HM

Chapter 6: Photosynthesis

Photosynthesis Overview

  • Definition: Process where sunlight energy is captured to synthesize carbohydrates.

  • Occurs In: Plants, algae, and some bacteria.

  • Importance: Provides energy for most Earth organisms.

  • Plant Organs: Leaves are primary photosynthetic organs.

  • Chloroplast Structure:

    • Thylakoids: Internal membranes.

    • Grana: Stacks of thylakoids.

    • Stroma: Liquid surrounding thylakoids.

Stages of Photosynthesis

  1. Capturing Energy: From sunlight using pigments in photosystems.

  2. Converting Energy: Light energy to chemical energy (ATP & NADPH) via light-dependent reactions.

  3. Synthesizing Carbohydrates: Using ATP and NADPH to make carbohydrates from CO_2 via light-independent reactions (Calvin cycle).

Light-Dependent Reactions

  • Location: Thylakoid membranes.

  • Components: Photosystems (pigments and protein complexes) capture light.

  • Photosystem II (PSII):

    • Captures photon, releases excited electron to Electron Transport System (ETS).

    • H2O is electron source; split to produce O2, H^+ ions, and electrons.

    • ETS uses energy to produce ATP (via proton gradient).

  • Photosystem I (PSI):

    • Absorbs another photon, releases excited electron to a second ETS.

    • Second ETS produces NADPH.

  • Overall Input/Output: Light energy + H2O \rightarrow ATP + NADPH + O2.

Light-Independent Reactions (Calvin Cycle)

  • Location: Stroma.

  • Purpose: Uses chemical energy (ATP and NADPH) to convert CO_2 into carbohydrates (e.g., glucose).

  • Also known as: C_3 photosynthesis.

  • Key Enzyme: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes carbon fixation.

  • Steps:

    1. Carbon Fixation: CO_2 added to RuBP (5-carbon molecule) by Rubisco, forming unstable 6-carbon molecule that splits into two 3-carbon molecules (3-phosphoglycerate).

    2. Reduction: ATP and NADPH energy convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P). Some G3P combines to make glucose.

    3. Regeneration: Remaining G3P uses ATP to regenerate RuBP, completing the cycle.

Photorespiration and Adaptations

  • Photorespiration: In hot conditions, plants close stomata, increasing leaf O2 and decreasing CO2. Rubisco binds O2 instead of CO2, wasting Calvin cycle efforts.

  • C_4 Photosynthesis: Adaptation for hot climates.

    • Carbon fixation occurs in a separate cell layer (mesophyll) to prevent Rubisco contact with O_2.

    • Examples: corn, sugarcane, pineapple.

  • Crassulacean Acid Metabolism (CAM): Adaptation for arid climates.

    • Plants use C4 pathway at night (cooler) to fix CO2.

    • Use C3 pathway during the day with stored CO2.

    • Examples: cacti, succulents.