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Chapter 1: Have The Ability

Autotrophic plant cells and the basis of plant nutrition

  • Autotrophs are organisms that synthesize organic compounds from inorganic sources; in plants, this is primarily through photosynthesis, enabling them to make their own food.

  • Key organelle: chloroplasts – site of photosynthesis. Structure includes:

    • Outer and inner membranes

    • Stroma: fluid-filled matrix

    • Thylakoids: flattened sacs organized into stacks called grana

    • Lumen: internal space of thylakoids

    • Pigments located in thylakoid membranes (chlorophylls and carotenoids)

  • Major products of photosynthesis: organic sugars (glucose, etc.) and O₂ as byproduct in aerobic environments.

Overview of photosynthesis (two main stages)

  • Stage 1: Light-dependent reactions (requires light)

    • Location: thylakoid membranes

    • Primary inputs: light energy, water

    • Primary outputs: ATP, NADPH, and O₂ (from water splitting)

    • Key processes:

    • Photolysis of water: 2 H2O ightarrow O2 + 4 H^+ + 4 e^-

    • Light absorption by photosystems II and I

    • Electron transport chain (ETC) transfers electrons, creates proton gradient

    • ATP synthase uses the proton gradient to synthesize ATP

    • NADP^+ is reduced to NADPH at the end of the chain (via PSI)

    • Overall stoichiometry (conceptual): captures energy from photons to generate carriers (ATP, NADPH) used in the next stage

    • Important outputs summarize energy currency: ext{ATP}, ext{NADPH}
      ightarrow ext{Calvin cycle}\

  • Stage 2: Calvin cycle (Light-independent reactions / carbon fixation)

    • Location: stroma

    • Primary inputs: CO₂, ATP, NADPH (from the light-dependent reactions)

    • Primary outputs: glyceraldehyde-3-phosphate (G3P), which can be used to synthesize glucose and other carbohydrates

    • Key steps:

    • Carbon fixation: CO₂ is fixed by RuBP (a 5-carbon sugar) via Rubisco to form 2 molecules of 3-PGA

    • Reduction: 3-PGA is phosphorylated and reduced to G3P using ATP and NADPH

    • Regeneration: Most G3P is recycled to regenerate RuBP using ATP; a portion exits the cycle to synthesize sugars

    • Net per three CO₂ fixed (to form one molecule of G3P capable of continuing to sugar synthesis):

    • Carried out using energy carriers: 3 CO2 + 6 NADPH + 9 ATP ightarrow G3P + 6 NADP^+ + 9 ADP + 9 Pi

    • This means one G3P (3 carbons) is produced for each three CO₂ fixed, and requires the equivalents of energy and reducing power above

    • Relation to glucose synthesis: two G3P molecules (from 6 CO₂) can be combined to form glucose and other hexoses

Chloroplast architecture and its relation to function

  • Thylakoid membranes house the photosystems (PSII and PSI) and the electron transport chain

  • Grana are stacks of thylakoids; stroma surrounds the thylakoids and contains enzymes of the Calvin cycle

  • Pigments absorb light and funnel energy to reaction centers in the photosystems

  • Light-harvesting complexes increase the efficiency of light capture

Photosystems and the electron transport chain (ETC)

  • PSII absorbs light and initiates electron transport by exciting P680

  • Electrons move through the ETC via plastoquinone, the cytochrome b6f complex, and plastocyanin to PSI

  • PSI absorbs light to re-energize electrons at P700 and transfers them to ferredoxin

  • NADP+ reductase catalyzes the final step to form NADPH

  • The flow of electrons pumps protons across the thylakoid membrane, generating a proton-m motive force used by ATP synthase to produce ATP

  • Oxygen is released from water at PSII as a byproduct of photolysis

Key molecules and concepts

  • Photosynthetic pigments: chlorophyll a, chlorophyll b, carotenoids; absorb light and drive electron transfer

  • Electron carriers: NADP+/NADPH, ATP, ADP, Pi (inorganic phosphate)

  • Carbon fixation enzyme: Rubisco, catalyzes the first stable step of the Calvin cycle

  • Energy coupling: photophosphorylation (ATP production via light-driven proton gradient)

Types of photosynthesis and adaptations

  • C3 photosynthesis: classic pathway; occurs in many plants; susceptible to photorespiration under high O₂, low CO₂, heat

  • C4 photosynthesis: spatial separation of steps; CO₂ initially fixed into a 4-carbon compound in mesophyll cells and released in bundle-sheath cells; reduces photorespiration; advantageous in hot, bright environments

  • CAM (Crassulacean Acid Metabolism): temporal separation; stomata open at night to fix CO₂ into organic acids, which are used during the day for the Calvin cycle; efficient water use in arid conditions

Pigments and light absorption

  • Primary pigment: chlorophyll a (absorbs best in red and blue light) with accessory pigments (chlorophyll b, carotenoids) that broaden the spectrum absorbed

  • Light-harvesting complexes collect photons and transfer energy to reaction centers

  • Absorption spectra explain why leaves appear green (reflect green wavelengths)

Limiting factors and regulation

  • Light intensity: drives the rate of the light-dependent reactions

  • CO₂ concentration: substrate for the Calvin cycle; lower CO₂ reduces carbon fixation

  • Temperature: affects enzyme kinetics (Rubisco activity, membrane fluidity, diffusion)

  • Water availability: stomatal closure reduces CO₂ uptake and can limit photosynthesis; excessive water loss risks

  • Photoinhibition: damage to PSII if excess light cannot be safely dissipated

Real-world relevance and applications

  • Photosynthesis underpins agricultural productivity and crop yields

  • Improving light capture, reducing photorespiration, or engineering more efficient Rubisco could boost plant growth

  • Bioenergy and carbon sequestration: plants as sinks and sources of atmospheric CO₂

  • Sustainability and ethics: access to seeds, ecological impact, and equitable benefits

Connections to foundational principles

  • Energy transduction: light energy is converted to chemical energy (ATP, NADPH)

  • Redox chemistry: electron transfer drives proton pumping and NADPH production

  • Metabolic integration: regenerated RuBP allows the cycle to continue; outputs feed into carbohydrate synthesis

  • System-level thinking: chloroplasts interact with mitochondria and overall leaf physiology to balance energy and carbon needs

Key formulas and equations

  • Overall photosynthesis (simplified):
    6 CO2 + 6 H2O + ext{light energy}
    ightarrow C6H{12}O6 + 6 O2

  • Water splitting in the light reactions (simplified):
    2 H2O ightarrow O2 + 4 H^+ + 4 e^-

  • Calvin cycle net (per G3P from CO₂):
    3 CO2 + 6 NADPH + 9 ATP ightarrow G3P + 6 NADP^+ + 9 ADP + 9 Pi

Study tips and quick recap

  • Remember two-stage structure: light-dependent reactions (ATP, NADPH, O₂) and Calvin cycle (CO₂ fixation to sugars)

  • Visualize chloroplast structure: thylakoids in grana for light reactions; stroma for Calvin cycle

  • Distinguish C3, C4, CAM as adaptations to environmental conditions (temperature, light, water availability)

  • Link pigments to energy capture and the efficiency of photosynthesis