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BIOLOGY IN FOCUS
MAINTAINING A BALANCE
Chapter 2 Transport—dissolved nutrients and gases
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HSCCOURSE
Student Worksheet: Transport in xylem and phloem
■
describe current theories about processes responsible
for the movement of materials through plants in xylem
and phloem tissue
")/,/'9
). &/#53
Linked to
page 67
On the figure below, highlight the xylem in pink and the phloem in green.
Complete the textboxes to outline the processes responsible for the treatment of
materials in the xylem and phloem.
TRANSPIRATION
MASS FLOW
sunlight
4
1
2
5
3
6
7
1
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
Use the text on pages 64–67 of Biology in Focus HSC Course textbook as well as
the material from the next worksheet ‘Assumed Knowledge: Diagrams of xylem
and phloem distribution and structure’ to answer the following questions.
1. Name the two transport tissues in plants.
_____________________________________________________________________
2. Identify the substances transported by each tissue and the direction of transport.
_____________________________________________________________________
_____________________________________________________________________
3. List the cells types of which xylem tissue is composed.
_____________________________________________________________________
4. Describe the structure of xylem vessels and tracheids and relate this to their
functions.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
5. List the two main types of cells of which phloem tissue is composed.
_____________________________________________________________________
6. Describe how the structure of each phloem cell type relates to its function.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
7. Complete the following passage describing how materials are transported in
xylem vessels.
Use the words below:
adhesion, passive, unloading at the sink, active, aplastic, pressure flow,
transpiration stream, source-path-sink, cohesion, loading at the source, symplastic,
decreases, increases
Xylem vessels are not alive so the movement of materials is ___________, not
active.
■ ___________: water is drawn up the xylem tubes to replace the loss of water
through ___________ from the stomates.
2
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
■
Movement through tubes by capillarity:
— ___________: water molecules are polar and stick together.
— ___________: water molecules stick to the walls of the vessels.
8. Insert the correct words to describe the theories used to explain how materials
are transported in the phloem.
Movement of materials is by a mechanism known as ___________ ___________.
The flow of materials in phloem is an ___________ process that requires
energy. Another name for the mechanism by which it occurs is the
________________________ system that is driven by pressure gradients
generated osmotically.
(a) ____________________________________________
Amino acids, sucrose and other mineral nutrients are loaded into the phloem in
the leaves. There are two theories as to how this may occur:
(i)
___________ loading: sugars and other nutrients move in the cytoplasm
from the mesophyll cells to the sieve elements.
(ii) ___________ loading: sugars and nutrients move along a pathway through
the cell walls until they reach the sieve element. Materials would pass into
the sieve cell by active transport.
(iii) As sugars enter the phloem the concentration of phloem sap ___________
and the osmotic pressure at the source end is high.
(b)
Materials flow to the sink. At the sink, sugars and materials are actively being
removed from the phloem (e.g. roots, stem, flowers or storage areas in the
plant). As sugars are actively removed from the phloem, water flows out with
them. This __________ pressure in the sieve cells at the sink region. This
pressure difference between the source and sink in the phloem drives the
phloem sap flow. The direction depends on where the sink areas of the plant
are operating.
3
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
Answers
TRANSPIRATION
MASS FLOW
sunlight
Chloroplast—site of sugar manufacture
by photosynthesis
Mesophyll cell of leaf
4
1
Loss of water by
evaporation
Sugar released from illuminated
chloroplast. More negative
osmotic potential here
Cytoplasmic
connections
between cells
5
Transpiration
stream of water up
the xylem vessels
2
High hydrostatic pressure in the
mesophyll cell leads to bulk
export along the sieve tubes
Phloem sieve
tube
Companion
cell
3
6
7
Less negative osmotic potential
in starch storage cell, e.g. in
root
Uptake of water
from soil
Starch grain
Water returns to the transpiration
stream
Cell of root
Use the text on pages 64–67 of Biology in Focus HSC Course textbook as well as
the material from the next worksheet ‘Assumed Knowledge: Diagrams of xylem
and phloem distribution and structure’ to answer the following questions.
1. Name the two transport tissues in plants.
Xylem and phloem
2. Identify the substances transported by each tissue and the direction of transport.
Xylem: transports water and mineral ions upwards from the roots to the leaves.
Phloem: transports organic materials (particularly sugar, amino acids and
hormones) up and down the stem to other parts of the plant.
