Biology Lecture: Photosynthesis and Chromosomes

Administrative Announcements and Schedule

• Upcoming Quiz: A quiz is scheduled for Thursday. It will cover the remaining material from Chapter 6 (discussed last Wednesday) and all of Chapter 7.
• Exam 2 Schedule: The second exam will take place a week from Wednesday.
• Exam 2 Coverage: This exam will encompass all material from Chapters 5, 6, and 7.

Overview of Photosynthesis (Chapter 7)

• General Definition: While grade school curricula often simplify photosynthesis as plants taking in carbon dioxide ($CO_2$) and water ($H_2O$) with sunlight energy to produce oxygen ($O_2$) and food (sugar, glucose), the process is significantly more complex.
• General Chemical Formula:   - $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
• Cellular Location: In eukaryotic organisms, photosynthesis occurs in a specialized organelle called the chloroplast.
• Chloroplast Structure:   - Outer Membrane: The outermost phospholipid bilayer.   - Inner Membrane: The second membrane, analogous to the mitochondrial inner membrane.   - Thylakoid: The third, green-pigmented membrane system. It is the site of sunlight absorption.   - Thylakoid Space: The internal region within the thylakoids.   - Stroma: The fluid-filled region surrounding the thylakoids, analogous to the matrix of a mitochondrion.
• Redox Nature of Photosynthesis: Photosynthesis is an oxidation-reduction reaction.   - Water ($H_2O$): Is oxidized, meaning it loses or donates electrons.   - Carbon Dioxide ($CO_2$): Is reduced, meaning it gains electrons.
• Two-Step Division of Photosynthesis:   - Light-Dependent Reactions: These occur on the thylakoid membrane and involve the absorption of sunlight.   - Light-Independent Reactions (Calvin Cycle): These occur in the stroma and involve the reduction of carbon dioxide.

Mechanisms of the Light-Dependent Reactions

• Light Absorption:   - Chlorophyll: A hydrophobic green dye embedded in the thylakoid membrane core. It absorbs all visible light colors except green, which it reflects.   - Electron Promotion: When chlorophyll absorbs a photon (packet of light), an electron is promoted to a higher quantum energy level.
• Energy Release Pathways:   - Fluorescence: If chlorophyll is isolated (non-biological setting), the excited electron falls back down, releasing energy as light and heat.   - Photosystem Organization: In the thylakoid, chlorophyll is organized by a protein scaffold called a photosystem.   - Photosystem Components: Consists of various chlorophyll units and a specialized central pair called the reaction center.
• Resonance Energy Transfer: The photosystem acts like an antenna. When one chlorophyll is excited, it transfers that energy to a neighbor, which excites the next neighbor. This "wave" of excitation continues until reaching the reaction center. This is called resonance energy transfer; electrons do not physically jump between chlorophylls here.
• Reaction Center Oxidation: At the reaction center, the excited chlorophyll does not release energy as heat. Instead, it is oxidized, losing its electron to a mobile electron carrier. This conversion marks the transition from light energy to chemical energy (reducing power).

Detailed Steps of the Light-Dependent Reactions ($Z$-Scheme)

• Requirement of Two Photons: A single photon does not provide enough energy to reduce $NADP^+$; therefore, electrons receive two "doses" of sunlight energy through two separate photosystems operating in series.
• Photosystem II (PSII):   - Despite the name, it occurs first in the sequence.   - It absorbs a photon, exciting the reaction center.   - Oxygen Generating Complex: A cluster of proteins at PSII that splits water ($H_2O \rightarrow \frac{1}{2}O_2 + 2H^+ + 2e^-$).   - The electrons from water replace those lost by the reaction center.   - Atmospheric Oxygen: This complex is the source of all oxygen on Earth. Geologists use oxygen levels in rocks to determine when photosynthesis evolved.
• Electron Transport Chain (ETC): Electrons move from PSII through mobile carriers to the ETC. As they move, the energy is used to pump hydrogen ions ($H^+$) from the stroma into the thylakoid space, creating a gradient.
• Photosystem I (PSI):   - Electrons arrive at the PSI reaction center and absorb a second photon.   - The re-energized electrons are passed to a final mobile carrier.
• $NADP^+$ Reductase: This protein uses the high-energy electrons to reduce $NADP^+$ into $NADPH$ (used for anabolic building reactions).
• ATP Synthesis: The high concentration of $H^+$ in the thylakoid space diffuses back to the stroma through ATP synthase. This protein captures the kinetic energy of the flow to catalyze $ADP + P_i \rightarrow ATP$.

