Transport in Plants

Question 1

What is osmosis?

Answer 1

The net diffusion of water across a partially permeable membrane, from a solution of high water potential to low water potential, down a concentration gradient

Question 2

Name an artificial partially permeable membrane, where is it used?

Answer 2

Visking tubing, it is used in kidney dialysis machines

Question 3

Define/explain ‘partially’ permeable:

Answer 3

Visking tubing has microscopic holes in it which let small molecules like water pass though, it is permeable to them. However it does not let larger molecules such as sucrose pass through so it is partially permeable.

Question 4

Explain an experiment to show the effects of osmosis, what are the expected results?

Answer 4

Fill a visking tubing ‘sausage’ with sucrose solution, attaching it to a capillary tube. Place this in a beaker of water. The level in the capillary tubing shoUld rise as water moves from the beaker to the inside of the visking tubing.

Question 5

Explain why the level in the capillary tube will rise in this experiment

Answer 5

The sucrose molecules are too big to pass through the holes in the partially permeable membrane. The water molecules can pass through the membrane in any direction, but those in the visking tubing are attracted to the sugar molecules. This slows them down and makes them less free to move. Therefore more water molecules diffuse from the more dilute solution, to the more concentrated solution.

Question 6

What is water potential?

Answer 6

How ‘free’ the water molecules are to move.

Question 7

What is the correlation between water potential and the concentration of a substance?

Answer 7

The more concentrated a solution is, the lower its water potential.

Question 8

What allows osmosis to occur in plant cells?

Answer 8

The cell surface membrane of both animal and plant cells are partially permeable and so is the inner membrane of the vacuole in a plant.

Question 9

What is the function of the cell wall? Is it partially permeable?

Answer 9

The cell wall is made from tough cellulose which keeps the shape of the cell and can resist changes in the pressure inside the cell. It is fully permeable to water and solutes as it has large holes in it.

Question 10

What would happen if you put a cell into a pure water or dilute solution?

Answer 10

The contents of the cell would have a lower water potential than the external solution, so the cell would absorb water by osmosis. The cell then swells up and pushes against the cell wall. A plant cell that has developed an internal pressure like this is turgid.

Question 11

What would happen if you put a cell into a concentrated sucrose solution?

Answer 11

The contents of the cell would have a higher water potential than the outside solution so the cell would lose water by osmosis. The cell decreases in volume and the cytoplasm no longer pushes against the cell wall, in this state the cell is flaccid.

Question 12

When is a cell plasmolysed?

Answer 12

When a cell is flaccid and the cytoplasm no longer pushes against the cell wall, eventually the cell contents shrink so much that the membrane and cytoplasm split away from the cell wall and gaps appear between the wall and the membrane. This is a plasmolysed cell.

Question 13

What is turgor and why is it important in plants?

Answer 13

Turgor is the state a plant is in when its cells are turgid. It is important because the pressure inside the cells pushes neighbouring cells together supporting the parts of the plant that don’t have hard tissue such as the young stems and leaves.

Question 14

Why do plants wilt?

Answer 14

If a plant loses too much water from its cells they will become flaccid and the plant will wilt. The leaves droop and collapse. This is a protective action as it cuts down water loss by reducing the exposed surface area of the leaves and closing the stomata.

Question 15

How does and can water travel across a plant tissue?

Answer 15

If a cell has a higher water potential than the cell next to it, water will move from the first cell to the second. This will dilute the contents of the second cell so that it has a higher water potential than the next cell. In this way the water moves across the plant tissue down a gradient of water potential.

Question 16

An experiment to test the effects of osmosis on onions:

expected results?

Answer 16

Place a drop of concentrated molar solution on a microscopic slip and a drop of tap water on the second slide. two small squares of inner epidermis are removed from the layers of an onions. Put one on the sucrose solution slide and one on the water slide. Do this as fast as possible so the cells don’t dry out.

Add a secondary drop of each solution on top of the respective slides, followed by a cover slip. Any excess liquid is blotted with paper.

The specimen with water will show turgid cells. The cells in the sucrose solution will gradually plasmolyse.

Question 17

experiment to test the effects of osmosis on potato tuber tissue:
expected results?

Answer 17

One boiling tube half filled with tap water, a second with sucrose solution and a third left empty.
Cut a potato into three 5cm x 1cm x 1cm chips and blot each gently to remove excess moisture.
Find the mass of each chip on a weighing scale. Then place a chip in each of the three boiling tubes.
Remove the chips after 30 minutes and blot again and reweigh.
Feel them and compare how flexible and stiff they are.

The chip in water should have gained mass and be stiff, sucrose solution lose mass and be flexible, and in air lose a little bit of mass (evaporation) and be stiff.

Question 18

In a plant what is the main site of water absorption?

Answer 18

The root hair cells in the roots.

Question 19

How are root hair cells specially adapted for their job?

Answer 19

The plant roots are covered in millions of cells which grow into long ‘hairs’ that stick out into the soil. This gives the plant a big surface area for absorbing water from the soil. Each hair is long and thin and can penetrate between the soil particles, reaching the soil water.

Question 20

How does the root hair cell absorb water? What does it do with it?

