3B - Transport in plants - Xylem

Question 1

What are the 2 types of tissues involved in transport in plants?

Answer 1

Xylem and phloem.

Question 2

What does xylem tissue transport?

Answer 2

Water and mineral ions in solution.

Question 3

In what direction do xylem tissues transport water and mineral ions in plants?

Answer 3

Up the plants from roots to leaves.

Question 4

What are the xylem and phloem?

Answer 4

Mass transport systems in plants.

Question 5

What is the structure of the xylem vessels?

Answer 5

Narrow tubes made of dead cells.
No end walls creating a continuous tube.
Strong walls, strengthened with lignin.

Question 6

What helps water move up plants from roots to leaves?

Answer 6

Cohesion and tension.

Question 7

What happens at the top of the xylem (at the leaf)?

Answer 7

The water evaporates.

Question 8

What is the evaporation of water at the top of the xylem known as?

Answer 8

Transpiration.

Question 9

How does water enter a plant?

Answer 9

Through the roots.

Question 10

Explain how cohesion and tension occur to pull water up the xylem

Answer 10

Water evaporates from the leaves at the top of the xylem (transpiration). This creates tension (suction), which pulls more water into the leaf. Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow. This means the whole column of water in the xylem from the leaves down to the roots moves upwards.

Question 11

What is transpiration?

Answer 11

The evaporation of water from a plant’s surface, especially the leaves.

Question 12

What is transpiration a side effect of?

Answer 12

Photosynthesis

Question 13

Explain why transpiration occurs

Answer 13

• Water readily evaporates from the cell walls of the mesophyll cells.
• This water vapour accumulates in the air spaces.
• When the stomata are open, water vapour diffuses out of the air spaces into the surrounding air.
• As water evaporates from the mesophyll cell walls it is replaced by water from the xylem vessels in the leaf.

Question 14

What is cohesion?

Answer 14

A force resulting from the attraction between molecules of the same substance.

Question 15

What is adhesion?

Answer 15

A force resulting from the attraction between molecules of different substances.

Question 16

What is the cohesion-tension theory?

Answer 16

• When water is drawn out of the xylem vessels, a tension (negative pressure) develops and a water potential gradient is established through the plant.
• Because of these two things, water is drawn up the xylem. The cohesive properties of water allow it to be pulled up in one continuous column.
• Adhesive forces between water and the xylem walls prevent the water column dropping under gravity.
• This mechanism of water movement is known as the transpiration pull.

Question 17

What evidence is there for the cohesion-tension theory?

Answer 17

Change in the diameter of tree trunks according to the rate of transpiration (when transpiration is at its greatest, the tree diameter decreases).

If a xylem vessel is broken and air enters it, the tree can no longer draw up water.

When a xylem vessel is broken, water doesn’t leak out, as would be the case if it were under pressure.

Question 18

What are the 4 factors that affect transpiration rate?

Answer 18

Light, temperature, humidity, wind.

Question 19

How does light affect transpiration rate?

Answer 19

The lighter it is - the faster the transpiration rate because the stomata open when it gets light to let in CO2 for photosynthesis and when it is dark transpiration rate is low as the stomata are usually closed.

Question 20

How does temperature affect transpiration rate?

Answer 20

The higher the temp, the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

Question 21

How does humidity affect transpiration rate?

Answer 21

The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpiration.

Question 22

How does wind affect transpiration rate?

Answer 22

The windier it is the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the concentration gradient, which increases the rate of transpiration.

Question 23

What can be used to estimate transpiration rate?

Answer 23

A potometer.

Question 24

What can a potometer be used to estimate?

Answer 24

Transpiration rate.

Question 25

Why is it not valid enough to estimate transpiration rates using a potometer?

Answer 25

As it assumes that water uptake of the plant (which is what it actually measures) is directly related to water loss by the leaves.

Question 26

How do you use a potometer to estimate transpiration rates in plants?

Answer 26

• Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the SA available for water uptake.
• Assemble the potometer in water and insert the shoot underwater, so no air can enter.
• Remove the apparatus from the water but keep the ned of the capillary tube submerged in a beaker of water.
• Check that the apparatus is watertight and airtight.
• Dry the leaves, allow time for the shoot to acclimatise, and then shut the tap.
• 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.
• Record the starting position of the air bubble.
• Start a stopwatch and record the distance moved by the bubble per unit of time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate.
• Only change one variable at a time and keep all other conditions constant.

Question 27

Describe how you would dissect a plant a prepare a section of tissue to look at under a microscope

Answer 27

• Use a scalpel (or razor blade) to cut a cross-section of the stem. Cut the section as thinly as possible - thin sections are better for viewing under a microscope.
• Use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out.
• Transfer each section to a dish containing stain, e.g. toluidine blue O (TBO), and leave for 1 min. TBO stains lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels and examine their structure.
• Rinse of the sections in water and mount each one onto a slide.

Question 28

What are the walls of the xylem vessels strengthened by?

Answer 28

Lignin.