mass transport in plants 3.3.4

Question 1

how is water transported in plants

Answer 1

xylem vessels

Question 2

what are xylem vessels

Answer 2

dead cells
long continuous tubes
provide structural support to the stem

Question 3

describe the cohesion tension theory of water transport in the xylem (5)

Answer 3

water lost from leaves because of transpiration
lowers water potential of mesophyll
water pulled up from xylem creating tension
water molecules cohere by H bonds: water column
adhesion of water to walls of xylem

Question 4

what are the three components of phloem vessels

Answer 4

sieve tube elements
companion cells
plasmodesmata

Question 5

what is plasmodesmata

Answer 5

gaps between cell walls where the cytoplasm links, allowing substances to flow

Question 6

what are companion cells

Answer 6

involved in ATP production for active loading of sucrose into sieve tubes

Question 7

what do sieve tube elements do

Answer 7

form a tube to transport sucrose in the dissolved form of sap

Question 8

name the process whereby organic materials are transported around a plant

Answer 8

translocation

Question 9

how does sucrose in the leaf move into the phloem

Answer 9

sucrose enters the companion cells by active loading using ATP and a diffusion gradient of H+ ions
sucrose then diffuses from the companion cells into the sieve tube elements via the plasmodesmata

Question 10

how do phloem vessels transport sucrose around the plant

Answer 10

as the sucrose moves into the sieve tube elements water potential in the phloem is reduced
water enters via osmosis from the xylem and increases hydrostatic pressure
water moves through sieve tubes to areas of lower hydrostatic pressure
sucrose diffuses into surrounding cells where it is needed

Question 11

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

Answer 11

• sap is released when a stem is cut therefore there must be pressure in the phloem
• there is a higher concentration gradient of sucrose in the leaves than the roots
• increasing sucrose levels in the leaves results in increased sucrose in the phloem

Question 12

give evidence against the mass flow hypothesis of translocation (3)

Answer 12

• the 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 areas with the lowest concentration first

Question 13

how can ringing experiments be used to investigate transport in plants

Answer 13

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

Question 14

how can tracing experiments be used to investigate transport in plants

Answer 14

plants are grown in the presence of radioactive CO2
will be incorporated into the plants sugars
using autoradiography, we can see the areas exposed to radiation correspond to where the phloem is

Question 15

what does the xylem transport

Answer 15

water and mineral salts

Question 16

what does the phloem transport

Answer 16

sugars and amino acids

Question 17

use understanding of mass flow hypothesis to explain how pressure is generated inside phloem tube

Answer 17

sucrose actively transported into phloem
reducing water potential
water moves into phloem by osmosis

Question 18

evidence for cohesion tension theory (2)

Answer 18

tree trunks are narrower in the day time when transpiration is occurring
broken xylem cannot draw up water but doesn’t look as if it drawing up air

Question 19

two water pathways

Answer 19

apoplast - through cell walls + plasmodesmata (usual)
symplast - through cytoplasm