- Autotroph means "self-feeder"
but photosynthesizing organisms do more than feed themselves: they
feed everything else.
- All green plants are autotrophs. They're
an amazingly diverse group.
- In happens in the green tissues of
plants: mostly leaves, but not always!
- The cells of the green tissues of plants
have chloroplasts. Double membrane-bound organelles containing
- pancake shaped "thylakoids"
- arranged in stacks called "grana"
- within a thick fluid called the
- These "Light reactions"
harvest energy by "photophosphoralyation" producing ATP and
by adding electrons to NADP+. Notice that CO_2 hasn't entered the
picture yet and there's no sugar yet.
- The light reactions produce ATP and
NADPH. The Calvin/Benson Cycle uses these energy carriers to
incorporate atmospheric carbon dioxide into G3P (glyaldehyde
- The Calvin/Benson Cycle:
- An enzyme called rubisco catalysizes
the reaction of 6 CO_2 with 6 molecules of a 5-carbon substance
called ribulose bisphosphate (RuBP), the product is 12 3-carbon
molecules called phosphoglyeric acid (aka phosphoglyerate: PGA)
- After energy inputs from ATP and
NADPH, the 12 PGAs form 12 3-carbon glyceraldehyde 3-phosphates (PGAL).
Two molecules of PGAL is "harvested." Forming a
- After energy inputs from ATP, 10
(3-carbon) PGAL react to form 6 (5-carbon) RuBP, and the cycle
- The sugar formed is transformed into
starch, cellulose or sucrose as needed.
- The light reactions provide the ATP
and NADPH that "fuels" the carbon fixation in the
- When its hot and dry, bad things can
- You lose water. Plants need water
for photosynthesis and to keep themselves erect.
- Ok, close your stomata: keep the
water in. Oops! that keeps the CO_2 out too!
- When this happens O_2 builds, up
and CO_2 supplies are quickly exhausted. O_2 competes with
CO_2 in reacting with RuBP, and entering the Calvin/Benson
cycle. This process called photorespiration, uses
oxygen, and produces a generally useless 2-carbon compound.
- Solution 1. Manage the CO_2
gradient by fixing atmospheric CO_2 (to a 4 carbon compound)
in tissues near the stomata, shunt this to photosynthesizing
vascular tissues, change and back to CO_2 and stick it the
Calvin cycle. C_4 plants do this.
- Solution 2. Open stomata at
night, Fix CO_2 to a 4 carbon compound in darkness, and close
the stomata during the day, feeding the fixed carbon into the
calvin cycle as required. CAM plants do this.
- Plants that do neither of these
tricks are called C_3 plants. Typified by cool season,
- Light (energy harvesting) and Dark
- Reactions. Light reactions couple two
Photosystems, the P680 splits water, for it's electrons, releasing
- The Calvin cycle fixes atmospheric
carbon dioxide to a three carbon compound.
- This process feeds the world!