Eukaryotic Cells

Characteristics:

• Nucleus with nuclear envelope
• Membrane-bound organelles
• Larger (10-100 μm)
• Examples: Animals, plants, fungi, protists

Structures:

• Nucleus: contains DNA
• Membrane-bound organelles:
  • Mitochondria
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes (animals)
  • Peroxisomes
• Ribosomes: 80S (larger than prokaryotic)
• Cytoskeleton: structural support, movement

Plant vs. Animal Cells

Unique to Plants:

• Cell wall: cellulose
• Chloroplasts: photosynthesis
• Central vacuole: storage, turgor pressure
• Plasmodesmata: channels between cells

Unique to Animals:

• Centrioles: organize spindle fibers
• Lysosomes: digestion (rare in plants)

Surface Area to Volume Ratio

Importance:

• Limits cell size
• Larger cells have lower SA:V ratio
• Less efficient diffusion as cells grow
• Cells divide to maintain high SA:V

Calculation: For a cube with side length a:

• Surface area = 6a²
• Volume = a³
• Ratio = 6a²/a³ = 6/a

As a increases, ratio decreases!

Key Concepts

1. Prokaryotes lack nucleus and organelles
2. Eukaryotes have nucleus and organelles
3. Plant cells have cell wall, chloroplasts, central vacuole
4. Surface area to volume ratio limits cell size
5. All cells have plasma membrane, ribosomes, DNA

(a) Membrane-bound organelles:

Prokaryotic: ✗ No compartmentalization

• All reactions occur in cytoplasm
• Plasma membrane folds may increase surface area (mesosomes)

Eukaryotic: ✓ Extensive compartmentalization

• Nucleus, mitochondria, ER, Golgi, lysosomes, peroxisomes
• Allows specialized functions in separate compartments

(b) DNA organization:

Prokaryotic:

• Single circular chromosome in nucleoid
• No histones (except Archaea)
• Plasmids (small circular DNA) common
• Genes lack introns (continuous coding)

Eukaryotic:

• Multiple linear chromosomes
• DNA wrapped around histone proteins → chromatin
• Genes contain introns and exons
• Much more DNA (genome size 1000x larger typically)

(c) Cell size:

Prokaryotic: 0.1-5.0 μm

• E. coli: ~2 μm long
• Small size = high surface area to volume ratio
• Efficient nutrient uptake

Eukaryotic: 10-100 μm

• Human cell: ~10-30 μm
• Larger, more complex
• Require internal membrane systems

(d) Examples:

Prokaryotic:

• Bacteria: E. coli, Streptococcus, Cyanobacteria
• Archaea: Methanogens, Halophiles, Thermophiles

Eukaryotic:

• Protists: Amoeba, Paramecium, Euglena
• Fungi: Yeast, mushrooms
• Plants: All plants
• Animals: All animals

$\boxed{\text{Prokaryotes: simple, no nucleus; Eukaryotes: complex, membrane-bound organelles}}$