptor protein systems are found across the three domains.
In archaea, bacteriorhodopsin molecules generate potential differences by absorbing light to pump protons across the membrane.
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.

18. Photoreception in animals; the human retina
The human retina contains two types of photoreceptors, rods and cones.

19. Rods and cones
Cones cells are used for colour vision and function in bright light. They contain different forms of a membrane protein called opsin that are sensitive to different wavelengths of light ranging through red, green, blue or UV.
Rod cells work over a wider range of wavelengths in weaker light, so involve amplification of the light signal.

20. Rhodopsin
In animals the light-sensitive molecule retinal is combined with a membrane protein opsin.
Retinal is a form of vitamin A and is acquired from the diet or synthesised from beta-carotenes.
This diagram of bovine rhodopsin shows the seven membrane-spanning alpha-helix domains of the opsin with retinal (red) complexed in a pit at its centre. These structures are common to all rhodopsin/photopsin molecules.
When stimulated by light the retinal undergoes photoisomerisation, changing from 11-cis-retinal into all-trans-retinal. This is referred to as bleaching.
This induces a conformation change in the opsin, which activates an associated G-protein, transducin, on the cytoplasmic side of the membrane.

21. Light
Photoisomerisation of retinal (bleaching)
Conformational change of opsin protein
Activation of associated G protein molecule
Activation of hundreds of molecules of enzyme
Enzyme generates products, if sufficient levels reached
A nerve impulse is generated and sent to the visual centre in the brain

22. The outer segment of rod cell membrane disks contain rhodopsin
The outer segment of rod cell membrane disks contain rhodopsin. The cells contain synaptic vesicles containing neural transmitters, adapted to a much lower resting potential (-10mV) than most neurones (-60 to -90mV).
So in the dark they are constantly secreting neurotransmitters and stimulating the neurons linked further back in the retina. Light hitting the rod cell causes hyperpolarisation (more negative) charge by triggering the closure of Na+ channels in the membrane, as more light is absorbed more channels close and so less and less neurotransmitter is released.

23. Creating the signal
The photoreceptor in rods is called rhodopsin, this is made up of a transmembrane protein - opsin (7 membrane spanning helices common to a lot of the transducing G proteins).
Opsin is bound to a light absorbing pigment 11-cis-retinal. This absorbs in the visible range ( nm) and drives isomerisation to 11-trans-retinal.

24. Creating the signal
When the membrane protein opsin binds to trans-retinal it becomes activated opsin and signal transduction causes Na+ channels to open. The signal transduction involves removing cGMP, a molecule that is normally at very high levels due to active phosphorylation enzymes in the membrane. The Na+ channels are allosteric channels, held open by binding to 3 cGMP molecules. Any drop in the cGMP level causes a rapid drop in the number of open channels. To make the system more sensitive the whole signal transduction is set up as a cascade.

25. Creating the signal
One activated opsin  activates around 500 activated transducins (a G protein)  cGMP phosphodiesterase  converts 3’,5’-cGMP to 5’GMP  lose allosteric modulator  closed Na+ channel.
To allow the system to reset the activated opsin radidly dissociates, and an enzyme converts the trans-retinal back to cis-retinal.

26. Creating the signal
A single photon causes a hyperpolarisation of around 1mV and lasts 1-2s. Humans can detect a flash of as few of 5 photons. 1 photon blocks inflow of about 10 million Na+ ions due to closure of hundreds of channels. If there is a constant exposure to light the cells become adapted – i.e. more light is required to have the response

27. Detecting and amplifying an environmental stimulus LOs 2
In animals the light-sensitive molecule ______ is combined with a membrane protein ______ and a cascade of proteins amplifies the signal.
In _____ cells, different forms of opsin give sensitivity to specific wavelengths (red, green, blue or UV).
In ____ cells, the _________ absorbs a wider range of wavelengths and a greater degree of amplification by the protein cascade results in sensitivity at ____ light intensities.
When stimulated by one photon, a _________ molecule activates hundreds of _________ molecules, which activate hundreds of molecules of an enzyme.
If the enzyme triggers a sufficient product formation, a _____ _______may be generated.

28. Detecting and amplifying an environmental stimulus Key Concepts
In animals the light-sensitive molecule retinal is combined with a membrane protein opsin and a cascade of proteins amplifies the signal.
In cone cells, different forms of opsin give sensitivity to specific wavelengths (red, green, blue or UV).
In rod cells, the rhodopsin absorbs a wider range of wavelengths and a greater degree of amplification by the protein cascade results in sensitivity at low light intensities.
When stimulated by one photon, a rhodopsin molecule activates hundreds of G-protein molecules, which activate hundreds of molecules of an enzyme.
If the enzyme triggers a sufficient product formation, a nerve impulse may be generated.

29. Key Area 1.4 Detecting and amplifying an environmental stimulus
Past Paper Practice
Advanced Higher
Key Area 1.4
Detecting and amplifying an environmental stimulus

34. Sheep Eye Dissection
Sheep Eye Dissection