Photosynthesis: Light Reaction

Photosynthesis: Light Reaction

• Photosynthesis is a biological process that utilizes energy from the sun in plants, algae, and cyanobacteria
• Light energy drives the synthesis of carbohydrates and oxygen from carbon dioxide and water
• Photosynthesis is essential for sustaining life on Earth
• The overall formula is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• The reaction involves transformation of light into chemical energy, oxidation of water, and reduction of CO₂ into sugars
• ATP and NADPH are required for reduction processes

Contributor to Planet's Photosynthesis

• Phytoplankton are major contributors to Earth's photosynthesis
• Aquatic and terrestrial ecosystems are significant for photosynthesis

Photosynthesis in Higher Land Plants

• Primarily occurs in chloroplasts, with the highest activity in the leaf mesophyll layer
• Light absorption occurs via green pigment chlorophylls and is most abundant in the leaves
• CO₂ enters leaves via stomata
• Thylakoids, often stacked into grana, are found in the inner membrane within stomata

Cellular Structures

• Stroma is the fluid within the chloroplast surrounding the thylakoid
• Grana lamellae are stacked membranes of a granum
• Stroma lamellae are exposed membranes without stacking

Thylakoid Reactions

• Occur in thylakoids within the chloroplast
• End products are ATP and NADPH
• Utilized in the synthesis of sugars (carbon fixation)

Light - Wave and Particle

• Light is both a wave and a particle (photon)
• Short wavelengths have high energy and high frequency
• Long wavelengths have low energy and low frequency
• Visible light is a small portion of the electromagnetic radiation spectrum
• Electromagnetic energy travels in waves
• Wavelength is the distance between two adjacent crests
• Short wavelengths have high energy and frequency
• Longer wavelengths have low energy and frequency

Chlorophyll

• Chlorophylls are most abundant in the thylakoid membranes of plants
• Chlorophylls capture blue and red wavelengths while reflecting green
• Blue light excites chlorophylls to a higher energy state
• Chlorophyll a is the primary pigment which is involved in energy conversion and absorbs blue and red light
• Chlorophyll b is an accessory pigment that broadens light absorption, absorbing blue and orange light and appearing olive-green

Light Absorption and Emission

• When absorbing certain light spectra, a pigment excites to a higher energy state, then returns by re-emitting a photon (fluorescence) OR converting excitation energy into heat
• Chlorophyll transfers its energy to the surroundings in light absorption and emission
• Chlorophyll enters a lowest excited state and has four alternative pathways for disposing of available energy
• Excited chlorophyll can re-emit a photon and return to its ground state through fluorescence
• Alternatively, excited chlorophyll can convert excitation energy into heat
• Chlorophyll participates in resonance energy transfer
• The energy of the excited state initiates photochemical reactions
• Visible radiation fuels the light reactions

Light Absorption and Chlorophyll

• Pigments are materials that absorb certain wavelengths.
• Plant pigments are integrated into the thylakoid membrane.
• Chlorophyll transmits green wavelengths.
• Blue light absorption boosts chlorophyll to a higher energy state more than red light.
• More light absorbed by the pigments causes a higher photosynthesis rate
• Enhancement effect: Greater photosynthesis rate under both red and far-red light

Chlorophyll and Accessory Pigments

• Chlorophyll a absorbs mainly blue and red light and reflects mainly green light.
• The action spectrum shows wavelengths most effective in powering processes.
• Absorption and action spectra will correspond if the pigment that obtains the absorption spectrum are the same as those causing response
• The red drop effect is when far-red light (>680 nm) is insufficient alone in driving photosynthesis
• Accessory Pigments strongly absorb wavelengths not absorbed by chlorophyll a functioning as antenna pigments
• Chlorophyll b absorbs blue and orange light and reflects olive green

Xanthophylls

• Xanthophylls are carotenoids absorbing light with a spectrum of 466-495 nm, thus are yellow
• Lutein, Violaxanthin, and Zeaxanthin are Xanthophylls
• They help harvest light and provide photoprotection from too much light that could inhibit photosynthesis
• High Light and Violaxanthin conversion Nonphotochemical quenching of excess exitation as heat

Photosynthetic Units

• Photosystems I and II (PSI and PSII): pigment-protein complexes
• Components: antenna pigment complex (LHC; accessory pigments and chlorophyll molecules) and photochemical reaction center (pair of chlorophyll molecules)
• The excited chlorophyll molecule transfers the energy to neighboring chlorophyll by resonance energy transfer
• Excited electrons from chlorophyll a are donated to the primary electron acceptor

