Transcript Photosynthesis

AP Biology
 Photosynthesis:
Reaction is the
opposite of glycolysis
6CO2 + 6H2O
→ C6H12O6 + 6O2
Photosynthesis in nature


Autotrophs:
biotic producers;
photoautotrophs;
chemoautotrophs; obtains
organic food without eating
other organisms
Heterotrophs:
biotic consumers; obtains
organic food by eating
other organisms or their byproducts (includes
decomposers)
The chloroplast
 Sites
of photosynthesis
 Occurs in specialized plant cell (mesophyll)
 Gas is exchanged in openings called
stomata
 Chloroplast is organelle responsible for
photosynthess
 Double membrane
 Contains thylakoids, grana, stroma
 Light is absorbed in pigments called
chlorophyll
Photosynthesis
2
Reactions
Light Reactions
Calvin Cycle
Photosynthesis

Light Reactions – Light energy is converted
to chemical energy to split hydrogen from
water.
 As the name implies, you MUST have light
for this reaction
 Light is first absorbed by the chloroplasts
 Takes place in the grana of the
chloroplasts (the coin-like stacks of sacs
. . . thylakoids).
Light absorption


Chloroplasts only absorb a portion of the sun
Sunlight is composed of white light which
contains all the colors (example: prism)
 The chloroplasts contain pigments that only absorb certain
“colors” of light
 Chlorophyll a: absorbs mostly “red” light
 Chlorophyll does not really absorb much “green” light so
this light is reflected (why many plants appear green)
 Carotenoids: absorb mostly “yellow”, “orange” and
“brown” light
 In the fall, many leaves lose their carotenoids and that is
why they take on the fall colors because that light is
reflected.
Purpose of the light reaction

The purpose of the light reaction is to
capture light energy and then
convert it to usable chemical energy
 Chemical energy is stored in the ATP
molecule and in NADPH molecules
 As a result of the light reaction, O2 is
released
PURPOSE OF CALVIN CYCLE
(C3 PATHWAY)

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Calvin cycle is the process by which atmospheric CO2 is
taken in by the plant and utilized to make the high
energy glucose molecule
In order to produce glucose, CO2 must be incorporated
into an organic compound, carbon fixation
In the first step, CO2 diffuses into the stroma (again, inside
the chloroplast) from the surrounding cytosol of the cell
The carbon dioxide then goes through a series of
“cycles” to create organic compounds
Photosynthesis: an overview
Redox process
 H2O is split, e- (along w/ H+)
are transferred to CO2,
reducing it to sugar
Light reactions:
 Uses 2 photosystems
 Creates ATP and NADPH
Calvin cycle:
 Fixes CO2 to create larger
(energy rich) organic
compounds
 Uses ATP and NADPH from light
reactions

Light energy to chemical
energy
 To
convert the sunlight to chemical energy (usable
by the plant and other organisms), a redox reaction
must occur in two:
photosystems : a cluster of pigment molecules and
proteins
 Photosystem
I – where electrons from photosystem II
continue to move electrons to reduce a molecule
 Photosystem II – where the light energy is initially
used to oxidize a molecule (release an electron)
Photosystems

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Light harvesting units of the
thylakoid membrane
Composed mainly of
protein and pigment
antenna complexes
Antenna pigment
molecules are struck by
photons
Energy is passed to reaction
centers (redox location)
Excited e- from chlorophyll
is trapped by a primary eacceptor
Photosystem II

Photosystem II (P680):
 photons excite
chlorophyll, e- to an
acceptor
 e- are replaced by
splitting of H2O (release of
O2)
 e-’s travel to Photosystem
II down an electron
transport chain
(cytochromes)
 as e- fall, ADP ---> ATP
(photophosphorylation)
START WITH PHOTOSYSTEM II

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Accessory pigments absorb light energy and transfer it to
chlorophyll a molecules
As a result of the light energy, electrons are released from
the chlorophyll a molecule in an oxidation reaction
The free electrons from the chlorophyll a molecule are then
“accepted” by a protein called a primary electron
acceptor which reduces the molecule
The electrons then move from one molecule to another in a
series of events called the electron transport chain
 Think of this as a water wheel with the electrons being the
water and it fuels a pump
Photosystem II
 The
electron transport chain uses its energy
to pump H+ into the thylakoid membrane
 This creates a high concentration of H+
inside the thylakoid
 Think of adding more and more air to a
balloon. The more you add, the more
pressure is inside the balloon. This
“pressure” created by a high
concentration of H+ is used to create ATP
Photosystem II
 The
high pressure created by the H+ pump is
“released” through a process called chemiosmosis
 An enzyme, called ATP synthase, is coupled to the
release of the H+ ions
 This enzyme uses the energy from the H+ pump to
add a phosphate molecule to ADP to form the
energy rich molecule ATP
ADP
+ Phosphate + Energy  ATP
Photosystem II
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Electrons from photosystem II can add to electrons used
in photosystem I (kind of a reserve supply)
If the electrons from photosystem II run out, the light
reaction stops
NOTE: another source of e-: there is an enzyme in the
thylakoid membrane that splits water molecules to
provide electrons for the electron transport chain and H+
for chemiosmosis
 2H2O  4H+ + 4e- + O2
Note: This is how a plant releases O2 so we can breathe
Photosystem I


Photosystem I (P700):
 ‘fallen’ e- replace
excited e- to primary
e- acceptor
 2nd ETC (NADP+
reductase) transfers
e- to NADP+ 
NADPH (...to Calvin
cycle…)
Photosystems produce
equal amounts of ATP
and NADPH
Photosystem I



