AP Biology
John D. O’Bryant School of
Mathematics and Science
November 8, 2012
AP Biology
Agenda
 Do Now (Table of Contents)
 HW discussion; Lab discussion
 Photosynthesis: Modeling
AP Biology
Table of Contents
(Notes/Classwork)
Date
Topic
11/1/12
Cellular Respiration summary; Photosynthesis:
Overview
11/2/12
Photosynthesis: Overview, Light Reactions;
Pigment lab
11/5/12
Photosynthesis: Calvin Cycle, Light Reactions
11/6/12
Photosynthesis: Modeling
11/7/12
Photosynthesis: Calvin Cycle, Modeling
11/8/12
Photosynthesis: Modeling
Page
number
HW
AP Biology
Modeling Photosynthesis
Task:
In pairs, build a dynamic model of C3 photosynthesis using cutout pieces
of paper to represent the molecules, ions, and membrane transporters or
pumps. You should be able to manipulate or move carbon dioxide and
water and its breakdown products through the various steps of the
process.
AP Biology
Quiz
 1. The sugar that results from three "turns" of the Calvin cycle is
glyceraldehyde-3-phosphate (G3P). Which of the following is a
consequence of this?

A) Formation of a molecule of glucose would require 9 "turns.”
B) G3P more readily forms sucrose and other disaccharides than it
does monosaccharides.
C) Some plants would not taste sweet to us.
D) The formation of starch in plants involves assembling many G3P
molecules, with or without further rearrangements.
E) G3P is easier for a plant to store.
AP Biology
Quiz
 2. How is photosynthesis similar in C4 and CAM plants?

A) In both cases, only photosystem I is used.
B) Both types of plants make sugar without the Calvin cycle.
C) In both cases, rubisco is not used to fix carbon initially.
D) Both types of plants make most of their sugar in the dark.
E) In both cases, thylakoids are not involved in photosynthesis.
AP Biology
Quiz
 3. Which process is most directly driven by light
energy?
 A) creation of a pH gradient by pumping protons
across the thylakoid membrane
B) carbon fixation in the stroma
C) reduction of NADP+ molecules
D) removal of electrons from chlorophyll molecules
E) ATP synthesis
AP Biology
Quiz
 4. Which of the following statements is a correct
distinction between autotrophs and heterotrophs?

A) Only heterotrophs require chemical compounds
from the environment.
B) Cellular respiration is unique to heterotrophs.
C) Only heterotrophs have mitochondria.
D) Autotrophs, but not heterotrophs, can nourish
themselves beginning with CO2 and other nutrients
that are inorganic.
E) Only heterotrophs require oxygen.
AP Biology
Quiz
 5. Which of the following does not occur during the
Calvin cycle?

A) carbon fixation
B) oxidation of NADPH
C) release of oxygen
D) regeneration of the CO2 acceptor
E) consumption of ATP
AP Biology
Quiz
 6. What is the purpose of creating a vacuum in the
syringe while the leaf pieces are inside the syringe?
 7. Why is it important to do #6 before using the
leaves in the experiment?
 8. How will you figure out the rate of photosynthesis
in this experiment?
AP Biology
AP Biology
2007-2008
Photosynthesis:
Variations on the Theme
AP Biology
2007-2008
Remember what plants need…
 Photosynthesis

light reactions
 light  sun
 H2O  ground

Calvin cycle
 CO2  air
What structures have
plants evolved to
APsupply
Biology
these needs?
O
C
O
vascular bundle
xylem (water)
Leaf Structure
phloem (sugar)
cuticle
epidermis
palisades
layer
spongy
layer
O2 H O
2
CO2
stomate
Transpiration
Gas exchange
AP Biology
O2
H 2O
CO2
guard
cell
Controlling water loss from leaves
 Hot or dry days
stomates close to conserve water
 guard cells

 gain H2O = stomates open
 lose H2O = stomates close

adaptation to
living on land,
but…
creates PROBLEMS!
AP Biology
When stomates close…
 Closed stomates lead to…
O2 build up  from light reactions
 CO2 is depleted  in Calvin cycle

 causes problems in Calvin Cycle
O2
xylem
(water)

CO2
AP Biology
O2
CO2
phloem
(sugars)

