3.4.2 Mass Transport in Plants

Question 1

Xylem structure and function

Answer 1

Transport ions and water through xylem vessels. Very long, tube-like structures formed from dead cells joined end to end. No end walls on these cells, so they form uninterrupted tubes which allow water to pass through

Question 2

Movement of water up a plant

Answer 2

1. Transpiration takes place at the top of xylem
2. Creates tension which pulls more water into leaf
3. Water molecules are cohesive, pulling more along into the leaf. Whole column of water from roots to leaves moves upwards
4. Water enters stem through roots

Question 3

Transpiration

Answer 3

Evaporation of water from a plant’s surface, evaporates from moist cell walls and accumulates in the spaces between cells in the leaf. Moves out of open stomata down water potential gradient

Question 4

Light intensity effect on transpiration

Answer 4

Stomata open up when it gets light to let in CO2, so more transpiration when more light

Question 5

Temperature effects on transpiration

Answer 5

Higher temperature gives water molecules more energy so they evaporate faster, increases water potential gradient so water diffuses out of leaf faster

Question 6

Humidity

Answer 6

If less humidity, increased water potential gradient between leaf and air, increases rate of transpiration

Question 7

Wind

Answer 7

Wind blows away water molecules from around the stomata. Increases water potential gradient which increases transpiration rate

Question 8

Phloem structure and function

Answer 8

Transports organic solutes (mainly sugars) around plant. Formed from sieve tube elements and companion cells

Question 9

Sieve tube elements

Answer 9

Living cells that form the tube transporting solutes, no nucleus and few organelles

Question 10

Companion cell

Answer 10

One for each sieve tube element, carry out living functions for sieve cells (e.g. providing energy needed for active transport of solutes)

Question 11

Translocation

Answer 11

Movement of solutes/assimilates around the plant, requires energy. Moves them from source to sink. Enzymes maintain a concentration gradient from source to sink, so always a lower concentration at the sink

Question 12

Mass flow (Source)

Answer 12

Active transport used to load solutes from companion cells to sieve tubes at source. Lowers water potential so water enters from xylem, creates high pressure inside sieve tubes at source

Question 13

Mass flow (Sink)

Answer 13

Solutes are removed from phloem to be used. Increases water potential inside sieve tubes so water leaves by osmosis. Lowers pressure inside sieve tubes

Question 14

Mass flow (Flow)

Answer 14

Results in a pressure gradient from source to sink. Pushes solutes to sink

Question 15

Ringing to evidence mass flow

Answer 15

If a ring of bark only including phloem is removed, a bulge forms above the ring which has a higher concentration of sugars than fluid below the ring. Sugars can’t move past the ring, evidence for downward flow of sugars

Question 16

Radioactive tracer

Answer 16

14C can be used to tack the movement of organic substances in a plant