Photosynthesis Notes
Autotroph—“self-feeders”; make organic molecules from inorganic raw materials; producers
Photoautographs—use light as a source of energy
Chemoautotrophs—oxidize chemistry as source of energy
Heterotroph—“other-feeders”; consume other organisms to obtain energy (decomposers and consumers)
Site of photosynthesis—chloroplast located in mesophyll (middle portion of leaf)
Photosynthesis—“light production”
6CO2 + 6H2O + light  C6H12O6 + 6 O2
--water is split in photosythesis
Reactants
6CO2
Products
C6H12O6
(reverse of cellular respiration)
12H2O
6 O2
6H2O
Two parts: light reactions and dark reactions (Calvin Cycle)
Light Reactions “photo”-part
 Light from the sun drives the transfer of electrons and H+ from H2O to NADP to make NADPH, which temporarily stores
energized electrons.
 Releases O2 as byproduct from the water being broken down. Water is oxidized, NADPH is reduced.
 Produces ATP by photophosphorylation (chemiosmosis) that is then used in the light independent reactions.
Calvin Cycle (dark reactions or light-independent reactions); “synthesis”/fixation -part
 Incorporates CO2 from air into organic molecules already present in the chloroplast. This is called carbon fixation.
and NADPH are oxidized. Carbon dioxide is reduced.
 This is powered by the reducing power of NADPH and by ATP, both created in the light reactions.
 Therefore, the Calvin cycle depends on energy produced or stored by the light reactions.
ATP
Light—Background Information
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Light travels as a wave and a particle (a photon); therefore, we can speak of a certain wavelength of light even though it has
many particle-like qualities.
Spectrophotometer—an instrument used to measure a pigments ability to absorb light.
Pigment—any substance that absorbs visible light.
If illuminated with white light, we see the colors that are reflected. Absorbed light is converted into kinetic energy that causes
the electrons to jump to higher energy levels.
Absorption spectrum—a graph of a pigments light absorption vs. wavelength
Action spectrum—a plot of wavelength against some measure of photosynthetic rate.
Chlorophyll a—a pigment that acts directly in the light reactions.
Accessory pigments—(chlorophyll b, carotenoids (organge), rhodopsin (red), xanthophylls (yellow)) pigments that absorb
light and transfer the energy to chlorophyll a (absorb wavelengths that chlorophyll a cannot). The cumulative total of this
network is called the photosystem.
All pigments are imbedded in the thylakoid membrane
Light Reactions—a closer look
 Occurs in the thylakoid
 Involves photosystems (“antennae” complex consisting of a few hundred chlorophyll a molecules and accessory pigments.
Step 1: Pigment absorbs a photon; energy is transmitted to chlorophyll a in photosystem II (P680). Yes, photosystem II is first.
Step 2: Energy causes an electron to leave chlorophyll a and jump to the primary electron accepter.
Noncyclic Electron Flow (See figure 10-12)
 P680 (PSI)absorbs lightelectron moves to higher energy level
 Enzyme “steals” electron from H2O to fill electron “hole” in chlorophyll a. This splits water; O2 is released.
 Electron from P680 passes via an electron transport chain to photosystem I (P700); the energy released as electrons fall to
lower energy is harnessed by the membrane to produce ATP (via chemiosmosis like respiration). H+ is pumped into the
thylakoid and is used to make ATP as it flows through ATP synthase out to the stroma. Production of ATP during noncyclic
flow is called noncyclic photophosphorylation. ATP produced here fuels the Calvin cycle.
 When electron reaches the bottom of the electron transport chain, it fills electron “hole” in P700 (PSII) produced when light
energy excited an electron here.
 Excited electron from P700 gets passed to electron carrier molecule NADP+ after going down a second e-transport chain.
Cyclic Electron Flow
 Uses P700 only
 Light excites electron in P700
 Electron is passes down electron transport chain back to chlorophyll a in P700 (where is came from) instead of being passed to
NADP+. Energy released on the way down is used to produce ATP—cyclic photophosphorylation
 Why? Calvin cycle needs more ATP than NADPH; cyclic electron flow makes up the difference
Chemiosmosis Comparison
 Both use proton-motive force (H+ gradient across membrane to make energy)
 Mitochondria uses chemical energy to fuel this; chloroplast uses light energy
 Spatial differences:
 Mitochondria—protons pumped from matrix to intermembrane space
 Chloroplast—protons pumped from stroma IN to thylakoid compartment
Calvin Cycle
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Produces a 3-carbon sugar (called G3P); needs three cycles to fix 3 carbons
Phase I—Carbon Fixation
 1 CO2 molecule joined to 1 5-carbon molecule (RuBP) forming 6-carbon molecule; extremely unstable and immediate
splits into two 3-carbon molecules.
Phase II—Reduction
 Each 3-carbon molecule gets a phosphate group added (1 ATP is “spent”)
 Pair of electrons for NADPH is used to reduce molecule to the higher energy G3P
Phase III—Regeneration of RuBP
 Requires ATP
PHOTORESPIRATION: sometimes when there is insufficient carbon dioxide, oxygen binds to the RUBP and it goes through
the whole cycle “burning” the NADPH and ATP energy, but produces NO glucose. This is wasteful and needs to be avoided by
the plant. Sufficient carbon dioxide levels must be maintained.
Alternative Methods For Carbon Fixation
C3 plants (rice, wheat, soybeans)
 Standard method of carbon fixation using RuBP (described above)
 Not good in hot, dry climates
 If they close stomata during the day to reduce water loss, the plant doesn’t get enough CO2
 Instead RuBP binds to excess O2 and photorespiration occurs (wasteful process)
C4 Plants (sugarcane, corn)
 PEP (a 3C molecule) is CO2 acceptor instead of RuBP and becomes a 4C molecule all within the mesophyll cells.
 From the mesophyll cells the 4C molecules will enter a special layers of cells (bundle sheath) where they are broken down to
release CO2 in order to keep CO2 levels high locally
 Therefore, C4 plants can partially close stomata during the day, decreasing both CO2 concentration and water loss, without
stopping the calvin cycle
CAM Plants (cacti, pineapples)
 Stomata only open at night
 Converts carbon dioxide and 3C molecules into 4C molecule organic acids.
 The acid is then periodically broken down the next day to release carbon dioxide and keep it at sufficient levels.