Chapter Summary

• The cell theory is the unifying theory of cell biology. All living things are composed of cells, and all cells come from preexisting cells.

• A cell is small and maintains a large surface area-to-volume ratio. This allows it to exchange adequate quantities of materials with its environment. Review Figures 5.1, 5.2, Activity 5.1

• Cell structures can be studied with light and electron microscopes. Review Figure 5.3, Activity 5.2

• All cells are enclosed by a selectively permeable cell membrane that separates their contents from the external environment.

• Whereas certain biochemical processes, molecules, and structures are shared by all kinds of cells, there are two categories of organisms—prokaryotes and eukaryotes—that can be distinguished by characteristic cell structures.

• Eukaryotic cells are generally larger and more complex than prokaryotic cells. They contain membrane-bound organelles, including the nucleus.

• Prokaryotic cells have no internal compartments but have a nucleoid region containing DNA. The rest of the material inside the cell membrane, including the ribosomes, is called the cytoplasm (or cytosol). Some prokaryotes have additional protective structures, including a cell wall, an outer membrane, and a capsule. Review Figure 5.4

• Some prokaryotes have folded internal membranes such as those used in photosynthesis, and some have flagella or pili for motility or attachment. Review Figure 5.5

• Filamentous proteins in the cytoplasm make up the cytoskeleton, which assists in cell division and the maintenance of cell shape.

• Eukaryotic cells are larger than prokaryotic cells and contain many membrane-enclosed organelles. The membranes that envelop organelles ensure compartmentalization of their functions. Review Figure 5.7

• The nucleus contains most of the cell’s DNA and participates in the control of protein synthesis. The DNA and the proteins associated with it form a material called chromatin. Each long, thin DNA molecule occurs in a discrete chromatin structure called a chromosome. Review Figure 5.8

• Within the nucleus is the nucleolus, where ribosome assembly begins. The ribosomes function as sites of protein synthesis.

• The endomembrane system—consisting of the cell membrane, the nuclear envelope, the endoplasmic reticulum, the 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 Figure 5.10, Activity 5.4, Animation 5.1

• Mitochondria and chloroplasts are organelles that process energy. Mitochondria are present in all eukaryotic organisms and contain the enzymes needed for cellular respiration. The cells of photosynthetic eukaryotes contain chloroplasts that harvest light energy for photosynthesis. Review Figures 5.11, 5.12

• Large vacuoles are present in many plant cells. A vacuole consists of a membrane-enclosed compartment full of water and dissolved substances.

• The microfilaments, intermediate filaments, and microtubules of the cytoskeleton provide the cell with shape, strength, and movement. Review Figures 5.14, 5.15, 5.16

• Motor proteins use cellular energy to change shape and move. They drive the bending movements of cilia and flagella, and transport organelles along microtubules within the cell. Review Figures 5.18, 5.19

• The plant cell wall consists principally of cellulose. Cell walls are pierced by plasmodesmata that join the cytoplasms of adjacent cells.

• In animal cells, the extracellular matrix consists of different kinds of proteins, including collagen and proteoglycans. Review Figure 5.22

• Infoldings of the cell membrane could have led to the formation of some membrane-enclosed organelles, such as the endomembrane system and the nucleus. Review Figure 5.23A

• Symbiosis means “living together.” The theory of endosymbiosis states that mitochondria and chloroplasts originated when larger cells engulfed, but did not digest, smaller cells. Mutual benefits permitted this symbiotic relationship to be maintained, allowing the smaller cells to evolve into the eukaryotic organelles observed today. Review Figure 5.23B