8. Osmosis And Plant Transport

Question 1

Animal and plant cells possess a cell membrane. What is the function of the cell membrane?

Answer 1

It controls what enters and leaves the cell. It is selectively permeable.

Question 2

There are three ways in which substances can pass into and out of a cell. Name these

Answer 2

1. Diffusion
2. Active uptake
3. Osmosis

Question 3

Explain what Osmosis is.

Answer 3

Osmosis is a special type of diffusion. It involves the movement of water molecules through a selectively permeable membrane. A selectively permeable membrane contains tiny pores, which allow small water molecules to rapidly pass through. Larger molecules such as sucrose (a type of sugar) and starch cannot pass through.

If pure water and a sugar solution are separated by a selectively permeable membrane then Osmosis will occur. The water will move from where it is in higher concentration (I.e the pure water) to where it is in lower concentration (I.e. The sugar solution)

Question 4

Define Osmosis

Answer 4

Osmosis is the movement of water molecules from a dilute solution to a more concentrated solution through a selectively permeable membrane.

Question 5

Explain the concentration gradient.

Answer 5

A concentration gradient exists across the selectively permeable membrane. The water molecules can move in any direction across the membrane but there will be a net movement of water to the more concentrated side. They will continue to move until there is the same concentration on each side.

Question 6

Draw out two diagrams showing test tubes with visking tubing describing what will occur when A. The sugar solution is inside the tubing and B. When it is outside

Answer 6

(A) there is a higher concentration of water molecules outside the visking tubing than inside it.
Water molecules move from the dilute solution to the more concentrated solution by Osmosis I.e. Into the visking tubing.
The visking tubing swells and increases in mass.

(B) there is a higher concentration of water molecules inside the visking tubing than outside it.
Water molecules move from the dilute solution to the more concentrated solution by Osmosis I.e. Out of the visking tubing.
The visking tubing shrinks and decreases in mass.

Question 7

(A) apparatus was set up. The visking tubing was filled with sucrose solution and placed in a beaker of pure water. There is a very low amount of water rising into the capillary tube.

(B) B shows what the apparatus looked like after several hours. The water had greatly raised within the capillary tube.
Explain the results

Answer 7

• There was a higher concentration of water molecules outside the visking tubing than inside it.
• water molecules move from the dilute solution to the more concentrated solution by osmosis I.e. Into the visking tubing.
• thus causes the liquid to rise up into the capillary tube.

Question 8

What would happen to the level of liquid inside a capillary tube where the visking tubing inside a beaker was surrounded by a very concentrated solution of sucrose?

Answer 8

It would drop as water would move out of the visking tubing ( to the more concentrated solution) by osmosis

Question 9

Explain the method used when investigating osmosis in potatoes.

Answer 9

1. Label three boiling tubes 0% (distilled water), 5% sucrose and 10% sucrose.
2. Using a measuring cylinder place 20cm3 of each solution into the relevant boiling tube.
3. Working on the white tile, cut the ends off a potato and use a cork borer to obtain three cylinders of potato tissue.
4. Cut the potato cylinders to exactly the same length (4cm), using a ruler and razor blade.
5. Weigh each cylinder and place one into each boiling tube. Cover the tubes with sealing film and leave for 24 hours
6. After 24 hours remove the potato cylinders, remove any excess solution using paper towels and weigh each cylinder.
7. Calculate the percentage change in mass for each cylinder and record your results.

Question 10

Investigating Osmosis in potatoes.
Questions

1. What process caused the potato cells to change mass
2. Why were the boiling tubes covered during the experiment?
3. Explain the change in mass of the potato cylinder in 0% sucrose solution
4. Explain the change in mass of the potato cylinder in 10% sucrose solution
5. Use your graph to determine the concentration of sucrose solution that is equal to the concentration of the potato cell cytoplasm? Explain your answer
6. Why was the percentage change in mass used rather than the actual change in mass?
7. Instead of measuring the mass of potato the length of the potato cylinder could be measured. Suggest two reasons why it is more accurate to measure the mass.
8. Suggest five ways to ensure this experiment is a fair test.

