BIO 2, Lecture 14
FIGHTING ENTROPY III:
PHOTOSYNTHESIS
• Autotrophs sustain themselves without
eating anything derived from other
organisms
• Autotrophs are the producers of the
biosphere, producing organic molecules
from CO2 and other inorganic molecules
• Almost all plants are photoautotrophs,
using the energy of sunlight to make
complex organic molecules from H2O and
CO2
• Photosynthesis occurs in plants, algae,
certain other protists, and some
prokaryotes
• These organisms feed not only
themselves but also most of the living
world
(a) Plants
(c) Unicellular protist
10 µm
(e) Purple sulfur
bacteria
(b) Multicellular alga
(d) Cyanobacteria
40 µm
1.5 µm
• Heterotrophs obtain their organic
material from other organisms
• Heterotrophs are the consumers of
the biosphere
• Almost all heterotrophs, including
humans, depend on photoautotrophs
for food and O2
• In plants, the work of photosynthesis is
done by organelles called chloroplasts
• Chloroplasts are structurally similar to
and likely evolved from photosynthetic
bacteria
• In plants, leaves are the major locations
of photosynthesis
• Their green color is from chlorophyll, the
green pigment within chloroplasts
• Light energy absorbed by chlorophyll
drives the synthesis of organic molecules
in the chloroplast
• CO2 enters and O2 exits the leaf through
microscopic pores called stomata
• Chloroplasts are found mainly in cells of
the mesophyll, an interior tissue of the
leaf
• A typical mesophyll cell has 30–40
chloroplasts
• The chlorophyll is in the membranes of
thylakoids (connected sacs in the
chloroplast); thylakoids may be stacked in
columns called grana
• Chloroplasts also contain stroma, a dense
fluid
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
Chloroplast
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
• Chloroplasts split H2O into hydrogen and
oxygen, incorporating the electrons of
hydrogen into sugar molecules
• Photosynthesis can be summarized as the
following equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
• Photosynthesis is a redox process in
which H2O is oxidized (to O2) and CO2 is
reduced (to C6H12O6)
Reactants:
Products:
6 CO2
C6H12O6
Reduced CO2
12 H2O
6 H2O
6 O2
Oxidized H2O
Photosynthesis consists of two parts:
1. “Photo” part = light-dependent
reactions
•
Require light; only occur in the daytime
2.“Synthesis” (of sugar) part = light
independent reactions
•
•
•
Also called the Calvin cycle
Occur both in the daytime and at night
Plant switches most energy to Calvin
Cycle at night
• The light reactions
– Occur in the thylakoid membrane and
thylakoid space (inside the thylakoid)
– Split H2O into H+ and O2 (gas)
– Reduce NADP+ to NADPH
– Generate ATP from ADP and Pi
• The Calvin cycle
• Occurs in the stroma
• Forms sugar from CO2, using the ATP and
NADPH generated in the light reactions
• Begins with carbon fixation, incorporating
CO2 into organic molecules
CO2
H2O
“Dark” (lightindependent)
reactions (occur in
the presence and
absence of light)
Light
NADP+
ADP
+ P
i
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
• Light is a form of electromagnetic
energy, also called electromagnetic
radiation
• Like other electromagnetic energy, light
travels in rhythmic waves
• Wavelength is the distance between
crests of waves
• Wavelength determines the type of
electromagnetic energy
• Shorter wavelength = higher energy
• The electromagnetic spectrum is the
entire range of electromagnetic energy,
or radiation
• Visible light consists of wavelengths
(including those that drive photosynthesis) that produce colors we can see
• Light also behaves as though it consists
of discrete particles, called photons
10–5 nm 10–3 nm
103 nm
1 nm
Gamma
X-rays
rays
UV
106 nm
1 m
(109 nm) 103 m
MicroInfrared waves
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
• Pigments are substances that absorb
visible light
• Different pigments absorb different
wavelengths
• Wavelengths that are not absorbed are
reflected back (and seen by observers)
• Leaves appear green because chlorophyll
reflects green light back to our eyes
• It is actually the combined wavelengths not
absorbed by chlorophyll that collectively
appear green
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
• Chlorophyll a is the main photosynthetic
pigment
• Accessory pigments, such as chlorophyll
b, broaden the spectrum used for
photosynthesis
• Accessory pigments called carotenoids
absorb excessive light that would damage
chlorophyll
• When a pigment absorbs light, it goes
from a ground state to an excited state,
which is unstable
• When excited electrons fall back to the
ground state, photons are given off, an
afterglow called fluorescence
• If illuminated, an isolated solution of
chlorophyll will fluoresce, giving off light
and heat
Energy of electron
e–
Excited
state
Heat
Photon
Photon
(fluorescence)
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
• Photosynthesis begins at photosystems
located in the thylakoid membrane
• A photosystem consists of a reactioncenter complex (comprised of several
proteins) surrounded by light-harvesting
complexes
• The light-harvesting complexes (pigment
molecules bound to proteins) funnel the
energy of photons to the reaction center
• There are two types of photosystems
in the thylakoid membrane:
• Photosystem II (PS II) functions first
(the numbers reflect order of discovery)
and is best at absorbing a wavelength of
680 nm
• Photosystem I (PS I) functions second
and is best at absorbing a wavelength of
700 nm
• At the beginning of photosynthesis, a
primary electron acceptor in the
reaction center of Photosystem II
accepts an electron from chlorophyll a
that has been excited by a photon
• Solar-powered transfer of an electron
from a chlorophyll a molecule to the
primary electron acceptor is the first
step of the light reactions
• The chlorophyll molecule that is now
missing an electron is a very strong
oxidizing agent
• It grabs an electron from H2O (in the
thylakoid space) and is reduced
• O2 is released as a by-product of this
splitting of H2O
• H+ is also formed, which begins to build
up in the thylakoid space (sound familiar?)
