Osmosis

Some students are conducting an osmosis investigation using cylinders of potato.

They cut cylinders of potato 50 mm long using the ruler shown in the diagram.

What is the percentage uncertainty in the length of the potato cylinders?

A 0.25%

B 0.5%

C 1%

D 2%

A student investigates water potential in the cells of a bell pepper, Capsicum annuum.

A slice of bell pepper is shown in the image.

The student:

• prepares several concentrations of sucrose solution

• cuts pieces of pepper and records the mass of each piece

• places pieces of pepper in each sucrose solution

• removes the pieces after 30 minutes and dries them

• records the mass of each piece.

The student makes sucrose solutions of different concentrations by diluting a stock solution.

State a piece of equipment, with a higher resolution than a measuring cylinder, that the student should use to transfer solutions during the dilution procedure.

The student controls temperature and the time that the pepper pieces are in the sucrose solution.

State one other variable that the student should control during the experiment.

The student calculates the change in mass of each piece of pepper during the experiment and converts these values to percentage change in mass.

Explain why the student converts the results to percentage change in mass.

The graph shows the results of the experiment.

Use the graph to estimate the concentration of sucrose solution that has the same water potential as the pepper tissue.

Concentration of sucrose solution =........ mol dm^–3

In 1933 two scientists published a study in which they investigated the movement of coloured dyes in the transpiration stream of the stems of sycamore trees growing in a wood.

They bored holes in the stems and poured dye solution into the borehole. Then they sealed the hole with impermeable tape.

Describe what is meant by the term transpiration stream.

Suggest why the scientists sealed the borehole with tape.

The scientists found that the dye moved both up and down the stems.

A student wanted to investigate this in the laboratory. They devised an experiment using water-soluble food dyes injected into the stems of sunflower plants growing in pots.

Suggest how they could observe the route taken by the dyes.

Plants are often watered with a solution containing minerals to help them grow.

While preparing a class practical a technician accidentally watered some sunflower plants with a mineral concentration that was 1000 times higher than normal.

Explain why the plants wilted, even though they were given a lot of water.

The teacher suggested that the same effect would be seen if the plants had been watered with a solution containing cyanide ions that inhibit ATP synthesis.

Explain whether the teacher was correct.

Explain why plants growing in high light intensity require more water than plants growing in shade, even when the temperatures are the same.

A student investigated osmosis in plant cells.

They used cylinders of potato cut with a cork borer and measured the change in length of the cylinders after they were placed in sucrose solutions and left overnight.

The student was given 100 cm³ of a solution of 1 mol dm^–3 sucrose and asked to prepare (a) 30 cm³ of each dilution.

Complete Table 18.1 to show how the student should prepare the solutions.

The solution given to the student was prepared using 34.23 g of sucrose in 100 cm³ (ii) water.

The sucrose was measured on an electronic balance using the following procedure:

• mass of weighing boat empty = 10.55 g

• mass of weighing boat plus sucrose = 44.78 g.

The balance recorded masses to two decimal places with an uncertainty of ± 0.01 g.

Calculate the percentage uncertainty in the mass of sucrose.

The student’s results are shown in Table 18.2.

The student was told that one of their results was anomalous.

Identify the anomalous result.

State one variable that should be controlled in this experiment and give a reason for your choice.

The formation of tissue fluid is an example of ultrafiltration. Osmosis plays an important part in ultrafiltration.

The water potential of the blood depends on the concentration of solutes such as glucose, amino acids and mineral ions as well as large plasma proteins.

State the effect on the water potential of the blood if the concentration of glucose increases.

Explain why the oncotic pressure of the blood depends only on the concentration of large plasma proteins.

The table below compares a capillary with the surrounding tissue fluid.

Net movement of fluid between the capillary and tissue fluid depends on the net driving force (J_v):

J_v = (P_c – P_i) – (_c – _i)

Where:

P_c = capillary hydrostatic pressure

P_i = tissue fluid hydrostatic pressure

_c = capillary oncotic pressure

_i = tissue fluid oncotic pressure

= reflectance factor

The reflectance factor is a measure of how permeable the capillary is to albumin. It varies between 0 (totally permeable) and 1 (totally impermeable).

Inflammation can reduce the value of the reflectance factor.

In one type of capillary the reflectance factor was found to be 0.75.

Use this information and the data in the table to calculate a value for the net driving force and predict the direction of movement.

Net driving force, J_v =........ kPa

Direction of movement........

Nephrotic syndrome is a kidney disease where protein is excreted in the urine.

One group of patients with nephrotic syndrome had a plasma albumin concentration of 0.015 g dm^–3.

Explain what effect this would have on the net driving force. (There is no need to perform a further calculation.)

A medical student concluded that a patient with kidney disease and a plasma albumin concentration of 0.015 g dm^–3 would show signs of swelling, such as swollen ankles.

Evaluate the student’s conclusion.

The table below contains statements about the composition of lymph and tissue fluid.

Place ticks (✓) in the boxes in the table to indicate whether the statements are true or false.

Water moves by osmosis in living organisms.

Define osmosis.

Plants rely on osmosis for support.

Explain the importance of osmosis in plant support.

The apparatus shown in Fig. 16 can be used to demonstrate osmosis.

When the capillary tube with visking tubing bag was placed in solution Y, the level of solution X inside the capillary tube rose from 10.5 mm to 26.5 mm.

The ruler used to measure the distance along the capillary tube was accurate to the nearest 0.5 mm.

Calculate the percentage uncertainty of the measurement.

What conclusions can be drawn about the composition of solutions X and Y ?

Explain why it was necessary to calculate percentage change in mass.

The students identified replicate 3 of the potato in 0.7 mol dm^–3 sucrose as anomalous. (ii)

Suggest a practical error by the students that might have caused this result to be anomalous and explain the likely effect of this error.

Use Table 16 to identify which plant cells contained the highest concentration of sucrose.

Justify your conclusion.

Water has many properties that are essential for living organisms.

Explain how properties relating to the density of water contribute to the survival of organisms.