Chapter 10: Photosynthesis (yt video)

Chapter 10: Photosynthesis

Overview of Photosynthesis

  • Definition: Photosynthesis is the primary process through which organisms in the biosphere obtain nourishment for cellular processes.

  • Energy Conversion: Organisms capable of photosynthesis convert solar energy into chemical energy.

  • Organisms Involved:

    • Includes plants, algae, some prokaryotes, and specific protists.

    • Autotrophs are organisms that obtain energy without ingesting it. They are known as producers in ecology.

    • Autotrophs synthesize organic molecules from carbon dioxide and inorganic materials.

  • Photoautotrophs: Specific to plants, they utilize sunlight in making larger molecules, requiring energy for endergonic processes.

  • Heterotrophs: Acquire organic materials from other organisms; includes primary and secondary consumers.

  • Importance: Without photoautotrophs, oxygen levels in the biosphere would be insufficient for survival.

Mechanism of Photosynthesis

  • General Process: Converts light energy into chemical energy in the form of food.

  • Chloroplasts:

    • Resemble mitochondria and are thought to have evolved from photosynthetic bacteria.

    • Main site of photosynthesis, particularly in the leaves.

  • Leaf Anatomy:

    • Contains mesophyll cells with chloroplasts, responsible for photosynthesis.

    • Stomata: Microscopic openings for gas exchange; allow oxygen to exit and carbon dioxide to enter.

    • Chlorophyll: Main pigment in chloroplasts, located in thylakoid membranes.

    • Thylakoids: Stacked to form grana; surrounded by stroma (fluid within chloroplasts).

Chemical Reactions of Photosynthesis

  • Photosynthesis Reaction: Opposite of cellular respiration; reactants of photosynthesis are products of respiration.

    • Redux Process: Oxygen and water are oxidized, while carbon dioxide is reduced.

    • Considered an endergonic and anabolic process, as it requires energy to produce larger sugar molecules.

Stages of Photosynthesis

Light Reactions

  • Location: Occur in the thylakoids.

  • Processes:

    • Water is split, producing oxygen.

    • NADP+ is reduced to NADPH and ATP is produced.

    • Called photophosphorylation, as it utilizes light energy.

Calvin Cycle

  • Location: Occurs in the stroma.

  • Functions:

    • Involves the fixation of carbon dioxide and uses ATP and NADPH produced in light reactions.

    • Primary product is glyceraldehyde-3-phosphate (G3P), which can then be converted into glucose.

Energy and Light in Photosynthesis

  • Nature of Light: Light is electromagnetic radiation, traveling in waves with varying wavelengths; shorter wavelengths are more energetic.

  • Photons: The smallest particles of light, carrying energy; absorbed by pigments.

  • Pigments in Chloroplasts: Include chlorophyll, which absorbs specific wavelengths (e.g., red and blue, reflects green).

  • Photometers: Measure light absorption by different wavelengths.

Photosystems in Light Reaction

  • Photosystems: Two types—Photosystem II (PSII) and Photosystem I (PSI).

    • Photosystem II (PSII): Absorbs light best at 680 nm, splits water, producing O2 and electrons for ATP synthesis.

    • Photosystem I (PSI): Absorbs light best at 700 nm, works in electron transport chain after PSII.

    • Electron Transport: Involves linear electron flow, moving electrons through the protein complexes to synthesize ATP and NADPH.

  • Chemiosmosis in the Chloroplast: Chemiosmosis is similar in chloroplasts and mitochondria but occurs in reverse—protons (H+) move from stroma to thylakoid lumen and flow back to stroma to generate ATP.

Detailed Steps of the Light Reactions

  • Excitation of Electrons: Electrons in chlorophyll are excited by photons, moving down the electron transport chain, forming a proton gradient.

  • Cyclic Electron Flow: Some electrons cycle back to PSI to produce more ATP if ATP is needed in greater quantities for the Calvin Cycle.

The Calvin Cycle Explained

  • Carbon Fixation: Involves CO2 binding to ribulose bisphosphate (RuBP) catalyzed by rubisco.

  • Reduction Phase: Conversion of 3-phosphoglycerate (3PG) to G3P using ATP and NADPH.

    • One G3P exits the cycle for glucose synthesis; five G3Ps are used to regenerate RuBP.

  • Importance of ATP and NADPH: Consumed in high amounts; 6 ATP and 6 NADPH are used for every six CO2 molecules integrated into the cycle.

  • Regeneration of RuBP: Requires additional ATP (3 ATP for rearrangement of G3P back to RuBP).

Adaptations in Photosynthesis

  • Photorespiration: Occurs when CO2 is low, and O2 levels are high (stomata closed); rubisco fixes O2 instead of CO2, resulting in no sugar production, wasting resources.

  • Types of Plants:

    • C3 Plants: Fix CO2 through rubisco and typically are found in moist environments.

    • C4 Plants: Utilize a different enzyme (PEP carboxylase) to fix CO2 in mesophyll cells (spatial separation) and then send it to bundle sheath cells for the Calvin Cycle.

    • CAM Plants: Store CO2 during night (temporal separation) and utilize it during the day when stomata are closed, minimizing water loss.

Summary of Photosynthesis

  • Overall Function: Utilize sunlight to convert into organic compounds, which serve as a framework for more complex molecules; oxygen is released as a byproduct.

  • Storage of Energy: Sugars stored in roots, tubers, seeds, and fruits are available for future use.

  • Ecological Impact: Oxygen produced is vital for many organisms in the biosphere.