I. Photosynthesis in nature
A. Autotrophs = “producers”, organisms that make their own
food. Making organic molecules from inorganic raw
materials obtained from the environment.
1. Auto = “self”;Troph = “feed”
2. Photoautotrophs = use light as source of energy to make
organic compounds
3. Chemoautotrophs = use energy by oxidizing inorganic
substances, such as sulfur or ammonia. Some bacteria
do this.
B. Plants, algae, certain protists, and some prokaryotes
C. Heterotrophs = obtain their organic compounds from other
organisms.
1. Hetero = “other, different”
2. consumers, decomposers
D. Chloroplasts are the sites of photosynthesis in plants
1. All green parts of plants have chloroplasts…leaves are major site.
a. color is from chlorophyll (green pigment)…absorbs light energy
(drives the making of food)
2. Leaf structure:
a. Mesophyll…type of cell where chloroplasts are found. This tissue
is found in the interior of the leaf.
b. Stomata…microscopic pores where CO2 enters and O2 exits
c. Veins…deliver water to leaves and sugar to rest of plant.
3. Chloroplast structure:
a. 2 membranes enclose the Stroma, dense fluid
b. interconnected thylakoid membranes (where chlorophyll is
located) segregates the stroma from the thylakoid space (or
lumen)
c. thylakoids can be stacked in columns called grana
II. The Process of Photosynthesis
A. Overall equation:
6 CO2 + 12 H2O + light energy  C6H12O6 + 6 O2 + 6 H2O
Can express it using the net consumption of water:
In this form, it is the reverse of respiration
B. Making food takes two processes:
1. Light reaction (in thylakoids)
a. Converts solar energy to chemical energy
b. NADP+ (like NAD+ , but with a phosphate) is reduced to
NADPH by oxidizing water (water splitting…where O2 comes
from) C.B. Van Niel used tracer to confirm this
c. ATP is made = “photophosphorylation”
2. Calvin cycle or the “dark reaction” (in stroma)
a. named after Melvin Calvin…1940’s
b. Carbon fixation take place = incorporating carbon (from CO2)
into organic compounds already present in the chloroplast.
c. by adding electrons (from NADPH) the fixed carbon is reduced
to a carbohydrate. ATP is also required to do this.
C. Properties of light (need to know to understand light reaction)
1. Light travels in waves = “electromagnetic waves”
2. Sometimes light behaves as though it consists of particles = photons
a. each photon has a fixed amount of energy.
b. amount of energy is inversely proportional to the wavelength
c. chlorophyll most effectively absorbs blue and red.
3. Light can be reflected, transmitted, or absorbed.
4. Pigments are substances that absorb light.
a. chlorophyll a (initiates light reaction)
b. chlorophyll b (accessory pigment)
c. carotenoids (photoprotective)
Chlorophylls
• Has CHON and Mg.
• Several types possible.
• Molecule has a lipophilic tail that allows it
to dissolve into membranes.
• Contains Mg in a reaction center.
Fall Leaf Colors
• Chlorophyll breaks down.
• N and Mg salvaged and moved into the
stem for next year.
• Accessory pigments remain behind, giving
the various fall leaf colors.
D. What happens when pigments absorb photons?
1. When a molecule absorbs a photon, one of the molecule’s
electrons is elevated to a higher energy level.
a. electron goes from ground state to excited state
2. Can only absorb photons whose energy is equal to the energy
difference between the ground state and excited state.
a. varies from atom or molecule to another
b. reason why each pigment is unique in which
wavelengths of light is absorbs.
3. The excited electron quickly falls to ground state releasing
light and heat. Glow is called “fluorescence”.
a. chlorophyll only fluoresces in isolation, not in the
chloroplast.
Seen when chlorophyll
is isolated.
E. Photosystems: light gathering complex
1. Chlorophyll, proteins, and other smaller organic molecules
organized in the thylakoid.
a. when pigment absorbs a photon, the energy is transmitted
from pigment to pigment until it gets to the chlorophyll a in
the “reaction center”.
2. Reaction center = where chlorophyll a is located and
where the first light-driven chemical reaction.
3. Primary electron acceptor = located next to chlorophyll a in the
reaction center. Traps an excited electron before it falls back
down to ground state.
