AH Biology: Unit 1 Cells and Proteins Photoreceptor Protein Systems Detecting and Amplifying an Environmental Stimulus:

Photoreceptor systems Photoreceptor systems are found across the three domains: Archaea, Prokaryota and Eukaryota. This topic consider the way light is detected and used in: Archaea Eukaryota – Animalia and Plantae

Archaea Single-celled organisms with no membrane-bound organelles or defined nucleus. Similar to Prokaryotes, but a separate evolutionary path. Occupy many niches. Including very high temp. and high conc. salt water regions. Some groups are capable of fixing carbon as an energy source e.g. the Haloarchae, which can photosynthesise. The Haloarchea rely 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. It changes conformation in response to light (photoisomeration)

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

Bacterial rhodopsin When sunlight strikes a bacterial rhodopsin the shape change causes the rhodopsin molecule to be activated. The activated rhodopsin pumps protons (H+) out of the cell. The electrochemical gradient causes protons to flow back into the cell, driving ATP synthase. ATP synthase complexes Pi and ADP. ATP is generated. GFDL image by Kirsten Carlson, MBARI (© 2001). http://www.mbari.org/twenty/proteorhodopsin.htm

Rhodopsin in animals 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 membrane-spanning seven 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.

Light transduction In 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.

Light transduction Cone cells are of 3 types, which contain photopsin (like rhodopsin) The retinal behaves identically to that in rhodopsin but the nature of the opsin associated 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.

Light transduction When stimulated by one photon, a rhodopsin (rod cells) or photopsin (cone cells) molecule activates hundreds of transducin molecules. Transducin activates an 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.

Light transduction This fall in cGMP levels closes ligand-gated Na+ channels in the light-sensitive cells of the retina. This 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.

Cone sensitivity Sensitivity of the three classes of cone cells, S, M and L. These are equated to: L: red M: green S: blue

Photosynthesis in plants Light energy converted into chemical energy, by generating 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

Photophosphorylation Photolysis NADP reduced with H to form NADPH. H and electrons delivered for use in carbon fixation (Calvin cycle). Photophosphorylation Light energy is also used to generate 2 molecules of ATP from ADP and Pi. The ATP is used in carbon fixation. Light energy absorbed in grana 2H2O 4H+ + 4e- + O2 Photophosphorylation ADP + Pi ATP Photolysis (water split)

Photosynthetic electron transport chain

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.