Photosynthesis
 Photosynthesis in Nature
 Capturing the Energy in
Light
 Light-dependent Reactions
The Calvin Cycle
 Light-independent Reactions
Photosynthesis
in Nature
• Autotrophy & Heterotrophy
• Chloroplast Structure &
Function
• Leaf Structure & Function
Trophic Processes
• Autotroph
 auto = “self”; trophos = “feeding”
 produce organic molecules from
inorganic substrates obtained
from the environment
 Types:
chemoautotrophy (prokaryotic
bacteria only)
photoautotrophy
Trophic Processes
• Photoautotroph
 obtains inorganic molecules from
environment
 energy source – radiant energy of
sunlight (photosynthesis)
 examples:
cyanobacteria (prokaryotes)
all algae & many other protists
all plants (w/ exceptions)
Prokaryotic
Photoautotrophs
Oscillatoria - filamentous
cyanobacterium (blue-green alga)
Eukaryotic
Protist Photoautotrophs
Euglena – a unicellular mixotroph
Eukaryotic
Protist Photoautotrophs
Laminaria – a multicellular
algal kelp
Eukaryotic
Plant Photoautotrophs
flowering
seed
plant
moss
fern
Trophic Processes
• Heterotroph
 hetero = “other”; trophos =
“feeding”
 obtain organic molecules from
feeding on other organisms or their
products
 Types:
Photoheterotrophs (prokaryotic
bacteria only)
chemoheterotrophs
Trophic Processes
• Chemoheterotroph
 obtains organic molecules by
ingestion of or absorption from
other organisms
 energy source – breakdown of
organic molecules ingested or
absorbed from other organisms
 examples:
most bacteria
many protists
all fungi
all animals
Prokayotic
Chemoheterotrophs
Salmonella – a parasitic
enteric bacterium
Eukaryotic Protist
Chemoheterotrophs
ingested
Paramecia
Amoeba – a free-living,
unicellular protist
Eukaryotic Fungal
Chemoheterotrophs
Mycena – a club fungus
Eukaryotic Animal
Chemoheterotrophs
Oh my!
Plant Leaf
Structure & Function
• Leaf
 plant organ specialized for
photosynthesis
• Leaf tissues
 epidermis – contains stomata (sing.,
stoma) to allow influx of CO2
 mesophyll – contains photosynthetic
cells
 vascular tissue:
xylem – carries H2O to leaf
phloem – carries photosynthetic products
away to plant stems & roots
Plant Leaf
Structure & Function
Plant Cell
Structure & Function
• Plastids
 chloroplasts
plant organelles specialized for
photosynthesis
contain chlorophyll a and accessory pigments
in thylakoid membrane
 amyloplasts – store starch
 chromoplasts – store colorful pigments
for animal attraction to flowers
• Central vacuole
 bounded by tonoplast
 stores H2O, waste products, nutrients,
protective toxins
Plant Chloroplast
Structure & Function
•
•
•
•
outer membrane
intermembrane space
inner membrane
stroma
 site of light-independent reactions
• thylakoid membrane
 grana (sing., granum)
Chlorophyll
molecule
 site of light-dependent
reactions
• thylakoid space
 site of [H+] (light-dependent reactions)
Capturing Light Energy
Light-dependent Reactions
• Energy for Life Processes
• Light Absorption in
Chloroplasts
• Electron Transport
• Chemiosmosis
Learning
Objectives
1. Describe the role of
chlorophylls & other pigments
in photosynthesis
2. Summarize the main events of
electron transport
3. Explain how the structure of the
chloroplast relates to its
function
Energy &
Life Processes
• Photosynthesis
 The conversion of light energy into
chemical energy stored in organic
compounds
glucose  starch
amino acids  proteins
• Biochemical Pathways
 A series of linked chemical reactions
 The products of one reaction are
consumed by the following reaction
Autotrophs &
Heterotrophs
Autotrophs
Autotrophs &
heterotrophs
Photosynthesis
Tracking Atoms Through
Overview
Photosynthesis
Chemical Equation:
6CO2 + 6H2O + Light Energy
 C6H12O6 + 6O2
Chloroplast Structure
•
•
•
•
•
Outer membrane
Intermembrane space
Inner membrane
Stroma
Thylakoid membrane
 Arranged in stacks: grana (sing.,
granum)
 Thylakoid space
Chlorophyll
Molecule – in Thylakoid
• Chlorophyll a
 primary photosynthetic pigment
molecule – in thylakoid
 absorbs light energy: peak λ
420 nm (violet)
680 nm (orange-red)
 reflects blue-green
Accessory
Pigments – in Thylakoid
• Chlorophyll b
 passes absorbed light energy to
chlorophyll a
 absorbs light energy: peak λ
480 nm (violet-blue)
650 nm (orange)
 reflects yellow-green
Accessory
Pigments – in Thylakoid
• Carotenoids
 absorbed light energy
passed to chlorophyll a
photoprotection – absorption &
dissipation of harmful, excess light
energy
 absorbs light energy: peak λ
470 nm (violet-blue)
650 nm (orange)
 reflects yellow-orange
Photoexcitation:
Chlorophyll in Thylakoid
Electromagnetic
Spectrum
Absorption & Action
Spectra of Chlorophyll
Absorption spectrum –
plots
a pigment’s light absorption against
the wavelength of light absorbed
Action spectrum –
plots the
wavelength of light absorbed by a
pigment against a measure of
photosynthetic rate (like CO2
