Lesson 3: Transport in Plants: Phloem

Question 1

Phloem

Answer 1

The function of phloem is to move organic
molecules throughout the plant. This is
known as translocation.

The organic molecules (sugars, amino acids,
hormones) are dissolved in water producing
a substance known as phloem sap.

Phloem consists of two types of living cells:

Sieve tube cells – stacked end to end,
these cells are perforated to allow
for the movement of solutes.

Companion cells – connected to
sieve tube cells by plasmodesmata,
they produce and transport proteins
to the sieve tube cells in order to
keep them alive.

Question 2

Translocation

Answer 2

Xylem moves water and dissolved minerals
in only one direction, from roots to leaves.

Phloem moves organic compounds in any
direction throughout the plant.

Translocation always moves organic
material from a source (where the
material is stored or made) to sink
(where the material is used)

This type of movement occurs through
a pressure flow mechanism.

Phloem sap moves passively
throughout the plant as a result of a
hydrostatic pressure gradient.

Question 3

At Source

Answer 3

The products of photosynthesis (sugars and amino acids) move from leaf cells
(source) to phloem by active transport.

This sugar loading raises the solute concentration at the source end of the
phloem tube

High solute concentration draws water into the tube from the xylem by
osmosis

This inflow of water raises water pressure at the source end of the phloem
tube.

Question 4

At Sink

Answer 4

Sugars and amino acids leaves the phloem by active transport.

This reduces the solute concentration at the sink end of the phloem tube.

Low solute concentration causes water to return to the xylem by osmosis.

The outflow of water lowers the water pressure at the sink end of the phloem
tube

Question 5

Proving Pressure Flow
Mechanism

Answer 5

Phloem sap moves around plants at a
faster rate than can be explained by
diffusion.

The pressure flow mechanism can be
difficult to test as mechanisms for
testing disrupt the structure and
function of the phloem tubes.

Natural phenomena have been
observed that prove this mechanism,
most notably the feeding of aphids.Aphids insert a stylet into a
sieve tube cell in the phloem
in order to feed off the
sugars produced by the
plant.

If the aphid stylet is inserted
closer to the source, the flow
of sap has been observed to
be faster than if the stylet is
inserted closer to the sink.

This increase in flow rate
seems to indicate that the
sap is under higher pressure
when closer to the source.

Question 6

Xylem and Phloem
Overview

Answer 6

Vascular bundles in the roots, stems
and leaves of plants are made up of
both xylem and phloem.

Question 7

Xylem

Answer 7

Composed of a column of dead
cells when mature
Continuous tube of cells enables
an unbroken column of water
(cohesive forces)

Thickened with lignin to
withstand negative pressure as
water vapour is lost

Transport water and minerals
from root to leaves

Question 8

Phloem

Answer 8

Composed of a column of living
cells with perforated cell walls

Living cells enable substances to
be loaded by active transport.

Companion cells carryout cell
functions and supply energy for
active transport

Transport sugars, amino acids
and hormones to all parts of the
plant by mass flow.