Photosynthesis and Plant Physiology Flashcards

Overview and General Process of Photosynthesis

• Definition: Photosynthesis is the biological process specifically utilized by plants, certain bacteria, and various protistans to convert sunlight energy into chemical energy. This involves producing glucose from the raw materials of carbon dioxide and water.

• Summary Equation: The process is summarized by the word equation: $\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$.

• Energy Conversion and Respiration: The resulting glucose can be further converted into pyruvate. Through the process of cellular respiration, pyruvate releases adenosine triphosphate (ATP), which is the primary energy currency for cellular activities.

• Stages of Photosynthesis:

  • Light-dependent reactions: These occur in the thylakoid membranes of chloroplasts, where sunlight is captured and converted to ATP and NADPH while splitting water molecules to release oxygen.

  • Light-independent reactions (Calvin Cycle): Taking place in the stroma, these reactions utilize ATP and NADPH to drive the synthesis of glucose from carbon dioxide.

• Primary Pigment: The conversion of solar energy into chemical energy is fundamentally linked to the green pigment known as chlorophyll. Chlorophyll is categorized as a complex molecule.

Chlorophyll and Accessory Pigments

• Chlorophyll Variants: While several modifications of chlorophyll exist across different photosynthetic organisms, all such organisms contain chlorophyll a.

• Accessory Pigments: These pigments function by absorbing light energy at wavelengths that chlorophyll a does not. Examples include:

  • Chlorophyll b: found in plants.

  • Chlorophyll c, d, and e: found specifically in algae and protistans.

  • Xanthophylls.

  • Carotenoids: A notable example is beta-carotene.

• Absorption Spectrum of Chlorophyll a: This molecule primarily absorbs energy from the violet-blue and reddish orange-red wavelengths. It absorbs very little energy from the intermediate spectrum, which includes green, yellow, and orange wavelengths.

• Molecular Structure of Chlorophylls: All chlorophyll molecules share a common structure:

  • A hydrocarbon tail: This is lipid-soluble and has the formula $(C_{20}H_{39}-)$.

  • A flat hydrophilic head: This structure contains a magnesium ion at its center. Variation between different chlorophyll types occurs due to different side-groups attached to this head.

  • Linkage: The tail and the head of the molecule are connected via an ester bond.

Leaf Anatomy and Gas Exchange

• Specialization: Plants are unique as the only photosynthetic organisms possessing leaves, although not every plant species has them. Leaves serve as specialized solar collectors densely packed with photosynthetic cells.

• Material Transport:

  • Water Inflow: Water is absorbed through the roots and transported to the leaves through specialized vascular tissue called xylem vessels.

  • Gas Entry/Exit: Carbon dioxide enters the leaf, and oxygen (a byproduct) exits the leaf.

• Stomata and Guard Cells: To prevent desiccation (drying out), land plants developed stomata (singular: stoma). These are pores flanked by two guard cells that regulate the entry and exit of gases.

• Cuticle: The leaf is covered by a protective waxy layer called the cuticle. This layer is impermeable to carbon dioxide, necessitating the role of the stomata.

• Transpiration Risk: When stomata open for gas exchange, significant water loss occurs. For instance, Cottonwood trees can lose approximately $100$ gallons (about $450\,dm^3$) of water per hour during intense heat in desert environments.

Structure of the Chloroplast and Thylakoid Membranes

• The Thylakoid: This is the fundamental structural unit of photosynthesis. Both prokaryotic and eukaryotic photosynthetic organisms contain these flattened sacs/vesicles which house photosynthetic chemicals.

• Eukaryotic Complexity: Only eukaryotes possess chloroplasts, which are organelles enclosed by a surrounding membrane.

• Grana and Stroma:

  • Grana: Thylakoids are organized into vertical stacks resembling pancakes.

  • Stroma: This term refers to the fluid-filled areas located between the grana.

• Membrane Systems: Unlike mitochondria, which have two membrane systems, chloroplasts possess three membrane systems, resulting in three distinct internal compartments.

Fundamental Chemical Stages of Photosynthesis

• Photoactivation of Chlorophyll a: When light energy is absorbed by chlorophyll a, an electron becomes 'excited' (gains energy) and is transferred to a primary electron acceptor. The chlorophyll molecule becomes oxidized (loses an electron) and carries a positive charge.

• Chemical Reaction Types:

  • Condensation Reactions: These involve the splitting out of water molecules and include phosphorylation (the addition of a phosphate group to an organic compound).

  • Oxidation/Reduction (Redox) Reactions: These involve the transfer of electrons between molecules.

