topic 9

Biology 107, Fall 2025

Topic 9: Photosynthesis Lecture Notes

This lecture outline is designed to help you take notes as you progress through the lecture materials for Topic 9:

1. Introduction to Photosynthesis

2. The Photosystems

3. Linear Electron Flow

4. Chemiosmosis

5. Cyclic Electron Flow

6. The Calvin Cycle

7. Putting It All Together!

Learning Objectives:

This topic will introduce you to photosynthesis, the process that plants and other photoautotrophic organisms use to convert light energy into chemical energy in the form of carbohydrates. At the end of this lecture topic, you should be able to:

• Write a summary reaction for photosynthesis.

• Identify the types of organisms that can perform photosynthesis.

• Identify the parts of the chloroplast and explain which processes of photosynthesis occur in each location.

• Distinguish between the light reactions and the Calvin cycle.

• Describe the types of pigments found in chloroplasts and explain the roles they play in photosynthesis.

• Explain what happens to electrons when an atom absorbs light energy.

• Describe the three ways that an excited electron can release or transfer its energy. Identify where each of these three processes occurs in photosynthesis.

• Explain the functions of an electron acceptor and an electron donor.

• Describe the composition of the light-harvesting complexes and the reaction center complex, and explain how they are combined to form a photosystem.

• Distinguish between the properties and functions of Photosystem II and Photosystem I.

• Trace the movement of electrons through linear electron flow and describe what happens to the electrons at each step.

• Explain the role of H2O in linear electron flow.

• Identify the high-energy molecules produced from linear electron flow.

• Describe the process of chemiosmosis and explain how it produces ATP from linear electron flow.

• Explain why cyclic electron flow is important to photosynthesis, and describe the conditions in which this type of electron flow would occur.

• Trace the movement of electrons through cyclic electron flow and describe what happens at each step.

• Identify the similarities and differences between linear and cyclic electron flow.

• Define the term carbon fixation.

• Trace the movement of carbon atoms through the three phases of the Calvin Cycle.

• Explain in general what happens in each phase of the Calvin Cycle (you do not need to memorize the chemical reactions).

• Describe the roles of ATP and NADPH in the Calvin Cycle.

• Name and describe the major product of the Calvin Cycle.

• List the possible fates of G3P once it has been synthesized in the chloroplast.

Background Reading for Topic 9 Lecture Notes:

The information covered in these lecture videos is based on your textbook, Biology: Exploring the Diversity of Life:

• Chapter 6, Sections 6.1 through 6.4 (5th Ed. pages 136-146; 4th Ed. pages 126-137)

For additional supplementary reading on these topics, you can consult the following websites:

• https://openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis

• https://openstax.org/books/biology-2e/pages/8-2-the-light-dependent-reactions-of-photosynthesis

• https://openstax.org/books/biology-2e/pages/8-3-using-light-energy-to-make-organic-molecules

• https://www-jove-com.ezproxy.macewan.ca/science-education-library/174/photosynthesis

Topic 9.1: Introduction to Photosynthesis

• Photosynthesis: Process that converts light energy into chemical energy (sugars)

  • Summary Reaction:
    $CO<em>2 + H</em>2O + ext{light} <br>ightarrow [C<em>nH</em>{2n}O<em>n] + O</em>2$

  • Done by photoautotrophs

  • Energy from sunlight, carbon from CO2

  • Organisms: Plants, algae (protists), some bacteria

  • Occurs in chloroplasts in eukaryotes

Phases of Photosynthesis:

1. Light Reactions: Energy conversion

   • Sunlight absorbed by thylakoid membranes

   • Energy converted into ATP and NADPH (No sugar produced)

   • H2O broken down and O2 produced

   • Site: Thylakoid membrane

2. Calvin Cycle: Carbon fixation

   • CO2 is incorporated into complex carbon molecules (sugars)

   • Uses energy from ATP and NADPH (from light reactions)

   • Does NOT require light

   • Site: Stroma

Topic 9.2: The Photosystems

• Reminder: Light-Dependent Reactions

  • Convert sunlight into chemical energy (ATP and NADPH)

  • Occurs in the thylakoid membrane

Photosynthetic Pigments

• Pigments in chloroplasts are used to absorb light energy

  • Absorption spectrum:

• Chlorophyll structure:

  • Porphyrin ring absorbs light

  • Hydrocarbon tail anchors pigment in thylakoid membrane

Application Question 1:

• Why do purple plants look purple?

  • Contain more anthocyanin (absorbs green light) than chlorophyll (reflects green light)

  • Still use chlorophyll for photosynthesis, but not all blue/red light is absorbed

• What chemical bonding arrangement would anthocyanin and chlorophyll have in common?

