In addition to nutrients, the pH of the growth medium is also important for growth rate and cell density. The optimal growth pH for is near neutral. cells can grow reasonably well over a range of three pH units (from pH to ). Extreme pH beyond this range will significantly decrease the cell growth rate and may sometimes even cause cell death. The minimum and maximum growth pHs for are pH and respectively. cells appear to tolerate a more neutral pH better than a high pH. This is clearly shown as grows well at pH 7 as shown on plate M2, M4,M6, and fact, extended exposure of cells to a high pH causes cell lysis. This is the reason why no colonies were found on plate M3 and M7. At the saturation or stationary phase, the pH of the culture in commonly used media is near its pH limits. pH is another limiting factor for cell growth in addition to nutrition exhaustion and accumulation of toxic metabolites. cells can also use sugars such as glycerol and glucose as carbon or energy sources. When the cells use these sugars as carbon sources, they will produce acetic acid and therefore lower the medium pH. Carefully balancing the sugar contents, and aeration conditions can maintain the culture medium pH near optimum growth pH or within the range of the three pH units. Low aeration conditions lead the cells to produce acids. High aeration conditions allow the cells to use organic acids as carbon source and increase medium pH. Selected aeration conditions can also help cells maintain its medium pH.
The protons then move down the concentration gradient from the space between the inner and outer membranes back into the matrix. However, they can only move back across via an enzyme embedded in the inner membrane. This enzyme is called ATP synthase. The protons are transported back into the matrix through the channels of ATP synthase and as they do so they release energy. This energy is then used by ATP synthase to convert ADP into ATP. Since the electrons come from previous oxidation reactions of cell respiration and the ATP synthase catalyses the phosphorylation of ADP into ATP, this process is called oxidative phosphorylation. Chemiosmosis is necessary for oxidative phosphorylation to work.