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.