Energy Acquiring Pathways

1
Energy - Acquiring Pathways

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 7

2
Key Concepts

• Photosynthesis is the pathway by which carbon and
  energy enter the web of life
• Survival of nearly all organisms depends on
  photosynthesis performed by autotrophs
• Photosynthesis converts solar energy to chemical
  energy
• Photosynthesis is summarized this way

3
The Equation
12H2O 6CO2
6O2 C6H12O6 6H2O
WATER
CARBON DIOXIDE
OXYGEN
GLUCOSE
WATER
in-text, p. 113
4
Factors that influence photosynthetic rates

• 12H20 6CO2 ? 602 C6H12O6 6H2O
• Factors that would impact it
• CO2 carbon dioxide concentration
• Light intensity
• Water availability
• Minerals
• Temperature

5
Where Photosynthesis Takes Place

• Chloroplasts
• Two outermost membranes surround interior stroma
• Inner thylakoid membrane system

6
upper surface of leaf
photosynthetic cells
two outer membrane layers
part of thylakoid membrane system(the
chloroplasts innermost membrane)
(see next slide)
Fig. 7.3a, p. 114
stroma
7
Inside the Chloroplast
Stroma
Thylakoid membranes
8
Sunlight as an Energy Source

• Visible light is 380 - 750 nm
• Photoautotrophs use visible light energy

9
Absorption Spectra

• Main pigments are Chlorophyll a and b
• Accessory pigments are carotenoids, anthocyanins,
  and phycobilins

10
Action Spectrum
http//cenocracy.topcities.com/spectrum.gif
11
Chlorophyll and Solar Energy

• Solar energy is captured by pigment molecules in
  the thylakoid membranes of chloroplasts.
• Examples chlorophyll a and b, beta-carotene
• Electrons in these molecules get excited by
  sunlight and move to higher energy levels
  photosynthesis gets started.

12
http//members.lycos.nl/rherold/science/extraction
/plantenextractie.htm
13
Pigments

• Pigment structure
• Hydrocarbon backbones dissolve readily in lipid
  bilayer of photosythetic cell membranes

14
Photosystems

• Pigments are organized in photosystems

15
What Happens to the Absorbed Energy?

• Energy flows randomly among pigments of a
  photosystem until trapped by reaction center
• Mixture of pigments
• Chloropyll a at center is important because of
  its location

16
Photosynthesis An Introduction
Light Dependent Rxns Thylakoid Membrane
Light Independent Rxns Stroma
17
Sun Energy Source
Light Dependent Rxns Thylakoid Membrane
Light Independent Rxns Stroma
18
Sun Energy Source
Chlorophyll absorbs solar energy
Light Dependent Rxns Thylakoid Membrane
Light Independent Rxns Stroma
19
Sun Energy Source
Chlorophyll absorbs solar energy
1) Some energy used to make ATP
ATP
ADP P
Light Dependent Rxns Thylakoid Membrane
Light Independent Rxns Stroma
20
Sun Energy Source
Chlorophyll absorbs solar energy
ATP
H2O reactant
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
21
Sun Energy Source
Chlorophyll absorbs solar energy
2) Some energy used to split water hydrolysis
O
H
H2O reactant
H
H
O
H
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
22
Sun Energy Source
Chlorophyll absorbs solar energy
H picked up by H carrier NADP
NADPH
O
H
H2O reactant
H
NADP
H
O
H
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
23
Sun Energy Source
Chlorophyll absorbs solar energy
H picked up by H carrier NADP
NADPH
O
H
H2O reactant
H
H
O
H
O2 waste
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
24
Sun Energy Source
CO2 reactant
Chlorophyll absorbs solar energy
NADPH
H2O reactant
O2 waste
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
25
Sun Energy Source
CO2 reactant
Chlorophyll absorbs solar energy
ATP energy used to fix carbon
NADPH
C
C
O
H2O reactant
O
O
O
O2 waste
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
26
Sun Energy Source
CO2 reactant
Chlorophyll absorbs solar energy
C
O
O
O
H
H
NADPH
H
NADPH drops off H
NADP
H2O reactant
H
C
O
O2 waste
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
27
Glucose made with ATP energy
Sun Energy Source
Chlorophyll absorbs solar energy
NADP
H2O reactant
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
O2 waste
28
Review

• Light-Dependent
• Sunlight drives ATP formation from ADP and Pi
• Water is split (hydrolysis)
• NADP picks up electrons and hydrogen
• Light - Independent
• ATP donates energy
• NADPH donates hydrogen
• CO2 donates carbon and oxygen
• Glucose (C6H12O6) is assembled

29
Light - Dependent Reactions

• Relies on proteins in thylakoid membrane
• Pigments absorb photon energy
• Flow of electrons generates power to pump H ions
  through membrane
• Hydrolysis provides replacement electrons to
  pigments
• Flow of electrons also used to build NADPH from
  NADP and H
• Cascade of H through ATP synthase makes ATP

