Is+Yeast+Alive?+(Practical)

Do you expect yeast to produce a gas when sugar is available? Sugar in the form of warm glucose solution is broken down into simpler substances during respiration, water and carbon dioxide gas is produced. The presence of carbon dioxide can be tested using limewater or calcium hydroxide and if a white precipitate is formed, then carbon dioxide is present. Respiration does not take place when the yeast is placed in hot water and therefore no white precipitate is formed in electrons the limewater.  Oxidation means losing. Reduction is gaining electrons. Blue methylene blue is reduced to a colourless methylene blue --> is an indicator that is used to see whether the chemical is reduced or oxidised --> reduced to colourless in the live yeast solution (methylene blue gain in electron; yeast loss of electron) --> oxygen is used for respiration by the yeast --> there is no oxidative reaction occurring in dead yeast; only possible in live yeast

Why shake gently? Breaking down granules of yeast so they are floating in the suspension --> surface area of yeast is increased when they are released. Vigorous shaking introduces air to the methylene blue and decolourizes it.

The solution becomes lighter blue in colour. ||
 * **Test-Tube** || **Contents** || **Observations** ||
 * A || 5 cm3 of warm yeast solution + 2 drops of methylene blue || The solution is foamy, frothy and translucent. It is filled with gas bubbles.
 * B || 5 cm3 of boiled yeast solution + 2 drops of methylene blue || No visible change. The solution remains dark blue in colour. ||

The foam produced --> carbon dioxide evolved.

Warm glucose temperature (about 40 degree celsius) --> reaction faster --> yeast is tolerant to heat --> add same amount of yeast to each test tube and measure height of foam to see how much carbon dioxide is produced (constants: volume of solution must, temperature, time measured for reaction)

Baking bread --> flour + sugar added for reaction to be faster

Dark blue yeast cells --> DEAD. Live yeast cells allow methylene blue to enter itself --> diffusion occurs within cell wall --> methylene blue is a small molecule that can pass through the cell membrane --> once it is inside, it will decolorize due to the reduction reaction that is taking place inside the yeast.