AH Biology: Unit 1

Cells and Proteins

Detecting and Amplifying an Environmental Stimulus:

Photoreceptor Protein Systems

Photoreceptor systemsPhotoreceptor systems are found across the three domains: Archaea, Prokaryota and Eukaryota.

We will consider the way light is detected and used in:

•Archaea

•Eukaryota

– Animalia

– Plantae

Archaea• 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 are capable of fixing carbon as an energy source and some, eg the Haloarchae, can photosynthesise. Unlike the Eukaryotes or Prokaryotes no species has been found that can do both.

• The photosynthetic nature of the Haloarchea relies on bacterial rhodopsin causing activation of ATP synthase.

Bacterial rhodopsin• Bacterial rhodopsin consists of

retinal, a light-sensitive chromophore, sitting within a transmembrane protein bacterial opsin.

• Bacterial opsin has a similar structure to that found in animal rhodopsin but is not associated with G-protein.

• The retinal–opsin complex is called rhodopsin.

• The photoisomerisation of retinal results in a conformational change in the opsin, causing it to pump protons from the intracellular compartment to the outside.

Bacterial rhodopsin1. When sunlight strikes a bacterial rhodopsin

molecule the bound retinal undergoes photoisomerisation. The resultant shape change causes the rhodopsin molecule to be activated.

2. The activated rhodopsin pumps protons (H+) out of the cell.

3. The electrochemical gradient causes the proton to flow back into the cell, driving ATP synthase.

4. ATP synthase complexes with Pi and ADP.

5. ATP is generated.

GFDL image by Kirsten Carlson, MBARI (© 2001). http://www.mbari.org/twenty/proteorhodopsin.htm

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.

Light transduction• When stimulated by one photon, a rhodopsin (rod cells) or photopsin (cone cells)

molecule activates hundreds of transducin molecules.

• In turn transducin activates a phosphodiesterase enzyme on the intracellular face of the plasma membrane.

• This can lead to the breakdown of a thousand cGMP molecules per second, which makes rhodopsin-based systems very sensitive.

• This fall in cGMP levels closes ligand-gated Na+ channels in the light-sensitive cells of the retina, which prevents the synaptic release of inhibitory neurotransmitters, allowing the adjoining sensory synapse to become excited and transmit a nerve impulse to the visual centres of the brain.

Light transductionIn the vertebrates retina rod cells and cone cells both contain opsins, which contain retinal.

Rods contain rhodopsin and many rods are connected to a single neuron, maximising the sensitivity to light. Rods do not detect colour.

Cone cells are of three types, each of which contains a rhodopsin analogue, photopsin. The retinal behaves identically to that in rhodopsin but the nature of the opsin it is associated with means it is sensitive to a narrower range of wavelengths. One cone cell is connected to a single neuron.

Each of the three classes of cone photopsins is sensitive to different ranges of light wavelengths. This allows the detection of colour by the eye.

Discs filled with photopsin

Light transductionSensitivity of the three classes of cone cells, S, M and L.

These are equated to:

L: redM: greenS: blue

This is not a precise representation of their sensitivities, as can be seen from their sensitivity spectra.

Photosynthesis in plants

By Kristian Peters -- Fabelfroh (photographed by myself) [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

Photosynthesis in plants relies on light energy being converted into chemical energy, which generates glucose from carbon dioxide and water.

Chloroplasts in the cell cytoplasm contain photosynthetic pigments, mainly chlorophyll a, and accessory pigments such as chlorophyll b, the phycobilins and carotenoids.

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.

Chloroplast structure

PhotolysisWater 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).

PhotophosphorylationLight 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.

Photolysis (water split)

Light energy absorbed in grana

2H2O 4H+ + 4e- + O2

Photophosphorylation

ADP + Pi ATP

Photosynthetic plant pigments

• Chlorophyll a is the main photosynthetic pigment, located on thylakoid membranes.

• Its role is to channel electrons excited by the absorption of photons to other parts of the photosynthetic electron transport chain.

• Accessory pigments transfer their trapped energy onto chlorophyll a.

• Two special subsets of chlorophyll a, P680 and P700, comprise the light-sensitive parts of photosystem 2 and photosystem 1.

• Photosystem 2 uses the energy of excited electrons to split water, which increases the thylakoid H+

concentration and generates O2. The electrons generated are passed onto other intermediaries such as plastiquinones, which result in more H+ accumulating in the thylakoid space.

• Various other intermediaries transfer the electrons onto photosystem 2, which is also light excitable. The high-energy electrons generated here, together with some of the thylakoid space H+, are used to convert NADP to NADPH for use in the Calvin cycle.

• The large pool of thylakoid H+ leaks back across the thylakoid membrane down its electrochemical gradient and back into the stroma.

• This drives ATP synthase to generate ATP from ADP and Pi.

Photosynthetic electron transport chain