Mass transport in plants

Question 1

What is the function of the xylem? (2)

Answer 1

Transports water and soluble mineral nutrients from the roots throughout the plant (in the stem and leaves of plant)

Question 2

What is the function of the phloem? (2)

Answer 2

Transport sugars and organic substances from leaves where they are formed (by photosynthesis) to where they are needed (for example shoots, flowers and fruits and in the roots)

Question 3

What are the adaptations of root hair cells? (4)

Answer 3

(Hair like) extensions which increase surface area so increased uptake of water and increase the area for channel and carrier proteins
Thin cell wall so shorter diffusion distance

Question 4

How does water enter the root hair cell? (3)

Answer 4

Root hair cells actively transport ions from the soil across their membranes into the cytoplasm via carrier proteins
This lowers the water potential of the cytoplasm, compared to the water potential of the soil
Water enters the root hair cells by osmosis down a water potential gradient

Question 5

How are xylem adapted to their function? (4)

Answer 5

Dead cells form hollow tubes with no cytoplasm - allows easier water flow
Cell walls strengthened with lignin (hydrophobic) - makes the xylem rigid and waterproof/withstand tension
End walls break down so xylem forms a continuous tube with no end walls - so water can form a continuous column
Xylem pits - allow water to move laterally between xylem vessels

Question 6

What is transpiration? (1)

Answer 6

Water leaves the plant through stomatal pores by diffusion

Question 7

Outline the transpiration-cohesion-tension theory (7)

Answer 7

Water evaporates/transpires out of the stomata
Water moves down its water potential gradient from mesophyll cells into air spaces
This lowers the water potential of the mesophyll cells so water moves by osmosis from adjacent mesophyll cells
This sets up water potential gradient across the leaf to the xylem vessels
The movement of water creates tension
Water moves up the xylem into the leaf in a continuous column
Water molecules form weak hydrogen bonds between them so they tend to stick together - they have cohesion
They are also attracted to the walls of the xylem - there are forces of adhesion between the water and the xylem

Question 8

Why is evaporation at the leaves important? (2)

Answer 8

Has a cooling effect
Reduce the chance of denaturation of enzymes

Question 9

What are the factors affecting rate of transpiration? (4)

Answer 9

Light
Temperature
Humidity
Air movement

Question 10

How does light affect transpiration? (2)

Answer 10

Stomata open in the light and close in the dark
The rate of transpiration is higher in the light

Question 11

How does temperature affect transpiration? (2)

Answer 11

Increasing temperature - increases the kinetic energy of molecules - they move more rapidly - rate of water diffusion increases - rate of transpiration increases

Question 12

How does humidity affect transpiration? (3)

Answer 12

The greater the difference in humidity between the air spaces and the air outside the leaf - the greater the rate of diffusion of water vapour out of the leaf - greater rate of transpiration
[When the air is more humid, it is harder for the water to evaporate into the air. (More humidity = less transpiration)]

Question 13

How does air movement affect the rate of transpiration? (2)

Answer 13

Air movement over a leaf moves the water vapour away from the stomatal pores
This increases the water potential gradient between the inside and the outside of the leaf
The greater the rate of movement of air - the faster the movement of water vapour - greater rate of transpiration

Question 14

How is the rate of transpiration measured? (2)

Answer 14

Use a potometer.
Measure how far the bubble moves in a set length of time e.g. 10 minutes.

Question 15

What are the precautions students take when setting up a potometer? (5)

Answer 15

A leafy shoot is diagonally cut under water - to stop xylem vessels sealing/closing
Care is taken to prevent getting water on the leaves - to prevent stomatal pores getting blocked with water
Potometer is filled completely with water making sure there are no air bubbles - air bubbles might get into xylem and block continuous column of water
Using a rubber tube, the leafy shoot is fitted to the potometer under water - water enters the xylem and water column remains
Potometer is removed from under the water and all joints are sealed with waterproof jelly - stop air getting in and water escaping from potometer

Question 16

What are the limitations of using a potometer? (4)

Answer 16

Volume of water taken up does not always equal volume of water lost through transpiration
Potometer only measuring the uptake of water through the stem and does not consider the uptake through the roots of a normal plant
some water is used in photosynthesis
Not all water taken up by plants is used in transpiration, for example some of water is used in cells to maintain turgidity

Question 17

What are the three components of phloem vessels? (3)

Answer 17

Sieve tube elements
Companion cells
Plasmodesmata (gaps between cell walls to where the cytoplasm links)

Question 18

What do companion cells contain and why? (2)

Answer 18

Many mitochondria to synthesise ATP through aerobic respiration for the active transport of solutes

Question 19

How is a concentration gradient maintained from the source to the sink? (2)

Answer 19

Enzymes maintain a concentration gradient from the source to the sink by converting the solutes at the sink to other storage substances
This makes sure there is always a lower concentration at the sink than the source

Question 20

Describe the mass flow hypothesis (7)

Answer 20

In the source, sucrose and amino acids are actively transported into the sieve tubes of the phloem from companion cells
This lowers the water potential in sieve tubes
Water enters the tubes by osmosis from xylem and companion cells
This creates a high pressure inside the sieve tubes at the source end of the phloem
The result is a pressure gradient from the source end to the sink end
This gradient pushes solutes along the sieve tubes towards the sink
Solutes used up in respiration or stored as starch at the sink

Question 21

How does rate of translocation increase (according to the mass flow hypothesis)? (1)

Answer 21

The higher the concentration of sucrose at the source the higher the rate of translocation

Question 22

Describe the mass flow hypothesis for the mechanism of translocation in plants (5)

Answer 22

In source sugars are actively transported into phloem
By companion cells
Lowers water potential of sieve tubes and water enters by osmosis
Increase in pressure causes mass movement towards sink
Sugars used/converted in root for respiration for storage

Question 23

Give evidence for the mass flow hypothesis of translocation (3)

Answer 23

Sap released when stem is cut - there must be pressure in the phloem
Higher sucrose concentration in the leaves than the roots
Increasing sucrose levels in the leaves results in increased sucrose in the phloem

Question 24

Give evidence against the mass flow hypothesis (3)

Answer 24

Structure of sieve tubes seems to hinder mass flow
Not all solutes move at the same speed as they would in mass flow
Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first

Question 25

How can ringing experiments be used to investigate transport in plants (3)

Answer 25

The bark and the phloem of a tree are removed in a ring, leaving behind the xylem
Eventually the tissue above the missing ring swells due to accumulation of sucrose as the tissue below begins to die
Therefore, sucrose must be transported in the phloem

Question 26

How can tracing experiments be used to investigate transport in plants (3)

Answer 26

Plants are grown in the presence of radioactive CO2 - which will be incorporated in the plants sugars
Using autoradiography we can see that the areas exposed to radiation correspond to where the phloem is