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BIOL 1000
Paula Wilson

Light and Life Physical nature of light Light serves 2 important functions: 1. source of energy that directly or indirectly sustains virtually all organisms (for ex: chloroplast uses light energy for photosynthesis) 2. provides organisms with information about the physical world that surrounds them (for ex: eyespot which allows the cell to tell the location and intensity of a light source) electromagnetic radiation: * converting hydrogen into helium 3.4 x 10 to energy * comes in form of a wave variable wavelength (distance between 2 peaks) and energy:  typically measured in nanometers (10 m)  wavelength and energy inversely related: shorter wavelength = high energy (red) ex: gemma rays, x-rays longer wavelength = lower energy (purple) ex: radiowaves, microwaves photon: * packet of energy with no mass * absorbed by electrons what is light? The portion of the electromagnetic spectrum that humans can detect with their eyes. It’s a wave of photons. Light interacts with matter A photon has three possible fates:  absorbed – must take place in order to be used as a source of energy (by pigments) !  reflected  transmitted absorption: when the photon is transferred to an electron within the molecule, moving it from ground state (low-energy level) to excited state (high-energy level) happens only if the photon energy = energy difference between the electron’s ground state and excited state, and if not – the photon is transmitted / reflected pigments  molecules that can absorb photons of light  shape: - all have a region where carbons are linked by single / double bonds - conjugated system of delocalized electrons  capture photons which can be used to do work / reflect a certain color (a pigment’s color is the result of light that it does not absorb) Light as a source of energy  the absorption of light by pigment results in electron becoming excited – represent a source of potential energy photosynthesis: the process whereby light energy is used by plants (pigments – chloroplast) to convert carbon dioxide into rich energy = sugars (carbs) = ATP (used in Calvin Cycle) cellular respiration: breaks down carbs  release ATP different kind of use (not by photosynthesis): halobacteria contains pigment called bacteriorhodospin and use it to capture light energy and generate ATP Light as a source of information Photoreceptor: light-sensing system. The most common kind is rhodopsin = opsin (membrane protein) + retinal (pigment molecule) Absorption of photon light causes the retinal molecule to change shape 2 kinds: rods – don’t perceive different colors (good vision during night time), cones – perceive colors (good vision during daylight) Light captured by the human eye: involves 125 million photoreceptors cells – rods and cones which line the retina each cell contains thousands of rhodopsin molecules C. reinhardtii : each cell contains eyespot (which built from photoreceptors) - allow the cell to sense light direction and intensity Phototaxis : the respond to light by swimming towards or away from the light In plants: photoreceptor called phytochrome – chromophore linked to a protein, senses the light environment, critical for phomorphogenesis (development process of seeds that are exposed to the sun), absorbs strongly in red light The eye: the organ animals use to sense light. Visions requires a brain to interpret signals sent from the eye(visual processing occurs in the brain) – we see with our brain, not with the eyes. Other light sensors: Ocellus – the simplest eye, found in planarians (ex: flatwarms) and enable them to sense light direction and intensity (away from a source of light into a dar
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