Answer to Question 1
Answer: The multiple nuclei model was developed by geographers C. D. Harris and E. L. Ullman in 1945. According to the multiple nuclei model, a city is a complex structure that includes more than one center around which activities revolve. Examples of these nodes include a port, neighborhood business center, university, airport, and park.
The multiple nuclei theory states that some activities are attracted to particular nodes, whereas others try to avoid them. For example, a university node may attract well-educated residents, pizzerias, and bookstores. An airport may attract hotels and warehouses. On the other hand, incompatible land-use activities will avoid clustering in the same locations. Heavy industry and high-class housing, for example, rarely exist in the same neighborhood.
Answer to Question 2
Answer: The concentric zone model was created by sociologist W.W. Burgess in 1923. According to the model, a city grows outward from a central area in a series of five concentric rings, like the growth rings of a tree.
- The innermost zone is the CBD, where nonresidential activities are concentrated.
- A second ring, the zone in transition, contains industry and poorer-quality housing. Immigrants to the city first live in this zone in high-rise apartment buildings.
- The third ring, the zone of working-class homes, contains modest older houses occupied by stable, working-class families.
- The fourth zone has newer and more spacious houses for middle-class families.
- A commuters' zone beyond the continuous built-up area of the city is inhabited by people who work in the center but choose to live in dormitory towns for commuters.
The double-helix structure of DNA.
Marx's Model of Historical Change
Drawing of spirochete's mobility system (with internal flagellae).
Structure of HIV.
Strand displacement model for mitochondrial DNA replication
Amino Acid Structure