Download Chapter 10: PHOTOSYNTHESIS

Chapter 10:
PHOTOSYNTHESIS
1. Overview of Photosynthesis
2. Light Absorption
3. The Light Reactions
4. The Calvin Cycle
1. Overview of Photosynthesis
Chapter Reading – pp. 185-190, 206-207
What is Photosynthesis?
The process of converting light energy (kinetic)
into energy stored in the covalent bonds of
glucose molecules (potential).
6 CO2
+ 6
Carbon dioxide
H2O
Light
energy
Water
C6H12O6 + 6
Glucose
O2
Oxygen gas
PHOTOSYNTHESIS
• carried out by photoautotrophs
• plants, phytoplankton, cyanobacteria (any
photosynthetic organism)
• the basis of almost all ecosystems
• all “food energy” ultimately comes from the sun
• source of all atmospheric oxygen (O2)
Photosynthetic Organisms
(a) Plants
(c) Unicellular protist
10 µm
(e) Purple sulfur
bacteria
(b) Multicellular alga
(d) Cyanobacteria
40 µm
1.5 µm
Photosynthesis occurs in Chloroplasts
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
Chloroplast
Chloroplast
Structure
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
The Fate of Atoms Involved
in Photosynthesis
Reactants:
Products:
6 CO2
C6H12O6
12 H2O
6 H 2O
6 O2
Revealed by experiments involving radioactive
isotopes in key molecules:
• 14C in CO2 and 18O in H2O and CO2
Two Stages of Photosynthesis
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
2. Light Absorption
Chapter Reading – pp. 190-193
The Electromagnetic Spectrum
10–5 nm
10–3 nm
103 nm 106 nm
1 nm
Gamma
X-rays
rays
UV
Infrared
1m
(109 nm) 103 m
Microwaves
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Chlorophyll absorbs “non-green”
light
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Green light passes
on through or is
reflected, causing
the leaves to
appear green
• only wavelengths
with exact amount
of energy to excite
an e- to a higher
orbital are absorbed
Spectrophotometry
TECHNIQUE
Refracting Chlorophyll Photoelectric
prism
solution
tube
White
light
Galvanometer
2
1
Slit moves to
pass light
of selected
wavelength
3
4
Green
light
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
Spectrophotometers
measure the amount
of light passing
through a sample:
• measures %
absorbance
OR
Blue
light
The low transmittance
(high absorption)
reading indicates that
chlorophyll absorbs
most blue light.
• measures %
transmittance
Light-Absorbing Pigments
Chlorophyll a & b,
and carotenoids
CH3 in chlorophyll a
CHO in chlorophyll b
RESULTS
Chlorophyll a
Chlorophyll b
Carotenoids
(a) Absorption spectra
400
500
600
700
Wavelength of light (nm)
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
(b) Action spectrum
Aerobic bacteria
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
Filament
of alga
(c) Engelmann’s
experiment
400
500
600
700
Electrons absorb Photons
• electrons excited to higher energy orbitals
Energy of electron
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
3. The Light Reactions
Chapter Reading – pp. 194-199
Photosystems
Photosystem
Photon
Thylakoid membrane
Light-harvesting Reaction-center
complex
complexes
STROMA
Primary
electron
acceptor
• an array of lightabsorbing pigments
e–
Transfer
of energy
Special pair of
chlorophyll a
molecules
Each photosystem
in the thylakoid
membrane
consists of:
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• a reaction center
containing 2
molecules of
chlorophyll a and a
primary e- acceptor
The Light Reactions
Produces ATP (chemical energy) & NADPH (reducing power).
4
Primary
acceptor
2
H+
+
1/ O
2
2
e–
H2O
2
Primary
acceptor
e–
Pq
Cytochrome
complex
7
Fd
e–
e–
8
NADP+
reductase
3
NADPH
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
NADP+
+ H+
Photosystem I
(PS I)
H 2O
½ O2 + 2 H+ + 2 *ePS I
1
PS II
2
e- transport chain
(ETC) pumps H+
into thylakoid
4
PS II
2 e- to
NADPH
PS I
3
ATP Synthase
uses H+ flow to
make ATP
4 Stages of the Light Reactions
1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II
H 2O
sunlight
O2 + H+ + *e-
2) Energy released by a series of *e- transfers is
used to generate H+ gradient
• H+ accumulates inside the thylakoid membrane
3) H+ gradient used to make ATP (chemiosmosis)
4) *e- “re-energized” in PS I, passed on to NADP+
• *e- ends up in NADPH (an electron carrier)
Electron Energy Levels
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
ATP in Respiration vs Photosynthesis
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
H+
Intermembrane
space
Inner
membrane
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Stroma
Matrix
Key
ADP + P
[H+]
Higher
Lower [H+]
i
H+
ATP
Summary of the Light Reactions
STROMA
(low H+ concentration)
Cytochrome
complex
Photosystem II
Light
4 H+
Light
Photosystem I
NADP+
reductase
Fd
NADP+ + H+
NADPH
Pq
H2O
THYLAKOID SPACE
(high H+ concentration)
e–
1
e–
1/
Pc
2
2
3
O2
+2 H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
4. The Calvin Cycle
Chapter Reading – pp. 199-204
Overview of the Calvin Cycle
A series of reactions called the Calvin cycle
that synthesize glucose from CO2 and H2O:
CO2 + H2O
ATP, NADPH
C6H12O6 (glucose)
• uses energy stored in ATP and NADPH
• produced by the light reactions
• can occur in dark (doesn’t require light directly)
• also occurs during daylight!
• takes place in the stroma of chloroplasts
• outside the thylakoids
Input
3
The Calvin
Cycle
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
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
C4 Pathway helps retain H2O
The C4 pathway
C4 leaf anatomy
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4
Bundleplant leaf
sheath
cell
CO2
PEP carboxylase
Oxaloacetate (4C)
PEP (3C)
ADP
Vein
(vascular tissue)
ATP
Malate (4C)
Stoma
Mechanism to store carbon
from CO2 in soluble form,
allowing stomata closure
during day to conserve water
Bundlesheath
cell
Pyruvate (3C)
CO2
Calvin
Cycle
Sugar
Vascular
tissue
C4 compared to CAM
Crassulacean
Acid
Metabolism
Sugarcane
Pineapple
C4
CAM
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
1 CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
CO2
Calvin
Cycle
Night
Organic acid
CO2
2 Organic acids
Sugar
(a) Spatial separation of steps
release CO2 to
Calvin cycle
Day
Calvin
Cycle
Sugar
(b) Temporal separation of steps
• a slightly
different way
to fix CO2 at
night for use
during the day
without
opening
stomata
Summary of Photosynthesis
H2O
CO2
Light
NADP+
ADP
+ Pi
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP
3-Phosphoglycerate
Calvin
Cycle
ATP
NADPH
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)
Key Terms for Chapter 10
• photoautotroph
• chloroplast, thylakoid, stroma
• chlorophyll, carotenoids
• ATP, NADPH, photosystem, reaction center
• electron transport chain (ETC)
• ATP synthase
• Light reactions, Calvin cycle
• C4 and CAM carbon fixation
Relevant
Chapter
Questions
1-8, 10, 12