  • Covalent bonds

  • Able to absorb visible light

  • Absorbed light excites electrons

• Why are purple plants commonly found in equatorial regions but are not native to Canada?

  • Anthocyanin thought to be plant sunscreen

  • Absorbs excess light energy

  • Too much light can damage the photosystems and reduce the efficiency of photosynthesis.

  • Purple plants are more common in areas that get a lot of sunlight – but not in Canada!

Excitation of Electrons

• When light is absorbed, electrons in porphyrin ring of chlorophyll are excited.

Three Ways Electrons Can Release or Transfer Energy:

1. Fluorescence:

   • Electron falls back to ground state

   • Releases energy as heat and light

   • Occurs only in isolated pigment molecules

2. Resonance Transfer:

   • Excited electron transfers its energy to another electron in a nearby chlorophyll molecule

   • Transfer of energy only

3. Electron Transfer:

   • High energy electron is transferred to a nearby molecule (electron acceptor)

   • Both energy and electron transferred

   • Low energy electron is taken from an electron donor to fill the electron “hole”

   • Electron donor in photosynthesis is water

The Photosystems

• Light-harvesting unit located in the thylakoid membrane

• Contains:

  1. Several light-harvesting complexes (Antenna complexes)

  • Many chlorophyll molecules attached to proteins

  • Gather light energy and transfer it by resonance

  1. One reaction centre complex in the middle

  • Mostly protein

  • Special chlorophyll a molecule accepts energy from light-harvesting complexes

  • Primary electron acceptor accepts excited electrons from the special chlorophyll a

Two Types of Photosystems:

• Different composition of reaction centre complexes

Topic 9.3: Linear Electron Flow

• Linear electron flow transfers energy absorbed from light to energy storage molecules (ATP and NADPH)

Steps of Linear Electron Flow:

1. Chlorophyll molecules in light harvesting complexes of PSII absorb photons of light:

   • 1 photon excites one electron

   • Energy transferred from one chlorophyll to another by resonance transfer

   • Energy reaches the reaction centre chlorophyll a (P680) in PSII and excites its electrons

   • Energy transfer, NOT electron transfer

2. P680 (PSII reaction centre chlorophyll a) transfers its high energy electron to the primary electron acceptor

   • Electron transfer occurs, maintaining high energy form of the electron

3. P680 chlorophyll now has an electron ‘hole’ that must be filled by an electron donor

   • Electron donor is water

     • e– fill P680 electron hole

     • O2 is released (1 O2 per 2 H2O)

     • H+ is released into thylakoid space and contributes to proton gradient

4. Excited electrons move from the primary electron acceptor of PSII through an electron transport chain (ETC)

   • Protein complexes in thylakoid membrane

5. Movement of electrons down the ETC releases energy at each step:

   • Exergonic process

   • Energy is used to make ATP through chemiosmosis

   • Components of the ETC:

     • Plastoquinone (Pq)

     • Cytochrome complex (cyt)

     • Plastocyanin (Pc)

6. In the meantime, chlorophyll a molecules in light-harvesting complexes of PSI absorb photons of light:

   • Transfer their energy to the P700 chlorophyll a of PSI by resonance transfer

   • P700 transfers its high energy electron to the primary electron acceptor of PSI

   • P700 now has an electron hole that must be filled

   • The low-energy electron released from the bottom of the ETC (Pc) fills the electron hole in P700

7. Excited electron moves from the primary electron acceptor of PSI through another ETC:

   • Ferredoxin (Fd) → NADP+ reductase

   • No chemiosmosis (no ATP)

8. NADP+ reductase transfers 2 electrons from ferredoxin to NADP+

   • Reduces NADP+ to NADPH in the stroma

   • Electrons in NADPH are much higher in energy than when they were stripped from H2O

Practice Question:

• Place the following locations in order to correctly outline the path of an electron as it moves through the light-dependent part of photosynthesis:

  • Cytochrome complex

  • NADP+

  • P700

  • H2O

  • P680

  • Plastocyanin

Topic 9.4: Chemiosmosis

• What is Chemiosmosis?

  • Makes ATP using energy from a proton gradient

  • Uses enzyme called ATP synthase

  • Protons diffusing through ATP synthase provide energy to form ATP from ADP + inorganic phosphate (Pi)

How Energy Gets from High Energy Electrons in PSII to ATP:

1. Energy is released during movement of electrons down the ETC from PSII

   • Plastoquinone uses the energy to actively transfer H+ into the thylakoid space from the stroma

2. Concentration gradient of protons is built up across the thylakoid membrane

   • High [H+] in thylakoid space

   • Low [H+] in stroma

3. Protons diffuse out of the thylakoid into the stroma through ATP synthase

   • Released energy causes ATP synthase to rotate

4. ATP is made in the stroma from ADP + Pi

   • Called photophosphorylation because energy originally came from light energy

• H+ released into the stroma can be:

  • Added to NADP+ to make NADPH

  • Actively transported back into the thylakoid space

Topic 9.5: Cyclic Electron Flow

• Why is it used?

