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Light and Life- Chapter 1 - 1st Lec.docx

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York University
BIOL 1000
Nicole Nivillac

BIO 1000 Light and Life (Ch.1) + Evolution (Ch.3) Why it Matters • Light is important as it is used in all areas of life! o Ex. Monet’s painting use to be bright and very vibrant, his later paintings were the total opposite of his original art o This was because he was diagnosed with “Cataract” – change in the lens of the eye, making it more opaque – proteins under the lens denature – the transmission of blue light is decreased … to a cataract suffer to world appears more yellow 1.1: The Physical Nature of Light • Light serves 2 important functions for life on Earth o 1. Source of Energy (directly or indirectly) o 2. Provides organisms w/ information about the physical world that surrounds them - provides awareness o Ex. C. reinhardtii – uses light e to make energy, light sensor = eyespot 1.1a: What is light? • Life on earth b/c of distance from sun (150 mill. Km) • By converting He into H @ fast rate, Sun converts over 4 mil tonnes of matter into e/ sec o This e is given off as “electromagnetic radiation” – which travels as a “wave” @ the speed of light (1, 079, 252, 848 km/h) & reaches Earth in just over 8 min • Wavelength range from less than 1 picometre (cosmic rays) to more than a km for radio waves • Light: portion of the electromagnetic spectrum that humans can detect w/ their eyes o Only spans from wavelengths of 400-700 nm (nanometer) • Light has no mass, travels through space (as wave) • Light is composed of a stream of e particles called photons o Thus, light as properties of both wave & stream of photons = particle- wave duality o Light best understood as wave of photons (longer the wavelength, lower the e of photons) 1.1b Light Interacts w/ matter • Even though light = no mass, can interact w/ matter to cause change o This change allows e of light to be used by living things • When photon hits object, 3 things possible: 1. Can be reflected off the object, 2. Transmitted through the object, or 3. Can be absorbed by the object • Absorption most important – must take place in order for organism to use light for source of e • Absorption takes place when e of photon transferred to e- in molecule – excites e- from ground state to higher-e level (called excited state) o *Photon can only be absorbed if photon e = e diff. between e- ground state & excited state o If e doesn’t match, then photon transmitted • Molecules responsible for absorbing photons = pigments! (Ex. Chlorp. a, retinal (vision), indigo) • All pigments share common feature imp. For light absorption o Region where C atoms are covalently bonded to each other w/ alternating single and double bonds – bond arrangement known as Conjugated System – results in delocalization of e- (these e- interact w/ photon of light) • Pigments differ in absorbing light @ diff wavelengths (diff in # of excited states) • Apigments colour is the result of photons of light that it does not absorb – reflected instead 1.2: Light as a Source of Energy BIO 1000 • E from sun enters biosphere via photosynthesis (light  CO2 & sugars) o Following light absorption, the potential e of excited e- w/ into pigment molecules (Chlorp a) used in electron transport to make e-rich molecules NADPH (nicotinamide adenine dinucleotide phosphate) &ATP – these consumed by Cal. Cycle to convert Co2 into sugars o Cellular respiration breaks down carbs intoATP • Not all organisms that use light as e are classified at photosynthetic – meaning these organisms don’t use light e to convert CO2 into carbs (Ex. Halobacterium) o HB- pigment bacteriorhodopsin – captures photons of light that provide e to pump protons out of cell – results of H+ conc. diff across plasma membrane = potential e used by enzyme ATP synthase (ATP ADP) 1.3: Light as a Source of Information 1.3a Rhodopsin, The Universal Photoreceptor • Photoreceptor: basic light sensing system, most common one in nature is Rhodopsin (basis of vision) – each photoreceptor cell contains thousands of rhodopsin molecules • Each rhodopsin consists of a protein called “opsin” – which binds a single pigment called retinal • Evolutionary similar to bacteriorhodopsin • Absorption of light causes the retinal to change shape – causing opsin to change – causing electrical signals (sent to brain) and intracellular ion conc. • Humans, light captured by eye = approx. 