Chapter 10 Adaptions For Transport (plants)

Question 1

What is the function of the xylem?

Answer 1

Transport water and dissolved minerals

Provide mechanical strength and support.

Question 2

What are the main cell types in the xylem?

Answer 2

Vessels (only in angiosperms)

Tracheids (in ferns and angiosperms)

Question 3

What are the functions of tracheids?

Answer 3

Tracheid walls contain lignin which is hard, strong and waterproof. The walls of gaps called pits through which the water travels. Tracheids are spindle shaped so water takes a twisting path up the xylem.

Question 4

What are the functions of vessels in the xylem?

Answer 4

Lignin walls cause the contents to die leaving an empty space in a tube shape. Water travels directly up the vessels.

Question 5

What are the three ways water moves through the root?

Answer 5

Apoplast pathway
Symplast pathway
Vacuolar pathway

Question 6

What is the apoplast pathway?

Answer 6

Water moves in the cell walls, by travelling through the spaces between cellulose fibres. However, water cannot enter the xylem through the apoplast pathway. This is because the xylem walls are made of lignin and are waterproof. The vascular tissue has a single layer of cells called the endodermis. The endodermis has a band of cells made of Suberin (waterproof waxy material) called the casparian strip which forces water into the symplast or vacuolar pathway.

Question 7

What is the symplast pathway?

Answer 7

Water moves through the cytoplasm and plasmodesmata.

Question 8

What is the vacuolar pathway?

Answer 8

Water moves from the vacuole of one cell to the vacuole of the next.

Question 9

How does water move up the xylem?

Answer 9

Cohesion-tension theory states that because water is bipolar,it produces adhesive forces which allow water molecules to attract to the lining of the xylem vessels. Water also produces cohesive forces which is where water molecules attract other water molecules which causes tension in the water column. Therefore, water moves up the xylem by ‘sticking’ to the lining of the xylem by adhesion and pulling up other water molecules by cohesion. Root pressure pushes water up the xylem.

Question 10

What is transpiration?

Answer 10

The evaporation of water vapour from the leaves out through the stomata into the atmosphere.

Question 11

What are the 4 factors effecting transpiration?

Answer 11

Light intensity
Temperature
Air movement
Humidity

Question 12

How does temperature affect rate of transpiration?

Answer 12

As temperature increases, the kinetic energy of water molecules increases causing them to vibrate rapidly causing the, to evaporate and if the stomata are open, speeds up their rate of diffusion out into the atmosphere.

Question 13

How does humidity affect the rate of transpiration?

Answer 13

The air inside a leaf is saturated with water vapour, so it has a high water potential. The humidity of the atmosphere surrounding the leaf varies. Therefore, if the atmosphere has a low humidity, there is a greater water potential gradient between the leaf and atmosphere causing water to move out the stomata by osmosis at a faster rate.

Question 14

How does air movement affect the rate of transpiration?

Answer 14

Movement of the surrounding air removes the humid microclimate at the leaf surface. The water potential outside the leaf decreases, increasing the water potential gradient between the inside and outside of the leaf. Therefore, water moves out of the stomata by osmosis at a faster rate.

Question 15

How does light intensity affect the rate of transpiration?

Answer 15

Increased light intensity, increases stomata opening. The open stomata provide a pathway for water to leave the leaf causing a faster rate of transpiration.

Question 16

What are adaptations of mesophytes?

Answer 16

Mesophytes grow on land

• shed their leaves in winter so they do not lose water by transpiration when liquid water may be scarce.
• annual mesophytes live as dormant seeds over winter, with such low metabolic rates that almost no water is required.

Question 17

What are adaptations of xerophytes?

Answer 17

Rolled leaves - reduces leaf area exposed to air
Sunken stomata - trap humid air
Hairs - trap water vapour
Thick cuticles - wax is waterproof reducing water loss
Streamlined bodies - reducing area exposed to light and high temperature.

Question 18

What are adaptations of hydrophytes?

Answer 18

No lignified support tissues (water acts as supportive medium)
Less developed xylem
No or little cuticle (no need to reduce water loss)
Stomata on upper surface (lower surface is under water)
Large air spaces forming a reservoir of oxygen and CO2 providing buoyancy.

Question 19

What is translocation?

Answer 19

The movement of the soluble products of photosynthesis such as sucrose and amino acids, through phloem, from source to sink.

Question 20

What is the phloem?

Answer 20

Plant tissue containing sieve tube elements and companion cells, translocating sucrose and amino acids from the leaves to the rest of the plant.

Question 21

What is mass flow theory?

Answer 21

At the source, such as the leaf, sucrose is produced by photosynthesis. The sucrose lowers the water potential and so water move into the cells by osmosis. As water enters, hydrostatic pressure increases forcing the sucrose solution into the phloem. The pressure pushes the solution down the phloem towards the sink. The increased pressure pushes the solution out into the cells and water moves back into the xylem by osmosis.

Question 22

What does mass flow not take into account?

Answer 22

Does not explain the purpose of sieve plates.

Companion cells are biochemically active, but mass flow does not suggest a role for them.

Question 23

What do other theories of translocation suggest?

Answer 23

Active processes - cyanide and low temperatures inhibitory bit translocation indicating energy is needed.
Protein filaments pass through the sieve pores suggesting different solutes are carried along different routes through the same sieve tube element
Cytoplasmic streaming causes the movement in different directions in individual sieve tube elements

Question 24

How do ringing experiments support mass flow theory?

Answer 24

Cylinders of outer bark tissue are removed in a ring. This removes the phloem, but the xylem remains. While the plant photosynthesised, the phloem contents above the ring swelled slightly. Above the ring, a lot of sucrose was found, but below the ring, no sucrose was found suggesting the sucrose had been used by the plant but not replaced because the ring prevented it from being moved downwards.

Question 25

How do radioactive tracers support mass flow?

Answer 25

A plant photosynthesises in the presence of a radioactive isotope such as 14C in CO2. A stem section is placed on a photographic film, which is exposed if there is a radiation source. The radioactivity coincides with the position of the phloem indicating that the phloem transports the sucrose made from radioactive 14CO2.

Question 26

How do aphid experiments support mass flow?

Answer 26

Aphids have a hollow needle-like mouthpart called a stylet. The stylet is inserted into a sieve tube and the phloem contents, the sap, exude under pressure into the aphids stylet. The sap was analysed showing the presence of sucrose. This indicates the phloem is under pressure when translocating sucrose.

Question 27

How do combined aphid and radioactive tracer experiments support mass flow?

Answer 27

The aphid experiments were used on plants which photosynthesised in 14CO2. These showed that the radioactivity and the sucrose moved much faster than the rate of diffusion alone and so additional mechanism has to be considered suggested by mass flow.