Post on 17-Jan-2018
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Photosynthesis
Section 3.1
Overview of Photosynthesis
General Equation:
Light Energy
6 CO2 + 6 H2O C6H12O6 + 6 O2
The Leaf
• The leaf is the photosynthetic organ of green plants.
• mesophyll cells– The cells that perform
photosynthesis– they contain the
largest number of chloroplasts
Vein
Leaf cross section
Mesophyll
CO2 O2Stomata
Chloroplast
Mesophyll
5 µm
Outermembrane
IntermembranespaceInner
membrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
ThylakoidSpace (thylakoid lumen)
Thylakoid
GranumStroma
1 µm
Stomata (pores in leaves)
• Three functions:– Allow CO2 in
– Allow O2 out– When water goes
out, the leaves are cooled so they do not overheat (called evaporative cooling) CO2 in
O2 out
H2O out
H2O in
• Plants regulate size of stomata opening in response to environmental conditions in order to:– Maximize CO2 uptake– Minimize water loss
• In general, stomata reduce in size:– Sunny, warm, dry and windy
Guard cells• The guard cells
control the stomata by opening and closing.
• This is controlled by osmosis:– When water is scarce,
the guard cells lose H2O and deflate, closing the stomata.
– When water is abundant, the guard cells swell, opening the stomata.
• Also, in general, stomata are open in the daytime and closed at night
Two Types of Reactions in Photosynthesis
• Light Dependent Rxns (Light Reactions) – require light, which includes exciting electrons and
splitting H2O.
• Light Independent Rxns (Dark Reactions) – do not directly require sunlight, includes Carbon
fixation, the Calvin Cycle.
Light Reactions
• Occur in the grana• Split water, release
oxygen, produce ATP, and form NADPH
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
Sucrose
NADPH
NADP
ADP+ P
O2
ATP
starch
NADH NADPH
Dark Reactions
• The Calvin cycle– Occurs in the
stroma– Forms sugar from
carbon dioxide, using ATP for energy and NADPH for reducing power
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
Sucrose
NADPH
NADP
ADP+ P
O2
ATP
starch
Electromagnetic Radiation
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energy
Pigments• Pigments
– Are substances that absorb visible light
– The reflected light and transmitted include the colors we see
– Absorbed light is important in driving photosynthesis
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
Pigments
• Chlorophyll a– Is the main
photosynthetic pigment
• Chlorophyll b– Is an accessory
pigment
C
CH
CH2
CC
CC
C
CNNC
H3C
C
CC
C C
C
C
C
N
CC
C
C NMgH
H3C
H
C CH2 CH3
H
CH3C
HHCH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll ain chlorophyll b
Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
Spectrophometer
Figure 10.8
Whitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength
Greenlight
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
The low transmittance(high absorption) readingchlorophyll absorbs most blue light.
Bluelight
1
2 3
40 100
0 100
Absorption Spectra
– Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
• They both have a double peak: in the red/orange and in the blue/violet areas.
• Between the two they absorb much of the V, B, O and R spectrums, leaving green and yellow (typical leaf colours).
Homework.
• Read Ch 3.1• Do p145 #4, 5, 6.