Photosynthesis Lecture Notes

Overview of Photosynthesis and Energy Conversion

  • Photosynthesis is the definitive process used by plants, certain bacteria, and specific protistans to convert energy from sunlight into chemical energy in the form of glucose (C6H12O6C_6H_{12}O_6).

  • The process utilizes carbon dioxide (CO2CO_2) and water (H2OH_2O) as primary reactants.

  • The resulting glucose serves as a versatile energy source that can be converted into pyruvate to release adenosine triphosphate (ATP) through the process of cellular respiration.

  • Aside from glucose, oxygen (O2O_2) is formed as a crucial byproduct.

  • The Summary Word Equation for Photosynthesis:

    • carbon dioxide+water→glucose+oxygen\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}

Photosynthetic Pigments and Chlorophyll Structure

  • Role of Chlorophyll: The conversion of sunlight into chemical energy is associated with the green pigment chlorophyll, a complex molecule found in all photosynthetic organisms.

  • Primary and Accessory Pigments:

    • Chlorophyll a: The essential primary pigment present in all photosynthetic organisms.

    • Accessory Pigments: These absorb light energy in wavelengths that chlorophyll a cannot. They include:

    • Chlorophyll b, c, d, and e: Found variously in plants, algae, and protistans.

    • Xanthophylls.

    • Carotenoids: Such as beta-carotene.

  • Absorption Spectrum:

    • Chlorophyll a absorbs energy primarily from the violet-blue and reddish orange-red wavelengths.

    • It absorbs very little from the intermediate wavelengths (green-yellow-orange).

  • Molecular Structure of Chlorophyll:

    • Hydrocarbon Tail: A lipid-soluble chain with the formula (C20H39−)(C_{20}H_{39}-).

    • Hydrophilic Head: A flat structure featuring a magnesium (Mg2+Mg^{2+}) ion at its center. This head is where different side groups are attached to distinguish between various types of chlorophyll.

    • Bonding: The tail and head are joined by an ester bond.

Leaf Morphology and Gas Exchange

  • Function of Leaves: In the plant kingdom, leaves act as specialized solar collectors packed with photosynthetic cells.

  • Material Transport:

    • Water: Absorbed by the roots and transported to the leaves through specialized vascular tissue known as xylem vessels.

    • Gas Exchange: Land plants have evolved structures called stomata (singular: stoma) to manage gas exchange while minimizing desiccation (drying out).

  • Structural Components:

    • Cuticle: A protective waxy layer covering the leaf surface that prevents the passage of CO2CO_2 and water.

    • Stomata: Pores flanked by two guard cells that regulate the entry of CO2CO_2 and the exit of O2O_2.

  • Transpiration Cost: A significant amount of water is lost during gas exchange. For example, Cottonwood trees can lose up to 100 gallons100\,\text{gallons} (approximately 450 dm3450\,dm^3) of water per hour during hot desert days.

Chloroplast Structure and Thylakoid Systems

  • The Thylakoid: The fundamental structural unit of photosynthesis. These are flattened sacs or vesicles containing photosynthetic chemicals.

    • Found in both photosynthetic prokaryotes and eukaryotes.

    • In eukaryotes, thylakoids are housed within a double-membrane-bound organelle called the chloroplast.

  • Internal Organization:

    • Grana: Thylakoids are organized in stacks (resembling pancakes) known as grana.

    • Stroma: The fluid-filled area surrounding the grana.

    • Membrane Systems: Unlike mitochondria which have two membrane systems, chloroplasts contain three, creating three distinct internal compartments.

The Two Stages of Photosynthesis

Photosynthesis is divided into two distinct but interconnected stages: the light-dependent reactions and the light-independent reactions.

1. Light-Dependent Reactions

  • Location: Occur within the grana of the chloroplast.

  • Requirement: Direct light energy is required to produce energy-carrier molecules.

  • Key Processes:

    • Photophosphorylation: Light energy is trapped by chlorophyll to synthesize ATP.

    • Photolysis: Light energy splits water molecules into oxygen, hydrogen ions, and electrons.

    • Reaction: 2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-

    • NADP Reduction: Free electrons react with the carrier molecule nicotinamide adenine dinucleotide phosphate (NADP+NADP^+) to form reduced NADPH.

