Photosynthesis Lecture Notes
Exam Information
Course: BLG143: Biology I
Date: Wednesday, October 29th, 2025
Time: 6:00 PM – 8:00 PM (Arrive 15-20 minutes early)
Coverage: Weeks 1-6 (Lectures 1-12)
Format: ~40 multiple choice questions and 4-5 short answer questions
Permitted Items: TMU OneCard, pen, pencil, whiteout/eraser, one-sided handwritten cheat sheet (8.5" by 11")
Prohibited Items: Electronic devices
Introduction to Photosynthesis
Definition: Photosynthesis is the process of converting sunlight into carbohydrates (energy).
Types of Organisms:
Autotrophs: Photosynthetic organisms (self-feeders). Use photosynthesis.
Heterotrophs: Non-photosynthetic organisms (different-feeders), obtain sugars from other organisms (e.g., humans).
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Types of Photosynthetic Organisms
Multicellular Eukaryotes: Include most land plants and seaweeds (plant-like protists).
Unicellular Eukaryotes: Best studied example is Euglena.
Prokaryotes: Include cyanobacteria, purple sulfur bacteria, and heliobacteria.
Photosynthesis Process
Energy Conversion: Converts electromagnetic energy (sunlight) into chemical energy. Requires sunlight, carbon dioxide ($CO2$), and water ($H2O$), producing oxygen ($O_2$) as a by-product.
Overall Reaction:
{6CO2 + 6H2O + light
ightarrow C6H{12}O6 + 6O2}Opposite to Cellular Respiration.
Energy demanding
Two Linked Sets of Reactions
Light-Dependent Reactions:
Produce $O2$ from $H2O$.
Water is split to form $O_2$ (gas); electrons excited by light energy.
Produces NADPH and ATP.
High energy electrons are transferred to the electron carrier NAD+, forming NADPH
ATP is also produced.
Calvin Cycle:
Produces sugar from $CO_2$, using electrons and ATP from the light-dependent reactions.
Photosynthesis Location
Occurs in chloroplasts of plants, algae, and other photosynthetic organisms.
Structure includes:
Two Membranes surrounding the chloroplast.
Thylakoids: Flattened sac-like structures that form stacks called grana.
Interior
Stroma: Space surrounding thylakoids.
Space inside thylakoid is lumen
Photosynthetic Pigments
Function: Molecules that absorb specific wavelengths of light (most common is chlorophyll, which reflects green light).
Thylakoid membrane contain large quantities of pigments.
Multicelluler and unicellular eukaryoutes photosynthesis takes place in the chloroplast.
In prokaryotes, the entire cell is involved in this process.
Photosynthetic pigments absorb light.
The electromagnetic radition is a form of energy
Light is a type of electromagenetic energy radiation
Acts both wavelike and particle like
As a wave light can be charachterized by its wavelenghts.
As a particle, light exists in discrete packets called photons, each carrying a specific amount of energy dependent on its wavelength.
The electromagnetic spectrum
Displays the range of wavelength of electro magnetic radiation
Visible light is the portion humans can see
Each photon and wavelength has a specific amount of energy
The energy of a photon is inversely proportional to its wavelengths
Shorter wave length have more energy than longer wavelenghts.
Photosynthetic pigments absorb light
Photons may be absorbed, transimitted, or reflected when they an object.
Chromatography separates different pigments from plants.
**Types of Pigments:
Chlorophyll a & b: Absorb red and blue light; reflect green.
Carotenoids: Absorb blue and green light; reflect yellow, orange, and red.
Diffeent pigmrent absorb differnt waveleghts of lights
Each pigment has an absorbtion spectrum that determines the specific wavelengths of light it can capture to facilitate the process of photosynthesis.
Each pigment has unique absorption characteristics; effectiveness at driving photosynthesis is determined by the wavelengths absorbed.
an action spectrum show the rate of phptosynthesis at each wavelength.
excited electrons are unstable
Pigments that absorb blue and red photons are the most effective at driving photosynthesis.
