Biology of Yeast Cells - Simplified
Yeast are a tiny form of fungi or plant-like microorganism (visible only under a microscope) that exist in or on all living matter i.e. water, soil, plants, air, etc. A common example of a yeast is the bloom we can observe on grapes. As a living organism yeast needs sugars, water and warmth to stay alive. In addition, albumen or nitrogenous material are also necessary for yeast to thrive.
There are hundreds of different species of yeast identified in nature, but the genus and species most commonly used for baking is Saccharomyces cereviae. The scientific name Saccharomyces cerevisiae, means 'a mold which ferments the sugar in cereal (saccharo-mucus cerevisiae) to produce alcohol and carbon dioxide'. The ultimate reaction of importance in this process is the conversion of simple sugars to ethyl alcohol and carbon dioxide. A more thorough set of conversion reactions is provided later.
Simple Sugar → Ethyl Alcohol + Carbon Dioxide
C6 H12 O6 → 2C H3 CH2 OH + 2CO2
Examining a yeast cell under a microscope will give a greater understanding of the composition and nature of yeast. The method for viewing a sample of yeast under a microscope is to disperse a small amount of yeast in water, causing the water to be slightly clouded, and then drop a spot of the liquid onto a glass slide. The drop is then covered and viewed with a 650 x magnification. The individual cells will take the general form illustrated in Figure 1.
When viewed under the microscope, one sees round or oval cells about 1/100 of a millimeter in diameter, which weigh about 8 to 10 billion to the gram. (Calvel et al) If individual yeast cells were placed side by side it would take approximately 1200 cells to measure 1cm in length. Inside each cell are the following:
- One or more dark patches called vacuoles.
A yeast cell has 6000 different yeast genes. Like any living thing, yeast is made up of chromosomes;
there are 16 different chromosomes i
n yeast compared with 23 in humans.
The double cell wall may have bud scars (seen in Figure 1 to the right), which are caused by budding, i.e. the cell reproducing itself. There can be up to ten such scars, which cover the cell totally, after which the cell expires.
This happens (generally speaking) as follows:
Compared with other plant organisms, yeast has a much better chance of survival in spite of harsh environmental conditions. It is independent from climate and soil conditions. It is not dependent on any location and can survive for hundreds of years as a spore.
Under favorable living conditions, yeast multiplies through the separation of cells (budding) or yeast multiplication. Under unfavorable living conditions, when water and nutrients are lacking, the yeast forms spores.
Cell Separation (Budding)
The cell core migrates to the cell wall of the yeast cell. It splits up and forms a daughter cell. The daughter cell multiplies in the same way while it is still growing and tied to the mother cell. A colony develops. Later, the daughter cell separates from the mother cell. The multiplication process continues for as long as the conditions for multiplication are present. This is depicted in Figure 2. As can be seen, a parent cell grows a protuberance, this swells as the bud forms, a neck develops between the parent cell and the bud, and they separate. The process starts again and, in ideal conditions, a cell can reproduce itself in 20 minutes so that numbers increase from one to two, then to four, to eight, to 16, and so on. If the numbers are plotted on a graph, the line would take an exponential form.
Sporulation
Spores form once the nutrients of a solution are used up. The yeast becomes dormant and feeds on its reserve material. When the nutrient solution and the yeast cells dry out, the cell core separates and forms spores. The spores are insensitive to heat and cold. The slightest breeze carries them anywhere. Under dry conditions, the spores can live forever. When spores fall into a nutrient solution, they germinate into yeast cells. Each yeast cell can give rise to four spores.