Download Chapter 10- Photosynthesis

Chapter 10- Photosynthesis
Organisms, Carbon, and Energy
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All life forms must obtain carbon and energy.
A. Autotrophs- “self-nourishing”, “self feeders”
1. They obtain carbon from carbon dioxide.
2. Photoautotrophs- harness light energy.
3. Chemoautotrophs- extract energy from chemical reactions by oxidizing inorganic
substances, such as sulfur or ammonia
B. Heterotrophs- feed on organisms or organic wastes, “other feeders”
1. Heterotrophs acquire carbon and energy from autotrophs.
2. Heterotrophs are animals, protistans, bacteria, and fungi.
Photosynthesis
A. Simplified picture- process by which light energy is captured and converted into chemical
energy of glucose
1. Light Dependent reactions- convert light energy to chemical energy stored in ATP
and NADPH. (“photo”)
2. Light- Independent reactions- assemble organic molecules using energy from ATP
and NADPH. (“synthesis”)
3. Overall, for glucose formation the equation is:
12H2O + 6CO2 ---sunlight
6O2 + C6H12O6 +6H2O
B. Chloroplast Structure and Function:
- Light-dependent reactions occur in
the thylakoid system.
o The thylakoids are folded
into grana (stacks of disks).
o The interior spaces of the
thylakoid disks are filled
with H+ needed during ATP
synthesis.
- Carbohydrate formation occurs in
the stroma (2 membrane) area that
surrounds the grana.
- Chloroplasts are in the mesophyll
1.
Light-Dependent Reactions
- Pigments absorb light energy and give up electrons.
- Electron and hydrogen transfers form ATP and NADPH.
- Electrons are replaced in the pigment molecules.
A. Light Absorption
- Light-Trapping Pigments
o Light is a form of energy- visible light spectrum ranges from red to violet
o Chlorophyll pigments absorb blue and red but reflect green. Chlorophyll a looks
blue/green and chlorophyll b is
yellow/green.
o Carotenoid pigments absorb violet and
blue but reflect yellow, orange, and red.
Function for photoprotection (keep from
having sun damage)
o Pigments are found in the thylakoids
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Photosystems (PS)
o A photo system is a cluster of 200 to 300
light-absorbing pigments located in the
thylakoid.
o The pigments "harvest" sunlight
B. Absorbed light boosts electrons to a higher level.
- The electrons quickly return to the lower level
and release energy.
- Released energy is trapped by chlorophylls,
which act as a sink for energy harvested by all
pigments.
- The trapped energy is then used to transfer a chlorophyll electron to an acceptor
molecule.
C. How ATP and NADPH Form in Chloroplasts
- Electrons expelled from chlorophyll go through one or two electron transport systems in
the thylakoid membranes.
- As the electron passes from one molecule to another in each system, phosphate is added
to ADP to form ATP by photophosphorylation.
D. Cyclic Pathway- Photo system I (P700) Only
- In cyclic photophosphorylation, electrons are excited, pass through an electron transport
system (forming ATP), and then return to the original photosystem.
- This photo system is characterized by the presence of chlorophyll P700.
- The cyclic pathway is an ancient way to make ATP from ADP; it was used by early
bacteria.
E. Noncyclic Pathway- Photo system II (P680), then
Photo system I (P700)
- Noncyclic photophosphorylation transfers electrons
through two photo systems and two electron
transport systems (ETS) in the thylakoid membranes.
- Pathway begins when chlorophyll P680 in photo
system II absorbs energy and has an e- become
excited (enters an ETS, forming ATP).
- Water is split by light (photolysis), and an e- jumps
from the water to fill up P680.
- P700 in photo system I absorbs energy, and has an ebecome excited (enters an ETS, forming NADPH).
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The e- from P680 fills up P700.
o ATP and NADPH contain energy that can be used to form organic molecules.
F. A Closer Look at ATP Formation
- Hydrogen ions from photolysis of water accumulate inside the thylakoid compartment of
chloroplasts to set up concentration and electric gradients.
- As the hydrogen ions flow out through channels into the stroma, enzyme action links
phosphate to ADP to form ATP.
2. Light-Independent Reactions (Carbon Fixation)
A. Overview
- Aka Dark reaction- can occur in the dark or in the light
- Aka C3 pathway
- Aka Calvin Cycle
- The participants & their roles in the synthesis of carbohydrates are:
a. ATP- provides energy,
b. NADPH- provides hydrogen atoms and electrons,
c. Atmospheric air- provides carbon dioxide
- The reactions take place in the stroma of chloroplasts and are not
dependent on sunlight directly.
B. Calvin-Benson Cycle
- Light energy is now stored as chemical energy in organic compounds.
- The cyclic pathway operates as follows:
a. Carbon dioxide (CO2) becomes attached to ribulose bisphosphate (RuBP) (5 Carbon
compound) to form a six-carbon intermediate.
b. The intermediate splits at once to form two PGA molecules.
c. Each PGA then receives a phosphate from ATP plus H+ and electrons from NADPH to
form PGAL.
d. Two PGAL join to form a sugar phosphate, which will be modified to sucrose, starch,
or cellulose.
3. Final tally= 12H2O + 6CO2 + 18ATP + 12NADPH
6O2 + C6H12O6 +18 ADP + 18 Pi + 12NADP+ +6H2O + 12H+
How Autotrophs Use Intermediates and Products of Photosynthesis
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Sugar phosphates are used as cellular fuel and as building blocks in synthesis of
carbohydrates.
Sucrose is transported from leaves to all parts of the plant.
Starch is the main storage form of carbohydrate in plants.
Photosynthesis also yields intermediates and products that can be used in lipid and amino
acid synthesis.
C4 Plants
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Plants in hot, dry environments close their stomata to conserve water but in doing so
slow carbon dioxide entry and permit oxygen buildup inside the leaves.
Photosynthesis continues improperly; this nonproductive process is called
photorespiration.
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To overcome this fate, C4 plants fix carbon twice to produce oxaloacetate (a four-carbon,
hence C4) compound, which can then donate the carbon dioxide to the Calvin-Benson
cycle
The steps are in different cells
Examples: crabgrass, sugarcane, corn, and other plants
CAM plants (Crassulacean Acid Metabolism)
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Open the stomata during the night and closes during the day
at night they take in CO2, store it to use during the day
Prevents the plants from drying out during the day
The steps are in the same cells at different times
Examples: cacti, pineapples, and other plants found in the desert
Chemosynthesis
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Chemosynthetic autotrophs obtain energy from oxidation of inorganic substances.
Some soil bacteria strip protons and electrons from ammonia