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Chapter 1

BIOL 1000 Chapter Notes - Chapter 1: Suprachiasmatic Nucleus, Folic Acid, Melanosome

Course Code
BIOL 1000
Nicole Nivillac

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Light and Life
Chapter One
Light sustains all life as it’s an energy source.
Light provides information to organisms about their surroundings.
Cells contain an eyespot which senses light intensity and direction.
We are able to see light due to the visible light region of the electromagnetic spectrum, which
contain wavelengths of different nanometers. Long wavelengths (red) are weak while short
wavelengths (blue) are strong.
Light is described using wavelengths but they behave as particles called photons which are
streams of energy.
Photons don’t have mass – they have an energy source that is equivalent to the inverse manner of
the wavelengths. Short is strong. Long is weak.
Electromagnetic Spectrum
Infrared Radiation -- Visible Light (ROY G. BIV) – Ultraviolet Radiation
When light hits an organism, three possible things can occur:
1. It reflects
2. It transmits through the object
3. It’s absorbed by the object which uses the energy to process biological functions
Pigments can only absorb photons with the designated wavelengths or else it reflects. The
designated wavelengths have to match the energy difference between the ground and excited
state. A single photon will only excite one electron of a pigment molecule.
Examples of Pigments
Chlorophyll A – used in photosynthesis
Retinal – used in vision
Indigo – used in dying jeans
Carmine – scale pigments in some insects
Beta-carotene – an orange photosynthetic pigment that leaves have when turning colours
Chemically, all pigments contain a structural similarity. They have a conjugated system – a
region where carbon atoms are in an alternating pattern of single and double bonds. The
system absorbs photons for energy use and this is possible because it allows for delocalized
electrons which interact more with the photons.
Chlorophyll is green because it reflects green wavelengths – hence giving plants their colour.
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Because not all pigments absorb every single photon, the action spectrum measures the
efficiency and productivity of a wavelength for biological processes.
An absorption spectrum measures the amount of light a pigment absorbs in relation to the
A photoreceptor is a basic light sensing system that contains a protein and a pigment responsible
for the detection and response to light. Almost all organisms contain the Rhodopsin
photoreceptor which provides vision and is a part of the eyespot.
One rhodopsin molecule will have a protein called opsin. They are membrane proteins that span
a membrane several times in a complex manner. Opsin binds to a pigment molecule called retinal
which absorbs photons. When it does absorb a photon, it will physically change and this
conformational change leads to a signal transduction cascade. The opsin protein then
physically changes to accommodate the retinal and after a bunch of more events, electrical
signals will be sent to the vision center of the brain and an image is formed.
Phototaxis is the movement in response to light. If it’s positive, the organism will move towards
it. If it is negative, it will move away from it. It is the process that allows the organism to absorb
the most light for photosynthesis.
In addition to the Rhodopsin photoreceptor, phytochrome is another type of photoreceptor
which is only present in the cytosol of plant cells. Once exposed to red wavelengths they become
active and allow seedlings in dark environments to proceed with normal development once it
senses light. This is called photomorphogenesis.
The eye cannot act on its own as it needs a brain to process and interpret the signals coming from
the eye. Hence, we “see” with our brain.
The simplest type of eye is called the ocellus eye (plural ocelli) and it contains up to only 100
photoreceptors. Connected to each other to the cerebral ganglion, it provides a simple image for
the organism, usually insects and anthropods have the ocellus eye.
There are two types of image-forming eyes called the:
Compound Eyes: They contain hundreds of thousands of eyes called ommatidium (plural
ommatidia) shaped like hexagons that fit close together. Each ommatidium contains a
photoreceptor that can detect the slightest movement as it environment changes, hence a mosaic
image is formed.
Single-Lens Eyes: Light enters the transparent cornea and a lens concentrates the light to the
retina so that the photoreceptors can process it and record an image.
An optically refined eye is no good unless the brain advances with it (coevolution) so that the
neural processing of information is being sent by the optic nerve.
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