Upload
griffin-king
View
230
Download
2
Embed Size (px)
Citation preview
AP Biology
Photosynthesis:Variations on the Theme
AP Biology
Remember what plants need… Photosynthesis
light reactions light H2O
Calvin cycle CO2
What structures have plants evolved to
supply these needs?
sun
ground
air
O
OC
AP Biology
Leaf Structure
H2O
CO2
O2 H2O
phloem (sugar)xylem (water)
stomate guardcell
palisadeslayer
spongylayer
cuticleepidermis
O2 CO2
Transpiration
vascular bundle
Gas exchange
AP Biology
Controlling water loss from leaves Hot or dry days
stomates close to conserve water guard cells
excess H2O = stomates open
conserve H2O = stomates close
adaptation to living on land, but…
creates PROBLEMS!
AP Biology
When stomata close…
xylem (water)
phloem (sugars)
H2OO2 CO2
Closed stomata lead to… O2 build up inside the plant from light reactions
CO2 is depleted /can’t enter for the Calvin cycle causes problems in Calvin Cycle
AP Biology
RuBisCo
Carbon Fixation enzyme in the Calvin Cycle
Fixes Carbon Dioxide onto RuBP
• When the concentration of oxygen is high/ CO2 is low, RuBisCo will fix Oxygen onto RuBP in a
process called photorespiration.
•The products of fixed O2 are not particularly healthy for the plant, and most assuredly don’t
lead to the formation of glucose, like the fixation of CO2 does.
AP Biology
6Cunstable
intermediate
1C CO2
Calvin cycle when CO2 is abundant
5CRuBP
3CPGA
ADP
ATP
3C
NADP
NADPH
ADP
ATP
Leaveto make glucose
3C
2 PGAL
5C
RuBisCo
C3 plants
AP Biology
Calvin cycle when O2 is high
5CRuBP
3C2C
to mitochondria
–––––––without
making ATP
photorespiration
O2
RuBisCo
1 PGA
AP Biology
Impact of Photorespiration Oxidation of RuBP
short circuit of Calvin cycle reduces production of photosynthesis
no C6H12O6 (food) produced
if photorespiration could be reduced, plant would become 50% more efficient pressure to evolve alternative carbon fixation systems
AP Biology
Reducing photorespiration Separate carbon fixation from Calvin cycle
C4 plants PHYSICALLY separate carbon fixation from Calvin cycle
different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds
different enzyme to capture CO2 (fix carbon) PEP carboxylase
CAM plants separate carbon fixation from Calvin cycle by TIME OF
DAY fix carbon during night
store carbon in 4C compounds
perform Calvin cycle during day
AP Biology
C4 plants are so named because instead of forming PGA’s in the beginning of the Calvin
cycle, they form a four-carbon compound OAA (Oxaloacetate)
Several thousand species use the C4 pathway.
Instead of being fixed by rubisco, CO2 combines with a 3Carbon molecule PEP to form OAA, using the fixing enzyme PEP carboxylase.
• OAA is then converted to malate, and that is shuttled to the bundle sheath cells.
• Here, malate is converted to pyruvate and CO2.
• The pyruvate moves back to the mesophyll cells where one ATP is broken down to form AMP (not ADP) which is required to convert the pyruvate back to PEP (to help continue the cycle)
• The overall effect of this process is to move CO2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient.
AMP
AP Biology
C4 plants A better way to capture CO2
1st step before Calvin cycle, fix carbon with enzymePEP carboxylase store as 4C compound
adaptation to hot, dry climates have to close stomates a lot
sugar cane, corn, other grasses…
sugar cane
corn
AP Biology
C4 leaf anatomy
PEP (3C) + CO2 oxaloacetate (4C)
CO2
CO2
O2
light reactions
C4 anatomy
C3 anatomy
PEP carboxylase enzyme higher attraction for CO2 than O2
better than RuBisCo
fixes CO2 in 4C compounds
regenerates CO2 in inner cells for RuBisCo keeping O2 away from RuBisCo
bundlesheath
cell RuBisCo
PEPcarboxylase
stomate
AP Biology
AP Biology
CAM (Crassulacean Acid Metabolism) plants
Adaptation to hot, dry climates separate carbon fixation from Calvin cycle by TIME
close stomates during day
open stomates during night
at night: open stomates & fix carbonin 4C “storage” compounds
in day: release CO2 from 4C acids into normal Calvin cycle increases concentration of CO2 in cells
succulents, some cacti, pineapple
AP Biology
CAM plants
succulents
cacti
pineapple
AP Biology
AP Biology
AP Biology
C4 vs CAM Summary
C4 plants separate 2 steps of C fixation anatomically in 2 different cells
CAM plants separate 2 steps of C fixation temporally =2 different times
night vs. day
solves CO2 / O2 gas exchange vs. H2O loss challenge
AP Biology
Characteristics of Photosynthesis in C3, C4 and CAM plants
Characteristic C3 Plant C4 Plant CAM Plant
Photorespiration Yes Little none
Rubisco present Yes Yes Yes
PEP Carboxylase present No Yes Yes
Initial CO2 fixation directly into Calvin Cycle via Rubisco
into OAA via PEP carboxylase, then to
malic acid which moves from mesophyll cell to bundle sheath cell and then releases
CO2.
into OAA via PEP carboxylase, then to malic acid which
moves into vacuole (during night). CO2 released during the
day.
Secondary CO2 fixation ---------------- In bundle sheath cell using Rubisco
In “mesophyll”* cell using Rubisco – in morning
Site of Calvin cycle mesophyll cells bundle sheath cells Mesophyll cells