Chapter 6: The Cell

Structural Organization of Cells

Cells are the fundamental units of life, and their structural organization varies between prokaryotic and eukaryotic types. Understanding these differences is essential for grasping cellular function and evolution.

• Prokaryotic vs. Eukaryotic Cells: - Prokaryotic cells lack a nucleus and membrane-bound organelles. Their genetic material is located in the nucleoid region. - Eukaryotic cells possess a true nucleus and various membrane-bound organelles. - Size: Prokaryotic cells are generally smaller (1-10 μm) than eukaryotic cells (10-100 μm). - Advantages of Small Cell Size: Higher surface area-to-volume ratio facilitates efficient exchange of materials with the environment.

• Example: Escherichia coli (prokaryote) vs. human epithelial cell (eukaryote).

Microscopy in Biology

Microscopy is essential for studying cells and their components. Different types of microscopes offer varying levels of resolution and magnification.

• Types of Microscopy:

  • Light Microscopy: Uses visible light to observe living cells; limited resolution (~200 nm).

  • Electron Microscopy: Uses electron beams for much higher resolution; includes Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM).

• Advantages and Disadvantages:

  • Light microscopes allow observation of living cells but have lower resolution.

  • Electron microscopes provide detailed images of cell ultrastructure but require fixed (dead) specimens.

• Formula: where is the wavelength of light, is the refractive index, and is the half-angle of the maximum cone of light that can enter the lens.

Eukaryotic Cell Structure

Eukaryotic cells contain various organelles, each with specialized functions. The plasma membrane separates the cell from its environment, while the cytoplasm houses organelles and the cytosol.

• Plasma Membrane: Phospholipid bilayer that regulates entry and exit of substances.

• Organelles and Components:

  • Nucleus: Contains genetic material (DNA).

  • Rough and Smooth Endoplasmic Reticulum (ER): Protein and lipid synthesis.

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

  • Mitochondria: Site of cellular respiration and ATP production.

  • Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

  • Transport Vesicles: Move materials between organelles.

• Cytosol: Fluid portion of the cytoplasm where metabolic reactions occur.

• Example: The mitochondrion is often called the "powerhouse" of the cell due to its role in energy production.

Cytoskeleton and Cell Motility

The cytoskeleton provides structural support and enables cell movement. It consists of microfilaments, intermediate filaments, and microtubules.

• Microfilaments: Composed of actin; involved in cell shape and movement.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Hollow tubes made of tubulin; involved in organelle movement and cell division.

• Extracellular Matrix (ECM): Network outside animal cells that provides structural and biochemical support.

• Motility Structures:

  • Cilia: Short, hair-like structures for movement or fluid transport.

  • Flagella: Longer, whip-like structures for cell locomotion.

• Centrioles: Organize microtubules during cell division.

• Example: Human respiratory tract cells use cilia to move mucus.

Plant vs. Animal Cells

Plant and animal cells share many features but also have distinct differences, especially in their structural components and organelles.

• Cell Wall: Present in plant cells; provides rigidity and protection.

• Central Vacuole: Large organelle in plant cells for storage and maintaining turgor pressure.

• Chloroplasts: Site of photosynthesis in plant cells (not present in animal cells).

• Lysosome Function: In animal cells, lysosomes digest macromolecules; in plant cells, the central vacuole can serve similar functions.

• Table: Comparison of Plant and Animal Cells

• Example: The central vacuole in plant cells stores water, nutrients, and waste products.