    • Reaction: NADP++2e−+2H+→NADPH+H+NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+

2. Light-Independent Reactions (The Calvin Cycle)

  • Location: Occur in the stroma of the chloroplast.

  • Process: Utilizes the ATP and NADPH produced in the light-dependent stage to reduce CO2CO_2 and manufacture carbohydrates.

  • Initial Product: A 3-carbon atom molecule called glyceraldehyde 3-phosphate (GALP).

The Z-Scheme and Photoactivation

  • Photoexcitation: When chlorophyll a absorbs light, an electron is boosted to a higher energy level and becomes "excited."

  • Photoionisation: If enough energy is absorbed, the electron is freed from the molecule, leaving the chlorophyll oxidized with a positive charge.

  • The Photosystem Core: A photosystem consists of three main components: a chlorophyll molecule, an electron acceptor, and an electron donor.

  • Photosystems II and I:

    • Photosystem II (PSII) / P680: Named second due to discovery order, but acts first in the sequence.

    • Photosystem I (PSI) / P700: Receives electrons from the transport chain after PSII.

  • The Z-Scheme: The energy changes during electron transfer follow a specific "Z" shape when mapped, releasing sufficient energy to facilitate the synthesis of ATP from ADP and inorganic phosphate.

Chemiosmosis and ATP Synthesis

  • Reaction Type: ATP is formed via a condensation reaction between adenosine diphosphate (ADP) and phosphoric acid, involving the elimination of a water molecule.

  • Electrochemical Gradient:

    • Electrons moving through the transport chain provide energy to pump hydrogen ions (H+H^+) from the stroma into the thylakoid compartment.

    • This creates a high concentration of H+H^+ inside the thylakoid compared to the stroma.

  • Chemiosmosis: The diffusion of H+H^+ ions back across the membrane (from high to low concentration) drives the enzymatic production of ATP.

Non-Cyclic vs. Cyclic Phosphorylation

Non-Cyclic Phosphorylation (The Z-Scheme)

  • Inputs: Light, water, NADP+NADP^+, ADP.

  • Outputs: ATP, NADPH, and Oxygen.

  • Pathway: Electrons move from water →\rightarrow PSII →\rightarrow Electron Transport Chain →\rightarrow PSI →\rightarrow NADPH.

Cyclic Phosphorylation

  • Purpose: Produces extra ATP required for the light-independent reactions without producing additional NADPH.

  • Involvement: Only involves Photosystem I.

  • Pathway: Excited electrons from PSI are transferred to the transport chain between PSII and PSI and then returned to PSI, completing a cycle.

The Light-Independent Process: Carbon Fixation

  • Carbon Fixation: The process of incorporating atmospheric CO2CO_2 into organic compounds.

  • The Step-by-Step Cycle:

    1. Carboxylation: CO2CO_2 combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP).

    2. Fragmentation: This forms an unstable six-carbon sugar which immediately breaks down into two molecules of glycerate 3-phosphate (GP).

    3. Phosphorylation and Reduction: GP is phosphorylated by ATP into glycerate diphosphate, which is then reduced by NADPH to form glyceraldehyde 3-phosphate (GALP).

  • Regeneration and Output:

    • For every pair of GALP molecules: one is used as the end product (to make glucose, lipids, or amino acids), and the other undergoes reactions to reform RuBP.

    • Specifically, to make one glucose molecule (6-C6\text{-C}), two molecules of phosphoglyceraldehyde (PGAL/GALP) are removed from the cycle. The remaining ten PGAL molecules are converted using ATP to reform six RuBP molecules.

Limiting Factors of Photosynthesis

  • Light Intensity: The rate of the light-dependent reaction increases proportionately with light intensity until another factor becomes limiting.

  • Wavelength: Photosynthesis is most efficient when light matches the absorption peaks of the photosystems: 680 nm680\,nm for PSII and 700 nm700\,nm for PSI.

  • Carbon Dioxide Concentration: Increasing CO2CO_2 increases the rate of carbon incorporation in the light-independent reaction until a plateau is reached.

  • Temperature: Because photosynthesis relies on enzyme-catalyzed reactions, the rate increases as temperature approaches the enzymes' optimum. Beyond the optimum temperature, the rate drops sharply as enzymes denature.