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Photosynthesis (Outline)
1. Overview of photosynthesis 2. Producers, consumers, and decomposers of the ecosystem (source
of carbon and energy) 3. Plant structures: organ, tissue, cells, sub-cellular organelle, and
molecules. 4. Visible Light and its wavelengths 5. Overview of the two pathways of photosynthesis Light reaction Substrates, products, cellular components and their location Calvin cycle Substrates, products, cellular components and their location 6. Specifics of the Light reaction and the Calvin Cycle - Light Reaction: The light receptors (Pigments), Photosystems,
Location within chloroplast, Photophosphorylation, Flow of energy 7. Comparison of chemiosmosis in respiration and photosynthesis - The Calvin cycle: Energy molecules, Carbon source, RuBp, G3P 8. Photorespiration Role of Rubisco Challenge to plants on hot dry days Adaptations of C4 and CAM plants
Photosynthesis
Life on the surface of Earth is powered by solar energy that is converted into chemical energy in organic molecules, for use in cellular respiration
Light
energy 6 CO2 6 + H2O
Carbon dioxide Water
C6H12O6 6 + O2
Glucose Oxygen gas
Producers of the ecosystem Autotrophs Produce their own food and sustain themselves without eating other organisms Bacteria Algae Plants
Based on source of energy: - Photoautotrophs solar energy - Chemoautotrophs chemical energy of inorganic
substances Bacteria can uniquely oxidize sulfur and ammonia
Heterotrophs
– consumers and decomposers of the biosphere.
– completely dependent on autotrophs
Leaf Cross Section
Leaf
Mesophyll Cell
Mesophyll
Vein Stoma CO2 O2
Chloroplast
Chloroplast
Grana Stroma
TEM
9,7
50 ×
Stroma
Granum Thylakoid Thylakoid space
Outer membrane Inner membrane Intermembrane space
LM 2
,600
×
In plants, photosynthesis takes place in green leaves Mesophyll Stoma
Necessary components for Photosynthesis 1.Light 2.Chloroplast Thylakoids (stacked as grana) The light Reaction Stroma The Calvin Cycle Chloroplast
Grana Stroma
TEM
9,7
50 ×
Stroma
Granum Thylakoid Thylakoid space
Outer membrane
Inner membrane
Intermembrane space
Light as a source of energy
- Solar radiation received by Earth. - Visible light consists of photons of different
wavelengths making up the colors of the rainbow.
Increasing energy
10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m
Gamma rays
X-rays UV Infrared Micro- waves
Radio waves
Visible light
400 500 600 700 750
650 nm
Wavelength (nm) 380
Light
CO2 H2O Chloroplast
LIGHT REACTIONS
(in thylakoids) CALVIN CYCLE
(in stroma)
NADP+
ADP +
Pi
ATP
NADPH
O2 Sugar
An Overview of Photosynthesis
The Light reactions - Converts light energy to chemical energy (ATP & NADPH) - Produces O2 from breakdown of water
The Calvin cycle - uses ATP & NADPH from light reaction to assemble sugar
molecules from CO2
Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules during the light reaction
Specific details of the Light reactions: The Light Receptors
• Photosynthetic Pigments • Different pigments absorb different
wavelengths • Wavelengths that are not absorbed are
reflected or transmitted • Pigments are present within photosystems
Chloroplast
Light
Reflected light
Absorbed light
Granum
Light-capturing Pigments: Chlorophyll
In chlorophyll an electron from magnesium in the
porphyrin ring that is excited.
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center
Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown
I. Chlorophyll a participates directly in the light
reactions II. Accessory photosynthetic pigments.
Photosynthetic Pigments
1. Chlorophyll b different absorption spectrum funnels the energy from these wavelengths to
chlorophyll a. 2. Carotenoids funnel the energy from other wavelengths to
chlorophyll a participate in photoprotection against excessive light. Chemistry of autumn leaf color http://chemistry.about.com/library/weekly/aa082602a.htm
II. Accessory Pigments
H2O
LIGHT REACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADP P + i CALVIN
CYCLE
[CH2O] (sugar)
Leaf Cross Section
Leaf
Mesophyll
Vein Stoma CO2 O2 Chloroplast
Chloroplast Components • Light reactions: - Structures: Thylakoid membrane
- Photosystems II & I - Photosynthetic pigments - Electron acceptor
- Electron transport chain - ATP Synthase - Electron transport chain - NADP + Reductase
- Energy and chemical factors: light, Water, ADP +Pi, NADP+
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#
Light-capturing Pigments: Chlorophyll
In chlorophyll an electron from magnesium in the
porphyrin ring that is excited.
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center
Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown
Thylakoid
Photon
Light-harvesting complexes
Photosystem
Reaction center
STROMA
Primary electron acceptor
e–
Transfer of energy
Special chlorophyll a molecules
Pigment molecules
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
Thyl
akoi
d m
embr
ane
Photosystem consists of a
reaction center surrounded by light-harvesting
complexes
Two types of photosystems that work together to use light energy to generate ATP and NADPH.
