EduNinja
[Maximum number: 7]

Phloem sap is transported from sources to sinks in phloem sieve tubes. Each sieve tube is constructed from phloem sieve tube elements.

(a)

The structure of a phloem sieve tube element is adapted to its function.

Each of explanations A to F describes how a particular structural feature of a phloem sieve tube element in a source is suited to the function of transporting phloem sap.

The matching structural feature for each explanation is listed in Table 1.1.

A

for entry of sucrose and other organic compounds

B

for rapid entry of water to create high hydrostatic pressure

C

provides pores to allow the flow of phloem sap from one sieve tube element to the next

D

to form very long tubular structures for the transport of phloem sap from source to sink E decreases resistance to the flow of phloem sap within each sieve tube element, so the speed of flow is maintained F provides more space to increase the volume of phloem sap transported per unit time Complete Table 1.1 by writing the correct letter from A to F in the last column of each row, so that each structural feature is matched to the correct explanation. Use each letter only once. The first row has been completed for you.

Table 1.1

Table 1.1

[ 4 ]
(b)

At the sink, sucrose and other organic compounds are unloaded from the phloem sieve tube element.

Explain why the process of unloading helps the mass flow of phloem sap from the source to the sink.

[ 3 ]
[Maximum number: 4]

Phloem transfer cells are modified companion cells that move sucrose and other assimilates from mesophyll tissue into phloem sieve tube elements.

Fig. 5.1 is an electron micrograph of a phloem transfer cell.

Fig. 5.1

Fig. 5.1

magnification =×10000=\times 10000

(a)

Explain how sucrose in the phloem sap is moved from sieve tube elements in leaves to cells in a sink, such as a root.

[ 4 ]
[Maximum number: 1]

Which changes to the water potential and the volume of liquid in the phloem occur when carbohydrate is taken out of a sink into a phloem sieve tube element?

water potential

in phloem sieve

tube element

volume of liquid

in phloem sieve

tube element

higher

decreases

higher

increases

lower

decreases

lower

increases

(a)

Viruses can infect plant cells.

[ 5 ]
(i)

Many plant viruses can travel through the plant to enter companion cells in the phloem tissue. The viruses then travel with assimilates in the phloem sap to other areas of the plant.

Explain the mechanism that allows assimilates and viruses to travel through phloem sieve tubes to other areas of the plant.

[ 5 ]
[Maximum number: 1]

The diagram shows a model which can be used to demonstrate mass flow.

Question image

X and Y are filled with sucrose solutions of different concentrations, causing water to move in or out of X and Y by osmosis or as a result of hydrostatic pressure. Sucrose solution then moves through tube T joining X and Y .

Which description is correct?

water potential in X compared with Y

direction of movement of sucrose solution in tube T

less negative

from X to Y

less negative

from Y to X

more negative

from X to Y

more negative

from Y to X

[Maximum number: 1]

The diagram represents part of the phloem pathway, with adjacent cells, from leaf to root in a plant.

Question image

Which process is occurring between 1 to 2 and what is the effect on the water potential at 2 ?

process from 1 to 2

water potential at 2

active transport of sucrose

becomes less negative

active transport of sucrose

becomes more negative

diffusion of sucrose

becomes less negative

diffusion of sucrose

becomes more negative

[Maximum number: 1]

What is the sequence of events in the translocation of sucrose?

A

active loading of sucrose into sieve elements at the source, increased hydrostatic pressure, mass flow, unloading at the sink

B

hydrolysis of storage compounds in sinks, lowered water potential, unloading of sucrose from sieve elements, mass flow from the source

C

lowered pressure in sieve elements at the source, movement of sucrose down the pressure gradient, mass flow down a diffusion gradient to the sink

D

mass flow of dissolved sucrose into the sieve element at the source, lowered hydrostatic pressure, diffusion of sucrose to sink, active unloading

[Maximum number: 1]

Which changes to the water potential and the volume of liquid in the phloem occur when sucrose is moved from the phloem sieve tube to an actively dividing root tip?

water potential in phloem sieve tube becomes

volume of liquid in phloem sieve tubes

less negative

decreases

less negative

increases

more negative

decreases

more negative

increases

[Maximum number: 1]

Which changes to the water potential and the volume of liquid in the phloem occur when amino acids are moved into a phloem sieve tube at a source?

water potential

in phloem sieve

tubes

volume of liquid

in phloem sieve

tubes

higher

decreased

higher

increased

lower

decreased

lower

increased

0