1. Transport in flowering plants
a summary for AS Biology

2. ***relate the structure of xylem vessel elements to their functions;
Objectives:
*describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants
**describe the structure of xylem vessel elements and be able to recognise these using the light microscope;
***relate the structure of xylem vessel elements to their functions;

3. 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

4. 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

5. Root structure
stele
Know these tissues!
structure
location
function

6. Root structure { Endodermis Epidermis Cortex parenchyma cell Air space
Stele
Pericycle
Phloem
TS buttercup root (low power)
TS buttercup root stele (high power)
Xylem
Know these tissues!
structure
location
function

7. Passage of water across a root
Root hair
Epidermis
Cortex
Endodermis
Pericycle
Xylem

8. Passage of water across a root
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.
Some water enters the root hair vacuole by osmosis, and travels by osmosis from vacuole to vacuole across the cortex.
This is the vacuolar 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.

9. Passage of water across a root
The Casparian strip completely blocks the apoplast pathway …
… so that only the symplast and vacuolar pathways are available.
Why is this important?
It allows the flow of water and dissolved minerals into the plant to be controlled.

10. 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.

11. 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)

12. Vessels and tracheids
Vessels 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.

13. Vessel with annular thickening
Xylem vessels
Xylem vessels show different patterns of woody thickening (lignification), giving them a function in support as well as water conduction. LS
Xylem parenchyma
Fibre
Pitted vessel
Vessel with annular thickening TS

14. The whole picture
1 Water evaporates from the surface of a mesophyll cell into the leaf air space 2
2 By cohesion, another water molecule is pulled into the cell from the leaf xylem 1 3
3 By cohesion, the pull is transmitted all the way down the stem and root xylem 3
5 … so that water flows down a water potential gradient from the soil across the vacuolar, symplastic and apoplastic pathways in the root 3
4 The upward pull lowers the water potential in the root xylem … 4 5

15. Use of a potometer to investigate water uptake
Why is a potometer like the one above usually assembled under water?
What is the function of the central reservoir?
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.

16. Use of a potometer to investigate water uptake
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.
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.

17. TRANSLOCATION
Objectives: * **
***
****

18. 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

19. Structure of phloem
In addition phloem contains parenchyma cells and phloem fibres
Phloem fibres
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 phloem are sieve cells, assisted by companion cells

20. Phloem structure LS Phloem parenchyma Sieve plate Plasmodesmata
Sieve cell. These join end to end to form sieve tubes, connected by perforated end walls (sieve plates)
Companion cell
Sieve plate
Sieve cells have little cytoplasm, no nucleus and very few organelles. Every sieve cell is closely associated with a companion cell, the two cells communicating through many plasmodesmata TS
Phloem parenchyma

21. TRANSLOCATION
Objectives: * **
***
****

22. Phloem structure Sieve cell
Sieve plates are associated with large amounts of a protein called P-protein. Its precise role is unknown.(It is now believed to be untrue)
Parenchyma cell
Companion cell
Sieve plate
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.

23. Companion cells In between sieve tubes Large nucleus, dense cytoplasm
Many mitochondria to load sucrose into sieve tubes
Many plasmodesmata (gaps in cell walls between companion cells and sieve tubes) for flow of minerals

24. Phloem Structure and Function:
Sieve Elements
Specialises in efficient transport of food.
Living cells but do not have a nucleus.
Long, narrow, thin walled living cells.
End walls are heavily perforated – called a sieve plate.
A series of sieve elements is called a sieve tube.
Companion Cells
Assist the sieve element in food transport.
Live narrow cells with a prominent nucleus.
Its nucleus also controls the sieve element.
Dense cytoplasm particularly rich in mitochondria.

25. Movement of Sugars
Translocation: movement of assimilates (sugars and other chemicals) through the plant
Source: a part of the plant that releases sucrose to the phloem e.g. leaf
Sink: a part of the plant
that removes sucrose from
the phloem e.g. root

26. The mass flow hypothesis
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.
The mass flow hypothesis
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.

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

28. Sucrose Entering the Phloem
Active process (requires energy)
Companion cells use ATP to transport hydrogen ions out of their cytoplasm
As hydrogen ions are now at a high concentration outside the companion cells, they are brought back in by diffusion through special co-transporter proteins, which also bring the sucrose in at the same time
As the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tubes through the plasmodesmata (gaps between sieve tubes and companion cell walls)

29. Sucrose movement through phloem
Sucrose entering sieve tube lowers the water potential (more negative) so water moves in by osmosis, increasing the hydrostatic pressure (fluid pushing against the walls) at the source
Sucrose used by cells surrounding phloem and are moved by active transport or diffusion from the sieve tube to the cells. This increases water potential in the sieve tube (makes it less negative) so water moves out by osmosis which lowers the hydrostatic pressure at the sink

31. Movement along the phloem
Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving at the sink produces a flow of water along the phloem that carries sucrose and other assimilates. This is called mass flow. It can occur either up or down the plant at the same time in different phloem tubes

32. Evidence for translocation
Radioactively labelled carbon from carbon dioxide can appear in the phloem
Ringing a tree (removing a ring of bark) results in sugars collecting above the ring
An aphid feeding on the plant stem contains many sugars when dissected
Companion cells have many mitochondria
Translocation is stopped when a metabolic poison is added that inhibits ATP
pH of companion cells is higher than
that of surrounding cells
Concentration of sucrose is higher at
the source than the sink

33. Evidence against translocation
Not all solutes move at the same rate
Sucrose is moved to parts of the plant at the same rate, rather than going more quickly to places with low concentrations
The role of sieve plates is unclear