FY

Third Canadian Edition: Biological Science

Authors

  • Freeman, Quillin, Allison, Black, Podgorski, Taylor, Harrington, Sharp

Course

  • BLG143: Biology I (Fourth Canadian Edition)

  • Faculty of Science

  • Department of Chemistry & Biology

  • Toronto Metropolitan University

BLG143 Midterm Information

  • Date: Wednesday, October 29, 2025

  • Time: 6:00 PM – 8:00 PM; Please arrive 15 – 20 minutes early

  • Room locations: Assigned by section; to be posted on D2L

  • Midterm Coverage: Content from Weeks 1 through 6 (Lectures 1 through 12)

  • Exam Format: Approximately 40 multiple choice and 4-5 short answer questions

  • Permitted Items:

    • TMU OneCard

    • Pen, pencil, whiteout and/or eraser

    • Cheat Sheet:

    • Size: 8 ½ by 11

    • Format: One-sided & handwritten ONLY

  • Note: No electronic devices permitted!

Chapter 10: Photosynthesis

Introduction to Photosynthesis

  • Definition: Photosynthesis is the process by which organisms use sunlight to manufacture carbohydrates.

  • Organisms:

    • Autotrophs: Organisms that use photosynthesis, defined as “self-feeders.”

    • Heterotrophs: Non-photosynthetic organisms that obtain sugars from other organisms (e.g., humans are heterotrophs).

Types of Photosynthetic Organisms

  1. Photosynthetic Multicellular Eukaryotes

    • Examples include most land plants and seaweeds (plant-like protists).

  2. Photosynthetic Unicellular Eukaryotes

    • The most studied organism in this category is the Euglena.

  3. Photosynthetic Prokaryotes

    • Includes cyanobacteria, purple sulfur bacteria, and heliobacteria.

Photosynthesis: Energy Conversion

  • Photosynthesis converts electromagnetic energy from sunlight to chemical energy stored as carbohydrates.

  • Requirements: Sunlight, carbon dioxide, and water.

  • By-product: Produces oxygen.

  • Overall Reaction:
    ext{CO}2 + ext{H}2 ext{O} + ext{light energy}
    ightarrow ext{C}6 ext{H}{12} ext{O}6 + ext{O}2

  • This process is the opposite of cellular respiration.

Two Linked Sets of Reactions in Photosynthesis

  1. Light-dependent Reactions

    • Produce O₂ from H₂O by splitting water molecules to form O₂ gas.

    • Light energy excites electrons, which are transferred to the electron carrier NADP⁺, forming NADPH.

    • ATP is also produced during this stage.

  2. Calvin Cycle

    • Reactions that produce sugars from CO₂ using the electrons and ATP generated in the light-dependent reactions.

Location of Photosynthesis

  • Photosynthesis occurs primarily in the chloroplasts of green plants, algae, and other photosynthetic organisms.

  • Chloroplast Structure:

    • Surrounded by two membranes with an interior filled with thylakoids, which are flattened, vesicle-like structures forming stacks called grana.

    • The space inside a thylakoid is called its lumen; the space surrounding thylakoids is referred to as the stroma.

Photosynthetic Pigments and Light Absorption

  • Function of Pigments:

    • Pigments absorb specific wavelengths of light and reflect or transmit others.

    • The most common pigment is chlorophyll, responsible for the green color of plants and algae by reflecting green light.

  • Types of Pigments:

    • Chlorophylls (a & b):

    • Absorb red and blue light; reflect and transmit green light.

    • Carotenoids:

    • Absorb blue and green light; reflect and transmit yellow, orange, and red light.

The Electromagnetic Spectrum

  • Displays the range of wavelengths of electromagnetic radiation.

  • Visible Light: The portion of the electromagnetic spectrum visible to humans.

  • Photon Energy: The energy of a photon is inversely proportional to its wavelength, where shorter wavelengths possess higher energy.

Pigment Absorption and Action Spectrum

  • Each pigment has an absorption spectrum, plotting the wavelengths of light absorbed.

  • Action Spectrum: Illustrates the rate of photosynthesis at various wavelengths of light; pigments absorbing blue and red light are most effective for photosynthesis.

Experimental Setup to Determine Effective Wavelengths

  • Experiment by Engelmann (1882):

    • Algal cells exposed to different wavelengths in the presence of oxygen-seeking bacteria.

    • Conclusion: Pigments absorbing violet-to-blue and red wavelengths most effectively drive photosynthesis.

Pigment Structure

  • Chlorophyll molecules feature a long “tail” that embeds within the thylakoid membrane and a “head” structure containing a magnesium atom where light absorption occurs.

Accessory Pigments

  • Role of Carotenoids and Xanthophylls:

    • Accessory pigments absorb light and transfer energy to chlorophyll, allowing photosynthesis to utilize a broader range of wavelengths.

Excitation of Electrons

  • When a photon strikes chlorophyll, its energy promotes an electron to a higher energy state.

    • Photon Types:

    • Red photons bump an electron up one energy level.

    • Blue photons bump an electron up two energy levels.

    • Green photons are not absorbed and do not promote electrons.

Fluorescence

  • Occurs when excited electrons return to the ground state, emitting light. Approximately 2% of red and blue photons produce fluorescence, while 98% are utilized for photosynthesis.

Photosystems

  • Formation of Photosystems: Chlorophyll molecules group together to form a photosystem, consisting of:

    1. An Antenna Complex: Composed of 200-300 chlorophyll and accessory pigments, transmitting energy via resonance energy transfer.

    2. A Reaction Centre: Where the excited electrons are transferred to an electron acceptor, converting electromagnetic energy into chemical energy.

Types of Photosystems

  1. Photosystem I (PSI)

  2. Photosystem II (PSII)

  • Both systems ultimately enhance photosynthesis efficiency when activated by different light wavelengths.

The Z Scheme

  • Describes the interaction between PSI and PSII during photosynthesis:

    1. Photons excite PSII electrons, which are processed through an electron transport chain (ETC).

    2. Water splitting replenishes the missing electrons in PSII.

    3. A proton gradient develops, facilitating ATP production while NADPH is generated through PSI.

Cyclic Electron Flow and Photophosphorylation

  • Cyclic electron flow occurs when PSI transfers excited electrons back to the ETC, increasing ATP production, and coexists alongside linear electron flow.

The Role of Oxygenic Photosynthesis

  • Oxygen released from this process was vital for the evolution of advanced life forms and the atmosphere as we currently know it, particularly through the action of cyanobacteria.

Check Your Understanding and Learning Objective

  • Questions: Exploring the role of water in replenishing electrons in PSII and the consequences of different experimental setups.

  • Learning Objective:

    • Summarize the process of photosynthesis.

    • Analyze how pigment structures help acquire energy from light.

    • Explain the role of photosystems in converting light energy to chemical energy.