Capturing Solar
Energy:
Photosynthesis
Photosynthesis
 Light energy captured and stored
as chemical potential energy in
the covalent bonds of
carbohydrate molecules
 6 CO2 + 6 H2O + light  C6H12O6
+ 6 O2
Photosynthesis
 Less Than 1% of the Sun's Energy Is Captured in
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Photosynthesis
Sun energy drives reduction of carrier molecules
Electrons in respiration loose energy going from
sugar to oxygen
Mitochondria use released energy to make ATP
Electrons in photosynthesis must gain energy
going from water to sugar
Energy provided by the sun
 Occurs in 1 million billionths of a second
Photosynthesis
 Light
 A. Light consists of units of energy called photons
 B. Photons possess differing amounts of energy
 C. Energy in visible light
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1. Violet has short wavelength and high energy photons
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2. Red has long wavelength and low energy photons
 D. Absorbed vs reflected
 E. Specific atoms can absorb only certain photons of light
Pigments-molecules that absorb
light
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A. Molecules that absorb light
B. Types
1. Carotenoids-absorbs some green
a. Absorb photons over a broad range, not highly efficient
b. Include beta-carotene, vitamin A and retinal
2. Chlorophylls
a. Absorb photons by excitation like the photoelectric effect
1. Complex ring structure called a porphyrin ring
2. Metal ion within a network of alternating single and double bonds(Fe)
b. Absorb photons over a narrow range
1. Chlorophyll a absorbs in violet-blue range
2. Chlorophyll b absorbs in the red range
3. Wavelength not absorbed by chlorophylls reflected as green
4. Chlorophyll absorbs in a narrow range, but with great efficiency
c. xanthrophyll
Life depends on
photosynthesis
 A. Foundation of energy for most
ecosystems
 B. Source of oxygen
 C. Key component of the carbon
cycle
The mechanism of
photosynthesis
 Chloroplasts are the sites of
photosynthesis
 Have a membrane system within internal
space (stroma)
 Arranged in disk-shaped sacks (thylakoids)
 The thylakoids contain light-harvesting
photosynthetic pigments & enzymes
 Internal membranes define space (lumen) that
is separate from the rest of the stroma
The mechanism of
photosynthesis
 Photosynthesis occurs in two steps
 1. Light-dependent reactions
 a. Provides the energy necessary to
fix carbon
 b. Occurs in the thylakoid membranes
 c. Generates ATP
LIMITATIONS
1. Geared only towards energy
production(ATP)
 2. Does not provide for
biosynthesis(glucose synthesis)
CYCLIC PHOTOSYNTHESIS
 PRIMATIVE FORM
 COMES IN TO PLAY ON ITS OWN
IN FALL IN HIGHER PLANTS.
Light-Dependent
Reactions
 What happens during light reactions?
 During transport of electrons from PS II to
PS I
 Some energy is harnessed to produce ATP
 Eventually, chlorophyll from PS II is
oxidized
 Gets replacement electrons from water-
photolysis
Light-Dependent
Reactions
 Energy of light has thus been captured
in two forms:
 The synthesis of NADPH from NADP+
 Proton gradient across the thylakoid
membrane
 Cannot be used directly to make food
 Must first be converted to ATP by chloroplast
ATP synthase
The mechanism of
photosynthesis
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Energy carriers ATP and NADPH
transport energy from the lightdependent reactions to the lightindependent reactions
The mechanism of
photosynthesis
 2. Light-independent reactions
a. Uses energy of the light-dependent
reaction to make sugar from CO2
b. Occurs in the stroma
Light-Independent
Reactions
 Steps in Light-Independent Reactions:
 CO2 joins with RuBP forming an unstable 6-
C molecule
 Breaks into two 3-C PGA molecules
 This first step in Calvin-Benson/C3 cycle is
catalyzed by enzyme
 Called ribulose biphosphate carboxylase
(Rubisco)
1
3
Carbon fixation
combines CO2
with RuBP.
RuBP
regeneration
uses energy
and 10 G3P.
2
G3P synthesis
uses energy.
2 G3P available
for synthesis of
organic molecules.
PHOTORESPIRATION
Many land plants take up oxygen and release CO2 in the
light. There is a superficial resemblance to true
respiration, but the process is much faster. However, it is
normally masked by photosynthesis, which is even
faster. Photorespiration differs from true respiration.
Although plants do respire normally (with mitochondria,
etc.) this is useful (produces ATP and NADH), and occurs
mostly in the dark. In contrast, photorespiration is
wasteful and occurs mostly in the light. Photorespiration
appears to serve no useful purpose. Its main effect is to
reduce the apparent rate of photosynthesis. Most of our
important crops photorespire about half of their potential
yield away!
PHOTORESPIRATION
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O2 competes for CO2 with RuBP oxidizes it-high
oxygen low carbon dioxide
CO2 released without ATP or NADPH
C3 lose 1/4 to 1/2 of carbon fixed-40%
C4 And CAM plants adapted to counter act this
problem
Rubisco takes oxygen makes phosphoglycerate and
glycolate
Goes to perioxisome-takes oxygen and makes a
compound that goes to mitochondria to make
carbon dioxide like respiration.
(a)
mesophyll cell in C3 plant
C3 plants use the C3 pathway
Much
photorespiration
occurs under hot,
dry conditions.
In a C3 plant, most chloroplasts are in
mesophyll cells.
(b) C plants use the C pathway
4
4
mesophyll cell in C4 plant
CO2 is captured
with a highly
specific enzyme.
bundlesheath
cells
In a C4 plant, both mesophyll
and bundle-sheath cells
contain chloroplasts.
Much glucose
synthesis occurs.
Almost no
photorespiration
occurs in hot,
dry conditions.
bundle-sheath cell in C4 plant
ADAPTATIONS
 C4 plants-Hatch Slack plants
 a. Different leaf structure
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b. Bundle sheath surrounded by palisade mesophyll
c. Grasses-Found in hot climates, lots of sun,above 300 C
d. Uses about 2x ATP but stores CO2 at night or anytime
stomates are open. Saves CO2 when plant can
e. Cycle
1. CO2 is picked up by PEP in mesophyll-no rubisco
2. Converted to oxaloacetic acid then malic acid
3. Stored in this stable form
4. Malic converted to Pyruvic acid + CO2
5. Pumped into bundle sheath thru plasmodesmata
6. Deeper than surface because there is less oxygen to
cause photorespiration to occur
C4 plants/Hatch Slack
 Utilize an alternate pathway to
make sugars in dry environments
 Closing stomata to conserve water
results in photorespiration in C3
plants
CAM PLANTS
 a. Hot(desert) climates-high daytime temps,
low soil moisture,intense light
 b. Stomates only open at night
 c. Central Vacuole stores malic acid
 d. Leaves vacuole and and releases CO2
 Diatoms
 1. Have both C3 and C4 cycles
 2. C3 in chloroplasts
 3. C4 in cytosol
 4. Uses because of low CO2 in the ocean