PHOTOSYNTHESIS

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PHOTOSYNTHESIS
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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 selfTroph
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
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D. Chloroplasts are the sites of photosynthesis
in plants 1. All green parts of plants have
chloroplastsleaves are major site. a. color is
from chlorophyll (green pigment)absorbs light
energy (drives the making of food) 2.
Leaf structure a. Mesophylltype of cell where
chloroplasts are found. This tissue is
found in the interior of the leaf. b.
Stomatamicroscopic pores where CO2 enters and O2
exits c. Veinsdeliver 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
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Site of Photosynthesis
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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
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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 splittingwhere 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 Calvin1940s 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.
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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)
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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.

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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.

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D. What happens when pigments absorb photons?
1. When a molecule absorbs a photon, one of the
molecules 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.
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Seen when chlorophyll is isolated.
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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
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Book pg. 193
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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 Hs 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
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h. e- is then passed to a second ETC i. Fd
(ferredoxin) receives e- first, then NADP
reductase (an enzyme) transfers e- to NADPH.
Noncyclic Electron Flow
Fd
NADP reductase
Pq
Cyt
Cyt
Pc
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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 madecyclic
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)
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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)
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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
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H. How is ATP made in the noncyclic and cyclic
pathways? Chemiosmosis
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I. Comparison of Chemiosmosis in chloroplasts and
mitochondria MITOCHONDRIA CHLOROPLAST use
food to make ATP use light to make ATP pumps
H from matrix pumps H from stroma

to intermembrane space into thylakoid space
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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 (PGA)
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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) 1,3-
bisphosphoglycerates 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 -gt 6G3Pbut 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)
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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)
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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
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• 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 (photolightrespirationconsu
  mes 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.

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• 3. C4 Plants (corn, sugar cane and grass
  familycrab 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.

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• g. Process of preparing sugars in C4
  plants
• In the mesophyll
• CO2 PEP ---gt 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
• (Pyruvatea 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)

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C3 Photosynthesis vs C4 Photosynthesis

• Photorespiration
• Shade to full sun
• High water use
• Cool temperatures
• Slow to moderate growth rates
• Cool season crops

• No Photorespiration
• Full sun only
• Moderate water use
• Warm temperatures
• Very fast growth rates
• Warm season crops

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• 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

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• C4
• 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.

• CAM
• 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

• C3/Photorespiration
• When Rubisco accepts O2 instead of CO2 as the
  substrate.
• Generates no ATP.
• Decreases Ps output by as much as 50.