4. Two kinds of photosystems, each having a unique reaction
center.
a. Photosystem I: reaction-center chlorophyll is P700
b. Photosystem II: reaction-center chlorophyll is P680
Book pg. 193
F. From the primary electron acceptor, the electron can go 2 ways:
1. Noncyclic electron flow pathway: this is the predominant route
a. photosystem II absorbs light (e- are excited and captured by
primary electron acceptor)
b. remaining chlorophyll (P680) is now a strong oxidizing agent.
c. water is split to obtain e- and H’s to reduce chlorophyll and
oxygen is released
d. e- are passed to photosystem I via electron transport chain.
e. as e- fall down ETC, the energy is harnessed by the thylakoid
membrane to make ATP...this is called “photophosphorylation”
f. at the bottom of chain, e- fill the “hole” in P700 (chlorophyll a
in photosystem I
g. e- are then excited and driven to the primary acceptor of
photosystem I
h. e- is then passed to a second ETC
i. Fd (ferredoxin) receives e- first, then NADP+ reductase
(an enzyme) transfers e- to NADPH.
Fd
NADP+
reductase
Pq
Cyt
Cyt
Pc
2. Cyclic Electron Flow
a. Uses photosystem I, not II.
b. e- cycled back from Fd to the cytochrome complex
c. Enters the P700 chlorophyll
d. No production of NADPH and no release of oxygen
e. ATP is made…”cyclic photophosphorylation”
f. Why? Calvin cycle used more ATP than NADPH.
g. What determines which pathway, noncyclic or cyclic, will
occur?
The concentration of NADPH in the chloroplast (when ATP
runs low, NADPH accumulates as the Calvin cycle slows
down. This stimulates shift from noncyclic, to cyclic until
ATP catches up)
G. The Splitting of Water in the light reaction
1. Oxygen given off by plants is from water, not carbon dioxide.
2. Plants split water as a source of hydrogen (discovered by C.B.
van Niel of Stanford University)
a. Sulfur bacteria gets hydrogen from hydrogen sulfide (H2S)
3. Electrons and H+ ions are transferred to CO2, reducing the
carbon dioxide to sugar.
4. The electrons increase in potential energy as they move from
water to sugar.
5. The required energy to do this is provided by light.
Photosynthesis
Cellular
Respiration
H. How is ATP made in the noncyclic and cyclic pathways? Chemiosmosis
I. Comparison of Chemiosmosis in chloroplasts and mitochondria
MITOCHONDRIA
CHLOROPLAST
use food to make ATP
use light to make ATP
pumps H+ from matrix
to intermembrane space
pumps H+ from stroma
into thylakoid space
J. The Calvin Cycle or The Dark Reaction:
1. Calvin Cycle Overview
a. Carbon enters cycle as CO2 ONE at a time
b. Cycle must go three times to make
1 Glyceraldehyde 3-phosphate (G3P)
c. Cycle must go 6 times to make glucose (combine 2
G3Ps)
2. Phase 1: Carbon fixation
a. (3) CO2 bond with a (3) 5C sugar called RuBP (ribulose
bisphosphate)
b. Enzyme Rubisco catalyzes this step (this is the most
abundant and important protein on Earth)
c. Products are highly unstable (3) 6C molecules that
immediately splits into (6) molecules of 3-phosphoglycerate
2. Phase 2: Reduction
a. An enzyme transfers a phosphate group from (6) ATP to
(6) 3-phosphoglycerate to make (6) 1,3-bisphosphoglycerate
b. (6) NADPHs are oxidized, reducing (6) 1,3bisphosphoglycerates to (6) G3Ps
(1,3 biphosphoglycerate + 2e-(from NADPH) =G3P
-Changes to G3P because it can store more energy
-G3P is found in step 4 of glycolysis
- 3CO2 -> 6G3P…but the NET gain is 1 G3P (the 5 other
molecules of G3P continue in the cycle)
-The cycle began with 15 carbons (3 molecules of
5C RuBP)
-Now there are 18 C (6 molecules of G3P)
3. Phase 3: Regeneration of CO2 acceptor (RuBP)
a. Add (3) ATPs to the (5) G3Ps remaining in the cycle
b. (5) G3Ps are rearranged into (3) RuBPs
(RuBPs receives CO2 to start cycle again)
K. Calvin Cycle Summary
1. Input
- 9 ATPs and 6 NADPHs (from the light reaction)
- 3 CO2 and 3 RuBP (5 Carbon molecule)
2. Output
-1 G3P molecule (this is the starting material for
metabolic pathways that synthesize other organic
