Chapter 1: Light and Life
- Light has 2 important functions: source of energy for all organisms and
provided info with world around them
o Eg green algae
- each cell has a large chloroplast that uses energy in which it uses to make
enrgy rich molecules through photosynthesis
- has eyespot( gathers information about location/intensity if light source)
- The only reason we have life is due due to the distance between earth and
the sun (150mil km)
- Sun converts 3.4x10^38H nuclei per second
- Light is defined as the visible part of the EM spectrum that we can detect
with out eyes. Spans between 400-700nm.
- Light has no mass; composed of photons (particle/wave duality)
- Light understood as wave of photons (inverse relationship between WL and
1.1b-Light interacts with Matter
- when photon hits an object, it’s either transmitted, reflected or absorbed.
- its to be used as an energy/source of info; has to be aborsorbed.
- occurs when energy of photon electron within molecule
pigments: molecules that absorb photons
- all pigments share a region where carbon atoms are covalently bonded to
each other with alternating single and double bonds --- called conjugated
- - pigments absorb light at different wavelengths (b/c they differ in ## of
wxcited states )
- a pigments color is the result of photons it didn’t absorb, this color is
reflected transmitted to our eyes.
1.2Light as Source of Energy
- photosynthesis (light energy used by plants to convert co2 to sugars)
- cellular respiration breaks down carbs, releasing energy as ATP
-this in turn is used by the energy requiring metabolic biosynthetic processes that
are fundamental to life.
-species of halobacterium contain pigment protein complex called
bacteriorhodopsin (functions as light driven photon pump)
-captures photon of light that provide energy supply to pump protons out of cell.
-the resulting difference is the H+ concentration across the plasma membrane as a
source of of potential energy used by ATP synthase to generate ATP from ADP and
1.3 Light as a source of info; 1.3a Rhodopsin, the universal photoreceptor
- basic light sensing system is termed the photo receptor.
-most common photoreceptor is rhodopsin (serves as light sensing unit of
reinhardtii) -each rhodopsin molecule contains of a protein called opsin (binds single molecule
-opsins are membrane proteins that span a membrane multiple times and form a
complex with retinal molecule at centre
-absorption of a photon causes the retinal pigment molecule to change shape, which
triggers changes to the opsin protein which causes alterations in intracellular ion
-in humans, light captured by the eye involves 125 million photoreceptors cells.
Each photoreceptor contains 1000’s of rhodopsin molecules.
1.3b-Sensing light without eyes
- eyespot is a sensitive structure that is approx. 1um in diameter and found within a
chloroplast of a cell.
- eyespot consists of 2 layers of carotenoid rich lipid globules that direct light
-200 different proteins assembled to produce eyespot
-eyespot doesn’t play a role in photosynthesis
-instead, the PR of the eyespot allow the cell to sense light direction and intensity
-photoaxis cells respond to light by swimming accordingly towards/away light
-allows cell to stay in optimal light capturing for photosynthesis.
-light absorption by eyespot is linked to swimming response by signal tramsduction
pathway; triggers changes in concentrations of ions (K and Ca), which generate
- in plants (PR called phytochrome senses light environment and is used for
phoromorphgenesis- normal development process when seedlings are exposed to
-phytochrome is present in the cytosol of plant cells and when the seedling is
exposed to red light, the pytochrome becomes active and starts a signal pathway
that goes to the nucleus where the signal activates 100’s of genes which code
protein involved in photosynthesis and leaf development
1.3c The Eye
- eye: the organ that senses light
-vision distinguishes eye of inverterbrate from others
- process of vision requires eye to absorb and focus light and a brain/NS that
interprets signal sent to eye.
-simplest eye is called ocellus, (lined with 100’s of PR cells)
-PR cell is a modified nerve cell that has 1000’s of PR molecules
image forming eyeS: compound eyes and single lens eyes.
-compound eyes: built on 100’s of ommatidia fitted together. Each ommatidium
samples only small part of visual field with incoming light focused on PR cells.
-from these signals, brain receives image.
-single eye: light enters through the cornea, lens conentrates light and focuses it
onto layer of PR cells at back of the eye where the retina uses its PR cells to send the
iformation to the brain via the optic nerve. 1.3d-Darwin and Evolution of eye
-darwin proposed eye would have problem; too complicated to have appeared
-he proposed that eye evolved from simple eye over changes in time
-study shows that light sensitive cells on skin would take over 500000 years along
with 2000 small improvements to yield a single lens eye.
1.4Uniqueness of Light
-light essential to life (400-700nm)- used for PS, vision, photoaxis, navigation, etc
-George Wald (1906-1997) reasoned that visible light used by organisms because
most dominant form of EM radiation reaching earth’s surface.
-Shorter wavelength absorbed by ozone layer, longer absorbed by co2/h20
-radiation of shorter wavelength of light contains energy to destroy chemical bonds
humans are made of
-radiation of longer wavelength wont have enough energy to move an electron from
its ground to higher, excited state. Also longer WL absorbed by h20
1.5-Light Can damage biological molecules
-PR cells that line human retina can be damaged by light
- high energy environment with pigment molecules along with excited electrons
results in photo oxidative damage
-excess light energy reacts with 02, results in reactive oxygen species
-these species include hydrogen peroxide which are damaging to proteins/other
biological molecules causing to lose their function and maybe lead to death.
-typical chloroplast contains 100’s of photosystem with each trapping energy
of1000’s of photons/second, converting them from light energy to chemical energy
-photosystem damage repaired by removing damaged proteins and replacing them
with new ones
-PS2 needs to be repaired ever 20 mins
1.5b-UV light harmful
-wavelenghts between 200-400nm and energy > & damaging to biological molecules
-earth protected from UV light due to ozone layer
-ozone (03) produced when UV light reacts with 02
-short UV rays are absorbed by ozone, longer UV reaches earth
-xrays, gamma rays, Uv rays are ionizing radiation-photons energetic enough to
remove electron from atom producing an ion (proton/electron not equal)
-structure of DNA can be damaged by DNA
-UV light reacts with the nucleotide bases which results in dimer which is when
neigbouring bases covalently bond.
-Dimers change the shape of the double helix structure and prevent replication and
-since exposure to UV is unavoidable, organism use range of bio, mech, struct,
mechanisms to protect themselves eg feathers 1.5c Melanin, sun tanning and Vitamin D
-organisms synthesize melanin pigment that absorbs UV light
-melanin also component of ink released by a squid
-it absorbs UV light, and releases 99% of it harmlessly as heat
-spectrophometer detects which WL of light are transmitted through sample and
determines which WL absorbed by pigment
-humans synthesize skin in skin cells called melanocytes
-melanin synthesizes increase upon exposure to sun, which increases dark skin,
-Africans have more melanin in their skin due to this
-Humans require some UV light as it makes vitamin D (crucial for bone
1.6 Using light to tell time
-Day/night and seaso