BIOL208 Lecture Notes - Lecture 9: Photosynthetically Active Radiation, Aphotic Zone, C3 Carbon Fixation
10 Oct 2018
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Lecture 9 Energy acquisition autotrophs
• Autotrophs use inorganic sources of carbon and energy
• Sources of energy
1. Solar radiation (sun)
2. Organic molecules (proteins, glucose)
3. Inorganic molecules (H2S)
• Autotrophy:
- Photosynthesis
Photon: particle of light, light moves as waves through space but interacts with matter as
particle
Visible light is photosynthetically active radiation (PAR)- (400-700 nm)
UV radiation is higher energy than visible light and can break covalent bonds in organic
compounds
PAR is measured as photo flux density (PFD)= number of photons striking a square meter
per second. Measured in umoles, when 1 mol = 1023photons
Amount of PAR a plant uses= amount of photosynthesis a plant does. Quality and quantity
of light both effect photosynthesis.
Top → 79% PAR
Middle → 7% PAR
Low → 2% PAR
Soil → 2% PAR
Canopy are adapted to high light. Middle trees and low trees are adapted to low light and
can use PAR more efficiently.
Euphotic zone → sunlight zone, sunlight rarely penetrates beyond this zone in the ocean.
Dysphotic zone → twilight, sunlight decreases rapidly with depth, no photosynthesis
Aphotic zone → midnight, sunlight doesn’t penetrate at all, darkness, chemosynthetic
organisms
How do plants ger carbon?
1. C3 photosynthesis- first kind of photosynthesis that evolved in the planet, wastes lot of
water and less efficient than other types, requires moderate light, most plants and algae
CO2 + ribulose bisphosphate (RuBP, 5 carbon sugar) + enzyme RUBISCO →
phosphoglyceric acid (PGA, 3 carbon sugar)
Plants open stomata to let CO2 in, allows higher rates of transpiration, water flows out
faster than CO2. One step photosynthesis: both light and dark reactions take place in
the mesophyll cells, RUBISCO can bind to O2 in higher temperatures and release CO2 as
a by product → photorespiration
When first plants evolved there was less O2 in the environment, so photorespiration
was not a major problem so there was no selection for efficiency, when O2
concentration increased plants growing in warmer climates selected for more efficient
enzymes than RUBISCO.
2. C4 photosynthesis- like C3 but uses different cells, arid environments, different
enzymes, limitation to water usage
Opens stomata less often, increases CO2 efficiency by using PEP, PEP carboxylates fixes
carbon efficiently and RUBISCO completes photosynthesis in bundle sheath cells where
Document Summary
Lecture 9 energy acquisition autotrophs: autotrophs use inorganic sources of carbon and energy, sources of energy, solar radiation (sun, organic molecules (proteins, glucose) Photon: particle of light, light moves as waves through space but interacts with matter as particle. Visible light is photosynthetically active radiation (par)- (400-700 nm) Uv radiation is higher energy than visible light and can break covalent bonds in organic compounds. Par is measured as photo flux density (pfd)= number of photons striking a square meter per second. Measured in umoles, when 1 mol = 1023photons. Amount of par a plant uses= amount of photosynthesis a plant does. Quality and quantity of light both effect photosynthesis. Middle trees and low trees are adapted to low light and can use par more efficiently. Euphotic zone sunlight zone, sunlight rarely penetrates beyond this zone in the ocean. Dysphotic zone twilight, sunlight decreases rapidly with depth, no photosynthesis. Aphotic zone (cid:373)id(cid:374)ight, su(cid:374)light does(cid:374)"t pe(cid:374)etrate at all, dark(cid:374)ess, chemosynthetic organisms.
