Photosynthesis involves two key sets of reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH. The Calvin cycle then uses these products to incorporate carbon from CO2 into organic molecules like glucose. Some plants have evolved adaptations to reduce oxygenation of RuBisCO and increase CO2 concentration, like C4 and CAM plants. C4 plants separate carbon fixation from the Calvin cycle across different cell types while CAM plants separate it across day and night periods. These adaptations help maximize carbon fixation and limit photorespiration.

1. Photosynthesis
Reactions

2. Electron carriers
 Electrons in chlorophyll
 Sun excites them
 Electrons gain E
 These high E electrons require special
carrier
 High E e- are similar to hot coals that need
to be transferred
 Electron carriers needed to transport high-
E e-
 NADP+
 Nicotinamide adenine dinucleotide
phosphate (Just remember NADP)
 Accepts 2 high-E e- to become
NADPH
 This is how energy (e-) from sun can
be trapped in chemical form
 NADPH carries high-E e- from
chlorophyll to other parts of the
chloroplast
 Help build molecules, such as glucose

4. Photosystems
 Clusters of chlorophyll and other pigments in the
thylakoid membrane (organized by a set of proteins in
the plant cell)
 Contain few hundred pigment molecules
 Chlorophyll a (absorbs 680 nm wavelengths)
 Chlorophyll b (absorbs 700 nm wavelengths)
 Carotene/carotenoids
 Light-collecting unit of the cell
 Solar panel
 Photosystem II (P680) and
Photosystem I (P700)

7. Light Dependent
Reactions
 Produce oxygen gas and convert ADP and NADP+ into the
energy carriers ATP and NADPH
 Take place in the THYLAKOID membrane of chloroplast
 Begins with photosystem II
 (this was discovered after photosystem I but actually occurs before it)
 Photosystem II traps light E and transfers excited e- to an ETC
 “water-splitting” photosystem
 Light absorbed by photosystem II is used to break-up water molecules
into high-E electrons, oxygen, and H+ ions
 2 electronsreplace lost e- in chlorophyll
 2 H+ ions released into the inside of the thylakoid membrane
 1 oxygen atom oxygen released into atmosphere
 Electrons in chlorophyll are excitedpassed along ETCdo
electrons in chlorophyll run out?
 No: the high-E electrons lost by the chlorophyll are replaced by the
electrons from “water splitting”

8.  High-E electrons move through the ETC from
photosystem II to photosystem I
 Energy from electrons is used by molecules in
the ETC to transport H+ ions from the stroma
into the inner thylakoid space

9.  Pigments in photosystem I use Energy from
light to re-energize the electrons
 NADP+ then picks up these high-E electrons
and H+ ions at the outer surface of the
thylakoid membrane
 NADP+ becomes NADPH

10.  As electrons are passed from chlorophyll to
NADP+, H+ ions are pumped across
membrane
 Inside of thylakoid membrane fills up with
positive H+ ions, outside in negative
 Chemosmosis occurs
 ATP synthase turns making ADPATP

11. ETC proteins
 Photosystem II (recieves light & splits water)
 Oxygen-evolving complex
 Plastoquinone
 Cytochrome
 Plastocyanin
 Photosystem I (receives more light)
 Ferredoxin
 Ferrdoxin-NADP reductase
 NADPNADPH
 ATP synthase
 ADPATP

17. Overview of Light
Dependent Rxn
 Use:
 ADP
 NADP+
 Water
 Produce:
 Oxygen
 ATP
 NADPH
 Why are these products important?
 Provide energy to build energy-containing sugars
from low-energy compounds in Calvin cycle

18. Calvin Cycle/ Light-
Independent Reactions
 So what do we have from pour light-dependent
rxns?
 High-E electrons stored in ATP and NADPH
 “chemical energy”
 But plants cannot store this chemical energy for
more than a few minutes…must change this
chemical energy into something that can be stored
for long periods of time
 Calvin cycle
 Uses ATP and NADPH from light-dependent rxn to
produce high-E sugars

22. Important concept!
 ONE carbon dioxide enters the Calvin
cycle at a time
 Calvin cycle must turn THREE times to
make the first small, three-carbon sugar
 Calvin cycle must turn SIX times to have
two small, three carbon sugars to throw
together to make GLUCOSE

23. Step 1
 THREE TURNS:
 Three carbon dioxide molecules enter cycle from atmosphere
 Enzyme adds each CO2 molecule to a Ribulose biphosphate,
RuBP molecule (a 5-carbon molecule) making three unstable 6-
carbon molecules
 The 3 unstable 6-carbon molecules immediately break off into six 3-
carbon molecules called 3-phosphoglycerate, 3-PGA
 Add six ATPs to make each of the 3PGAs into DPGA
 3-PGAs + ATP1, 3-biphosphoglycerate

