BIOA01H3 Chapter Notes - Chapter 1: Photochemistry, Melanocyte, Light Pollution

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Published on 3 Nov 2012
School
UTSC
Department
Biological Sciences
Course
BIOA01H3
Professor
Notes Biology chapter 1 light and life
15th 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(10-12m) to a kilometer
(106m) 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?
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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
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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
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Document Summary

Notes biology chapter 1 light and life. Also it contains light sensor called eyespot that allows it to sense light direction and light intensity. Wavelength of electromagnetic radiation ranges from one picometre(10-12m) to a kilometer (106m) 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. Photons have no mass but have a specific, precise amount of energy which is inversely related light can be described as a wave, behaves as a stream of energy particles called photons 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.

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