Post on 10-May-2015
description
BIOL 101
Chapter 10
Photosynthesis
Rob Swatski Associate Professor of Biology
HACC – York Campus
The Process That Feeds the Biosphere
Photosynthesis:
- converts solar energy into chemical energy
- directly or indirectly feeds entire living world
- in plants, algae & other protists, some prokaryotes
Autotrophs: survive without eating anything derived from other organisms
- Producers: make organic molecules from inorganic molecules (H2O and CO2)
- Photoautotrophs: use solar energy to make organic molecules
(a) Plants
(c) Unicellular protist
10 µm
1.5 µm
40 µm (d) Cyanobacteria
(e) Purple sulfur bacteria
(b) Multicellular alga
Heterotrophs: obtain their organic material from other organisms
- Consumers of the biosphere
- heterotrophs depend on photoautotrophs for food and O2
Can an organism be both autotrophic and heterotrophic?
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
- their structure enables the chemical reactions of photosynthesis to occur – how so?
Leaves: the major organs of photosynthesis
- chlorophyll: green pigment inside chloroplasts
- absorbs light energy that powers the synthesis of organic molecules
CO2 enters and O2 exits the leaf through stomata
Chloroplasts: found in mesophyll cells in the leaf’s interior
- 30-40 chloroplasts per mesophyll cell
Chlorophyll is in the thylakoid membranes of chloroplasts
- thylakoids are stacked into grana (granum)
- stroma
5 µm
Mesophyll cell
Stomata CO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section
1 µm
Thylakoid space
Chloroplast
Granum
Intermembrane space
Inner membrane
Outer membrane
Stroma
Thylakoid
Photosynthesis summary equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
Photosynthesis is a redox process:
- H2O is oxidized
- CO2 is reduced
Chloroplasts split H2O into hydrogen and oxygen, incorporating the e- of H into glucose
Reactants: 6 CO2
Products:
12 H2O
6 O2 6 H2O C6H12O6
The Two Stages of Photosynthesis: A Preview
Light reactions: the “photo” part
Calvin cycle: the “synthesis” part
Light Reactions (in thylakoids):
- split H2O
- release O2
- reduce NADP+ to NADPH
- generate ATP from ADP by photophosphorylation
Light
H2O
Chloroplast
Light Reactions
NADP+
P
ADP
i +
Light
H2O
Chloroplast
Light Reactions
NADP+
P
ADP
i +
ATP
NADPH
O2
Calvin cycle (in stroma)
- forms glucose from CO2
- uses ATP and NADPH
Begins with carbon fixation
- incorporate CO2 into organic molecules
Light
H2O
Chloroplast
Light Reactions
NADP+
P
ADP
i +
ATP
NADPH
O2
Calvin Cycle
CO2
Light
H2O
Chloroplast
Light Reactions
NADP+
P
ADP
i +
ATP
NADPH
O2
Calvin Cycle
CO2
[CH2O] (sugar)
The Nature of Sunlight
Light is a form of electromagnetic energy (radiation)
- travels in rhythmic waves
Wavelength: distance between crests of waves
- determines the type of electromagnetic energy
Electromagnetic spectrum: the entire range of electromagnetic radiation
Visible light: the wavelengths that produce colors we can see
- includes wavelengths that drive PSN
- behaves as though it consists of discrete energy “particles” (photons)
UV
Visible light
Infrared Micro- waves
Radio waves X-rays Gamma
rays
103 m 1 m
(109 nm) 106 nm 103 nm 1 nm 10–3 nm 10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energy Higher energy
Shorter wavelength
Photosynthetic Pigments: The Light Receptors
Pigments: chemicals that absorb visible light
- different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
- leaves appear green because chlorophyll reflects and transmits green light
Reflected light
Absorbed light
Light
Transmitted light
Granum
Spectrophotometer: measures a pigment’s ability to absorb different wavelengths
- sends light through pigments
- measures the fraction of light transmitted at each wavelength
Galvanometer
Slit moves to pass light of selected wavelength
White light
Green light
Blue light
The low transmittance (high absorption) reading -chlorophyll absorbs most blue light
High transmittance (low absorption) reading -chlorophyll absorbs very little green light
Refracting prism Photoelectric
tube
Chlorophyll solution
TECHNIQUE
1
2 3
4
Absorption spectrum: graph plotting a pigment’s light absorption vs. wavelength
- the absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for PSN
Action spectrum: profiles the relative effectiveness of different wavelengths of radiation in driving PSN
Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’s experiment
Aerobic bacteria
RESULTS
Filament of alga
Chloro- phyll a Chlorophyll b
Carotenoids
500 400 600 700
700 600 500 400
Chlorophyll a: the main photosynthetic pigment
Accessory pigments:
- Chlorophyll b: broadens the PSN spectrum
- Carotenoids: absorb excess light that would damage chlorophyll
Porphyrin ring: light-absorbing “head” of molecule -Mg atom at center
in chlorophyll a CH3
Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts (H atoms not shown)
CHO in chlorophyll b
Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a stable ground state to an unstable excited state
When excited e- fall back to the ground state, photons are given off (fluorescence)
- if illuminated, a chlorophyll solution will fluoresce, giving off light and heat
(a) Excitation of isolated chlorophyll molecule
Heat
Excited state
(b) Fluorescence
Photon Ground state
Photon (fluorescence)
Ene
rgy
of
ele
ctro
n
e–
Chlorophyll molecule
e–
Photosystem consists of:
- Reaction-center complex surrounded by
- Light-harvesting complexes (pigments bound to proteins)
Funnels energy of photons to the reaction center
Primary electron acceptor (in reaction center complex)
