Photosynthesis and Plant Biology

Definition and Fundamental Processes of Photosynthesis

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

• The energy conversion involves transforming usable sunlight energy into chemical energy, a process fundamentally associated with the green pigment chlorophyll.

• The general word equation summarizing photosynthesis is: carbon dioxide + water $\rightarrow$ glucose + oxygen.

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

• Oxygen ($O_2$) is formed as a byproduct of these reactions.

Photosynthetic Pigments and Chlorophyll Structure

• Chlorophyll is a complex molecule and is the primary pigment responsible for photosynthesis.

• There are several modifications of chlorophyll across different plants and photosynthetic organisms, but chlorophyll a is universal to all photosynthetic organisms.

• Accessory pigments function to absorb light energy at wavelengths that chlorophyll a cannot. These include:

  • Chlorophyll b (found in plants).

  • Chlorophyll c, d, and e (found in various algae and protistans).

  • Xanthophylls.

  • Carotenoids, such as beta-carotene.

• Chlorophyll a specifically absorbs energy from the violet-blue and reddish orange-red wavelengths of the light spectrum. It absorbs very little energy from the intermediate wavelengths, which include green, yellow, and orange.

• The molecular structure of all chlorophylls consists of:

  • A lipid-soluble hydrocarbon tail with the formula $C_{20}H_{39}-$.

  • A flat hydrophilic head featuring a magnesium ion ($Mg^{2+ polarization}$) at its center.

  • Different types of chlorophyll are distinguished by different side-groups attached to the hydrophilic head.

  • The tail and the head are connected by an ester bond.

Leaf Structure and Environmental Adaptation

• Plants are the unique photosynthetic organisms that possess leaves, though not all plant species have them.

• A leaf acts as a solar collector, densely packed with photosynthetic cells.

• Resource Exchange:

  • Raw materials ($H_2O$ and $CO_2$) enter leaf cells.

  • Products (sugar and $O_2$) exit the leaf.

• Water Transport:

  • Water enters the plant through the roots and is transported upward to the leaves via specialized cells called xylem vessels.

• Gas Exchange and Stomata:

  • Land plants have evolved specialized structures called stomata (singular: stoma) to manage gas exchange while preventing desiccation.

  • A protective waxy layer called the cuticle covers the leaf; while it prevents drying out, it is impermeable to $CO_2$.

  • $CO_2$ enters the leaf through the stoma, which is flanked by two guard cells.

  • Oxygen produced during photosynthesis exits through these same opened stomata.

• Transpiration and Water Loss:

  • A significant amount of water is lost while stomata are open for 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 and Thylakoid Morphology

• The thylakoid is the fundamental structural unit of photosynthesis. These are flattened sacs or vesicles containing the necessary photosynthetic chemicals.

• Both photosynthetic prokaryotes and eukaryotes possess thylakoids, but only eukaryotes have them enclosed within a membrane-bound organelle called the chloroplast.

• Thylakoids are organized into stacks resembling pancakes, known as grana (singular: granum).

• The fluid-filled space surrounding the grana is referred to as the stroma.

• Unlike mitochondria, which have two membrane systems, chloroplasts contain three membrane systems, resulting in three distinct internal compartments.

The Light-Dependent Reactions

• The light-dependent reactions take place within the grana and require direct light energy to produce energy-carrier molecules.

• Photoactivation and Photoionisation:

  • When chlorophyll a absorbs light, its electrons gain energy and reach an "excited" state (photoexcitation).

  • If sufficient energy is absorbed, the molecule is ionized, and the electron is freed, leaving a positively charged chlorophyll ion (photoionisation).

  • The excited electron is transferred to a primary electron acceptor.

• Photosystems:

  • Chlorophyll molecules are organized into photosystems, consisting of the chlorophyll molecule, an electron acceptor, and an electron donor.

  • Photosystem II (PSII) is also known as P680.

  • Photosystem I (PSI) is also known as P700.

  • Note: PSII occurs before PSI in the reaction sequence; it was named second because it was discovered after PSI.

• The Z Scheme:

  • The energy changes during the electron transfer process follow a Z-shaped pattern when graphed, often called the Z scheme.

  • Electrons are carried across the thylakoid membrane by an electron transfer system.

• Key Chemical Processes:

  • Photophosphorylation: Light energy is trapped by chlorophyll to synthesize ATP. This involves a condensation reaction where phosphoric acid and ADP combine, eliminating a water molecule to form ATP.

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

  • Reduction of NADP: The freed electrons react with the carrier molecule nicotinamide adenine dinucleotide phosphate ($NADP^+$), reducing it to NADPH: $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$.

ATP Synthesis and Chemiosmosis

• Non-cyclic phosphorylation produces both ATP and NADPH.

• Mechanism of Chemiosmosis:

  • As electrons move through the transport chain in the thylakoid membrane, they provide energy to pump $H^+$ ions from the stroma into the thylakoid compartment.

  • This creates an electrochemical gradient, with a higher concentration of $H^+$ inside the thylakoid than in the stroma.

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

Cyclic Phosphorylation

• While non-cyclic phosphorylation transfers electrons from water to NADP, generating ATP in the process, the light-independent reactions require more ATP than this process provides.

• Cyclic phosphorylation generates this extra ATP.

• This process involves only Photosystem I (PSI).

• Excited electrons from PSI are transferred back to the electron transport chain located between PSII and PSI instead of being passed to $NADP^+$.

• No NADPH is formed during cyclic phosphorylation; the cycle is completed as electrons return to PSI via the electron transport system.

The Light-Independent Reactions (The Calvin Cycle)

• This stage is also known as the Dark Reaction and occurs within the stroma.

• Carbon Fixation:

  • Atmospheric (or aquatic) carbon dioxide is captured and added to organic compounds.

  • $CO_2$ combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP).

  • This creates an unstable six-carbon sugar which immediately breaks down into two molecules of glycerate 3-phosphate (GP), a 3-carbon compound.

• Reduction and Synthesis:

  • Energy from ATP and NADPH (produced in the light-dependent stage) is used to phosphorylate and reduce GP.

  • GP is converted into glycerate diphosphate and then into glyceraldehyde 3-phosphate (also known as GALP, PGAL, or phosphoglyceraldehyde, a 3-carbon molecule).

• Cycle Regeneration and Output:

  • The first stable product of the cycle is phosphoglycerate (PGA).

  • In a full cycle, $12$ molecules of glyceraldehyde phosphate are produced.

  • Two ($2$) of these molecules are removed from the cycle to synthesize one molecule of glucose.

  • The remaining molecules (one from each pair produced) are converted using ATP to reform six ($6$) molecules of RuBP to restart the cycle.

  • GALP serves as the initial end product and is quickly converted into glucose, other carbohydrates, lipids, or amino acids.

Factors Affecting the Rate of Photosynthesis

• Limiting Factors: The overall rate of photosynthesis is determined by light intensity, $CO_2$ concentration, and temperature.

• Light Intensity:

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

  • Wavelength Sensitivity: PSI is most efficient at $700\,nm$, and PSII is most efficient at $680\,nm$. Light containing high proportions of these wavelengths yields higher photosynthetic rates.

• Carbon Dioxide Concentration:

  • Increasing $CO_2$ concentration increases the rate of carbon incorporation into carbohydrates during the light-independent reaction, until limited by another factor.

• Temperature:

  • Since photosynthesis is catalyzed by enzymes, the rate increases as temperatures approach the enzymes' optimum.

  • Above the optimum temperature, the rate decreases until the process stops due to enzyme denaturation.