Lecture 9: Photosynthesis
I.
A.
1.
Characteristics of Light
Light is composed of particles that travel as waves
Comprises a small part of the electromagnetic spectrum
B.
1.
Radiation varies in wavelength
Visible spectrum
a.
Red has longer wavelengths
b.
Violet has shorter wavelengths
C.
1.
2.
Light is comprised of photons
Photons are carriers of electromagnetic radiation
The energy of a photon is inversely related to its wavelength
a.
Short wavelengths have high energy photons
D.
Photon’s energy can be transferred to matter
1.
2.
Energize electrons
Potential outcomes of being energized
a.
Energized electron may return to the ground state
i.
Energy will dissipate as heat
ii.
Energy be emitted as light—fluoresce
b.
Electron may leave the atom and be accepted by an electron acceptor
i.
Electron acceptors are the basis of photosynthesis
II.
A.
1.
Chloroplasts
Photosynthesis in eukaryotes takes place in chloroplasts
Chloroplasts typically contain chlorophyll
B.
1.
Photosynthesis is primarily associated with mesophyll cells
Mesophyll cells contain numerous chloroplasts
C.
Structure of the chloroplast
1.
3.
4.
Double membrane
a.
Inner membrane encloses the stroma
Thylakoid
a.
Third set of membranes enclosing the thylakoid interior space
Stacks of thylakoids are known as grana
Chlorophyll and other pigments are embedded in the thylakoid membranes
D.
1.
Prokaryotes lack chloroplasts
Thylakoid membranes are formed from in-folding of the plasma membrane
2.
E.
1.
2.
Chlorophyll
Contained in the thylakoid membrane
Photopigment that absorb light with particular wavelengths
a.
Chlorophyll absorbs light in the red and blue regions of the spectrum
i.
Appears green to our visual system
3.
Structure of chlorophyll molecules
a.
4.
Porphyrin ring
i.
Absorbs light energy
b.
Tail
i.
Embeds the molecule in the thylakoid membrane
c.
At the center of the porphyrin ring is a magnesium atom
Types of chlorophyll
a.
Chlorophyll a contributes most to light-dependent photosynthetic reactions
b.
Chlorophyll b is an accessory pigment
i.
Similar to chlorophyll a
ii.
Differs in functional groups of the porphyrin ring
5.
Carotenoids absorb different wavelengths than the chlorophylls
a.
Act as accessory pigments
b.
Appear yellow and orange
c.
Widen the action spectrum for photosynthesis
III.
Overview of Photosynthetic Reaction
A.
1.
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O
Water appears as both a reactant and a product
a.
No net yield of water
Photosynthesis involves light dependent reactions and the carbon fixation reactions
a.
Carbon fixation does not require light
2.
B.
Light-dependent reactions
1.
2.
3.
Produce ATP and NADPH
Occur in the thylakoid membranes
Overview
a.
Energy from light causes chlorophyll to expel a high-energy electron
b.
This electron is transferred to an acceptor molecule
i.
Free energy state is systematically decreased in electron transport chains
ii.
Free energy drives the formation of ATP and NADPH
c.
Electron lost is replaced by an electron from water
C.
1.
2.
3.
Carbon fixation reactions
Produce carbohydrates
Products of the light-dependent reactions drive the carbon fixation reactions
Fixation
a.
Process that converts an inorganic atom to an organic compound
i.
Carbon atom to glucose
IV.
A.
1.
B.
Light-Dependent Reactions
Convert light energy to chemical energy
12 H2O + 12 NADP+ + 18 ADP + 18 Pi → 6 O2 + 12 NADPH + 18 ATP
Require light and chlorophyll
C.
1.
Photosystems I and II
Include antenna complexes that trap light
a.
Antenna complexes are aggregations of pigment molecules and electron acceptors
The reaction center is made of a complex of chlorophyll molecules and proteins
a.
The reaction centers are characterized by chlorophyll a molecules with slightly different
absorption spectra
i.
In photosystem I absorption peaks at 700 nm—P700
ii.
In photosystem II absorption peaks at 680 nm—P680
2.
D.
1.
2.
Production of ATP and NADPH
Requires both photosystems (I and II)
2 ATP molecules and 1 NADPH molecule are produced for every 2 electrons
3.
NADPH is formed by transfer of high energy electrons to NADP+
a.
Pigments in photosystem I absorb energy and transfer it to the reaction center
b.
A molecule of P700 is excited and emits an electron
c.
The electron is transferred to the primary acceptor
d.
The electron is then transferred to ferredoxin
i.
Sequentially through an electron transport chain
e.
