Cell Structure and Function

Why Biology for Engineers?

Biology underpins:

• Wastewater treatment (microbial processes)
• Bioremediation (bacteria degrading pollutants)
• Environmental impact assessment (ecosystems)
• Biomaterials and bioengineering

Cell Types

Prokaryotic Cells

• No nucleus — DNA is free in cytoplasm
• No membrane-bound organelles
• Smaller (1–10 μm)
• Examples: bacteria, archaea
• Importance: activated sludge in wastewater, nitrogen cycling, decomposition

Eukaryotic Cells

• Have nucleus and membrane-bound organelles
• Larger (10–100 μm)
• Examples: animals, plants, fungi, protists

Key Organelles (Eukaryotic)

| Organelle | Function | |-----------|----------| | Nucleus | Contains DNA, controls cell activity | | Mitochondria | Energy production (ATP) via respiration | | Ribosomes | Protein synthesis | | Endoplasmic Reticulum (ER) | Protein and lipid processing | | Golgi Apparatus | Packages and ships proteins | | Cell Membrane | Controls what enters/exits | | Vacuole | Storage | | Chloroplast (plant) | Photosynthesis |

Cell Membrane

Phospholipid bilayer — hydrophilic heads face water, hydrophobic tails face inward.

Functions:

• Selective permeability
• Cell communication
• Transport (passive, active, facilitated)

Cellular Respiration

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36–38 ATP

• Glycolysis (cytoplasm) → 2 ATP
• Krebs cycle (mitochondria) → 2 ATP
• Electron transport chain → 32–34 ATP

Anaerobic respiration (no O₂): produces less ATP + fermentation products (e.g., ethanol, lactic acid).

Relevance to Wastewater Treatment

• Aerobic bacteria break down BOD using oxygen
• Anaerobic bacteria digest sludge in digesters → produce methane (biogas)
• Nitrifying bacteria convert NH₄⁺ → NO₂⁻ → NO₃⁻
• Denitrifying bacteria convert NO₃⁻ → N₂ gas

Summary

Cells are the building blocks of life. Bacteria are the workhorses of biological treatment. Understanding metabolism and microbial processes is essential for environmental engineers.