The Light-Independent Reactions (The Calvin Cycle)

• Location: Occurs in the stroma of the chloroplast.
• Pre-existing Intermediate: Ribulose 1,5-bisphosphate ($RuBP$), a 5-carbon carbohydrate.
• Carbon Fixation: Three units of $CO_2$ from the atmosphere are attached to three units of $RuBP$ (totaling 15 carbons from $RuBP$ + 3 from $CO_2$ = 18 carbons).
• Rubisco: The enzyme responsible for fixing $CO_2$ to $RuBP$. It is remarkably inefficient and slow, so the chloroplast produces it in massive quantities. It is considered the most abundant enzyme on Earth.
• Reduction Phase: The 18-carbon intermediate is processed into six units of Glycerate 3-phosphate, which are then reduced using ATP and $NADPH$ into six units of Glyceraldehyde 3-phosphate (G3P).
• Cycle Output:   - One unit of G3P (3 carbons) leaves the cycle to be used for food and biosynthesis.   - The remaining five units of G3P (15 carbons) stay in the cycle.
• Regeneration Phase: The five remaining G3P units are converted back into three units of $RuBP$ (15 carbons) through a series of steps powered by ATP hydrolysis, allowing the cycle to repeat.

The Significance of the Photosynthetic Output (G3P)

• Glyceraldehyde 3-phosphate (G3P): This is the immediate product of photosynthesis (the same molecule found in glycolysis).
• Biosynthetic Pathways: Plants use G3P to create all necessary organic macromolecules:   - Glucose and Starch: G3P units are joined to make glucose, then starch for energy storage.   - Cellulose: Used for structural support in plant cell walls.   - Lipids: G3P is converted to Acetyl-CoA, then into fatty acids and lipids.   - Proteins and Nucleic Acids: Precursors are pulled from the citric acid cycle to build amino acids and nucleotides.
• Philosophical/Biological Implications:   - Heterotrophs (Humans): Can only survive by destroying and consuming other living things (plants or animals) to obtain high-energy molecules.   - Autotrophs (Plants): Perform a "miracle" by taking dead, inert material ($CO_2$, $H_2O$) and diffuse solar energy and assembling them into new living flesh (low entropy, high energy) from the void.

Introduction to DNA Organization and Cell Division (Chapter 8)

• Chromosomes: DNA in eukaryotic cells is organized into finite bodies called chromosomes ("colored bodies").
• Structural Components:   - Centromere: The central region where DNA strands are joined.   - Telomeres: The protective ends of the chromosome.
• Diploid State: Most human cells are diploid ($2n$), meaning chromosomes exist in pairs (one from the biological father, one from the mother).
• Human Chromosome Count: Humans have 23 pairs of chromosomes, totaling 46.
• Genes and Alleles:   - Genes: Instructions for specific proteins (e.g., insulin, keratin); both homologous chromosomes carry the same genes in the same locations.   - Alleles: Variations of a specific gene (e.g., a variant for straight vs. wavy hair).
• Autosomal vs. Sex Chromosomes:   - Autosomes: Chromosome pairs 1 through 22; they function normally as homologous pairs.   - Sex Chromosomes (Pair 23): Determine biological sex.     - X Chromosome: A normal, gene-rich chromosome.     - Y Chromosome: A small, degenerate chromosome containing the SRY gene, which triggers male development.     - XX: Biologically female (default path).     - XY: Biologically male (due to SRY activation).     - YY: Non-viable; a cell cannot survive without the genes found on the X chromosome.

DNA Replication and Nomenclature

• Chromatin: The diffuse, uncoiled state of DNA found in the nucleus for most of the cell's life.
• Sister Chromatids: When a cell prepares to divide, every chromosome replicates. The two identical copies remain attached at the centromere as "sister chromatids."
• Homologous vs. Sister:   - Homologous Chromosomes: Two separate chromosomes representing the same gene set (one from each parent).   - Sister Chromatids: Two identical copies of a single chromosome produced by DNA replication.
• Visual Representation: Popular culture images of "X-shaped" chromosomes actually depict the replicated form (two sister chromatids), not the single-strand form existing during most of the cell cycle.