Not all Plants are Created Equal!
“Photosynthesis is an ancient process that
originated not long after the origin of life and
has evolved via a complex path to produce the
distribution of types of photosynthetic
organisms and metabolisms that are found
today.”
http://www.plantphysiol.org/content/154/2/434.full
Plant Evolution
Examples
C3 plants
Are the most numerous on the planet
Thrive during conditions that are cool and moist
with normal light
Get its name because the carbon dioxide is
incorporated into a 3-carbon compound.
RUBISCO, the enzyme involved in photosynthesis,
is also the enzyme involved in the uptake of
CO2.
It takes place throughout the leaf.
C4 Photosynthesis :
• CO2 is first incorporated into a 4-carbon compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in
the uptake of CO2. This enzyme allows CO2 to be
taken into the plant very quickly, and then it
"delivers" the CO2 directly to RUBISCO for
photsynthesis.
• Photosynthesis takes place in inner cells
C4 Photosynthesis :
Adaptive Value:
•Photosynthesizes faster than C3 plants under
high light intensity and high temperatures
because the CO2 is delivered directly to RUBISCO,
not allowing it to grab oxygen and undergo
photorespiration.
•Has better water-use-efficiency because PEP
Carboxylase brings in CO2 faster, and the stomata
does not need to be open as much (less water lost
by transpiration) for the same amount of CO2.
CAM Photosynthesis or Crassulacean Acid
Metabolism
• Named after the plant family in which it was first
found (Crassulaceae) and because the CO2 is
stored in the form of an acid before use in
photosynthesis.
• Stomata open at night (when evaporation rates
are usually lower) and are usually closed during
the day.
• The CO2 is converted to an acid and stored
during the night. During the day, the acid is broken
down and the CO2 is released to RUBISCO for
photosynthesis
CAM Photosynthesis or Crassulacean Acid
Metabolism
•Adaptive Value:
• Better water-use-efficiency than C3 plants under arid
conditions due to opening stomata at night when
transpiration rates are lower (no sunlight, lower
temperatures, lower wind speeds, etc.).
• When conditions are extremely arid, CAM plants can
just leave their stomata closed night and day. Oxygen
given off in photosynthesis is used for respiration and
CO2 given off in respiration is used for photosynthesis.
•This allows the plant to survive dry spells, and for the
plant to recover very quickly when water is available
again (unlike plants that drop their leaves and twigs and
go dormant during dry spells).
EVOLUTION OF PHOTOSYNTHESIS & AEROBIC
RESPIRATION
by C. Grier Sellers
Doctoral Candidate in Biology
NSF GK12 Fellow
Temple University
History of the Earth: Four Eons:
• Hadean Eon: 4500 - 3800 millions of years ago
(mya)
• Archean Eon: 3800 - 2500 mya
• Proterozoic Eon: 2500 - 542 mya
• Phanerozoic Eon: 542 mya - Present
http://en.wikipedia.org/wiki/Geologic_time_scale
Hadean Eon:
• For its first billion
years, the Earth’s
surface was molten
rock
• It was too hot for life
to exist or to originate
Archean Eon:
(Image: American Museum of Natural History)
• Prokaryotic life originated
soon after Earth cooled
and oceans were formed
• Earth was anoxic
• Photosynthetic bacteria,
including cyanobacteria
evolved (bumps are
“stromatolites”, made by
them)
Photosynthetic Bacteria:
• Some photosynthetic bacteria (such as green
sulfur bacteria) use Hydrogen sulfide (H2S) & do
not produce oxygen:
H2S + CO2 + Light --> Sugars + Sulfur
• Cyanobacteria (& in eukaryotes, chloroplasts)
use water instead, & produce oxygen:
H2O + CO2 + Light --> Sugars + O2
Cyanobacteria:
http://universe-review.ca/F11-monocell.htm
Key Dates in the Early Evolution of Life:
• Origin of Life: (the first prokaryotes) c. 3800 mya
• Origin of Photosynthesis: c. 3500 mya ??
• Origin of Oxygenic Photosynthesis: (oldest
cyanobacterial fossils) c. 2900 mya
• Origin of Aerobic Respiration: c. 2400 mya??
• Origin of Eukaryotes: (the first protists) c. 2000 mya
Oxygen Terminology:
• An environment
lacking oxygen is:
Anoxic.
• An environment
containing oxygen is:
Oxic.
• Anaerobe: an
organism that lives
• Aerobe: an organism
without oxygen (many
that requires oxygen
are killed by oxygen).
to live.
The Oxygen Revolution:
• The first O2 on Earth was a waste product of a
new type of photosynthesis invented by
cyanobacteria (oxygenic photosynthesis)
• This caused formation of oxidized compounds
(banded iron formations - or “rusty rocks”)
• Oxygen first dissolved in the oceans, then
accumulated in the atmosphere - it currently
makes up 21% of the air.
Gradual Increase in Atmospheric O2:
http://www3.ncc.edu/faculty/bio/fanellis/biosci119/marineorg.html
Banded Iron Formation: (c. 2400 mya)
oxidation of iron, forming rust
http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html
formed by the
Consequences of Oxygen in the Environment:
• Caused a mass extinction of anaerobic
prokaryotes, for whom it is toxic
• Made evolution of aerobic respiration in bacteria
possible (later, some of these oxygen-respiring
bacteria were eaten by eukaryotes, and became
mitochondria)
• These aerobic bacteria were very successful,
since aerobic respiration produces much more
ATP than anaerobic bacteria can produce