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Chapter 1

Chapter 1 Light and Life Summary notes of the entire chapter, covers important topics in detail, with definitions, and focus on important topics covered in lecture


Department
Biology
Course Code
BI110
Professor
Diane Williams
Chapter
1

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BI-110
CHAPTER 1 - LIGHT & LIFE
1.1 The Physical Nature of Light
-Light sustains all life, and provides organisms with information about the physical world
1.1a What is Light
-light is energy created by the sun, by converting hydrogen into helium at such an
extreme rate that it converts 4 million tons of matter per second
-energy is released as electromagnetic radiation, which comes in two forms, electrical
and magnetic, traveling at 90 degree to one another
-Light can be defined as the portion of the electromagnetic spectrum that humans can
detect with their eyes. or as a narrow bang of the electromagnetic spectrum spanning
the wave lengths in nanometres. approx. 380nm-750nm.
-Light also behaves as a stream of photons. Photons have no mass but contain a
precise amount of energy which is inversely related to its wavelength
-SHORT WAVELENGTH --------------------> HIGH ENERGY (blue light)
-LONG WAVELENGTH ---------------------> LOW ENERGY (red light)
1.1b Light Interacts With Matter
-Although light has no mass, it is able to interact with, and change, matter, allowing it to
be used by living things
-Light can do one of three things when it comes in contact with an object:
1. Reflect off the object
2. Transmit through the object
3. By absorbed by the object
-Pigment a molecule that can absorb photons of light. Individual pigments differ in the
wavelengths of light they can absorb
-Pigments share a common feature critical to light absorption called a conjugated
system
-Conjugated System a region where carbon atoms are covalently bonded with
alternating single and double bonds. Results in the delocalization of electrons (thus they
more readily interact with the photons)
1.1c Why is Chlorophyll Green?
Absorption of light occurs when the energy of a photon is transferred to an electron of
the pigment molecule.
1. A single photon results in the excitation of ONE electron in a pigment
molecule
2. The energy of the photon must match the energy difference between
the ground state and one of the excited states in order for the photon
to be absorbed
*recall that each photon has a precise amount of energy
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BI-110
-COLOUR IS DETERMINED BY THE LIGHT THAT AN ELECTRON CANNOT
ABSORB
ie. Chlorophyll is green because it can absorb both red and blue light, because their
energies match the energy difference between its ground and excited states, however,
gree light does not, thus it is reflected. Other colours on the spectrum are absorbed by
accessory pigments not associated with photosynthesis.
Action Spectrum the plot of effectiveness of different wavelengths of light on a
biological process, such as photosynthesis. ie. above.
1.2 Light as a Source of Energy
-excited state electrons are a source of potential energy that can be used for work.
Used in photosynthesis, synthesis of other biological molecules (lipids, proteins, nucleic
acids)
1.3 Light as a Source of Energy
-not every person or species sees the world the same way
1.3a Rhodopsin, a Highly Conserved Photoreceptor
-the photoreceptor is the basic light-sensing system, found almost universally in all
organisms
-most common photoreceptor in nature
-basis of vision in animals, used in many other organisms
-consists of a protein (OPSIN) that binds a single pigment molecule (RETINAL)
-OPSINS are membrane proteins that span a membrane multiple times and form a
complex with the RETINAL molecule at the centre
-Rhodopsin is very similar to the bacteriorhodopsin found in Halobacteria and other
prokaryotes
-absorption of a photon of light causes the retinal pigment molecule to change shape
causing alterations to the opsin protein which, it turn, causes downstream events
-in humans, capturing light in the eye involves about 125 million photoreceptor cells
(rods and cones) each of which contains thousands of individual rhodopsin molecules
-Rhodopsin is NOT the only photoreceptor found in nature
-Similar opsins are involved in different senses (smell, taste, etc.)
1.3b Sensing Light Without Eyes
-many organisms can sense the light in their surroundings even though they lack eyes
-PHOTOTAXIS - cells respond to light by “swimming” towards or away. allows the cell to
stay in the optimum light environment to maximize light capture for photosynthesis
-light absorption by the eyespot is linked to the swimming response. Light absorption
triggers rapid changes in the concentrations of ions (K+Ca) which generates a cascade
of electrical events that causes “beating” of the flagella (phototaxis)
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