- The effectiveness of light depends on its wavelength.
*An Action Spectrum is a plot of the effectiveness of different wavelengths of light on a biological process.
1.2 Light as a Source of Energy
- When an e- within a pigment molecule is in its excited state, it becomes a source of potential energy that can be use to do work. This energy is used in photosynthetic e- transport to synthesize the energy-rich compounds NADPH & ATP >> Then, they are used to convert CO2 into carbohydrates.
- Some of this energy is also used to to synthesize other biological molecules such as lipids, proteins, and nucleic acids, form simple building blocks found in the environment.
- Millions of photons are absorbed each second.
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Prokaryotes - Organisms in which their DNA is suspended in the cell interior w/o separation from other cellular components.
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Eukaryotes - Organisms in which their DNA is enclosed in a nucleus.
1.3 Light as a Source of Information
Rhodopsin, a HIghly Conserved Photoreceptor
- The most common photoreceptor in nature.
- Consists of a protein called opsin that binds a single pigment molecule called retinal.
* Opsins -membrane proteins that span a membrane multiple times & form a complex with the retinal molecule at the centre.
- Photon of light is absorbed >> the retinal pigment molecule changes shape >> alterations to the opsin protein are triggered >> more downstream events are triggered: alteration in intracellular ion concentrations and electrical signals. The electrical signals are sent to the visual centres of the brain.
Sensing Light WIthout Eyes
- In some organisms, photoreceptors of the eyespot allow the cell to sense light direction & intensity.
* Phototaxis - moving toward or away from a light source >> allows the cell to stay in the optimum light environment to maximize light capture for photosynthesis.
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In plants, the photoreceptor phytochrome senses the light environment and is critical for photomorphogenesis (shifting of seedlings towards the light while growing).
- Phytochrome is present in the cytosol of all plant cells. Plant is exposed to wavelengths of red light >> phytochrome becomes active >> initiates a single transduction pathway that reaches the nucleus. In the nucleus, these signals activate hundreds of genes, many of which code for proteins involved in photosynthesis and development.
The Eye
- Can be defined as the organ animals use to sense light.
- Requires a brain or a simple nervous system that interprets signals sent from the eye.
- We see not with our eye, but with our brain.
- Simplest eye: ocellus (pl. ocelli) - consists of up to 100 photoreceptor cells lining a cup or pit.
- in planarians the photoreceptor cells in a cup-like depression bellow the epidermis are connected by bundles of nerves to the cerebral ganglion.
- Ocelli are covered on one side by a layer of pigment cells that block most of the light waves arriving from the opposite side of the animal. As a result, most of the light received by the pigment cells enters the ocellus from the side it faces. By interpreting info. transmitted to the cerebral ganglion from the eyecups, planarians orient themselves so that light falls equally on their two ocelli, and reduces as they swim.
- This carries them directly away from the source of light, and away from predators.
- Image forming eyes are found in compound eyes and in single-lens eyes
- Compound eyes are common in arthropods such as insects & crustaceans (crabs).
- Contain hundreds to thousands of ommatidia (omma - eye) units fitted closely together. Each ommatidium provides with only a small part of the visual field. The brain receives a mosaic image of the world from all the photoreceptor cells from the ommatidia.
- Since even the slightest motion is detected by many ommatidia at once, compound eyes are extraordinarily adept at detecting movement.
- Single-lens eyes (“camera eyes) are common among vertebrates & some invertebrates.
- When light enters the eye through the cornea, it is concentrated by a lens, and then a layer of photoreceptors at the back of the eye (the retina) records the image of the surroundings.
Darwin and the Evolution of the Eye
- Proposed that the eye as it exists in humans and other animals didn’t appear suddenly but evolved by mutation and natural selection over time from a simple, primitive eye.
- Recent study had predicted that about 2000 small improvements over time would gradually yield a camera-type eye in less than 0.5 million years.
- Eye evolution is explained by the huge advantage an improved eye would provide for an organism.
- Improvement in visual ability of a predator would force comparable improvements in both prey and other predators. ∴ Rapid eye development would be critical for survival.
- The brain must improve at the same time, allowing for more advanced neural processing of info. sent from the optic nerve in order for the eye-improvements to be any good.
1.4 Light Can Damage Biological Molecules
- Only a very small portion of the electromagnetic spectrum is essential to life on Earth. The wavelengths from about 400-700 nm are used for photosynthesis, vision, phototaxis, navigation, and many other light-driven processes.
- It may not be a coincidence that these processes depend on such a narrow band of the Electromagnetic spectrum. Light is the most dominant form of electromagnetic radiation reaching Earth. Shorter wavelengths are absorbed by the ozone layer, yet longer wavelengths are absorbed by water vapour & CO2 in the atmosphere.
- Also, other wavelengths of electromagnetic radiation contain different amounts of energy.
- Shorter wavelengths contain enough energy to destroy the chemical bonds that hold together the molecules living things are made up from.
-Absorption of high-energy photons wouldn’t just excite e- within pigments, but actually oxidize the molecule producing ions >> ∴ Shorter wavelengths of electromagnetic radiation are often referred to as ionizing radiation
- Longer wavelengths would not supply enough energy to excite the e- necessary for photochemistry.
- Longer wavelengths are easily absorbed by water, which makes up the bulk of all living things.
Damage by Light: Direct Effects
- Light is a form of energy with the potential to damage biological molecules: directly & indirectly.
- All organisms that are exposed to sunlight have developed a mechanism to either help prevent light-induced damage, or if already occurred, to help repair it quickly.
