Chapter 10: Photosynthesis
Photosynthesis: The conversion of light energy to chemical energy that is stored in glucose or
other organic compounds; occurs in plants, algae, and certain prokaryotes
I. Photosynthesis in Nature
A. Autotrophy: Organism that obtains organic food molecules without eating other
organisms or substances derived from other organisms. Use energy from the sun or
from the oxidation of inorganic substances to make organic molecules from inorganic
ones.
1. Producers
2. Photoautotrophs: Light as source to synthesize organic substances
3. Chemoautotrophs: Produce organic compounds without the help of light
B. Heterotrophs: Obtain organic material by eating other organisms or their by-products
(other feeding)
1. Consumers
2. Decomposers consume remains of dead organisms
II. Chloroplasts; the sites of photosynthesis
A. Chlorophyll: Green pigment located within chloroplasts
1. Absorbs light energy that drives synthesis of food molecules
B. Mesophyll: Tissue in the interior of the leaf
1. Area where chloroplasts are found
2. Each mesophyll cell has about 30 – 40 chloroplasts
C. Stomata: Microscopic pores on underside of leaf
1. CO2 enters, O2 exits
D. Veins
1. Deliver water absorbed by roots to leaves
2. Exports sugar from leaves to roots and other non-photosynthetic parts
E. Chloroplasts
1. Watermelon-shaped organelle (2-4 um by 4-7 um)
2. An envelope of 2 membranes
3. Stroma: Dense fluid within
4. Elaborate system of interconnected thylakoid membranes
1.Chlorophyll resides
5. Thylakoid space or lumen
6. Grana: Thylakoid sacs stacked in columns
III. Pathways of Photosynthesis
A. Tracking atoms
6CO2 + 12H2O + Light energy  C6H12O6 + 6O2 + 6H2O
1. Water on both sides
a. 12 molecules are consumed, 6 newly formed
2. 6CO2 + 6H2O+ light energy  C6H12O6 + 6O2
3. Divide by 6 for simplest form
a.CO2 + H2O  CH2O + O2
b.CH2O is not an actual sugar: Represents carbohydrate
B. The splitting of water
1. O2 given off by plants is derived from water not CO2
a. Chloroplast splits H2O into H2 and O2
2. Prevailing hypothesis: Photosynthesis split CO2 and added water to carbon
a. Step 1: CO2  C + O2 (O2 from CO2)
b. Step 2: C + H2O  CH2O
c. Challenged in 1930s by CB van Niel
3. Studied bacteria that make carbohydrate from CO2 but do not release O2
a. Bacteria that use H2S instead of water
b. CO2 + 2 H2S  CH2O + H2O + 2S
c. Bacteria split H2S: Used it to make sugar
d. Generalized that all photosynthetic organisms require a H source
1.General: CO2 + 2H2X  CH2O + H2O + 2x
2. Sulfur bacteria:
CO2 + 2H2S  CH2O + H2O + 2S
3. Plants: CO2 + 2 H2O  CH2O + H2O + O2
4. Confirmed 20 years later using O-18 as tracer
a. Water labeled with O-18: O2 showed tracer
CO2 + 2H2O  CH2O + H2O + O2
b. CO2 labeled with O-18: Tracer did not show on O2
CO2 + 2H2O  CH2O + H2O + O2
C. Photosynthesis as a redox Process
1. Photosynthesis reverses direction of e- flow
a. Water is split
b. e- and H+ are transferred from H2O to CO2 reducing it to sugar
c. e- increase PE as they move from water to sugar
1. Energy provided by light
IV Light Reactions and the Calvin cycle convert light energy to the chemical energy of food
A. Light Reactions: Steps that convert solar energy to chemical energy
1. Light absorbed by chlorophyll drives a transfer of e-s and H from water to
NADP+
a. NADP+: (Nicotinamide adenine dinucleotide phosphate) Acceptor
which temporarily stores energized e-s
1. Solar power reduces NADP+ to NADPH
2. Adds a pair of e-s and a H+
b. Water is split
c. O2 is given off as a by-product
d. Photophosphorylation: Light reactions generate ATP by powering
addition of a phosphate group to ADP
2. Occurs in thylakoids
B. Calvin Cycle: Second stage of photosynthesis involving CO2 fixation and reduction of
the fixed C into carbohydrate
1. Melvin Calvin and colleagues studied steps in 1940s
2. Carbon fixation: Incorporation of carbon into organic compounds
1. CO2 from air into organic molecules present in chloroplast
2. Reduces C by addition of e—s
a. Provided by NADPH
b. Requires ATP
3. Dark reactions or light independent reactions
4. Occurs in stroma
1.NADP+ and ADP bump into thylakoid membrane
a. Pick up e- and phosphate
b. Transfer high energy cargo to Calvin cycle
C. Light Reaction
1.The nature of sunlight
a. Electromagnetic radiation
b. Waves: Dropping a pebble in the water
1.Disurbance of electric and chemical fields (not water)
2.Wavelength: Distance between crests of electromagnetic waves
1.Gamma rays (less than a nm)
2.Radiowaves (more than a km)
c. Electromagnetic spectrum: The entire range of radiation
1.Visible light: Narrow band of electromagnetic spectrum (380750 nm) detected as various colors by the human eye.
