Chapter 1-Exploring the Diversity of Life

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29 Sep 2011

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Biology: Exploring the Diversity of Life
Chapter 1: Light and Life
1.1 The Physical Nature of Light
Light serves two important functions for life on earth
o1st → it is a source of energy that sustains all life.
o2nd → light provides organisms with information about the physical world.
Each cell contains…
oA single large chloroplast that harvests light energy and uses it to make energy rich molecules through
the process of photosynthesis.
oA light sensor called an eyespot that allows it to sense both light direction and light intensity.
Wavelength : the distance between two successive peaks. The wavelength of electromagnetic radiation ranges
from less than one picometre for cosmic rays to more than a kilometer for radio waves.
Light : the portion of the electromagnetic spectrum that humans can detect with their eyes.
oLight is a narrow band of the electromagnetic spectrum spanning the wavelengths in nanometers from
400nm (blue light) to about 700nm (red light).
oWavelengths outside this range should not be referred to as light but rather as ultraviolet and infrared
oLight can be described as a wave but also behaves as a stream of energy particles. These particles or
packets of energy are referred to as photons.
oLight has no mass but it is able to interact with and change matter. These changes allow light to be used
by living things.
Photons : particles or packets of energy
oPhotons have no mass, but each contains a precise amount of energy.
oThe amount of energy in a photon is inversely related to its wavelength. This means that blue light, with
a shorter wavelength consists of photons that have higher energy than longer wavelength red light.
oWhen photons of light hit an object, the photons have 3 possible fates:
1) Reflected off the object
2) Transmitted through the object
3) Absorbed by the object.
Pigment : A molecule that can absorb photons of light
oIndividual pigments differ in the wavelengths of light they can absorb. (Ex some
pigments absorb only blue light, some only green and some can absorb light of a number of different
oAll pigments share a common feature critical to light absorption: a region where carbon
atoms are covalently bonded with alternating single and double bonds. This bonding arrangement is
called a conjugated system and results in the delocalization of electrons.
Absorption of light occurs when the energy of a photon is transferred to an electron of the pigment molecule.
oBefore absorbing a photon of light, an electron exists in the ground state, at 0.
oUpon absorption of a photon of light, the energy is transferred to the electron moving it
from the ground state to a higher energy, excited state.
oFor a chlorophyll molecule, the electron involved in photon capture can exist in 2 and
only 2 excited states.
oThe lower excited state, at 1 is reached by chlorophyll absorbing a photon of red light.
The higher excited state, at 2 is reached by the absorption of an electron to a higher energy state than
absorption of red light because blue photons contain more energy.
Two Important Principles :
o1st → a single photon results in the excitation of one and only one election in a pigment
o2nd → 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. (If the energies do not match
the photon is not absorbed)
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The Color of a Pigment : is determined by the wavelengths of light it cannot absorb.
oWhy is chlorophyll green in color? → Chlorophyll is green because although it can trap
photons of blue light and red light, it cannot absorb photons of green light.
oA chlorophyll molecule cannot absorb a photon of green light because it does not have an
energy level matching that of a green photon.
Action Spectrum : A plot of the effectiveness of different wavelengths of light on a biological process
1.2 Light as a Source of Energy
Potential Energy: After a photon of light is absorbed, an electron within a pigment molecule is raised to a higher
excited state. This excited state electron is a source of potential energy that can be used to do work.
oThis potential energy is used in photosynthetic electron transport to synthesize energy-
rich compounds NADPH and ATP, which are used to convert carbon dioxide into carbohydrates.
1.3 Light as a Source of Information
Photoreceptor : The basic light sensing system that is found almost commonly in all organisms
oThe most common photoreceptor in nature is rhodopsin which is not only the basis of
vision in animals but also is used as the light sensing unit of the eyespot.
oEach rhodopsin molecule consists of a protein called opsin that binds a single pigment
molecule called retinal.
Opsin : membrane proteins that span a membrane multiple times and form a complex with the retinal molecule
at the centre.
oAbsorption of a photon of light causes the retinal pigment molecule to change shape.
oThis change triggers alterations to the opsin protein which in turn, triggers downstream
events including alterations in intracellular ion concentrations and electrical signals.
Rhodopsin : is the most common photoreceptor found in nature, but isn’t the only one.
oIn humans, capturing of light by the eye involves about 125 million photoreceptor cells
that line the retina. Each photoreceptor cell contains thousands of individual rhodopsin molecules.
oBoth plants and animals have a range of other photoreceptors that absorb light of
particular wavelengths.
Sensing Light Without Eyes
oMany organisms can sense the light in their surroundings even though they lack eyes.
This includes plants, algae, invertebrates and some prokaryotes.
oIn the eyespot of C. reinhardtii. The eyespot does not play a role in photosynthesis;
instead the photoreceptors of the eyespot allow the cell to sense light direction and intensity. Using a
pair of flagella, the cells can respond to light by swimming toward or away from the light source. This is
a process called phototaxis.
oThis allows the cell to stay in the optimum light environment to maximize light capture
for photosynthesis.
oLight absorption by the eyespot is linked to the swimming response by a signal
transduction pathway, in which light absorption triggers rapid changes in the concentrations of ions,
including potassium and calcium which generate a cascade of electrical events.
oIn plants, a different photoreceptor called phytochrome, senses the light environment and
is critical for photomorphogenesis, the normal developmental process activated when seedlings are
exposed to light.
oPhytochrome is present in the cytosol of all plant cells and when the plant is exposed to
wavelengths of red light, phytochrome becomes active and initiates a signal transduction pathway that
reaches the nucleus.
oIn the nucleus, these signals activate hundreds of genes, many of which code for proteins
involved in photosynthesis and leaf development.
The eye : defined as the organ animals use to sense light. What distinguishes the eye of a simple invertebrate, for
example, from the eyespot of C. reinhardtii is vision.
- The process of vision not only requires an eye but it also requires a brain or at least a simple nervous
system that interprets signals sent from the eye.
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