3. List the cells types of which xylem tissue is composed.
Parenchyma and fibre cells
4
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
4. Describe the structure of xylem vessels and tracheids and relate this to their
functions.
Xylem vessels are hollow—their end walls break to form a tube for easy flow
of water and dissolved mineral ions. Vessels can be up to several metres long
and are strengthened with lignin (in spiral, ring or net patterns), making them
impermeable to water and giving them support. Pits in the walls enable water
and solutes to pass through from one xylem element to another.
Tracheids transport water and mineral ions. They are long, thin tubes with
overlapping cells. They have pits in the walls so water can pass through from
one tracheid to another.
5. List the two main types of cells of which phloem tissue is composed.
Sieve cells and companion cells
6. Describe how the structure of each phloem cell type relates to its function.
Sieve cells are joined end-to-end to form a series of connecting tubes. Sieve
plates (membranes with pores) are found at the junction of sieve cells. This
allows for the transport of phloem sap from one sieve cell to another. Mature
sieve cells have a large internal space to allow for movement of sap.
Companion cells are linked to sieve cells by plasmodesmata. They are thought
to take on metabolic functions for the sieve plate.
7. Complete the following passage describing how materials are transported in
xylem vessels.
Use the words below:
adhesion, passive, unloading at the sink, active, aplastic, pressure flow,
transpiration stream, source-path-sink, cohesion, loading at the source, symplastic,
decreases, increases
Xylem vessels are not alive so the movement of materials is passive, not active.
Transpiration: water is drawn up the xylem tubes to replace the loss of water
through evaporation from the stomates.
■ Movement through tubes by capillarity:
— cohesion: water molecules are polar and stick together.
— adhesion: water molecules stick to the walls of the vessels.
■
8. Insert the correct words to describe the theories used to explain how materials
are transported in the phloem.
Movement of materials is by a mechanism known as translocation.
The flow of materials in phloem is an active process that requires energy.
Another name for the mechanism by which it occurs is the source–path–sink
system that is driven by pressure gradients generated osmotically.
5
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
(a) Loading at the source
Amino acids, sucrose and other mineral nutrients are loaded into the phloem in
the leaves. There are two theories as to how this may occur:
(i)
Symplastic loading: sugars and other nutrients move in the cytoplasm from
the mesophyll cells to the sieve elements.
(ii) Apoplastic loading: sugars and nutrients move along a pathway through
the cell walls until they reach the sieve element. Materials would pass into
the sieve cell by active transport.
(iii) As sugars enter the phloem the concentration of phloem sap increases and
the osmotic pressure at the source end is high.
(b)Path–sink
Materials flow to the sink. At the sink, sugars and materials are actively being
removed from the phloem (e.g. roots, stem, flowers or storage areas in the
plant). As sugars are actively removed from the phloem, water flows out with
them. This reduces pressure in the sieve cells at the sink region. This pressure
difference between the source and sink in the phloem drives the phloem sap
flow. The direction depends on where the sink areas of the plant are operating.
Note to teacher: An additional worksheet is provided overleaf to assist weaker
students create a flow chart showing pressure flow in the xylem and phloem.
6
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
MAINTAINING A BALANCE
Chapter 2 Transport—dissolved nutrients and gases
i˜`>Ê
,7Ê>À}>ÀiÌÊ,"-" Ê
܈̅Ê-Ìi«…>˜ˆiÊ"-
HSCCOURSE
Additional worksheet: Pressure flow in the xylem
and phloem
")/,/'9
). &/#53
Linked to
page 67
Student task
Select words from the list below to complete the flow chart provided.
pressure difference, loading, sink, decreases, increases, removed
Flow chart outlining pressure flow in phloem
______________ occurs at the source (symplastic/apoplastic)
Ð
As sugar enters the phloem, the concentration of the fluid in the phloem
______________, and so the osmotic pressure increases
Ð
Materials flow to the ______________
Ð
Sugars and water are ______________ at the sink
Ð
Pressure ______________ in sieve cells in sink region
Ð
______________ ______________ drives the phloem sap flow
1
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.
BIOLOGY IN FOCUS
Answer
Loading occurs at the source (symplastic/apoplastic)
Ð
As sugar enters the phloem, the concentration of the fluid in the phloem
increases, and so the osmotic pressure increases
Ð
Materials flow to the sink
Ð
Sugars and water are removed at the sink
Ð
Pressure decreases in sieve cells in sink region
Ð
Pressure difference drives the phloem sap flow
2
Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.