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I was reading some questions in the test bank. Then in Chapter 27, I wonder why the answer is A. (undergo death by plasmolysis). Is the high concentration of sugar that kills them or the effects of sugar on bacteria? enter image description here

Reece et al. Campbell Biology Test Bank. 9th ed. Boston: Benjamin Cummings / Pearson, 2011: 563.

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In (very) short: enzymes are necessary for life. Cytoplasm is necessary for enzymes to function. Water is necessary for cytoplasm to exist. Cells have osmotic membranes.

Given a cell in a solution (even a goopy one, like jam) with a very high concentration of sugar, water will move along a gradient of low to high sugar concentration, in other words, water will move out of the cell and into the jam, thus stopping all major activity in the bacterium (basically stopping it from reproducing, if not exactly killing it. The killing actually comes from heating the jam and canning it while still hot enough to kill most bacteria.)

Plasmolysis: The shrinking of protoplasm away from the cell wall of a plant or bacterium due to water loss from osmosis.

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In short our environment always tries to maintain an equilibrium, a stable state. When microbes encounter a surrounding like a high concentrated sugar state or solution (hypertonic), water diffuses out of the permeable cellular membrane of the microbe to its outer environment with high sugar concentration causing dehydration of the microbe and it leads to shrinking of cell. As water is necessary for chemical reactions inside the cell, the natural metabolism is hindered.The phenomenon of contraction of a cell as a result of water loss is called plasmolysis and the food preservation technique is called sugar curing. Actually we can say a different environment created for microorganism (here for the purpose of food preservation) with high sugar concentration has effect of plasmolysis in microbe resulting inhibition of its natural metabolic activity that results its destruction.

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Actually, it doesn't kill them. As far as I am aware people with weak immune responses shouldn't eat honey or jams because of risk of fungal or bacterial infections.

Resource: http://www.mayoclinic.org/drugs-supplements/honey/evidence/hrb-20059618

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  • $\begingroup$ I can't find anything in your source about people with weak immune response and a risk of bacterial infection from honey. Can you point to where it supports that? $\endgroup$
    – YviDe
    Commented Jan 29, 2016 at 20:38
  • $\begingroup$ In the side effect section, it is mentioned that spore producing microorganisms like Clostridium botolinum have a chance to grow in it.and also I did some research and the reason many microorganisms can't live in high sugar concentration is low water activity level because of sugar molecules which is needed for microorganisms' activity.but it is not simply the case for all microorganisms. $\endgroup$
    – Shayan Kabiri
    Commented Jan 29, 2016 at 21:49
  • $\begingroup$ @ShayanKabiri - While you're correct that immunocompromised people should avoid many foods, jams, jelly, syrups (refrigerated after opening) are allowed; raw or unpasteurized honey - and raw soft berries - usually are not, though. See, for example, Diet for Immunosuppressed and other sources. $\endgroup$
    – anongoodnurse
    Commented Jan 30, 2016 at 5:18

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