TOPIC 9.2 – TRANSPORT IN PHLOEM

9.2 – A - Phloem

INTRO

IB BIO – 9.2 3

https://i.ytimg.com/vi/LQ03xIkLLQU/maxresdefault.jpg

As noted in 9.1, plants transport water and nutrients throughout

their body in vascular bundles. These bundles contain the xylem

and phloem tissues.

Skills

S1: Identification of xylem and phloem in microscope images of stem and root.

Key Terms

Xylem

Phloem

IB BIO – 9.2 4

http://media.gettyimages.com/photos/alfalfa-herbaceous-dicot-20x-shows-vascular-bundles-arranged-in-a-picture-id139803630

Xylem and phloem can be visualized using different types of

microscopy. Phloem are typically smaller than xylem and are

located towards the outside of bundles in stems and roots.

Skills

S1: Identification of xylem and phloem in microscope images of stem and root.

Key Terms

Xylem

Phloem

IB BIO – 9.2 5

http://plantphys.info/plant_physiology/images/stemvb.jpg

Xylem & Phloem Images

To the left is a light microscope

image of a stem cross section.

• Phloem are the small tubes

towards the top of the image.

• Xylem are the larger tubes

closer to the center of the

stem.

The xylem and phloem are

separated by the vascular

cambium

Skills

S1: Identification of xylem and phloem in microscope images of stem and root.

Key Terms

Xylem

Phloem

IB BIO – 9.2 6Xylem & Phloem Images

To the left is a scanning electron

micrograph of vascular bundle.

• Xylem are the large green

tubes to the right.

• Phloem are the small yellow

tubes on the left.

The xylem and phloem are

separated by the vascular

cambium shown in orange.

https://s-media-cache-ak0.pinimg.com/originals/d4/6f/5c/d46f5c9eee8c6e18497d4a9d60675381.jpg

Understandings

U1: Plants transport organic compounds from sources to sinks.

Phloem tissues are found throughout plant bodies and are

responsible for transporting organic compounds.

Phloem structures include:

Key Terms

Phloem

Sieve Tube Cells

IB BIO – 9.2 7

https://online.science.psu.edu/sites/default/files/biol011/Fig-9-12-Sieve-Tubes.jpg

• Columns of sieve tube cells,

separated by perforated walls

called sieve plates.

• Companion cells, which

provide ATP and nutrients for

cell functions.

Applications

A1: Structure–function relationships of phloem sieve tubes.

Key Terms

Phloem

Sieve Tube Cells

IB BIO – 9.2 8

https://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Apoplast_and_symplast_pathways.svg/2000px-Apoplast_and_symplast_pathways.svg.png

The structure of phloem sieve tube cells is related to their function.

The cytoplasm of companion cells is connected with sieve tubes cell

via the plesmodesmata.

Sieve tube cell walls provide the rigidity necessary to increase

internal pressure. The perforation of sieve plates also reduces

resistance to sap flow through tube.

Understandings

U1: Plants transport organic compounds from sources to sinks.

Key Terms

Phloem

Source

Sink

IB BIO – 9.2 9

http://plantsinaction.science.uq.edu.au/edition1/files/Fig%205.20.png

Phloem components transport

organic solutes in a process called

translocation. Compounds are

transported from source to sink:

• Source – tissues with surplus

organic compounds

Ex. green leaves/stems, storage

tissues, tap roots, tubers, etc.

• Sink – tissues that consume and

need organic compounds

Ex. roots, developing fruits/seeds,

growing leaves

Understandings

U1: Plants transport organic compounds from sources to sinks.

Key Terms

Phloem

Source

Sink

IB BIO – 9.2 10

http://www.wineland.co.za/wp-content/uploads/2017/08/Sugars-on-move-through-vine-Figure-1_WEB.jpg

Sometimes, plant sinks can turn into sources and vice versa.

Because of this, the phloem must be able to transport compounds in

both directions. This is an active process and requires energy.