Answer 10

1. Osmosis
2. To prevent evaporation of water surrounding the potato.
3. The mass increases. The solution surrounding the potato had a higher concentration of water than inside the potato. Therefore water moved from the more dilute solution to the concentrated solution inside the potato by Osmosis.
4. The mass decreased. Inside the potato had a higher concentration of water than the surrounding solution. Therefore water moved from the more dilute solution in the potato into the concentrated solution surrounding the potato by Osmosis.
5. 4% sucrose. At this concentration the potato cylinder would remain the same mass I.e. It would not increase or decrease. There would not be any overall net movement of water into or out of the potato cylinder by Osmosis. This is because the concentration of the surrounding solution is the same as the potato cell cytoplasm.
6. The initial sizes of potato cylinders were probably not all the same. Percentage change enables the results to be more easily compared with each other.
7. • a balance is more accurate than the human eye
     - as well as becoming shorter/longer, the potato cylinders may become narrower/wider - measuring the mass will take this into account
     - when measuring lengths inaccuracies occur due to the end of the cylinder not being perfectly smooth/straight.
8. • same temperature
     - same type of potato
     - same volume of surrounding solution
     - same mass/length of potato
     - immerse the potatoes in solution for the same length of time.

Question 11

What does cytoplasm contain?

Explain Osmosis in cells.

Answer 11

Dissolved sugars and salts. Think of it as a weak solution. The direction of osmosis depends on the concentration of the fluid surrounding the cell.

• if the surrounding solution is more concentrated than inside the cell, water will move out of the cell by osmosis.
• if the surrounding solution is more dilute than inside the cell, water will move into the cell by osmosis.
• if the surrounding solution has the same concentration of water as inside the cell, there will be no net movement of water in any direction.

Question 12

If an animal cell (e.g. Red blood cell) is placed in pure water or a very weak/dilute solution, water will enter it. Explain why this occurs.

Answer 12

• there’s a higher concentration of water in the surrounding liquid than inside the red blood cell
• water will move into the red blood cell by osmosis (from a dilute solution to a more concentrated one!)
• through a selectively permeable membrane

Question 13

Uptake of water into animal cells.

What happens if too much water enters a cell?

Answer 13

If too much water enters an animal cell by osmosis it will swell up and eventually burst - this is known as cell lysis. This occurs because animal cells do not have a cell wall to limit the amount of water entering the cell.

Question 14

Loss of water from animal cells

What happens if too much water leaves a cell?

Answer 14

If too much water leaves an animal cell by osmosis it will shrink and shrivel up.

Question 15

Why does cell lysis or shrivelling happen to cells in a healthy body?

Answer 15

Because the concentration of the blood is carefully controlled to ensure that it does not vary too much.

Question 16

Explain the uptake of water into plant cells

Answer 16

When water moves into a plant cell by osmosis the vacuole increases in size, pushing the cell membrane against the cell wall. This force makes the cell firm or turgid. This turgor is important as it gives the cell support and helps to keep plants upright.
The cell will continue to take in water until it is stopped by the cell wall. The cell wall stops the membrane expanding too far to cause damage and therefore limits the water intake.

Question 17

Draw a diagram of turgid cell

Answer 17

• arrows to show water moving into vacuole
• vacuole fills with water
• cell wall stretches slightly
• nucleus
• cytoplasm
• chloroplasts

Question 18

Explain the loss of water from plant cells

Answer 18

When plant cells lose water by osmosis they cannot remain turgid and wilting occurs. Cells that are not turgid are described as being flaccid.

If a plant cell loses too much water by osmosis it will shrink because the vacuole decreases in volume. This condition is known as plasmolysis. During plasmolysis, the cell membrane is pulled away from the cell wall. A cell with this condition can be described as plasmolysed .

Question 19

What happens when animal and plant cells are placed in a solution which has the same concentration as inside the cells?

Answer 19

There will be no net movement of water. The cells will not gain or lose water.

