transport in plant notes apr13 -1

8/10/2019 Transport in Plant Notes Apr13 -1

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G TRANSPORT IN PLANTS

Learning Outcomes

Topic G( )

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2/18

Transport Systems in PlantsPlants dont have a circulatory system like animals, but they do have a sophisticated transport system for

carrying water and dissolved solutes to different parts of the plant, often over large distances.

Stem Structure

epidermiscortexphloemcambiumxylempith

v a s c ul ar

b un d l e

Epidermis. One cell thick. In young plants the

epidermis cells may secrete a waterproof cuticle, and

in older plants the epidermis may be absent, replaced

by bark.

Cortex. Composed of various packing cells, togive young plants strength and flexibility, and are the

source of plant fibres such as sisal and hemp.

Vascular Tissue. This contains the phloem andxylem tissue, which grow out from the cambium. In

dicot plants (the broad-leafed plants), the vascular

tissue is arranged in vascular bundles, with phloem on

the outside and xylem on the inside. In older plants the

xylem bundles fuse together to form the bulk of the

stem.

Pith. The central region of a stem, used for food

storage in young plants. It may be absent in older plants (i.e. theyre hollow).

Root Structure

epidermiscortexendodermispericyclephloemcambiumxylem

v a s c ul ar

t i s s u e

roothairs

Epidermis. A single layer of cells often with longextensions called root hairs, which increase the

surface area enormously. A single plant may have

1010 root hairs.

Cortex. A thick layer of packing cells oftencontaining stored starch.

Endodermis. A single layer of tightly-packedcells containing a waterproof layer called the

casparian strip. This prevents the movement of

water between the cells.

cellwall

cellmembrane

cytoplasm vacuolecasparian

strip

Pericycle. A layer of undifferentiated

meristematic (growing) cells. Vascular Tissue. This contains xylem and phloem

cells, which are continuous with the stem

vascular bundles. The arrangement is different,

and the xylem usually forms a star shape with 2-6

arms.


3/18

(a) Explain the need for transport systems in multicellular plants and animals in terms of size and surface area tovolume ratios

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(e) *describe the structure of xylem vessel elements, sieve tube elements and companion cells and be able to recognise these usingthe light microscope;

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4/18

(f) relate the structure of xylem vessel elements to their functions;

Differences between Xylem vessel & tracheids

lignified walls at maturation; pits on wall ; Transport water

long, thin cells w tapered ends.

Do not have open ends ,do notform vessels/ cont tube

elongated vessel elements arrangedend-end/ no end walls

form a continuous tube

Hollow lumen

Large lumen

Lignified wall

Continuous tube/ no end wall

Pits


5/18

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Companion cell

Cellulose cell wall

Cell plate/ sieve pores

Little cell content

plasmodesmata


6/18

Features of Sieve Tube element & Companion cells related to its function.

Sieve Tube element Companion cells.

i) Alive

ii) No nucleus, No ribosomes

iii) Cytoplasm is reduced to strandsaround the cell edge.

plasma membrane ,mitoc, plastids & ER

iv) Center of tubes is empty.

v) no secondary wall thickening

vi) sieve plates

allowing free flow of liquids

i) normal cells with nucleiand organelles.

ii) connected to the sieve tube

cells by plasmodesmataiii) Metabolically very active

provide proteins, ATP andnutrients.


7/18

(g) explain the movement of water between plant cells and between them and their environment, in terms of water potential

1) Soil to root cells Osmosis gradient Partially permeable membran

2) root cells to xylem Osmosis gradient Apoplast & Symplast Plasmodesmata; Cytoplasm Cell wall Casparian strip Endodermis Pericycle

3) along Xylem Transpiration Water loss Hydrostatic pressure Mass flow Capillary action(adhesion,cohesion)

Transpirational pull

4,5,6) xylem to leaf to atmosphere**Apoplast & symplast

gradient Evaporation Spongy Mesophyll cell Water vapour Moist cell wall Large SA of spongy mesophyll cells Diffusion Stomata


8/18

Factors affecting TranspirationThe rate of transpiration can be measured in the lab using a potometer (drinking meter):

speed of movement of air bubble (mm/s)

x cross-sectional area of capillary tube (mm )

= rate of water uptake (mm /s)

2

3


9/18

The potometer can be used to investigate how various environmental factors affect the rate of transpiration.

Explain how each of the following factor can affect the rate of photosynthesis. (ref pg 18 in reference material)

1. Light.

2. Temperature.

3. Humidity.

4. Air movements.

Many plants are able to control their stomata, and if they are losing too much water and their cells are

wilting, they can close their stomata, reducing transpiration and water loss. So long periods of light, heat, or

dry air could result in a decrease in transpiration when the stomata close.

(b)define the term transpiration and explain that it is an inevitable consequence of gaseous exchange in plants;

(i)*describe how the leaves of xerophytic plants are adapted to reduce water loss by transpiration;

Adaptations to dry habitats

Plants in different habitats are adapted to cope with different problems of water availability.

Mesophytes plants adapted to a habitat with adequate water

Xerophytes plants adapted to a dry habitat

Halophytes plants adapted to a salty habitat

Hydrophytes plants adapted to a freshwater habitat


10/18

Some adaptations of xerophytes are:

Adaptation How it works Example

thick cuticle most dicots

small leaf surface area conifer needles, cactus spines

low stomata density

stomata on lower surface ofleaf only

most dicots

shedding leaves in dry/cold

season

deciduous plants

sunken stomata marram grass, pine

stomatal hairs marram grass, couch grass

folded leaves marram grass,

succulent leaves and stemcacti

extensive roots cacti


11/18

(j) explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between the

leaves (sources) and other parts of the plant (sinks);(k) explain the translocation of sucrose using the mass flow hypothesis;

Solute Transport in Plants

The phloem contains a very concentrated solution of dissolved solutes, mainly sucrose, but also other sugars, amino

acids, and other metabolites. This solution is called the sap, and the transport of solutes in the phloem is called

translocation.

