Visible Light
Where It Starts –
Photosynthesis
Chapter 6
Electromagnetic Spectrum
Shortest
wavelength
Longest
wavelength
Gamma rays
X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Radio waves
• Wavelengths humans perceive as
different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy
Pigments
• Light-absorbing molecules
• Absorb some wavelengths and
transmit others
• Color you see are the
wavelengths not absorbed
1
Light-Dependent Reactions
Main pigments in most photoautotrophs
• Pigments absorb light energy, give up ewhich enter electron transfer chains
Wavelength absorption (%)
Chlorophylls
chlorophyll a
chlorophyll b
Wavelength (nanometers)
Light-Independent Reactions
• Water molecules are split, ATP and
NADH are formed, and oxygen is
released
• Pigments that gave up electrons get
replacements
Photosynthesis Equation
• Synthesis part of
photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle
2
Chloroplasts
Inside the Chloroplast
Organelles of photosynthesis
leaf’s upper surface
photosynthetic cells
central vacuole
chloroplast
one photosynthetic cell inside the leaf
vein
stoma (gap) in lower epidermis
• Two outer
membranes
enclose a
semifluid
interior, the
stroma
• Thylakoid
membrane
inside the
stroma
two outer membranes
thylakoid
membrane
system
chloroplasts
see next slide
stroma
section from the leaf, showing its internal organization
Linked Processes
Photosynthesis
• Energy-storing pathway
Aerobic Respiration
• Energy-releasing
pathway
Two Stages of Photosynthesis
sunlight
energy
ATP
lightdependent
reactions
• Releases oxygen
ADP + Pi
lightindependent
reactions
NADPH
• Requires oxygen
• Requires carbon
dioxide
CO2
(carbon dioxide)
H2O
(water)
NADPH+
glucose
• Releases carbon
dioxide
O2
H2O (metabolic water)
3
Photosystem Function:
Reaction Center
Inside the Chloroplast
• Photosystems
are embedded
in thylakoids,
containing 200
to 300
pigments and
other molecules
that trap sun’s
energy
• Two types of
photosystems: I
and II
light
harvesting
complex
electron
transfer
chain
PHOTOSYSTEM II
thylakoid
membrane
• Molecule of chlorophyll a (P700 or
P680) is the reaction center of a
photosystem
PHOTOSYSTEM I
• Reaction center accepts energy and
donates electron to acceptor molecule
thylakoid
compartment
Electron Transfer Chains
ATP and NADPH Formation
• Adjacent to photosystem
LIGHTHARVESTING
COMPLEX
photon
• Acceptor molecule donates electrons
from reaction center
PHOTOSYSTEM II
PHOTOSYSTEM I
NADPH
NADPH + H+
H+
H+
H+
• As electrons flow through chain, energy
they release is used to produce ATP
and, in some cases, NADPH
sunlight
a light-harvesting
complex has a
ring of pigment
molecules
A photosystem is surrounded by densely
packed light harvesting complexes.
H+
H+
H+ H+
H+
H+
H+
H+
thylakoid
compartment
thylakoid
membrane
ADP + Pi
ATP
stroma
4
ATP Formation
• When water is split during photolysis,
hydrogen ions are released into
thylakoid compartment
• More hydrogen ions are pumped into
the thylakoid compartment when the
electron transfer chain operates
Calvin-Benson Cycle
ATP Formation
• Electrical and H+ concentration gradient
exists between thylakoid compartment
and stroma
• H+ flows down gradients into stroma
through ATP synthesis
• Flow of ions drives formation of ATP
Calvin-Benson Cycle
6CO2
• Overall reactants
• Overall products
– Carbon dioxide
– Glucose
– ATP
– ADP
– NADPH
– NADP+
ATP
6 RuBP
12 PGA
12
6 ADP
Calvin-Benson
cycle
ATP
12 ADP +
12 Pi
12 NADPH
4 Pi
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is regenerated
12 NADP+
10 PGAL
12 PGAL
1 Pi
1
glucose-6-1-phosphate
5
Building Glucose
Using the Products of
Photosynthesis
• Phosphorylated glucose is the building
block for:
• PGA accepts
– phosphate from ATP
– hydrogen and electrons from NADPH
• PGAL (phosphoglyceraldehyde) forms
• When 12 PGAL have formed
– 10 are used to regenerate RuBP
– Sucrose
• The most easily transported plant carbohydrate
– Starch
• The most common storage form
– 2 combine to form phosphorylated glucose
Summary of Photosynthesis
sunlight
LightDependent
Reactions
12H2O
6O2
ADP + Pi
ATP
6CO2
6 RuBP
LightIndependent
Reactions
NADPH
CalvinBenson
cycle
NADP+
The evolution of oxygen
About 3.8 billion years ago, the first organisms appeared on the young planet Earth. They were
able to use the water vapor, nitrogen, methane and ammonia that made up Earth's atmosphere for
food and energy, probably through a process facilitated or catalyzed by metals such as iron and
magnesium.
Between 3.3 and 3.5 billion years ago, cyanobacteria (blue-green algae) appeared. These
single-celled organisms had the ability to convert energy from the sun into chemical energy
through photosynthesis using hydrogen sulfide (H2S).
Between 1 and 2 billion years ago, some bacteria adapted to use water (H2O) in
photosynthesis. Oxygen, which is released as a byproduct of photosynthesis, appeared
12 PGAL
in Earth's atmosphere.
About 500 million years ago, hemoglobin and myoglobin proteins evolved.
