Photosynthesis and Plant Biology

Overview of Photosynthesis

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

• Energy Transformation: This process converts usable sunlight energy into chemical energy, which is initially associated with chlorophyll and subsequently stored in the bonds of glucose.

• Post-Photosynthetic Metabolism: The glucose produced can be converted into pyruvate. This conversion, occurring during cellular respiration, facilitates the release of adenosine triphosphate (ATP), the primary energy currency of the cell.

• Byproducts: Oxygen ($O_2$) is formed as a byproduct of this process.

• Word Equation:

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

Chlorophyll and Accessory Pigments

• Chlorophyll Function: Chlorophyll is a complex molecule and the primary green pigment responsible for the conversion of light energy into chemical energy.

• Ubiquity of Chlorophyll a: All photosynthetic organisms contain chlorophyll a.

• Accessory Pigments: These pigments absorb light energy at wavelengths that chlorophyll a does not absorb, broadening the spectrum of light available for photosynthesis. They include:

  • Chlorophyll b: Found in plants.

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

  • Xanthophylls.

  • Carotenoids: Examples include beta-carotene.

• Absorption Spectrum:

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

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

• Chemical Structure of Chlorophylls:

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

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

  • Bonding: The head and tail of the molecule are joined via an ester bond.

Leaf Structure and Physiological Mechanisms

• Leaves as Solar Collectors: Leaves are unique to plants (though not all plants possess them) and function as specialized organs packed with photosynthetic cells.

• Resource Transport:

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

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

• Gas Exchange and Stomata:

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

  • Stoma (plural: stomata): Openings in the leaf surface flanked by two guard cells that regulate gas exchange ($CO_2$ in, $O_2$ out).

  • Transpiration: The inevitable loss of water during gas exchange. For example, Cottonwood trees can lose up to $100\,\text{gallons}$ (approximately $450\,dm^3$) of water per hour during hot desert days.

Chloroplast Structure and Membranes

• The Thylakoid: The fundamental structural unit of photosynthesis. Both prokaryotic and eukaryotic photosynthetic organisms possess these flattened sacs or vesicles containing photosynthetic chemicals.

• Chloroplast Differences: Only eukaryotes possess chloroplasts, which are organelles surrounded by a membrane.

• Internal Organization:

  • Grana: Stacks of thylakoids (resembling pancakes).

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

• Membrane Systems: While mitochondria have two membrane systems, chloroplasts have three, which define three distinct internal compartments.

The Two Stages of Photosynthesis

Photosynthesis occurs in two distinct but interdependent stages:

• Light-Dependent Reactions:

  • Occur within the grana.

  • Require direct sunlight energy to produce energy-carrier molecules (ATP and NADPH).

• Light-Independent Reactions (Dark Reactions/Calvin Cycle):

  • Occur in the stroma.

  • Use the products of the light-dependent reactions (ATP and NADPH) to reduce carbon dioxide into carbohydrates (initially glyceraldehyde 3-phosphate).

The Light-Dependent reactions: Photoactivation and Photolysis

• Photoexcitation: When chlorophyll a absorbs light, its electrons gain energy and move to higher energy levels.

• Photoionisation: If an electron gains sufficient energy, it is freed from the chlorophyll molecule, leaving behind a positively charged chlorophyll ion.

• Core of a Photosystem: Consists of a chlorophyll molecule associated with an electron acceptor and an electron donor.

• Photolysis of Water: To replace electrons lost during photoionisation, water molecules are split using light energy:

  • $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$

• NADP Reduction: Freed electrons react with the carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), reducing it from its oxidized state ($NADP^+$) to its reduced state (NADPH):

  • $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$

• Reaction Types involved:

  • Condensation Reactions: Responsible for water splitting and phosphorylation (adding a phosphate group to an organic compound).

  • Oxidation/Reduction (Redox): Involves the transfer of electrons between molecules.

The Z-Scheme (Non-cyclic Photophosphorylation)

• Photosystems: Energy is captured and transferred by two photosystems:

  • Photosystem II (PSII): Also known as P680 (absorbs most efficiently at $680\,nm$). Despite the name, it occurs first in the sequence.

