5.1 All organisms are composed of cells.
Characteristics of Cells
• Prokaryotes have a single circular molecule of DNA, while eukaryotic cells have a nucleus, bounded by a nuclear membrane and containing DNA. (p. 80)
• Cytoplasm fills the interior of the cell, exclusive of the nucleus, and a plasma membrane encloses the cell and separates it from its surroundings. The plasma membrane contains several types of proteins that allow the cell to interact with its environment. (p. 80)
• Early cell theory included three principles: (1) All organisms are composed of one or more cells; (2) cells are the smallest living things; and (3) cells arise only by division of previously existing cells. (p. 81)
• Smaller cells are more advantageous than larger cells due to the limitations of communication in relation to surface area-to-volume ratios. (p. 81)
Visualizing Cells
• Resolution refers to the minimum distance two points can be apart and still be distinguished. (p. 82)
• Compound microscopes use magnifying lenses to achieve high magnification and clarity and to increase resolution. (p. 82)
• Other types of microscopes, including transmission electron microscopes and scanning electron microscopes, as well as staining procedures, are employed to view cells. (pp. 82—83)
5.2 Eukaryotic cells are more structurally complex than prokaryotic cells.
Prokaryotes: Simple Cells with Complex Physiology
• Most prokaryotic cells are encased by a strong cell wall and can be classified using the Gram staining procedure. (p. 84)
• Some prokaryotes use a flagellum for locomotion. (p. 84)
• Because prokaryotic cells lack membrane-bounded organelles, all cytoplasmic constituents have access to all areas of the cell. (p. 85)
Eukaryotes: Cells with Complex Interiors
• Eukaryotic cells have extensive internal compartmentalization, with multiple membrane-bounded organelles carrying out specific functions. (pp. 86—87)
5.3 Take a tour of a eukaryotic cell.
The Nucleus: Information Center for the Cell
• The nucleus is the repository for the genetic information that directs cell activities. (p. 88)
• The surface of the nucleus is bounded by two phospholipid bilayer membranes that form the nuclear envelope. (p. 89)
• DNA is organized into chromosomes located in the nucleus. (p. 89)
The Endomembrane System
• The endoplasmic reticulum is an extensive system of folded membranes that compartmentalizes the cell's interior. (p. 90)
• The Golgi apparatus collects, packages, modifies, and distributes molecules throughout the cell. (p. 92).
• Lysosomes contain digestive enzymes that catalyze the rapid breakdown of proteins, nucleic acids, and carbohydrates. (p. 94)
Ribosomes: Sites of Protein Synthesis
• Two ribosomal subunits join to form a ribosome when they attach to messenger RNA to direct protein synthesis. (p. 95)
Organelles That Contain DNA
• Mitochondria and chloroplasts have their own DNA specifying proteins for their unique functions. (p. 96)
The Cytoskeleton: Interior Framework of the Cell
• The cytoplasm of all eukaryotic cells is meshed by a network of protein fibers called the cytoskeleton, which supports the shape of the cell and anchors its organelles. (p. 98)
• Eukaryotic cells may contain three types of cytoskeletal fibers: actin filaments, microtubules, and intermediate filaments. (p. 98—99)
5.4 Not all eukaryotic cells are the same.
Vacuoles and Cell Walls
• Plant cells usually contain a large central vacuole for storage, and are encased by thick cell walls and often even by strong secondary walls. (p. 101)
The Extracellular Matrix
• Animal cells lack the thick cell walls of plants, but secrete a mixture of glycoproteins to form an extracellular matrix, which helps coordinate the behavior of all the cells in a particular tissue. (p. 102)