Unit 3 - Photosynthesis
The Basis of Life
Overall Process
6CO2 + 12H2O + Light Energy  C6H12O6 + 6O2 + 6H2
Structure of a Leaf
• Stomata
• Mesophyll
Chloroplast
• Site of photosynthesis
– Light Reaction – thylakoids
– Calvin Cycle – stroma (fluid)
Redox in Biology
• OIL – Oxidation Is Loss
• RIG – Reduction Is Gain
• In chemistry, “loss” or “gain” refers to e• In biology, “loss” or “gain” refers to H atom
– Ex:
• NADPH is reduced (because it gained an H)
• NADP+ is oxidized (because it lost the H)
Phosphorylation
• Accomplished by enzymes called kinases
– Attach a phosphate group (PO4) to a molecule
• Makes molecule less stable due to increase in
free energy (more ordered)
ADP Phosphorylation
Two stages:
• Light Rxn:
– Reactants – Light (photons) and H2O
– Products – O2, ATP, and NADPH
• Calvin Cycle:
– Reactants – ATP, NADPH, and CO2
– Products – CH2O (sugar!), ADP + Pi, NADP+
Light Rxn is divided into 2 parts
• Photosystem II
• Photosystem I
• Photosystem – reaction center located on
thylakoid membrane
– Contain many light-harvesting complexes (contain
chlorophyll)
Photosystem II (PSII)
1. Light, in the form of photons, hits leaf and is
absorbed by chlorophyll (usually chlorophyll a)
in PSII rxn center
2. e- with chlorophyll a gets excited and moves to
higher energy state (gains PE)
3. @ same time, enzymatic rxn splits H2O  2e-,
2H+, and ½ O2
– e-’s from splitting of water (photolysis) replace e- that
was excited in chlorophyll a
4. Electron Transport Chain (ETC) – e- is passed
from PSII to PSI via a series of molecules
– Each “pass” results in e- falling to a lower energy
state
– Exergonic process provides energy to pump H+
(protons) into the thylakoid space.
Chemiosmosis
• Light rxn creates a proton gradient due to ETC
– Results in a pH, charge, and [] gradient
• ATP Synthase – couples the diffusion of H+
ions back down their concentration gradient
to stroma with phosphorylation of ADP to
make ATP
pH gradient
Concentration gradient
Charge gradient
Photosystem I (PSI)
5. Light energy transfers from PSII  PSI which
excites e6. e- travels through PSI and enters another ETC
7. As e- travels down ETC, the exergonic process
produces energy to convert NADP+ to NADPH
(reduction!).
– NADPH is the final electron acceptor!
Thus….
• Light rxns convert light energy to chemical
energy stored in NADPH (@PSI) and ATP (via
chemiosmosis).
• Light rxns also produce O2 (@PSII) as a byproduct.
Calvin Cycle
(aka dark rxns aka light-independent rxns)
• ATP and NADPH from light reactions provide
energy to drive carbohydrate synthesis.
– CO2 (from air) is reduced and “fixed” into
carbohydrate
– NADPH and ATP provide reducing power
• Anabolic due to building up of sugar from smaller
molecules and consumption of energy
Fates/uses of glucose
• Provides plant with chemical energy and
carbon (for synthesis of all organic
macromolecules)
– Proteins, lipids, nucleic acids
– STARCH and CELLULOSE
• Heterotrophs (us) consume autotrophs
(plants)
Evolutionary background
• Photosynthesis first evolved in prokaryotes
– Evidence supports that prokaryotic photosynthesis
was responsible for production of oxygenated
atmosphere
– Big point – prokaryotic photosynthetic pathways
were foundation of eukaryotic photosynthesis