Download SAPS - Vascular tissue microscopy

Dissection and microscopy of plant vascular tissue
Learning outcomes
Introduction
These learning outcomes aim to cover subject knowledge requirements for this part of A-level Biology in
three specifications (OCR, AQA and Edexcel). You may need to adapt this information for your
specification.
This information can be used to produce learning resources, revision materials, quizzes etc.
Students should be able to:
1. List the types of cells found in plant stems (*=cell types named in specifications)
*Xylem vessels
*Phloem – sieve tube elements and companion cells
*Sclerenchyma fibres
Parenchyma (make up cortex and pith – which can be mainly hollow)
Collenchyma
Epidermis
Cambium
2. State the function of xylem vessels, phloem and sclerenchyma fibres in plant stems
Xylem vessels – transport water and mineral ions from the roots to the rest of the plant. They also provide
some structural support for the plant.
Phloem – transport sugars from a source to a sink. Photosynthetic cells are sources, cells requiring sugars
are sinks (root cells, growing regions). Areas of a plant that store carbohydrates are sinks when they are
building up a store but sources when that store is being utilised by the plant.
Sclerenchyma fibres – provide structural support to the plant.
3. List the two components of phloem and explain why both are needed
Sieve-tube elements – provide the vessels for transport of carbohydrates (relatively hollow tubes provide
reduced resistance to flow).
Companion cells – provide the metabolic needs of the sieve-tube elements.
4. Describe the structure of xylem vessels, phloem and sclerenchyma fibres
Xylem vessels are hollow tubes made up of dead cells. The cells are arranged end on end and the cell
walls between each cell are broken down to produce the long hollow tube. The cells have lost all cell
contents and have thickened cell walls that have been impregnated with lignin. Xylem vessels also have
holes in their walls that connect adjacent vessels.
Phloem are hollow tubes made of up many connected cells (sieve tubes elements). The cell walls between
each of the cells are perforated into structures called sieve plates. Each cell contains very little cytoplasm
and no nucleus. These cells have cytoplasmic connections with companion cells.
Sclerenchyma fibres are separate cells, pointed at each end, attached together to form fibres. The cells are
dead and hollow and have very thickened cell walls that are impregnated with lignin.
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 1
5. Identify xylem vessels, phloem and sclerenchyma fibres in preparations, photos and
diagrams of plant stems
A
Sclerenchyma
fibres
Phloem
Xylem
B
Sclerenchyma
fibres
Phloem
Xylem
Photomicrographs
A: Transverse section of part of a young
stem of a buttercup (Ranunculus sp.). Image
by John Bebbington FRPS
C
B: Transverse section of a vascular bundle of
a young stem of a buttercup (Ranunculus
sp.). Image by John Adds
Xylem
C: Longitudinal section of xylem vessels from
stem of Sunflower (Helianthus annuus).
Image by Leighton Dann
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 2
Preparations of celery petioles (leaf stalks) using this method (Apium graveolens
var dulce)
A and B: Preparations of transverse
sections through celery stalks [petioles]
(Apium graveolens var dulce)
C and D: Preparations of longitudinal
sections through celery stalks [petioles]
A
Phloem
B
Xylem
C
Xylem
D
Phloem
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 3
Diagrams
A: Cross-section through a plant stem (from http://www.saps.org.uk/secondary/teachingresources/1324)
B: Overview of a longitudinal section of a plant stem (image from
www.saps.org.uk/animations)
C: Detailed longitudinal section of a plant stem (image from
www.saps.org.uk/animations)
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 4
6. Describe how the structure of xylem vessels, phloem and sclerenchyma fibres are adapted
for their specific function
Xylem
Feature
Dead cells
Hollow cells
Cells connected end on end
Cell walls between cells broken down
Thickened cell walls
Lignified cell walls
Holes connecting adjacent tubes
Function
Allows the vessels to be hollow
Provides a hollow vessel that provides minimum
resistance to the flow of water (also reduces
weight)
Provides a continuous vessel for the transport of
water
Reduces the resistance to the flow of water
Gives the vessels extra strength
Waterproofs areas of the vessels to reduce water
loss and give the vessels extra strength
Allows for horizontal movement of water to
maintain vertical movement of water even if there
are blockages in the vessels
Phloem
Feature
Living cells
Cells contain little cytoplasm and no nucleus
Relatively hollow cells
Cells connected end on end
Perforated cell walls between cells (sieve
plates)
Cytoplasmic connections with companion
cells
Function
Allows the cells to take part in active processes
such as the loading of sucrose into the tubes
Allow the cells to be relatively hollow
Provides a relatively hollow vessel that provides
minimum resistance to the flow of dissolved
sugars (also reduces weight)
Provides a continuous vessel for the transport of
dissolved sugars
Reduces the resistance to the flow of dissolved
sugars
Allow the companion cells to have metabolic
control over the sieve tube elements such as
providing the sieve tube element with ATP
Sclerenchyma
Feature
Dead cells
Hollow cells
Pointed connected cells
Very thickened cell walls
Lignified cell walls
Function
Allows the vessels to be hollow
Reduces weight (increases strength to weight
ratio)
Provides a large surface area for one cell to be
connected to another to give the fibres greater
tensile strength
Give the vessels extra strength to support the
plant
Give the vessels extra strength to support the
plant
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 5
7. Identify similarities and differences between xylem vessels, phloem and sclerenchyma fibres
Feature
General
structure
Dead / Alive
Function
Cell wall
Cell
contents
Xylem vessels
Vessels = hollow tubes
made of many connected
cells
Dead cells
Transport of water and
mineral ions
Structural support
Thickened and possess
lignin
Completely hollow – some
perforated or slitted end
plates of cells remain
Phloem
Vessels = hollow tubes
made of many connected
cells
Living cells – sieve tube
elements kept alive by
connections to companion
cells
Transport of dissolved
carbohydrates
Sclerenchyma fibres
Separate cells joined
together to form fibres
No special modifications
(plasmodesmata between
sieve tube element and
companion cells, also
sieve plates at the end of
cells)
Hollow centre with thin
layer of cytoplasm around
the edge. Sieve plates at
the end of each cell.
Thickened and possess
lignin
Dead cells
Structural support
Hollow dead cells
Science & Plants for Schools: www.saps.org.uk
Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 6