Photosynthesis
Lecture 13
Outline
▪ Photosynthesis
– Overview
– The photo part
– The synthesis part
– C3 vs C4 plants
– Cam plants
Light
energy
Figure 9.2
ECOSYSTEM
CO2  H2O
Photosynthesis
in chloroplasts
Cellular respiration
in mitochondria
ATP
Heat
energy
Organic  O
2
molecules
ATP powers
most cellular work
Review - Cellular Respiration – Redox reactions
▪ During cellular respiration – Glucose is oxidized and O2 is
reduced
– OIL RIG (or for biology, reduced – gains hydrogens; oxidized loses
hydrogens)
becomes oxidized
becomes reduced
Review - Cellular Respiration – The stages
▪ Three steps to get there:
1.
Glycolysis (http://www.youtube.com/watch?v=EfGlznwfu9U)
▪
2 ATPs
2.
Pyruvate Oxidation,Citric Acid (Kreb’s) Cycle – 2 ATPs
3.
Oxidative phosphorylation
▪ (Electron Transport Chain/Chemiosmosis/ATP synthase) - 26 – 28 ATPs
Figure 9.16
Electron shuttles
span membrane
2 NADH
Glycolysis
2 Pyruvate
Glucose
2 NADH
or
2 FADH2
2 NADH
Pyruvate oxidation
2 Acetyl CoA
6 NADH
Citric
acid
cycle
 2 ATP
 2 ATP
Maximum per glucose:
CYTOSOL
MITOCHONDRION
About
30 or 32 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
 about 26 or 28 ATP
Photosynthesis - Overview
▪ Very important!
– Without it, there’s no us
– Nourishes almost the entire living world
Photosynthesis - Overview
▪ Autotrophs – make their own food
– Almost all plants are autotrophs
– Algae, some protists, some prokaryotes
▪ Heterotrophs – get their food from other organisms
Photosynthesis - Overview
▪ Light energy
Chemical energy
▪ CO2 and H2O and Sunlight
Carbohydrate and O2
KEY FOR LIFE
Photosynthesis - Overview
▪ “Fixing” Carbon
▪ 6 CO2 + 12 H2O + Photons (light energy) C6H12O6 + 6 O2 + 6 H2O
▪ A redox reaction:
becomes reduced
Energy  6 CO2  6 H2O
C6 H12 O6  6 O2
becomes oxidized
Figure 10.UN01
Photosynthesis – Overview – Two Stages
▪ Light Reactions
– Need sunlight (photons) to occur
– Needs Water
– Produces O2, ATP and NADPH (a reducing agent)
Photosynthesis – Overview – Two Stages
▪ Light Independent Reactions
– Occur when the sun is out, but they don’t need the sun
– Called the Calvin Cycle
– Uses CO2 ATP and NADPH to produce Phosphoglyceraldehyde (G3P)
– G3P can be used to produce glucose
CO2
H 2O
Light
NADP
ADP
+ Pi
Light
Reactions
Calvin
Cycle
ATP
NADPH
Figure 10.6-4
Chloroplast
O2
[CH2O]
(sugar)
Photosynthesis – Overview – Where it happens
Leaf cross section
Chloroplasts Vein
Mesophyll
Stomata
Figure 10.4a
CO2
O2
Chloroplast
Outer
membrane
Stroma
Thylakoid
Granum
Thylakoid
space
Figure 10.4b
1 m
Intermembrane
space
Inner
membrane
CO2
H 2O
Light
NADP
ADP
+ Pi
Light
Reactions
Calvin
Cycle
ATP
NADPH
Figure 10.6-4
Chloroplast
O2
[CH2O]
(sugar)
Photosynthesis – Review of Light
▪ Electromagnetic energy
▪ Photons
▪ Travels in waves
– Measured from crest to crest
105 nm 103 nm
Gamma
rays
103
1 nm
X-rays
UV
nm
1m
(109 nm)
106 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
Figure 10.7
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Photosynthesis – Review of light
▪ Pigments
– Molecules that absorb visible light
▪ Different pigments absorb different wavelengths
▪ Wavelengths that are not absorbed are reflected
▪ We see the reflected color
– Color can also be transmitted if it shines through and is not absorbed
Photosynthesis – Light – Where it happens
▪ Chlorophyll a is the main pigment of chloroplasts
– It reflect the green wavelength
▪ Other pigments help to broaden the absorbed spectrum
– Chlorophyll b and Cartenoids
▪ Chlorophylls are part of a complex within the thylakoid
membrane
CH3
CH3 in chlorophyll a
CHO in chlorophyll b
Porphyrin ring
Figure 10.11
Hydrocarbon tail
(H atoms not shown)
Light
Reflected
light
Chloroplast
Figure 10.8
Absorbed