• Photosystem II • Photosystem I
Splitting of water
e–
ATP
Mill makes
ATP
Photosystem II Photosystem I
NADPH e–
e– e–
e–
e–
e–
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#
STROMA (Low H+ concentration)
Light
Photosystem II Cytochrome
complex
2 H+
Light
Photosystem I NADP+
reductase Fd
Pc Pq
H2O O2 +2 H+
1/2 2 H+
NADP+ + 2H+
+ H+ NADPH
To Calvin cycle
THYLAKOID SPACE (High H+ concentration)
STROMA (Low H+ concentration)
Thylakoid membrane
ATP synthase
ATP
ADP + P H+
i
[CH2O] (sugar) O2
NADPH
ATP
ADP
NADP+
CO2 H2O
LIGHT REACTIONS
CALVIN CYCLE
Light
Light P680
e–
Photosystem II (PS II)
Primary acceptor
Ener
gy o
f ele
ctro
ns
[CH2O] (sugar)
NADPH
ATP
ADP CALVIN CYCLE
LIGHT REACTIONS
NADP+
Light
H2O CO2
O2
e–
e–
+
2 H+
H2O
O2 1/2
Pq
Cytochrome complex
Pc
ATP
P700
e–
Primary acceptor
Photosystem I (PS I)
e–
e–
NADP+ reductase
Fd
NADP+
NADPH + H+
+ 2 H+
Light
Electron Flow during the light reaction
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
• Chloroplasts and mitochondria generate ATP by chemiosmosis, using different sources of energy
• Mitochondria transfer chemical energy from
food to ATP; chloroplasts transform light energy into the chemical energy of ATP
MITOCHONDRION STRUCTURE
Intermembrane space
Membrane
Electron transport
chain
Mitochondrion Chloroplast
CHLOROPLAST STRUCTURE
Thylakoid space
Stroma
ATP
Matrix
ATP synthase
Key
H+ Diffusion
ADP + P
H+
i
Higher [H+]
Lower [H+]
O2
The Calvin Cycle
• Uses ATP and NADPH • CO2 enters the cycle and leaves as a 3C
sugar, glyceraldehyde-3-phosphate (G3P).
• Regenerates RuBP
The Calvin Cycle
Light
CO2 H2O
Light reactions Calvin cycle
NADP+
RuBP
G3P ATP
Photosystem II Electron transport
chain Photosystem I
O2
Chloroplast
NADPH
ADP
+ P i
3-Phosphoglycerate
Starch (storage)
Amino acids Fatty acids
Sucrose (export)
Leaf Cross Section
Leaf
Mesophyll
Vein Stoma
CO2 O2
Chloroplast
• Terrestrial plants face dehydration. • The stomata close to limit loss of water. • This limits amount of CO2 while O2 production continues • A wasteful process called photorespiration • On a hot dry day photorespiration drains 50% of the carbon
that could be fixed by the Calvin Cycle.
Challenge to plants in hot and dry area
Photorespiration
Rubisco can use both O2 and CO2 as substrates and can catalyze a wasteful reaction known as photorespiration when O2 is the substrate.
O2 and organic fuel are consumed without
producing sugar Photorespiration is an evolutionary relic
• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
• In C3 plants, a drop in CO2 and rise in O2 when stomata close on hot dry days divert the Calvin cycle to photorespiration
• C3 leaf anatomy
Leaf Cross Section
Leaf
Mesophyll
Vein Stoma
CO2 O2
Chloroplast
Some plants have special adaptations to save water and limit “Photorespiration” by separating CO2 fixation from sugar formation
1. C4 plants (spatial separation) corn, sugar cane 2. CAM plants (temporal separation) succulents (cacti and pineapple)
Photosynthetic cells of C4 plant leaf
Mesophyll cell
Bundle- sheath cell
Vein (vascular tissue)
C4 leaf anatomy
Stoma Bundle- sheath cell
Pyruvate (3 C) CO2
Sugar
Vascular tissue
CALVIN CYCLE
PEP (3 C)
ATP ADP
Malate (4 C)
Oxaloacetate (4 C)
CO2 PEP carboxylase Mesophyll cell
C4 leaf anatomy and the C4 pathway
• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 for use in the Calvin cycle
C4 Plants
Sugarcane
C4 plant
CALVIN CYCLE
3-C sugar
CO2
4-C compound
CO2 Mesophyll cell
Bundle-sheath cell
• CAM plants open their stomata at night, incorporating CO2 into organic acids.
• Stomata close during the day, and CO2 is released from organic acids are used in the Calvin cycle
CAM Plants
CAM plant
Day
CALVIN CYCLE
3-C sugar
CO2
4-C compound
Night
Pineapple
CO2
Comparing C4 and CAM photosynthesis
Bundle- sheath cell
Mesophyll cell Organic acid
C4 CO2
CO2
CALVIN CYCLE
Sugarcane Pineapple
Organic acids release CO2 to Calvin cycle
CO2 incorporated into four-carbon organic acids (carbon fixation)
Organic acid
CAM CO2
CO2
CALVIN CYCLE
Sugar
Spatial separation of steps Temporal separation of steps
Sugar
Day
Night
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