chapter 10: photosynthesis - untitled page chapter... · chapter 10: photosynthesis 2. ... other...
TRANSCRIPT
Chapter 10:
PHOTOSYNTHESIS
2. Light Absorption
1. Overview of Photosynthesis
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).
• 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)
Light
energy
PHOTOSYNTHESIS
6 CO2 6+ H2O
Carbon dioxide Water
C6H12O6 6+ O2
Glucose Oxygen gas
(a) Plants
(c) Unicellular protist10 µm
1.5 µm
40 µm(d) Cyanobacteria
(e) Purple sulfur
bacteria
(b) Multicellular alga
Photosynthetic Organisms
5 µm
Mesophyll cell
StomataCO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section
Photosynthesis occurs in Chloroplasts
1 µm
Thylakoid
space
Chloroplast
Granum
Intermembrane
space
Inner
membrane
Outer
membrane
Stroma
Thylakoid
Chloroplast
Structure
Reactants: 6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
The Fate of Atoms Involved
in Photosynthesis
Revealed by experiments involving radioactive
isotopes in key molecules:
• 14C in CO2 and 18O in H2O and CO2
Light
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)
Two Stages of Photosynthesis
2. Light Absorption
Chapter Reading – pp. 190-193
UV
Visible light
InfraredMicro-waves
RadiowavesX-rays
Gamma
rays
103 m1 m
(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energyHigher energy
Shorter wavelength
The Electromagnetic Spectrum
Reflectedlight
Absorbedlight
Light
Chloroplast
Transmittedlight
Granum
Chlorophyll absorbs “non-green”
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
Galvanometer
Slit moves topass lightof selectedwavelength
Whitelight
Greenlight
Bluelight
The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
Refractingprism
Photoelectrictube
Chlorophyllsolution
TECHNIQUE
1
2 3
4
Spectrophotometry
Spectrophotometers
measure the amount
of light passing
through a sample:
• measures %
absorbance
• measures %
transmittance
OR
Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’sexperiment
Aerobic bacteria
RESULTS
Filamentof alga
Chloro-
phyll a Chlorophyll b
Carotenoids
500400 600 700
700600500400
Light-Absorbing Pigments
Chlorophyll a & b,
and carotenoids
Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center
in chlorophyll aCH3
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown
CHOin chlorophyll b
(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
PhotonGroundstate
Photon(fluorescence)
En
erg
y o
f ele
ctr
on
e–
Chlorophyllmolecule
Electrons absorb Photons• electrons excited to higher energy orbitals
3. The Light Reactions
Chapter Reading – pp. 194-199
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
STROMA
e–
Pigmentmolecules
Photon
Transferof energy
Special pair ofchlorophyll amolecules
Th
yla
ko
id m
em
bra
ne
Photosystem
Primaryelectronacceptor
Reaction-centercomplex
Light-harvesting
complexes
Photosystems
Each photosystem
in the thylakoid
membrane
consists of:
• an array of light-
absorbing pigments
• a reaction center
containing 2
molecules of
chlorophyll a and a
primary e- acceptor
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fd
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
Photosystem II(PS II)
The Light Reactions
Produces ATP (chemical energy) & NADPH (reducing power).
1
H2O ½ O2 + 2 H+ + 2 *e-
2e- transport chain
(ETC) pumps H+
into thylakoid
PS I
PS II
3 ATP Synthase
uses H+ flow to
make ATP
42 e- to
NADPHPS II
PS I
4 Stages of the Light Reactions
1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II
H2O O2 + H+ + *e-sunlight
2) Energy released by a series of *e- transfers is
used to generate H+ gradient
• H+ accumulates inside the thylakoid membrane
• *e- ends up in NADPH (an electron carrier)
3) H+ gradient used to make ATP (chemiosmosis)
4) *e- “re-energized” in PS I, passed on to NADP+
Mill
makes
ATP
e–
NADPH
e–
e–
e–
e–
e–
ATP
Photosystem II Photosystem I
e–
Electron Energy Levels
Key
Mitochondrion Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
Electrontransport
chain
H+ Diffusion
Matrix
Higher [H+]
Lower [H+]
Stroma
ATP
synthase
ADP + Pi
H+ATP
Thylakoid
space
Thylakoid
membrane
ATP in Respiration vs Photosynthesis
Light
Fd
Cytochrome
complex
ADP
+
iH+
ATP
P
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
Summary of the Light Reactions
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 C6H12O6 (glucose)ATP, NADPH
• can occur in dark (doesn’t require light directly)
• also occurs during daylight!
• takes place in the stroma of chloroplasts
• outside the thylakoids
• uses energy stored in ATP and NADPH
• produced by the light reactions
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
6
6 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 P
Output G3P(a sugar)
Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
The Calvin
Cycle
C4 leaf anatomy
Mesophyll cellPhotosyntheticcells of C4
plant leafBundle-sheathcell
Vein(vascular tissue)
Stoma
The C4 pathway
Mesophyllcell CO2
PEP carboxylase
Oxaloacetate (4C)
Malate (4C)
PEP (3C)
ADP
ATP
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
C4 Pathway helps retain H2O
Mechanism to store carbon
from CO2 in soluble form,
allowing stomata closure
during day to conserve water
CO2
Sugarcane
Mesophyllcell
CO2
C4
Bundle-sheathcell Organic acids
release CO2 to Calvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Pineapple
Night
Day
CAM
SugarSugar
Calvin
Cycle
CalvinCycle
Organic acid Organic acid
(a) Spatial separation of steps (b) Temporal separation of steps
CO2 CO2
1
2
C4 compared to CAM
Crassulacean
Acid
Metabolism
• a slightly
different way
to fix CO2 at
night for use
during the day
without
opening
stomata
LightReactions:
Photosystem IIElectron transport chain
Photosystem IElectron transport chain
CO2
NADP+
ADP
Pi+
RuBP3-Phosphoglycerate
CalvinCycle
G3PATP
NADPHStarch(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2
Summary of Photosynthesis
Key Terms for Chapter 10
• chlorophyll, carotenoids
• ATP synthase
• ATP, NADPH, photosystem, reaction center
• electron transport chain (ETC)
• photoautotroph
• chloroplast, thylakoid, stroma
• Light reactions, Calvin cycle
• C4 and CAM carbon fixation
Relevant
Chapter
Questions 1-8, 10, 12