light
Granum
Transmitted
light
Photosynthesis – Light Dependent Reactions
▪ Electrons in chlorophyll get “excited” by
photons of light
– They go to a higher energy state
– Become less stable
▪ These electrons get transferred to a protein
complex
– Called the reaction center
▪ An electron acceptor in the middle of the
complex is reduced
Figure 10.13a
Lightharvesting
complexes
Thylakoid membrane
Photon
Photosystem
Reactioncenter
complex
STROMA
Primary
electron
acceptor
e
Transfer
of energy
Pigment
Special pair of
molecules
chlorophyll a
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
Photosynthesis – Light Dependent Reactions
▪ Two Different photosystems (complexes) involved
– Photosystem II
▪ The first in the reaction
▪ Absorbs wavelength of 680nm best
▪ Oxidizes H2O, Reduces the electron acceptor of P680 (P680+)
– O2 is released as the by-product
– Photosystem I
▪ Absorbs 700nm wavelength best
Figure 10.13b
STROMA
Thylakoid membrane
Chlorophyll
Protein
subunits
(b) Structure of photosystem II
THYLAKOID
SPACE
Photosynthesis – Light Dependent Reactions
▪ Linear electron flow
– Electrons fall down an electron transport chain from the primary
acceptor of PS II to PS I
▪ The Energy released by the flow of electrons drives proton
pumps
– Sets up a proton gradient across the thylakoid membrane
▪ Diffusion of H+ across the membrane drive ATP synthesis
Photosynthesis – Light Dependent Reactions
▪ Primary electron acceptor of PS I transfers the electron to
Ferredoxin (Fd)
▪ Then transferred to NADP+
– Reduced to NADPH
▪ NADPH becomes available for reactions of the Calvin cycle
– Also removes a proton from the stroma
e
e
e
Mill
makes
ATP
e
e
NADPH
e
Figure 10.15
e
ATP
Photosystem II
Photosystem I
Figure 10.14-1
Primary
acceptor
e
2
P680
1 Light
Pigment
molecules
Photosystem II
(PS II)
Figure 10.14-3
Primary
acceptor
2
1/
H
+
O
2 2
H2 O
e
2
3
4
Pq
Cytochrome
complex
Pc
e
e
5
P680
1 Light
ATP
Pigment
molecules
Photosystem II
(PS II)
Figure 10.14-5
Primary
acceptor
2
H
+
1/ O
2 2
H2 O
e
2
3
Primary
acceptor
4
e
Pq
Cytochrome
complex
7
Fd
e 
e
8
NADP
reductase
Pc
e
e
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
NADP
+ H
NADPH
Photosynthesis – Light Dependent Reaction –
Electron flow
Photosynthesis – Light Dependent Reactions
▪ Linear vs Cyclic Electron Flow
– Linear is the main process
▪ Results in ATP and NADPH production
– Cyclic produces surplus ATP
▪ Only uses photosystem I
▪ Only makes ATP (no NADPH)
▪ No oxygen is released.
Figure 10.16
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP
reductase
Cytochrome
complex
Pc
Photosystem I
Photosystem II
ATP
NADP
+ H
NADPH
Photosynthesis – Overview – Two Stages
▪ Light Reactions
– Need sunlight (photons) to occur
– Needs Water
– Produces O2, ATP and NADPH (a reducing agent)
▪ Light Independent Reactions
– Occur when the sun is out, but they don’t need the sun
– Called the Calvin Cycle
– Uses CO2 ATP and NADPH to produce Phosphoglyceraldehyde (G3P)
– G3P can be used to produce glucose
Figure 10.18
STROMA
(low H concentration)
Photosystem II
Light
4 H+
Cytochrome
complex
Light
NADP
reductase
3
Photosystem I
Fd
Pq
H2 O
THYLAKOID SPACE
(high H concentration)
1/
2
O2
+2 H+
Pc
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H concentration)
NADPH
2
1
NADP + H
ATP
synthase
ADP
+
P i
ATP
H+
Photosynthesis – Light Independent Reaction –
The Calvin Cycle
▪ Calvin Cycle builds sugar from smaller molecules
– Uses the ATP and the reducing power of NADPH
– Called Carbon Fixation
Photosynthesis – Light Independent Reaction –
The Calvin Cycle – 3 Phases
▪ 3 phases
1.
Carbon Fixation Phase
▪ CO2 + RuBP (Ribulose 1,5 bisphosphate) start the cycle
▪ Driven by the enzyme RuBisCo
2.
