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

  1. 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.

  1. 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.

  1. 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.

  1. Identify xylem vessels, phloem and sclerenchyma fibres in preparations, photos and diagrams of plant stems



  1. Describe how the structure of xylem vessels, phloem and sclerenchyma fibres are adapted for their specific function

Xylem

Feature / Function
Dead cells / Allows the vessels to be hollow
Hollow cells / Provides a hollow vessel that provides minimum resistance to the flow of water (also reduces weight)
Cells connected end on end / Provides a continuous vessel for the transport of water
Cell walls between cells broken down / Reduces the resistance to the flow of water
Thickened cell walls / Gives the vessels extra strength
Lignified cell walls / Waterproofs areas of the vessels to reduce water loss and give the vessels extra strength
Holes connecting adjacent tubes / Allows for horizontal movement of water to maintain vertical movement of water even if there are blockages in the vessels

Phloem

Feature / Function
Living cells / Allows the cells to take part in active processes such as the loading of sucrose into the tubes
Cells contain little cytoplasm and no nucleus / Allow the cells to be relatively hollow
Relatively hollow cells / Provides a relatively hollow vessel that provides minimum resistance to the flow of dissolved sugars (also reduces weight)
Cells connected end on end / Provides a continuous vessel for the transport of dissolved sugars
Perforated cell walls between cells (sieve plates) / Reduces the resistance to the flow of dissolved sugars
Cytoplasmic connections with companion cells / Allow the companion cells to have metabolic control over the sieve tube elements such as providing the sieve tube element with ATP

Sclerenchyma

Feature / Function
Dead cells / Allows the vessels to be hollow
Hollow cells / Reduces weight (increases strength to weight ratio)
Pointed connected cells / Provides a large surface area for one cell to be connected to another to give the fibres greater tensile strength
Very thickened cell walls / Give the vessels extra strength to support the plant
Lignified cell walls / Give the vessels extra strength to support the plant
  1. Identify similarities and differences between xylem vessels, phloem and sclerenchyma fibres

Feature / Xylem vessels / Phloem / Sclerenchyma fibres
General structure / Vessels = hollow tubes made of many connected cells / Vessels = hollow tubes made of many connected cells / Separate cells joined together to form fibres
Dead / Alive / Dead cells / Living cells – sieve tube elements kept alive by connections to companion cells / Dead cells
Function / Transport of water and mineral ions
Structural support / Transport of dissolved carbohydrates / Structural support
Cell wall / Thickened and possess lignin / No special modifications (plasmodesmata between sieve tube element and companion cells, also sieve plates at the end of cells) / Thickened and possess lignin
Cell contents / Completely hollow – some perforated or slitted end plates of cells remain / Hollow centre with thin layer of cytoplasm around the edge. Sieve plates at the end of each cell. / Hollow dead cells

Science & Plants for Schools:

Dissection and microscopy of plant vascular tissue – Learning outcomes: p. 1