Photosynthesis and Glucose Metabolism

• Photosynthesis involves producing glucose using sunlight.

Glucose Catabolism

• Glucose combines with oxygen to produce carbon dioxide, water, and energy.
• The chemical equation for glucose catabolism is: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
• The change in Gibbs free energy (ΔG) is -686 kcal per mole of glucose.

Glucose Anabolism

• Carbon dioxide and water combine with energy to produce glucose and oxygen.
• The chemical equation for glucose anabolism is: C6H12O6 + 6O2 ← 6CO2 + 6H2O + energy.
• The change in Gibbs free energy (ΔG) is 686 kcal per mole of glucose.
• The energy needed to synthesize glucose originates from sunlight via photosynthesis.
• During cellular respiration, energy released from the C-H bonds of glucose is transferred via electron carriers, which donate electrons to oxygen, generating water.
• Photosynthesis is a redox process with electron flow occurring in the opposite direction of cellular respiration.
• Water is split, and electrons from the polar H-O bonds are transferred with protons to carbon dioxide, reducing it to sugar and forming less polar C-H bonds.
• The energy is stored as potential energy in the C-H bonds of the sugar.

Leaves and Chloroplasts

• Leaves are the primary sites of photosynthesis.
• Chloroplasts are mainly located in the cells of the mesophyll tissue.
• Carbon dioxide enters and oxygen exits the leaf through stomata.
• Water absorbed by roots is transported to leaves via veins, which are also used to export sugar to other parts of the plant.
• A chloroplast features two membranes enclosing a dense fluid called the stroma.
• Suspended within the stroma are thylakoids, which form a third membrane system.
• Thylakoids segregate the stroma from the thylakoid space.
• Thylakoid sacs form stacked columns called grana.
• Chlorophyll, which gives leaves their color, is located in the thylakoid membranes.
• Chloroplasts have inner and outer membranes and phospholipid bilayers.
• Thylakoid discs are enclosed in a granum.
• The thylakoid contains a fluid-filled space.
• The stroma is the location of the Calvin cycle.
• The inner membrane regulates material passage into the chloroplast, and the outer membrane controls molecule movement in and out.

Photosynthesis Reactions

• Photosynthesis involves two sets of reactions occurring in chloroplasts: light reactions and dark reactions (Calvin Cycle).
• Light reactions split water molecules, releasing oxygen.
• Electrons from this reaction are captured by chlorophyll increasing their energy level, moved down electron transfer chains, releasing energy used to generate ATP and NADPH.
• Dark reactions fix carbon dioxide, reducing C-O to C-H and generating carbohydrates, similar to the Krebs cycle as a cyclic metabolic pathway known as the Calvin Cycle.
• Solar energy is converted to chemical energy through light reactions.
• Light absorbed by chlorophyll drives the movement of electrons and hydrogen ions from water to NADP+, which is reduced to NADPH.
• Light reactions also generate ATP through chemiosmosis, involving the addition of a phosphate group to ADP (phosphorylation).
• Light energy is initially converted to chemical energy stored in NADPH and ATP.
• NADPH acts as "reducing power" by passing electrons to an electron acceptor, while ATP is the energy currency of cells.
• Light reactions do not produce sugar; sugar production occurs in the Calvin Cycle.

Calvin Cycle

• The Calvin Cycle begins by incorporating carbon dioxide from the air into existing organic molecules. This process is known as carbon fixation.
• The Calvin cycle reduces carbon to carbohydrate with the addition of electrons.
• NADPH power facilitates the reduction.
• The Calvin cycle also requires ATP
• Thus, the Calvin Cycle makes sugar working in tandem with help from NADPH and ATP produced by light reactions
• No steps require light directly.
• Leaves appear green because chlorophyll does not absorb green light.
• Green light passes through the leaf.
• Plants contain two types of chlorophyll: chlorophyll a and chlorophyll b, which differ slightly in structure and absorbance properties.
• Plants also contain carotenoids, which can change the absorbance properties of photosynthetic mechanisms.
• In chloroplasts, light-absorbing pigments in the thylakoid membrane are organized with proteins into photosystems.
• Photosystem I (PSI) absorbs light most efficiently at 700nm (P700)
• Photosystem II (PSII) absorbs light most efficiently at 680 nm (P680).