Accessory pigments absorb light energy and
transfer it to chlorophyll a molecule (just like in
photosystem II) simultaneously with photosystem II
As a result of the light energy, electrons are released
from the chlorophyll a molecule in an oxidation
reaction
The free electrons from the chlorophyll a molecule
are then “accepted” by a protein called a primary
electron acceptor which reduces the molecule

NOTE: So far this is just like
photosystem II (it’s the same process)
Photosystem I
 The
electrons then move from one molecule to
another in a second electron transport chain
 This electron transport chain ends on the side of the
thylakoid membrane that faces the stroma (a
solution that surrounds the grana)
 At this point the electron is used to combine with H+
and NADP+ to form NADPH (a high energy
molecule)


NADP+ + H+ + 2e- + Energy NADPH (reduction)
NADPH is then used in the Calvin cycle
Summary
Light is absorbed by pigments in the grana of the
thylakoid (inside a chloroplast)
The pigment chlorophyll releases electrons to
move through an electron transport chain
Photosystem II creates a H+ pump that creates an
uneven distribution of H+ on each side of the
thylakoid membrane
1.
2.
3.

4.
ATP synthase uses the “pressure” create from this H+
pump to create ATP
Photosystem I uses the electron transport to
combine with H+ and NADP+ to form NADPH
Calvin cycle
Calvin
Cycle – ATP and NADPH from
the light reactions are used along with
CO2 to form a simple organic
compound (made of carbon)
Takes place in the stroma of the
chloroplasts (the liquid filling).
Byproducts are C6H12O6 (glucose, sort
of), ADP, and NADP+ (which return to
the light reactions).
The Calvin cycle


3 molecules of CO2 are
‘fixed’ into glyceraldehyde
3-phosphate (G3P)
Phases:
 1- Carbon fixation~ each
CO2 is attached to RuBP
(rubisco enzyme)
 2- Reduction~ electrons
from NADPH reduces to
G3P; ATP used up
 3- Regeneration~ G3P
rearranged to RuBP; ATP
used; cycle continues
The Calvin cycle
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
With the aid of an enzyme, CO2 is combined with a
5-C molecule called ribulose-bisphosphate (RuBP)
to form 2 molecules called 3-phosphoglycerate (3PGA) . . . 3-C organic compound
 CO2 + RuBP  2 3-PGA
The 3-PGA is converted to another molecule called
glyceraldehyde 3-phospate (G3P)
 2 3-PGA  2 G3P (3 carbon organic compound)
 This process uses up one ATP and one NADPH
created in the light cycle
The Calvin cycle
 One
molecule of G3P leaves the Calvin cycle
and is used to make organic compounds
(carbohydrates)
 The other molecule of G3P remains in the Calvin
cycle to make more RuBP
 Summary:
Now the CO2 is fixed into an organic
compound that can be used by the cell to
make either energy or structural molecules
Calvin Cycle
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IMPORTANT NOTE: Glucose is not directly created by
photosynthesis
The G3P molecule is the one created
This acts as a precursor to MANY organic molecules
 Glucose is emphasized because it is most important to us
Breakdown of photosynthesis:
6CO2 + 6H2O + energy  C6H12O6 + 6O2
 H2O is used in the light cycle to provide electrons for electron
transport chain
 CO2 is absorbed from atmosphere for Calvin cycle
 O2 is produced as a by-product in light cycle
 Glucose is eventually created from G3P molecules
Calvin Cycle, net synthesis
 For
each G3P (and for 3 CO2)
 Consumption of 9 ATP’s & 6
NADPH (light reactions regenerate
these molecules)
 G3P can then be used by the plant
to make glucose and other organic
compounds
Sunlight
O2
NADP+
ADP
ATP
H2O
NADPH
CO2
CHLOROPLAST
Cyclic electron flow
 Alternative
cycle
when ATP is deficient
 Photosystem II used
but not I; produces
ATP but no NADPH
 Why? The Calvin
cycle consumes more
ATP than NADPH…….
 Cyclic
photophosphorylation
Alternative carbon fixation methods, I
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Photorespiration: hot/dry
days; stomata close; CO2
decrease, O2 increase in
leaves; O2 added to
rubisco; no ATP or food
generated
Two Solutions…..
1- C4 plants: 2
photosynthetic cells,
bundle-sheath & mesophyll;
PEP carboxylase (instead of
rubisco) fixes CO2 in
mesophyll; new 4C
molecule releases CO2
(grasses)
Alternative carbon fixation methods, II
 2-
CAM plants: open
stomata during night,
close during day
(crassulacean acid
metabolism); cacti,
pineapples, etc.
Factors that affect photosynthesis

Light intensity
 In general, the higher the light intensity, the faster the light cycle
 Eventually all electrons are being used and you hit a maximum rate
for photosynthesis

Carbon dioxide levels
 Works like light intensity: increased CO2 levels increase the rate of
photosynthesis
 When the maximum level of CO2 is used, the rate does not increase
further

Temperature
 Increasing temperature increases the rate of reactions and
photosynthesis
 At a point, the heat denatures (or breaks apart) the enzymes
needed for photosynthesis and the rate decreases
A review of photosynthesis
Sunlight
Heat
Photosystem
II
NADP+
ADP
PhotoSystem
I
ATP
O2
H2O
NAD+
NADPH
Calvin
Cycle
ATP
NADH
CO2
Glycolysis
Glucose
CHLOROPLAST
Electron
Transport
System
Citric
Acid
Cycle
Pyruvate
ATP
MITOCHONDRION
ATP