3.3 Transport in plants

Question 1

What does xylem tissue transport?


Answer 1

Water and mineral ions

Question 2

What does phloem tissue transport?


Answer 2

Assimiliates (sucrose)

Question 3

What do xylem and phloem tissues make up?


Answer 3

Vascular bundle

Question 4

Where is the vascular bundle in the root of a plant?


Answer 4

Xylem is in the centre in an X shape, phloem is in the gaps between the X shape.

Question 5

Where is the vascular bundle in the stem of a plant?


Answer 5

Both xylem and phloem are near the outside

Question 6

Where is the vascular bundle in the leaf of a plant?


Answer 6

Majoritively everywhere, form a vein network through the leaves.

Question 7

How are xylem vessels adapted for water transport?


Answer 7

Long, tube like structures
Joined end to end with no end walls to make a continuous stream of water (mass flow)
Cells are dead so contain no organelles
Walls thickened with a woody substance called lignin in a spiral formation

Question 8

How does water move out of the xylem vessel?


Answer 8

Small pits in the walls where there is no lignin

Question 9

How are phloem vessels adapted for assimilate transport?


Answer 9

Sieve tube elements and companion cells

Question 10

How is a sieve tube element adapted for assimilate transport?


Answer 10

Living cells so they can perform active processes
Joined end to end with sieve plates in between
No nucleus, few organelles, and a thin layer of cytoplasm

Question 11

How are companion cells adapted for assimilate transport?


Answer 11

Carry out the living functions for themselves and their sieve cells
Have modified cytoplasm
Plasmodesmata in between them and sieve cells for effective movement

Question 12

How does water enter a plant via the root hair cells?


Answer 12

Through the root hair cells then passes through the root cortex, including the endodermis to get to the xylem
Water is drawn into the root hair cells via osmosis
Goes through one of three pathways; apoplast, symplast or vacuolar

Question 13

What is the symplast pathway?

Answer 13

Water travels through the cytoplasm of plants
Cytoplasms of neighbouring cells are connected via plasmodesmata
Moves through via osmosis

Question 14

What is the apoplast pathway?

Answer 14

Water travels through cell walls
Walls are very absorbpent and water can diffuse through as well as pass through the spaces between cells
Water can carry solutes

Question 15

What is the vacuolar pathway?

Answer 15

Water travels through the vacuoles of the cells

Question 16

What does the casparian strip do?


Answer 16

In the endodermis
It is an impermeable strip of suberin
Water in the apoplast pathway is stopped as it cannot continue and so it diverts to the symplast pathway

Question 17

What happens to water after the casparian strip?


Answer 17

Water moves into the xylem
Xylem transports it up the stem to the leaves
Water leaves the xylem and moves into cells for photosynthesis
Water diffuses out of the cell via the stomata and into the surrounding air
This is transpiration

Question 18

What is the cohesion-tension theory?


Answer 18

Helps water move up the roots to leaves, against gravity
Water evaporates from the leaves
This creates tension, which pulls more water into the leaves
Water molecules are cohesive so when some are pulled into the plant due to tension, they pull others with them
This creates a continous column of water

Question 19

Define cohesion vs adhesion (of water)


Answer 19

Cohesion = water molecules form hydrogen bonds with other water molecules
Adhesion = water molecules form hydrogen bonds with lignin or the xylem vessels

Question 20

How can adhesion be partly responsible for the movement of water?


Answer 20

As the water is ‘attracted’ to the xylem vessels, it helps the water rise

Question 21

How does light affect the transpiration rate?


Answer 21

The lighter it is, the faster the rate of transpiration
The stomata opens when it is light so CO2 can diffuse easier for photosynthesis, increasing transpiration

Question 22

How does temperature affect the transpiration rate?



Answer 22

Higher temperature means faster transpiration
Warmer water molecules have more energy so they evaporate from cells faster
Increase water potential gradient so water vapour diffuses quicker

Question 23

How does humidity affect the transpiration rate?



Answer 23

Lower humidity means faster transpiration
Air around the plant being dry means that the water potential outside is lower than inside the plant
Water vapour diffuses quicker in attempt at creating equilibrium

Question 24

How does wind affect the transpiration rate?


Answer 24

Faster wind means faster transpiration
Wind blows away any water vapour, increasing water potential gradient

Question 25

What does a potometer do?


Answer 25

Measures water uptake by a plant, but it is assumed that water uptake is directly related to water loss by the leaves This can be used to estimate transpiration rate

Question 26

How is a potometer set up?


Answer 26

Cut the stem of the plant underwater and at an angle Assemble apparatus in water Dry the leaves Remove the end of the capillary tube from the beaker of water until one air bubble is formed Record starting position of the air bubble Start stopwatch to record distance moved by the bubble per unit of time

Question 27

Why is it essential for the potometer to be water and air tight?


Answer 27

To avoid any air bubbles in the xylem which would disrupt the transpiration stream

Question 28

What is a xerophytic plant?


Answer 28

Plants adapted to living in dry conditions.

Question 29

What are the adaptations of a xerophytic plant?


Answer 29

Smaller leaves which reduce the surface area for water loss Both densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation Respond to low water availability by closing the stomata to prevent water loss Contain hairs and pits which serve as a means of trapping moist air, Roll their leaves to reduce the exposure of lower epidermis to the atmosphere

Question 30

What is a hydrophytic plant?


Answer 30

Plants that actually live in water.

Question 31

What are the adaptions of hydrophytic plants?

Answer 31

Have a very thin or absent waxy cuticle as they don’t need to conserve water Many constantly open stomata are found on the upper surfaces of leaves to maximise gas exchange Wide, flat leaves give a large surface area for light absorption Air sacs are found in some to enable leaves to stay afloat as any large air spaces to make leaves and stems more buoyant.

Question 32

What is translocation?


Answer 32

Movement of assimilates (dissolved substances) from source cells to sink cells

Question 33

What is active loading used for?


Answer 33

Move substances into the companion cells from surrounding tissues and from the companion cells into the phloem

Question 34

How does active loading work?


Answer 34

A H+ ion binds to a contransporter protein in the companion cell membrane and enters the cell A sucrose molecule binds to the cotransporter protein at the same time Movement of H+ ion is used to move the sucrose molecule into the cell Sucrose molecules then go through the plasmodesmata from the companion cells and into the phloem

Question 35

What follows after active loading in the transport of assimilates?


Answer 35

Mass flow Sucrose that has now moved into the phloem lowers the water potential Water moves into the phloem by osmosis from the xylem and companion cells

Question 36

What is the process of unloading at the sink cell?


Answer 36

Assimilates are removed from the phloem to be used up This increases the water potential inside the phloem, so water leaves the phloem via osmosis Lowers the pressure inside the phloem

Question 37

What does unloading result in?


Answer 37

Pressure gradient in the phloem from source to sink This gradient pushes assimilates along the phloem