Question 20

Draw a diagram of a flaccid cell.

Answer 20

• smaller vacuole
• cell membrane pulled away from the cell wall as the vacuole loses water.
• cell wall becomes soft
• cell becomes plasmolysed
• some of the surrounding solution is found in this space.
• water moving out of cell
• chloroplasts
• nucleus

Question 21

Draw a diagram of:

• turgid cell
• cell in same concentration of solution
• flaccid cell
• plasmolysed cell

Answer 21

.

Question 22

If you wash lettuce leaves in slightly salted water, you kill any insects on the leaves but the leaves go limp.
Explain.

Answer 22

The concentration of water is higher inside the lettuce cells than outside (more dilute) therefore water leaves the lettuce by Osmosis.

Question 23

For you sprinkle sugar onto a dish of strawberries, liquid collects in the dish.
Explain

Answer 23

Sugar dissolve on the surface of the strawberries and creates a very concentrated sugar solution.therefore water leaves the strawberries by osmosis and liquid collected in the dish.

Question 24

On a Monday morning the plants in the laboratory are soft and floppy (they have wilted) following watering they quickly recover and become firm.
Explain

Answer 24

The vacuoles will fill up with water and the cells become turgid.

Question 25

Explain the importance of water to plants.

Answer 25

Water is a very important compound in plants. It is necessary for many purposes including:
support - water keeps plant cells turgid

Transport - as water moved up through a plant it carries minerals

…….. - water moves up through a plant, evaporated from the leaf cells and diffuses out of the stomata.

……….. - water is used as a raw material for this process.

Question 26

Explain transpiration

Answer 26

Much of the water that enters the leaves will evaporate into the atmosphere. This loss of water by evaporation is called transporation.

• water evaporates from the spongy mesophyll cells in plant leaves
• the water vapour diffuses through the intercellular airspaces.
• the water vapour diffuses out through the stomata (small pores).

Question 27

Define transporation.

Answer 27

Transporation is the evaporation of water from the spongy mesophyll cells, followed by diffusion through airspaces and stomata

Question 28

Learn diagram of evaporation from the spongy mesophyll cells and diffusion from the stomata.

Answer 28

.

Question 29

What is the transpiration stream

Answer 29

The continuous movement of water into, through and out of a plant

Question 30

What are stomata and where are they located?

Answer 30

Small pores and majority are found on the underside of the leaf

Question 31

Name the pair of cells that control the opening and closing of the stomata

Answer 31

Guard cells

Question 32

will transpiration occur at (A) Night (B) during the day

Answer 32

Stomata are closed during the night and open during daylight hours, so the answer is (A) no. (B) yes.

Question 33

Draw diagram of a bubble potometer.

Answer 33

• transpiring shoot
• water
• reservoir/syringe for resetting the bubble to zero
• air bubbles move along the tube.
• scale to measure the distance the bubble moves over a period of time.

Question 34

What is a bubble potometer.

Answer 34

The bubble potometer is a piece of apparatus designed to measure water uptake in a leafy shoot. As water evaporates from the leaves of the cut shoot, the shoot sucks water up through the potometer. This causes the air bubble to move towards the leafy shoot.

Question 35

How could you introduce the bubble into the water column?

Answer 35

Lift the potometer out of the beaker to allow it to suck in some air. Then place it back in the beaker and water will continue to be taken up but an air bubble will be trapped.

Question 36

How can the rate of water uptake be calculated using a potometer?

Answer 36

• record the initial position of the air bubble
• after a certain period of time record the final position of the air bubble
• calculate the distance moved by the air bubble per minute

Question 37

Explain the reservoir/syringe

Answer 37

• this allows the apparatus to be reset (I.e. To return the air bubble to zero) so that replicate results can be recorded or the water uptake measured in different environmental conditions.

Question 38

Explain air leaks

Answer 38

• the uptake of water into the plant will be hindered if there are air leaks. Therefore it is important that the potometer apparatus is properly sealed (using Vaseline), particularly at the junction between the shoot and the neck of the potometer.