Pigments

• Chlorophyll molecules in plants are arranged in clusters within the thylakoid membrane.
• Chlorophyll features porphyrin ring and a hydrophobic tail.
• The hydrocarbon tail anchors chlorophyll into the thylakoid membrane.
• Excitation of isolated chlorophyll returns to ground state by heat or fluorescence.
• When the isolated chlorophyl absorbs a photon (light energy), an electron (e-) is excited to a higher energy level.
• The excited electron is unstable and returns to its ground state, releasing energy in two ways: Heat and fluorescence.
• Fluorescence emits energy as a lower-energy photon.
• A photosystem contains a reaction-center complex surrounded by light-harvesting complexes.
• *Reaction-center complex: an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor.
• Each light-harvesting complex has pigment molecules like chlorophyll a and b, and multiple carotenoids bound to proteins.
• The number and variety helps a photosystem gather light from a larger area and get larger portions of the spectrum
• The light-harvesting complexes act as an antenna for the complex.
• Energy from a photon absorbed by a pigment molecule is transferred from pigment to pigment until it reaches the chlorophyll a molecules in the reaction-center.
• These molecules boost one of their electrons to a higher level of energy, and transfer it to the primary electron acceptor.

Photosystem II (PSII) versus Photosystem I (PSI)

• P680 reaction-center in PSII.
• P700 reaction-center in PSI.
• Both contain chlorophyll a, nearly identical, but their association w/ different proteins in the thylakoid membrane affects electron distribution, and light-absorbing properties.

Light Reactions and Chemiosmosis

• Light energy excites electrons in photosystem II to plastoquinone (Pq).
• To replenish electrons, water (H₂O) is split into electrons, protons (H+), and oxygen (O2).
• Excited electrons travel from PSI through the electron transport chain (ETC).
• As electrons move, protons (H+) are actively pumped from the stroma into the thylakoid space, creating a proton gradient.
• Electrons pass through the cytochrome complex and are transferred to plastocyanin (Pc).
• Pc carries the electrons through the thylakoid lumen to PSI.
• Electrons reach photosystem I (PSI) and are re-energized by another photon.
• The high-energy electrons are passed to NADP+ reductase, reducing NADP+ to NADPH.
• NADPH carries these high-energy electrons to the Calvin Cycle.
• The proton gradient drives H+ ions (protons) through ATP Synthase, which phosphorylates ADP into ATP.
• ATP is sent to the Calvin Cycle.
• Light energy absorbed by pigment molecules in the complex and transfers between pigments until it reaches P680.
• Incoming energy excites P680 which boosts an electron and transfers to a high energy level.
• The energized electron is transferred to the primary electron acceptor, ionizing P680 to P680+
• Because P680 has lost an electron, P680+ becomes a very strong oxidizing agent (P680+)
• To replenish the electron, water is split into protons, electrons and oxygen.
• Electrons from water reduce P680+ back to P680, returning its stability.
• P680 is ionized by the loss of electrons.
• P680 is the strongest oxidizing agent known in biology.
• It's electron vacancy needs to be filled, and so the splitting of water releases oxygen, and hydrogen.
• The oxygen is released to the atmosphere.
• Generated H+ is released into the thylakoid lumen and contribute to the proton gradient.
• Linear electron flow involves the flow of electrons through the photosystems building onto the thylakoid membrane.

Linear Electron Flow

• A photon of light excites a pigment raising the electron energy level.
• As this electron falls back down it hits another pigment.
• The process continues, and it hits another electron, passing one by one electron pairing and excites one and the now energized electron is transferred from P680 to a primary electron receptor.
• Then P680 is ionized, and is now the strongest oxidizing agent ready to grab the next electron.

Linear Electron Flow (cont.)

• An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions.
• Those electron pairs fuel incoming electrons and are accepted to a primary electron acceptor, while ions are released and O2 is formed.
• Electrons are passed primary acceptors through PsII and PsI via an electron transport chain to pump protons and contributing to a proton gradient.
• Light energy transferred excites P700 pair of chlorophyll molecules.
• Next electron is passed through primary electrons, now calling the vacancy P700+.
• Excited e- passing from electron acceptor through a secondary chain via ferredoxin, but does not crate a proton gradient.
• The enzyme NADP+ reduces the transfer of electrons from fd to NADP+, needing 2 electrons for this reaction.
• The process removes H+ from the stroma, and the energized electrons are used to catalyze ATP formation.

Cyclic Electron Flow

• Cyclic electron flow is when electrons can take an alternative path using only photosystem I
• The electrons cycle back from ferredoxin (Fd) to the cytochrome complex, then via, a plastocyanin (Pc) molecule to P700 chlorophyl.
• There is no prodution of NAPDH and no release of oxygen, but generates ATP.

Chemiosmosis

• Both orgenelles use electron transport Chains-pump protons (h+) across a membrane from a low to a high concentration.
• The protons then diffuse back across the membrane through ATP Synthase