d. Light also behaves as particles
1.Photons: Discrete particles of light
a. Fixed quantity of energy
b. Energy is inversely related to wavelength
1.The shorter the wavelength the greater the energy
of the photon
2.Photon of violet light packs almost twice the
energy as a photon of red light
2. Sun radiates full spectrum of electromagnetic energy
a. Atmosphere filters out much
b. Visible light drives photosynthesis
D. Pigments: Substances that absorb visible light
1. Different pigments absorb different wavelengths
2. Absorbed disappear
3. Color we see is the one that is reflected
a. Leaf: Red and blue wavelengths are absorbed
1. Most effective in photosynthesis
2. Green wavelengths are transmitted
a. Least effective
4.Spectrophotometer: Measures ability of a pigment to absorb various
wavelengths
a. Directs beams of light of different wavelengths through a solution of
pigment
1.Measures light transmitted at each wavelength
b. Absorption spectrum: Graph plotting pigments; (light absorption vs
wavelength)
c. Chlorophyll a: Blue and red light
3.Action spectrum: Profiles relative performance of different wavelengths
a. Illuminate chloroplasts with different color lights
b. Wavelength vs CO2 consumption or O2 release
d. Action spectrum is broader than absorption spectrum
1.Only chlorophyll a participates directly in light reactions
2.Other pigments absorb light
a. Transfers energy to chlorophyll a
b. Chlorophyll b: Structural difference
1. Different absorption spectra
2. Different colors
a. a: blue-green
b. b: yellow-green
c. Carotenoids: Hydrocarbons (yellow and orange)
1. Photoprotection: Absorb and dissipate
excessive light
E. Excitation of chlorophyll by light
1. Absorbed colors
2. Molecules absorb a photon
a. Elevated to an orbital of higher PE
1. Ground state: Normal orbital
2. Excited state: e- boosted to higher energy orbital
3. Specific compounds absorb only photons corresponding to specific
wavelengths
4. Some pigments emit light and heat after absorbing photons
a. As e- falls to ground state a photon is given off
b.Fluorescence: After glow
F. Photosystems: Light harvesting unit consisting of proteins and smaller organic
molecules
1. Light-gathering antenna complex
a. Cluster of a few hundred chlorophyll a, b and carotenoid
b. Photon absorbed: Energy transmitted from one pigment molecule to
another
1. Reaction center: Location of a particular chlorophyll a where
the first light driven chemical reaction occurs
2. Primary e- acceptor: Specialized molecule that shares reaction center with
chlorophyll a
a. Loses 1 e- to primary e- acceptor
b. Light excites e- to higher energy level
c. e- acceptor traps e- before it returns to ground state
d. Isolated chlorophyll fluoresces
e. Acceptor molecule functions like dam
3. Photosystem I and Photosystem II: 2 types of photosystems that cooperate in
light reactions within the thylakoid membrane
a. Photosystem I: Reaction-center: P700
b. Photosystem II: Reaction-center: P680
c. Identical chlorophyll a molecules
1. Associated with different proteins in thylakoid membrane
2. Affects e- distribution
a. Slight differences in light absorbing properties
G. Noncyclic e- flow: Predominant route
1. Photosystem II absorbs light
a. e- excited to higher energy level in reaction center (P680)
b. Captured by primary acceptor
c. Chlorophyll is oxidized
2. Enzyme extracts e- from H2O
a. Applies them to P680
b. Splits water into 2 H+ and O
c. O combines to form O2
3. e- passes from II to I via e- transport chain
a. Plastoquinone (Pq): 2 cytochromes and plastocyanin (PC): Copper
containing protein
4. e- cascade down chain to lower energy level
a. Energy harnessed to produce ATP
b. Photophosphorylation
5. P700: e- acceptor
a. Hole created when light energy drives e- to primary acceptor of
photosystem I
6. Primary e- acceptor of photosystem I passes e- to 2nd e- transport chain
a. Ferredoxin (Fd): Iron containing protein
b. NADP+ reductase
c. NADP+
d. NADPH: Reducing power for synthesis of sugar
H. Cyclic electron flow: Photoexcited e- s use photosystem I but not photosystem II
1. e- cycle back from Fd to cytochrome complex to P700 chlorophyll
a. No Production of NADPH
b. No release of O2
c. ATP
1. Cyclic photophosphorylation: Generates ATP by cyclic e- flow
2. Non-cyclic e- flow produces ATP and NADPH in equal quantities
a. Calvin cycle consumes more ATP than NADPH
b. NADPH regulate which pathway e-s take
c. Low on ATP for Calvin cycle
1. NADPH accumulates
2. Shift to cyclic flow until ATP catches up
I. Comparison of chemiosmosis in chloroplasts and mitochondria
