Photosynthesis and Plant Physiology

Overview of Photosynthesis

• Definition: Photosynthesis is the biological process by which plants, certain bacteria, and various protistans utilize energy from sunlight to synthesize glucose from carbon dioxide ($CO_2$) and water ($H_2O$).

• Energy Conversion: The process converts usable sunlight energy into chemical energy, which is initially associated with the green pigment chlorophyll.

• Metabolic Context: The glucose produced can be converted into pyruvate, which subsequently releases adenosine triphosphate (ATP) through the process of cellular respiration.

• By-products: Oxygen ($O_2$) is formed as a byproduct of this reaction.

• General Word Equation:     $\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$

Photosynthetic Pigments and Chlorophyll Structure

• Chlorophyll Variants: Chlorophyll is a complex molecule existing in several modifications across different photosynthetic organisms.

  • Chlorophyll a: Present in all photosynthetic organisms; it absorbs energy primarily from violet-blue and reddish orange-red wavelengths.

  • Accessory Pigments: These absorb light energy in wavelengths that chlorophyll a does not (specifically intermediate green-yellow-orange wavelengths). These include:

    • Chlorophyll b: Found in plants.

    • Chlorophylls c, d, and e: Found in algae and protistans.

    • Xanthophylls.

    • Carotenoids: Such as beta-carotene.

• Molecular Structure of Chlorophyll: All chlorophyll molecules consist of two main components linked by an ester bond:

  • Hydrocarbon Tail: A lipid-soluble chain with the formula $C_{20}H_{39}-$.

  • Hydrophilic Head: A flat structure with a magnesium ion ($Mg^{2+}$) at its center. Different types of chlorophyll are distinguished by different side-groups attached to this head.

Leaf Anatomy and Resource Management

• Leaves as Solar Collectors: Only plants possess leaves (though not all plants do). A leaf is specialized as a solar collector densely packed with photosynthetic cells.

• Material Transport:

  • Water ($H_2O$): Enters the plant via the roots and is transported upward to the leaves through specialized vascular cells called xylem vessels.

  • Carbon Dioxide ($CO_2$): Enters the leaf from the atmosphere through stomata (singular: stoma).

  • Products: Sugar and oxygen exit the leaf after synthesis.

• Gas Exchange and Water Loss:

  • Cuticle: A protective waxy layer covering the leaf that prevents water loss but also blocks $CO_2$.

  • Stomata and Guard Cells: Stomata are openings flanked by two guard cells that regulate gas exchange. While they allow $CO_2$ in and $O_2$ out, they also facilitate significant water loss.

  • Example of Water Loss: Cottonwood trees can lose up to $100\,\text{gallons}$ (approximately $450\,dm^3$) of water per hour during hot desert days.

Chloroplast Structure and Thylakoid Membranes

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

  • Prokaryotes vs. Eukaryotes: Both possess thylakoids; however, only eukaryotes have chloroplasts, which are organelles surrounded by a membrane.

• Internal Organization:

  • Grana: Thylakoids are stacked together like pancakes into units called grana.

  • Stroma: The fluid-filled areas between the grana.

• Membrane Systems: Unlike the mitochondrion, which has two membrane systems, the chloroplast has three membrane systems forming three distinct compartments.

The Two Stages of Photosynthesis

• Stage 1: Light-Dependent Reactions:

  • Location: Occur in the grana.

  • Requirement: Requires direct light energy.

  • Function: Traps light energy to produce ATP (photophosphorylation) and reduced nicotinamide adenine dinucleotide phosphate (NADPH).

  • Photolysis: Water is split into oxygen, hydrogen ions, and free electrons:         $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$

  • Reduction of NADP: Electrons and hydrogen ions react with $NADP^+$:         $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$

• Stage 2: Light-Independent Reactions (The Dark Reaction):

  • Location: Occur in the stroma.

  • Function: Utilizes ATP and NADPH from the light-dependent stage to reduce $CO_2$ into carbohydrates.

  • Initial Product: Glyceraldehyde 3-phosphate (a 3-carbon molecule).

