Photosynthesis 2 Flashcards
Light Independent Reactions names
- Calvin cycle
- Calvin Benson cycle
- Carbon fixing reactions
- Carbon reducing reactions
- Dark reactions
Dark reactions
a misnomer as the reactions
do not occur in the dark. They are sometimes
called this as the reactions do not TRAP light
but they occur in the light.
B) Light Independent Reactions what happens
-Carbon fixation:
B) Occurs in light (not in dark)
C) The energy that was trapped as ATP and NADPH in light reactions
powers the Calvin cycle and ends up in carbohydrates
D) Regeneration of substrate =
RuBP
Carbon fixation:
Carbon dioxide that enters through stomates is trapped.
Carbon from CO2 is fixed into chemical bonds of carbohydrates e.g. glucose
Regeneration of substrate in the light-independent reactions =
RuBP (Ribulose bisphosphate)
rubisco
*Rubisco is the most abundant enzyme in the world!!
*Rubisco is partially coded for by chloroplast DNA and nuclear DNA.
*Rubisco is light activated (why the reactions do not occur in dark)
*All photosynthesizing plants use Rubisco in the Calvin Cycle to make carbohydrates
( but not all plants use Rubisco to fix atmospheric CO2!)
do all plants use rubisco the fox atmospheric co2
no
Calvin Cycle step 1
Six molecules of CO2 combine with six
molecules of RuBP (ribulose 1,5-bisphosphate)
in the carbon fixing step.
calvin cycle step 2
For every 6 CO2’s fixed, one molecule of glucose
(which contains 6 carbons) is produced
calvin cycle step 3
NADPH and ATP from the light dependent
reactions supply energy and electrons that
result in the production of glucose
calvin cycle step 3
The cycle regenerates the substrate RuBP.
Photorespiration
A special type of respiration that occurs in plants
in the light (in addition to aerobic cellular
respiration)
RuBP carboxylase/oxygenase
has 3 possible substrates
- RuBP (always one of the substrates)
- CO2
- O2
co2 in
Calvin cycle (Rubisco acting as carboxylase)
o2 in
Photorespiration (Rubisco acting as oxygenase)
what 2 chemicals Compete for the Active Site
co2 and o2
Photorespiration versus Aerobic Cellular Respiration
photoresperation
Photorespiration is a seemingly
wasteful process
1. No ATP is produced
2. Release of CO2 (loss of CO2 is of
significance to plants)
3. No net carbon fixation.
4. When rubisco is active in
photorespiration, it is not
available for the Calvin cycle.
5. However allows plants that have
it (C3 plants) to survive under
hot dry conditions and helps
prevent photooxidative damage
Photorespiration versus Aerobic Cellular Respiration
Aerobic cellular respiration
- ATP produced
- Release of CO2
Does Rubisco tend to act more as a
carboxylase or as an oxygenase?
calvin cycle
carboxylase
Does Rubisco tend to act more as a
carboxylase or as an oxygenase?
photoresperation
oxygenase
Atmospheric concentrations
of o2
20-21 percent o2
Atmospheric concentrations
of co2
0.04 percent and raising
Does Rubisco tend to act more as a
carboxylase or as an oxygenase?
The high O2 concentration in the atmosphere means
that Rubisco often acts as an oxygenase Photorespiration is promoted by the relative O2 /CO2
concentrations in atmosphere
Different Photosynthetic Strategies
A. C3
B. C4
C. CAM
C4 and CAM plants have
evolved strategies
to deal with photorespiration
C3 Plants
-Produce 3-carbon compounds during first step of
atmospheric carbon fixation (3PGA)
-Use Calvin cycle to fix atmospheric CO2
* C3 plants can have much photorespiration
* If one pumps CO2 into a greenhouse or a growth
chamber with C3 plants, that will cut down on
photorespiration.
C3 Plants
examples
- Spinach
- Soybeans
- Oats
- Wheat
- Rye
- Many grasses (not all)
- Oaks
- Maples
Some plants don’t use Rubisco to
fix
atmospheric CO2 .
