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Chapter Seven
Pathways of Photosynthesis
I. Introduction
–
–
–
–
Organisms that photosynthesize
Equation for Photosynthesis
Terms: Oxidation vs Reduction
Requirements: solar energy; photosynthetic pigments; carbon
dioxide; water
II. Structure & Function of Chloroplast
III. Light (Dependent) Reactions of Photosynthesis
(Photosystems) – First Stage
– Noncyclic Electron Pathway
IV. Calvin Cycle Reactions of Photosynthesis – Second Stage
– CO2 Fixation, CO2 Reduction, Regeneration of RuBP
V. Photorespiration
VI. Photosynthesis in C3, C4, and CAM plants
I. Introduction
• Photosynthesis - type of energy
transformation
• Solar energy converted into Chemical energy
(glucose / C6H1206)
• Photosynthetic organisms are AKA autotrophs
and/or producers.
• Examples of Photosynthetic organisms:
– Plants (Plantae)
Eukaryotes - chloroplast
– Algae (Protista)
– Cyanobacteria (Domain Bacteria)
Prokaryotes –
no chloroplast
• Photosynthesis takes place in three stages
1. Capturing energy from Sunlight
2. Using the energy to make ATP and to reduce
NADP+, an electron carrier, to NADPH
3. Using the ATP and NADPH to power the
synthesis of organic molecules from CO2 in the
air.
• Chemical Equation for Photosynthesis:
2nd stage - reduction
Solar Energy
CO2 + H20
(CH2O)n + O2
Pigments
1st stage - oxidation
• Oxidation - loss of electrons (or hydrogen atoms)
• Reduction - gain of electrons (or hydrogen atoms)
H atom = H+ and eProton
water
• Which reactant is being oxidized?
Carbon dioxide
• Which reactant is being reduced?
• CONCEPT: Oxidation/reduction reactions occur together called reox
reactions
Role of NADP in photosynthesis:
Oxidation
Reduction
NADP + H → NADPH
Becomes
reduced
Picks up
“charged up”
electron
NADPH → NADP + H
Becomes
oxidized
Drops off
“charged up”
electron
FOUR things are required for photosynthesis:
– Solar energy
– Photosynthetic pigments
– Water (H2O)
– Carbon Dioxide (CO2)
1. Solar Energy (pg. 124)
• Solar (radiant) energy travels in waves.
• Wavelengths of solar energy is measured in nanometers (billionth)
Short
Long
High
Low
• It is the solar radiation in the visible light range that is absorbed and used
during the process of photosynthesis.
• Visible light spectrum ranges from 380 to 750 nm. (PBGYOR)
• Solar energy is captured by photosynthetic pigments
• Captured energy is used to excite (“charge up”) electrons
2. Photosynthetic pigments (found chloroplast)
• Function: absorbs wavelengths of visible light spectrum
• Because there are various wavelengths of visible light to absorb,
there are various pigments present in photosynthetic organisms:
– Chlorophylls - Green pigments (e.g. a & b); Major pigments
– Carotenoids - Yellow / orange pigments (e.g. beta-carotenes,
xanthophylls); Accessory pigments
• In fall you can see the different pigments found in leaves
(chlorophyll breaks down and no longer masks the other pigments)
• Plants absorb & use ONLY solar energy in the visible light range
• White light contains all colors (wavelengths) of the visible light
spectrum
• Wavelengths in visible light spectrum MOST efficiently
absorbed/used:
– Purple/blues & reds
• Wavelength in visible light spectrum LEAST efficiently used:
– green
• (NOTE: Plants are green because green is reflected NOT absorbed.)
3.
Get from
environment
Water (H2O) - Reactant required during 1st phase of
photosynthesis.
–
4.
Absorbed by roots and transported up to leaves
Carbon dioxide (CO2) - Reactant required during 2nd phase
of photosynthesis.
–
Absorbed via stoma of leaves
CONCEPTS:
• Plant pigments absorb the wavelengths of solar energy
from the visible light spectrum.
