3.5 Photosynthesis

The highly complex organization of living systems requires constant input of energy and the exchange of macromolecules. Describe the photosynthetic processes that allow organisms to capture and store energy. Organisms capture and store energy for use in biological processes-- Photosynthesis captures energy from the sun and produces sugars. Photosynthesis first evolved in prokaryotic organisms. Scientific evidence supports the claim that prokaryotic (cyanobacterial) photosynthesis was responsible for the production of an oxygenated atmosphere. Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture energy present in light to yield ATP and NADPH, which power the production of organic molecules. Explain how cells capture energy from light and transfer it to biological molecules for storage and use. During photosynthesis, chlorophylls absorb energy from light, boosting electrons to a higher energy level in photosystems I and II. Photosystems I and II are embedded in the internal membranes of chloroplasts and are connected by the transfer of higher energy electrons through an electron transport chain (ETC). When electrons are transferred between molecules in a sequence of reactions as they bass through the electron transport chain, an electrochemical gradient of protons (hydrogen ions) is established across the internal membrane. The formation of the proton gradient is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthesis. The energy captured in the light reactions and transferred to ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast.

photosynthesis

• conversion of light energy to chemical energy/organic molecules

• first evolved in prokaryotic organisms, cyanobacteria, causing oxygenation of atmosphere

• occurs in chloroplasts in mesophyll, gases enter/exit via stomata, light absorbed by chlorophyll pigment

• light depend reactions and light independent reactions

photosynthesis equation

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

light dependent reactions of photosynthesis

• take place in grana/thylakoids

• light energy captured, used to generate ATP and NADPH, oxygen released as byproduct

• photolysis

• electron arrives at photosystem I via electron transport chain, boosted to a high energy level and transferred to acceptor molecule also via ETC

• high-energy electron travels down ETC, passed to NADP+ (terminal receptor) → NADPH (needs two electrons)

• protons pumped into thylakoid lumen (interior), creating proton gradient essential for ATP synthesis

• photophosphorylation, protons go through ATP synthase back to stroma

• cyclic electron flow

photolysis

photons (particles of light) make contact with Photosystem II, light energy splits water molecules into hydrogen ions, electrons, and oxygen gas

• H2O → O2 + H⁺ + e-

photosystems I and II

embedded in the internal membranes of chloroplasts, connected by the transfer of higher energy electrons through an electron transport chain (ETC)

• during photosynthesis, chlorophylls absorb energy from light, boosting electrons to a higher energy level in photosystems I and II

photophosphorylation

light-driven addition of phosphate group to ADP, synthesizing ATP, catalyzed by ATP synthase

cyclic electron flow

protons are recycled around Photosystem I, generating ATP without also generating NADPH

• C4 plants

light independent reactions in photosynthesis/the Calvin Cycle

• synthesis takes place in the stroma

• 3CO2 + 9ATP + 6NADPH → G3P, building block of glucose

• carbon fixation - production of carbohydrates from carbon cycle is powered by energy captured in light reactions and transferred to ATP and NADPH

• NADP+ and ADP/P_i sent back to light reaction to be reused

different photosynthetic pathways of different plants

• C3 plants - most common type plants, stomata closure may lead to high oxygen concentrations and photorespiration

  • wasteful pathway competing with the Calvin cycle, begins when rubisco acts on oxygen instead of carbon dioxide and produces no sugar

• C4 plants - has chloroplasts in both mesophyll and bundle sheath cells, undergoes Calvin Cycle in bundle sheath cells, requires more energy but minimizes photorespiration

• CAM pathway - used by desert plants, carbon fixation/stomata opening only occurs at night