7.8 Transport in phloem

Question 1

What is translocation

Answer 1

The transport of organic molecules and mineral ions through the phloem

Eg sucrose, amino acids and inorganic ions like k+ or Cl-

Question 2

What is the phloem made up of

Answer 2

Sieve tube elements are the cells that make it up, long structures
They contain very few organelles
Their end walls are perforated to form sieve plates with pores

Next to sieve tube elements are companion cells

Question 3

What are sources and sinks

Answer 3

Sources are the site of production of sugars during photosynthesis, so eg the leaves

The sinks are where these sugars are transported to, where they will be used in respiration, or will be stored as starch. Eg in the roots
However sinks can be above or below the source which is why transport in phloem goes in both directions

Question 4

Mass flow theory can be divided into 3 phases

Describe phase 1 where sucrose is transported into sieve elements from source cells

Answer 4

. Sucrose is manufactured from products of photosynthesis in cells with chloroplasts
. The sucrose diffuses down a concentration gradient by facilitated diffusion from the source cells to companion cells
This is because there is a higher concentration of them in source cells.

. Using co transport, the sucrose are transported with the H+ ions into the sieve tube elements with co transport carrier proteins

H+ ions are actively transported from companion cells into the spaces within cell walls using ATP
. These H+ ions then diffuse down concentration gradient by facilitated diffusion using carrier proteins into sieve tube elements

Question 5

What is stage 2 of mass flow theory

Where mass flow of sucrose through sieve tube elements

Answer 5

. Sucrose is now in the sieve tube elements, so lowers the water potential there
. So water moves in from the xylem by osmosis, creating a high hydrostatic pressure in that region

. At the respiring/ sink cells there is a low concentration of sucrose as it is used up in respiration
. Sucrose is actively transported into the cell from the phloem
This lowers their water potential so water moves into the cells from the sieve tubes by osmosis

This reduces the hydrostatic pressure of sieve tubes in this region

So sucrose flows down this pressure gradient in the sieve tubes

Question 6

What is mass flow

Answer 6

. Mass flow is the bulk movement of a substance through a given channel or area at a given time

Question 7

Give 3 pieces of evidence for mass flow hypothesis

Answer 7

. Concentration of sucrose is higher in leaves (source) than in roots (sink)

. Downward flow of sucrose in the phloem occurs in daylight but ceases at night

. There is a pressure in the sieve tubes as shown by sap being released when they are cut

Question 8

Give 2 pieces of evidence against mass flow

Answer 8

. Function of sieve plates is unclear as they would seem to hinder mass flow
But perhaps they’re there to support and structure the phloem

. Sucrose is delivered at the same rate to different regions but surely if mass flow was correct it would be delivered at different rates if there are different pressures in the regions

Question 9

What is step 3 of mass flow hypothesis

Where sucrose is transferred from sieve tube elements to other sink cells or for storage

Answer 9

. Sucrose is actively transported by companion cells out of the sieve tubes and into sink cells

Question 10

When sucrose enters sink cells, what is it used for

Answer 10

. Can be stored as starch
. Converted to glucose for respiration