REVIE

WIB BIO – 9.2 111. State the function of phloem

2. Outline how to identify xylem and phloem in

images.

3. Describe how the structure sieve tube cells is

adapted to their function.

9.2 – B - Translocation

Understandings

Key Terms

Translocation

IB BIO – 9.2 13

http://biology-igcse.weebly.com/uploads/1/5/0/7/15070316/499195_orig.png

Translocation is the process in plants that transports organic

materials between tissues (sources to sinks). The overall movement

is the result of both active and passive processes.

Understandings

U3: Active transport is used to load organic compounds into phloem sieve tubes at the source.

Key Terms

Translocation

PhloemLoading

IB BIO – 9.2 14

http://classconnection.s3.amazonaws.com/193/flashcards/1242193/jpg/bulk_flow1330467690160.jpg

Sucrose is the most abundant sugar in phloem sap. Molecules are

transported from source cells into the phloem in an active process

called loading. This increases the concentration in companion cells.

Understandings

U4: High concentrations of solutes in phloem at the source lead to water uptake by osmosis.

Key Terms

Translocation

PhloemLoading

IB BIO – 9.2 15

http://classconnection.s3.amazonaws.com/193/flashcards/1242193/jpg/bulk_flow1330467690160.jpg

Sucrose molecules then diffuse down the concentration gradient into

the phloem sieve tube. A high concentration of sugar results in the

movement of water from the xylem into the phloem via osmosis.

Understandings

U2: Incompressibility of water allows transport along hydrostatic pressure gradients.

Key Terms

Translocation

IB BIO – 9.2 16

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Because of cell wall rigidity and the

incompressibility of water, the influx

of water increases hydrostatic

pressure within the phloem.

This pressure results in the movment

of sap throughout the plant.

Understandings

U5: Raised pressure causes the content of phloem to flow towards sinks.

Key Terms

Translocation

IB BIO – 9.2 17Differences in pressure results in

sap being transported away from

areas of high pressure (sourcea).

When contents reach a sink, sucrose

is absorbed and used or converted

to starch for storage.

The resulting decrease in solute

concentration reduces osmotic

pressure.

http://classconnection.s3.amazonaws.com/193/flashcards/1242193/jpg/bulk_flow1330467690160.jpg

Understandings

U5: Raised pressure causes the content of phloem to flow towards sinks.

Key Terms

Translocation

IB BIO – 9.2 18This movement of organic

compounds is collectively

referred to as the

pressure-flow hypothesis.

For more information,

follow the link below:

• McGraw-Hill Animation

http://classconnection.s3.amazonaws.com/193/flashcards/1242193/jpg/bulk_flow1330467690160.jpg

Skills

S2: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide.

Key Terms

IB BIO – 9.2 19

http://bio1152.nicerweb.com/Locked/media/ch36/36_19PhloemSapFlow.jpg

Compared with other plant products, phloem sap is very nutrient-

rich. Aphids are insects that use mouth stylets to penetrate plant

tissues to reach the sap.

Skills

S2: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide.

Key Terms

IB BIO – 9.2 20

http://bio1152.nicerweb.com/Locked/media/ch36/36_19PhloemSapFlow.jpg

Radioactive CO2 and aphids can be used to detect transport rates in

plants. After being exposed for short periods, plants uptake the CO2

and synthesize sucrose which is then transported.

Aphids colonies are placed at measured distances along the stem.

Then, the amount of radioactive CO2 that has been taken up is

measured.

REVIE

WIB BIO – 9.2 211. Discuss the translocation of sugars in plants.

Include the following key terms in your discussion.

- Xylem / Phloem

- Active transport

- Sucrose

- Hydrostatic pressure

- Osmosis

- Loading

- Source / Sink

- Sap

VID

EO

SIB BIO – 9.2 22• Fuse School: Translocation

https://www.youtube.com/watch?v=QXdujo4PZ7c

• Aphid Stylectomy to Get Pure Phloem Saphttps://www.youtube.com/watch?v=i6OVYj9Ia2Y