Passage IV

Biological aging is the process by which the functions within an animal cell gradually decline, causing the cell to age. Four students each proposed an explanation for how this process occurs.

Student 1

Biological aging is caused solely by the reactive oxygen species (ROS) produced by cellular respiration. ROS are molecules that damage the proteins and lipids in a cell. A cell produces antioxidants, which eliminate ROS before they cause cell damage. However, the amount of antioxidants produced by a cell is always less than what is needed to eliminate all the ROS produced by that cell. Therefore, ROS damage accumulates in a cell, causing it to age.

Student 2

Biological aging is caused solely by the formation of cross-links (a type of chemical bond) between the proteins in a cell, causing these proteins to form clumps. These clumps accumulate in a cell, interfering with the cell's functions, causing it to age. Although ROS is damaging to proteins and lipids, this damage never occurs in a cell because the amount of antioxidants produced by a cell is always greater than what is needed to eliminate all the ROS produced by that cell.

Student 3

Biological aging is caused solely by the DNA damage that results from cell exposure to certain environmental agents. The extent of DNA damage caused by these agents eventually exceeds the cell's ability to repair this damage. Therefore, DNA damage accumulates in a cell, causing it to age. Although cells do produce ROS, ROS damage never accumulates in a cell. While cross-linked proteins do form clumps in a cell, these clumps never affect cell function.

Student 4

Biological aging is caused solely by the lipofuscin (a brown pigment made of oxidized lipids) produced by cellu­lar respiration. Lipofuscin forms clumps that accumulate in a cell, interfering with the cell's functions, causing it to age. Although cells do produce ROS, ROS damage never accumulates in a cell. Because protein cross-links are short-lived, protein clumps never accumulate in a cell. The extent of DNA damage that occurs in a cell never exceeds the cell's ability to repair that damage.