25. Step 2
 THREE TURNS:
 Six 3-carbon molecules are converted into
higher energy forms using energy from ATP
and high-E electrons from NADPH
 Six 3-PGAs are converted into Six
energized Glyceraldehyde 3-phosphates
(G3P).
 1, 3-biphosphoglycerate + NADPH
Glyceraldehyde 3-phosphate

27. Step 3
 One of the G3Ps (we had a total of 6 of
these 3-carbon molecules) is removed
from the cycle
 This G3P waits for the Calvin cycle to
turn again and release another one to
make GLUCOSE (or other
sugars, lipids, amino acids, and other
compounds plant needs for metabolism
and growth)

29. Step 4
 Remaining five G3Ps (3-carbon
molecules) use ATP and rearrange
themselves
 ADP and NADP+ go back to light rxns
 Converted back into RuBP molecules
(three 5-carbon molecules)
 Calvin cycle begins again with the next
three CO2 molecules

31. Calvin cycle overview
 Three turns…  Six turns…
 Uses:  Uses:
 Three molecules of  Six molecules of CO2
CO2  NADPH
 NADPH  ATP
 ATP  Produces:
 Produces:  Two G3P’s (PGALs)
 One 3-carbon sugar that we put together to
called G3P or make …One 6-carbon
PGAL sugar “glucose”

35. Photosynthesis overview
 Two sets of reactions work together
 Light dependent
 Trap energy of sunlight into chemical form
 Calvin cycle/light independent
 Use chemical energy to produce stable-high
energy sugars from carbon dioxide and water

36. Factors that effect rate of
photosyntehsis
 Water availability
 Shortage of water can slow or stop photosyn.
 Adaptations
 Desert plants and conifers
 Waxy coating
 Temperature
 Photosyn. Depends on enzymes that function between 0*C
and 35*C
 Low temp. may cause photosyn. to stop
 Intensity of light
 Increase light intensity=increase rate of photosynthesis
 After a certain level of intensity, plant reaches its max rate of
photosynthesis

38. Problems
 Calvin cycle- RuBP binds with CO2 to make 6-C compound
that changes immediately into 3- Carbon compounds that
eventually make G3P then sugar
 Enzyme that catalyzes this reaction is called Rubisco
 Problem with Rubisco is that its not good at grabbing
CO2…gets easily confused by oxygen
 When levels of CO2 inside cell are low, Rubisco starts
grabbing oxygen
 This cause photorespiration
 When plant uses light and consumes oxygen, producing
carbon dioxide
 This uses up energy…NOT good
 Big problem on hot, dry days when stomata close and
can’t get CO2
 So how do plants deal?

40. 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

41. Comparative anatomy
Location,
location,location!
C3 C4
PHYSICALLY separate C fixation from Calvin cycle

42. C4 plants
 A better way to capture CO2
 1st step before Calvin cycle,
fix carbon with enzyme
PEP carboxylase
 store as 4C compound corn
 adaptation to hot,
dry climates
 have to close stomates a lot
 different leaf anatomy
 sugar cane, corn,
other grasses…

43. Comparative anatomy
Location,
location,location!
C3 C4
PHYSICALLY separate C fixation from Calvin cycle

44. Special Plants
 C4 plants
 "four-carbon”
 plants initially attach CO 2 to PEP
(phosphoenolpyruvate) to form the four-
carbon compound oxaloacetate using
the enzyme PEP carboxylase.
 This takes place in mesophyll cells.
 Oxaloacetate is then pumped to another
set of cells, the bundle sheath cells,
 In the bundle sheath cells it releases the
CO 2 for use by Rubisco.
 Now plant can use Calvin Cycle
 By concentrating CO 2 in the bundle
sheath cells, C4 plants promote the
efficient operation of the Calvin cycle
and minimize photorespiration.
 C4 plants include corn, sugar cane, and
many other tropical grasses

45. 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
 succulents, some cacti, pineapple

46. Special Plants
 CAM (crassulacean acid metabolism) plants
 plants initially attach CO 2 to PEP (phosphoenolpyruvate) to form the four-
carbon compound oxaloacetate using the enzyme PEP carboxylase.
 CAM plants fix carbon at night and store the oxaloacetete (organic acid)
in large vacuoles within the cell.
 During the day, CO2 is released from these organic acids and used in the
Calvin cycle
 Then they can open their stomatas at night (cool weather) and lets CO2
 The avoid water loss and to use the CO 2 for the Calvin cycle during the
day when it can be driven by the sun's energy.
 CAM plants are more common than C4 plants
 Ex. Are cacti, pineapples and other succulent plants.

47. CAM plants
cacti
succulents
pineapple

48. C4 vs CAM Summary
solves CO2 / O2 gas exchange vs. H2O loss challenge
C4 plants CAM plants
separate 2 steps separate 2 steps
of C fixation of C fixation
anatomically in 2 temporally =
different cells 2 different times
night vs. day