-The photosynthetic apparatus of photosystems: pigment-protein complexes that trap energy of light & convert it to chemical energy. Chloroplasts contain hundreds of photosystems that trap the energy about ~10 000 photons/sec. The high-energy environment within a photosystem’s core often results in damage to its proteins. The damage is unavoidable, but rapid repair was developed early during the evolution of life to maintain the rate of photosynthesis even under high-light conditions.
Damage by Light: Indirect Effects
- Ultraviolet radiation ( 200-400 nm) accompanies light from the sun.
- The atmosphere’s ozone layer protects life on Earth from the most damaging form of UV light, UV-C. Longer wavelengths, UV-B and UV-A do reach the Earth’s surface.
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Because of its high energy, UV radiation can randomly damage atoms in a range of molecules such as pigments and proteins. DNA is particularly vulnerable to damage, and the interaction of UV light with nucleotide bases can result in the formation of a “dimer” when 2 neighboring bases become covalently linked. Dimers can change the shape of the DNA double-helix, preventing its replication and slowing down gene-expression. This can result in harmful genetic mutations.
- Organisms use a range of behavioral, physiological, and biochemical mechanisms to protect themselves.
- May avoid intense sunlight/shield skin with fur or feathers.
- Naked organisms rely on producing melanin as a protective mechanism.
- Melanin - a pigment that absorbs UV radiation.
- Humans synthesize melanin in specialized skin cells called melanocytes.
- Presence of melanin prevents the DNA damage in skin cells >> linked to the development of skin cancer. Melanin prevents UV radiation from penetrating the skin & destroying the essential B vitamin folate.
- Humans require some UV radiation to synthesize vitamin D >> critical for normal bone development.
1.5 Role of Life in Ecology and Behaviour
- Organisms adapt to the specific light environment of particular habitats >> may lead to unique colorations that may serve to attract members of the same species while being potentially less visible for predators.
Using Light to Tell Time: Circadian Rhythms
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Circadian Rhythms - physiological & behavioral responses geared to Earth’s day-night cycle (with the
period of approx. 24 hours). (circa = “around”; diem = “day”)
- Are NOT direct responses to changes in external light environment, but are controlled by an internal organism-based clock >> set by the external light environment, but can run a long time without any input from outside the organism.
- The free-running nature of Circadian Rhythms was discovered in 1729 by the French astronomer Jean-Jacques d’Ortous de Mairan.
- Found in all forms of life.
- Allows organisms to anticipate when a process occurs most efficiently in the 24-hour day and prepare accordingly.
- In most animal the central biological clock is found within the suprachiasmatic nucleus (brain > within the hypothalamus). The suprachiasmatic nucleus receives light inputs directly from the eye via the optic nerve, using it to set the biological clock.
- In turn, this clock regulates a wide range of bodily functions, such as the secretion of the melatonin hormone from the pineal gland.
- Melatonin is thought to have a role in our sleep-wake cycles since its synthesis is active at night but inhibited during the day.
- Cannot be automatically reset to the new conditions, may take a few days to become readjusted.
Avoiding Detection: Camouflage
- When one animal fails to distinguish another from the background.
- Pattern and behaviour = central roles in camouflage.
Using Colour as Signals
- Colour is often used by animals to signal that they are distasteful, armed, and/or dangerous.
- If the signal-receiver is blind there is no point.
- noise! rattle snakes, bees.. smell! skunk, pheromones...
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Flowers are colourful in order to attract pollinators to move pollen from the male parts (anthers) of one flower to the female parts (stigma) of the same or other flowers for fertilization & production of seeds.
- Pollinators are happy too - they get sugar-rich nectars & protein-rich pollen from flowers.
Light in Aquatic Habitats
- Water “swallows” light rapidly, and almost no light penetrates below ~150 metres.
- Water scatters & absorbs longer wavelengths of light more effectively than short ones >> below ~30 metres, light is essentially monochromatic >> only consisting of blue wavelengths.
- Red algae contain the accessory photosynthetic pigment phytoerythrin, which is not found in land plants and absorbs blue wavelengths of light >> ∴ the distinctive colour of red algae.
Ecological Light Pollution
- The presence of artificial light disrupts orientation in nocturnal animals, that are used to operate in the dark. Affects baby sea turtles, frogs, salamanders, migrating birds, and more.
1.6 Life in the Dark
- With decreasing light levels we first loose our ability to see colour and the the ability to distinguish shapes. Nocturnal animals see very well under dim light.
- 25 million years of adaptation to life in the dark has resulted in the natural degeneration of animals’ visual system to sometimes the point of complete blindness.
- Studies have shown that the photoreceptors of the eye remain functional even though the image-forming part of the brain is dramatically reduced.
- The photoreceptors (for the mole rats, at least, since they are exposed to light for brief periods) allow the setting of their biological clocks & control their circadian rhythms.
1.7 Organisms Making Their Own Light: Bioluminescence
- Has developed many times during evolution but always involves same basic biochemical reaction:
Chemical energy in the form of ATP excites an e- in a substrate molecule to a higher excited state >> the e- returns to ground state >> energy is released as a photon of light.
- Remarkably efficient: 95% of the energy of a light bulb is lost as heat, while less than 5% of the energy used for bioluminescence is given off as heat. High heat production would be incompatible with life.
- Bioluminescent organisms use light to attract a mate, attract prey, for camouflage, or to communicate.
- Organisms that use bioluminescent signals must have light-sensing organs since light must be perceived by another organism to be useful for the organism producing the light.
- Most biolum. organisms are marine and most are very common bellow 800 metres, where sunlight does not penetrate.
