There are limits to how large cells can be because metabolic needs are related to cell volume, and abilities to satisfy those needs are determined by cell surface area. Cell volume grows faster than cell surface area. Multicellularity and cell specialization enabled organisms to grow larger. Groups of similar cells form tissues, and multicellular animals are composed of four tissue types: epithelial, connective, nervous, and muscle. Individual organs include multiple tissue types and are part of organ systems.

Question 1

Cells are limited in size by whether their surface area is sufficient to meet their metabolic needs by diffusion and transport of nutrients and wastes across the cell membrane.

Question 2

The major advantage of being multicellular is that every cell does not have to provide for all of its metabolic needs. Different cells can become specialized to perform specific functions that contribute to the survival and health of the whole organism. With that specialization comes greater efficiency. Also, multicellularity avoids the problem of the decreasing surface area-to-volume ratio that accompanies an increase in size of the organism. A third advantage is that bigger organisms can more easily avoid predation and in turn prey on smaller organisms.

Question 3

The gut is a good example of how multiple tissue types contribute to the structure and function of an organ. Epithelial tissues provide the boundary between the gut and its lumen where food is digested, and also provide transport mechanisms for the uptake of nutrients. Muscle tissues provide the motility necessary to move the gut contents from mouth to anus. Nervous tissues control motility of the gut and also provide sensory information used to control gut functions. Connective tissues give structural integrity to the gut, and one connective tissue—blood—carries nutrients absorbed from the gut to all the other cells of the body.