Photosynthesis
the conversion of light energy to chemical energy
Photosynthetic organisms build carbohydrates using sunlight and CO2 from the air
It is a major entry point of energy into biological systems
Supporting Photosynthesis
Photosynthetic organisms in diverse environments can carry out photosynthesis → if sunlight is available
E.g., In the ocean, photosynthesis occurs in the surface layer about 100m deep → photic zone
Food webs are supported by the biomass of photosynthetic organisms
Photosynthesis Reactions
Overall, photosynthesis is a redox reaction
CO2 is reduced to form high energy carbohydrate molecules
This requires energy in the form of sunlight
Can be divided into 2 stages
Light Capture
Carbon Fixation
The electrons donor in these reactions is → H2O
The oxidation of water results in the production of
Electrons
Protons
O2
Thus, the oxidation of water & reduction of CO2 are linked through the photosynthetic electron transport chain
Photosynthesis: Eukaryotic Cells
In photosynthetic eukaryotes, both stages take place in the → chloroplast
Chloroplasts are semi-autonomous & self-replicating → bound by 2 membranes separated by narrow space
In the center of the chloroplast are highly folded flattened membrane sacs → thylakoid
Space inside the thylakoid membrane is a fluid filled space → lumen
Orderly stacks of thylakoids → Grana
Space surrounding thylakoids → Stroma
Carbon fixation occurs here
Overview of Photosynthesis
6 CO2 + 12 H2O = C6H12O6 + 6O2 + 6 H2O
* goes by light
Photosynthesis: 2 Series of Reaction
- Light-dependent reaction
Sunlight energy converted into chemical energy → occurs in thylakoid membranes
Products: ATP & NADPH - Light-independent reaction
ATP & NADPH used to synthesize carbohydrates → occurs in stroma
How is light absorbed?
Energy from sun is a form of electromagnetic radiation → travels in photons
When a photons become absorbed, the compound is converted to a higher-energy state → excited state
3 ways to re-establish ground state
- Dissipate energy as heat
- Re-emit energy in a longer wavelength → fluorescence
- Transfer energy to another molecule → happens with photosynthetic pigments
Chlorophyll
Major light capturing molecules
Absorb light of blue and red λ → reflects green λ
2 parts
Porphyrin Ring → Light absorption
Specific atom in center of ring → Mg
Different side groups on ring → gives different types of chlorophyll
Phytol Side Chain
Insertion of Chlorophyll in lipid bilayer → thylakoid membrane
Photosynthetic Pigments
Pigments are molecules that contain a chromophore → chemical group capable of absorbing light of specific wavelengths (λ)
The leaf of a green plant efficiently absorbs light energy over most of the spectrum
Why Green?
Because the pigment chlorophyll, is poor at absorbing green λ
Accessory Pigments
Found in the thylakoid membrane
E.g., carotenoids
Accessory pigments can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll
Increases efficiency of light absorption
Protects photosynthetic components from damage
Light Absorption
When chlorophyll absorbs light → one of its electrons is elevated to a higher-energy state
If the chlorophyll is beside other chlorophyll molecules → energy can be transferred
The energy is passed from chlorophyll to chlorophyll until it reaches a pair of chlorophyll molecules → The Reaction Center
Remember, the antenna chlorophylls transfer energy → Not Electrons
Reaction-center chlorophyll → a specific chlorophyll capable of transferring electrons to an electron acceptor
Before the reaction center can take in more light energy → it must first acquire an electron from an electron donor since it lost one electron
Where does this electron come from?
H2O
Role of Photosystems
Photosynthesis begins w/ absorption of light by protein-pigment complexes → Photosystems
Absorbed light drives redox reactions
2 photosystems
Photosystem II → PSII
Photosystem I → PSI
The 2 are connected by photosynthetic ETC
Drives formation of → ATP & NADPH
These are energy sources needed to synthesize carbohydrates from CO2 in the Calvin Cycle
Each photosystem has a reaction center chlorophyll
The electrons move from water to PSII → initial high energy level, but this drops as it goes through the ETC
More light energy coming in PSI raises the electron energy high enough to be used to reduce NADP+
The energy trajectory resembles a “Z” → called the Z scheme
2 Photosystems act in series
→ linked by ETC
Flow of e-
1. Between H2O
2. Between PSII and PSI → ETC
3. Between PSI and NADP+
As e- flow along the Z pathway → H+ ions are moved from stroma to inner compartment of thylakoids → Sets up a proton gradient
Proton Concentration → Increase in lumen of thylakoid, decrease in stroma
ATP Production
In the Chloroplast
Thylakoid membrane contains embedded ATP synthase enzymes
The ATP produced in the chloroplast remains in the chloroplast → use in carbon fixation reactions
Plants also have mitochondria to produce ATP needed elsewhere in the cell
b/c ATP produced in the chloroplast stays in the chloroplast
ATP Production
ETC
E- pass PSII and PSI through cytochrome b6f complex
Plastoquinone (PQ) → carries e- from PSII to the cytochrome b6f complex
Plastocyanin (Pc) → carries e- from cytochrome b6f complex to PSI by diffusing through the thylakoid lumen
Formation of ATP → results from e- moving through PSII & PSI
Proton gradient drives ATP formation → Enzyme is ATP Synthase
Protons pass through the enzyme ATP synthase → ATP is generated
The Calvin Cycle
Introduction
Takes low potential energy carbon compound (CO2) and converts it into a higher-energy carbon compound
Requires NADPH & ATP → 15 chemical reactions that synthesize carbohydrates from CO2
These reactions are grouped into 3 main steps
1. Carboxylation
2. Reduction
3. Regeneration
Step 1 - Carboxylation
This is where CO2 from the air is combined with RuBP to form a 6-carbon molecule → Catalyzed by Rubisco
The 6-carbon molecule is then broken down into 2 3-carbon molecules of PGA
Step 2 - Reduction
The potential energy of PGA needs to be increased → it must be reduced
The reduction occurs in 2 steps
The PGA is phosphorylated by ATP → Produce Triose Phosphates
The triose phosphate molecules are then reduced by NADPH → exported out of the chloroplast
Step 3 - Regeneration
For every 6 triose phosphate molecules that are produced, only one can be withdrawn from the Calvin cycle
This is because RuBP (a 5-carbon molecule) needs to be regenerated → using the other triose phosphates
That regeneration requires energy in the form → ATP
Regeneration produces three 5-C RuBP molecules to be used in the carboxylation portion of the Calvin Cycle
the Calvin Cycle: Excess Carbohydrates
Produced in Calvin Cycle are converted into storage → as starch
Formation of starch granules during the day is a source of carbohydrates that can be used during the night when there is no sunlight
Evolution of Photosynthesis
Early photosynthetic bacteria had only a single photosystem → cannot capture enough energy with one photosystem
Evolution of 2 PS could have occurred in the following 2 possibilities
- Horizontal Gene Transfer
- Gene duplication and divergence of one of the genes
Photosynthetic Eukaryotes
1st organisms to use water as the electron donor were the cyanobacteria
The endosymbiotic theory proposes that a cyanobacterium was engulfed by a eukaryotic cell
Over time, the cyanobacterium lost the ability to live outside the host → thus it became the organelle we know as the chloroplast
