Chapter 10: Photosynthesis
Word Roots:
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auto- = self
-troph = food
chloro- = green
-phyll = leaf
hetero- = other
meso- = middle
photo- = light
Autotroph – self feeder – an
organism that can make organic
compounds from inorganic raw
materials.
Heterotroph – other feeder – an
organism that must obtain pre-made
organic compounds from other living
organisms.
Some introductory vocabulary
•Autotroph: sustain life without eating; “photoautotrophs” include
plants , algae, protists, cyanobacteria, sulfur bacteria
•Heterotroph consumers, must eat
•Photosynthesis: light energy of the sun is converted to the chemical
energy of sugar and other organic nutrients
•6CO2 + 12H2O + Light Energy
•6CO2 + 6H2O + Light Energy
C6H12O6 + 6O2 + 6H2O
C6H12O6 + 6O2
Plant Structures
•Chlorophyll: green pigment in
chloroplasts
•Mesophyll: leaf tissue containing
most chloroplasts
•Stomata: site of gas exchange
•Chloroplast: site of
photosynthesis
•Stroma: dense fluid within
chloroplast
•Thylokoid: membranous sacs
containing chlorophyll
•Grana: stack of thylakoids
Where does the O2 come from?
•Two possibilities
•CO2
•H2O
•C.B. van Niel
•Bacteria with H2S – create yellow sulfur globules
•Hypothesized O2 from H2O
•Supported with radioactive isotope labeling
•Oxygen-18 (18O)
•Plant given radiolabeled water did not have the labeled water
in their tissues
2 Major Processes of Photosynthesis:
•Light reactions (or Light Dependent Reaction)
•Calvin cycle (occurs in the Light Independent or Dark Reaction)
More helpful vocabulary…
•NADP+ (nicotinomide adenine dinucleotide phosphate)
•E- acceptor, very similar to NAD+
•Reduces to NADPH
•Photophosphorylation: occurs in Light Dependent Reaction,
generates ATP via chemiosmosis
•Carbon fixation: C of CO2 is assimilated into organic compounds
by adding e- to them (reducing them)
•Part of the Light Independent Reaction
The Nature of Sunlight
•Wavelength – the distance between two crests of an
electromagnatic wave (λ).
•Electromagnetic Spectrum – entire range of electromagnetic
radiation (Gamma rays – Radio waves).
•Visible Light – the electromagnetic waves that are detectible by the
human eye, about 380 nm – 750 nm
•Photons – a fixed quantity of energy resulting from
electromagnetic radiation.
•Photon energy is indirectly related to λ (wavelength)
•Decrease wavelength, increase energy
•Ex: a photon of violet light has 2x the energy of a photon of
red. Violet light has a shorter wavelength.
Spectrophotometry
•Spectrophotometer – measures the ability of a pigment to absorb
wavelengths of light.
•Absorption Spectrum – a graph showing a pigment’s light
absorption verses the wavelength.
•Action Spectrum – shows the relative effectiveness of a different
wavelengths of light in photosynthesis.
Photosynthetic Pigments
•Chlorophyll a – used in light
reaction, blue-green
•Chlorophyll b – olive green
•Photoabsorption
•Carotenoids – yellow orange,
absorb violet and blue green
•Photoprotection: absorb and
dissipate excessive light
energy
•Has antioxidant properties
Chlorophyll
•Amphipathic
•Porphyrin ring
•Mg center
•Hydrocarbon tail
Excitation of Chlorophyll by Light
•Colors disappear when absorbed but energy does not.
•Photon absorbed – electron excited to higher energy level
(increased PE)
•Very unstable
•Returns to ground state
•Energy is released as heat
•In chloroplasts
•Photon absorbed – electron excited to higher energy level.
•Captured by Primary Electron Acceptor
•Transferred down ETC
Photosystems: 2 Components
1.Light-harvesting complexes
•Chlorophylls and Proteins
2. Reaction center
•2 special chlorophyll a
•Primary electron acceptor
•Photosystem I (PS I)
•Chlorophyll a = P700
•Absorbs 700 nm best
•Photosystem II (PSII)
•Chlorophyll a = P680
Light drives synthesis of ATP and NADPH by
energizing 2 photosystems embedded in
the thylakoid membranes of chloroplasts
1. Photon excited e- in chlorophylls of P680 in PSII
2. e- captured by 1° e- acceptor (leaving P680 as P680+)
3. Photolysis – 2e -, 2H+, 1O produced
•
2e - go to P680+, O joins another O, leaving cell as O2
4. e- transferred from PS II to PS I by ETC
•
Plastoquinone (Pq) - cytochrome
•
Plastocyanin (Pc) – protein
5. ATP generated by exergonic fall
of e- accompanied by H+
gradient in thylakoid and
chemiosmosis. ATP
synthesized in stroma.
6. Photon excited e- in P700, ecaptured by 1° e- acceptor,
leaving P700
7. e- transferred from PS I to NADP+
by ETC
Ferredoxin (Fd) - protein
8. NADP+ reductase transfers efrom Fd to NADP+ → NADPH
NADPH goes on to the Calvin
Cycle…
Cyclic Electron Flow
•Only involves PSI to create ATP (no NADPH or O2) because Linear
Electron Flow alone does not produce enough ATP to power the Calvin
Cycle
•The number of ATP produced is not sufficient enough allow the
Calvin cycle to create sugar
•NADPH is produced by Linear Electron Flow
•E- cycle back to Fd, then back to cytochrome complex and continue to
P700 in PSI
Electron Flow During Light
Dependent Reaction
• Water → PS II (P680 and primary e- acceptor)
→ Plastoquinone → Cytochrome Complex →
Plastocyanin → PS I (P700 and primary eacceptor) → Ferredoxin → NADP+ reductase
→ NADPH
Comparing Chemiosmosis in Mitochondria and Chloroplasts
• Both use chemiosmosis to
generate ATP
– Electrons are passed
through a series of carriers
while protons are pumped
through the membrane,
creating an electrochemical
gradient.
