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BIOL 1000 Midterm: Midterm 1

Course Code
BIOL 1000
Nicole Nivillac
Study Guide

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1. What is Light?
Light has 2 functions:
source of energy (directly or indirectly sustain all organisms)
provides information about the physical world around organisms
Definition of light:
the portion of the electromagnetic spectrum that humans can detect with their eyes
wavelength from 400nm to 700nm (purple to red)
shorter wavelength = more energy, longer wavelength = less energy
gamma rays, xrays, ultraviolet radiation, visible light, near-infrared radiation,
infrared radiation, microwaves, radiowaves
particle-wave duality: light behaves as particles called photons a wave of photons,
a stream of photons
Interaction with matter
Reflection (off the object)
Transmission (through the object)
Absorption (by the object)
a. Pigments and Absorption of Photons
Absorption of photons by electrons
All electrons have a ground state
Delocalized electrons can absorb a photon and the energy from the photon is
transferred to the electron
This energy allows the electron to move up to an excited state
The energy in the photon has to be equal to the energy between the two orbits to
allow the electron to move up (not enough energy = no excited state)
Pigments and absorption of photons
Pigments are the only molecules that can absorb photons
Every pigment absorbs a different wavelength (colour)
- exemple of pigment: chlorophyll A, involved in photosynthesis
If a pigment can have multiple excited states, it will be able to absorb multiple
colours. A pigment colour is the colour it does not absorb the colour is reflected
off the pigment and reaches our eyes.
- for a red shirt: pigments absorb blue, green and yellow.
Pigments Conjugated System
All pigments have a conjugated system: a region of covalent bonds alternating
double and single bonds between the C atoms.
Because of the alternative bonding, some electrons have a weak association and are
considered delocalized -> this delocalization allows them to absorb energy
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The conjugated system is only found in pigment molecules. This is the only region
where absorption and energy transfer can happen.
Examples of pigments: chlorophyll, retinal, indigo
2. Light as a Source of Energy
Light enters the biosphere through photosynthesis (direct).
All living organisms require light, directly or indirectly.
Photosynthesis (through pigments in chloroplasts)
The synthesis of carbohydrates (sugars) from carbon dioxide and water, using light
energy to synthetize molecules that will help provide energy for the reaction
Chlorophyll (pigment) captures energy and this energy is used to synthetize energy-
rich molecules like NADPH and ATP.
NADPH and ATP are involved in the Calvin cycle of photosynthesis to convert CO2
into carbohydrates
Cellular Respiration
Break down carbohydrates and other molecules, trapping the released energy as
ATP and uses the energy for other biological processes
Both photosynthesis and cellular respiration occur in plants.
a. Not all organisms that use light as a source of energy are classified
as photosynthetic
Contains a pigment-protein complex called bacteriorhodopsin, which functions like
a proton-pump. This is a passive event.
Rhodopsin is made of a pigment (retinal) and a protein (opsin) located in the plasma
Concentration gradient: high concentration of H+ on the outside of the membrane,
low concentration on the inside. Protons follow the concentration gradient (from +
to area) and create a source of energy for the enzyme ATP synthase to generate
ATP from ADP and inorganic phosphate.
3. Light as a Source of Information
a. Rhodopsin, the universal photoreceptor
Used for vision in all animals and insects
Consists of a protein (opsin) that binds to a pigment (retinal)
As soon as photons are absorbed by the pigment, this causes a conformational change
(shape change) from cis-retinal to trans-retinal. This triggers alterations to the opsin
protein, which triggers downstream events up until the electric signals sent to the brain.
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Proteins similar to opsins are used in olfaction, suggesting that opsin proteins are useful
for sensory perception.
b. Sensing Light without eyes
Ma orgaiss like plats, algae ad soe ateria do’t hae a aera ee like
humans but are still able to sense light
How Plants See (but not form an image) still contains rhodopsin
Eyespot: light-sensitive structure present in chloroplasts containing phytochrome
It allows plants to gather information on light direction and intensity
Phototaxis: cells respond to light by swimming (with flagella) toward or away from it
to maximize light capture for photosynthesis
c. The Eye
Eye: the organ animals use to sense light
Vision: involves processing of light information (requires a brain to interpret electric
signals sent from the eye)
Different types of eyes
Ocelli, simple eye: a cup lined with up to 100 photoreceptor cells. Found in insects,
molluscs and arthropods. Gives information on light intensity and direction.
Compound eyes, like flies: hundreds of individual units, ommatidia, fitted closely
together. The organisms are excellent at detecting movement. Pigments on the side
prevent light from scattering laterally.
Camera Eye or Single-Lens Eye, like humans and most vertebrates: light enters the
cornea, is focused on the retina at the back of the eye (where photoreceptors like
rods and cones are). Rods sense light in low light conditions and cones sense
colour. Rhodopsin molecules are present in rods and cones. Photoreceptors send
information to the brain through the optic nerve.
Types of opsins
3 types of opsins: short wave sensitive (blue), medium wave (green), long wave (red)
An individual only possessing 2 types will have dichromatic vision.
An individual possessing all 3 types will have trichromatic vision.
The opsin genes (3 genes for 3 types) code for opsin proteins. Genes came from
random genetic mutation during duplication and stuck around because of selection
(some force or phenomenon impacting the survival of organisms)
How light impacts behaviour
signals, mate attraction, pollination, camouflage...
d. Darwin’s Theory
The eye as it exists now evolved over time from a simple, primitive eye.
Natural Selection: evolution of the eye can be explained by the improvement of
eyesight it would give organisms
Predators have better sight -> forces preys to have better sight for survival
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