Photoactivation and Photoionisation

• Photoactivation: When chlorophyll a absorbs light energy, an electron enters an "excited" state and is transferred to a primary electron acceptor. The chlorophyll molecule becomes oxidized (loses an electron) and carries a positive charge.

• Photoionisation: If the absorbed energy is sufficient, the electron is freed entirely, leaving a positively charged chlorophyll ion.

• The Photosystem Core: In a chloroplast, each chlorophyll is associated with an electron donor and an electron acceptor.

• Electron Replacement: The positively charged chlorophyll ion retrieves a pair of electrons from a neighboring donor, such as water.

The Electron Transfer System and the Z-Scheme

• Mechanism: A series of chemical reactions carries two electrons across the thylakoid membrane.

• Photosystems:

  • Photosystem II (PSII): Also known as P680. Despite the name, it occurs first in the sequence.

  • Photosystem I (PSI): Also known as P700.

• The Z-Scheme: The energy changes occurring during these transfers form a "Z" shape when charted. The energy released during this electron transfer is sufficient to synthesize ATP from ADP and phosphate.

ATP Synthesis and Chemiosmosis

• Phosphorylation: ATP is formed via a condensation reaction between adenosine diphosphate (ADP) and phosphoric acid, resulting in the elimination of a water molecule ($H_2O$).

• Chemiosmotic Mechanism:

  • Electrons passing through the transport chain provide energy to pump $H^+$ ions from the stroma across the thylakoid membrane into the thylakoid compartment.

  • This creates an electrochemical gradient where $H^+$ ions are more concentrated inside the thylakoid.

  • The diffusion of $H^+$ ions from high to low concentration (back to the stroma) drives the production of ATP.

Cyclic vs. Non-Cyclic Phosphorylation

• Non-Cyclic Phosphorylation (The Z-Scheme):

  • Produces both ATP and NADPH.

  • Involves both PSII and PSI.

  • Electrons move from water (donor) to $NADP^+$ (final acceptor).

• Cyclic Phosphorylation:

  • Produces only ATP; no NADPH is formed.

  • Involves only Photosystem I (PSI).

  • Excited electrons from PSI are transferred to the electron transport chain between PSII and PSI and then back to PSI.

  • Purpose: Provides the extra ATP required to drive the light-independent reactions, which consume more ATP than the non-cyclic process provides.

The Light-Independent Reactions (The Calvin Cycle)

• Carbon Fixation: The process of incorporating $CO_2$ from the atmosphere (or water for aquatic systems) into organic compounds. This converts light energy into C-C bond energy.

• Step 1: Carboxylation: $CO_2$ combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP), to form an unstable six-carbon intermediate.

• Step 2: Breakdown: The six-carbon sugar breaks down into two molecules of glycerate 3-phosphate (GP) (a 3-carbon molecule).

• Step 3: Reduction:

  • GP molecules are phosphorylated by ATP to form glycerate diphosphate.

  • These are then reduced by NADPH to form glyceraldehyde 3-phosphate (GALP), also known as phosphoglyceraldehyde (PGAL).

• Outcomes of GALP:

  • Some GALP molecules are converted into glucose, other carbohydrates, lipids, or amino acids. (Specifically, 2 out of every 12 PGAL molecules are removed to make glucose).

  • The remaining GALP molecules are converted back into RuBP using energy from ATP to restart the cycle.

Limiting Factors of Photosynthesis

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

• Wavelength of Light:

  • PSI is most efficient at $700\,nm$.

  • PSII is most efficient at $680\,nm$.

  • Light concentrated in these specific wavelengths yields higher photosynthetic rates.

• Carbon Dioxide Concentration: Increasing $CO_2$ levels speeds up the rate at which carbon is incorporated into carbohydrates during the light-independent reactions, until another factor limits the process.

• Temperature:

  • Photosynthesis is an enzyme-catalyzed reaction.

  • The rate increases as temperature approaches the enzymes' optimum.

  • Above the optimum temperature, enzymes degrade, and the rate decreases until the process stops.