BIO 217 B S15 - ST

Photosynthesis

Learning Outcomes

  • Define Key Terms:

    • Heterotrophs: Organisms that obtain food from autotrophs.

    • Autotrophs: Organisms that produce food from simple compounds (e.g., CO2 and water) using an energy source.

  • Photosynthesis Overview:

    • Understand photosynthesis as a form of autotrophy.

    • Explain key photosynthetic structures and processes in plants, including light reactions and light-independent reactions (Calvin cycle).

    • Describe the role of different photosynthetic pigments and cyclic electron flow in ATP production.

Photosynthesis: Introduction

  • Definition:

    • Photosynthesis converts solar energy into chemical energy, where "photo" means light and "synthesis" means to produce.

  • Nutrition Mode:

    • Organisms categorized as:

      • Heterotrophs: Obtain food from others (e.g., humans, animals).

      • Autotrophs: Produce food from simple compounds like CO2 and water (e.g., most plants, algae).

        → Photoautotrophy: The process of producing food using solar energy.

Importance of Photosynthesis

  • Oxygen Production: Main source of O2 in the atmosphere.

  • Fossil Fuels: Majority of fossil fuel sources are derivates of ancient photosynthesis.

  • Organism Examples:

    • Heterotrophs: Animals, fungi, some prokaryotes, protists.

    • Autotrophs: Almost all plants, algae, some prokaryotes, and protists.

      • bacteria do the most photosynthesis

Photosynthetic Structures in Plants

  • Chloroplasts:

    • Structurally and genetically similar to photosynthetic bacteria, potentially evolved from them.

    • Located mainly in leaves, but also in other green plant areas.

  • Chloroplast Structure:

    • Enveloped by two membranes, surrounding a dense fluid called stroma.

    • Contains thylakoids, organized in stacks called granum.

Photosynthetic Pigments

  • Chlorophyll a: Main photosynthetic pigment (CH3)

  • Chlorophyll b: Broadens the light absorption spectrum (CHO)

    • makes photosynthesis possible

  • Accessory Pigments (Carotenoids): Absorb light energy and protect chlorophyll from damage.

  • Function: Pigments are crucial for absorbing light energy used in photosynthesis.

→ green light is not useful for photosynthesis, it is transmitted

→ light reaction are similar to oxidative phosphorylation into thee mitochondrion, they happen inside the phospholipids molecules of inner membrane of chloroplast

Photosynthesis as a Redox Process

  • Redox Reaction:

    • Photosynthesis involves oxidation of water (H2O) and reduction of CO2 to glucose.

  • Energy Source: Light energy is utilized to convert CO2 into organic molecules.

The Calvin Cycle

Calvin Cycle is the core biochemical cycle in photosynthesis

  • Location: Takes place in the chloroplast stroma.

  • Function: Uses ATP and NADPH to reduce CO2 into glucose.

Stages of Photosynthesis

  • Light Reactions:

    • Occur in thylakoids, split H2O, produce O2, NADPH, and ATP.

  • Light-Independent Reactions (Calvin Cycle):

    • Occur in stroma and utilize products from light reactions (NADPH, ATP) to synthesize glucose.

    • Input: CO2, NADPH, ATP

    • Output: Glucose

Photophosphorylation

  • Definition: ATP production using solar energy during photosynthesis.

Pigments Role

  • Chlorophyll are key to conversion of light energy to chemical energy

  • When a pigment absorbs light, it goes from a ground state to an excited state (unstable)

  • When excited electrons fall back to ground state, heat and photons (fluorescence)

Photosystems

Pigments inside thylakoid membrane are organized in structures called photosystems

  • Composition: Photosystems contain proteins and pigments that absorb light energy.

  • Types:

    • Photosystem II (PS II): Best at absorbing light at 680 nm.

      → functions first

    • Photosystem I (PS I): Best at absorbing light at 700 nm.

      → after PSII

Photosystem II

Starts process of photosynthesis

  • P680 gets excited

    • e- goes up, primary electron acceptor catches e-

  • P680 loses e- (+ charged)

    • P680+, strongest biological OA

  • H2O is split by P680+ (photolysis)

    • P680+ replenishes e- by taking e- from H2O

    • e- from hydrogen atoms are transferred to P680+, reducing it back to P680

      → O2 released as by-product

      → H+ ions accumulate in lumen

  • Each electron “falls” down ETC

    • Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

    • Diffusion of H+ (protons) across the membrane drives ATP synthesis

Photosystem I

  • PSI absorbs light and re-excites the e- passed to it from PSII. Its chlorophyll molecules (P700) gets energized

  • NADPH Production

    • The excited e- from PSI are transferred to NADP+, forming NADPH, a molecule that carries high-energy e- for the Calvin cycle

Cyclic and Linear Electron Flow

  • Cyclic Electron Flow:

    • Electrons cycle back through photosystem I, producing only ATP, with no O2 release.

      • Instead of being transferred to NADP+ to form NADPH, the electrons are cycled back to cytochrome complex (ETC between PSII and PSI)

      • This process helps to generate a proton gradient that drives ATP synthesis via ATP synthase, providing energy for cellular activities.

      • The e- complete the cycle by returning to PSI to repeat the process

        → a lot of ATP

        → no NADPH

      • → no O2 released

    • Plants don’t have to go thru Calvin cycle to produce ATP

      • does Kelvin cycle only to store energy in the form of sugar

  • Linear Electron Flow:

    • Involves both PS II and PS I, produces ATP and NADPH, and releases O2 from water splitting.