Photosynthesis Cheatsheet

Cheatsheet Content

1. Introduction to Photosynthesis Definition: Process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Overall Equation: $6CO_2 + 6H_2O \xrightarrow{\text{Light Energy}} C_6H_{12}O_6 + 6O_2$ Site: Chloroplasts in eukaryotic cells (mesophyll cells of leaves). Pigments: Chlorophyll a (main), Chlorophyll b, Carotenoids (accessory pigments). 2. Chloroplast Structure Outer Membrane: Permeable. Inner Membrane: Less permeable, contains transport proteins. Stroma: Fluid-filled space, site of dark reactions (Calvin cycle). Thylakoids: Flattened sacs, site of light reactions. Grana: Stacks of thylakoids. Stroma Lamellae: Connect grana thylakoids. Thylakoid Lumen: Space inside thylakoid. 3. Light-Dependent Reactions (Light Reactions) Location: Thylakoid membranes. Requirements: Light, Water ($H_2O$), Photosynthetic pigments. Products: ATP, NADPH, $O_2$. Key Processes: Light Absorption: Pigments absorb light energy. Water Splitting (Photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$. Releases $O_2$, electrons, and protons. Electron Transport Chain (ETC): Electrons move through carriers (Plastoquinone, Cytochrome b6f complex, Plastocyanin). ATP Synthesis (Photophosphorylation): Non-cyclic Photophosphorylation: Both PS-I and PS-II involved. Electrons from $H_2O$ pass through ETC to PS-I, then to $NADP^+$ to form NADPH. Produces ATP and NADPH. Cyclic Photophosphorylation: Only PS-I involved. Electrons cycle back from PS-I to the ETC, then back to PS-I. Produces only ATP. Occurs when $NADP^+$ is low or more ATP is needed. NADPH Formation: $NADP^+ + 2e^- + H^+ \rightarrow NADPH$. 4. Light-Independent Reactions (Dark Reactions/Calvin Cycle) Location: Stroma of chloroplasts. Requirements: ATP, NADPH (from light reactions), $CO_2$. Product: Glucose ($C_6H_{12}O_6$). Phases: Carbon Fixation: $CO_2$ combines with Ribulose-1,5-bisphosphate (RuBP, a 5-C sugar) catalyzed by RuBisCO to form two molecules of 3-Phosphoglycerate (PGA, a 3-C compound). Reduction: PGA is converted to Glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. For every $CO_2$ fixed, 2 ATP and 2 NADPH are consumed. Regeneration: RuBP is regenerated from G3P using 1 ATP. For 6 $CO_2$ molecules to produce one glucose, 18 ATP and 12 NADPH are required. First stable product: PGA (3-C compound). Therefore, it's called the $C_3$ pathway. 5. Photorespiration Definition: A wasteful pathway that occurs when RuBisCO binds with $O_2$ instead of $CO_2$. Conditions: High $O_2$ concentration, high temperature, low $CO_2$ concentration. Process: RuBisCO binds $O_2$ to RuBP, producing one molecule of PGA (3-C) and one molecule of Phosphoglycolate (2-C). Phosphoglycolate is then metabolized, releasing $CO_2$ but consuming ATP and NADPH without producing sugar. Outcome: Reduces photosynthetic output. 6. $C_4$ Pathway (Hatch-Slack Pathway) Adaptation: To minimize photorespiration in hot, dry climates. Plants: Maize, Sugarcane, Sorghum. Anatomy: Kranz anatomy (bundle sheath cells around vascular bundles with thick walls, no intercellular spaces, many chloroplasts). Process: Mesophyll Cells: $CO_2$ is fixed by PEP Carboxylase (PEPcase) to Phosphoenolpyruvate (PEP) to form Oxaloacetate (OAA, a 4-C compound). This is the first stable product. OAA is converted to Malate (or Aspartate) and transported to bundle sheath cells. Bundle Sheath Cells: Malate releases $CO_2$ (decarboxylation). This concentrated $CO_2$ is then fixed by RuBisCO into the Calvin cycle. Pyruvate is returned to mesophyll cells to regenerate PEP. Advantages: Higher photosynthetic efficiency, no photorespiration, better water use efficiency. 7. Factors Affecting Photosynthesis Light: Intensity: Increases rate up to a saturation point. Quality (Wavelength): Blue and red light are most effective (absorption spectrum of chlorophyll). Duration: Longer duration increases total photosynthesis. Carbon Dioxide Concentration: Increases rate up to a saturation point. $C_3$ plants are more sensitive than $C_4$. Temperature: Optimal range for enzyme activity. High temps can denature enzymes. $C_4$ plants have higher optimal temp. Water: Deficiency causes stomatal closure (reducing $CO_2$ uptake) and wilting. Mineral Elements: E.g., Magnesium (Mg) for chlorophyll synthesis, Nitrogen (N) for proteins/enzymes. Blackman's Law of Limiting Factors: When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor. 8. Chemiosmotic Hypothesis (ATP Synthesis) Proposed by Peter Mitchell. Proton gradient across the thylakoid membrane drives ATP synthesis. Steps: Water splitting on the inner side of the membrane releases protons into the lumen. Electron transport chain moves protons from stroma to lumen. $NADP^+$ reductase enzyme on the stroma side removes protons from stroma for NADPH synthesis. Accumulation of protons in the lumen creates a proton gradient (high $H^+$ in lumen, low $H^+$ in stroma). Protons flow from lumen to stroma through $CF_0-CF_1$ ATP synthase, driving ATP synthesis from ADP and inorganic phosphate.