Biol 103: Lecture 7

Introduction to Photosynthesis

Course Information

  • Course: Biol 103: Introductory Biology I
  • Lecture 7 Topic: Photosynthesis
  • Presenter: Dr. Michael D. Preston
  • Position: Assistant Professor, Ecosystem Science and Management
  • Email: michael.preston@unbc.ca
  • Office Hours: 12:20-1:00 pm Mon/Wed/Fri (Room 6-213) or by appointment

Relevance of Oil Production

  • Oil and gas represent energy derived from sunlight, with carbon originating from the atmosphere around 252-66 million years ago, along with older deposits.

  • Formation of Oil and Natural Gas:

    • Requires an anaerobic environment.
    • Organic matter must be buried and mixed with sediment.
    • Need heating to the range of 90°C - 160°C to produce oil, with higher temperatures yielding natural gas.

Learning Objectives

  • Describe the redox reactions involved in photosynthesis.
  • Summarize the photosynthetic apparatus.
  • Explain how light reactions generate a proton gradient.
  • Describe the Calvin Cycle.
  • Outline photorespiration.
  • Differentiate between C3, C4, and CAM pathways.
  • Compare photosynthesis and cellular respiration.
  • Required Reading: Chapter 6 – Photosynthesis

Types of Organisms

Autotrophs

  • Definition: Organisms that produce their own food.
    • Utilize photosynthesis or chemosynthesis.
    • Examples:
    • Grass
    • Other plants
    • Bacteria and algae (in various environments)

Heterotrophs

  • Definition: Organisms that consume other organisms for energy.
    • Examples:
    • Grasshopper
    • Toad
    • Most animals, bacteria, and fungi

Photosynthesis Overview

  • Definition:
    • Photosynthesis is the process by which light energy is utilized to convert carbon dioxide (CO2) into organic compounds.
    • Photoautotrophs: - The primary producers of Earth, utilizing sunlight to drive the synthesis of organic molecules.
    • Organic molecules consist of C-H bonds and serve as fuel for photosynthetic organisms as well as the primary energy source for heterotrophs.

Autotrophs vs Heterotrophs

Autotrophs

  • Make their required organic molecules from inorganic sources, such as CO2 and water.
  • Photoautotrophs:
    • Use light energy to synthesize organic molecules through photosynthesis.

Heterotrophs

  • Require organic molecules sourced from the food they consume for survival (consumers and decomposers).

The Nature of Light and Electromagnetic Spectrum

  • The sun converts approximately 4 million tonnes of matter into energy each second, emitting electromagnetic radiation that travels to Earth in about 8 minutes.

Interaction with Light

  • Upon striking an object, photons can undergo three potential outcomes:
    1. Reflected: Light bounces back.
    2. Transmitted: Light passes through the object.
    3. Absorbed: Energy is taken up by the object.

Photosynthesis as a Redox Process

  • Overall Reaction:
    6 CO2 + 12 H2O
    ightarrow C6H{12}O6 + 6 O2 + 6 H_2O.
  • Oxidation Process:
    • Water undergoes oxidation, losing electrons to form oxygen:
      2 H2O + ext{light energy} ightarrow O2 + 4 H^+ + 4 e^-.
    • Electron transport continues with CO2 gaining electrons to form organic compounds.

Stages of Photosynthesis

  • Light Reactions (Light-dependent):

    • Occur in the thylakoid membrane; produce ATP and NADPH from sunlight and water.

  • Calvin Cycle (Light-independent):

    • Occurs in the stroma; uses ATP and NADPH to convert CO2 to carbohydrates (monosaccharides).

Chloroplasts and Leaf Structure

  • Leaf Structure:
    • Composed of photosynthetic cells with chloroplasts, vacuoles, and stomata (for gas exchange).
    • Chloroplast Structure:
    • Outer membrane, inner membrane, thylakoids (absorb light and produce energy), and stroma (site for Calvin cycle).

Global Photosynthesis Statistics

  • Approximately 110extbillionkg110 ext{ billion kg} of CO2 are fixed by photosynthetic processes annually; about 50 ext{ ext{%}} of this occurs in oceans.

Light Energy and Absorption in Plants

  • Light is captured by pigment molecules, specifically chlorophylls and carotenoids, which absorb specific light wavelengths.
  • Wavelength and Energy Relationship:
    • Light energy is inversely proportional to wavelength:
      Eextextproportionalto1extwavelengthext(whereextwavelength=extextnm)E ext{ } ext{proportional to} \frac{1}{ ext{wavelength}} ext{ (where } ext{wavelength} = ext{ } ext{nm}).

Why are Plants Green?

  • Plants appear green due to the absorption spectrum of chlorophyll, which captures light in certain wavelengths while reflecting green light.

Photosystem Structure and Function

Components of Photosystem

  • Antenna Complex: Absorbs light energy and transfers it to the reaction center.
  • Reaction Center: Special chlorophyll a pigment that donates electrons to electron transport carriers.

Types of Photosystems

  • Photosystem II (P680): Captures light and begins water splitting, producing oxygen.
  • Photosystem I (P700): Steps in transferring electrons to NADP+ to form NADPH.

Cyclic Electron Transport

  • Allows ATP production to catch up with NADPH during light reactions, ensuring energy balance.

The Calvin Cycle: Key Processes

  • Location: Stroma of chloroplasts.
  • Key Enzymes: Rubisco, facilitated conversion of CO2 to G3P (glyceraldehyde 3-phosphate) and the regeneration of RuBP (ribulose bisphosphate).

Photorespiration

  • Occurs when oxygen binds to Rubisco's active site, competing with CO2, making the process less efficient.
Causes of Photorespiration
  • High levels of O2 and low levels of CO2; especially significant with modern atmospheric conditions.

Solutions to Photorespiration

  • Mechanisms to increase CO2 availability include:
    • CO2 pumping in aquatic plants.
    • C4 carbon transport in terrestrial plants.
    • CAM pathways in desert plants.

C4 Pathway and Adaptation

Leaf Anatomy
  • C4 plants have distinct leaf anatomy with mesophyll and bundle-sheath cells for efficient CO2 capture and Calvin cycling in low O2 conditions.
C4 Metabolic Pathway
  • Calvin Cycle Dynamics:
    • CO2 combines with PEP to produce oxaloacetate, which eventually leads to the production of sugars while minimizing photorespiration.

CAM Pathway in Desert Plants

  • Temporal Separation:
    • Stomata open at night to capture CO2, which is stored as malate and used during the day for photosynthesis.

Overall Summary of Photosynthesis

  • Contrast between Photosynthesis and Cellular Respiration focusing on reactants, products, and the direction of reactions with climate impacts.

Key Questions

  • Discuss the adaptive significance of the C4 and CAM pathways.
  • Understand the competitive inhibition role of O2 on the active site of Rubisco.

Conclusion and Exam Preparation

  • Key concepts and terms to be reviewed include redox reactions, phases of photosynthesis, and differences between paths and their implications, relating back to adaptive functionalities of plants in diverse environments.