Honors Biology: Photosynthesis, Transpiration & Cellular Respiration
Nutrition & Energy Basics
Definition: Organisms obtain energy by consuming organic molecules (food) produced by other organisms.
Mechanism: Ingest and digest complex organic compounds (e.g., glucose, proteins) into smaller molecules that can be absorbed and used in cellular respiration.
Examples: Animals, fungi, most bacteria.
Connection: Relies on autotrophs as the base of the food chain.
Definition: Organisms produce their own organic molecules (glucose) from inorganic sources using energy from light or chemicals.
Mechanism: Use photosynthesis (light energy) or chemosynthesis (chemical energy) to convert CO₂ and H₂O into glucose.
Examples: Green plants, algae, some bacteria.
Connection: Primary producers in ecosystems; foundation of all food webs.
Reason: Releasing all of glucose’s energy at once would be explosive and wasteful (mostly lost as heat and light).
Mechanism: Cellular respiration breaks glucose down in small, controlled steps using enzymes to capture energy in ATP.
Connection: ATP is the universal energy currency; glucose is a storage molecule (stores ~90x more chemical energy than one ATP).
Structure: Adenine + ribose + three phosphate groups.
Function: High-energy bonds between phosphate groups store and release energy when broken (ATP → ADP + Pᵢ + energy).
Mechanism: Energy from food (glucose) is transferred to ATP during cellular respiration; ATP powers nearly all cellular work.
Connection: Cells convert glucose to ATP to use energy in small, usable packets.
Photosynthesis
Purpose: Convert light energy into chemical energy stored in glucose.
Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Who: Autotrophs (plants, algae, some bacteria).
When: Only in the presence of light.
Location: Chloroplasts (specifically thylakoids and stroma).
Connection: Produces glucose (food) and oxygen (waste); reverse of cellular respiration.
Definition: Green pigment in chloroplasts that absorbs light energy (mainly red and blue wavelengths).
Mechanism: Excites electrons when hit by photons, initiating the light-dependent reactions.
Location: Embedded in thylakoid membranes (in photosystems).
Connection: Reflects green light (why plants appear green); critical for capturing light energy.
Stroma: Fluid-filled space; site of light-independent reactions (Calvin cycle).
Thylakoids: Flattened membrane sacs; contain chlorophyll; site of light-dependent reactions.
Granum (Grana): Stacks of thylakoids; increase surface area for light absorption.
Connection: Double membrane isolates reactions; thylakoid lumen builds H⁺ gradient for ATP synthesis.
Answer: Blue (400–500 nm) and Red (600–700 nm).
Reason: Chlorophyll a and b absorb most strongly in these ranges; peak absorption at ~430 nm and ~660 nm.
Connection: Action spectrum matches absorption spectrum of chlorophyll.
Answer: Green (500–600 nm).
Reason: Chlorophyll reflects green light; very little is absorbed.
Connection: Explains why plants appear green; accessory pigments (carotenoids) absorb some green/orange.
Stages of Photosynthesis
Where: Thylakoid membranes.
Why: Capture light energy and convert it to chemical energy (ATP and NADPH).
When: Requires light.
Inputs: Light, H₂O, NADP⁺, ADP + Pᵢ.
Outputs: O₂ (waste), ATP, NADPH.
Mechanism: Photosystems II → I → electron transport chain → chemiosmosis; water split to replace electrons.
Where: Stroma.
Why: Use ATP and NADPH to fix CO₂ into glucose (carbon fixation).
When: Can occur in light or dark (if ATP/NADPH available).
Inputs: CO₂, ATP, NADPH, RuBP.
Outputs: Glucose (G3P), ADP, NADP⁺, Pi.
Mechanism: 3 phases: Carbon fixation → Reduction → Regeneration of RuBP.
Definition: Incorporation of inorganic CO₂ into organic molecules (e.g., glucose).
Mechanism: Enzyme Rubisco attaches CO₂ to RuBP → unstable 6-carbon compound → splits into two 3-PGA.
Connection: First step of Calvin cycle; builds carbon skeletons for sugars; stores energy in C–C bonds.
Plant Metabolism & Leaf Anatomy
1. Cellular Respiration: Broken down in mitochondria to produce ATP.
2. Polymer Synthesis: Dehydration synthesis to form starch, cellulose, etc.
3. Building Blocks: Rearranged into lipids, amino acids, proteins, nucleic acids.
Extra Elements: N and P absorbed from soil for proteins and DNA.
Connection: Glucose is the central molecule in plant metabolism.
Cuticle: Waxy layer; prevents water loss, allows light penetration.
Palisade Mesophyll: Columnar cells; high chloroplast density; maximizes light absorption.
Spongy Mesophyll: Air spaces; gas diffusion (CO₂ in, O₂ out); supports transpiration.
Vein (Xylem & Phloem): Transports water/minerals up, sugars down; structural support.
Stomata: Pores for gas exchange and transpiration.
