Transcript PLANTS: ANATOMY, FUNCTION & GROWTH

Photorespiration
• When weather is hot and dry, guard cells decrease the size of
stomata, also decreasing the concentration of carbon dioxide.
• Oxygen (O2) starts to compete with CO2 for rubisco’s active site
• RRubisco adds O2 to RuBP forming one PGA molecule (3C) and one
glycolate molecule (2C)
• GGlycolate leaves the Calvin Cycle and is partially converted to CO2
• PPhotorespiration consumes O2 and revolves CO2 while generating
no ATP
• It is believed that Rubiso’s inability to distinguish between CO2 and
O2 is an evolutionary remnant of an earlier mechanism, better
suited to an early Earth whose atmosphere was rich in CO2 and
poor in O2
THE PROBLEM
1. It occurs when the stoma close in an
effort to save water on high light, low
water availability days (like some hot
dry summer days around here).
2. Oxygen concentration goes up in the
closed off leaf and competes with the
dwindling CO2 for the active site on the
enzyme “Rubisco”
3. This short circuits the Calvin Cycle and
no glucose is made.
4. Why is “Rubisco” so dumb that it can’t
tell CO2 from O2? It evolved when
oxygen level in the atmosphere was
very low; before photosynthesis raised
oxygen levels in the atmosphere.
Plants are in a way a victim of their
own success.
THE SOLUTIONS:
1. Ignore It: C3 Plants (So called
because the first compound CO2 is
fixed in is a 3-carbon PGA) can survive
around here because the days
promoting photorespiration are few
enough that the plant will not starve.
Solutions
2. C4 SOLUTION (A solution of location):
•
a. C4 plants have evolved a way to survive
in areas that have a significant number of
Photorespiration causing days in the
growing season; too may for a C3 to
survive.
•
b. Their leaf and chlorophyll anatomy are
different. “Kranz (German for wreath)
Anatomy”
i.
I. No Palisade cells, only spongy mesophyll cells.
ii.. Bundle sheath cells have chloroplasts but without
thylakoids (thus can only do dark reactions).
Alternative Carbon
Fixation Reactions
• There are some alternative
methods for carbon fixation, e.g. C4
photosynthesis:
• 3C sugar + CO2 _ 4C sugar
• CO2 is fixed into a 4 carbon sugar
molecule
• Prevalent in tropical grasses such
as sugar cane and maize
• C4 photosynthesis is typically found
in plants that grow at higher
temperatures and under high light
intensity
• c. In the spongy mesophyll cells a 3carbon compound (PEP) picks up a CO2
to form a 4-carbon compound (hence
the name C4). The enzyme that fixes
the CO2 to the PEP in the spongy cells
has a particularly high affinity for CO2 –
PEP Carboxylase. Unlike Rubisco, this
enzyme does not react with O2 at all.
•
• d. This 4-carbon compound is shuttled
into the bundle sheath cells where it
releases the CO2. The reformed 3carbon PEP goes back out into the
spongy cells and picks up another CO2.
Calvin Cycle then takes place in the
bundle sheath cells. The object is to
get CO2 to the Rubisco in a protected
place so that it doesn’t compete with
the O2.
• e. So efficient is this CO2 “shuttle” that
the Bundle Sheath cells have 10 to 60
times to CO2 compared to the spongy
mesophyll cells. The concentration
favors the productive reaction with
Rubisco.
• F. Bundle Sheath cell chloroplasts have
no thylakoids because they are
specialized to do Light Independent
Reactions. The spongy mesophyll cells
do all the light dependent reactions the
plant needs.
• g. If this is so good why don’t all the
plants do it? Because it takes about 30
ATP to make a molecule of glucose by
this extra pathway compared to the 18
ATP per glucose in plain old C3. So a
cost benefit analysis would show if only
happens where it is a matter of survival.
C4 Plants
• C4 plants minimize respiration even in hot, dry climates
• C4 plants have a 4C stable intermediate oxaloacetate
• C4 plants carry out the first stages of carbon fixation in
the mesophyll cells
• CO2 is fixed into oxaloacetate, which is converted to
malate
• Malate is transferred to the bundle sheath tissue. CO2 is
released and used in the normal Calvin cycle
• This is used to increase the CO2 concentration within the
bundle sheath tissue to prevent photorespiration
• Ex: sugar cane, corn, sorghum
C4 Pathway
Occurs in the cytoplasm, not in the chloroplast
CAM Plants
• Water- storing plants (known as succulents) such as cacti
and pineapples
• Open their stomata at night and close them during the day
• When stoma open at night CO2 is incorporated into C4
organic acids by the enzyme PEP carboxylase
• These organic acids are stored in vacuoles until the
morning
• The stoma close and the organic acids release CO2 that
enter the C3 Calvin cycle to be fixed into carbohydrates
CAM Solution(A solution of
time):
•
a. CAM plants have the same problem
and live in areas so dry even C4 plants
would die.
CAM SOLUTION
• b. CAM plants grow in “Xeric” areas like
deserts. They are all about conserving water.
• i. Their body structure is often very different
(have a low s/v ratio).
• ii.Their leaves are minimized in terms of
surface area and do not do photosynthesis
(often form spines or thorns).
• iii.The stem is the main photosynthetic
structure.
iv. Stomata are located in deep pits to
minimize evaporation.
CAM Solutions
• C. AND they open their stomata at night so
they can get CO2 and minimize the loss of
water.
• D. During the night they fix CO2 that comes
into the plant in a 4-carbon compound and
store it in vacuoles.
• E. During the day the stomata close, to save
water and the plant uses the 4-carbon
compound made last night just like the C4
plant does.
C4 utilizes a spatial separation of steps. The two cycles, carbon
fixation and Calvin Cycle occur in different types of cells. The
carbon fixation step occurs in the mesophyll cell, where CO2 is
incorporated into 4-carbon organic acids. The CO2 is then released
into the bundle-sheath cell into the Calvin Cycle. CO2 is taken in
• PEP carboxylase (an enzyme that has greater affinity for
by the
CO2 than rubisco and no affinity for O2), and released into the
bundle-sheath cells next to the mesophyll cells.
CAM utilizes a temporal separation of steps
The two cycles occur in the same cells but at
different times, with carbon fixation during
the night, when CO2 is taken up, and
changed into organic acids.
•
These organic acids are saved in vacuoles
in the mesophyll cells until morning, when
the stomata close to conserve water in dry
regions. In the morning, the light reactions
allow photosynthesis to occur- providing
NADPH and ATP so that the Calvin Cycle
can occur.
This releases CO2 from the organic acids
made during the night, to be combined with
other elements within the chloroplast to
form sugar.
Fun Facts!!
1. All three plant types (C3, C4, and CAM)
make glucose by Calvin Cycle. The
difference is how the CO2 gets to Rubisco.
•
a. C3 use CO2 directly from the air
• b. C4 and Cam first combine CO2 with PEP
making a 4-carbon compound.
2. The C4 and CAM solutions are similar
chemically but the main difference is:
• a. C4 plants solve the problem by location of
the reactions.
• b. CAM plants solve the problem by the
timing of the reactions
More FUN!
3. FAMOUS PLANTS THAT ARE
• a. C3: Wheat, oats, rice, regular
grass, most deciduous trees (oak,
maple, elm)
• b. C4: Corn, crabgrass, Sugar Cane,
Bermuda grass
• c. CAM: Cactus, pineapple, vanilla,
orchids
SUMMARY OF CO2 FIXATION