- accepts an excited e- from chlorophyll a
- the 1st step of the light reactions
THYLAKOID SPACE (INSIDE THYLAKOID)
STROMA
e–
Pigment molecules
Photon
Transfer of energy
Special pair of chlorophyll a
molecules (P680 or P700)
Thyl
ako
id m
emb
ran
e
Photosystem
Primary electron acceptor
Reaction-center complex
Light-harvesting complexes
Two Types of Photosystems:
Photosystem II (PS II) functions first:
- best at absorbing wavelengths of 680 nm
P680: the reaction-center chlorophyll a of PS II
Photosystem I (PS I) functions second:
- best at absorbing wavelengths of 700 nm
P700: the reaction-center chlorophyll a of PS I
Linear Electron Flow
Two possible routes for e- flow during the light reactions: cyclic and linear
Linear electron flow: the primary pathway
- involves both photosystems I and II
- produces ATP and NADPH using light energy
A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
- an excited e- from P680 is transferred to the primary electron acceptor
Pigment molecules
Light
P680
e– 2
1
Photosystem II (PS II)
Primary acceptor
e-
P680+ = P680 with one less e-
- is a very strong oxidizing agent
- H2O is split by enzymes
- its e- are transferred from H atoms to P680+
- reduces P680+ to P680
- O2 is released as a by-product of this reaction
Pigment molecules
Light
P680
e–
Primary acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
Photosystem II (PS II)
e-
e-
e-
Each e- “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
- energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
- the diffusion of H+ (protons) across the membrane drives ATP synthesis
Pigment molecules
Light
P680
e–
Primary acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome complex
5
ATP
Photosystem II (PS II)
e-
e-
e-
e-
e- e-
e-
e-
e-
e-
e-
In PS I (like PS II), transferred light energy excites P700, which loses an e- to an electron acceptor
P700+: P700 that is missing an e-
- accepts an e- passed down from PS II via the electron transport chain
Pigment molecules
Light
P680
e–
Primary acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome complex
5
ATP
Photosystem II (PS II)
e-
e-
e-
e-
e- e-
e-
e-
e-
e-
e-
Photosystem I (PS I)
Light
Primary acceptor
e–
P700
6
e-
e-
Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
- the e- are then transferred to NADP+ and reduce it to NADPH
- the e- of NADPH are available for the reactions of the Calvin cycle
Pigment molecules
Light
P680
e–
Primary acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome complex
5
ATP
Photosystem I (PS I)
Light
Primary acceptor
e–
P700
6
Fd
NADP+ reductase
NADP+
+ H+
NADPH
8
7
e– e–
6
Photosystem II (PS II)
e-
Mill makes
ATP
e–
NADPH
e– e–
e–
e–
e– ATP
Photosystem II Photosystem I
e–
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Both 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
Spatial Organization of Chemiosmosis:
Mitochondria: protons are pumped into the intermembrane space
- diffusion of protons back into the matrix drives ATP synthesis
Chloroplasts: protons are pumped into the thylakoid space
- diffusion of protons back into the stroma drives ATP synthesis
Key
Mitochondrion Chloroplast
CHLOROPLAST STRUCTURE
MITOCHONDRION STRUCTURE
Intermembrane space
Inner membrane
Electron transport
chain
H+ Diffusion
Matrix
Higher [H+]
Lower [H+]
Stroma
ATP synthase
ADP + P i
H+ ATP
Thylakoid space
Thylakoid membrane
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
In summary:
- light reactions generate ATP
- they increase the potential energy of e- by moving them from H2O to NADPH
Light
Fd
Cytochrome complex
ADP +
i H+
ATP P
ATP synthase
To Calvin Cycle
STROMA (low H+ concentration)
Thylakoid membrane
THYLAKOID SPACE (high H+ concentration)
STROMA (low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
Light NADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2O O2
e– e–
1/2 1
2
3
The Calvin cycle regenerates its starting material as molecules enter and leave the cycle
- analagous to the citric acid cycle
- builds sugar from smaller molecules using ATP and the reducing power of e- carried by NADPH
Carbon enters the Calvin cycle as CO2
Carbon leaves as the sugar, glyceraldehyde-3-phospate (G3P)
For net synthesis of 1 G3P:
- the Calvin cycle must turn 3X, fixing 3 molecules of CO2
Three Phases of the Calvin Cycle
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
Short-lived intermediate
Phase 1: Carbon fixation
(Entering one at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
P P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
Short-lived intermediate
Phase 1: Carbon fixation
(Entering one at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
P P P
ATP 6
6 ADP
P P 6
1,3-Bisphosphoglycerate
6
P
P 6
6
6 NADP+
NADPH
i
Phase 2: Reduction
Glyceraldehyde-3-phosphate (G3P)
1 P Output G3P
(a sugar)
Glucose and other organic compounds
Calvin Cycle
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
Short-lived intermediate
Phase 1: Carbon fixation
(Entering one at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
P P P
ATP 6
6 ADP
P P 6
1,3-Bisphosphoglycerate
6
P
P 6
6
6 NADP+
NADPH
i
Phase 2: Reduction
Glyceraldehyde-3-phosphate (G3P)
1 P Output G3P
(a sugar)
Glucose and other organic compounds
Calvin Cycle
3
3 ADP
ATP
5 P
Phase 3: Regeneration of the CO2 acceptor (RuBP)
G3P
Light Reactions:
Photosystem II Electron transport chain
Photosystem I Electron transport chain
CO2
NADP+
ADP
P i +
RuBP 3-Phosphoglycerate
Calvin Cycle
G3P ATP
NADPH Starch
(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2