• The Two Stages:

  • The Light-Dependent Reactions: Occur in the grana. They use direct light energy to produce energy-carrier molecules (ATP and NADPH).

  • The Light-Independent Reactions: Occur in the stroma. They use the products of the light reactions (ATP and NADPH) to reduce carbon dioxide into carbohydrates.

The Light-Dependent Reactions and Photosystems

• Photoexcitation and Photoionisation: When light energy hits chlorophyll, electrons move to higher energy levels (photoexcitation). If energy is sufficient, the electron is freed, leaving a positively charged ion (photoionisation).

• Photosystem Core: Each chlorophyll molecule is part of a core consisting of the chlorophyll, an electron acceptor, and an electron donor. During photoionisation, two electrons are transferred to the acceptor. The positive chlorophyll ion then captures a pair of electrons from a donor, such as water.

• The Electron Transfer System: This is a chain of reactions that moves electrons across the thylakoid membrane.

• Photosystems:

  • Photosystem II (PSII): Also known as P680. Despite the name, this process occurs first (it was discovered second).

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

• The Z Scheme: The energy changes associated with electron transfer follow a distinct Z-shaped pattern. This process releases enough energy to synthesize ATP from ADP and phosphate.

Specific Processes in the Light-Dependent Stage

• Photolysis of Water: Water is split to provide electrons to the system: $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$.

• Reduction of NADP+: Free electrons react with the carrier molecule nicotinamide adenine dinucleotide phosphate (NADP+), converting it to its reduced state (NADPH): $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$.

• Non-cyclic Phosphorylation (Z-Scheme) Summary:

  1. Photoionisation in PSII transfers electrons to an acceptor.

  2. Photolysis of water releases oxygen, hydrogen ions ($H^+$), and electrons to replace those lost by PSII.

  3. Electrons move through the transport chain to PSI.

  4. Absorbed light in PSI further excites electrons to reduce $NADP^+$ to $NADPH$.

• ATP Synthesis (Phosphorylation Details): Phosphoric acid and ADP undergo a condensation reaction where water is eliminated to form ATP.

Chemiosmosis and Cyclic Phosphorylation

• Chemiosmosis: Components for non-cyclic phosphorylation reside in the thylakoid membranes.

  • Energy from the electron transport chain is used to pump $H^+$ ions from the stroma into the thylakoid compartment.

  • This creates an electrochemical gradient ($H^+$ concentration is higher inside the thylakoid).

  • The diffusion of $H^+$ ions back out drives the production of ATP.

• Cyclic Phosphorylation: While non-cyclic phosphorylation produces both ATP and NADPH, the light-independent reactions require more ATP than is produced in that ratio.

  • Mechanism: Involves only Photosystem I.

  • Route: Excited electrons are transferred to the transport chain between PSII and PSI and then back to PSI, rather than being used to make NADPH.

  • Result: Generates extra ATP without forming NADPH or releasing oxygen.

The Light-Independent Reactions (The Calvin Cycle)

• Atmospheric Carbon Fixation: Carbon dioxide (from the atmosphere or water for aquatic organisms) is captured and modified by adding hydrogen to form carbohydrates. This converts light energy into stable $C-C$ bond energy.

• The Process Steps:

  1. Carboxylation: Carbon dioxide combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP), to form an unstable six-carbon sugar.

  2. Cleavage: This six-carbon sugar breaks down into two molecules of glycerate 3-phosphate (GP).

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

• Fate of GALP (Phosphoglyceraldehyde/PGAL):

  • For every pair of GALP molecules, one is used as the initial end product of photosynthesis (to be converted into glucose, other carbohydrates, lipids, or amino acids).

  • The other is recycled to reform RuBP via ATP-driven reactions.

• Cycle Stoichiometry: 12 molecules of PGAL are formed; 2 are removed to produce one glucose molecule ($C_6H_{12}O_6$), while the remaining 10 are used to regenerate 6 RuBP molecules.

Limiting Factors of Photosynthetic Rate

• Primary Factors: Light intensity, carbon dioxide concentration, and temperature.

• Light Intensity: As intensity increases, the rate increases proportionally until another factor becomes limiting. The wavelength is also critical; PSI is most efficient at $700\,nm$ and PSII at $680\,nm$.

• Carbon Dioxide Concentration: Increased $CO_2$ levels raise the rate of carbon incorporation in the light-independent phase until limited by other factors.

• Temperature: Since photosynthesis is an enzyme-catalyzed process, the rate increases as temperatures approach the optimum temperature. Beyond this point, enzymes may denature, causing the rate to decrease and eventually stop.