Transport in Plants

Content: Students will be able to...

_Explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio;

_Describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants

_Describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells

_Define the term transpiration

_Explain why transpiration is a consequence of gaseous exchange;

_Describe the factors that affect transpiration rate

_Describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates

_Explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment. (No calculations involving water potential will be set

_Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and the stomata;

_Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream

_Describe, with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration

_Explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between sources (eg leaves) and sinks (eg roots, meristem);

_describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and the evidence for and against this mechanism

_HSW: Use a potometer to investigate the effects of environmental factors on water uptake

HSW: Calculate water uptake rates.

_HSW: Identify the limitations of using a potometer.

_

Vascular Tissues

Key Term

Definition, diagrams, examples

_Xylem

_Phloem

_Endodermis

_Meristem

_Cambium

_Vascular bundle

_

Xylem and Phloem in the young root

Your drawing:

_

Xylem and Phloem in the stem

Your drawing:

Xylem and phloem in the leaf

Your drawing:

_Past Paper Question:

1.The diagram below is a vertical section through part of a leaf of a dicotyledonous plant.

Reproduced by kind permission of D.G. Mackean

Complete the table below to identify xylem and phloem from the tissues labelled E to I.

tissue

letter

_xylem

_phloem

_[Total 2 marks]

Xylem and Phloem Investigation

Apparatus and materials

a length of fresh celery that has been left standing in water with a little added stain for 2448 hours

slides

cover-slips

tile

prepared slide of TS of a young stem

single-edged razor blade or sharp scalpel

forceps

prepared slide of LS of a stem

mounted needles

microscope

Introduction

In this practical, you will study the microscopic structure of xylem and phloem.

Procedure

1. You are provided with a petiole of celery that has been standing in water containing a coloured dye. Use a single-edged razor blade or a sharp scalpel to cut a thin cross-section of the petiole. Place the cross-section on a microscope slide and examine it at lower power under the microscope

2. Draw the cross-section of the celery petiole to show the distribution of the areas that are stained by the dye. Label these areas.

3. Use the low power of your microscope to examine the prepared slide of a transverse section (TS) of a stem. Find the xylem. Use high power to examine carefully the xylem tissue. Compare what you can see with Figure 6.9b on page 92 of Biology 1.

4. Examine a prepared longitudinal section (LS) of stem under high power for xylem vessels.

5. Look again at the prepared slide of a transverse section of a stem. This time find phloem tissue. Use high power to examine the phloem tissue carefully. Look for any sieve plates with sieve pores sometimes these are visible in cross-sections.

6. Make a drawing to show the details of three adjacent phloem sieve tube elements and their companion cells.

7. Examine a longitudinal section of stem under high power for phloem tissue. Compare what you can see with Figure 6.17 on page 99 of Biology 1.

8. Construct a table to show the differences between xylem and phloem tissue that are visible through the light microscope.

Diagrams:

T.S

L.S

_

Notes:

Properties of Xylem as seen under a light microscope

Properties of phloem as seen under a light microscope

_

_Past Paper Questions

1.Below is a diagram of a cross section of an organ from a dicotyledonous plant showing some of the tissues.

(i)Name the plant organ shown in the diagram above.

.........................................................................................................................

[1]

(ii)State which of the regions A to D is phloem tissue.

.........................................................................................................................

[1]

[Total 2 marks]

2.Flowering plants have two tissues to transport materials, xylem and phloem. The diagram below shows the outline of a transverse section of the root of a dicotyledonous flowering plant.

Sketch in and label the areas occupied by the xylem and phloem.

