Photosynthesis Lecture Practice Flashcards

Fundamentals of Photosynthesis and Energy Conversion

• Photosynthesis is the biological process through which plants, certain bacteria, and specific protistans utilize energy from sunlight to synthesize glucose from carbon dioxide and water.

• The chemical energy stored in glucose can be converted into pyruvate, a process that releases adenosine triphosphate (ATP) via cellular respiration.

• A byproduct of this process is the formation of oxygen.

• The summary word equation for photosynthesis is:     $carbon dioxide + water \rightarrow glucose + oxygen$

• The transformation of usable sunlight energy into chemical energy is facilitated by the green pigment known as chlorophyll.

• Chlorophyll is a complex molecule existing in several modifications across different plants and photosynthetic organisms.

• All photosynthetic organisms possess chlorophyll a.

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

  • Chlorophyll b (found in plants and certain algae).

  • 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 green-yellow-orange wavelengths.

Molecular Structure of Chlorophyll

• Every chlorophyll molecule consists of two primary components:

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

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

• While the general structure is consistent, different types of chlorophyll possess different side-groups attached to the head.

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

Leaf Anatomy and Physiological Transport

• Among photosynthetic organisms, only plants possess leaves, though this is not universal to all plant species.

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

• The leaf manages the influx of raw materials and the efflux of products:

  • Raw materials: Water and carbon dioxide enter the leaf cells.

  • Products: Sugar and oxygen exit the leaf.

• Water is absorbed by the roots and transported to the leaves through specialized plant cells called xylem vessels.

• Land-dwelling plants have evolved specialized structures to prevent desiccation (drying out):

  • Cuticle: A protective waxy layer covering the leaf that carbon dioxide cannot penetrate.

  • Stomata (singular: stoma): Specialized pores that allow gas exchange. Carbon dioxide enters and oxygen exits through these openings.

  • Guard Cells: Two cells flanking each stoma that regulate its opening and closing.

• A significant amount of water is lost as a side effect of gas exchange. For example, cottonwood trees can lose approximately $100\,gallons$ (roughly $450\,dm^3$) of water per hour during hot desert days.

Chloroplast Structure and Compartmentalization

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

• Thylakoids are found in both photosynthetic prokaryotes and eukaryotes, but only eukaryotes house them within a membrane-bound organelle called the chloroplast.

• Thylakoids are organized into stacks called grana (singular: granum).

• The fluid-filled regions between the grana are known as the stroma.

• Unlike the mitochondrion, which contains two membrane systems, the chloroplast contains three membrane systems, resulting in three distinct internal compartments.

The Two-Stage Process of Photosynthesis

• The overall process is divided into light-dependent and light-independent reactions.

• When chlorophyll a absorbs light energy, an electron is promoted to a higher energy state, becoming "excited."

• This excited electron is transferred to a primary electron acceptor, leaving the chlorophyll molecule oxidized and positively charged.

• Photoactivation of chlorophyll a drives the splitting of water and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).

• Primary chemical reactions involved include:

  • 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 Light-Dependent Reactions

• These reactions occur within the grana and require direct light energy to produce energy-carrier molecules.

• Photoexcitation: Light energy causes electrons in chlorophyll to move to higher energy levels.

• Photoionisation: When sufficient energy is absorbed, an electron is freed from the chlorophyll molecule, resulting in a positively charged ion.

• Photosystem structure: Each chlorophyll molecule in a chloroplast is part of a photosystem core consisting of the chlorophyll, an electron acceptor, and an electron donor.

• Photosystems utilized:

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

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

  • Note: PSII occurs first in the sequence, though it was named second because it was discovered after PSI.

• Key mechanisms of the light-dependent stage:

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

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

  • NADP Reduction: Electrons react with the carrier molecule $NADP^+$, converting it to its reduced state (NADPH) according to the equation: $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$

The Z Scheme and Non-Cyclic Phosphorylation

• The energy changes during electron transfer follow a "Z" shape, referred to as the Z scheme.

• Process Flow:

  1. Photoionisation occurs in PSII, and two electrons are passed to an electron acceptor.

  2. Wait is split (photolysis), and two electrons are transferred to fill the "positive holes" left in the chlorophyll molecules of PSII.

  3. Electrons move through an electron transfer chain (a series of redox reactions) across the thylakoid membrane.

  4. Energy released during this transfer enables the synthesis of ATP from ADP and phosphate.

  5. The electrons reach PSI, where further light absorption increases their energy levels again.

  6. High-energy electrons from PSI are used to reduce $NADP^+$ to $NADPH$.

ATP Synthesis and Chemiosmosis

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

• Chemiosmosis mechanism:

  • As electrons pass through the transport chain in the thylakoid membrane, the released energy is used to pump hydrogen ions ($H^+$) 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 down this gradient (from high to low concentration) drives the production of ATP.

Cyclic Phosphorylation

• The light-independent reactions require more ATP than non-cyclic phosphorylation provides.

• Cyclic phosphorylation serves as an additional source of ATP.

• In this cycle, only Photosystem I (PSI) is involved.

• Excited electrons from PSI are transferred to the electron transport chain located between PSII and PSI, rather than being used to reduce $NADP^+$.

• The electrons are then transported back to PSI.

• This process produces ATP but does not generate NADPH.

The Light-Independent Reactions (Carbon Fixation)

• Also referred to as the "Dark reaction," these occur in the stroma of the chloroplast.

• Carbon fixation involves capturing atmospheric carbon dioxide (or dissolved carbon dioxide for aquatic organisms) and adding hydrogen to form carbohydrates.

• The energy for these reactions is provided by the ATP and NADPH generated in the light-dependent phase.

• The Calvin Cycle steps:

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

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

  3. The GP molecules (also known as phosphoglycerate or PGA) are phosphorylated by ATP into glycerate diphosphate.

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

• Fate of the 3-carbon GALP/PGAL molecules:

  • For every 12 molecules of PGAL produced, 2 are removed from the cycle to synthesize one glucose molecule or other carbohydrates, lipids, or amino acids.

  • The remaining 10 PGAL molecules are converted using ATP energy to reform 6 molecules of RuBP, allowing the cycle to restart.

Factors Affecting the Rate of Photosynthesis

• The rate is determined by "limiting factors," which include light intensity, carbon dioxide concentration, and temperature.

• Light Intensity:

  • The rate increases proportionately with light intensity until another factor becomes limiting.

  • Wavelength is critical: PSI absorbs most efficiently at $700\,nm$, and PSII at $680\,nm$.

• Carbon Dioxide Concentration:

  • Increasing $CO_2$ concentration increases the rate of carbon incorporation in the light-independent reactions until the rate plateaus due to other limiting factors.

• Temperature:

  • Photosynthesis is an enzyme-catalyzed reaction.

  • The rate increases as temperatures approach the enzymes' optimum.

  • Once the optimum temperature is exceeded, the rate decreases and eventually stops due to enzyme denaturation.