consumption or O2 release)
Photosystems
• light harvesting complexes of
the thylakoid membrane
• Structure
 antenna complex:
clusters of 100s of chl a, chl b, &
carotenoid molecules
proteins
 reaction center:
chl a near a protein primary electron
acceptor
Photosystems
• Function
 antenna complex:
captures photons from light
energy passed from pigment
molecule to pigment molecule until it
reaches a reaction center
 reaction center:
chl a absorbs energy and boosts
electron to higher energy level where
it is captured by the primary electron
acceptor
Pathways of
Photosynthesis
• Equation for Photosynthesis
• Light-dependent Reactions
– Non-cyclic electron flow
– Cyclic electron flow
• Light-independent Reactions
– Calvin cycle (C3)
– C4 processes
– CAM (crassulacean acid metabolism)
Photosynthesis
Overview
In thylakoid
membrane
& space
In stroma
Noncyclic
Electron Flow
Light-dependent
Reaction: Requirements
• requires radiant energy from
sunlight
• Involves 2 photosystems
 photosystem I (P700)
 photosystem II (P680)
• requires H2O from environment
as source of H+ & e• utilizes electron transport
chains (ETCs)
Light-dependent
Reaction Steps
(1) Photoexcitation step: light energy
forces a pair of e- to enter a higher
energy level in two chlorophyll
molecules of photosystem II
(2) Electron capture step: excited e- leave
chlorophylls & are captured by a
primary e- acceptor
Light-dependent
Reaction Steps
(3) Pq-Pc ETC step: photoexcited e- pass
from primary e- acceptor to
photosystem I via “Pq-Pc” ETC; e- lose
energy which is used to pump protons
(H+) into thylakoid space; Note photosystem I has previously lost and
e- by photoexcitation
(4) Photoexcitation step: light energy
forces a pair of e- to enter a higher
energy level in two chlorophyll
molecules of photosystem I; e- move
to another primary e- acceptor
Light-dependent
Reaction Steps
(5) NADPH formation step: primary
electron acceptor of photosystem I
passes photoexcited e- to “Fd” ETC; epass down ETC to enzyme; e- from
chain & H+ are attached to NADP+ to
produce NADPH. Note: NADPH will be
used in the light-independent
reactions of photosynthesis
Light-dependent
Reaction Steps
(6) Restoration of photosystem II e-: 2 H2O
are split by a water-splitting enzyme
to produce:
2H2O  4H+ + 4e- + O2
Chemiosmosis
Mechanical Analogy
for Light Reactions
Light-dependent
Reaction: Products
• produces O2 as byproduct
• produces energy from e- moving
down ETCs
 used to pump H+ across a membrane
 creates a proton motive force
• produces ATP
 from flow of H+ through ATP synthase
• produces NADPH
 as final electron acceptor
Organization of the
Thylakoid Membrane
The “Light” Reactions
• To View Video:
– Move mouse cursor over slide titlelink
– When hand appears, click once
• MOV Video plays about 1-1/2 min
Light-dependent Reactions
The Calvin Cycle
• Carbon Fixation by the Calvin
Cycle
• Alternative Pathways
• Rate of Photosynthesis
Learning
Objectives
1. Summarize the main events of the
Calvin cycle
2. Describe what happens to the
compounds made in the Calvin
cycle
3. Distinguish btw/ C3, C4, & CAM
plants
4. Explain how environmental factors
influence photosynthesis
Light-independent Reaction:
The Calvin Cycle
Light-independent
Reaction: Requirements
• requires 6 CO2 from environment
as C source
• requires 6 NADPH
 noncyclic e- flow of light-dep. Rx
• requires 9 ATP
 6 from noncyclic photophosphorylation
 3 from cyclic photophosphorylation
• requires 3 RuBP molecules
Calvin Cycle
Reactions
• Carbon fixation
 incorporation of CO2 into RuBP –
ribulose biphosphate
 enzyme that fixes (attaches) CO2
to RuBP: rubisco – RuBP
carboxylase
most abundant protein on earth
• Cycles 2 times using 3 CO2 @
time to make C6H12O6
Light-independent
Reaction: Products
• produces a sugar G3P
(glyceraldehyde-3-phosphate)
 2 G3Ps  combine  C6H12O6
• regenerates 6 NADP+
• regenerates 9 ADP + 9 Pi
• regenerates 3 RuBP
Light-independent
Reaction Steps
(1) Phase 1: Carbon fixation
3 CO2 + 3 RuBP –rubisco 3 unstable
intermediate  6 3-phosphoglycerate
(2) Phase 2: Reduction
6 3-phosphoglycerate + 6 ATP 
6 1,3-biphosphoglycerate + 6 ADP
6 1,3-biphosphoglycerate + 6 NADPH
 6 G3P + 6 NADP+ + 6 Pi
Light-independent
Reaction: Fate of G3P
Note:
(a) one of the 6 G3P shunts off to be
used by plant in biosynthesis of
glucose, a.a., F.A., or other organic
compounds
(b) the other 5 G3Ps will be used to
regenerate RuBP
Light-independent
Reaction Steps
(3) Phase 3: Regeneration of RuBP
complex set of reactions
 rearrange the 3-C skeletons of 5 G3Ps
(15 C total)  3 RuBP (@ w/ 5-C skeleton =
15 C total)
3 ATP
–interact in Rx 3 ADP + 3 Pi
The Calvin Cycle
• To View Video:
– Move mouse cursor over slide titlelink
– When hand appears, click once
• MOV Video plays about 2 min
Review of
Photosynthesis