Practice question

Late ripening grapes undergo water loss and sugar concentration contributing to raisin formation and wine quality concentration. Berry pericarp consists of epidermal cells covered by waxy cuticle containing embedded aquaporins of PIP1, PIP2 and TIP families forming selective water pores. Water loss process involves evaporation from skin surface lowering apoplastic water potential creating gradient that draws cellular water outward. Movement across plasma membrane and tonoplast occurs via facilitated diffusion through aquaporin pores functioning as channels allowing single file diffusion at billion molecules per second down water potential without ATP coupling or conformational shuttling distinct from simple lipid diffusion alone. Although osmosis term commonly used at tissue level, molecular mechanism qualifies as facilitated diffusion because protein accelerates polar molecule traversing hydrophobic core. Reverse osmosis requires applied hydrostatic pressure exceeding osmotic pressure to force water opposite natural direction, not relevant here. Active transport requiring ion gradients or ATP would transport solutes uphill, not observed. Ion channels for sodium or potassium selective do not transport substantial water. Therefore facilitated diffusion via aquaporins drives berry shriveling water loss.