Sugars transport into yeast cells

Carrier-mediated movement of sugars across the plasmalemma of yeasts involves the combination of the sugar with a protein on one side of the plasmalemma, followed by release of the sugar into the cytoplasm on the other side. Such movement is described either as (t) facilitated diffusion, when the movement requires no metabolic energy, or (ii) active transport, which involves the expenditure of metabolic energy. Sugars entering yeast cells by active transport may be accumulated within the cells to a concentration many hundred times the external level. This subject has been reviewed by... 

The yeast cell membrane may be envisioned as a selectively permeable barrier that serves a vital role in the organism s ability to maintain osmotic balance and regulate transport of essential nutrients into and metabolites (including ethanol) out of the cell. Ethanol is soluble in both aqueous and lipid phases of the cell membrane and its formation and passive effusion eventually interferes with structure and function of the membrane. Particularly important in this regard are the cell-membrane-associated transport enzymes such as those responsible for uptake of sugars and critical amino acids. During active fermentation at warm temperatures, ethanol accumulates intracellularly faster than it can be eliminated from the cell. This situation worsens as extracellular concentrations increase. Thus, temperature- and ethanol-directed inhibition is likely the result of the time delay arising from passive diffusion coupled with impaired membrane function. 

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