• In order for plants to be efficient at photosynthesis, plants
have different pigments to absorb the different
wavelengths of the visible light spectrum.
• Solar energy captured by the photosynthetic pigments is to
be converted into chemical energy (e.g. carbohydrate).
II. Structure & Function of Chloroplast (pg.121)
– Structure: (double membrane, stroma, thylakoids,
grana)
Stroma
Grana
Thylakoid
O2
1st Stage
(CH2O)n
2nd Stage
Thylakoids
Stroma
1st Stage of Photosynthesis occurs here
2nd Stage of Photosynthesis occurs here
Content: Photosynthetic Pigments
Content: Enzyme-rich Fluid
Associate with Photosystems
Associate with Calvin Cycle
(antennae)
Generates ATP & NADPH
Uses ATP & NADPH
First Stage
H2O
Second Stage
O2
CO2
Oxidation
(CH2O)n
Reduction
Light Dependent (runs only in
Light Independent (can run
day)
day or night)
Absorbs solar energy and transforms to
chemical energy
ATP and NADPH
Drives
Converts inorganic molecules (CO2)
to organic molecules (CH2O)n - food
III. Light (Dependent) Reactions of Photosynthesis
• FIRST STAGE: H2O→ O2 (plus hydrogen; supplies
electrons to generate energy)
• Occurs in the thylakoids of chloroplast.
– Visible wavelengths of solar energy are trapped by the
photosynthetic pigments in the thylakoids of chloroplast
and transferred to the photosystems
– Most plants use two photosystems; (PS II& PS I)
• Antenna complex – hundreds of pigment molecules that gather
photons and feed the captured light energy to the reaction center
(light-harvesting complex)
• Reaction center – consists of chlorophyll a molecules in a matrix of
proteins that passes excited electrons out of the photosystem
– Two major purposes of photosystems
• Capture solar energy
• “charge up” low e- to high e-
– Photosystems in the thylakoid membrane of chloroplast
generates ATP via chemiosmosis coupled with the ETS
e- enters
PS II
e- leaves
PS I
NON-CYCLIC ELECTRON PATHWAY – (Tracking
of electron into/out of PS) – pg. 125
1. Electron supplied by splitting of water Oxygen released
2. Low energy electron is charged up in PS II
3. High energy electron goes thru ETS & low
energy leaves – ATP produced*
4. Low energy electron gets recharged in PS I
5. High energy electron + H ion → H + NADP
→ NADPH produced
– (*used to generate more ATP via
chemiosmosis)
http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie.htm
High e- + H + NADP = NADPH
High e-
High electron carrier
of energy
H2O
PS II
Low eH+
O2
ATP
PS I
Low eH+
H
Protein – ATP
Synthase
L
Thylakoid
ADP + P
ATP
ATP produced via chemiosmosis
(AKA photophosphorylation)
IV. Calvin Cycle Reactions of Photosynthesis
• SECOND STAGE: CO2 → (CH2O)n
• Occurs in stroma of chloroplast
• AKA Dark Reaction
• Requires energy input; (NADPH & ATP) generated from light
(dependent) reactions or the First stage.
• Second phase does NOT DIRECTLY require light but
USUALLY occurs during day because that is when the
energy (NADPH & ATP) is generated to run the Calvin cycle.
• Reduction of CO2 into (CH2O)n occurs in three stages: (pg.
128)
– CO2 fixation
– CO2 reduction
– Regeneration of RuBP
1. Carbon fixation
• CO2 enters the cycle and reacts with ribulose 1,5bisphosphate (RuBP) to form a transient 6-carbon
intermediate.
• This intermediate splits into two molecules of the 3-carbon
acid, 3-phosphoglycerate (PGA)
• Enzyme that aids in this reaction is rubisco.
2. Reduction
• With the addition of energy from ATP and the hydrogen’s
from NADPH, PGA is reduced (addition of hydrogen) to
glyceraldehyde 3-phosphate (G3P)
• Six turns of the cycle produce two G3P required to make a
single glucose molecule.
