chapter 8 Photosynthesis

AN INTRODUCTION TO PHOTOSYNTHESIS

• Photosynthesis fuels the biosphere:

  • Autotrophs:

    • Sustain themselves.

    • Examples are plants.

    • Do not consume other organisms; make their own food through photosynthesis.

  • Photoautotrophs:

    • Use light energy to produce organic molecules.

  • Chemoautotrophs:

    • Prokaryotes that use inorganic chemicals as an energy source.

  • Heterotrophs:

    • Consumers that feed on other organisms.

    • Decompose organic material.

  • Importance of Photoautotrophs:

    • Feed, clothe, and house us.

    • Provide energy for warmth, light, transport, and manufacturing.

PHOTOSYNTHESIS IN PLANT CELLS

• Photosynthesis occurs in chloroplasts:

  • Pancreas of plants where photosynthesis actually occurs.

  • Chlorophyll:

    • Light-absorbing pigment responsible for the green color in plants.

    • Converts solar energy to chemical energy.

  • Key Structures:

    • Mesophyll:

      • Interior of the leaf where chloroplasts are located.

    • Stomata:

      • Tiny pores in leaves that allow CO2 to enter and O2 to exit.

    • Veins:

      • Transport water absorbed by roots to the leaves.

    • Stroma:

      • Thick fluid within chloroplasts where the Calvin Cycle takes place.

    • Thylakoids:

      • Interconnected membranous sacs within the chloroplasts, containing chlorophyll.

    • Thylakoid Space:

      • Internal compartment of the thylakoid membranes, site of light reactions.

PHOTOSYNTHESIS PROCESS

• Basic Equation of Photosynthesis:

  • 6 CO2 + 6 H2O
    ightarrow C6H{12}O6 + 6 O2

  • Original discovery: Oxygen produced comes from H2O, not CO2.

• Photosynthesis as a redox process:

  • Redox means oxidation-reduction process.

  • Description of processes:

    • CO2 is reduced to create glucose ($C6H{12}O_6$).

    • Water (H2O) is oxidized to produce oxygen (O2).

TWO STAGES OF PHOTOSYNTHESIS

• Linked by ATP and NADPH:

  1. Light Reactions:

     • Occur in the thylakoid membranes.

     • Water is split, releasing O2 as by-product.

     • ATP is generated as a product.

     • Light energy is absorbed by chlorophyll; energy is transferred to NADP+, reducing it to NADPH.

  2. Calvin Cycle:

     • Occurs in the stroma of the chloroplast.

     • Assembles sugar molecules, incorporating CO2 and utilizing the energy-rich products from light reactions.

     • Known as light-independent or dark reactions since none of the steps require light.

LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

• Electromagnetic energy:

  • The energy in sunlight; includes a spectrum of light wavelengths.

• Photon:

  • Discrete packets of energy; shorter wavelengths have greater energy.

• Plant Pigments:

  • Absorb some wavelengths of light and reflect/transmit others:

    • Chlorophyll a: Absorbs blue-violet and red light; reflects green light.

    • Chlorophyll b: Absorbs blue and orange; appears olive-green due to reflection.

    • Carotenoids: Provide protection by dissipating excess light energy.

PHOTOSYSTEMS

• Functions of Photosystems:

  • Capture solar energy through pigment absorption.

  • Pigments increase the potential energy of electrons, sending them to an unstable state.

• Structure of Photosystems:

  • Consist of light-harvesting complexes surrounding a reaction-center complex.

  • Light-harvesting complex: Contains various pigment molecules bound to proteins that gather light.

  • Solar-powered transfer of electrons is the first step in transforming light energy to chemical energy.

• Types of Photosystems:

  • Photosystem I and Photosystem II work cooperatively in the light reactions.

ELECTRON TRANSPORT CHAIN AND ATP SYNTHESIS

• Connecting Photosystems:

  • Light energy transforms into chemical energy of ATP and NADPH.

  • Electrons are removed from H2O, passed from Photosystem II to Photosystem I, and accepted by NADP+, thus reducing it to NADPH.

  • The electron transport chain facilitates ATP synthesis through chemiosmosis, leading to ATP generation (photophosphorylation).

THE CALVIN CYCLE: REDUCING CO2 TO SUGAR

• Key Ingredients Needed:

  • Atmospheric CO2, ATP, and NADPH (produced in the light reactions).

• Production of G3P:

  • Glyceraldehyde 3-phosphate (G3P) can lead to glucose and other organic molecules, including disaccharides like sucrose.

• Steps in the Calvin Cycle:

  1. Carbon fixation (catalyzed by Rubisco enzyme).

  2. Reduction phase.

  3. Release of one molecule of G3P.

  4. Regeneration of RuBP (ribulose bisphosphate).

EVOLUTION CONNECTIONS: CARBON FIXATION OPTIONS IN CLIMATE VARIATIONS

• C3 plants:

  • First product of carbon fixation is a three-carbon compound (3-PGA).

  • In hot, dry conditions, C3 plants may undergo photorespiration which reduces efficiency.

• C4 plants:

  • Fix CO2 into a four-carbon compound; reduce water loss by closing stomata in heat.

• CAM plants:

  • Open stomata at night; fix CO2 into a four-carbon compound and release it to the Calvin cycle during the day.

GLOBAL SIGNIFICANCE OF PHOTOSYNTHESIS

• Role in Ecosystems:

  • Photosynthesis makes sugar from CO2 and H2O, providing energy and O2 for nearly all living organisms.

  • About 50% of carbohydrates generated by photosynthesis are subsequently used in cellular respiration by plants.

  • Provides materials for building other essential organic molecules and stores excess glucose.

SCIENTIFIC THINKING: IMPACT OF CLIMATE CHANGE ON PLANTS

• Research on CO2 impacts:

  • Studies indicate that rising atmospheric CO2 can enhance plant productivity, but can also lead to adverse effects such as increased toxicity levels in certain plants (e.g., poison ivy).

  • Long-term experiments like FACE monitor the effects of elevated CO2 on ecosystems.

SCIENTIFIC RESEARCH AND OZONE LAYER PROTECTION

• Ozone layer significance:

  • Protects earth from UV radiation; depleted by chlorofluorocarbons (CFCs).

  • International treaties have been implemented to reduce CFC emissions, although recovery of the ozone layer is expected to take several decades.