Biology 1000 chapter 1 – summary notes
1.1a what is light
Light serves as two important functions for life on Earth.
1. First, it is a source of energy that directly or indirectly sustains virtually all organisms.
2. Second, light provides organisms with information about the physical world that surrounds them.
Sun converts over 4 million tonnes of matter into energy every second.
This energy is given off as electromagnetic radiation, which travels in the form of a wave at a
speed of 1 079 252 848km/h (speed of light)
The wavelength of electro- magnetic radiat6on ranges from less than one picometre (10 m) for
cosmic rays to more than a kilometre (10 m) for radio waves.
So what is light? Light is commonly defined as the portion of the electromagnetic spectrum that
humans can detect with their eyes.
Light has no mass, some experiments suggest that light behaves as a wave as it travels through
space, the results of other experiments are best explained by light being composed of a stream of
energy particles called photons.
shorter-wave- length are the blue light, consists of photons of higher energy than red light, which
has a longer wavelength and photons of lower energy.
1.1 b Light interacts with matter
light has no mass, it is still able to interact with matter and cause change.
This change is what allows the energy of light to be used by living things.
When a photon of light hits an object, the photon has three possible fates:
1. It can be reflected off the object
2. Transmitted through the object
3. It can be absorbed by the object, to be used as a source of energy or information by an organism,
The absorption of light occurs when the energy of the photon is transferred to an electron within a
A photon can be absorbed by an electron of a molecule only if the photon energy equals the
energy difference between the electron’s ground state and an excited state.
If the energies don’t match, then the photon is trans- mitted through the molecule or reflected.
A major class of molecules that are very efficient at absorbing photons are called pigments.
Absorption: a region where carbon atoms are covalently bonded to each other with alternating
single and double bonds.
This bonding arrangement is called a conjugated system, and it results in the delocalization of
A pigment’s colour is the result of photons of light that it does not absorb.
Instead of being absorbed, these photons are reflected off the pigment or transmitted through the
pigment to reach your eyes 1.2 Light as a Source of Energy
Following light absorption, the potential energy of excited electrons within pigment molecules
such as chlorophyll is used in photosynthetic electron transport to synthesize the energy-rich
compounds NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine
molecules are in turn consumed in the Calvin cycle of photosynthesis to convert carbon dioxide
While photosynthesis converts carbon dioxide into carbohydrates, it is the process of cellular
respiration that breaks down carbohydrates and other molecules, trapping the released energy as
energy-requiring metabolic and biosynthetic processes that are fundamental to life.
1.3 light as a source of information
1.3a Rhodopsin, the Universal Photoreceptor
The basic light-sensing system is termed the photo- receptor
The most common photoreceptor in nature is rhodopsin
Each rhodopsin molecule consists of a protein called opsin that binds a single pigment molecule
Absorption of a photon of light causes the retinal pigment molecule to change shape.
triggers alterations to the opsin protein, which, in turn, trigger downstream events, including
alterations in intracellular ion concentrations and electrical signals.
Each photoreceptor cell contains thousands of individual rhodopsin molecules.
1.3 b Sensing Light without Eyes
Many organisms can sense the light in their surroundings even though they lack organs that we
would considered to be eyes
These organisms include plants, algae, invertebrates, and even some prokaryotes.
In plants, a photoreceptor called phytochrome senses the light environment and is critical for
photo- morphogenesis, the normal developmental process activated when seedlings are exposed
1.3 c The Eye
The eye can be defined as the organ animals use to sense light.
The process of vision requires not only an eye to focus and absorb incoming light but also a brain
or at least a simple nervous system that interprets sig- nals sent from the eye.
Eye and brain are thought to have co-evolved
Essentially, we see not with our eyes but, rather, with our brain.
The simplest eye is the ocellus
Which consists of a cup or pit lined with up to 100 photoreceptor cells.
A common group of organisms that contain ocelli are flatworms of the genus Planaria
Compound eyes, which are common in arthropods such as insects and crustaceans, are built of
hundreds of individual units called ommatidia (omma = eye) fitted closely together organisms with com- pound eyes are extraordinarily good at detecting movement, a lesson soon
learned by fly-swatting humans.
the other major type of eye is called the single- lens eye or camera-like eye and is found in some
invertebrates and most vertebrates, including humans.
in a single-lens eye, as light enters through the trans- parent cornea, a lens concentrates the light
and focuses it onto a layer of photoreceptor cells at the back of the eye, the retina
The photoreceptor cells of the retina send information to the brain through the optic nerve.
1.4 The Uniqueness of Light
Visible light is used by organisms because it is the most dominant form of electromagnetic
radiation reaching Earth’s surface.
1.5 – Light Can Damage Biological Molecules
For light to be used as energy, photons must contain the same energy as the difference in
energy from one energy level to another in an atom
Absorbing excessive light can result in damage (permanent)
1.5a – Damage Is an Unavoidable Consequence of Light Absorption
High energy light can negatively affect pigment molecules. Excessive light energy can cause the
atoms of the pigment molecule to reach an excited state; when there are too many excited
electrons, some can react with oxygen. This produces reactive oxygen species which include
molecules such as hydrogen peroxide that can damage proteins and other essential components
of the pigment. This is called photo-oxidative damage. Particularly, this can occur within the
retina of the eye
Unlike the photosynthetic apparatus of humans, those of plants are exposed to the high energy
environment, that’s facilitated by the sun, all day long. Plants, however, contain photo systems
(found in chloroplasts) that can absorb large quantities of photons for creating chemical energy.
However, photo systems can still be affected by photo-oxidative damage, which is why plants
have a repair system that replaces harmed proteins.
1.5b – Ultraviolet Light is Particularly Harmful
The photons of U.V light react with oxygen to form O , thus the Ozone layer – an imperative