– This creates a proton
motive force, driving the
phosphorylation of ADP.
• Electrons carriers
(cytochromes) and ATP
synthases, are very similar in
chloroplasts and mitochondria.
Figure 10.16
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
H+ Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
Thylakoid
space
Stroma
P
H+
ATP
Comparing Chemiosmosis in Mitochondria and Chloroplasts
• Mitochondria: e- source is an
organic molecule
• Chloroplast: e- source is water
• Mitochondrial matrix is
analogous to chloroplast
stroma
• Proton gradient in
mitochondria is established in
the intermembrane space
• Proton gradient in thylakoid is
in the stroma
• However, ATP is generated on
the inside of mitochondria
(matrix) but the outside of the
thylakoid
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
MITOCHONDRION
STRUCTURE
Intermembrance
space
Inner Membrane
Matrix
H+ Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
CHLOROPLAST
STRUCTURE
Thylakoid
space
Thylakoid
Membrane
Stroma
P
H+
ATP
Path of Protons
•H+ pumped from stroma to Thylakoid
•ATP synthase generates ATP in stroma
•pH in stroma = 8
•pH in Thylakoid = 5
•NADPH also produced on stromal side
The Calvin Cycle
•AKA:
•Light Independent Reation
•The Dark Reaction
•The Carbon Fixation Reaction
•Anabolic
•3-C sugar produced
•Glyceraldehyde-3-phosphate (G3P)
•Requires 3 turns of the Calvin Cycle
•Glucose would require 6 turns
The Calvin Cycle
Three Phases of the Calvin
Cycle
Phase 1: Carbon Fixation
•3CO2 + 3 Ribulose
bisphosphate (RuBP)
•CO2 through
stomates
•Catalyzed by
Ribulose-1,5bisphosphate
carboxylase oxygenase
– Rubisco
•Creates an unstable 6-C
intermediate
•6 molecules of 3phosphoglycerate
Phase 2: Reduction
• 3-phosphoglycerate
phosphorylated to 1,3bisphosphoglycerate (ATP
used)
• NADPH reduces 1,3bisphosphoglycerate to
G3P (carboxyl to
aldehyde)
• G3P stores more
potential energy
Phase 3: Regeneration
•5 G3P converted to 3 RuBP using 3 ATP
Recap of 3 Phases:
•3 CO2 fixed
•1 G3P synthesized
•9 ATP used
•6 NADPH used
Photorespiration
• Transpiration: evaporative
water loss from leaves
– On hot days, stomates will
close for water conservation
• Therefore, CO2 concentration in
the leaf decreases as O2
increases
• A wasteful process called
photorespiration is favored
• Rubisco binds oxygen and then
creates 2-C molecule while
wasting important resources
Photorespiration
Occurs because Rubisco has an affinity for both CO2 and O2
When CO2 is in low concerntrations, Rubisco will bind oxygen
1. Rubisco binds O2 producing a 2-C compound that leaves the
chloroplast. This occurs when stomates are closed.
2. Peroxisomes and mitochondria rearrange the compound forming
CO2.
•
No ATP is generated but ATP is consumed
•
No sugar is produced – decreases photosynthetic output
Usually occurs in C3 plants when stomata are partly closed
•
Normally Fix CO2 to RuBP
•
Examples – Rice, wheat, soy
Avoiding Photorespiration
• There are 2 alternate pathways of in which
plants are better adapted for carbon fixation
in hot, arid climates (when stomates are
closed more often)
1.C4 plants
2.CAM plants (Crassulacean Acid Metabolism)
C4 Plants
•C4 Pathways uses carbon fixation to create an intermediate 4-Carbon
molecule in the mesophyll cells
•Sugarcane and corn
•Leaf Anatomy
•Mesophyll cells contain C4 pathway, Bundle Sheath contain CalvinCycle
•Mesophyll cells
•Relies on Cyclic Electron Flow (PSI only)
•PEP carboxylase adds CO2 to phosphoenolpyruvate (PEP – 3C) to make
oxaloacetate (4C)
•Transferred via plasmodesmata to Bundle Sheath Cells
•Bundle-sheath cells
•Found around the veins of the leaf
•4-C molecule release CO2
•Pyruvate regenerated, goes back to mesophyll and ATP is oxidized to
generate PEP
C4 mechanism
• Mesophyll of C4 pump CO2 into Bundle Sheath Cells
• Increasing the concentration of CO2 allows rubisco to bind
carbon dioxide over oxygen
• Minimizes photorespiration and increases sugar production
CAM Plants
•Occurs in succulents (water storing plants)
•Cacti, pineapples
•Stomata open at night and closed during the day
•CO2 incorporated into organic acids overnight and stored in
vacuoles until daylight
•Crassulacean acid metabolism (CAM)
•Once light is present, ATP and NADPH are available to power the
Calvin Cycle
•CO2 releases from organic acids (carboxyl groups)
•Sugar is made in the chloroplasts
Photosynthesis – Review
•50% sugar produced used by plant
•Respiration
•Anabolism
•Sugar transported as sucrose through veins (vascular tissue)
•Used for anabolism of proteins, lipids and other molecules
•Glucose is synthesized into cellulose, especially in growing
plants
•Sugar is stockpiled and stored as starch
•Starch is found in tubers, roots, seeds, fruits
•Creates atmospheric O2