Chapter
5. Energy Conservation in
Photosynthesis: CO2 Assimilation
- More about stomata:
- Table 5.1 show typical
density and distribution of stomata. The range is about 20-400/mm2.
- Pores occupy 0.2-2% of
surface area, but diffusion through pores is very efficient and may be
equivalent to rate expected from 70% of surface area.
- Guard cell movement in brief:
due to orientation of cellulose microfibrils in guard cells, when guard
cells take up water and become turgid, they expand asymmetrically.
This opens the stoma. See Figure 5.5.
- Now for the details:
- Uptake of water is due
to increase in guard cell water potential, this in turn is due to
accumulation of K+, Cl- and malate ions in
vacuole.
- An ATP-ase proton pump produces proton efflux. This
results in passive uptake of K+ and cotransport
of Cl-. Malate is produced by hydrolysis
of starch. See Figure 5.6.
- Environmental cues:
- Low CO2 content in substomatal cavity triggers opening.
- Light, especially
blue light, triggers opening. Specific blue light receptors are
involved.
- Drought stress
triggers closure:
- Hydropassive
closure - guard cells lose water, become
flaccid.
- Hydroactive
closure - plant senses water deficit, stomata close. The hormone ABA
is involved, and water deficit can be sensed in leaves or roots.
- All these processes
require complex signal transduction pathways, ending with activation of
the proton pumps.
·
PCR (Calvin) Cycle -
review
- Carboxylation
of C5 sugar to produce unstable C6 compound, dissociates to C3 compound, PGA.
- Reduction
of C3 compound to C3 sugar (a triose,
G3P). This process consumes ATP and NADPH (from the light dependent
reaction).
- Some of the triose is exported, the rest is used to regenerate C5
compound.
- This is C3 metabolism
and is universal in photosynthetic cells.
- This process is
illustrated in more detail in Figures 5.9,10,11.
- Photorespiration
- Respiration in the
sense that is an O2-consuming and CO2-generating process. There is
no net gain of ATP.
- Due to oxygenase
function of Rubisco.
- C5 compound oxidized,
some of the carbon lost as CO2. Regeneration of C5 requires
ATP. This is the glycolate cycle.
- Why? Some
hypotheses:
- "Oxygenase
function of Rubisco is
inescapable". This is a vestigial trait, an echo of the
Precambrian world with an atmosphere low in oxygen.
- The glycolate cycel produces
useful compounds (e.g. amino acids) as intermediates.
- Photorespiration can
dissipate excessive excitation energy when light is present but CO2
isn't (closed stomata).
- C4 and CAM
pathways may be ways of compensating for wasteful nature of
photorespiration.
- C4 cycle
- Uses PEPcase to capture CO2.
- C1 (CO2) + C3 (PEP)
makes C4 (e.g. malate), hence the term.
- C4 is decarboxylated, CO2 is passed to the Calvin cycle.
- Benefits:
- PEPcase
has a higher affinity for CO2 than Rubisco,
can scavenge CO2 at low concentration when stomata are closed (or nearly
so) to conserve H2O.
- PEPcase
is more heat tolerant than Rubisco
- Physical separation
of carboxylation (in mesophyll) and carbon
reduction (in bundle sheath) pathways inhibits photorespiration.
- Ecology of C4
metabolism
- Has independently
evolved in many plant families, but most common in desert and tropical
species.
- High water use
efficiency that C3 plants.
- Better able to exploit
high light intensities.
- CAM, Crassulacean Acid Metabolism
- CAM
plants (typically desert succulents) use the C4 pathway but separate
carbon fixation and carbon reduction in time, not space.
- During the night: stomata
open, CO2 diffuses in, C4 pathway fixes carbon in C4 acids.
- Sunrise:
stomata close to reduce water loss.
- During the day: light
dependent reactions turn on, resulting ATP
and NADPH used by PCR (Calvin) cycle to make
sugar.
- CAM and C4 metabolism are
illustrated in Figures 5.22,23,26.