• The electron “falls” down an electron
transport chain from the primary
electron acceptor of PS II to PS I
• Energy released by the fall is used by
proteins in the electron transport chain
to drive H+ ions from the stroma into the
inner space of the thylakoid
• Thus, there are two sources of build-up
of H+ ions in the inner space: from the
splitting of water and from the electron
transport chain
• ATP synthase, embedded in the thylakoid
membrane, coverts ADP + Pi to ATP using
the energy generated by the rush of the
H+ ions in the inner compartment out into
the stroma (with their concentration
gradient)
• Meanwhile, the electrons passing through
the electron transport chain of Photosystem II are eventually dumped off at
Photosystem I
• When photons hit chlorophyll molecules in
Photosystem I, electrons are kicked off
chlorophyll to a second electron transport
chain
• Chlorophyll molecules in Photosystem I (now
strong oxidixing agents) grab electrons
dumped dumped off from Photosystem II to
return to their reduced state
• Water is not split at Photosystem I
• Electrons excited by photons at
Photosystem I “fall” down a second
electron transport chain and are
eventually dumped onto NADP+, reducing
it to NADPH
• The electrons of NADPH are available for
the reactions of the Calvin cycle (that
drive the endergonic reactions that
create sugar and starch)
• ATP produced through the proton motive
force are also used in the Calvin Cycle
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
STROMA
(low H+ concentration)
Cytochrome
Photosystem I
complex
Light
Photosystem II
4 H+
Light
Fd
NADP+
reductase
H2O
THYLAKOID SPACE
(high H+ concentration)
e–
1
Pc
2
1/
2
O2
+2 H+
NADP+ + H+
NADPH
Pq
e–
3
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
• Chloroplasts and mitochondria generate
ATP by chemiosmosis, but use different
sources of energy
• Mitochondria transfer chemical energy
from food to ATP; chloroplasts transform
light energy into the chemical energy of
ATP, which is used to produce food
• Spatial organization of chemiosmosis
differs between chloroplasts and
mitochondria but also shows similarities
• In mitochondria, protons are pumped out
of the inner space (matrix) and into the
intermembrane space
• ATP synthesis occurs as they diffuse back
into the mitochondrial matrix
• In chloroplasts, protons are pumped into
the inner thylakoid space and out of the
stroma
• ATP synthesis occurs as they diffuse back
out of the inner thylakoid space (into the
stroma)
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
H+
Intermembrane
space
Inner
membrane
Matrix
Key
[H+]
Higher
Lower [H+]
Diffusion
Electron
transport
chain
Thylakoid
membrane
ATP
synthase
ADP + P i
Thylakoid
space
Stroma
H+
ATP
• The Calvin cycle, like the citric acid
cycle, regenerates its starting material
after molecules enter and leave the cycle
• The cycle builds sugar from smaller
molecules by using ATP and the reducing
power of electrons carried by NADPH
• The ATP and NADPH come from the lightdependent reactions
• Carbon enters the cycle as CO2 and leaves
as a sugar named glyceraldehyde-3phospate (G3P)
• For net synthesis of 1 G3P, the cycle
must take place three times, fixing 3
molecules of CO2
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P
P
3P
Ribulose bisphosphate
(RuBP)
P
Short-lived
intermediate
6
P
3-Phosphoglycerate
Input 3
CO2
(Entering one
at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
3 P
Ribulose bisphosphate
(RuBP)
P
6
P
3-Phosphoglycerate
P
6
ATP
6 ADP
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
Output
1
G3P
(a sugar)
P
Glucose and
other organic
compounds
Phase 2:
Reduction
Input
3
CO2
(Entering one
at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
3P
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
ATP
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADPH
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
Output
1
G3P
(a sugar)
P
Glucose and
other organic
compounds
Phase 2:
Reduction
H2O
Light
CO2
NADP+
ADP
+ P
i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvin
Cycle
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)