1. Chloroplasts do not need food to make ATP
a. Light drives e- s to top of transport chain
2. Thylakoid space is H+ reservoir
a. H+ diffuses to stroma through ATP synthase
1.Catalytic knob on stroma side
V. Calvin Cycle Uses ATP and NADPH to convert CO2 to sugar
A. Glyceraldehyde-3-phosphate (G3P): 3-C sugar produced by Calvin cycle
B. Phase 1: Carbon fixation
1.Attaches CO2 to a 5 C sugar
a. RuBP Ribulose bisphosphate
b. Rubisco (RuBP) carboxylase: Enzyme that catalyzes first step
c. Product: 6 C intermediate
1. Unstable; immediately splits into
2. 2 molecules of 3-phosphoglycerate
B. Phase 2: 3-phosphoglcerate receives phosphate group from ATP
1. Becomes 1,3-biophosphoglycerate
2. NADPH donates a pair of e—
a. Reduces 1,3-biophosphoglycerate to G3P
1. For every 3 molecules of CO2  6 G3Ps
a. One counted as net gain of
carbohydrate
3. Cycle began with 3 molecules of RuBP
4. Now; 6 molecules of RuBP
a. One exits to by used by plant cell
b. 5 recycled to regenerate 3 RuBP
C. Phase 3: Regeneration of RuBP (CO2 acceptor)
1. C skeletons of 5 molecules of G3P are rearranged
a. 3 molecules of RuBP
b. Spends 3 ATPs
c. RuBP can now receive CO2 again
D. Summary
1.1 G3P molecule produced
2. 9 ATP and 6 NADPH are consumed
VI. Alternative mechanisms of C-fixation
A. C3 Plants: Plants whose first organic product is the 3-C compound; 3phosphoglycerate
1. Most plants
a. Rice, wheat, soybeans
2. Produce less food when stomata close on hot, dry days
3. Declining level of CO2 starves Calvin cycle
4. Rubisco can accept O2 in place of CO2
a. Product splits
1. One piece: 2 C compound exported
2. Mitochondria and peroxisomes break 2-C molecule down to
CO2
b. Photorespiration: Process that occurs in light and consumes O2
1. Produces no ATP
2. Produces no food
3. Decreases photosynthetic output
c. Hypothesis: Photorespiration is evolutionary baggage
1. Atmosphere had less O2 and more CO2
2. Rubisco first evolved
3.Inability of enzymes’ active site to exclude O2 4. Now, so
concentrated, some photorespiration inevitable
5. If reduced crop yields would increase
B. C4 Plants: Alternate mode of C-fixation that forms a 4-C compound as its first product
1. Sugar cane and corn: Members of grass family
2. 2 distinct types of photosynthetic cells
a. Bundle-sheath cells: Arranged into tightly packed sheaths around veins
b. Mesophyll cells: More loosely arranged cells, found between bundle
sheath and leaf surface
3. Calvin cycle: Chloroplasts of bundle sheath
a. Preceded by incorporation of CO2 into organic compounds in mesophyll
b. Addition of CO2 to PEP (phosphoenolpyruvate)
1. Forms 4-C oxaloacetate
2. PEP carboxylase: Adds CO2 to PEP
a. Can fix CO2 when rubisco cannot
b. Exports 4-C product to bundle sheath cells through
plasmodesmata
a. 4-C compounds release CO2
b. Reassimilated into organic material by rubisco
and Calvin cycle
b. Mesophyll pumps CO2 into bundle sheath
1. Keeps CO2 concentration high for
2. Rubisco accepts CO2 rather than O2
C. Cam Plants
1. Succulent (water-storing) plants
a. Ice plants, cacti, pineapples,
b. Open stomata and night, close them during day
2. Night: Take up CO2 and incorporate it into organic acids
a. Crassulacean acid metabolism (CAM): Mode of C fixation in which
CO2 is taken up at night and incorporated into a variety of organic acids
b. Mesophyll cells store organic acids in vacuoles until morning
1. Day: Light reactions supply ATP and NADPH for Calvin cycle
2. CO2 is released from organic acids
3. In both C4 and CAM plants CO2 is first incorporated into organic intermediates
before entering Calvin cycle
a. C4 plants, C-fixation steps are separated structurally from Calvin cycle
b. CAM plants: Steps occur at different times
3. C3, C4 and CAM plants all use Calvin cycle
VII. Summary
A. 50% of organic material made by photosynthesis is consumed as fuel for cellular
respiration in mitochondria
B. Sometimes loss of products to photorespiration
C. Carbohydrate is transported out of leaves in form of sucrose to nonphotosynthetic
cells
1. Raw materials for cellular respiration
2. Anabolic pathways that synthesize proteins, lipids, other products
3. Lots of glucose is linked together to form cellulose
a. Cell walls
b. Most abundant molecule on surface of planet
c. Each day plants make more organic material than they need
1. Stockpile
a. Synthesize starch
b. Store some in chloroplasts, storage cells of roots,
tubers, seeds, and fruits
2. Photosynthesis makes 160 billion metric tons of
carbohydrate/year
3. No process is more important to welfare of life on earth