[Total 2 marks]

Structure of Xylem

Key Term

Definition/Diagram/Example

_Parenchyma

_Xylem Vessels

_Lignin

_Pits/Bordered pits

_

Label diagram

One directionperforation platesligninthin cytoplasmpits

No cell contentswater and minerals

Adaptation of Structure to function

In the table briefly explain how each structure/feature of the xylem helps it to do its function

Structure/Feature

How adaptation works

_Dead cells aligned end to end in a continuous column

_Narrow tubes

_Lignin deposited in the walls

_Pits in the lignified walls

_No end walls or cell content

_

Structure of Phloem

Key Term

Definition/Example/Diagram

_Sieve Tube element

_Companion Cell

_Plasmodesmata

_ATP

_

Label diagram

Thin layer of cytoplasmdense cytoplasmplasmodesmata

(Draw in and label mitochondria where appropriate)sieve elementnucleus

Adaptation of Structure to function

In the table briefly explain how each structure/feature of the xylem helps it to do its function

Structure/Feature

How adaptation works

_Sieve tube element

_Mitochondria in companion cell

_Little cytoplasm

_plasmodesmata

_

Xylem and Phloem Comparison

(Both Xylem and Phloem) (Phloem) (Xylem)

Exam Questions

1.(a)From the list below, circle the carbohydrate that is transported in phloem.

auxinfructoseglucoseglycogenstarchsucrose

[1]

(b)Phloem is responsible for the transport of carbohydrate in plants. The diagram below shows the structure of the cells in phloem.

(i) Name the cells P and Q in the diagram.

P .............................................................................................................

Q ............................................................................................................

[2]

2.The diagram below is a scanning electron micrograph of some xylem vessels.

Select two features that are visible in the electron micrograph and explain how these features help with the functioning of xylem vessels.

feature 1 ...................................................................................................................

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

feature 2 ...................................................................................................................

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

[Total 4 marks]

Plant Cells and Water

Draw what you would expect onion the cells to look like in each set of conditions

Using the terms water potential, plasmolysis, turgid and pressure potential explain these results.

Movement between cells

Key term

Definition/Diagram/Example

_Apoplast Pathway

_Symplast Pathway

_Vacuolar Pathway

_

Label the diagram with the following:

PlasmodesmataVacuoleCell wallCasparian StripCytoplasm

Use three different coloured arrows to show

Sympast pathway

Apoplast pathway

Vacuolar pathway

Past Paper Question

1.Complete the following passage on water uptake by the root using the most appropriate word or words.

Water moves from the soil to the xylem in the root down a.............................................. gradient.

The root hairs provide a large surface area for water uptake and once water has been taken up it crosses to the xylem by two pathways. The .............................................. pathway goes via the cell walls but is blocked at the .............................................. by the ................................................................ The other route involves the water crossing cell membranes by the process of osmosis and entering the cytoplasm. This is called the .............................................. pathway.

[Total 5 marks]

Water Uptake and Movement

Key term

Definition/Diagram/Example

_Root hair cells

_Water Potential

_Cortex

_Cohesion

_Cohesion-Tension

_Adhesion

_Guard Cells

_

Diagram 1. Water uptake in the roots

Diagram 2. Movement across root cortex

Process

Explanation

_Root Pressure

_Transpiration Pull

_Capillary Action

_Movement of water through plant

Fill in the gaps and label the diagram:

Past Paper Question

1.The figure below is a diagram showing some of the cells in the root of a dicotyledonous plant.

(a)Complete the table below by indicating which of the letters A to E indicates:

a cell from the endodermis

a cell from the phloem.

letter

_endodermis

_phloem

_[2]

(b)State two features of root hair cells which adapt them for water uptake.

1 ......................................................................................................................

2 ......................................................................................................................

[2]

(c)After water has entered the xylem vessels in the root, it passes through them to the rest of the plant.

Describe how two features of xylem vessels adapt them for water transport.

1 ......................................................................................................................

.........................................................................................................................

2 ......................................................................................................................

.........................................................................................................................

[4]

(d) In this question, one mark is available for the quality of spelling, punctuation and grammar.

Plants absorb water from the soil via their roots.

Describe the pathways and mechanisms by which water passes from the soil to the xylem vessels in the root.

(Allow one lined page).

[6]

Quality of Written Communication [1]

[Total 15 marks]

Investigating Water Uptake using a Potometer

Safety:

If the sap from the plants irritates your skin, let your teacher know.

