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Chapter 1

Chapter 1: Light and Life

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Wilfrid Laurier University
Matthew Smith

1.1 The Physical Nature of Light Light has two important functions for life on earth 1. It is a source of energy that sustains virtually all organisms. 2. It provides information about the physical world that surrounds them. Example- the green alga (Chlamydomonas reinhardtii) - a single-celled photosynthetic eukaryote. - single large chloroplast, harvests light energy and uses it to make energy rich molecules. - eyespot: light sensor, allows individual cells to gather information about location and intensity of light. - flagella move to increase or decrease the amount of light that gets absorbed. 1.1a What is Light? - The sun converts over 4 million tonnes of matter into energy every second. This energy is given off as electromagnetic radiation, which travels at the speed of light and reaches Earth in just over 8 minutes. - Light is commonly defined as the portion of the electromagnetic spectrum that humans can detect with their eyes. (only from 400nm- 700nm) - Light has no mass, has properties of both a wave and a stream of photons, which in other words is referred to as the particle-wave duality. - The relationship between the wavelength of light and the energy of the photons it carries is an inverse: the longer the wavelength the lower the energy of the photons it contains. Shorter wavelength= higher energy Longer wavelength= lower energy 1.1b Light Interacts with Matter - Through light interacting with matter change can occur. This change is what allows light energy to be used by living things. - When a photon hits an object it has three possible fates: 1. it can be reflected off the object. 2. transmitted through the object. 3. absorbed by the object. - The absorption of a photon by a molecule results in the energy being transferred to an election. This results in the electron being raised to a higher energy level (excited state) Remember: a photon can be absorbed by an electron of a molecule only if the photon energy equals the energy difference between the electron’s ground state and an excited state. If energies don’t match the photon is either transmitted through the molecule or reflected. Pigments: class of molecules that are very efficient at absorbing photons. a) chlorophyll a- involved in photosynthesis b) retinal- involved in vision c) indigo- used to dye jeans ^ Have a common region for light absorption. This is called a conjugated system where carbon atoms are covalently bonded to each other with alternating single and double bonds. ( results in delocalization of electrons) - None of these electrons are closely associated with a particular atom or involved in bonding and thus are available to interact with a photon of light. - Most pigments absorb light at distinct different wavelengths. This is because they differ in the number of excited states available to the excitable electrons. - Colors appear to the eye because they are not absorbed but rather reflected or transmitted off the pigment. 1.2 Light as a Source of Energy Photosynthesis- sustains almost all life. - Light energy is used to convert carbon dioxide into sugars (carbohydrates). - The potential energy of excited electrons within pigment molecules is used in photosynthetic electron transport to synthesize the energy rich compounds NADPH and ATP. (consumed in the Calvin cycle to convert carbon dioxide into carbohydrates) - Cellular respiration breaks down carbohydrates & other molecules trapping the released energy as ATP ^ used in the energy-required metabolic and biosynthetic processes that are fundamental to life. - Not all organisms use light energy to convert carbon dioxide into carbohydrates. Ex. Genus of organisms within the Archaea called Halobacterium ^ thrive in habitats that contain salt levels that are lethal to most other forms of life. ^ contain a pigment-protein complex called bacteriorhodopsin, which functions as a light-driven proton pump. ^ captures photons of light that provide the energy supply need to pump protons out of the cell. ^ resulting difference in H concentration across the plasma membrane represent a source of potential energy that is used by the enzyme ATP synthase to generate ATP from ADP & inorganic phosphate. - This is used for a range of
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