All photosynthesizing plants use Rubisco in
the Calvin cycle to make carbohydrates
C4 Plants
C4 plants have way to deal with photorespiration
* They have additional pathway
additional pathways c4 plants
Have C3 pathway (Calvin cycle just like all
photosynthesizing plants) and C4 pathway
When c4 plants fix atmospheric co2 they produce
they produce a 4 carbon compound
C4 Plants
number of species and famalies
how many monocots
dicots
> 1000 species in 18 different plant families
– 3 monocot families, 15 dicot families
– All of these 18 plant families have C3 and C4 members
– Suggests C4 syndrome has arisen a number of times
The 4-Carbon Pathway (C4 pathway)
Produces 4-carbon compound OAA
instead of 3-carbon PGA during fixation of atmospheric CO2
c4 plants ex
C4 Plants:
– Corn
– Sugar cane
– Cat tails
– Crab grass (weedy), more of a problem in heat of summer
– Pigweed (weedy)
– Many tropical grasses and arid-region plants
Kranz anatomy
C4 plants have Kranz anatomy (Kranz = German for wreath).
– Enlarged bundle sheath cells with large chloroplasts and
numerous starch grains
* © Kingsley Stern
what is combine, with aid of the
enzyme PEP carboxylase in mesophyll cells
PEP (phosphoenolpyruvate) and CO2
PEP carboxylase acts as a
carboxylase only, not an oxygenase
step 1 of c4 pathway
Oxaloacetic acid (4C) is produced as first stable product of
atmospheric CO2 fixation instead of PGA (3C)
step 2 of c4 pathway
CO2 is transported as malic acid to bundle sheath cells
step 3 of c4 pathway
CO2 is released and is re-fixed in Calvin cycle.
step 4 of c4 pathway
CO2 is concentrated in bundle sheath cells around Rubisco, so
Rubisco acts like a carboxylase and photorespiration is
minimized.
C4 plants have spatial separation between:
Calvin cycle in Bundle Sheath cells & C4 pathway in
Mesophyll cells
c4 or c3 plants which is better
c4 plants perform better in hotter weathere
At low temperatures, C3
plants are more
efficient, and
they outcompete C4 plants because Costs extra ATPs for C4
photosynthesis
C4 plants do best when it is
hot and dry. They can have
stomates partially open to
conserve water but they do
well since they are so
efficient at trapping CO2
* E.g. Crab grass does better in
heat of summer
At lower temperatures,
C3 plants outcompete C4 plants
At higher temperatures,
C4 plants outcompete C3 plants
Research to try to Convert C3 Plants to C4
Plants
- Need to change anatomy of leaf to Kranz
anatomy - Need to introduce more PEP carboxylase as
well as the C4 pathway into mesophyll cells - Need to take Rubisco out of the mesophyll
cells and put in bundle sheath cells
CAM Plants
Tend to grow in dry environments
* In about 30 plant families
CAM Plants plant famalies
Crassulaceae
Orchidaceae
Cactaceae
Bromeliaceae
Crassulaceae
- Jade plants
- Mother of thousands
Orchidaceae
orchids
Cactaceae
cacti
Bromeliaceae
- Bromeliads
- A lot of air plants = epiphytes; e.g. Spanish moss
step 1 CAM Photosynthesis stomates
are opened at night
step 2 CAM photosynthesis Atmospheric CO2 is fixed at
night by PEP
carboxylase into malic acid (4C
step 3 cam photosynthesis Malic acid is stored in the
vacoule
step 4 Stomates are closed during day to (CAM photosynthesis)
conserve water
step 5 (CAM Photosynthesis ) CO2 is released from malic acid during the
day
step 6 of cam photosynthesis
Now there is a high concentration of CO2
inside the plant, and
Rubisco will act like a
carboxylase and re-fix the CO2 into
carbohydrates Organic acids accumulate at night (stomata
open).
CAM plants have time-based separation between activity
of:
PEP carboxylase fixes CO2 at night and Rubisco fixes CO2 during day
CAM photosynthesis allows plants to function well under
limited water supply, as well as high light intensity.
However CAM plants have low
productivity
CAM plants Grow slowly
under natural conditions (in hot, dry
environments)
Obligate CAM =
always show CAM photosynthesis
Facultative CAM plants exhibit
CAM photosynthesis under
certain conditions (e.g. hot, dry) and the rest of the time
they act like C3 plants
– E.g. Mother of thousands
