Chapter 1: Light and Life
1.1 The Physical Nature of Light
• Light serves two important functions
1. It is a source of energy that directly or indirectly sustains virtually all organisms
2. Light provides organisms with information about the physical world that
• C. reinhardtii (green alga) has a light sensor called an eyespot that allows individual
cells to gather information about the location and intensity of a light source
1.1a What Is Light?
• The reason there is life on Earth
• The Sun converts over 4 million tonnes of matter into energy every second. This ener-
gy is given off as electromagnetic radiation
• Scientists often distinguish different types of electromagnetic radiation by their wave-
length, the distance between two successive peaks
• Light is most commonly defined as the portion of the electromagnetic spectrum that
humans can detect with their eyes. This is a very narrow portion of the electromagnetic
• Light has no mass
• It is composed of a stream of energy particles called photons
• Light is best understood as a wave of photons
• The longer the wavelength, the lower the energy of the photon it contains
1.1b Light Interacts with Matter
• When a photon of light hits an object, the photon has three possible fates; it can be re-
flected off the object, transmitted through the object or it can be absorbed by the object
• To be used as a source of energy or information by an organism, it is absorption that
must take place.
• The absorption of light occurs when the energy of the photon is transferred to an elec-
tron within a molecule.
• This excites the electron, moving it from its ground state to a higher energy level that is
referred to as an excited state
• 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 • The excited state electron that represents the source of energy required for processes
such as photosynthesis and vision
• Molecules that are very efficient at absorbing light are called pigments.
• What is it about pigments that enable them to capture light?
• A region where carbon atoms are covalently bonded to each other with alternating sin-
gle and double bonds
• This bonding arrangement is called a conjugated system, and it results in the delocal-
ization of electrons.
• None of these electrons are closely associated with a particular atom or involved in
bonding and thus are available to interact with a photon of light
• A pigment’s colour is the result of photons of light that it does not absorb
1.2 Light as a Source of Energy
• 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 and ATP
• Not all organisms that use light as a source of energy are classified as photosynthetic.
• Halobacterium contain a pigment-protein complex called bacteriorhodopsin, which
functions as a light-driven proton pump.
• The pigment component of bacteriorhodopsin captures photons of light that provide
the energy supply needed to pump protons out of the cell
• The resulting difference in H+ concentration across the plasma membrane represents
a source of potential energy that is used by the enzyme ATP synthase to generate ATP
1.3a Rhodopsin, the Universal Photoreceptor
• The basic light-sensing system is termed the photoreceptor
• The most common photoreceptor in nature is rhodopsin which is the basis of vision in
animals but is also very common in other organisms, including C. reinhardtii, where it
serves as the light-sensing unit of the eyespot
• Consists of a protein called opsin that binds a single pigment molecule called retinal
• Absorption of a photon of light causes the retinal pigment molecule to change shape.
• This change triggers alterations to the opsin protein, which in turn, trigger downstream
events, including alterations in intracellular ion concentrations and electrical signals
1.3b Sensing Light without Eyes • The eyespot is a light-sensitive structure that is approximately 1um in diameter and is
found within the chloroplast of the cell, in a region closely associated with the cell mem-
• 200 different proteins are assembled to produce the eyespot apparatus, including spe-
cific opsin proteins that are the basis of the rhodopsin-based photoreceptors.
• The photoreceptors of the eyespot allow the cell to sense light direction and intensity.
• Cells can respond to light by swimming toward or away from the light source in a
process called phototaxis.
• This allows the cell to stay in optimum light environment to maximize light capture for
• In plants, a photoreceptor called phytochrome senses the light environment and is crit-
ical for photomorphogensis, the normal developmental process activated when
seedlings are exposed to light.
1.3c 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 signals sent from the
• Essentially, we don’t see with our eyes but rather with our brains
• the simplest eye is the ocellus in flatworms.
• Information sent to the cerebral ganglion from individual eyes enables the worms to
orient themselves so that the amount of light falling on the two ocelli remains equal and
diminishes as they swim.
• This reaction carries them directly away from the source of the light and toward darker
• Compound eyes, which are common in arthropods such as insects and crustaceans,
are built of hundreds of individual units called ommatidia fitted closely together.
• Ommatidium samples only a small part of the visual field with incoming light being fo-
cused onto a bundle of photoreceptor cells.
• From these signals, the brain receives a mosaic image of the world
• Even the slightest motion is detected simultaneously by many ommatidia, organisms
with compound eyes are extraordinary good at detecting movement
• 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. • Light enters through the transparent cornea, a lens concentrates the light and focuses
it onto a layer of photoreceptor cells at the back of the eye, the retina.
• Sends information to the brain through the optic nerve.
1.3d Darwin and the Evolution of the Eye
• Darwin found the eye as it exists in humans and other animals did not appear sudden-
ly but evolved over time from a simple, primitive eye.
• The single lens eye found in humans, could have evolved more than 1000 times.
• Gene called Pax6 that has been identified as a master control gene that is almost uni-
versally employed for eye formation in animals
• Eye evolution in many different animal phyla is the huge advantage eyesight
1.4 The Uniqueness of Light