Concept 4.1 Cells Provide Compartments for Biochemical Reactions

      See WEB ACTIVITY 4.1

• Cell theory states that the cell is the fundamental unit of biological structure and function.
   
• Cells are small because a cell's surface area must be large compared with its volume to accommodate exchanges between the cell and its environment. Review Figure 4.2
   
• All cells are enclosed by a selectively permeable plasma membrane that separates their contents from the external environment.
   

Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus

• Prokaryotic cells usually have no internal compartments, but have a nucleoid containing DNA, and a cytoplasm containing cytosol, ribosomes (the sites of protein synthesis), proteins, and small molecules. Many have an extracellular cell wall. Review Figure 4.5
   
• Some prokaryotes have folded membranes, for example photosynthetic membranes, and some have flagella for motility. Review Figure 4.6
   

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

• Eukaryotic cells contain many membrane-enclosed organelles that compartmentalize their biochemical functions. Review Figure 4.7 (Animal Cell), Figure 4.7 (Plant Cell) and ANIMATED TUTORIAL 4.1
   
• The nucleus contains most of the cell's DNA.
   
• The endomembrane system —consisting of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes —is a series of interrelated compartments enclosed by membranes. It segregates proteins and modifies them. Lysosomes contain many digestive enzymes. Review Figures 4.8 and 4.9, WEB ACTIVITY 4.2, and ANIMATED TUTORIAL 4.2
   
• Mitochondria and chloroplasts are semiautonomous organelles that process energy.
   
• A vacuole is prominent in many plant cells. It is a membrane-enclosed compartment full of water and dissolved substances.
   

Concept 4.4 The Cytoskeleton Provides Strength and Movement

• The microfilaments, intermediate filaments, and microtubules of the cytoskeleton provide the cell with shape, strength, and movement. Review Figure 4.10
   
• Microfilaments and microtubles have dynamic instability and can grow or shrink in length rapidly.
   
• Cilia and flagella are microtubule-lined extensions of the plasma membrane that produce movements of cells or their surrounding fluid medium. Review Figures 4.11 and 4.12
   
• Motor proteins move cellular components, such as vesicles, around the cell by "walking" along microtubules. Review Figure 4.13
   
• Biologists establish cause-and-effect relationships by manipulating biological systems. Review Figure 4.14
   

Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment

• The plant cell wall consists principally of cellulose. Cell walls are pierced by plasmodesmata that join the cytoplasms of adjacent cells. Review Figure 4.15
   
• In animals, the extracellular matrix consists of different kinds of proteins, including collagen and proteoglycans. Integrins connect the cell cytoplasm with the extracellular matrix. Review Figures 4.16 and 4.17
   
• Specialized cell junctions connect cells in animal tissues. These include tight junctions, desmosomes, and gap junctions. The last are involved in intercellular communication. Review Figure 4.18 and WEB ACTIVITY 4.3