Overview
This lecture covers the structure, function, and organization of cells, focusing on differences between prokaryotic and eukaryotic cells, the role of cell membranes, membrane transport, osmosis, and water potential.
Introduction to Cells
- Cells are the basic unit of structure and function in all organisms.
- All cells have a membrane, cytoplasm, and genetic material (DNA).
- Proteins, especially enzymes, are vital for cell function and metabolism.
- Genes in DNA are transcribed into messenger RNA, then translated into proteins by ribosomes.
Prokaryotic vs. Eukaryotic Cells
- Prokaryotic cells are small, simple, lack a nucleus, and have circular DNA (chromosomes and plasmids).
- Eukaryotic cells are larger, complex, have a nucleus, linear chromosomes, and many membrane-bound organelles, including mitochondria.
Cell Size and Surface Area
- Cells must be small to maintain a high surface area-to-volume ratio for efficient diffusion.
- Surface area increases by thin sheets, folds (e.g., villi, mitochondria), or large flat structures (elephant ears).
- Large marine mammals lose less heat due to a lower surface area-to-volume ratio.
Compartmentalization and Endomembrane System
- Compartmentalization in eukaryotic cells creates distinct internal environments for specialized functions.
- The endomembrane system includes nuclear membrane, ER, Golgi, lysosomes, and vesicles.
- Mitochondria and chloroplasts originated via endosymbiosis and have their own DNA and ribosomes.
Cell Organelles and Their Functions
- Nucleus stores genetic information and controls gene expression.
- Ribosomes synthesize proteins, either free in cytoplasm or bound to rough ER.
- Mitochondria produce ATP from food; they have a highly folded inner membrane.
- Rough ER makes proteins; smooth ER synthesizes lipids and detoxifies substances.
- Golgi complex modifies and packages proteins for transport.
- Lysosomes (animal cells only) digest materials and recycle organelles.
- Cytoskeleton provides structure and enables cell movement.
- Centrosomes organize spindle fibers for cell division.
- Central vacuole (plant cells) stores water and maintains pressure.
- Chloroplasts conduct photosynthesis.
- Plant cell walls, made of cellulose, prevent overexpansion from osmotic pressure.
Membrane Structure and Function
- Cell membrane is a selectively permeable phospholipid bilayer with embedded proteins and cholesterol (fluid mosaic model).
- Proteins may span (transmembrane), partially embed (integral), or attach to surfaces (peripheral).
Membrane Transport
- Passive transport (diffusion, facilitated diffusion) moves substances down concentration gradients without energy.
- Active transport uses ATP to move substances against gradients.
- Bulk transport: endocytosis brings materials in; exocytosis exports them.
Osmosis and Water Potential
- Osmosis is water movement from high to low concentration (hypotonic to hypertonic).
- In plants, osmotic pressure maintains turgor; plasmolysis occurs in hypertonic environments.
- In animals, isotonic conditions prevent cell shrinkage or bursting.
- Contractile vacuoles in protists regulate water balance.
- Stomata are pores for gas exchange, regulated by guard cells via osmotic changes.
- Water potential (Ψ) quantifies water's tendency to move; solute lowers Ψ, pressure increases Ψ.
- Water flows from areas of higher to lower water potential.
Key Terms & Definitions
- Prokaryote — Cell lacking a nucleus and membrane-bound organelles.
- Eukaryote — Cell with a nucleus and membrane-bound organelles.
- Endosymbiosis — Process by which organelles like mitochondria originated from engulfed prokaryotes.
- Phospholipid Bilayer — Dual-layered membrane structure with hydrophilic heads and hydrophobic tails.
- Osmosis — Diffusion of water across a semipermeable membrane.
- Turgor Pressure — Pressure of water inside plant cells against the cell wall.
- Water Potential (Ψ) — Measurement of water's potential to move; Ψ = solute potential + pressure potential.
Action Items / Next Steps
- Review the checklist at apbiosuccess.com/checklist.
- Complete interactive tutorials, quizzes, and reviews at learn-biology.com.
- Study the formulas for surface area/volume and water potential.
- Prepare for unit 3 by reviewing related songs and videos.