Reduction Phase
▪ Produces Glyceraldehyde 3-phosphate (G3P)
– 2 molecules of G3P can be used to build glucose or other sugars
▪ ATP is converted to ADP
▪ NADPH is oxidized to NADP+
3.
Regeneration Phase
▪ 2 molecules of G3P are used to regenerate RuBP
Input
(Entering one
CO2 at a time)
3
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
3P
Ribulose bisphosphate
(RuBP)
Figure 10.19-1
P
6
P
3-Phosphoglycerate
Input
(Entering one
CO2 at a time)
3
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
P
3P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
6
NADPH
6 NADP
6 Pi
Figure 10.19-2
6
P
Glyceraldehyde 3-phosphate
(G3P)
1
G3P
(a sugar)
Output
P
Glucose and
other organic
compounds
Phase 2:
Reduction
Input
(Entering one
CO2 at a time)
3
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
P
3P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
Figure 10.19-3
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)
1
G3P
(a sugar)
Output
P
Glucose and
other organic
compounds
Phase 2:
Reduction
Photosynthesis – C3 Reveiw
Photosynthesis - Photorespiration
▪ RuBisCO
– Ribulose Biphosphate Carboxilase Oxygnease
– Can react with Carbon or Oxygen
Photosynthesis - Photorespiration
▪ When RuBisCO fixes O2, you get an in efficient reaction called
Photorespiration
– RuBP combines with O2 to make 3-phosphoglycerate (3 carbons)
and 2-phosphoglycolate (2 carbons)
– Phosphoglycolate gets processed in the peroxisome (a waste product)
Photosynthesis - Photorespiration
▪ Could be an evolutionary artifact from a time when there was
less oxygen in the atmosphere
▪ Could be a way to handle too much oxygen and prevent it
from causing other problems
Photosynthesis – C4 Plants
▪ During the first step of carbon fixation, they end up with a 4
carbon molecule
– CO2 combines with phosphoenol pyruvate (PEP, 3 carbons)
– Facilitated by PEP carboxylase NOT RuBisCO
▪ Can only fix carbon
– Produces Oxaloacetate
Photosynthesis – C4 Plants
Oxaloacitate
– Gets converted to malate or aspartate
– Then converted to PEP and CO2
Photosynthesis – C4 Plants
– Malate gets transported to the bundle sheath cells via tubes called
plasmodesmoda
▪ No oxygen in the bundle sheath cells
▪ Malate is converted into CO2 and pyruvate
▪ Pyruvate goes back to regenerate PEP
▪ CO2 gets fixed by Calvin cycle without the presence of oxygen
The C4 pathway
C4 leaf anatomy
Photosynthetic
cells of C4
plant leaf
Mesophyll
cell
PEP carboxylase
Mesophyll cell
Bundlesheath
cell
Oxaloacetate (4C)
Vein
(vascular tissue)
PEP (3C)
ADP
Malate (4C)
Stoma
Bundlesheath
cell
CO2
ATP
Pyruvate (3C)
CO2
Calvin
Cycle
Figure 10.20
Sugar
Vascular
tissue
Photosynthesis – C4 plants
▪ More efficient way of producing sugar
▪ About 3% of all terrestrial plants
▪ All are angiosperms
▪ Most are monocots
▪ 46% of grasses are C4 (crab grass)
▪ Food Crops include Maize, sugar cane and millet
Photosynthesis – Cam plants
▪ Evolved to conserve water
– Many succulents, cactus, pinapple
▪ Sub set of C4 photosynthetic plants
▪ Close their stomata during the day
▪ Open them at night
–
–
–
–
Lets in CO2
Fixes it in the same way that C4 plants do
However, malate gets stored in vacuoles
During the day it gets pumped back into the mesophyll cell and the
calvin cycle can proceed.
Sugarcane
Pineapple
C4
CAM
CO2
Mesophyll Organic acid
cell
CO2
1 CO2 incorporated
(carbon fixation) Organic acid
Night
Figure 10.21
CO2
CO2
Bundlesheath
cell
Calvin
Cycle
Sugar
(a) Spatial separation of steps
2 CO2 released
to the Calvin
cycle
Calvin
Cycle
Day
Sugar
(b) Temporal separation of steps
Photosynthesis - Review
▪ Light Dependent Reactions
– Electron Transport sets up chemiosmotic gradient
▪ Also produces NADPH
– Diffusion of protons drives ATP synthasis
▪ NADPH and ATP are used in the Calvin Cycle to fix CO2
– RuBisCo enzymes drives the conversion of CO2 and Ribulose 1,5bisphosphate to make glyceraldehyde 3 phosphate (GP3)
– 3 GP3 molecules are made per turn
▪ 2 are regenerated back into RuBP
▪ 1 is used to build glucose or other sugars