Water and membranes — questions

1. state

Which statement describes osmosis?

Circle one answer.

• Movement of sugar molecules through a membrane from a high to a low concentration
• Net movement of water through a partially permeable membrane, from a dilute to a more concentrated solution
• Net movement of water from a concentrated solution to a more dilute solution
• Movement of any substance through a membrane using energy from respiration

Nudge (only if you are stuck): Which way does the water go: towards the dilute solution or towards the concentrated one? And what crosses the membrane?

2. identify

Water is a solvent. Match each example to the job that water does there.

Write the number of the matching option.

1. Digestion
2. Excretion
3. Transport

• Glucose from food dissolves in the water in the gut, then is absorbed: ______
• Urea leaves the body dissolved in urine: ______
• Carbon dioxide is carried dissolved in blood plasma: ______
• Mineral ions are carried up from a plant’s roots in water: ______

Nudge (only if you are stuck): Digestion breaks food down and absorbs it; excretion removes waste; transport carries substances around the organism.

3. complete

Complete the description of osmosis through a membrane that lets water through but not sugar.

Osmosis is the … movement of water molecules

from a … solution (higher water potential)

to a … solution (lower water potential)

through a … membrane

The sugar molecules …

Nudge (only if you are stuck): Think of the membrane picture: overall, which way do more water molecules cross, and what happens to the sugar?

4. explain

Bag A (sugar solution inside, standing in water) went from 10.0 g to 11.3 g in 20 minutes. Bag B (water inside, standing in sugar solution) went from 10.0 g to 9.1 g. (a) Explain the change in bag A. (b) Explain the change in bag B. (c) Why is a bag of water standing in water also set up?

(a) Bag A

(b) Bag B

(c) Bag of water in water

Nudge (only if you are stuck): Compare the water potential inside and outside each bag. Which way is the net movement of water?

5. calculate

Bag B started at 10.0 g and ended at 9.1 g. Bag C, holding a stronger sugar solution and standing in water, started at 12.0 g and ended at 13.5 g. Calculate the percentage change in mass of each bag.

Bag B: ______________ unit ________

Bag C: ______________ unit ________

Nudge (only if you are stuck): Percentage change = (final mass − starting mass) ÷ starting mass × 100. A loss gives a negative answer.

6. explain

A plant takes in nitrate ions from the soil and uses them in its leaves to make proteins. Explain how water acting as a solvent lets the nitrate ions get from the soil to the leaves.

Nudge (only if you are stuck): Think of the job water does in transport: what happens to the ions in the soil water, and what carries them up?

7. explain

Slices of cucumber are put on a plate and sprinkled with salt. After 10 minutes there is a pool of liquid on the plate and the slices are limp. Explain where the liquid came from.

Nudge (only if you are stuck): The salt dissolves in the moisture on the slices. Compare the water potential of that salty solution with the cucumber cells.

8. explain

A kangaroo rat lives in the desert and drinks almost no water. Its kidneys make very concentrated urine, but it still loses some water in its urine every day. Explain why some water must be lost to excrete urea.

Nudge (only if you are stuck): How is urea carried out of the body?