Cell Structure and Classification

Distinguishing Prokaryotes and Eukaryotes

Cells are classified into two major types: prokaryotic and eukaryotic. Understanding their differences is fundamental in biology.

• Prokaryotes (e.g., Escherichia coli): Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region.

• Eukaryotes (e.g., amoeba, yeast): Possess a true nucleus and various membrane-bound organelles.

• Key distinguishing features: Presence/absence of nucleus, organelle complexity, cell size, and reproduction methods.

Composition of the Cell Membrane

The cell membrane is a selectively permeable barrier that surrounds all cells, maintaining homeostasis and mediating transport.

• Phospholipid bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, functioning in cell recognition.

• Cholesterol: Stabilizes membrane fluidity (mainly in animal cells).

Features of Prokaryotic Cells

Prokaryotic cells have unique structural and functional features.

• Nucleoid: Region containing circular DNA.

• Cell wall: Provides structural support (composed of peptidoglycan in bacteria).

• Ribosomes: Smaller (70S) than eukaryotic ribosomes.

• Additional info: Some prokaryotes have flagella or pili for movement and attachment.

Features of Eukaryotic Cells

Eukaryotic cells are more complex and contain various organelles.

• Nucleus: Contains genetic material (DNA).

• Mitochondria: Site of cellular respiration.

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes.

• Cytoskeleton: Provides structural support and facilitates movement.

Comparative Table: Prokaryotes vs. Eukaryotes

The following table summarizes the main differences between prokaryotic and eukaryotic cells.

Cell Membrane Structure and Function

Plasma Membrane Composition and Function

The plasma membrane controls the movement of substances in and out of the cell, maintaining the internal environment.

• Phospholipid bilayer: Provides a semi-permeable barrier.

• Proteins: Transport, signal transduction, and cell recognition.

• Carbohydrates: Cell-cell recognition and communication.

• Fluid mosaic model: Describes the dynamic arrangement of lipids and proteins.

Glycolipids and Glycoproteins

Glycolipids and glycoproteins are important for cell recognition and signaling.

• Glycolipids: Lipids with carbohydrate chains attached; involved in cell recognition.

• Glycoproteins: Proteins with carbohydrate chains; function as receptors and in immune response.

Membrane Transport Mechanisms

Passive Transport

Passive transport involves the movement of substances across the membrane without energy input.

• Simple diffusion: Movement of small, nonpolar molecules down their concentration gradient.

• Facilitated diffusion: Movement of larger or polar molecules via transport proteins.

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

Example: Oxygen and carbon dioxide move into and out of cells by simple diffusion.

Equation for diffusion rate:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from the movement of another substance.

Example: The sodium-potassium pump maintains ion gradients in animal cells.

Equation for sodium-potassium pump:

Types of Osmotic Solutions

Osmosis can occur in different types of solutions, affecting cell volume.

• Isotonic: No net movement of water; cell volume remains constant.

• Hypotonic: Water enters the cell; cell may swell and burst.

• Hypertonic: Water leaves the cell; cell shrinks.

Endocytosis and Exocytosis

Endocytosis

Endocytosis is the process by which cells take in large particles or fluids by engulfing them in vesicles.

• Phagocytosis: "Cell eating"; uptake of large particles (e.g., bacteria).

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

Example: White blood cells engulf bacteria by phagocytosis.

Exocytosis

Exocytosis is the process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Function: Secretion of hormones, neurotransmitters, and waste products.

• Example: Release of insulin from pancreatic cells.

Comparison Table: Endocytosis vs. Exocytosis

Summary of Key Terms

• Prokaryote: Cell without a nucleus or membrane-bound organelles.

• Eukaryote: Cell with a nucleus and membrane-bound organelles.

• Phospholipid bilayer: Main component of cell membranes.

• Passive transport: Movement of substances without energy input.

• Active transport: Movement of substances requiring energy.

• Endocytosis: Uptake of materials into the cell.

• Exocytosis: Release of materials from the cell.

Additional info: The notes also reference the ability to recognize and describe the differences between types of endocytosis and exocytosis, and to use diagrams and examples for illustration. For exam preparation, students should be able to compare and contrast these processes, and understand their biological significance.