Guard Cells: Regulate stomatal opening via turgor pressure (K⁺ pump).
Transpiration & Stomatal Regulation
Definition: Evaporation of water from plant leaves (mainly through stomata).
Purpose: Cools plant; drives transpirational pull; delivers minerals.
Mechanism: Water evaporates → creates negative pressure → pulls water up xylem via cohesion-tension.
Connection: Links photosynthesis, water uptake, and nutrient transport.
Definition: Upward movement of water through xylem driven by evaporation at leaves.
Mechanism: Water molecules stick together (cohesion) and to xylem walls (adhesion); evaporation creates tension pulling water column up.
Connection: Allows water transport from roots to leaves against gravity; no energy required from plant.
Open Stomata: When guard cells take in K⁺ → water follows → turgid → stomata open.
When: Daytime, high light, low CO₂, adequate water.
Why: Allows CO₂ entry for photosynthesis.
Closed Stomata: K⁺ pumped out → water leaves → flaccid → stomata close.
When: Night, water stress, high temperature/humidity.
Why: Prevents water loss.
Connection: Feedback mechanism balances CO₂ uptake and water conservation.
Light: Opens stomata → increases evaporation.
Temperature: Higher temp → faster evaporation and diffusion.
Wind: Removes humid air → increases concentration gradient.
Humidity: High humidity → slows diffusion out of leaf.
Connection: All affect the rate of water vapor loss and thus transpirational pull.
Cellular Respiration
Purpose: Extract maximum energy from glucose using oxygen.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36 ATP (+ heat)
Who: Most eukaryotes and some prokaryotes.
When: Continuously, when O₂ is available.
Location: Cytoplasm (glycolysis) → Mitochondria (Krebs + ETC).
Connection: Reverse of photosynthesis; releases energy stored in food.
Where: Cytoplasm.
Why: Split glucose into two pyruvate; generate small ATP and NADH.
Inputs: Glucose, 2 ATP, 2 NAD⁺.
Outputs: 2 Pyruvate, 4 ATP (net 2), 2 NADH.
Mechanism: 10 enzyme-catalyzed steps; anaerobic; universal in all life.
Where: Mitochondrial matrix.
Why: Oxidize pyruvate → produce CO₂, NADH, FADH₂, and 2 ATP.
Inputs: 2 Pyruvate → 2 Acetyl-CoA → enters cycle.
Outputs (per glucose): 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.
Mechanism: Turns twice per glucose; generates electron carriers for ETC.
Where: Inner mitochondrial membrane.
Why: Use high-energy electrons from NADH/FADH₂ to pump H⁺ and create gradient for ATP synthesis.
Mechanism: Electrons passed through protein complexes → O₂ accepts electrons → H₂O formed.
Outputs: ~32 ATP via chemiosmosis; H₂O.
Connection: Oxygen is final electron acceptor; without it, ETC stops.
Role: Final electron acceptor in the ETC.
Mechanism: Accepts electrons from ETC → combines with H⁺ → forms H₂O.
Without O₂: NADH/FADH₂ cannot unload electrons → Krebs and ETC halt → only 2 ATP from glycolysis.
Connection: Explains why we breathe and why anaerobic respiration yields far less ATP.
Anaerobic Respiration & Fermentation
Definition: Energy production without oxygen.
Location: Cytoplasm.
Yield: Only 2 ATP per glucose.
Types: Alcohol fermentation, lactic acid fermentation.
Connection: Allows survival in low-oxygen conditions; regenerates NAD⁺ for glycolysis.
Equation: C₆H₁₂O₆ → 2CO₂ + 2 ethanol + 2 ATP
Who: Yeast, some bacteria.
Mechanism: Pyruvate → acetaldehyde → ethanol; regenerates NAD⁺.
Connection: Used in bread (CO₂ rises dough) and alcohol production.
Equation: C₆H₁₂O₆ → 2 lactic acid + 2 ATP
Who: Human muscle cells (during intense exercise), some bacteria.
Mechanism: Pyruvate reduced to lactate; regenerates NAD⁺.
Connection: Causes muscle fatigue and soreness; lactate sent to liver for recycling.
Electron Carriers
Function: Transport high-energy electrons between reactions.
NADP⁺/NADPH: Used in photosynthesis (light reactions); accepts electrons from PS I.
NAD⁺/NADH & FAD/FADH₂: Used in cellular respiration (Krebs & ETC).
Mechanism: Accept electrons (reduced) → donate to next step (oxidized).
Connection: Shuttle energy without losing it as heat.
Photosynthesis vs. Cellular Respiration
Energy Source: Light (PS) vs. Glucose (CR).
Equation: Opposite reactions.
Location: Chloroplasts vs. Mitochondria.
Organisms: Autotrophs vs. All organisms.
Timing: Light only vs. Continuous.
Energy Form: Light → Chemical (glucose) vs. Chemical (glucose) → ATP.
Connection: Interdependent; O₂ and CO₂ cycle between processes.