Light Flashcards
The electromagnetic spectrum
Radio, microwave, infrared, visible, ultraviolet, xray, gamma ray
Visible light
400-700nm
Red to Violet (violet shortest wavelength, highest energy)
- Just right for: photosynthesis, phototaxis and phototropism, photomorphogenesis, biological rhythms, vision in most animals
Infrared
- 700nm - 1mm
- too little energy to be useful biologically
- thermal radiation
- some animals can “see” IR: some snakes have thermal receptors around their noses
Photosynthetic rate and light intensity
- rate of photosynthesis generally increases with intensity of visible light, although shade tolerant plants have a point of photoinhibition after which photosynthetic rate slows due to closure of stomata and other regulatory mechanisms
- Shade tolerant plants use far-red light
Light used in photosynthesis
- Chlorophyll a
- Chlorophyll b: green algae and plants
- Chlorophyll c: chromista
- Carotenoids: photosynthetic (PSC) and photoprotectant (PPC)
Aquatic light limitations
- water attenuated light (decreases with depth)
- water absorbs wavelengths differentially (salinity affects light penetration)
- lots of green and blue light in deep water, chlorophylls and carotenoids mostly absorb blue-violet and some red light
- So light available at greater depths in coastal marine systems is not absorbed by chlorophyll or crotenoids
Phycobilins
- phycocyanin and phycoerythrin
- found in cyanobacteria and red algae (not green algae or plants)
- allow some photosynthetic organisms to live at greater depths in coastal waters
- Have phycobilins so can live at greater depths
Terrestrial adaptations (plants)
- higher light environment than in water
- light attenuated by vegetation; benefits to growing taller, above competition; C4 plants do better than C3 plants under high light conditions
C3 plants
- Most plants
- Fix carbon: light reactions then Calvin cycle; producing sugar
- C3 because first product of Calvin cycle gives you 3PGA which has 3 carbons
- Limitation: Close stomata when hot and dry, reduces photosynthesis
- Process happens continuously; no storing of carbon dioxide
- Energy efficient, but water lost to evaporation in hot climates
C4 plants
- Have a step before Calvin cycle and form a 4th carbon molecule
- Fix CO2 in mesophyll cell, and then combine with PEP (not rubisco) –> get 4 carbon molecule which can be STORED in the mesophyll cell
- Adaptation to minimize photorespiration
- Minimizes water loss
- Sugar cane, corn
- Stores carbon because stomata is closed often in hot, dry environments
- Water loss minimized in warm climates BUT requires more energy
CAM plants
- Pineapple, cactus, succulents
- Only open stomata at night
- Similar to C4 but longer-term storage
- Take up CO2 at night and store it until morning
- Photosynthesis still goes on when stomata is closed
- Water loss minimized BUT more energy and slower growth
Other terrestrial adaptations (plants)
• Leaves at bottoms of plants may be larger than those at top
• Waxes, hairs
- Reflect or prevent absorption of radiation
- Mediate heat and UV
- BUT may block too much radiation (no photosynthesis)
Phototaxis
- Phototaxis: movement of organisms towards or away from the sun (or artificial light) – ex: moth fly to lights
- Helps maximize photosynthetic potential
- May be important in thermoregulation
- May affect migration
Phototropism
- Phototropism: orientations of plants towards/away from light
- Due to auxins that causes cell elongation on one side
- Maximize photosynthesis
- Minimize water loss
- Sometimes flowers orient to sun to provide warmth to pollinators
- Shoots and leaves generally move toward light (positive) and the roots it’s negative (away)
- Darwin’s study: phototropism is due to light detection in shoot tip – for that species at least
- Boyce Jensen: actually, the point is a little bit below the tip
Photomorphogenesis
- Control of morphogenesis (structural development) by light
- Can be studied by comparing plants grown under different light conditions
- In light: promote development of chlorophyll, root system, green leaves
- In dark: taller, yellow/white, no leaves, less developed root system
Photoperiodism
(Biological rhythms)
- Circadian rhythms
• 24-hour cycle; periods of light and dark
- Seasonal rhythms
• Length of days in a season
• Migration
- Plants as well; dispersal of seeds to wider area
• Germination (photodormant plants)
• Physiological changes
- In plants:
• Photoreceptive protein that detects length of night; many plants will only flower under specific light conditions
• Longer days vs shorter days (cocklebur vs clover) – critical period of light; too little it won’t, too much it won’t
- Cocklebur: short day, long night
- Clover: long day, short night
Photoperiodism involved in fall leaves
• During leaf death or senescence, the green pigment chlorophyll degrades and the other colours (reds, yellows) start to show
• Shorter days and cold; photoinhibition, loss of chlorophyll
• Anthocyanins produced
- Helps shut down photosynthesis by absorbing some light while protecting against UV
- Study: soil nutrient availability can affect anthocyanin production
Photoperiodism in mammals and animals
In mammals: • Registered in suprachiasmatic nucleus in hypothalamus, informed by ganglion in retina In animals • May be responsible for colour change (ex snowshoe hare) • Onset of hibernation • Sexual behaviour • Shedding, molt • Migration
Vision in animals
- Light bounces off or is produced by objects
- Photoreceptors in eyes that transmit message to brain
- Single-lens or compound
Vision in invertebrates
- Simplest eyes: pigment spot ocelli just detect light
- Spiders: several pairs of simple ocelli, only the central ones with moveable retina
- Compound eyes: thousands of individual photoreceptor (omatidia) units on a convex surface; more ommatidia, better resolution
Vision in Vertebrates and some molluscs
• Complex eyes • Can distinguish shpaes and colours • May confer depth perception – binocular vision • Light enters and projects onto retina • Control of light (iris contracts and dilates; not in most fish) • Cone cells: - In centre of retina - Used for central vision - Three pigments (S,M, L) - Used for colour vision - Requires lots of light: used mostly in the day • Rod cells: - Edge of retina - Peripheral vision - One pigment NOT for colour - Requires less light than cones; night vision
Why is vision important?
• Foraging • Mate recognition • Predator avoidance • Photoperiodism** - Blind fox in ecomuseum can’t control fur color and would die in wild • Habitat selection
Ultraviolet
100-400nm
- lots of energy, cane damage DNA, proteins
- Leads to free radicals, unstable oxygen molecules (only 1 electron)
- Bees can see UV
UVC
- Produces ozone
- Pale blue gas with pungent smell
- Doesn’t penetrate ozone layer
- Effects of ozone:
• In atmosphere = protective
• Ground level; it irritates respiratory system, produced by burning fossil fuels, interferes with photosynthesis and plants growth (because enters plants through stomata and ends up inhibiting opening of pores and decreases CO2 absorption reducing plant productivity and growth)
• Physiologically produced by white blood cells - Ozone interferes with photosynthesis and can cause crop yields to decrease
- Germicidal; sterilizes by damaging DNA