  • Photosystem I (PSI): Also known as P700 (absorbs most efficiently at $700\,nm$).

• The Sequence:

  1. PSII captures light energy, leading to photoionisation of chlorophyll.

  2. Electrons from PSII are passed through an electron transport chain (a series of chemical reactions across the thylakoid membrane).

  3. Photolysis of water provides replacement electrons for PSII.

  4. Energy released during the electron transfer enables the synthesis of ATP from ADP and phosphate (phosphorylation through a condensation reaction).

  5. Electrons reach PSI, where further light absorption increases their energy level again.

  6. The high-energy electrons are used to reduce $NADP^+$ to NADPH.

• Nomenclature: It is called the Z-scheme because the plotted energy changes of the electrons form a "Z" shape.

ATP Synthesis and Chemiosmosis

• Mechanism: ATP is synthesized via chemiosmosis during photophosphorylation.

• Electrochemical Gradient: As electrons pass through the transport chain, energy is used to pump hydrogen ions ($H^+$) from the stroma across the thylakoid membrane into the thylakoid compartment (internal space).

• Diffusion: A high concentration of $H^+$ ions builds up inside the thylakoid. The $H^+$ ions then diffuse back into the stroma through specific channels, providing the drive to produce ATP.

• Synthesis Reaction: Phosphoric acid and ADP undergo a condensation reaction, eliminating water ($H_2O$) to form ATP.

Cyclic Photophosphorylation

• Purpose: Non-cyclic phosphorylation produces both ATP and NADPH. However, the light-independent reactions require more ATP than non-cyclic processes provide. Cyclic phosphorylation generates this extra ATP.

• Process: This involves only Photosystem I (PSI). Excited electrons are transferred to the electron transport chain between PSII and PSI instead of being passed to $NADP^+$.

• Result: Electrons return to PSI in a cycle. This process produces ATP but does not produce NADPH or oxygen.

The Light-Independent Reactions: The Calvin Cycle

• Carbon Fixation: The incorporation of atmospheric (or aquatic) $CO_2$ into organic compounds.

• Energy Conversion: Light energy is converted into C-C bond energy, which organisms can release later through metabolism (e.g., glycolysis).

• Step-by-Step Sequence:

  1. Carboxylation: Carbon dioxide ($1$-C) combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP).

  2. Instability: An unstable six-carbon intermediate forms and immediately breaks down into two molecules of glycerate 3-phosphate (GP), a three-carbon molecule.

  3. Phosphorylation: ATP is used to phosphorylate the GP molecules into glycerate diphosphate molecules.

  4. Reduction: NADPH reduces these molecules into glyceraldehyde 3-phosphate (GALP), also known as phosphoglyceraldehyde (PGAL) or phosphoglycerate (PGA).

• Molecule Distribution:

  • For every $12$ molecules of PGAL/GALP produced:

    • $2$ molecules are removed from the cycle to produce one glucose molecule or other organic compounds (lipids, amino acids).

    • $10$ molecules are converted using ATP energy to reform $6$ molecules of RuBP, allowing the cycle to repeat.

Factors Affecting the Rate of Photosynthesis

Photosynthesis is governed by several "limiting factors":

• Light Intensity:

  • The rate of the light-dependent reaction increases proportionately with light intensity.

  • Eventually, the rate levels off when another factor (like $CO_2$ or temperature) becomes limiting.

• Wavelength of Light:

  • Efficiency is highest at specific wavelengths: $680\,nm$ (for PSII) and $700\,nm$ (for PSI).

• Carbon Dioxide Concentration:

  • Increased $CO_2$ concentrations increase the rate of carbon incorporation in the light-independent reaction until limited by another factor.

• Temperature:

  • Photosynthesis is an enzyme-catalyzed process.

  • Rate increases as enzymes approach their optimum temperature.

  • Beyond the optimum temperature, enzyme function declines, and the rate of photosynthesis decreases until it eventually stops.