Carbon Reactions:
CO2 is fixed by Rubisco located in the stroma. The molecule that is
carboxylated is RuBP.
RuBP has 5 carbons and is regenerated in the Calvin cycle. In the Calvin
cycle, carbon is conserved, ATP is used and NADPH is used.
The result is that some extra triose phosphate is generated that can be exported
to the cytosol or used in the chloroplast stroma to make starch.
cytosol
stroma
Photorespiration:
This occurs when Rubisco acts as an oxygenase rather than a carboxylase
How does Rubisco normally function in C3 photosynthesis?
Ribulose bisphosphate carboxylase/oxygenase (Rubisco)
Ribulose bisphosphate carboxylase/oxygenase (Rubisco)
When rubisco acts as an oxygenase the products are one
3-phosphoglycerate (C3) and one phosphoglycolate (C2).
The net cost of oxidation of RuBP is 1 ATP (however 1 C is also lost
and the cost of fixing that was at least 1.5 ATP and 1 NADPH)
Photorespiration = the recovery of carbon due to oxygenase activity of
Rubisco.
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Photorespiratory cycle:
For this class let’s just say it
is a way for plants to recover
carbon after rubisco acts as
an oxygenase and that it
involves multiple
compartments.
When is photorespiration a problem?
Photorespiration is more predominant at high temperature because
the ratio of CO2 to O2 in solution decreases.
So far we have discussed C3 photosynthesis, so named because the
first stable product in C3 photosynthesis is 3-phosphoglycerate (3
carbons).
Some plants have evolved strategies to avoid photorespiration.
These plants are specialized for growth at high temperatures and
under dry conditions. These are C4 and CAM plants. C4 and CAM
are add-ons to C3 photosynthesis.
C3 Plant (Poa)
Kranz anatomy in maize
C4 Plant (Saccharum - sugarcane)
C4 photosynthesis
is an add-on to
C3 photosynthesis.
The purpose is to increase
the CO2 concentration in
chloroplasts where the Calvin
cycle takes place.
PEP carboxylase
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Differences between C3 and C4 photosynthesis
C4 Pathway
In C4, the substrate for primary carboxylation is phosphoenolpyruvate
(PEP) (C3). What is the substrate for carboxylation for C3
photosynthesis?
In C4, the primary carboxylation is done by PEP carboxylase,
the first product is oxaloacetate (C4). The first (stable) product of
C3 photosynthesis is 3-phosphoglycerate (C3).
The cost of accumulating fixed carbon in bundle sheath cells
is 2 ATP per CO2.
C4 is more efficient than C3 under conditions where photorespiration
would predominate.
Figure 10.27b: Diagram of bundle sheath cells.
The C4 pathway imports CO2 and NADPH (indirectly) into bundle sheath cells
where the Calvin cycle takes place. Chloroplasts in bundle sheath mainly use
cyclic electron transport (PSI) to make ATP so there is little O2 evolution.
CAM (crassulacean acid
metabolism) plants
CAM is a form of C4
photosynthsis. In CAM plants
CO2 is taken up during the
night and the first step of C4
(carbon fixation by PEP
carboxylase) occurs. Malate
is stored in the vacuole.
Quantum yield for C3 plants depends on temperature. If CO2
concentration increases how does it affect the efficiency of C3 vs. C4
plants.
The benefit of CAM photosynthesis is that stomata are open at night when
water loss through evaporation is less.
During the day, the
stomata are closed.
Malate is exported from the
vacuole and decarboxylated.
CO2 enter the Calvin cycle.
However, it is expensive to store carbon as malate during the night,
at least 2 ATP per CO2 fixed.
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CAM plants
Ananas comosus
Next topic: Nutrient transport, moving liquids
Pathway for water from the soil through the plant and out to the
atmosphere.
Agave deserti
Aquaporins
Water potential
How does transpiration drive transport in the xylem?
What are the main mechanisms of nutrient uptake?
How does phloem work?
Symplastic pathway vs. apoplastic pathway. (symplasm is within cells, apoplasm is
outside cells.)
plasmodesma
xylem
root
hair
symplastic pathway
apoplastic pathway
cell wall
cytoplasm
epidermis
symplast
of endodermis
cortex
Casparian strip
of endodermis
stele
water-filled
leaf cells
substomatal
cavity
(intercellular
space)
water-filled
xylem in vein
cell wall
permeated
with H2O
cuticle relatively
impermeable
to H2O
air not saturated
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Negative pressure potential is generated in leaves in response to
water evaporation from the cell wall surrounding mesophyll cells.
Water remaining in the cell wall is restricted to smaller pores, and
due to adhesion, cohesion and surface tension the curvature of the
water surface increases.
Ψp = -2T/r
T is the surface tension of water,
7.28 x 10-8 MPa m, and r is the radius of
curvature in meters.
The direction of water transport is determined by water potential Ψw.
Water moves from higher to lower water potential.
Three components of water potential
Ψw = Ψs + Ψp + Ψg
1. Ψs (or Ψπ) Solute potential depends on solutes in solution (the higher the solute
concentration the more negative the solute potential).
2. Ψp Pressure potential is the hydrostatic pressure and can be
negative (in xylem) or positive (turgor pressure). Atmospheric pressure is defined
as Ψp = 0.
3. Ψg Gravity potential depends on the vertical height, the density of water
and gravity.
Solute potential in the cell is
negative and the pressure
potential is positive. The
water potential is equal to the
surrounding solution.
Osmosis: water movement across a semi-permeable membrane.
The direction of water movement across the membrane depends on
the water potential Ψw. But the rate across biological membranes
depends on aquaporins.
The water potential inside the cell and outside the cell always equilibrates because
biological membranes are permeable to water.
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Aquaporins are membrane proteins that transport water.
From: Fu and Lu (2007) Molecular Membrane Biology 24: 366-374
Peter Agre discovered aquaporins in 1991,
he received the Nobel prize in chemistry for
this work in 2003.
Peter Agre was born in Northfield MN, and he went to Theodore
Roosevelt High School in Minneapolis. He graduated
from Augsburg College in 1970 with a BS in chemistry. He is
a professor at Johns Hopkins University.
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