Baker’s yeast is a commonly used leavening agent to make dough expand or rise. This is expanding effect occurs when the yeast converts the sugar that is present in dough into carbon dioxide through the process of fermentation. Yeast are single-celled microorganisms that are classified in the Kingdom Fungi. Yeast is also a very sensitive organism. Elements such as exposure to light and heat can kill yeast. This is the reason why most bakers keep their yeast in a dark place. The intense rays emitted by light can damage cells when the energy absorbed cannot be effectively dispersed by the organism. Molecular biologist have found that blue light can affect the respiration of yeast. Exposure to light can affect metabolic rhythms of yeast, affect its oxygen consumption and growth rates. This effect is caused by light because in its presence the activity of cytochromes in yeast is inhibited. This leads to the inability of yeast to perform electron transport. Respiratory functions are limited due to these effects of light. This has been found by using yeast respiratory oscillation, or YRO, as a measurement to find the extent of impact on yeast metabolism. YRO is the movement of respiratory activity, which is measured by yeast oxygen consumption rates. Blue/green wavelengths emitted from light have more of an effect on yeast cells because in its presence, the period of YRO and its amplitude are lessened. In the research conducted to test lights effect on yeast cells, blue light was especially damaging to yeast when tested. These results are what guided me to form my hypothesis that blue light would affect the yeast samples the most in my experiment. Another sensitive part of yeast organisms is its cell membrane. When there is enough damage from light to yeast, the cell membrane is killed. The photoreceptors that are present in organisms that can safely absorb light are not abundant enough in yeast cells to survive in light. Photoreceptors are structures in living organisms that respond to the presence of light. These cells include sense organs and sensory cells. This experiment discloses the challenges yeast is subjected to in its natural environment and also gives insight into how a baker can take the best care of their yeast. The materials for this experiment include 5 yeast packets, 5 Petri dishes or small bowls, a white light bulb, blue light bulb, red light bulb and warm water. A stopwatch is also needed to record times during the observation periods of the experiments. To conduct this experiment, I poured each packet of yeast into a small bowl and let them sit under either the blue, red, or white light for 12 hours each. One bowl of yeast sat in a dark cupboard as the control sample. The rooms in which the samples were for the 12 hours were undisturbed by any other light source. After the 12 hours, I heated up water to a lukewarm temperature. It is important the water is lukewarm because water that is too hot will kill the yeast. For each sample, about 2 tablespoons of the water was mixed with a sample of the yeast. I observed the sample and recorded its state at 10, 15, and 20 minutes. I repeated this three times for each yeast mixture. Yeast that is alive should begin to bubble at about 10 minutes. In my observations, I was looking to see how long it took the sample to bubble or if it bubbled at all to determine the amount of damage the light did to the yeast. The results of the experiment showed that all colors of light will kill yeast cells if the yeast is exposed to it for extended amounts of time. The blue light and the white light damaged the yeast at a faster rate than the red light. The yeast that was not in any light was the only sample that fully bubbled.