LAB 12 - Osmotic Pressure

Osmotic Pressure

solutes

dissolved salts and sugars

osmosis

is the movement of water (NOT SOLUTE) through a semi-permeable membrane - like a cell membrane

water flows from _____ solute concentration to _____ solute concentration

low, high

what are the 3 osmotic environments?

1. hypotonic

2. isotonic

3. hypertonic

hypotonic

• solute concentration is lower outside of the cells

• water flows into cells expanding membrane

• cells do best if the environment is slightly hypotonic

• if the difference is too large, the cell will burst!!

isotonic

• solute concentration is equal in and out of the cell

• no net movement of water (the water movement is balanced)

hypertonic

• solute concentration is higher outside of cells

• water flows out of cells, causing plasmolysis

• plasmolysis: cell membrane collapses inward (shrinks away from rigid cell walls)

• can stop metabolism (water is necessary for metabolism)

_______ are added to jams, fruits, and condensed milk to control bacterial growth

• very effective at controlling bacterial growth

• fungi are usually more tolerant of hypertonic environments than bacteria

• known as “osmophilic” or “osmotolerant”

sugars

_______ are added to meat and fish

• more effective than sugar

• effective at lower concentrations due to denaturation of enzymes and disruption of membrane transport

• some bacteria are adapted to high salt concentration

• known as “halophilic” or “halotolerant”

• example: staphylococcus in salt ham

salt

which was more effective at inhibiting the growth of microbes, 15% NaCl or 15% sucrose? cite data to support your answer

the 15% NaCl was more effective at inhibiting the growth of microbes based on my data. for 15% NaCl, all the bacteria besides penicillium had 0 growth observed and penicillium was marked as a 1.

explain why one was more effective than the other, despite the fact that the concentrations were the same in both cases.

both NaCl and sucrose result in osmosis, but because NaCl is a salt, its hypotonic, meaning it draws water out of cells due to the diffusion gradient. This can easily denature proteins and kill the cells.

which species used in this lab is (are) likely to spoil foods preserved with high concentrations of salt (you can name more than one species)? what is the evidence for your conclusion?

penicillium would be most likely to spoil foods preserved with high concentrations of NaCl. for example, my observations show that penicillium growth is around levels 3 and 2 at lower levels of NaCl, meaning it is very likely it would spoil foods preserved at high concentrations of salt.

grape juice has a concentration of up to 25% sucrose. assume, for the moment, that E. coli, S. epidermidis, and S. cerevisiae all fermented sucrose to produce ethanol and carbon dioxide. if all of these cell types produced the same fermentation products, would there be any reason to use S. cerevisiae to make wine instead of the other two species? cite evidence from lab to back up your answer.

S. cerevisiae should still be used to make wine because it is more osmotolerant compared to E. coli and S. epidermidis. This means that it can handle hypotonic environments really well. Due to the high sugar content of grape juice, the microbe used for wine-making must be able to thrive in conditions with high solute concentration. in the lab, S. cerevisiae was a level 3 growth at 30% sucrose which is a close concentration to the sucrose in grape juice (25%) whereas E. coli and S. epidermidis are level 1.

based solely on data from this lab, describe two very specific ways to prevent the growth of E. coli in food. we want to see some concentrations here.

1. E. coli did not grow at all in the NaCl concentrations of 7.5% and 15%, therefore, using any NaCl concentration greater than 7.5% should prevent growth

2. E. coli did not grow at all in the sucrose concentration of 60%, therefore, any concentrations of sucrose greater than 60% will prevent growth of E. coli

________ damages cell walls (as do many other antibiotics)

penicillin

would penicillin kill bacterial cells in a hypotonic environment? if so, how or why would exposure to this antibiotic kill bacterial cells?

yes, penicillin would kill bacterial cells in a hypotonic environment because without a cell wall, cells in a hypotonic environment burst due to increase of fluids in cell, thus killing bacterial cells

would penicillin kill bacterial cells in an isotonic environment? if so, how or why would exposure to this antibiotic kill bacterial cells?

no, penicillin most likely wouldn’t kill bacterials cells in an isotonic environment, as long as they have their cytoplasmic membranes. without osmotic pressure, since its isotonic, there would be no rupturing of the membrane allowing bacterial cells to survive.