1. Transport in Plants
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;

2. Transport in Plants
Plants need a transport system so that cells deep within the plants tissues can receive the nutrients they need for cell processes
The problem in plants is that roots can obtain water, but not sugar, and leaves can produce sugar, but can’t get water from the air

3. What substances need to be moved?
The transport system in plants is called vascular tissue
Xylem tissue transports water and soluble minerals
Phloem tissue transports sugars

4. The Vascular Tissues
Xylem and phloem are found together in vascular bundles, that sometimes contain other tissues that support and strengthen them

5. Root vs. stem vs. leaf
The vascular bundle differs depending on if it is a root or stem

6. Root The vascular bundle is found in the centre
There is a large central core of xylem- often in an x-shape
This arrangement provides strength to withstand the pulling forces to which roots are exposed
Around the vascular bundle are cells called the endodermis which help to get water into the xylem vessels
Just inside the endodermis is the periycle which contains meristem cells that can divide (for growth)

7. Stem The vascular bundles are found near the outer edge of the stem
The xylem is found towards the inside of each vascular bundle, the phloem is found towards the outside
In between the xylem and phloem is a layer of cambium
Cambium is a layer of meristem cells that divide to make new xylem and phloem

8. Leaf
The vascular bundles (xylem and phloem) form the midrib and veins of the leaf
A dicotyledon leaf has a branching network of veins that get smaller as they branch away from the midrib
Within each vein, the xylem can be seen on top of the phloem

9. Phloem
Xylem
Stem

10. A = Xylem
B = Phloem
C/D = Upper/Lower epidermis
Leaf

11. Xylem vessel wall
Xylem vessel lumen
Phloem
Endodermis
Starch grains
Root

13. Structure of Xylem
Used to transport water and minerals from roots to leaves
Consists of tubes for water, fibres for support and living parenchyma cells

14. Xylem vessels Obvious in dicotyledonous plants
Long cells with thick walls containing lignin
Lignin waterproofs walls of cells and strengthens them
Cells die and ends decay forming a long tube
Lignin forms spiral, annular rings or broken rings (reticulate)
Some lignification is not complete and pores are left called pits or bordered pits, allowing water to move between vessels or into living parts

15. Adaptations of Xylem to Function
Xylem can carry water and minerals from roots to shoot tips because:
Made of dead cells forming continuous column
Tubes are narrow so capillary action is effective
Pits allow water to move sideways
Lignin is strong and allows for stretching
Flow of water is not impeded as: there are no end walls, no cell contents, no nucleus, lignin prevents tubes collapsing

16. Structure of Phloem
Function to transport sugars from one part to another
Made of sieve tube elements and companion cells

17. Sieve Tubes
Sieve tube elements not true cells as they have little cytoplasm
Lined up end to end to form a tube
Sucrose is dissolved in water to form a sap
Tubes (known as sieve tubes) have a few walls across the lumen of the tube with pores (sieve plates)

18. 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

19. Water route between cells
Apoplast: between cell walls of neighbouring cells
Symplast: through plasma membrane and plasmodesmata to cytoplasms from cell to cell
Vacuolar: same as symplast, but also through vacuoles

20. Water uptake from the soil
Epidermis of roots contain root hair cells
Minerals absorbed by active transport using ATP
Minerals reduce the water potential in the cell cytoplasm (more negative) so water is taken up by osmosis

22. Movement across the root
Active process occurring at the endodermis (layer of cells surrounding the xylem, some containing waterproof strip called casparian strip)
Casparian strip blocks the apoplast pathway (between cells) forcing water into the symplast pathway (through the cytoplasm)
The endodermis cells move minerals by active transport from the cortex into the xylem, decreasing the water potential (more negative), thus water moves from the cortex through the endodermal cells to the xylem by osmosis
A water potential gradient exists across the whole cortex, so water is moved along the symplast pathway (through cytoplasm) from the root hair cells across the cortex and into the xylem

23. Casparian Strip Blocks the apoplast pathway (cell walls)
Water and dissolved nitrate ions have to pass into the cell cytoplasm through cell membranes
There are transporter proteins in the cell membranes that actively transport nitrate ions into the xylem lowering the water potential (more negative)
Water enters xylem down concentration gradient and cannot pass back

24. Water movement up stem
Root pressure: minerals move into xylem by active transport, forcing water into xylem and pushes it up the stem
Transpiration Pull: loss of water at leaves replaced by water moving up xylem. Cohesion-tension theory- cohesion between water molecules and tension in the column of water (which is why xylem is strengthened with lignin) means the whole column of water is pulled up in one chain
Capillary action: adhesion of water to xylem vessels as they are narrow

25. How water leaves the leaf
Through stomata
Tiny amount through the waxy cuticle
Water evaporates from the cells lining the cavity between the guard cells, lowering water potential and meaning that water enters them by osmosis from neighbouring cells, which is replaced by further neighbouring cells and so on

26. Transpiration Loss of water vapour from upper parts of the plant
Water enters leaf from xylem and passes to mesophyll cells by osmosis
Water evaporates from surface of mesophyll cells to form water vapour (air spaces allow water vapour to diffuse through leaf tissue)
Water vapour potential rises in air spaces, so water molecules diffuse out of the leaf through open stomata

27. Transpiration: three processes
Osmosis from xylem to mesophyll cells
Evaporation from surface of mesophyll cells into intercellular spaces
Diffusion of water vapour from intercellular spaces out through stomata

28. Water use in plant Photosynthesis Cell growth and elongation Turgidity
Carriage of minerals
Cools the plant

29. Measuring transpiration
Potometer is used to estimate water loss

30. Factors affecting transpiration
Leaf number: more leaves, more transpiration
Number, size, position of stomata: more and large, more transpiration, under leaf, less transpiration
Cuticle: waxy cuticle, less evaporation from leaf surface
Light: more gas exchange as stomata are open
Temperature: high temperature, more evaporation, more diffusion as more kinetic energy, decrease humidity so more diffusion out of leaf
Humidity: high humidity, less transpiration
Wind: more wind, more transpiration
Water availability: less water in soil, less transpiration (e.g. in winter, plants lose leaves)

31. Too much water loss Less turgidity Non-woody plants wilt and die
Leaves of woody plants die first then it will die if water loss continues

32. Xerophytes Smaller leaves reducing surface area e.g. pine tree
Densely packed spongy mesophyll to reduce surface area, so less water evaporating into air spaces
Thick waxy cuticle e.g. holly leaves to reduce evaporation
Closing stomata when water availability is low
Hairs on surface of leaf to trap layer of air close to surface which can become saturated with water, reducing diffusion
Pits containing stomata become saturated with water vapour reducing diffusion
Rolling the leaves so lower epidermis not exposed to atmosphere also traps air which becomes saturated
Maintain high salt concentration to keep water potential low and prevent water leaving

33. Marram Grass Leaf rolled up to trap air inside
Thick waxy cuticle to reduce water evaporation from the surface
Trapped air in the centre with a high water potential (less negative)
Hairs on lower surface reduce movement of air
Stomata in pits to trap air with moisture close to the stomata

34. 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

35. 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)

36. 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

37. 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

38. 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

39. 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

40. Useful Revision Sites http://scienceaid.co.uk/