3. Autotrophic organisms use an
inorganic form of carbon, e.g.
carbon dioxide, to make up
complex organic compounds, with
1) light
energy from two sources:
2) chemicals
Rhizobium
in root nodules
5. Photosynthesis
Photosynthesis is more common and
important because:
1. It provides a source of
complex organic molecules
for heterotrophic
organisms.
2. It releases oxygen for use
by aerobic organisms.
6. Photosynthesis
Photosynthesis
Photosynthesis takes place in the green
portions of plants
Leaf of flowering plant contains mesophyll
tissue
Cells containing chloroplasts
Specialized to carry on photosynthesis
CO2 enters leaf through stomata
Diffuses into chloroplasts in mesophyll cells
In stroma, CO2 combined with H2O to form
C6H12O6 (sugar)
Energy supplied by light
6
8. Photosynthetic Reactions:
Overview
Light Reaction:
Photosynthesis
8
Chlorophyll absorbs solar energy
This energizes electrons
Electrons move down electron transport chain
- Pumps H+ into thylakoids
- Used to make ATP out of ADP and NADPH out
of NADP
Calvin Cycle Reaction
CO2 is reduced to a carbohydrate
Reduction requires the ATP and NADPH
produced above
9. Photosynthesis
Light Dependent Reactions
A. Light absorption
1. As chlorophyll absorbs light its electrons
are raised to a higher energy level by
photons at certain wavelengths
2. The electrons at higher energy levels are
said to be excited electrons
3. The excited electrons cause the
chlorophyll to become photoactivated
4. Photoactivation is the activation of a
particular pigment’s electrons (It is
caused by absorbing energy from
photons.)
11. Photosynthesis 11
Light Reactions:
The Noncyclic Electron Pathway
Takes place in thylakoid membrane
Uses two photosystems, PS-I and PS-II
PS II captures light energy
Causes an electron to be ejected from the reaction
center (chlorophyll a)
Electron travels down electron transport chain to PS I
Replaced with an electron from water
Which causes H+ to concentrate in thylakoid
chambers
Which causes ATP production
PS I captures light energy and ejects an electron
Transferred permanently to a molecule of NADP+
Causes NADPH production
13. Photosynthesis 13
Light Reactions:
The Cyclic Electron Pathway
Uses only photosystem I (PS-I)
Begins when PS I complex absorbs solar
energy
Electron ejected from reaction center
Travels down electron transport chain
Causes H+ to concentrate in thylakoid
chambers
Which causes ATP production
Electron returns to PS-I (cyclic)
Pathway only results in ATP production
15. Chapter 7
15
Light-Independent Reactions
NADPH and ATP from lightdependent reactions used to power
glucose synthesis
Light not directly necessary for lightindependent reactions if ATP &
NADPH available
Light-independent reactions called
the Calvin-Benson Cycle or C3 Cycle
16. The C3 Cycle
Chapter 7
16
6 CO2 used to synthesize 1 glucose
(C6H12O6)
Carbon dioxide is captured and
linked to ribulose bisphosphate
(RuBP)
ATP and NADPH from light
dependent reactions used to power
C3 reactions
17. Organization of the
Thylakoid Membrane
Photosynthesis
PS II:
Pigment complex and electron-acceptors
Adjacent to an enzyme that oxidizes water
Oxygen is released as a gas
Electron transport chain:
Consists of cytochrome complexes
Carries electrons between PS II and PS I
Also pump H+ from the stroma into thylakoid space
PS I:
Pigment complex and electron acceptors
Adjacent to enzyme that reduces NADP+ to NADPH
ATP synthase complex:
Has a channel for H+ flow
Which drives ATP synthase to join ADP and Pi
17
18. Photosynthesis
C3 Cycle Has Three Parts
2. Synthesis of Glyceraldehyde 3Phosphate (G3P)
Energy is donated by ATP and NADPH
Phosphoglyceric acid (PGA)
molecules are converted into
glyceraldehyde 3-Phophate (G3P)
molecules …
Ch
18
ap
ter
7
19. Photosynthesis
C3 Cycle Has Three Parts
3. Regeneration of Ribulose bisphosphate (RuBP)
10 of 12 G3P molecules converted into
6 RuBP molecules
2 of 12 G3P molecules used to
synthesize 1 glucose
ATP energy used for these reactions
Ch
19
ap
ter
7
20. Photosynthesis
A Summary of Photosynthesis
Lightdependent
reactions
occur in
thylakoids
Lightindependent
reactions (c3
cycle) occur
in stroma
Ch
20
ap
ter
7
22. Photosynthesis 22
Calvin Cycle Reactions:
Overview of C3 Photosynthesis
A cyclical series of reactions
Utilizes atmospheric carbon dioxide to
produce carbohydrates
Known as C3 photosynthesis
Involves three stages:
-Carbon dioxide fixation
-Carbon dioxide reduction
-RuBP Regeneration
23. Calvin Cycle Reactions:
Carbon Dioxide Fixation
Photosynthesis
23
CO2 is attached to 5-carbon RuBP molecule
Result in a 6-carbon molecule
This splits into two 3-carbon molecules (3PG)
Reaction accelerated by RuBP Carboxylase
(Rubisco)
CO2 now “fixed” because it is part of a
carbohydrate
25. Calvin Cycle Reactions:
Carbon Dioxide Reduction
Photosynthesis
3PG reduced to BPG
BPG then reduced to G3P
Utilizes NADPH and some ATP produced in
light reactions
25
27. Calvin Cycle Reactions:
Regeneration of RuBP
Photosynthesis
27
RuBP used in CO2 fixation must be replaced
Every three turns of Calvin Cycle,
Five G3P (a 3-carbon molecule) used
To remake three RuBP (a 5-carbon molecule)
5X3=3X5
29. Photosynthesis
Importance of Calvin Cycle
G3P (glyceraldehyde-3-phosphate) can be
converted to many other molecules
The hydrocarbon skeleton of G3P can form
Fatty acids and glycerol to make plant oils
Glucose phosphate (simple sugar)
Fructose (which with glucose = sucrose)
Starch and cellulose
Amino acids
29
30. Photosynthesis
30
C4 Photosynthesis
In hot, dry climates
Stomata must close to avoid wilting
CO2 decreases and O2 increases
O2 starts combining with RuBP instead of CO2
Photorespiration, a problem solve in C4 plants
In C4 plants
Fix CO2 to PEP a C3 molecule
The result is oxaloacetate, a C4 molecule
In hot & dry climates
- Avoid photorespiration
- Net productivity about 2-3 times C3 plants
In cool, moist, can’t compete with C3
33. Photosynthesis
CAM Photosynthesis
Crassulacean-Acid Metabolism
CAM plants partition carbon fixation by time
- During the night
CAM
plants fix CO2
Forms C4 molecules,
Stored in large vacuoles
- During daylight
NADPH
and ATP are available
Stomata closed for water conservation
C4 molecules release CO2 to Calvin cycle
33