Photosynthesis in Higher Plants - Unit 4 Review

UNIT 4: Structure and Physiological Variation of Living Organisms

  • This unit discusses biological variation over time through two perspectives:
    • Organismic level (ecology, related disciplines)
    • Cellular and molecular level (physiology, biochemistry)

PLANT PHYSIOLOGY

  • Focuses on:
    • Photosynthesis
    • Respiration
    • Plant growth and development
  • Discusses physiological processes in relation to the environment

MELVIN CALVIN

  • Born April 1911 in Minnesota
  • Ph.D. in Chemistry from the University of Minnesota
  • Professor at the University of California, Berkeley
  • Contributed to understanding photosynthesis post-WWII using radioactive carbon ($C^{14}$)
  • Proposed the mechanism of how plants convert light energy into chemical energy
  • Awarded the Nobel Prize in 1961 for mapping carbon assimilation in photosynthesis
  • His research is foundational for renewable energy studies

PHOTOSYNTHESIS IN HIGHER PLANTS

  • Green plants are autotrophs, synthesizing food via photosynthesis, essential for all life forms.
    • Photosynthesis: Light-driven synthesis of organic compounds (mainly glucose) from CO2 and H2O.
  • Two Types of Organisms:
    • Autotrophs: produce their food (e.g., green plants)
    • Heterotrophs: depend on others for food
  • Importance of Photosynthesis:
    • Primary source of food and oxygen on Earth

What Do We Know?

  1. Experiments Demonstrating Photosynthesis:
    • Chlorophyll Requirements: Green parts of leaves perform photosynthesis when exposed to light and CO2.
    • Priestley’s Experiment: Mint plant in a bell jar demonstrates plants restore air quality.
    • Ingenhousz's Experiment: Identified that sunlight is required for oxygen production by plants.
    • Sachs' Contribution: Showed glucose production correlates with starch formation in green parts.
    • Engelmann's Experiment: Developed action spectrum showing which light wavelengths facilitate photosynthesis.

PHOTOSYNTHESIS REACTIONS

  1. Light Reactions:

    • Occur in thylakoid membranes of chloroplasts.
    • Key products: ATP, NADPH + H+.
    • Excited electrons travel through two photosystems (PS I and PS II).
    • Water is split, releasing oxygen.

  2. Dark Reactions (Calvin Cycle):

    • Occur in the stroma of chloroplasts.
    • Involves fixation of CO2 into sugars.

  3. Chemiosmotic Hypothesis:

    • Describes how ATP is formed through proton gradients across thylakoid membranes.

PIGMENTS IN PHOTOSYNTHESIS

  • Four primary pigments:
    • Chlorophyll a (blue-green)
    • Chlorophyll b (yellow-green)
    • Carotenoids (yellow, yellow-orange)
    • Xanthophylls (yellow)
  • These pigments absorb light and transfer energy to chlorophyll a.

PHOTOSYNTHESIS PATHWAYS

  1. C3 Pathway:

    • First product: 3-phosphoglycerate (PGA).
    • Occurs in mesophyll cells using the enzyme RuBisCO.

  2. C4 Pathway:

    • First product: Oxaloacetic acid (OAA).
    • More efficient in hot, dry environments.
    • Uses PEP carboxylase in mesophyll cells; Calvin cycle occurs in bundle sheath cells.

FACTORS AFFECTING PHOTOSYNTHESIS

  • Factors:
    • Light Quality, Intensity, Duration: Affects photosynthesis rates; light saturation occurs at 10% of full sunlight.
    • Carbon Dioxide Concentration: Essential for photosynthesis; C4 plants have higher CO2 saturation.
    • Temperature: Influences dark reactions predominantly; C4 plants tolerate higher temperatures better.
    • Water Availability: Critical; water stress can limit CO2 availability due to stomatal closure.

SUMMARY OF PHOTOSYNTHESIS

  • Involves two stages:
    1. Light Reactions: Convert light to chemical energy (ATP and NADPH).
    2. Calvin Cycle: Fixes CO2 into sugars using ATP and NADPH from light reactions.