Photosystems I and II Differences

• PSI possesses a lower level of chlorophyll b and has a pair of Chlorophyll a that absorbs strongly to far-red light (greater than 680 nm)
• PSII possesses almost an equal amount of Chlorophyll a as Chlorophyll b but with more accessory pigments and has a pair of Chlorophyll a that absorbs strongly to red light (680 nm)
• PSI functions focus on cyclic electron flow
• PSII drives non-cyclic electron flow with electrons are extracted from water
• In PSI electrons return to the starting point after generating ATP without O₂ prouction
• PSII electrons are extracted, which releases oxygen and is used for chemical energy production

Z-Scheme in Photosynthesis

• Electron Pathway through PSI and PSII using electron carriers toward the production of NADPH and ATP

Cyclic Electron Pathway (PSI)

• Process of cyclic electron pathway, from direct absorption or antenna pigment to Fe4S4, then to ferredoxin, then to the enzyme ferredoxin-NADP + reductase, which then reduces NADP + to NADPH in the stroma
• By P700's chlorophyll a is excited through absorption/antenna, Fe, then Fd is added to the enzyme that reduces NADP to NADPH
• Electrons return and cycle

Non-Cyclic Electron Pathway (PSII)

• P680 transfers an electron to pheophytin
• Two plastoquinones receive electrons from phaeophytin
• Reduced QB2 takes 2 protons from the stroma and yields plastohydroquinone complex
• Cytochrome complex receives electrons, and transfers electrons to plastocyanin
• In plants, the ultimate electron donor is water, and the ultimate electron acceptor is NADP+
• P680 chlorophyll a is excited and transfers an electrons to pheophytin

Cytochrome Cycle

• Oxidized PQH2 molecule near near complex; transfers two e- to the Rieske Fe-S protein/one of the B-type cytochromes and simultaneously pushes two proteins to lumen
• Electrons from Fe-S protein are now cytochrome f/plastocyanin Oxygen-evolving complex exists on the internal surface of the thylakoid membrane, and contains 4 Mn, Cl, and Ca2+ ions for O2 evolution

Electrons and Protons

• Water oxidation yields generates oxygen and four hydrogen ions, or four e- are removed
• Oxidation/reduction balance between Photosystems I and II drives electron and proton transport and NADPH production

Photosystems and Organization of the Thylakoid Membrane

• Photosystem II mostly in the stacked regions/grana of thylakoid membrane
• Photosystem I, ATP synthase found in unstacked/stroma protruding towards the storma
• Proportional ratio of 1.5:1 of photosystems II to I
• The antenna pigment increases with increase distance
• In antennae, greater than 95% efficiency in absorbing photons under optimal
• In reactions center reactions have to occurs with chemical
• LHC reactions centers of proteins is the inner part

ATP Synthase

• Cytochrome b6f complex transports two protons into the lumen
• Proton flow is channeled through an enzyme complex, ATP synthetase (CF0-CF1 complex).
• ATP synthetase phosphorylates ADP to ATP
• CF0 forms a channel through which protons can pass
• Hydrogens will use ATP synthase to move into stroma

Dark Reaction Synthesis

• The light-dependent reactions provide ATP and NADPH and create high electrochemical potential
• The Dark reactions uses light-dependent reactions to create NADPH and ATP inside the Thylakoids
• Major reactants in dark reactions are ribulose-1,5-bisphosphate and CO2
• Outer product is G3P
• Takes place in the stroma and is independent of light as well as carbon reactions of the electron transport and provides substrate for enzymes
• Inorganic Co2 products are then transformed to create an electron transport chain called the Calvin Cycle
• Starch, a polysaccharide, is used as reserve
• Pi has more triose but less cytosolic
• Sucrose is a disaccharide used by leaves

Calvin-Benson Cycle

• Discovered Melvin Calvin, Nobel Winner
• A endergonic reaction requires energy from light that are found in prokayrotes
• The pathway decreases oxidation and contains 3 stages

Calvin Cycle Players and steps

• Carbon dioxide is captures from the atmosphere
• RuBP (ribulose-1,5-bisphosphat) is created when the reaction occurs, and it serves as the acceptor for CO2
• RuBisCO converts to (ribulose-1,5-bisphosphat

Additional Calvin Cycle Steps and Players

• 3-phosphoglycerate is converted to three-carbon carbohydrates
• ATP and NADPH acts as the primary driver of the reaction, these main source reduce ATP/NADPH

Photorespiration

• CO2 creates less photosynthesis
• RuBisCO releases CO2 and binds oxygenated