IGCSE Biology 0610: Osmosis in Reviving a Wilted Flower Stem (Core)

Cut flowers are often sold without any water around their stems. If a bunch of flowers is left out of water for several hours, the stems become soft and bend over, and the flowers droop. A florist can often revive a drooping bunch by cutting a fresh end from each stem and standing the stems upright in a container of fresh tap water. Within about an hour, the stems become firm and upright again.

(a) State what is meant by osmosis. [2]

(b) The cell sap inside the stem's cells contains dissolved sugars and other solutes, so it is more concentrated than the tap water in the container. Explain, in terms of osmosis, why standing the stems in tap water makes them firm and upright again. [3]

(c) Instead of tap water, the florist stands a second group of drooping stems in a strong sugar solution, which is more concentrated than the cell sap inside the stem's cells. Predict what would happen to these stems, and explain your prediction in terms of osmosis. [3]

Part (a): Defining osmosis

Osmosis is the net movement of water molecules through a partially permeable membrane, from a region where water molecules are more concentrated to a region where they are less concentrated.

Part (b): Why standing the stems in tap water revives them

Tap water contains a very high proportion of water molecules. Much higher than the proportion of water in the cell sap inside the stem’s cells, since the cell sap also contains dissolved sugars and other solutes. Because of this difference, water molecules move from the tap water into the stem’s cells by osmosis, through the partially permeable cell membranes. As each cell takes in water, it swells and presses outward on its surrounding cell wall. This build-up of pressure inside the cells is what makes the plant tissue feel firm, so the stem becomes firm and upright again.

Part (c): Predicting the effect of the concentrated sugar solution

The sugar solution is more concentrated than the cell sap inside the stem’s cells, so it has a lower proportion of water molecules than the cell sap. This means the direction of osmosis is reversed compared with part (b): water molecules now move out of the stem’s cells, through the partially permeable cell membranes, into the surrounding sugar solution. As the cells lose water, they can no longer press firmly against their cell walls, so the tissue loses its firmness. Rather than reviving the stems, standing them in the concentrated sugar solution would make them droop even further.

Final answers

(a) Osmosis: the net movement of water molecules through a partially permeable membrane, from a region of higher water-molecule concentration to a region of lower water-molecule concentration.
(b) The stems become firm again, because water moves into their cells from the more dilute tap water by osmosis, and the cells swell and press on their cell walls.
(c) The stems would droop further, because water moves out of their cells into the more concentrated sugar solution by osmosis, so the cells cannot press firmly against their cell walls.