Absorption of Light

Absorption of Light

  • Light Energy in Photosynthesis

    • Light energy is absorbed by pigments during photosynthesis.

    • Plants use visible light; this light appears white but contains various colors.

    • Light can be dispersed into a spectrum using a prism or water droplet.

    • Electromagnetic Spectrum: Visible light is perceived as colors with varying wavelengths:

      • Violet and Blue: Shorter wavelengths, higher energy.

      • Red: Longer wavelengths, lower energy.

Understanding Pigments

  • Types of Pigments: Each pigment absorbs specific wavelengths of visible light and reflects non-absorbed colors.

    • Chlorophyll a:

      • Reflects green light; absorbs blue and red wavelengths.

    • Chlorophyll b:

      • Absorbs blue and red-orange light.

    • Carotenoids: Another type of pigment found in plants.

    • Each pigment has a unique absorption spectrum indicating the wavelengths absorbed.

    • Light Limitations: Not all organisms have access to full sunlight:

      • Some grow underwater, where light is limited and certain wavelengths are absorbed by water.

      • Rainforest plants must utilize any available light due to competition from taller trees.

How Light-Dependent Reactions Work

  • Purpose: Light-dependent reactions convert light energy into chemical energy, powering the Calvin cycle (sugar assembly).

Photosystems and Their Function

  • Photosystems Locations: Found within thylakoid membranes - groupings of pigments and proteins.

  • Photon Interaction:

    • A photosystem absorbs photons, causing chlorophyll electrons to become 'excited.'

    • Excited electrons break free, leading to water molecule splitting, releasing oxygen and hydrogen ions.

    • Each split water molecule provides two electrons to replenish chlorophyll.

Energy Transfer within Photosystems

  • Electron Pathway:

    • Light energy excites electrons in chlorophyll, which are transferred to an electron acceptor.

    • Water splitting results in oxygen release and hydrogen ion formation.

Electron Transport Chain

  • Photosystem II (PSII): Initially receives the photon-excited electron, passing it through an electron transport chain.

    • The chain pumps hydrogen ions from stroma to thylakoid lumen, creating an electrochemical gradient.

    • This process is analogous to mitochondrial operations, where hydrogen ions are also pumped across membranes.

    • Photosystem I (PSI): Accepts the electron after it traverses the electron transport chain.

Generation of Energy Carriers

  • Two key energy carriers formed during light-dependent reactions: ATP and NADPH.

    • ATP Formation:

      • ATP is formed through chemiosmosis, utilizing the electrochemical gradient of hydrogen ions, facilitated by ATP synthase (similar to mitochondrial processes).

    • NADPH Formation:

      • Electrons at PSI are re-energized and used to convert NADP+ and H+ into NADPH, which stores energy for the Calvin cycle.

Calvin Cycle Overview

  • Purpose: Uses ATP and NADPH to convert CO2 into glucose and other carbohydrates.

  • CO2 Entry: Carbon dioxide enters chloroplasts via stomata, diffusing into the stroma where the Calvin cycle occurs.

  • Reactions:

    • Carbon Fixation: CO2 combines with Ribulose bisphosphate (RuBP) catalyzed by enzyme RuBisCO.

    • The formation of a six-carbon compound is followed by splitting into two three-carbon molecules (3-PGA).

    • ATP and NADPH convert 3-PGA to G3P (a three-carbon sugar).

    • Regeneration: Remaining G3P molecules regenerate RuBP, allowing the cycle to continue.

Energy Input Requirements

  • Each complete cycle takes six turns to fix six carbon atoms from CO2:

    • Requires 12 ATP and 12 NADPH for reduction.

    • Needs 6 ATP for regeneration of RuBP.