3. Regeneration of RuBP
• PGA is also used to regenerate RUBP.
• Six turns also regenerates six RuBP required for the Calvin
cycle to continue.
MAJOR INPUTS
(Intermediate
Product)
ADP*
CO2 + RuBP
(C1)
(C5)
ATP
MAJOR
OUTPUT
NADP*
PGA
(2 - C3)
G3P**
NADPH
* Gets recharged in 1st stage of PS
• * *For every 6 molecules of G3P generated:
– 5 G3Ps used to regenerate RuBP
– 1 G3P leaves Calvin Cycle (used by cell)
• G3P is an important product of the Calvin Cycle.
• G3P serves as the building block for many important organic
molecules.
• Example of the fate of G3Ps that leave the Calvin Cycle (pg. 115,
Figure 6.10)
G3P
Glucose**
Fatty Acids
Sucrose (transport sugar)
Starch (storage)
Cellulose (structural)
(** Takes 2 G3P to form a glucose molecule.)
Amino Acid
V. Photorespiration
– Rubisco, the enzyme required for carbon fixation
in the Calvin cycle, has a second enzymatic activity
that can interfere with photosynthesis (more
specifically carbon fixation).
– When O2 is incorporated into RuBP, CO2 is
released and not used for carbon fixation. This
process is called photorespiration.
• The carboxylation and oxidation of RuBP are catalyzed
by the same active site on rubisco.
• This means that CO2 and O2 compete for the active site
of the enzyme.
• Under normal conditions (moderate temperatures)
CO2 out competes O2for the active sites.
• However when temperatures are high, the leaves are
forced to close their stomata in order to conserve
water.
• This means the plant cannot take in CO2 or release O2.
• This creates a low- CO2 level and a high- O2 level which
favors the process of photorespiration thus reducing
the yield of photosynthesis.
• Plants that do normal photosynthesis are
called C3 plants and are subject to
photorespiration.
• Plants that live in warmer climates must have
adaptations to help them overcome this
problem of photorespiration
V. Photosynthesis in C3, C4, and CAM plants
– Plants fix carbon dioxide into Calvin cycle
differently due to the different environments in
which they live.
– Plants that fix carbon using only C3
photosynthesis (Calvin Cycle) are called C3 plants.
Mesophyll
cell
CO2
RuBP
Calvin
Cycle
G3P
3PG
(C3)
• Plants adapted to warmer climates add CO2 to
phosphoenolpyruvate (PEP) to form a 4-carbon
molecule with the help of the enzyme PEP carboxylase
• The 4-carbon compound produced by PEP carboxylase
allows CO2 to be stored in an organic form, to then be
released in a different cell or at a different time to keep
the level of CO2 high relative to O2 when the plants
stomata are closed.
• C4 plants capture CO2 in one cell (mesophyll) and the
decarboxylation occurs in an adjacent cell (bundleshealth cells). This represents a spatial solution to the
problem of photorespiration.
Mesophyll
cell
CO2
C4
BundleSheath Cell
CO2
Calvin
Cycle
G3P
• CAM plants perform both reactions in the same
cell but capture CO2 using PEP carboxylase at
night, then decarboxylate during the day. CAM
stands for crassulacean acid metabolism. This
represents a temporal solution to the
photorespiration problem.
CO2
Mesophyll cell
Night
C4
CO2
Calvin
Cycle
G3P
Day
C3
C4
Moderate temp & rainfall
Warmer/dryer conditions than
C3
Tulips, maples, azaleas
Sugarcane, corn, Bermuda
Flowering Succulents (cacti)
Fixes C O2 in day
Fixes C O2 in day
Fixes C O2 at night
RuBP carboxylase
PEPCase
PEPCase
C O2 + PEP → Oxaloacetate
(4 C chain)
C O2 + PEP → Oxaloacetate
(4-C chain)
C O2 + RUBP → PG
(3 C chain)
CAM
Hot/dry climates