Uptake of water: the transpiration stream

• The transpiration stream is the one-way movement of water:

from the soil into root hairs; across the root into xylem vessels; through root stem and leaf xylem into

mesophyll cells; by evaporation from mesophyll cell surfaces into

leaf air spaces; by diffusion from leaf air spaces through

stomata into the atmosphere

Uptake of water : the transpiration stream

• The movement of water in the transpiration stream is down a water potential gradient from soil solution to atmosphere

• The transpiration stream is ‘driven’ by the evaporation of water from mesophyll cell surfaces, each evaporating molecule ‘pulling’ another one behind it because of the cohesion of water molecules (due to hydrogen bonding)

• The ‘pull’ is transmitted from molecule to molecule in an unbroken chain all the way down to the root: this is the cohesion-tension hypothesis

• As well as cohesion, the adhesion of water molecules to the vessel walls and the cellulose molecules in mesophyll cell walls supports the column of water and keeps it from breaking

• Mineral ions taken by active transport into root hairs are carried passively in the transpiration stream

Root structure

stele

Know these tissues!

•structure

•location

•function

Root structure

TS buttercup root (low power)

Epidermis

Cortex

{Stele

TS buttercup root stele (high power)

Cortex parenchyma cellAir space

Endodermis

Pericycle Phloem

XylemKnow these tissues!

•structure

•location

•function

Passage of water across a root

Root hair

Epidermis

Cortex

Endodermis

Pericycle

Xylem

Passage of water across a rootSome water enters the root hair vacuole by osmosis, and travels by osmosis from vacuole to vacuole across the cortex.

This is the vacuolar pathway.

Some water (blue line) crosses the cell surface membrane into the cytoplasm and passes from cell to cell via plasmodesmata: this is the symplastic pathway.

Most water (red line) does not enter the living cells at all but passes along cells walls and intercellular spaces: this is the apoplastic pathway.The vacuolar

pathway presents the most resistance to water flow (because of the number of membranes to be crossed), the apoplastic pathway the least …

… but at the endodermis the apoplastic pathway is completely blocked by a strip of corky material (the Casparian strip) around the walls of the endodermal cells.

Passage of water across a root

The Casparian strip completely blocks the apoplast pathway …

… so that only the symplast and vacuolar pathways are available.

It allows the flow of water and dissolved minerals into the plant to be controlled.

Why is this important?

Movement through the xylem

• Water enters the xylem because its water potential is reduced by the upward ‘pull’ (tension) on the water column it contains

• Adhsion of water molecules to the xylem vessel walls also helps maintain the column.

Structure of xylem

• Xylem is a compound tissue, consisting of:

• two types of conducting cell, vessels and tracheids

• fibres (thin elongated cells with thick woody walls and no living contents)

• Xylem parenchyma (living cells with thin cellulose cell walls)

Vessels and tracheidsVessels are short hollow cells with woody (lignified) cell walls and no living contents at maturity.

Their end walls break down, so that water can flow freely from one to the next.

Many vessels have pits allowing sideways movement of water from vessel to vessel: this can help by-pass blockages.

Tracheids are narrower lignified cells with tapered ends that overlap, transferring water from cell to cell via pits.

Xylem vessels

LS

TS

Xylem vessels show different patterns of woody thickening

(lignification), giving them a function in support as well as

water conduction.

Xylem parenchymaFibre

Pitted vessel Vessel with annular thickening

1

1 Water evaporates from the surface of a mesophyll cell into the leaf air space

22 By cohesion, another water molecule is pulled into the cell from the leaf xylem

3 By cohesion, the pull is transmitted all the way down the stem and root xylem

3

4 The upward pull lowers the water potential in the root xylem …

4

5 … so that water flows down a water potential gradient from the soil across the vacuolar, symplastic and

apoplastic pathways in the root

5

3

3

The whole picture

Use of a potometer to investigate water uptake

1. Why is a potometer like the one above usually assembled under water?

2. What is the function of the central reservoir?

3. Describe how you would use the above apparatus to investigate the effect of moving air on the rate of water uptake by a leafy shoot.

Use of a potometer to investigate water uptake

4. The graph shows the results of an experiment in which a potometer was used to measure the uptake of water by a leafy shoot in three different conditions: still dry air, still humid air and dry air blown by a fan. Suggest which curve was obtained in which condition. Give reasons for your answers.

5. Calculate (a) the mean rate of water uptake by the shoot in moving dry air, (b) the percentage increase in mean rate of uptake when changing from still dry air to moving dry air.

Translocation in phloem

• Phloem transports organic products of photosynthesis from leaves or storage organs to sites of use

• It also transports plant growth substances and mineral ions

• Unlike xylem, the conducting elements of phloem are living cells, and the transport (called translocation) is an active process

Structure of phloem

Phloem

In roots phloem is found between the ‘arms’ of the star-shaped xylem

In stems phloem is found on the peripheral side of xylem in vascular

bundles

The conducting elements of xylem are sieve cells, assisted by companion cells

In addition phloem contains parenchyma cells and phloem fibres

Phloem fibres

Phloem structure

Sieve cell. These join end to end to form sieve tubes, connected by perforated end walls (sieve plates)

Sieve plate

Sieve cells have no nucleus and very few organelles. Every sieve cell is closely associated with a companion cell, the two cells communicating through many plasmodesmata

Companion cell

Plasmodesmata

Phloem parenchyma

LS

TS

Sieve plate

Phloem parenchyma

Phloem structure Sieve cell

Companion cell

Sieve plate

Sieve plates are associated with large amounts of a protein called P-protein. Its precise role is unknown.

Parenchyma cell

A sieve cell and its adjacent companion cell are produced by division of the same parent cell.

The companion cell probably carries out metabolic functions for the sieve cell, compensating for the sieve cell’s lack of organelles.

At the tips of leaf veins, companion cells have folded surfaces and act as transfer cells, actively transporting sucrose from mesophyll cells into sieve cells.

The mass flow hypothesis for the movement of organic solutes in phloem suggests that as sucrose is actively transferred into sieve cells at its source, water follows it by osmosis, raising the pressure in the sieve cells at that point.

Where sucrose is actively transferred out of sieve cells (a sucrose ‘sink’), water again follows by osmosis, reducing the pressure in the sieve cells at that point.

There is therefore a pressure gradient pushing sucrose and other solutes from source to sink.

The mass flow hypothesis

The contents of sieve cells are under positive pressure, as is shown by the feeding of aphids.

Aphids plug their piercing mouthparts (stylets) into sieve cells, and the pressure in the phloem pushes its contents into the insect’s gut – sometimes so quickly that it exudes from the aphid’s anus.

The mass flow hypothesis