calvin cycle

      • Introduction to Photosynthesis and Membrane Functions

        • Topic: Discussing another function of the membrane: photosynthesis.

        • Importance of understanding the relationship between drought and climate change.

        Relationship Between Drought and Climate Change

        • Multiple connections between drought and climate change:

          • Drought reduces plant growth, leading to:

          • Fewer plants on the planet.

          • Reduction in CO₂ absorption since:

            • Plants function as CO₂ sinks, therefore:

      • Less plants result in higher CO₂ levels, further worsening climate change.

    • Drought leads to stomata closing as a stress response:

    • Stomata are openings in leaves that regulate gas exchange (CO₂ intake and H₂O evaporation).

    • Closing stomata leads to:

      • Less CO₂ absorption (despite atmospheric levels not changing).

    • Water is directly involved in photosynthesis:

    • Water provides electrons necessary for photosynthesis.

    • Less water availability results in:

      • Reduced ATP and NADPH production, affecting CO₂ fixation and glucose formation.

  • Conclusion: Drought directly impacts carbon fixation processes, worsening climate change.

Photosynthesis Overview

  • Photosynthesis occurs in chloroplasts found in:

    • Plant cells.

    • Algae.

    • Cyanobacteria.

  • Structures in chloroplasts:

    • Chloroplasts appear green due to chlorophyll.

    • Chlorophyll is a pigment that absorbs light.

  • Electron microscopy identifies two membranes: outer and thylakoid membranes where photosynthesis takes place.

Stages of Photosynthesis

1. Light-Dependent Reactions

  • Required components:

    • Light.

    • Water.

    • Thylakoid membranes.

  • Outcomes:

    • Production of ATP and NADPH (activated carriers).

    • Oxygen as a byproduct (essential for aerobic organisms).

    • Key locations involved: thylakoid membranes and stroma.

2. Light-Independent Reactions (Calvin Cycle)

  • Utilizes ATP and NADPH from the light-dependent reactions:

    • Main goal: Fix CO₂ to produce sugar.

  • Process involves:

    • CO₂ attaching to ribulose bisphosphate (RuBP) to form a six-carbon intermediate that splits into two three-carbon molecules.

  • Outcomes include:

    • Formation of glyceraldehyde-3-phosphate (G3P), a precursor for glucose, starch, and other metabolites.

Functions of Pigments in Photosynthesis

  • Definition of pigments:

    • Molecules that absorb visible light.

    • Key examples include:

    • Chlorophyll - absorbs red and blue light; reflects green light (hence green leaf coloration).

    • Carotenoids - absorb light and can reflect yellow/orange.

    • Anthocyanins - appear in fall, synthesized to reflect colors.

  • Light behavior:

    • Composed of photons with dual properties (particle and wave).

    • Energy varies by wavelengths (400 nm higher energy than 700 nm).

Electron Excitation in Pigment Molecules

  • Process upon photon absorption:

    • Electron transitions from ground state to an excited state.

    • Outcomes can either be:

    • Electron loss via transfer to other molecules, or

    • Energy loss via heat and reversion to ground state.

  • Function of chlorophyll involves:

    • Absorption of specific light wavelengths and electron excitation.

Photosystem Structure and Function

  • Photosystems consist of:

    • Light-harvesting complexes (containing chlorophylls) and reaction centers (with special chlorophylls).

  • Photosystem types:

    • Photosystem I (PSI).

    • Photosystem II (PSII).

  • Functionality:

    • Light harvesting involves energy transfer between chlorophylls, culminating in electron excitation and transfer to the electron transport chain (ETC).

Mechanism of Light-Dependent Reactions

  1. Photosystem II:

    • Absorbs light energy, excites electrons, deposits electrons in the ETC.

    • Water molecules are split to replenish lost electrons.

  2. Electron Transport Chain:

    • Movement through various proteins (e.g. plastoquinone, cytochrome b6f).

    • Proton (H⁺) pumping creates an electrochemical gradient.

    • Proton movement drives ATP synthase to produce ATP.

  3. Photosystem I:

    • Absorbs light to re-excite electrons from PSII.

    • Electrons transferred to NADP⁺, reducing it to NADPH.

Summary of Calvin Cycle

  • Phase details:

    1. Carbon Fixation: CO₂ is attached to the ribulose bisphosphate (RuBP).

    2. Sugar Formation: G3P is formed; precursor for glucose.

    3. Regeneration: RuBP is regenerated to continue carbon fixation.

  • Inputs and outputs for each cycle:

    • Input: 3 CO₂, 9 ATP, 6 NADPH.

    • Output: glyceraldehyde-3-phosphate (G3P).

Linking Light-Dependent and Independent Reactions

  • Connection via production of ATP and NADPH during light-dependent reactions sustaining Calvin Cycle.

  • Change in pH and calcium ions:

    • Protons pumped into the thylakoid space and flow create a gradient affecting stroma pH.

    • Certain Calvin Cycle enzymes are pH-sensitive, enhancing efficiency during times of active photosynthesis.

Cellular Energy Pathways: Comparison of Chloroplasts and Mitochondria

  • Both utilize membrane-bound processes for ATP production.

  • Main differences:

    • Electron donor: Water in chloroplasts vs NADH in mitochondria.

    • Electron acceptor: NADP⁺ vs O₂ (forming H₂O).

  • ATP generated in the stroma of chloroplasts vs the mitochondria matrix.

Exam Preparation

  • Structure of exams:

    • Multiple-choice questions.

    • Short answer questions, with emphasis on clarity and relevance.

  • Techniques:

    • Read questions and familiarize with possible answers without looking.

    • Relevant information for short answers: focus on specified structures or functions as per question requirement.