Take care if you cut the plant shoot yourself.

Take care when assembling and handling the glass potometer: it is easy to break the long glass tubes and cut yourself with the broken ends.

Make sure you know how to deal with breakages.

Investigation

Set up the apparatus as in the diagram

Leave undisturbed for 5 minutes or until the shoot equilibrates to the conditions.

Starting about 2 cm from the free end of the capillary tubing, mark the tubing at 1 cm intervals using the ruler and marker pen. Make as many marks as possible (at least 6).

Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do this, loosen the screw on the boss and slide the boss up the clamp stand so that the capillary tube comes out of the water in the beaker. Retighten the screw on the boss.

Gently blot the end of the capillary tube with a piece of paper towel and an air bubble should appear in the capillary tube.

Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes back into the water in the beaker. Retighten the screw on the boss.

There are two ways of taking measurements:

Start the stop clock when the bubble of air touches the first marked line. Stop the clock when the bubble has travelled a fixed distance (2 or 3 cm) and touches the appropriate marked line.

OR:

Start the stop clock when the bubble of air touches the first marked line. Allow the bubble to travel upwards for a fixed period of time (depending on the rate of movement, this could be 10 or 30 seconds or longer) and mark how far the bubble has moved with a different coloured pen. Measure the distance between the first mark and this second coloured mark.

Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the formula given here. SI units are cm/s.

Distance moved by the air bubble

Time taken for the air bubble to move that distance

Change a factor that might affect how quickly the plant loses water by transpiration.

Repeat from d to i.

QUESTIONS

1 Draw a graph of your results.

2 Describe the shape of your graph and note any anomalies or inconsistencies.

3 Explain in biological terms what your graph shows.

4 The apparatus measures the uptake of water. What could happen to the water once it enters the plant shoot?

5 Suggest some ways you could improve this procedure, what are the limitations?

Factors that affect the rate of transpiration

Factor that affects the rate of water loss

How it affects water loss

_Number of leaves

_Number, size and position of stomata

_Presence of cuticle

_Light

_Temperature

_Relative humidity

_Air movement/wind

_Water availability

_

Past Paper Questions

1.Transpiration is the loss of water from plants by evaporation. The diagram below shows a potometer, an apparatus used to estimate transpiration rates.

(i)Describe how the apparatus should be set up to ensure that valid measurements can be obtained.

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

[4]

(ii)Transpiration itself is not measured by the potometer. State precisely what is measured by using the apparatus.

.........................................................................................................................

.........................................................................................................................

[1]

[Total 5 marks]

Xerophytic Plants

Key term

Definition/Diagram/Example

_Xerophyte

_Water Vapour Potential

_Marram grass

Cactus

Past Paper Question

1.Xerophytes are plants that are adapted for survival in areas where there is not much water available in the soil. Xerophytes have various modifications that reduce water loss from their leaves. State two such adaptations that reduce water loss and explain how the reduction in loss is achieved.

adaptation 1 .............................................................................................................

..................................................................................................................................

adaptation 2 .............................................................................................................

..................................................................................................................................

[Total 4 marks]

Translocation

Key term

Definition/Diagram/Example

_Phloem

_Source

_Sink

_Companion Cell

_Co-transporter proteins

_Hydrostatic Pressure

_

Sourcesinksucrosephloemwaterxylemcompanion cellroot cell

Past Paper Questions

1.There is evidence that sugar transport from sources to sinks in plants does not only involve passive movement by mass flow. There is also an active part to the mechanism.

(i)State one piece of evidence for the involvement of an active process.

.........................................................................................................................

.........................................................................................................................

[1]

(ii)Describe an active mechanism which could possibly be involved in the transport of sugars from sources to sinks.

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

[3]

[Total 4 marks]

2.Carbohydrate moves from regions of plants called sources to regions called sinks.

Explain how, at different times, the same plant root may be a source or a sink.

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

[Total 2 marks]

(XerophytesXylem and PhloemMovement root and stemTranslocationTranspirationTransport in Plants)

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