Biology Exam III Study Guide

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants use solar energy to convert carbon dioxide (CO₂) and water (H₂O) into sugars and other organic molecules, releasing oxygen gas (O₂) as a by-product. The general chemical reaction is represented as:

$\text{Light energy} + 6\text{CO}<em>2 + 6\text{H}</em>2\text{O} \rightarrow \text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\text{O}</em>2$

Key Components

• Reactants:
  • 6 CO₂
  • 6 H₂O
• Products:
  • C₆H₁₂O₆ (glucose)
  • O₂ (oxygen gas)

Types of Organisms

• Autotrophs: Organisms that produce their own food, primarily plants, which are the source of organic molecules for nearly all life.
• Photoautotrophs (Producers): Organisms that utilize light energy to synthesize their own food, converting it into glucose as their energy source.
• Heterotrophs: Organisms that cannot self-synthesize food and must consume plants, animals, or decomposed organic matter.

Examples of Photoautotrophs

• Plants
• Algae
• Cyanobacteria (photosynthetic bacteria)

Location of Photosynthesis

Photosynthesis primarily occurs in the leaves, which are the main sites of this process in most plants. All green parts of the plant contain chloroplasts, which are essential for energy conversion.

Chlorophyll

This is a light-absorbing pigment critical for converting solar energy into chemical forms. It is responsible for the green color of leaves.

Structure of Chloroplasts

• Chloroplasts are concentrated in the cells of the mesophyll layer.
• Stomata: Tiny pores through which CO₂ enters and O₂ exits the leaf.
• Veins: Transport water from roots to leaves and export sugars produced in photosynthesis.
• A typical mesophyll cell contains 30-40 chloroplasts.

Components of Chloroplasts

• Envelope of Two Membranes:
  • Outer Membrane: Protective layer.
  • Inner Membrane: Regulates the entry and exit of molecules.
  • Intermediate Membrane Space: Located between outer and inner membranes.
• Stroma: Fluid where the Calvin cycle occurs.
• Lumen: Fluid inside the thylakoid.

Thylakoids

These membranous sacs float in the stroma and enclose the thylakoid space.

• Granum (singular): A stack of thylakoids.
• Grana (plural): Multiple stacks.

Photosystems

• Light Reactions Occur Here: The thylakoid membranes, where chlorophyll captures light energy.

Light Reactions of Photosynthesis

1. Stage of Photosynthesis: Occurs in the thylakoids.
2. Function: Convert light energy into chemical energy (ATP and NADPH).
3. Process Summary:
   • Light energy excites electrons in chlorophyll.
   • Water is split (photolysis) to replace lost electrons.
   • Oxygen is released as a by-product.
   • An electron transport chain moves the excited electrons, releasing energy for ATP synthesis and forming NADPH, an electron carrier.

Chemical Reactions

• Oxidation and Reduction:
  • Oxidation: Loss of electrons (water to O₂).
  • Reduction: Gain of electrons (CO₂ to glucose).
• This simultaneous process is known as redox reactions.

Calvin Cycle (Light-Independent Reactions)

Overview

1. Location: Occurs in the stroma.
2. Inputs: Carbon dioxide and energy from light reactions (ATP and NADPH).
3. Carbon Fixation: CO₂ is fixed into organic molecules.
4. Reduction Phase: Energy from ATP and electrons from NADPH reduce these molecules into glucose (G3P).
5. Regeneration Phase: Remaining G3P molecules regenerate RuBP, enabling the cycle to continue.

Key Outcomes of the Calvin Cycle

• Output: G3P, which eventually forms glucose.
• For every 3 CO₂ that enter, 1 G3P leaves the cycle.
• The cycle must turn 6 times to produce enough G3P to generate 1 glucose molecule.

Energy Requirements for the Calvin Cycle

• Total Energy Consumption: Involves using 9 ATP and 6 NADPH molecules to synthesize one G3P.

C4 vs CAM Plants

• C4 Plants:
  • Separate CO₂ fixation and the Calvin cycle by location.
  • Examples: Corn, sugarcane.
  • Aim to reduce photorespiration.
• CAM Plants:
  • Separate CO₂ fixation and Calvin cycle by time.
  • At night: Stomata open for CO₂ storage.
  • During the day: Conduct the Calvin cycle.
  • Examples: Cactus, pineapple.
  • Purpose is to conserve water.

Cellular Respiration

Overview

Cellular respiration is an exergonic process that releases energy from glucose to produce ATP. It involves the consumption of O₂ as organic molecules are degraded to CO₂ and H₂O.

• Energy Yield: Can produce up to 32 ATP molecules per glucose molecule, utilizing approximately 34% of the energy stored in glucose with the others lost as heat.

Phases of Cellular Respiration

1. Stage 1: Glycolysis
   • Location: Cytosol
   • Breakdown of glucose to form two molecules of pyruvate.
2. Stage 2: Pyruvate Oxidation and Krebs Cycle
   • Location: Mitochondria
   • Completes glucose breakdown to CO₂.
3. Stage 3: Oxidative Phosphorylation and Electron Transport Chain
   • Location: Inner mitochondrial membrane.
   • Produces the majority of ATP.

Key Components of Mitochondria

• Outer Membrane: Protective barrier.
• Intermembrane Space: Area where protons accumulate.
• Inner Membrane: Contains electron transport chain and ATP synthase.
• Cristae: Folds that increase surface area for ATP production.
• Matrix: Innermost space, holds enzymes and mitochondrial DNA.

Redox Reactions in Cellular Respiration

• In cellular respiration, glucose (C₆H₁₂O₆) is oxidized to CO₂ while O₂ is reduced to H₂O, facilitating energy release utilized in ATP production.
• NAD⁺: Acts as an electron carrier, accepting high-energy electrons during glycolysis and the Krebs cycle to become NADH. The reaction is:

$\text{NAD}^+ + 2e^- + H^+ \rightarrow \text{NADH}$

Glycolysis

• Step 1: Energy Investment Phase
  • Uses 2 ATP to prepare glucose.
• Step 2: Energy Payoff Phase
  • Produces 4 ATP and 2 NADH.
• Outputs: For every glucose, you obtain 2 pyruvate, 2 NADH, and a net gain of 2 ATP.

Krebs Cycle

• Location: Mitochondrial matrix.
• Input: Acetyl-CoA enters the cycle and is oxidized.
• Outputs: For one glucose, yields 6 NADH, 2 FADH₂, 2 ATP, and 4 CO₂.

Oxidative Phosphorylation

1. Electron Transport Chain:
   • NADH and FADH₂ donate electrons, powering proton pumps across the inner membrane.
2. Chemiosmosis:
   • Flow of protons through ATP synthase generates ATP.
3. Overall Products: Approximately 26–28 ATP, along with water as a by-product when oxygen serves as the final electron acceptor.

Fermentation

When oxygen is absent, fermentation allows glycolysis to continue by regenerating NAD⁺. This process also produces small amounts of ATP compared to aerobic respiration.

• Types of Fermentation:
  • Lactic Acid Fermentation: Occurs in muscle cells, producing lactate.
  • Alcohol Fermentation: Occurs in yeast, producing ethanol and CO₂.

Summary of Processes

• Cellular Respiration: Fully breaks down glucose in the presence of oxygen to produce ~30–32 ATP.
• Fermentation: Occurs without oxygen, yielding significantly less ATP (2 ATP from glycolysis).

Comparison Table: Cellular Respiration vs. Fermentation

Conclusion

Understanding the detailed mechanisms of photosynthesis and cellular respiration is essential for grasping the fundamental biochemical processes that sustain life, bridging energy capture and utilization throughout ecosystems.