Cell Structure and Compartmentalization

Overview of Eukaryotic Cell Organization

Eukaryotic cells are highly organized structures that use internal membranes to create distinct compartments, allowing specialized chemical reactions to occur efficiently. This compartmentalization is essential for the complex functions required for life.

• Compartmentalization: Internal membranes divide the cell into organelles, each with specific roles.

• Energy and Matter Transformations: Organelles such as mitochondria and chloroplasts convert energy and synthesize biomolecules.

• Genetic Information Storage and Transmission: The nucleus stores DNA, and ribosomes synthesize proteins.

• Interactions with the Environment: The plasma membrane regulates the movement of substances in and out of the cell.

Example: Chloroplasts in plant cells convert light energy to chemical energy via photosynthesis, while mitochondria generate ATP through cellular respiration.

Organelles and Their Functions

Major Eukaryotic Organelles

Each organelle within a eukaryotic cell performs specific functions necessary for cell survival and activity.

• Nucleus: Contains genetic material (DNA) and is the site of transcription.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or bound to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Rough ER has ribosomes and synthesizes proteins; Smooth ER is involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Vacuoles: Storage organelles; central vacuole in plants maintains turgor pressure.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plant cells.

Example: The rough ER produces membrane-bound proteins, which are then processed in the Golgi apparatus and sent to their destinations.

Cell Membranes and Transport

Structure of Biological Membranes

Cell membranes are composed of a phospholipid bilayer with embedded proteins, creating a selectively permeable barrier.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Fluid Mosaic Model: Membranes are dynamic, with lipids and proteins moving laterally.

• Membrane Proteins: Serve as channels, carriers, receptors, and enzymes.

• Cholesterol: Modulates membrane fluidity.

Example: Integral membrane proteins span the bilayer and facilitate transport of molecules.

Membrane Transport Mechanisms

Cells use various mechanisms to move substances across membranes, maintaining homeostasis.

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

• Diffusion: Spontaneous movement from high to low concentration.

• Facilitated Diffusion: Uses protein channels or carriers for polar or charged molecules.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement against the concentration gradient using energy (usually ATP).

• Bulk Transport: Endocytosis brings materials into the cell; Exocytosis expels materials.

Equation:

Example: The sodium-potassium pump uses ATP to maintain ion gradients across the plasma membrane.

Osmosis and Tonicity

Osmosis affects cell volume and function depending on the surrounding solution's tonicity.

• Hypotonic Solution: Lower solute concentration outside; water enters cell, causing swelling.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves cell, causing shrinkage.

Example: Plant cells become turgid in hypotonic solutions due to water influx and cell wall resistance.

Prokaryotic Cell Structure and Adaptations

Prokaryotic Cell Features

Prokaryotes, including bacteria and archaea, are unicellular organisms with simple cell structure but remarkable adaptability.

• Cell Wall: Maintains shape, protects, and prevents lysis in hypotonic environments.

• Peptidoglycan: Main component of bacterial cell walls; absent in archaea.

• Capsule: Polysaccharide or protein layer for protection and adhesion.

• Fimbriae: Hair-like structures for attachment.

• Pili: Longer structures for DNA exchange (conjugation).

• Endospores: Dormant, resistant cells for survival in harsh conditions.

• Flagella: Structures for motility; differ in structure between bacteria, archaea, and eukaryotes.

Example: Gram-positive bacteria have thick peptidoglycan walls, while Gram-negative bacteria have an outer membrane and are often more resistant to antibiotics.

Adaptations and Diversity

Prokaryotes thrive in diverse environments, including extreme conditions, due to their structural and metabolic adaptations.

• Shape: Spherical (cocci), rod-shaped (bacilli), spiral.

• Taxis: Movement toward or away from stimuli (e.g., chemotaxis).

• Endospores: Allow survival in extreme environments.

Example: Prokaryotes in the Great Salt Lake survive high salinity due to specialized adaptations.

Comparing Prokaryotic and Eukaryotic Cells

Key Differences

Prokaryotic and eukaryotic cells differ in complexity, organization, and the presence of membrane-bound organelles.

• Prokaryotes: No nucleus, no membrane-bound organelles, smaller size (0.5–5 μm).

• Eukaryotes: Nucleus, membrane-bound organelles, larger size (10–100 μm).

Additional info: Eukaryotic cells can be multicellular, while prokaryotes are typically unicellular but may form colonies.