Unlike the water in the xylem, the contents of the phloem can move both up and down a plant stem, often

simultaneously. It helps to identify where the sugar is being transported from (the source), and where to (the sink).

During the summer sugar is mostly transported from the leaves, where it is made by photosynthesis (the source)to the roots, where it is stored (the sink).

During the spring, sugar is often transported from the underground root store (the source) to the growing leaf buds(the sink).

Flowers and young buds are not photosynthetic, so sugars can also be transported from leaves or roots (thesource) to flowers or buds (sinks).

Pressure flow modelhttp://bcs.whfreeman.com/thelifewire/content/chp36/36020.html

( , // . . . . / / / /00 .

Sucrose transporthttp://www.tvdsb.on.ca/westmin/science/sbioac/plants/sucrose.htm

** .// . . / / / 100/ 06 /36 01.

Keywords for 1-4 Source, mesophyll cell Companion cell H+ pump; Active transport, ATP;

H+

gradient, [ ] gradient; (H+-sucrose) Co-transporter; Sucrose diffuse Plasmodesmata Sieve tube Active loading

Keywords for 5-7 Water potential Osmosis xylem Hydrostatic pressure

Sink cell Sucrose actively unloaded Pressure difference Mass flow


12/18

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13/18

MCQ

1. The diagram below is a 3D representation of a cell found in many herbaceous plants.

Which one of the following descriptions applies to the cell shown here?

A A xylem vessel with secondary thickening

B An endodermal cell with a Casparian stripC An immature sclerenchymous fibreD A collenchyma cell with tangential thickening

2. The diagram below represents those parts of a plant associated with the transferof materials during daylight.

Which one of the arrows A, B , C or D indicates mass flow of organic solutes?

3. A region of a stem of a plant is heated to kill the cells in the living vascular tissues.

How will this treatment affect the transport between roots and leaves via xylem and phloem?

Xylem Phloem Key: transport

continues transport stops

A

B C D

4. The diameter of a tree is reduced slightly during the day and increased at night. Which of the following changes inenvironmental conditions cause the greatest reduction in diameter?

A Increase in wind velocity, temperature, humidity & light intensityB Increase in temperature humidity & light intensityC Increase in wind velocity, temperature & light intensityD Increase in wind velocity, temperature & humidity

5. The diagram shows a longitudinal section through transport tissue in a plant stem.What are the names of the structure labelled X and the tissue in which it is found?


14/18

6. What is responsible for the movement of water up xylem vessels in plants?

A active loading of water against the water potential gradient in the roots and osmosis in the vesselsB increasing water potential at the top of xylem vessels, and osmosis in the rootsC decreasing water potential at the top of the xylem vessels, with cohesion of water in the vesselsD translocation in the leaves, with capillarity in the xylem vessels.

7. The diagrams show transverse sections of parts of a plant.

In the cross sections, what do 1, 2, 3, 4, 5 and 6 represent?

8. The movement of water through xylem vessels is affected by external factors.A decrease in which external factor would result in an increase in water movement?

A atmospheric humidityB external temperatureC light intensityD wind velocity

9. How is sucrose transported into companion cells and then into phloem sieve tubes?

10. Why is the mass flow of sap through sieve elements described as an active process?

A Phloem sap is able to flow in sieve elements against the pull of gravity.B Sucrose is loaded into a sieve element against a concentration gradient.C Sucrose passes out of the phloem into regions where cells are dividing.D Water follows sucrose into a sieve element down a water potential gradient.


15/18

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

X and Y are filled with sucrose solutions of different concentration, 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 the tube T joining X and Y.Which description of this is correct?

Topic G- PLANT TRANSPORT Structured QuestionsQuestion 1

Fig.1 below shows transverse sections of a root and a stem. (J03/4)

Fig.1

(a)(i) Shade in an area in the transverse section of the root where there are cells specialised for the transport of water. [1 ]

(ii) Shade in an area in the transverse section of the stem where there are cell specialised for the transport of sucrose.[1 ]


16/18

(b) Suggest why the vascular bundles in the stem are situated towards the outside.

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(c) Describe the process by which water passes from the soil into the root hairs.

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(d) Explain how water passes from the stem to the air surrounding a leaf.

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Question 2

Fig. 2

(a) Identify cells A to D

A C .

B D . [4]


17/18

(b) Explain the mechanism by which water moves in the direction shown by the black arrows.

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

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..[4]

(c) Explain the significance of Casparian strips in the movement of ions across a root.

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.....[2]

Question 3

Figure 3.1 below is a diagram of a section through a leaf, showing the pathways followed by water molecules, as black arrows, and passage of sugar molecules manufactured during photosynthesis, as white arrows.

Fig. 3.1

(a) Identify cells A to D.

A.. B

C.. D... [2]


18/18

(b) Describe the methods by which water moves in the direction shown by the black arrows in Fig.3.1.

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

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..[4]

(c) Outline how sucrose is loaded into the cell C.

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..[4]

Fig.3.2 below is a diagram of a section through part of a single xylem vessel.

(d) On Fig. 3.2., label two structural features of the xylem vessels that are related to water transport. [2]

Fig. 3.2

transport in plant notes apr13 -1

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