Chlorophylls are the main photosynthetic pigment.
Structure of chlorophyll
Chlorophyll a and b are similar in structure and absorption spectra
They have long tail, keep the molecule embedded in thylakoid membrane
A head consisting of large ring structure with a magnesium atom in the middle
Light is absorbed in the head.
What is the role of Caortenoids and other Accessory Pigments?
Carotenoids and xanthophylls are accessory organs found in chloroplasts
Absorbed light and pass energy to chlorophyll
Since they absorb wavelength of light not absorbed by chlorophyll, they appear yellow, orange and red.
They extend range of wavelength that can drive photosynthesis.
Light Absorption and Excitation of Electrons
Photon Interaction: When photons strike chlorophyll, energy excites electrons to higher states.
Types of Photons:
Red photons bump electrons 1 energy level; blue photons bump electrons 2 energy levels.
Green photons are not absorbed.
Excited electrons are unstable
Fluroescence ocur when the electrons emit light as it falls back to its ground state.
About 2% of absorbed red and blue photons in chlorophyll produce fluorescence .
98% drive photosynthesis.
Photosystems in Photosynthesis
Chiorophyll work together ingroups and they form complex called Photosystem.
Photosystems: Composed of a reaction center and antenna complex that houses chlorophyll and other pigments.
Annetena complex is composed of 200-300 chlorophyll molecules, as well as other accessory pigments such as carotenoids.
When pigment is in antenna complex absorb photons.
The energy but not the the electrons, is passed to to nearby chlorophyll molecule.
After transferring its energy, the electrons falls back to its ground state.
Energy is trnsferred from one molecule to the next ubtil it reaches reaction center. In the reaction centre, excited electrons are transferred to a speacilaized chlorophyll , transfers an excited molecule to electron acceptor, this is where electromagnetic energy is transformed to chemical energy.
Photosystem I (PSI) and Photosystem II (PSII) work together to facilitate photosynthesis.
They produce enhancement efect.
Pheyophytin is the primary electron acceptor in Photosystem II, playing a crucial role in capturing the high-energy electrons generated by the excitation of chlorophyll before they are transferred through the electron transport chain. In addition, the energy released during this electron transport is used to pump protons into the thylakoid lumen, creating a proton gradient that ultimately drives ATP synthesis via ATP synthase.
Z Scheme: Model illustrating the interaction between PSI and PSII; enhancement effect observed when both systems operate together at maximum rates.
Electron Transport Chain (ETC)
Function in Thylakoids: Cy
Photons excite electrons that are then shuttled via plastoquinone (PQ) to the cytochrome complex, creating a proton gradient that drives $ATP$ production via ATP synthase (chemiosmosis).
Photophosphorylation: Energy from light used to form $ATP$.
Water Splitting: Replenishes PSII’s electrons; releases $O_2$ as a byproduct (oxygenic photosynthesis).
Role of NADP+
Reduction to NADPH: PSI transfers electrons to NADP+ to become NADPH, which is utilized in the Calvin Cycle.
Cyclic vs Linear Electron Flow
Linear Electron Flow: Electrons flow continuously from water to NADP+ in a linear fashion.
Cyclic Electron Flow: PSI can recycle electrons back through the ETC, increasing $ATP$ production alongside linear flow.
Importance of Oxygenic Photosynthesis
O2 produced from photosynthesis was critical for the evolution of life; prior, atmospheric O2 was virtually nonexistent.
Cynanobacteria, which are among the earliest organisms to perform oxygenic photosynthesis, played a vital role in transforming the Earth's atmosphere and paving the way for complex life forms to thrive.
Summary of Key Learning Objectives
Understand the process of photosynthesis: pigment structure, organization, and energy acquisition.
Know how photosystems utilize light energy to produce chemical energy (NADPH and ATP).
Analyze the biochemical processes involved in different photosynthetic reactions, including the light-dependent reactions and the Calvin Cycle.