Approximately how many doublings can normal human fibroblasts undergo in culture?

  50 to 100.

After 50 to 100 doublings, the cells will stop growing and die. The original cell was thought to have arisen from the enclosure of self-replicating by a phospholipid membrane..

photosynthesis makes the glucose that is used in cellular respiration to make atp. the glucose is then turned back into carbon dioxide, which is used in photosynthesis. while water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.