Introduction: The Basic Unit of Life
Welcome to Lesson 6. The cell is the fundamental structural and functional unit of all known organisms. In this lesson, we will explore the two primary types of cells—prokaryotic and eukaryotic—and then take a detailed tour of the sophisticated internal structures, or organelles, that allow eukaryotic cells to perform their complex functions.
Learning Objectives
- LO 6.1: Compare and contrast the key features of prokaryotic and eukaryotic cells.
- LO 6.2: Describe the structure and function of the nucleus, endomembrane system (ER, Golgi, lysosomes), mitochondria, and peroxisomes.
- LO 6.3: Identify the three main components of the cytoskeleton and describe their respective roles in cell shape, motility, and organization.
- LO 6.4: Distinguish between a typical animal cell and a plant cell, explaining the functions of the cell wall, chloroplasts, and central vacuole.
Part 1: Prokaryotes vs. Eukaryotes
All life can be categorized into two major domains based on cellular structure: Prokaryotes (bacteria and archaea) and Eukaryotes (protists, fungi, plants, and animals). The primary distinction is the presence of a true, membrane-bound nucleus and other organelles in eukaryotes.
Diagram: Prokaryotic vs. Eukaryotic Cell Structure
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Nucleus | Absent. Genetic material is in a nucleoid region. | Present. A double membrane encloses the DNA. |
| Membrane-Bound Organelles | Absent (no mitochondria, ER, Golgi, etc.). | Present. Allows for compartmentalization of functions. |
| DNA Structure | Single, circular chromosome. | Multiple, linear chromosomes complexed with proteins (chromatin). |
| Size | Typically small (1-5 μm). | Typically larger (10-100 μm). |
| Ribosomes | Smaller (70S). | Larger (80S in cytoplasm; 70S in mitochondria/chloroplasts). |
| Cell Wall | Usually present, made of peptidoglycan (in bacteria). | Present in plants (cellulose) and fungi (chitin); absent in animals. |
Part 2: The Eukaryotic Cell: A Detailed Tour
2.1 The Nucleus & Ribosomes: Information and Synthesis
The nucleus contains the cell's genetic material (DNA) and controls cell activities by regulating gene expression. The nuclear envelope is a double membrane with nuclear pores that regulate transport. The nucleolus is where ribosomal RNA (rRNA) is synthesized. Ribosomes, made of rRNA and protein, translate genetic information into proteins, either free in the cytosol or bound to the ER.
2.2 The Endomembrane System: Manufacturing and Shipping
This is a network of interconnected internal membranes that work together to synthesize, modify, package, and transport lipids and proteins.
- Endoplasmic Reticulum (ER): A network of membranous tubules and sacs. The Rough ER has bound ribosomes and produces proteins and membranes. The Smooth ER synthesizes lipids, detoxifies poisons, and stores calcium.
- Golgi Apparatus: A stack of flattened sacs (cisternae). It receives vesicles from the ER at its cis face, modifies their contents, sorts them, and ships them from its trans face.
- Lysosomes: Membranous sacs of hydrolytic enzymes found in animal cells. They digest macromolecules via phagocytosis (engulfing food particles) and recycle the cell’s own damaged organelles via autophagy.
Diagram: The Endomembrane System Workflow
2.3 Mitochondria & Peroxisomes: Energy and Metabolism
The mitochondrion is the site of cellular respiration. It has a double membrane; the inner membrane is folded into cristae. The inner compartment is the mitochondrial matrix. Peroxisomes are metabolic compartments that use oxygen to break down fatty acids and detoxify harmful substances, producing and then neutralizing hydrogen peroxide.
Diagram: Structure of Mitochondrion and Peroxisome
2.4 The Cytoskeleton: Support and Motility
The cytoskeleton is a network of protein fibers that organizes the cell’s structures and activities. It is composed of three main types of fibers:
Diagram: Components of the Cytoskeleton
Part 3: Plant Cells vs. Animal Cells
Plant cells have several unique structures not found in animal cells, which are related to their different lifestyles.
| Structure | Function | Found In |
|---|---|---|
| Cell Wall | Maintains cell shape, protects the cell, prevents excessive water uptake. Made of cellulose. | Plants, Fungi, Prokaryotes (not animals) |
| Chloroplasts | Site of photosynthesis, converting solar energy to chemical energy. | Plants and some protists |
| Central Vacuole | Stores water, ions, and nutrients; maintains turgor pressure; breaks down waste. | Mature plant cells |
| Lysosomes | Digestion and recycling of macromolecules. | Animal cells (rare in plants) |
| Centrosomes | Region where microtubules are initiated; contains centrioles in animal cells. | Animal cells |
Diagram: Unique Structures of a Plant Cell
Interactive Practice Quiz
Test your understanding of cell structure. Choose the best answer for each question (A-E) and then submit to see your results.