Mass Transport in Plants

Question 1

define transpiration

Answer 1

the process whereby water is lost from the leaves via evaporation

Question 2

define translocation

Answer 2

the process whereby the phloem transports sugars and organic substances from the leaves to where they are needed

Question 3

what is the job of a root hair cell?

Answer 3

actively transport ions from the soil, across their membranes, and into their cytoplasm

via carrier proteins

Question 4

what is the function of the xylem?

Answer 4

• transports water and mineral ions from the roots to the rest of the plant

Question 5

how is the structure of the xylem adapted in order to carry out it’s function? (4)

Answer 5

no cytoplasm and no organelles
no obstruction to flow of water

no end walls
allows continuous columns of water to form

lignin
strengthens and waterproofs the vessels; prevents collapse of the vessels under tension

lateral pits
allows lateral movement around any blockages

Question 6

describe and explain the cohesion-tension theory (5)

Answer 6

• stomata open to enable carbon dioxide to enter the leaf; at the same time, water diffuses from the air spaces to the air outside the leaf
• loss of water from the air spaces causes water to move down a water potential gradient from the mesophyll cells to the air spaces
• this lowers the water potential of the mesophyll cells; water moves by osmosis to adjacent mesophyll cells
• water potential gradient is established and tension is created
• water moves up the xylem in a continuous column which is held together by hydrogen bonds ; cohesion

Question 7

what exactly is tension?

Answer 7

a pulling force

Question 8

what is adhesion (2)

Answer 8

• where water molecules are attracted to the walls of the xylem
• this helps to narrow the walls and contributes to negative pressure

Question 9

what is the evidence for cohesion-tension theory? (2)

Answer 9

during the daytime, a tree trunk will shrink due to increased transpiration rates that creates more tension and negative pressure in the xylem

if the xylem vessel is broken, air is drawn into the xylem vessel rather than water spurting out

Question 10

list the factors that affect the rate of transpiration (4)

Answer 10

• light intensity
• temperature
• humidity
• air movement / speed

Question 11

how does light intensity affect the rate of transpiration? (2)

Answer 11

• increasing light intensity increases the rate of transpiration
• because stomata open in the light and close in the dark

Question 12

how does temperature affect the rate of transpiration? (3)

Answer 12

• increasing temperature increases the rate of transpiration
• because it increases kinetic energy and therefore increases the movement of water molecules
• causes the rate of water evaporation to increase

Question 13

what is humidity?

Answer 13

refers to the % of water vapour in the air

Question 14

how does humidity affect the rate of transpiration? (3)

Answer 14

• the greater the difference in humidity between the air spaces and the air outside the leaf, the faster the transpiration
• because there’s a greater rate of diffusion of water vapour out of the leaf
• and water leaves the leaf down a water potential gradient

Question 15

how does air movement / wind speed affect the rate of transpiration? (3)

Answer 15

• the faster the wind speed, the faster the rate of transpiration
• because air movement over a leaf moves water vapour away from the stomatal pores
• this increases the water potential gradient

Question 16

what piece of equipment is used to measure the rate of transpiration?

Answer 16

a potometer

Question 17

describe and explain how a potometer is set up to measure the rate of transpiration? (5)

Answer 17

• a leafy shoot is cut under water ; care is taken to prevent getting water on the leaves
• potometer is filled completely with water, making sure there are no air bubbles
• using a rubber tube, the leafy shoot is fitted to the potometer under water
• potometer is removed from under the water and all joints are sealed with waterproof jelly
• an air bubble is introduced into the capillary tube

Question 18

what occurs in the potometer as transpiration occurs?

Answer 18

• water moves through the capillary tube, and the bubble of air moves with it

Question 19

what are the limitations to this experiment? (2)

Answer 19

• doesn’t tell us how much water came out
• some of the water could have been used for photosynthesis

Question 20

how is the structure of the phloem adapted in order to carry out it’s function? (4)

Answer 20

sieve tube elements
living cells

no nucleus and few organelles

sieve plates

companion cells
contain many mitochondria to produce ATP for the active transport of solutes

Question 21

describe and explain the process of translocation in the phloem (5)

Answer 21

• sucrose is actively transported into the sieve tube by companion cells
• this lowers the water potential in sieve tubes and water enters by osmosis from the xylem
• this produces higher hydrostatic pressure inside the sieve tubes at the source end of the phloem
• mass flow to respiring tissues
• sucrose moved into the sinks by active transport and facilitated diffusion