Light and Life review

For organisms, light serves as a source of energy and as a source of information.
Light can be defined as electromagnetic radiation that humans can detect with their eyes.
Light can be thought of as a wave of discrete particles called photons.
Light energy must be absorbed by molecules called pigments to be used.
The colour of a pigment includes all wavelengths of light that are not absorbed.

When a pigment absorbs light, electrons become excited, which represents a source of potential energy.
Photosynthesis is the dominant process on Earth. It uses pigments to capture light energy and converts carbon dioxide into energy-rich carbohydrates.
Hatobacteria use bacteriorhodopsin to harvest light energy and to generate ATP.

A photoreceptor (e.g., rhodopsin) consists of a pigment molecule (retinal) bound to a protein opsin).
The C. reinhardtii eyespot allows the organism to sense both light direction and intensity and respond by swimming toward or away from the light (phototaxis).
The eye can be defined as the organ animals use to sense light. Vision requires a brain to interpret signals sent from the eye.
The simplest eye is the ocellus found in planarians. It enables the sensing of light direction and intensity.
Image-forming eyes include compound eyes found in arthropods and single-lens eyes found in some invertebrates and most vertebrates, including humans.
Because the eye was thought to be an organ of “extreme perfection," Darwin initially had a difficult time explaining how it could have arisen by evolution.
The relatively rapid evolution of the eye is explained by the huge advantage an improved eye would give an organism.

Photosynthesis, vision, and most other light-driven processes use only a narrow band of the electromagnetic spectrum. This may be because shorter wavelengths are more harmful (higher energy)

Light is a form of energy; thus, the absorption of too much light can damage biological molecules.
The photosynthetic apparatus is constantly being damaged by light, and the damage repaired.
Because of its high energy, ultraviolet radiation is particularly harmful to biological molecules, particularly DNA.
Human skin cells are protected by the pigment melanin, which absorbs ultraviolet radiation.

Many physiological and behavioural responses are geared to the daily changes in light and darkness and are called circadian rhythms.
Circadian rhythms are found in all life forms and evolved to enable organisms to anticipate changes in the light environment.
Jet lag is caused when your biological clock is out of synchronization with the external light environment.

Many organisms use colour to attract, warn, or hide from other organisms.
Bright coloration is thought to convey good health.
The widespread use of artificial lighting has been shown to disrupt numerous biological phenomena, including bird migration and the orientation of nocturnal animals.

Unlike humans, many nocturnal animals (moths, fish, bats, frogs) see very well under dim light conditions.
Some animals, such as the blind mole rat, are functionally blind yet are descended from ancestors who had functional eyes.

A range of organisms can use chemical energy to make light—this is called bioluminescence.
Bioluminescent organisms use light to attract a mate, for camouflage, to attract prey, or to communicate.