BIOA01- chapter 1 note - light and life.pdf

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Biological Sciences
Mary Olaveson

Notes – Biology chapter 1 – light and life 15 September 1, 2011 1.1 the physical nature  light’s two important functions for life on earth 1) source of energy that sustains all life 2) provides organisms with information about the physical word e.g. green alga called chlamydomonas reinhardtii , singled –cell photosynthetic eykaryote, it has large chloroplast that receives light and use it to make molecules with high energy. Also it contains light sensor called eyespot that allows it to sense light direction and light intensity 1.1a what is light?  Wavelength of electromagnetic radiation ranges from one picometre(10 m) to a kilometer 6 (10 m) for cosmic rays and radio waves respectively  light is the range of the electromagnetic spectrum that humans can detect with their eyes  wavelength outside this range is not considered as light but Ultraviolet and infrared radiation  light can be described as a wave, behaves as a stream of energy particles called photons  photons have no mass but have a specific, precise amount of energy which is inversely related to its wavelength  a shorter wavelength consists of photons that have higher energy than longer wavelength (e.g. blue vs. red) 1.1b light interacts with matter  light has no mass but is able to interact with and change matter  these changes allow light to be used by living things  3 possible outcomes when photos of light hit an object 1) Reflected off the object 2) Transmitted through the object 3) Absorbed by the object  Pigment is a molecule that can absorb photos of light  Different types of pigment absorb different wavelengths of light. Some can absorb a certain wavelength of light, some can absorb multiple wavelengths of light e.g. chlorophyll a (photosynthesis), retinal (vision), indigo(dye jeans)  Pigments are able to absorb light because of their common structure that the carbon atoms are covalently bonded with alternating single and double bonds. this is called a conjugated system which allows the delocalization of elections, thus available to interact with a photon of light 1.1c Why is chlorophyll green?  Energy of photon transferred to an electron of the pigment molecule = absorption of light  From chemistry we know that elections have their ground state and excited state  Electrons that are involved in photon capture in chlorophyll ONLY HAVE TWO EXCITED STATES  absorbing red light results in a lower excited state whereas blue light results in a higher excited state because blue photons contain more energy  in order for the photon to be absorbed, the energy of the photo must match the energy difference between the ground state and excited state  Chlorophyll is green because light that is not absorbed determines the color of it. Chlorophyll does not absorb green light because it does not have an energy level that of a green photon. Green light is either transmitted or reflected whereas blue and red lights are absorbed  Effectiveness of light in processes that use the absorbed light varies depending on the wavelength of light  Action spectrum shows the effectiveness of various wavelengths light on biological process e.g. blue and red are more effective in photosynthesis note: some photosynthesis still occurs under green light because some ACCESSORY pigment can absorb wavelengths of light BETWEEN red and blue 1.2 Light as a source of energy  Excited state electron is a source of potential energy used to do work  Photosynthetic electron transport to synthesize energy-rich compounds such as NADPH and ATP  Some chemical energy is used to synthesize other biological molecules such e.g. lipids, proteins, nucleic acids  Energy of a single photon is really small. However, photosynthetic apparatus in the chloroplast of a single C. reinhardtii cell absorbs a massive amount of photons each second  Organisms use light as a source of energy, but not in photosynthesis. For example: halobacterium found in prokaryotes contains a protein complex called bacteriorhodopsin, which function as a light-dependent proton pump 1.3 light as a source of information 1.3a rhodopsin, a highly conserved photoreceptor  organisms use light to sense their environment  photoreceptor is the basic light-sensing system in all organisms  the most common photoreceptor is rhodopsin which is not only the basis of vision in animals but also in many other organisms e.g. C.reinhardtii, serves as the light-sensing unit of the eyespot each rhodopsin has a protein called opsin that binds a single pigment molecule called retinal  opsins are mebrain proteins that pan a membrane and form a complex with the retinal molecule at the centre  absorption of photon of light causes the retinal pigment to change shape -> triggers alternation to the opsin protein -> causes alterations in intracellular ion [ ] and electrical signals -> signals are then sent to visual centres of the brain  125 million of photoreceptor cells (rods and cones) line the retina, each photoreceptor cell has thousands of rhodopsin  Plants and animals have range of other photoreceptors that absorb light of particular wavelength despite the fact that rhodopsin is the most common photoreceptor 1.3b sensing light without eyes  Many organisms can sense the light in their surroundings even though they lack eyes e.g. plants, algae, some prokaryotes  Specifically looking at eyespot of C.reinhardtii which is 1 m in diameter and is located in the chloroplast  Phototaxis – a process allows the cell to stay in the optimum light environment to maximize light capture for photosynthesis  Light absorbed by the eyespot causes rapid changes in the [ions] which generate electrical events. There, in turn chage the beating pattern of the flagella used for locomotion  Phytochrome is sa photoreceptor in plants that senses the light environment and is critical for photomorphogenesis  Photomorphogenesis is a process activated when seedlings are exposed to light  Phytochrome is present in the cytosol, and is activated when plant is exposed to wavelengths of red light -> signal pathways to nucleus is initiated -> activates genes which code for proteins involved in photosynthesis and leaf development 1.3c The eye  The organ animals use to sense light  The process of vision requires at least a simple nervous system that interprets signals sent from the eye  Detailed visual processing occurs in the brain  Ocellus, the simplest eye.  In planarians, photoreceptor cells below the epidermis are connected by nerves to the cerebral ganglion  Each ocellus is covered on one side by a layer pigment cells that blocks most of the light rays arriving from the opposite side of the animal. Therefore, most of the light enters from the side  Planarians orient themselves so that the amount of light falling on the two ocelli is equal and diminishes  Carries them away from source of light and towards darker areas to prevent predators  Some other organisms also have ocellus e.g. insects, arthropods, and mollusks  Eye is used to sense light intensity and direction. However, some of them can produce an actual image of the lighted environment. Two types of“image-forming” eyes: 1) Compound eyes - common in arthropods e.g. insects and crustaceans - Contains hundreds to thousands of ommatidia, fitted closely together - Each ommatidium samples only a small part of the visual field as light entering an ommatidium is focused onto a bundle of photoreceptor cells - The brain receivs a mosaic image of the world from these signals - Motion is detected simultaneously 2) Single-lens eyes - Light enter the eye through the transparent cornea - A lens that concentrates the light - A layer of photoreceptors at the back of the eye - Retina records the image (chapter 34) 1.3d Darwin and the evolution of the eye  There would be a problem when Darwin presented his theory of evolution by natural selection  Eyes haveb inimitable contrivances that let them adjust their focus and amount of the light admitted. However these are said to be formed from natural selection by variation (mutation)  Eyes started with a patch of light-sensitive cells on the skin, gradually yielding a camera-type eye
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