3.1.3 - Transport in Plants

Question 1

Why do multicellular plants need transport systems?

Answer 1

As multicellular plants become larger their surface area to volume ratio becomes smaller so transport systems are needed to supply nutrients to and remove waste from individual cells.

Question 2

What is the function of xylem vessels?

Answer 2

They transport water in the roots, stem and leaves of the plant in one direction from the roots to the top of the plant

Question 3

What is the function of the phloem?

Answer 3

They transport organic molecules such as sucrose from photosynthesis in all directions

Question 4

Which two cells does the phloem contain?

Answer 4

• Sieve tube elements
• Companion cells

Question 5

What are sieve tube elements?

Answer 5

They are living cells which have perforated cell walls called sieve plates between them and companion cells beside them. They contain no nucleus and few organelles .

Question 6

What do companion cells do?

Answer 6

They regulate the movement of solutes and provide ATP for active transport

Question 7

What connects the sieve tube element and the companion cell?

Answer 7

Strands of cytoplasm called plasmodesmata

Question 8

What is the structure of the xylem?

Answer 8

Xylem cells are made from dead lignified cells with no organelles that form continuous tubes and they contain pits which connect one xylem vessel to another to allow lateral water movement.

Question 9

What is transpiration?

Answer 9

The loss of water vapour from the stomata by evaporation

Question 10

How can transpiration be measured?

Answer 10

By using a potometer

Question 11

What are the 2 limitations of using a potometer to measure the rate of water uptake?

Answer 11

• The plants roots are removed so the calculated rate doesn’t account for the rate of water uptake in the roots
• This approach assumes that all of the plants water will be transpired which is unlikely to be the case

Question 12

What are the four key factors that affect the rate of transpiration?

Answer 12

• Light intensity
• Temperature
• Humidity
• Wind

Question 13

How does light intensity affect the rate of transpiration?

Answer 13

• Positive correlation
• More light causes more stomata to open so there is a larger surface area for evaporation

Question 14

How does temperature affect the rate of transpiration?

Answer 14

• Positive correlation
• More heat means more kinetic energy, faster moving molecules and therefore more evaporation

Question 15

How does humidity affect the rate of transpiration?

Answer 15

• Negative correlation
• More water vapour in the air will make the water potential more positive outside of the leaf which reduces the water potential gradient

Question 16

How does wind affect the rate of transpiration?

Answer 16

• Positive correlation
• More wind will blow away humid air containing water vapour maintaining the water potential gradient

Question 17

How is water transported into the plant?

Answer 17

Water is absorbed into plants through the root hair cells by osmosis

Question 18

How is water transported to the xylem?

Answer 18

Once the water has moved into the root hair cell by osmosis it then travels to the xylem by either the symplast or apoplast pathway

Question 19

What is the symplast pathway?

Answer 19

• The symplast pathway is through the cytoplasm of a cell
• The water moves from cell to cell towards the xylem by osmosis through the cytoplasm and through gaps in each cell wall called plasmodesmata
• Each successive cell’s cytoplasm has a lower water potential which is why the water is able to move by osmosis

Question 20

What is the apoplast pathway?

Answer 20

• The apoplast pathway is through the cell walls
• It travels through the cell walls until it reaches the Casparian strip
• The Casparian strip is impermeable to water so it forces water into the cell where it travels to the xylem via the symplast pathway
• Water can enter the cell wall and move due to the cohesive force of water
• The water molecules stick together forming a continuous stream of water which move toward the xylem
• This pathway transports the water faster as there is little resistance to the water in the cell wall

Question 21

What is the vacuolar pathway?

Answer 21

Water passes through cell vacuoles

Question 22

How is it possible for water to move up a plant from the roots against gravity?

Answer 22

• Cohesion-tension theory
• Water is a dipolar molecule which enables hydrogen bonds to form between the hydrogen and oxygen of different water molecules. This creates cohesion between water molecules so they stick together and travel up the xylem as a continuous water column
• Adhesion is when water sticks to other molecules. Water adheres to the xylem walls and the narrower the xylem the bigger the impact of adhesion
• As water moves into the roots by osmosis it increases the volume of liquid inside the root and therefore the pressure inside the root increases which is known as root pressure. This increase in pressure in the roots forces water above it upwards (positive pressure)

Question 23

How does water move up the xylem?

Answer 23

• Water vapour evaporates out of the stomata on leaves
• This loss in water volume creates a lower pressure
• When this water is lost by transpiration more water is pulled up the xylem to replace it
• Due to the hydrogen bonds between water molecules they are cohesive which creates a column of water within the xylem
• Water molecules also adhere to the walls of the xylem which helps to pull the water column upwards
• As this column of water is pulled up the xylem it creates tension pulling the xylem in to become narrower

Question 24

What are two examples of xerophytes?

Answer 24

• Cacti
• Marram grass

Question 25

What are the adaptations of xerophytes?

Answer 25

• Grow in dry habitats
• Thicker cuticle to reduce evaporation
• Reduced leaf surface area
• Few sunken stomata to reduce evaporation and to trap moisture to increase local humidity
• Rolled leaves to trap moisture to increase local humidity
• Hairs to trap moisture to increase local humidity
• Longer root network to reach more water

Question 26

What is an example of a hydrophyte?

Answer 26

Water lillies

Question 27

What are the adaptations of hydrophytes?

Answer 27

• Grow in or on water
• Little or no waxy cuticle to ensure no additional water is retained in the plant
• Large flat leaves to ensure light is absorbed
• Minimal root system for efficient water loss
• Air pockets to aid flotation and flexible stems

Question 28

What is translocation?

Answer 28

An energy requiring process which transports assimilates such as sucrose in the phloem between sources (leaves) and sinks (roots and meristems)

Question 29

What is a source?

Answer 29

Any part of the plant that loads sucrose into the sieve tube such as the leaves

Question 30

What is a sink?

Answer 30

Anywhere that removes sucrose from the phloem sieve tubes such as in a meristem or in a root

Question 31

How is sucrose loaded at the source?

Answer 31

• A carrier protein moves protons from the companion cell’s cytoplasm to the cell wall using active transport creating a higher concentration of hydrogen ions outside the companion cell
• A co transporter protein in the companion cell membrane then transports these hydrogen ions back into the cell cytoplasm in conjunction with sucrose molecules
• Sucrose then diffuses from the companion cells into the sieve tube elements via plasmodesmata
• Water moves via osmosis from the companion cell to the sieve tube elements due to the water potential gradient set up by the movement of sucrose into the sieve elements previously

Question 32

How is sucrose unloaded at the sink?

Answer 32

• The sucrose moves out of the sieve tube elements by diffusion
• The concentration gradient is maintained by converting sucrose into glucose and fructose
• This loading and unloading causes mass flow through the phloem via hydrostatic pressure and a pressure gradient