Cell Structure and Function: Eukaryotic and Prokaryotic Cells, Organelles, and Osmoregulation

Cell Types: Prokaryotic vs. Eukaryotic

Overview of Cell Types

Cells are the basic units of life and can be classified into two main types: prokaryotic and eukaryotic. Understanding their differences is fundamental in biology.

• Prokaryotic Cells: Cells that lack a membrane-bound nucleus and organelles. Bacteria are prokaryotic.

• Eukaryotic Cells: Cells that have a true nucleus and membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotic.

Key Differences:

• Genetic Material: Prokaryotes have circular DNA in the nucleoid; eukaryotes have linear DNA within a nucleus.

• Organelles: Eukaryotes possess membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum); prokaryotes do not.

• Cell Membrane: Both have a plasma membrane, but only eukaryotes have internal membranes.

• Cell Wall: Present in most prokaryotes and some eukaryotes (plants, fungi).

Example: Escherichia coli is a prokaryote; Homo sapiens cells are eukaryotic.

Cell Organelles and Their Functions

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes within eukaryotic cells, involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes proteins, especially those for secretion or membrane insertion.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals, and stores calcium ions.

Example: Liver cells have abundant smooth ER for detoxification.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Receives vesicles from the ER.

• Processes and tags proteins for their final destinations.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes. They break down macromolecules, old organelles, and foreign substances.

• Important for cellular digestion and recycling.

• Abundant in animal cells; rare in plant cells.

Vacuoles

Vacuoles are storage organelles found in both plant and animal cells, but are especially large in plant cells.

• Plant Vacuole: Maintains turgor pressure, stores nutrients and waste products.

• Contractile Vacuole: Found in some protists; pumps excess water out of the cell to maintain osmotic balance.

Example: Paramecium uses a contractile vacuole to expel water in hypotonic environments.

Cell Membranes and Cell Walls

Structure and Function

The cell membrane (plasma membrane) is a phospholipid bilayer that controls the movement of substances in and out of the cell.

• Phospholipid Bilayer: Provides fluidity and selective permeability.

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

The cell wall is a rigid structure outside the plasma membrane, providing support and protection. Found in plants, fungi, and most prokaryotes.

• Plant Cell Wall: Composed mainly of cellulose.

• Bacterial Cell Wall: Composed of peptidoglycan.

Osmoregulation in Cells

Maintaining Water Balance

Osmoregulation is the process by which cells control water balance to prevent excessive swelling or shrinking.

• In freshwater environments, cells may gain water by osmosis.

• Contractile vacuoles in protists pump out excess water to prevent lysis.

Example: If a protist is placed in pure water, it must use its contractile vacuole to expel water and avoid bursting.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Cell Wall	Usually (peptidoglycan)	Plants (cellulose), fungi (chitin), not in animals
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Key Terms

• Prokaryote: An organism whose cells lack a nucleus.

• Eukaryote: An organism whose cells contain a nucleus.

• Organelle: Specialized structure within a cell that performs a specific function.

• Osmoregulation: The control of water balance within a cell or organism.

• Contractile Vacuole: Organelle in some protists that expels excess water.