30
Light Dependent Reactions
http//www.stolaf.edu/people/giannini/flashanimat/
metabolism/photosynthesis.swf
Fig. 7.12a, p. 121
31
ATP Formation in Chloroplasts

• Chemiosmotic Theory
• H released by hydrolysis (photolysis) of water
• More H accumulates as electron transport
  systems operate
• H concentration and electric gradients form
  across the thylakoid membrane
• Flow of ions from thylakoid compartment into the
  stroma drives ATP formation

http//www.stolaf.edu/people/giannini/flashanimat/
metabolism/atpsyn1.swf
http//www.stolaf.edu/people/giannini/flashanimat/
metabolism/atpsyn2.swf
32
Light-Independent Reactions

• Synthesis
• ATP delivers energy
• NADPH delivers hydrogen and electrons
• CO2 provides carbon and oxygen

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Calvin-Benson Cycle

• Carbon Fixation
• Occurs in the stroma
• Carbon atom of CO2 is attached to RuBP (ribulose
  biphosphate) by the enzyme rubisco
• Unstable six-carbon intermediate forms
• Intermediate splits to form two three-carbon
  molecules of phosphoglycerate (PGA)

34
Calvin-Benson Cycle

• Building Glucose
• Each PGA accepts a phosphate group from ATP and
  electrons from NADPH
• Two PGAL are formed
• To build one six-carbon sugar phosphate, twelve
  PGAL must form
• 10 PGAL rearrange to regenerate RuBP
• 2 PGAL combine to form phosphorylated glucose

35
Glucose made with ATP energy
Sun Energy Source
Chlorophyll absorbs solar energy
NADP
H2O reactant
Light Independent Rxns Stroma
Light Dependent Rxns Thylakoid Membrane
O2 waste
36
Pathways of ATP Formation

• Noncyclic Pathway (just studied)
• Electrons flow from water, through Type I and
  Type II photosystems, then to NADP
• ATP and NADH form
• Oxygen (O2) is a by-product

37
Pathways of ATP Formation

• Cyclic Pathway
• ATP forms
• Electrons cycle from and back to a Type I
  photosystem

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Photosynthetic Styles

• There are three styles of photosynthesis
• C3
• C4
• CAM
• They reflect the tension between a plant needing
  carbon dioxide, but losing water

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Avoiding Photorespiration
If stomata close, CO2 levels fall, O2 levels rise
and a plant starts to do photorespiration fixes
O2 and not CO2
40
C3 Photosynthesis

• Fix C once
• Performed by rice, wheat, soybeans, KY bluegrass
• Starts with 3-C molecule (3-PGA) in Light
  Independent Reactions

41
C4 Photosynthesis

• Fix C twice (once in the mesophyll, once in
  bundle sheath)
• Performed by crabgrass, corn, others
• Starts with 4-C oxaloacetate
• Twice the C fixation means that plants can extend
  their sugar production
• Guards against photorespiration - when Rubisco
  accepts O2 vs. CO2 and attaches O2 to RuBP vs. CO2

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Fig. 7.16b, p. 124
43
PEP and its enzyme have a high affinity for CO2
and work well when rubisco does not-when it is
hot and dry and the stomata are partially closed
http//www.bios.niu.edu/sims/metabolism/c038f3b.gi
f
44
CAM Photosynthesis

• Fix CO2 at night!
• Performed by plants in deserts or dry
  environments
• C is stored in intermediate molecules and then
  Calvin/Benson cycle occurs during the day.
• This guards against water loss!

http//hcs.osu.edu/hcs200/images/leaves/SUCCUl.JPG
45
Bottom Line

• All plants get CO2 through stomata. Stomata close
  in hot, dry times.
• Plants want to conserve H2O, but there is a
  tension. Plants need CO2 to do photosynthesis.

http//gcte-focus1.org/activities/activity_13/task
_133/basin/tutorial/gifs_2/stomata.jpg
46
upper leaf surface
vein
mesophyll cell
lower leaf surface
bundle-sheath cell
CO2 moves through stoma, into air spaces in leaf
Fig. 7.16b, p. 124
47
CO2 or O2
CO2 H2O
Rubisco affixes O2 to RuBP.
one glycolate only one PGA (not two) decreased
CO2 uptake, fewer sugars can form
CALVIN- BENSON CYCLE
CO2
carbon fixation in mesophyll cells
oxaloacetate
C3 PLANTS. With low CO2 / high O2,
photorespiration predominates.
that carbon fixed again in bundle-sheath
cells CO2 level in leaf enhanced no
photorespiration
CALVIN- BENSON CYCLE
CO2
stomata open at night CO2 uptake but no water
loss
C4 PLANTS. With low CO2 / high O2, Calvin-Benson
cycle predominates.
CALVIN- BENSON CYCLE
stomata close during day CO2 in leaf used
Fig. 7.16a, p. 124
CAM PLANTS. With low CO2 / high O2, Calvin-Benson
cycle predominates.
48
Englemanns Observational Test

• Oxygen-requiring bacteria congregated where
  oxygen was being produced by algae