  • Linear electron flow creates equal amounts of ATP and NADPH

  • Calvin cycle uses more ATP than NADPH, creating an imbalance

  • Cyclic flow allows cell to make extra ATP without making more NADPH

  • Use is controlled by ATP and NADPH levels in the chloroplast

What Does Cyclic Electron Flow Do?

• Uses only PSI

• Produces ATP but NO NADPH

Steps of Cyclic Electron Flow:

1. Electrons in P700 are excited and transferred to PSI primary electron acceptor

2. High energy electrons move to ferredoxin

3. Electrons move backwards to plastoquinone

   • Protons are pumped and ATP is produced by chemiosmosis

4. Electrons move down ETC to plastocyanin (Pc), and then back to P700 to fill the electron hole

   • Electrons are recycled

Topic 9.6: The Calvin Cycle

• Occurs in the stroma

• No light involved

• Synthesizes sugars using energy from the light reactions (ATP and NADPH)

• A carbon fixation process

  • CO2 is incorporated into G3P (Glyceraldehyde-3-phosphate)

  • An endergonic and exergonic process

  • Products (sugar) have more free energy than reactants (CO2)

  • ATP and NADPH provide the energy needed to make the sugar

G3P Produced in Calvin Cycle:

• The carbohydrate produced is glyceraldehyde-3-phosphate (G3P)

  • A 3 carbon sugar

  • High energy

  • Need 3 molecules of CO2 to be fixed to make 1 G3P (3 turns of the cycle)

Overall Formula for 3 Turns of the Cycle:

$3 CO_2 + 9 ATP + 6 NADPH <br>ightarrow 1 G3P$

Phases of the Calvin Cycle:

1. Carbon Fixation:

   • RUBISCO:

     • Ribulose Bisphosphate Carboxylase-Oxygenase

     • Enzyme that catalyzes reaction between CO2 and RuBP

     • RuBP = Ribulose bisphosphate (5C sugar)

2. Reduction:

   • Role of ATP: Phosphates added to reactants provide energy to drive reactions

   • Role of NADPH: Each molecule donates 2 electrons for reduction of reactants

   • Produces 6 high-energy G3P:

     • 1 G3P is released for use (net gain = 3 C)

     • 5 G3P are used to regenerate RuBP (15 C reused)

3. Regeneration:

   • 3 CO2 + 3 RuBP → 6 3-phosphoglycerate → 6 G3P + 6 ATP → 5 G3P + 3 ATP → 3 RuBP

     • G3P to RuBP regeneration completed with 3 RuBP available to fix 3 more CO2 for the next cycle

What Happens to G3P?

• Can be used to make:

  • Carbohydrates such as glucose

    • Fuel for cellular respiration

  • Energy storage compounds, like starch

  • Cell wall constituents, like cellulose

Application Question 2:

• Atrazine: A commonly used herbicide in the U.S. blocks electron transfer from Photosystem II to plastoquinone during light reactions.

  • Poll: Would a plant cell exposed to Atrazine be able to produce NADPH using excited electrons from Photosystem I?

    • Yes or No?

    • Answer: Yes; electrons from PSI absorb light energy, but calvin cycle requires fixed carbon.

  • Explain why a susceptible plant would die following exposure to Atrazine:

    • Can’t make NADPH (no reduced NADP+)

    • Can’t perform Calvin Cycle because no fixed carbon is available

    • No carbon or energy to make cell structures and carry out functions

Poll Question:

• Would a plant cell exposed to Atrazine be able to carry out cyclic electron flow to produce ATP?

  • Yes or No?

  • Answer: No; electrons not passed from PS2, as they are supplied by ferredoxin, reducing efficiency of ATP synthesis.

Topic 9.7: Putting It All Together!

Summary of Photosynthesis:

• Light Reactions = Energy Conversion

  • Occur in thylakoid membrane

  • Light energy absorbed by chlorophyll

  • H2O is split by PSII (produces O2)

  • ATP produced by PSII and chemiosmosis

  • NADPH produced by PSI

  • Exception: In cyclic electron flow, ATP but no NADPH is produced by PSI

• Calvin Cycle = Carbon Fixation

  • Occurs in stroma

  • ATP and NADPH used as energy source

  • CO2 incorporated into the high-energy sugar G3P