Question 39

Explain Unwanted air bubbles

Answer 39

To prevent the development of unwanted air bubbles in the water column entering the plant, it is necessary to assemble the apparatus under water.

Question 40

The potometer can accurately measure the volume of water taken up by the shoot but it cannot give an absolute value for transpiration itself I.e. How much of the water taken up by the plant is actually transpired through the leaf surface. Why is this?

Answer 40

Some of the water will be used in photosynthesis and in providing support through turgor, so the volume transpired will be less than the volume taken up in the shoot.

Question 41

The potometer can be used to measure how conditions affect the rate of water uptake.
These conditions include:

Answer 41

• temperature
• wind speed
• humidity
• surface area
  These are environments factors

Question 42

Explain using the potometer and effect on transpiration for:

• temperature
• wind speed
• humidity
• surface area

Answer 42

Temperature;
Using the potometer - place the shoot in areas where the temperature differs
Effect on transpiration - higher temperatures increase the rate of transpiration as there is a faster rate of evaporation of water from the leaves of the shoot. The air bubble will move further as water uptake will be greater.

Wind speed;
Using the potometer - use a fan with different settings
Effect on transpiration - higher wind speeds increase the rate of transpiration as the water vapour surrounding the stomata on the leaves is blown away and replaced by dry air. This maintains a steep gradient of moisture. The air bubble will move further as water uptake will be greater.

Humidity;
Using the potometer - cover the shoot with a polythene bag
Effect on transpiration - high humidity levels decrease the rate of transpiration as the surrounding air already contains a lot of water vapour I.e. There is a decrease in the moisture gradient between the leaf surface snd the surrounding air. The air bubble will not move as far as water uptake will be less.

Surface area;
Using a potometer - have shoots with different sizes/numbers of leaves
Effect on transpiration - larger (or more) leaves will have a larger surface area and so there will be more stomata through which water can evaporate. The air bubble will move further as water uptake will be greater.

Question 43

What is it important to do when changing a particular environmental factor to show how it affects water uptake?

Answer 43

It is necessary to keep other environmental factors constant. This ensures that the results are valid. Replicated results will help ensure that the results are reliable.

Question 44

Describe how you could use a bubble potometer to compare the rates of water uptake in still and windy conditions.

Answer 44

• measure the distance the bubble moves in still conditions over a period of time
• use the reservoir to reset the potometer by returning the bubble to zero.
• use a fan to create windy conditions and using the same shoot measure the distance the bubbles move over the same period of time.
• other conditions (e.g. Light & temperature) must be kept the same
• repeat experiment in each condition to increase reliability

Question 45

How else can transpiration rates be measured?

Answer 45

Using a !!!!!!!. This involves weighing how much water a plant loses over a period of time. Normally the plant is placed on a balance for at least 24 hours and the mass recorded at intervals.

Question 46

Draw a diagram of the weighing method

Answer 46

• transpiring plant
• flask of water
• film of all to prevent evaporation of water from flask
• balance

Question 47

What is important when using the weighing method?

Answer 47

When using the weighing method, it is important that water can only escape by transpiration (through the leaves). The top of the flask must be covered (using polythene or oil) to prevent the evaporation of water from the flask.

Question 48

Describe how you would use the weighing method to compare the loss of water from a pot plant in humid and non-humid conditions.

Answer 48

• record the initial mass of the pot plant
• leave the pot plant in non-humid conditions for a certain period of time
• record the final mass of the pot plant
• create humid conditions ( by covering the plant with a clear polythene bag) and measure the weight loss of the pot plant over the same period of time.
• to stop evaporation of soil water ( and ensure that the loss of mass is only due to water evaporating through the leaves) the soil in the pot should be covered with polythene
• other conditions (e.g light, wind speed, temp) must be kept the same to create valid results
• repeat the experiment in each condition to increase reliability

Question 49

What needs to move in and out of cells and why

Answer 49

Substances need to move in and out of cells to stay healthy e.g nutrients, oxygen and waste products