6H2O
phosphorylated glucose
http://www.hawaii.edu/ur/heme.html
end products (e.g., sucrose, starch, cellulose)
6
Making ATP
Glucose metabolism
• Plants make ATP during photosynthesis
• Cells of all organisms make ATP by
breaking down carbohydrates, fats, and
protein
• Cellular respiration
– Aerobic
– Produces 36 ATP
– Takes place within
mitochondrion
http://staff.jccc.net/PDECELL/cellresp/respintro.html#stages
Overview of Aerobic
Respiration
Main Pathways Start
with Glycolysis
• Glycolysis occurs in cytoplasm
• Reactions are catalyzed by enzymes
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Glucose
(six carbons)
2 Pyruvate
(three carbons)
7
Overview of Aerobic Respiration
glucose
cytoplasm
2
Glucose metabolism
ATP
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
mitochondrion
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs
Cycle
2
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
• Glycolysis
– Converts one molecule
of glucose to two
molecules of pyruvate
– Anaerobic
– Produces 2 molecules
ATP (net)
– Cytoplasmic
TYPICAL ENERGY YIELD: 36 ATP
www.sirinet.net/jgjohnso/respiration.html
Net Energy Yield
from Glycolysis
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed
Second-Stage Reactions
• Occur in the
mitochondria
• Pyruvate is broken
down to carbon
dioxide
• More ATP is formed
• More coenzymes
are reduced
inner
mitochondrial
membrane
outer
mitochondrial
membrane
inner
outer
compartment compartment
Net yield is 2 ATP and 2 NADH
8
Results of the Second Stage
• All of the carbon molecules in pyruvate
end up in carbon dioxide
• Coenzymes are reduced (they pick up
electrons and hydrogen)
• One molecule of ATP is formed
• Four-carbon oxaloacetate is
regenerated
Second Stage of
Aerobic Respiration
Acetyl-CoA
Formation
pyruvate
coenzyme A
(CO2)
NADH
CoA
acetyl-CoA
Krebs Cycle
CoA
oxaloacetate
• Occurs in the mitochondria
• Coenzymes deliver electrons to electron
transfer chains
• Electron transfer sets up H+ ion
gradients
• Flow of H+ down gradients powers ATP
formation
citrate
NAD+
NADH
NADH
NAD+
FADH2
FAD
NAD+
NADH
ATP
Electron Transfer
Phosphorylation
NAD+
ADP +
phosphate
group
Electron Transfer
Phosphorylation
glucose
GLYCOLYSIS
pyruvate
• Electron transfer chains
are embedded in inner
mitochondrial
compartment
• NADH and FADH2 give up electrons that they picked up
in earlier stages to electron transfer chain
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
• Electrons are transferred through the chain
• The final electron acceptor is oxygen
9
ATP Formation
Summary of Transfers
glucose
ATP
2 PGAL
ATP
2 NADH
2 pyruvate
glycolysis
ATP
INNER
COMPARTMENT
ADP
+
Pi
2 CO2
2 FADH2
e–
2 acetyl-CoA
2 NADH
H+
H+
2
ATP
6 NADH
Krebs
Cycle
ATP
2 FADH2
4 CO2
KREBS
CYCLE
H+
H+
ATP
36 ATP
H+
H+
ADP
electron
+ Pi
transfer
phosphorylation
H+
H+
H+
Importance of Oxygen
Summary of Energy Harvest
(per molecule of glucose)
• Electron transfer phosphorylation
requires the presence of oxygen
• Glycolysis
• Oxygen withdraws spent electrons from
the electron transfer chain, then
combines with H+ to form water
• Krebs cycle and preparatory reactions
– 2 ATP formed by substrate-level
phosphorylation
– 2 ATP formed by substrate-level
phosphorylation
• Electron transfer phosphorylation
– 32 ATP formed
10
Anaerobic Pathways
Fermentation Pathways
• Do not use oxygen
• Begin with glycolysis
• Produce less ATP than aerobic pathways
• Do not break glucose down completely
• Two types of fermentation pathways
– Alcoholic fermentation
to carbon dioxide and water
• Yield only the 2 ATP from glycolysis
– Lactate fermentation
Yeasts
• Single-celled fungi
• Carry out alcoholic fermentation
• Saccharomyces cerevisiae
– Baker’s yeast
– Carbon dioxide makes bread dough rise
• Saccharomyces ellipsoideus
– Used to make beer and wine
Evolution of Metabolic
Pathways
• When life originated, atmosphere had little
oxygen
• Earliest organisms used anaerobic pathways
• Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
• Cells arose that used oxygen as final
acceptor in electron transfer
11
Processes
Are Linked
Aerobic Respiration
• Reactants
Summary
Aerobic Respiration
Photosynthesis
• Reactants
2
ATP
– Carbon dioxide
– Oxygen
– Water
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
2 NADH
8 NADH
2 FADH2
• Products
ATP
GLYCOLYSIS
energy input to
start reactions
mitochondrion
– Sugar
• Products
glucose
cytoplasm
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs
Cycle
2
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
– Carbon dioxide
– Sugar
– Water
– Oxygen
Why do animals inhale oxygen
and exhale carbon dioxide?
• Aerobic cellular respiration
– Oxygen acts as electron acceptor
– O2 combines with hydrogen ions to form
water
– Carbon dioxide is waste product
– Produces 36 ATP
Why is ATP important?
• High energy bonds hydrolyzed by
ATPases to produce ADP + Pi + energy
• Kinases phosphorylate (add Pi) to other
enzymes to activate them
• Facilitates muscle contraction, active
transport, etc.
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