Photosynthesis
Photosynthesis: Converting light energy into chemical energy
6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O
Summary Formula:
6CO2 + 6H2O C6H12O6 + 6O2
Photosythesis provides Green Energy
One mature maple tree makes about 2 tons of sugar per season
Photoautotrophs capture sunlight and convert it to chemical energy
Photosynthesis is carried out by
• Cyanobacteria • 7 groups of algae • All land plants –
chloroplasts
In plants, photosynthesis occurs in chloroplasts
Mesophyll cells contain chloroplasts.
Outer membrane
Cutaway view of a chloroplast
Thylakoid membrane
Thylakoid lumen
Stromal lamellae
Granum
Stroma (space around thylakoids) • light-independent reactions
Thylakoids • Sites of light-
dependent reaction
Inner membrane
H2O
LIGHT REACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADP P + i CALVIN
CYCLE
[CH2O] (sugar)
Overview Photosynthesis
The light reaction occurs in the Thylakoids.
The light-independent Calvin cycle occurs in the Stroma.
Where does the oxygen come from?
Water is split during the light reaction
What is Light?
! Light comes as electromagnetic waves. Visible light ranges from wavelengths of about 400 nm (violet) to 700 nm (red)
! The shorter the wavelength, the greater the energy
! Photons are discreet units of energy carried in light
Light is a form of electromagnetic radiation
Engelmann Experiment 1880: What part of light is necessary for Photosynthesis?
Strand of Spirogyra
Light A glass prism breaks up a beam of light into a spectrum of colors, which are cast across a microscope slide.
Bacteria
Chlorophyll Absorption Spectrum
! Chlorophyll absorbs blue and red wavelengths most strongly
! Chlorophyll a absorbs best at 425 & 680 nm
! Chlorophyll b absorbs at 460 & 645 nm
! Carotenoids absorb best at 450 & 490 nm
Action Spectrum
! Chlorophylls and carotenoids work together to absorb photons during photosynthesis
Leaves are green because we see the reflected -not the absorbed- light
Structure of Chlorophyll
! Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria
Light-absorbing head
in chlorophyll a in chlorophyll b
Hydrophobic side chain
The Light Reaction
Light
H2O CO2
O2
LIGHT REACTIONS
CALVIN CYCLE
ATP
NADPH
ADP NADP+!
[CH2O] (sugar)
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
Photosystems
! Photosystems are large complexes of light-absorbing pigments and proteins embedded in the thylakoid membranes to absorb light efficiently
Noncyclic Electron Flow
Ener
gy le
vel o
f ele
ctro
ns
Light energy
Photosystem II Primary acceptor
Plasto-quinone
pool
ATP synthase
Light energy
Plastocyanin
Photosystem I
Ferredoxin Primary acceptor
NADP+ reductase
To light-independent
reactions (Calvin cycle)
Cytochrome complex
ATP
Photosystem II
e–
e–
e–
e–
Mill makes
ATP
e–
e–
e–
Phot
on
Photosystem I
Phot
on
NADPH
Cyclic Electron Flow
ATP synthase To light-independent reactions
(Calvin cycle)
Plastocyanin
Ferredoxin
Cytochrome complex
Plasto-quinone
pool
Photosystem I Primary acceptor
Ferredoxin NADP+ reductase
Electron Transport Chain, Chemiosmosis and ATP synthesis
Light energy
Stroma (low proton concentration) Electron transfer
Antenna complex
Photosystem II
Thylakoid lumen (high proton concentration)
Thylakoid membrane
ATP synthase
To light-independent
reactions (Calvin cycle)
NADP+ reductase
Ferredoxin
Stator
Plastocyanin Water-splitting complex
Plastoquinone
Primary acceptor
Pigment molecules
Cytochrome complex
Light energy
Photosystem I
The Calvin Cycle (light-independent)
Light
H2O CO2
O2
LIGHT REACTIONS
CALVIN CYCLE
ATP
NADPH
ADP NADP+!
[CH2O] (sugar)
Calvin Cycle Summary
! Carbon fixation – CO2 added to RuBP by Rubisco to produce two 3PGA molecules
! Reduction – NADPH and ATP used to convert 3PGA into G3P, a higher energy molecule used to build sugars
! Regeneration – remaining G3P molecules are used to recreate the starting material RuBP
Rubisco
Photosynthesis and Cellular Respiration both occur in Plant Cells
In hot and dry climates, plants convert to photorespiration
C4 plants separate carbon fixation and Calvin Cycle into different cells
Sugarcane
C4 plant
CALVIN CYCLE
3-C sugar
CO2
4-C compound
CO2 Mesophyll cell
Bundle-sheath cell
C4 plants
• Include Corn, sugarcane, sorghum, and a number of other grasses
• They initially fix carbon using PEP carboxylase in mesophyll cells
• Oxaloacetate is produced (C4 compound), converted to malate, then transported to bundle-sheath cells
• Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2
• Now Rubisco uses the released CO2, binds it to RuBP and the Calvin cycle can progress
CAM plants separate Carbon fixation and Calvin Cycle by time
Comparison C4 versus CAM plants
Sugarcane Pineapple
C4 CO2 CO2 CAM
Organic acid
Organic acid
Night
Day
CO2 CO2
Calvin Cycle
Calvin Cycle
Sugar Sugar
Bundle- sheath cell
(a) Spatial separation of steps
(b) Temporal separation of steps
Mesophyll cell
2
1 1
2
CAM plants
• Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
• Stomata open during the night and close during the day – Reverse of that in most plants
• Fix CO2 using PEP carboxylase during the night and store in vacuole
Adaptations for Photosynthesis in hot, dry climates
• C3 – Plants that fix carbon using only C3
photosynthesis (the Calvin cycle)
• C4 and CAM – Add CO2 to PEP to form 4 carbon molecule – Use PEP carboxylase – Greater affinity for CO2, no oxidase activity – C4 –two pathways occur in different cells – CAM – C4 pathway at night and the C3 pathway
during the day
Greenhouse Effect and Global Warming
Sunlight
ATMOSPHERE
Some heat energy escapes into space
Radiant heat trapped by CO2 and other gases
The Excess of CO2 in the atmosphere Is contributing to global warming
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