H2O
The best laid
schemes of
mice and men…
and plants!
Inefficiency of RuBisCo: CO2 vs O2
 RuBisCo in Calvin cycle

carbon fixation enzyme
 normally bonds C to RuBP
 CO2 is the optimal substrate
 reduction of RuBP
 building sugars

photosynthesis
when O2 concentration is high
 RuBisCo bonds O to RuBP
 O2 is a competitive substrate
 oxidation of RuBP
 breakdown sugars
AP Biology
photorespiration
Calvin cycle when CO2 is abundant
1C
ATP
RuBP
CO2
5C RuBisCo
ADP
G3P
to make
glucose
5C
G3P 3C
AP Biology
6C
unstable
intermediate
C3 plants
3C
PGA
NADPH
ATP
NADP
ADP
3C
Calvin cycle when O2 is high
O2
RuBP
Hey Dude,
are you high
on oxygen!
It’s so
sad to see a
good enzyme,
go BAD!
AP Biology
5C RuBisCo
to
mitochondria
–––––––
lost as CO2
without
making ATP
2C
3C
photorespiration
Impact of Photorespiration
 Oxidation of RuBP
short circuit of Calvin cycle
 loss of carbons to CO2

 can lose 50% of carbons fixed by Calvin cycle

reduces production of photosynthesis
 no ATP (energy) produced
 no C6H12O6 (food) produced

if photorespiration could be reduced,
plant would become 50% more efficient
 strong selection 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
 different leaf structure

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
 A better way to capture CO2

1st step before Calvin cycle,
fix carbon with enzyme
PEP carboxylase
corn
 store as 4C compound

adaptation to hot,
dry climates
 have to close stomates a lot
 different leaf anatomy

sugar cane, corn,
other grasses…
AP Biology
sugar cane
PEP (3C) + CO2  oxaloacetate (4C)
C4 leaf anatomy
O2
light reactions
CO2
PEP
carboxylase
C3 anatomy
stomate
 PEP carboxylase enzyme

bundle
sheath
cell
CO2
RuBisCo
higher attraction for CO2 than O2
 better than RuBisCo


fixes CO2 in 4C compounds
regenerates CO2 in inner cells for
RuBisCo
AP Biology
 keeping
O2 away from RuBisCo
C4 anatomy
Comparative anatomy
C3
Location,
location,location!
C4
PHYSICALLY separate C fixation from Calvin cycle
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 carbon
in 4C “storage” compounds
 in day: release CO2 from 4C acids
to Calvin cycle

 increases concentration of CO2 in cells

AP Biology
succulents, some cacti, pineapple
It’s all in
the timing!
CAM plants
cacti
succulents
AP Biology
pineapple
C4 vs CAM Summary
solves CO2 / O2 gas exchange vs. H2O loss challenge
C4 plants
CAM plants
separate 2 steps
of C fixation
anatomically in 2
different cells
separate 2 steps
of C fixation
temporally =
2 different times
night vs. day
AP Biology
Why the C3 problem?
We’ve all got
baggage!
 Possibly evolutionary baggage

Rubisco evolved in high CO2 atmosphere
 there wasn’t strong selection against active site of
Rubisco accepting both CO2 & O2
 Today it makes a difference

21% O2 vs. 0.03% CO2

photorespiration can drain away 50% of carbon
fixed by Calvin cycle on a hot, dry day

strong selection pressure to evolve better way
to fix carbon & minimize photorespiration
AP Biology
It’s not so easy as it looks…
Any Questions??
AP Biology
2007-2008
Ghosts of Lectures Past
(storage)
AP Biology
2007-2008
A second look inside a leaf…
 Gas exchange & water flow
CO2 in  for Calvin cycle
 O2 out 
waste from light reactions
 H2O out  for light reactions

photosynthesis
xylem
(water)
O2 CO
2
gas exchange
water loss
AP Biology
phloem
(sugars)
H2O
O2
CO2
C4 photosynthesis
PHYSICALLY separated C fixation from Calvin cycle
 Outer cells
light reaction &
carbon fixation
 pumps CO2 to inner
cells
 keeps O2 away from
inner cells

 away from RuBisCo
 Inner cells
CO2
O2
AP Biology
O2
CO2
Calvin cycle
 glucose to veins

Supporting a biosphere
 On global scale,
photosynthesis is the
most important process
for the continuation of life on Earth

each year photosynthesis…
 captures 121 billion tons of CO2
 synthesizes 160 billion tons of carbohydrate

AP Biology
heterotrophs are dependent on plants as
food source for fuel & raw materials
The poetic perspective…
 All the solid material of every plant
was built by sunlight out of thin air
 All the solid material of every animal
was built from plant material
air
AP Biology
sun
Then all the plants, cats,
dogs, elephants & people …
are really particles of air woven
together by strands of sunlight!
If plants can do it…
You can learn it!
Ask Questions!!
AP Biology
2007-2008
Plant pigment lab
AP Biology