Photosynthesis PDF

Heterotrophic organisms (like E. coli and humans) get their energy from carbohydrates. Carbohydrates are mainly made through photosynthesis. Photosynthesis converts sunlight into chemical energy and is done by: ○ Phototrophic organisms (mainly plants but also some...

Heterotrophic organisms (like E. coli and humans) get their energy from carbohydrates. Carbohydrates are mainly made through photosynthesis. Photosynthesis converts sunlight into chemical energy and is done by: ○ Phototrophic organisms (mainly plants but also some microbes). Microbial photosynthesis is important for many ecosystems. Key Points About Photosynthesis Two Stages of Photosynthesis: ○ Light-dependent reactions: Sunlight is absorbed by pigments in plant membranes. Converts sunlight into stored chemical energy (ATP and NADPH/NADH). ○ Light-independent reactions: Use the chemical energy from the first stage to assemble sugar molecules from CO2. Still depend on the products from light-dependent reactions, which are short-lived. Main Outputs: ○ ATP (energy carrier) ○ NADPH or NADH (energy carriers) ○ Sugars (organic forms of energy) Eukaryotic Photosynthesis: Occurs in chloroplasts (organelle from photosynthetic bacteria). Chloroplasts have a double membrane. Contains thylakoids (stacked structures called granum) and stroma (surrounding space). Prokaryotic Photosynthesis: Occurs in infolded regions of the plasma membrane (thylakoids). Photosynthetic pigments in membranes convert light energy into chemical energy. Photosystems: Composed of light-harvesting complex and reaction center. Energy absorbed by pigments is transferred to the reaction center, leading to electron release. Pigments: Different pigments absorb specific light wavelengths. Examples: ○ Bacteriochlorophylls (green, purple, red)  ○ Carotenoids (orange, red, yellow) ○ Chlorophylls (green) ○ Phycocyanins (blue) ○ Phycoerythrins (red) Diverse pigments allow absorption of more light energy. Electron Transport System (ETS): High-energy electrons move to ETS, producing NADH or NADPH. ATP is generated via chemiosmosis. Photosynthesis Needs Electrons: Electrons lost from the reaction center must be replaced for photosynthesis to continue. Types of Photosynthesis: Oxygenic Photosynthesis: ○ Occurs in plants and cyanobacteria. ○ Uses water (H2O) as the electron source. ○ Produces oxygen (O2) as a byproduct. Anoxygenic Photosynthesis: ○ Carried out by some bacteria. ○ Uses other compounds (e.g., hydrogen sulfide (H2S) or thiosulfate (S2O3²⁻)) as electron donors. ○ Does not produce oxygen; generates elemental sulfur or sulfate ions instead. Discovery of Photosystems: Joan Mary Anderson proved there are two types of photosystems: Photosystem I (PSI) and Photosystem II (PSII). Used experiments with chloroplasts to show that different parts produce different products. Cyanobacteria and plant chloroplasts have both photosystems; anoxygenic bacteria use only one. Energy Production: Both photosystems are activated by light. If both ATP and NADPH are needed, cells use noncyclic photophosphorylation: ○ Electrons flow from PSII to PSI through an electron transport system (ETS). ○ Water splitting replenishes lost electrons in PSII. ○ PSI produces NADPH by reducing NADP+. ○ This process is called the Z-scheme. If more ATP is needed than NADPH, cells use cyclic photophosphorylation: ○ Only PSI is used.  ○ High-energy electrons return to PSI, generating ATP without NADPH. Light-Independent Reactions (Calvin Cycle) Simplified Key Information: Purpose: Converts CO2 into carbohydrates for long-term energy storage. Energy Source: Uses ATP and NADPH generated from sunlight. Carbon Source: Carbon comes from CO2, a waste product of cellular respiration. Calvin-Benson Cycle Overview: Location: Occurs in the cytoplasm of photosynthetic bacteria and in the stroma of chloroplasts in eukaryotes. Stages: ○ Fixation: Enzyme RuBisCO adds CO2 to RuBP, producing 3-PGA. ○ Reduction: Uses 6 ATP and 6 NADPH to convert 3-PGA into G3P; some G3P is used to make glucose. ○ Regeneration: Remaining G3P regenerates RuBP using 3 ATP, allowing the cycle to continue. Additional Information: RuBisCO is the most abundant enzyme on Earth, making up 30%-50% of soluble proteins in plant chloroplasts. The Calvin cycle is used by: ○ Plants ○ Photoautotrophic bacteria ○ Some non-photosynthetic chemoautotrophs Other bacteria and archaea may use different systems for CO2 fixation.