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Chapter 1

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Wilfrid Laurier University
Matthew Smith

 1.1 - The Physical Nature of Light  Light serves 2 important functions on Earth  Directly/indirectly sustains life (organisms)  Provides organisms w/ info about surrounding, physical world  E x. Green alga (Chlamydomonas reinhardtii) uses light for energy and info  Single-celled, photosynthetic eukaryote; found in ponds/lakes  Contains one chloroplast to harvest light energy  Contains eyespot (light sensor); allows cell to gather info about location and intensity of light source  Light is present on Earth due to distance  By converting hydrogen into helium, Sun converts tonnes of matter into energy  Energy emitted as electromagnetic radiation; travels as wave  Light = visible light spectrum (b/w 400 nm and 700 nm)  Light has no mass  Photons --> stream of energy particles; light composed of photons  Particle-wave duality --> light has properties of waves and streams  Light is a wave of photons  Longer wavelength = less energy; shorter wavelength = more energy  lower energy (of contained photons)  When light hits an object:  Reflected off object  Transmitted through object  When photon energy does not equal energy difference b/w ground state and excited state  Absorbed by object  Absorbed light is used as energy/info  Light absorption causes electron to enter excited state (higher energy level/orbital)  Photon can only be absorbed by electron if photon energy = energy difference b/w ground state and excited state  Pigments are very efficient photon-absorbers  Chlorophyll a --> photosynthesis  Retinal --> vision  Indigo --> dye  Feature critical to light absorption => alternating single and double bonds among carbons (conjugated system) --> results in delocalization of electrons (aren't linked to specific atom, thus able to interact w/ photons of light)  A pigment's colour is result of colour it doesn't absorb  Photons can be transmitted through pigment to eyes  1.2 - Light as a Source of Energy  Bacteriorhodopsin --> pigment-protein pump  Functions as light driven proton-pump  Pigment component captures photons of light that supply energy to pump protons out of cell  Resulting difference in H + concentration across plasma membrane represents potential energy source used by ATP synthase to make ATP from ADP and Pi  1.3 - Light as a Source of Information  Photoreceptor --> light-sensing system  Rhodopsin = universal photoreceptor  Rhodopsin consists of protein (opsin) and pigment (retinal)  Absorption of light causes change in shape (cis-->trans and vice versa) of retinal (pigment)  Shape change induces alterations to opsin protein; this triggers alterations in intracellular ion concentrations and electrical signals  Electrical signals sent to vision centres of brain  Eyespot made of two lipid globules (play role of focusing and directing incoming light toward photoreceptors)  Eyespot located in chloroplast; doesn't play role in photosynthesis  Eyespot sense light direction and intensity  Phototaxis --> cells respond to light by swimming toward/away from light source  Allows cell to remain in optimum light environment; maximizes light capture for photosynthesis  Light absorption through eyespot linked to swimming response by signal transduction pathway  Light absorption triggers rapid concentration change in ions; changes flagella beating- pattern (used for locomotion)  Phytochrome --> plant photoreceptor; sense light, critical for photomorphogenesis (normal developmental process activated when seedlings are exposed to light)  Present in cytosol of plant cells  When seedling exposed to red light, phytochrome is activated which initiates a signal transduction pathway that reaches nucleus; these signals activate a multitude of genes (many of which code for proteins involved w/ photosynthesis and leaf development  Vision distinguishes an invertebrate's eye and an eyespot  Process of vision requires an eye to focus and absorb light, and a brain or simple nervous system (like a jellyfish) that interprets signals sent from eye  Eye and brain thought to have coevolved since detailed visual processing occurs in brain rather than eye  Eyes:  Ocellus (Simplest Eye) --> insects, arthropods, molluscs, flatworms  Consists of cup/pit lined w/ up to 100 photoreceptors  Photoreceptor cell is modified nerve cell containing thousands of individual photoreceptor cell  Flatworms (genus Planaria) known for having ocelli eyes  Info sent to cerebral ganglion from individual eyes enables worms to orient themselves so amt. of light falling on two ocelli remains equal and diminishes as they swim; this reaction carries them directly away from source of light and toward darker areas where risk of predation is lower  Image-Forming Eyes  Greatest advancement in eye development  Produced an actual image of lighted environment which allowed objects and shapes to be discerned  Compound eyes (ex. Fly's eye)  Common in arthropods (insects and crustaceans) built of hundreds of individual units called ommatidia (omma = eye)  Incoming light focused onto bundle of photoreceptor cells; brain receives mosaic image of world  Slightest motion is detected by many ommatidia, compound eyes are extremely good at detecting movement (fly-swatting)  Single-lens eyes (or camera-like eye)  Humans have these eyes  Light enters through transparent cornea, lens concentrates light and focuses it onto layer of photoreceptor cells at back of eye (retina); photoreceptor cells of retina send info to brain through optic nerve  1.4 The Uniqueness of Light  Electromagnetic spectrum (visible light) necessary to life on Earth  Shorter wavelengths absorbed by ozone  Longer wavelengths absorbed by water vapour and carbon dioxide in atmosphere  Visible light spectrum (Electromagnetic spectrum) absorbed by Earth  Shorter wavelengths contain enough energy to
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