Light

Question 1

The electromagnetic spectrum

Answer 1

Radio, microwave, infrared, visible, ultraviolet, xray, gamma ray

Question 2

Visible light

Answer 2

400-700nm
Red to Violet (violet shortest wavelength, highest energy)
- Just right for: photosynthesis, phototaxis and phototropism, photomorphogenesis, biological rhythms, vision in most animals

Question 3

Infrared

Answer 3

• 700nm - 1mm
• too little energy to be useful biologically
• thermal radiation
• some animals can “see” IR: some snakes have thermal receptors around their noses

Question 4

Photosynthetic rate and light intensity

Answer 4

• 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

Question 5

Light used in photosynthesis

Answer 5

• Chlorophyll a
• Chlorophyll b: green algae and plants
• Chlorophyll c: chromista
• Carotenoids: photosynthetic (PSC) and photoprotectant (PPC)

Question 6

Aquatic light limitations

Answer 6

• 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

Question 7

Phycobilins

Answer 7

• 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

Question 8

Terrestrial adaptations (plants)

Answer 8

• 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

Question 9

C3 plants

Answer 9

• 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

Question 10

C4 plants

Answer 10

• 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

Question 11

CAM plants

Answer 11

• 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

Question 12

Other terrestrial adaptations (plants)

Answer 12

• 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)

Question 13

Phototaxis

Answer 13

• 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

Question 14

Phototropism

Answer 14

• 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

Question 15

Photomorphogenesis

Answer 15

• 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

Question 16

Photoperiodism

Answer 16

(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

Question 17

Photoperiodism involved in fall leaves

Answer 17

• 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

Question 18

Photoperiodism in mammals and animals

Answer 18

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

Question 19

Vision in animals

Answer 19

• Light bounces off or is produced by objects
• Photoreceptors in eyes that transmit message to brain
• Single-lens or compound

Question 20

Vision in invertebrates

Answer 20

• 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

Question 21

Vision in Vertebrates and some molluscs

Answer 21

• 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

Question 22

Why is vision important?

Answer 22

• Foraging 
• Mate recognition 
• Predator avoidance 
• Photoperiodism** 
- Blind fox in ecomuseum can’t control fur color and would die in wild  
• Habitat selection

Question 23

Ultraviolet

Answer 23

100-400nm

• lots of energy, cane damage DNA, proteins
• Leads to free radicals, unstable oxygen molecules (only 1 electron)
• Bees can see UV

Question 24

UVC

Answer 24

• 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

Question 25

UVB

Answer 25

• Erythermal; can cause sunburns and other skin damage
• Needed for vitamin D production
  • Absorption of calcium in bones and intestines
  • Sources: fatty fish, dietary supplement (milk) and our bodies created vitamin D in the presence of UVB
  • So technically a hormone
  • Most people in northern hemisphere don’t make vitamin D naturally because don’t get enough UVB

Question 26

UVA

Answer 26

• Most of UV that reaches the earth (90-95%)
• Used in tanning beds
• Penetrates deeper in skin = melanin which darkens us AND because it doesn’t cause burning was thought to be better BUT now we know it causes cancer
• Causes aging, sagging and cancer

Question 27

Preventing damage

Answer 27

• Move out of sun
• Immune system destroys affected cells
• Antioxidants: scavengers of free radicals
• Repair mechanisms especially for DNA
  • Up or down regulate expression of genes coding for repair and replication enzymes
• Sun screen (ex: Pigments)
  • Chlorophyll: photosynthesis in plants (green)
  • Anthocyanins: photoprotective in plants (purple and red)
  • Carotenoids: phoroprotective (red) and animals aphids and spider mites
  • Melanin
• Blocks UV physically and chemically
• Neutralized free radicals
• Activated during melanogensis, initiated after sun damage
• Produced by melanocytes then transferred to skin cells where they spread out around nuclei to protect DNA
• All people have bout same ratio of melanocytes to skin cells, but people with darker skin have larger melanocytes and produce more melanin
• Too much UV destroys folic acid