compounds including glucose and other carbohydrates)
L. Alternative methods to Carbon Fixation
1. Problems with land plants (Dehydration and Reproduction)
a. stomata are the sites of gas exchange (take in CO2 and
release O2)
b. stomata are also the site of transpiration (evaporative
loss of water in leaves)
c. Plant closes stomata on a hot, dry day which decreases
photosynthesis because CO2 intake is decreased
d. Plants need to balance between open and closed stomata
e. Three options:
Most plants go through “photorespiration” (C3 plants)
Plants adapted to this are C4 plants and CAM plants
2. C3 plants going through photorespiration
a. most plants
b. Named because the first product after carbon fixation
is a
3 carbon molecule (3-phosphoglycerate)
c. Photorespiration- uses O2 in the Calvin cycle instead
of
CO2 (photo=light…respiration=consumes oxygen and
gives off Carbon dioxide)
This process generates NO ATP (actually uses it) or Food
Declining level of CO2 due to closing the stomata starves
the Calvin Cycle
Rubisco accepts O2 and product splits. One piece, a 2
Carbon compound, leaves chloroplast where Mitochondria
and Peroxisomes break it down to CO2
RuBP is not recycled
May reflect a time when O2 was less plentiful and CO2 was
more common.
3. C4 Plants (corn, sugar cane and grass family…crab grass)
a. Seen in 19 families of plant
b. Characteristic of hot regions with intense sunlight
c. Have a unique leaf anatomy; contains 2 types of
photosynthetic cells
• Mesophyll cells- between bundle sheath and leaf
surface (prep for Calvin cycle)
• Bundle-sheath cells- tightly packed sheaths around
veins of leaf (Calvin cycle occurs here)
d. Uses a different enzyme to initially capture CO2
(PEP Carboxylase)
e. Separates CO2 capture from carbon fixation into sugar.
f. Still uses C3 Photosynthesis to make sugar, but only does so in
the bundle sheath cells.
g. Process of preparing sugars in C4 plants
• In the mesophyll:
CO2 + PEP ---> 4 C product (oxaloacetate)
(PEP Carboxylase does this)
 PEP has a higher affinity for CO2 than Rubisco and no
affinity for O2 (this is beneficial in hot environments because
the stomata are closed to hold in water)
 PEP prevents photophosphorylation
• In the bundle-sheath:
4 C products (malate for example) are transported here via
plasmodesmata
Here, the 4C compound releases CO2
(Pyruvate…a 3 C molecule...goes back into the
mesophyll cells to be converted to PEP)
High concentration of CO2 in the bundle sheath cells
allows Rubisco to accept it (instead of O2 and the Calvin
cycle can take place)
C3 Photosynthesis
Photorespiration
Shade to full sun
High water use
Cool temperatures
Slow to moderate
growth rates
Cool season crops
vs
C4 Photosynthesis
No Photorespiration
Full sun only
Moderate water use
Warm temperatures
Very fast growth rates
Warm season crops
4. CAM plants
a. Crassulacean Acid Metabolism
b. Found in plants from arid conditions where water stress is a
problem.
c. Examples - cacti, succulents, pineapples, many orchids.
d. Organic acid and sugar production occur at different times
• Open stomata at night and close them during the day
Helps conserve water (but limits the CO2 intake)
Take up CO2 at night and incorporate it into a variety of
organic acids
These acids are stored in the vacuole of mesophyll cells at
night
During the day, ATP and NADPH produced, CO2
released from organic acid and incorporated into sugar
C3/Photorespiration
• When Rubisco accepts O2
instead of CO2 as the
substrate.
• Generates no ATP.
• Decreases Ps output by as
much as 50%.
C4
CAM
• Uses a different enzyme to
initially capture CO2
• Separates CO2 capture
from carbon fixation
• Still uses C3 Ps to make
sugar, but only does so in
the bundle sheath cells.
• Open stomata at night
to take in CO2.
• The CO2 is stored as a
C4 acid.
• During the day, the acid
is broken down and CO2
is fixed into sugar.
• Still uses C3 Ps to
make sugar.
• Slow growth