125 mill. Photoreceptor cells (rods & cones) that line retina • Vision & smell = diff senses but proteins similar to opsin used in olfaction (smell perception) 1.3b Sensing Light without Eyes • Many organisms like plants, algae don’t have eyes, but still sense light o Ex. C. reinhardtii eyespot – doesn’t play role in photosynthesis but allows for sense of light & direction o Using flagella, it can respond to light by swimming toward or away from source via process called phototaxis – allows cells to stay in optimum light to maximize light capture for photosynthesis • Plants- photoreceptor called phytochrome (in cytosol of cells) senses the light environment and is critical for photomorphogensis – shifting a seedling from darkness to light triggers this – light sensed by phytochrome initiates the program that involves activation of hundreds of genes 1.3c The Eye • Eye: organ animals use to sense light • Process of vision- focusing & absorbing incoming light – also a brain or nervous system to interpret signals sent from eye – essentially we “see” with our brains • Simplest eye – ocellus- cup lined w/ up to 100 photoreceptor cells – (Ex. Flatworms) o Information sent to cerebral ganglion from individual eyes allows worms to orient themselves so that amount of light falling on the two ocelli remains equal o Allows them to swim to darker areas & escape predators • Greatest advancement in eye came when it produced an actual image of the lighted environment, which allowed objects & shapes to be separated o Image forming eyes are found in 2 types: compound eye and single-lens eye o Compound eyes: (common in arthropods) Built of hundreds of individual units called ommatidia (omma=eye) fitted closely – samples small part of visual field and sent to brain as mosaic images of world BIO 1000 o Single-lens/ camera-like eye: found in some invertebrates, most vertebrates - as light enters through cornea, a lens concentrates the light & focuses it onto a layer of photoreceptor cells @ back of eye – retina – send info to brain via optic nerve 1.3d Darwin & Evolution of Eye • Recognized that “organs of extreme perfection” (I.e.- eye) could present problem • Said the amazing properties and functions of an eye formed via natural selection – functions critical to survival needed thus through natural selection – evolution of the eye occurred • Evolved from simple eye to complex – could have evolved over 1000 times, but fossil evidence shows eye evolved independently over 40 times • Many similarities between eyes, (Ex. Gene called Pax6) –master control gene almost universally employed for eye formation in animals 1.4: The Uniqueness of Light • Visible light 400-700nm – used for photosynthesis, vision, phototaxis, etc. • George Ward says visible light used by all b/c most dominant form of electromagnetic radiation reaching Earth o Shorter wavelengths absorbed by ozone layer, whereas longer absorbed by water vapor and CO2 in atmosphere • Also has to do w/ e contained, note: all molecules made up of bonds – shorter waves (more photons) contain enough e to break these bonds, longer waves = not enough e to break bonds 1.5: Light can Damage Biological Molecules 1.5a Damage is an unavoidable consequence of light absorption • Photoreceptor cells that line the human retina can be damaged by exposure to bright light • Photo-oxidative damage: high-e environment associated with pigment molecules and excited electrons • Excess light e can result in excited e- reacting w/ o2 creating reactive oxygen species • These forms of O2 (Ex. H2O2) damage proteins and cause loss of function • Plants, algae exposed to light all day – more susceptible to damage o Photosystems are repaired very quickly though – mechanism which has developed 1.5b UV Light is Particularly Harmful • 400 (blue light) -200 (X-rays) nm • Shorter wavelengths = more e = more damage! • We are protected from shortest wavelengths (approx. 200nm) by Ozone – but longer ones reach • UV light classified as form of “ionizing radiation” o Ions are formed as a result of electrons being removed from atoms (UV strong enough)  Ions: protons & electrons not equal • DNAsusceptible to damage and dimers can form – when 2 neighbouring bases become covalently linke
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