NADP+ is the terminal electron acceptor
f.
NADP+ is reduced to form NADPH
i.
Released into the stroma
4.
Water is the electron source for non-cyclic photophosphorylation
a.
Photosystem II is also activated by light
i.
Emits an electron to a primary electron acceptor
b.
The electron is transferred to a series of acceptors, and ultimately to photosystem I
c.
Electrons in photosystem II are replaced by electrons from water
i.
Photolysis involves splitting of water into oxygen, 2 protons and 2 electrons
d.
Non-cyclic photophosphorylation is a continuous linear process
i.
Requires continuous supply of new reactants
E.
1.
2.
3.
Cyclic photophosphorylation produces ATP
Process only occurs in photosystem I
Electrons from P700 are returned to P700
No oxygen is released
i.
Photolysis does not occur
ATP is formed but not NADPH
Not a predominant mechanism for ATP formation
a.
May occur when NADP+ is not available
4.
5.
F.
ATP synthesis occurs by chemiosmosis
1.
Process requires both photosystem II and photosystem I
a.
Pigments in photosystem II absorb energy and transfer it to the reaction center
b.
A molecule of P680 is excited and emits an electron
c.
The electron is transferred to the primary acceptor
d.
Electron moves sequentially through an electron transport chain
i.
Energy is used to pump protons into the thylakoid space
e.
Chemiosmosis couples ATP synthesis and electron transport
i.
As electrons are transferred between carriers, protons are pumped into the
thylakoid interior space
ii.
Proton gradient causes a difference of about 3 pH units across the thylakoid
membrane
f.
Protons diffuse through channels formed by ATP synthase
i.
Catalyze the phosphorylation of ADP to form ATP
g.
ATP is ultimately released into the stroma
V.
A.
1.
Carbon Fixation Reactions
Energy captured during light reaction used to synthesize glucose
12 NADPH + 18 ATP + 6 CO2 → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O
B.
Most plants use the Calvin (C3) cycle to fix carbon
1.
CO2 with ribulose bisphosphate (RuBP)
a.
RuBP is a highly reactive 5-carbon molecule
b.
Reaction is catalyzed by rubisco
Forms an unstable 6-carbon molecule
i.
Immediately breaks down into 2 molecules of the 3-carbon phosphoglycerate (PGA)
PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate
(G3P)
G3P is rearranged into RuBP or glucose
a.
10 of 12 molecules of G3P are converted into RuBP
b.
2 of 12 molecules of G3P are converted into glucose or fructose
2.
4.
5.
C.
1.
2.
3.
4.
5.
D.
Photorespiration reduces photosynthetic efficiency
In C4 plants, during hot, dry periods, oxygen molecules in the chloroplast bind with Rubisco,
preventing carbon fixation
RuBP and oxygen combine, and the intermediates in the Calvin cycle degrade to form carbon
dioxide and water
Like anaerobic respiration, photorespiration produces carbon dioxide and water and requires
oxygen
Unlike aerobic respiration, photorespiration does not produce ATP
Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is
always high in the bundle sheath cells
The initial carbon fixation step differs in C4 plants
1.
2.
3.
E.
1.
2.
3.
4.
Carbon dioxide is relatively sparse in the atmosphere
a.
To obtain carbon dioxide as a carbon source, plants must keep their stomata open but
risk water loss
The C4 pathway efficiently fixes CO2 at low concentrations
a.
This pathway precedes the C3 pathway
b.
Carbon dioxide is first fixed into a 4-carbon molecule
i.
Pxaloacetate
ii.
Occurs in the mesophyll cells
c.
PEP carboxylase catalyzes the fixation of carbon dioxide
i.
High affinity for carbon dioxide
d.
Bundle sheath cells surround the vascular bundles in a leaf
e.
Oxaloacetate is converted into malate
i.
Malate is passed to the bundle sheath cells
ii.
Requires NADPH
f.
Malate is decarboxylated to form carbon dioxide and pyruvate
i.
NADPH is regenerated
g.
Pyruvate returns to the mesophyll cell
i.
Regenerates phosphoenol pyruvate
ii.
Requires ATP
C4 pathway requires 30 ATPs per sugar molecule
a.
More efficient at high light intensities
b.
C3 pathway is optimal at lower light intensities
Carbon fixation in CAM plants
Crassulacean acid metabolism
a.
Succulent plants (family Crassulaceae)
CAM plants fix CO2 at night
CAM plants fix carbon dioxide at night
a.
Form malate
i.
Stored in vacuoles
CAM plants close their stomata during the daytime to reduce water loss
a.
Allows plants to live in highly xeric conditions