Answer 20

The soil water has a higher water potential than the inside of the cell which allows water to enter the root hair by osmosis. This in turn dilutes the contents of the cell, increasing the water potential. Water then moves out of the root hair cell into the outer tissue of the root. Again a gradient of water potential is set up across the root cortex, continued by the water being taken up by the xylem in the middle of the root.

Question 21

What is transpiration?

Answer 21

The evaporation of water from the surface of a plant

Question 22

Describe the process of transpiration

Answer 22

1) transpiration is caused by the evaporation and diffusion of water from a plant’s surface. Most transpiration happens at the leaves
2) this evaporation creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it
3) this in turn means that more water is drawn up from the roots, and so there is a constant transpiration stream of water through the plant

Question 23

What are the functions of the transpiration stream:

Answer 23

• supplies water for the leaf cells to carry out photosynthesis
• carries mineral ions dissolved in water
• provides water to keep the plant cells turgid
• allows evaporation from the leaf surface, which cools the leaf, in a similar way to sweat cooling the human skin

Question 24

What is a vascular bundle?

Answer 24

A group of vessels or tubes

Question 25

Why is there usually more stomata at the bottom of a leaf?

Answer 25

If they were mainly on the upper leaf surface, the leaf would lose too much water because the stomata would be exposed to direct sunlight which would produce a high rate of evaporation from them. There is also less air movement on the underside of leaves.

Question 26

Describe the process of the opening and closing of stomata:

Answer 26

The part of the guard cell’s cell wall nearest the stoma is particularly thick. In light water enters the guard cell by osmosis from the surrounding epidermis cells. They become turgid and as they swell up their shape changes. They bend outwards opening the stoma. In the dark the guard cells lose water again. They become flaccid and the stoma closes.

Question 27

Why do the stomata open and close?

Answer 27

It is a useful adaption. Without the sun there is no need for loss of water vapour from the stomata to cool the leaves. In addition the leaves cannot photosynthesise in the dark so they don’t need water therefore it doesn’t matter if the transpiration stream is shut down by the closure of the stomata.

Question 28

What is a potometer?

Answer 28

A special piece of apparatus used to estimate transpiration rates by measuring water uptake

Question 29

Describe an experiment, using a potometer, to test the rate of transpiration in different conditions:

Answer 29

1) cut a shoot underwater to prevent air from entering the xylem - cut it at a slant to increase the surface area available for water uptake
2) assemble the potometer in water and insert the shoot underwater, so no air can enter
3) remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
4) check that the apparatus is water tight and air tight
5) dry the leaves, allow time for the shoot to acclimatise and then shut the tap
6) remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water
7) record the starting position of the air bubble
8) start a stopwatch and record the distance moved by the bubble per unit time (e.g. per hour)
9) keep the conditions constant throughout the experiment e.g. temp and humidity

Question 30

Factors that affect the rate of transpiration:

Answer 30

• Light intensity
• Temperature
• Humidity
• Wind speed

Question 31

How does the rate of transpiration change with light intensity?

Answer 31

The rate of transpiration increases in light, because the stomata open in the light, allowing water to escape

Question 32

How does the rate of transpiration change with temperature?

Answer 32

High temperature increase the rate of transpiration, as when it’s warm, water particles have more kinetic energy to evaporate and diffuse out of the stomata

Question 33

How does the rate of transpiration change with humidity?

Answer 33

When the air is less humid, the rate of transpiration is faster. There is a lower concentration of water outside the leaf, therefore a steeper concentration gradient, so diffusion happens more quickly from an area of high concentration in the leaf to low concentration outside the leaf

Question 34

How does the rate of transpiration change with wind speed?

Answer 34

The rate of transpiration increases with a higher wind speed and therefore faster air movements. The moving air removes any water vapour outside the leaf, so there is a lower concentration of water outside the leaf, therefore a steeper concentration gradient. So diffusion happens more quickly from an area of high concentration in the leaf to low concentration outside the leaf

Question 35

How can root cells uptake mineral ions?

Answer 35

Active transport or diffusion depending on the concentration of mineral ions in the soil.

Question 36

Why is transpiration a ‘side effect’ of the way leaves are adapted for photosynthesis?

Answer 36

They have to have stomata for easy gaseous exchange
Because there’s more water inside the plant than in the outside, the water escapes from the leaves through the stomata by diffusion

Question 37

Why is water important to plants?

Answer 37

So it can carry out photosynthesis

To keep plant cells turgid

Question 38

What is the role of the xylem?

Answer 38

The xylem tubes carry water and mineral salts from the roots up the stem to the leaves (mesophyll cells) in the transpiration stream

Question 39

What is the role of the phloem?

Answer 39

Phloem transport sucrose and amino acids from where they are made in the leaves to other parts of the plant (e.g. storage and growth areas)

Question 40

How are plants adapted to reduce water loss through transpiration?

Answer 40

They have a thick waxy cuticle which reduces evaporation from the epidermis
Stomata have sunken pits which trap a pocket of humid air
They can have less or smaller leaves which reduces the surface area where evaporation can occur
Have fewer stomata which reduces evaporation rates