1. Ch. 7: Transport in Plants
AICE Biology 2017

2. Introduction
Plants use photosynthesis to convert light energy to chemical energy
Simple organic substances, such as CO2, H2O and ions are used in their raw form to produce glucose and other carbohydrates.

3. Introduction How does the plants obtain H2O and CO2?
Does the plants have a circulatory system like us?
How these trees lift water against the force of gravity?
(i.e. American sequoias -giant redwood; Height of 60 meters)

4. Objectives
Explain the need for transport systems in multicellular plants
Describe the distribution of xylem and phloem tissue in roots, stems and leaves
Explain the absorption process in roots
Describe transport mechanisms (various ways of movement; both water, food and assimilates- compounds that the plant makes)
List factors that affects rate transpiration
Describe xerophyte properties
List the series of events that leads to translocation

5. Types of Plant Tissues

6. Plants have 2 separate transport tissues (VASCULAR TISSUE!)
Xylem tissue: Water and ions travel upwards
(1 direction)
Roots Stems Leaves Flowers Fruits
Phloem tissue: Sucrose and other assimilates travel upwards and downwards
Movement of water in the xylem and phloem is by mass flow. Everything travels in the same direction within each of column of xylem or phloem
Note that neither plant transport system carries O2 or CO2

8. Xylem and Phloem
In both the walls of the tubes are further thickened by the addition of:
Cellulose (organic compound –polysaccharide)
Lignin (woody material)

9. Water transport in 3 parts
Transpiration (or evapo-transpiration) is the transport of water and minerals from roots to leaves.
It involves 3 basic steps:
1- Absorption at the roots.
2 - Capillary action in the xylem vessels.
3 - Evaporation at the leaf.

10. Xylem (vascular tissue)
Function?
Types of cells?
Vessel elements
Tracheids
Direction of movement?
Transpiration
Stomata
Guard Cells

12. Roots Root hair Single-celled extensions of some cells
Very thin ( µm)
A single root can have thousands
Increases the surface area
Absorbs water by osmosis

13. Roots
Osmosis: Movement of H2O molecules from an area of high concentration to an area of lower concentration
Lower solute concentration in the soil
(high water potential)
Higher solute concentration in the root (lower water potential)

14. Two different routes for water to move into root:
Apoplast pathway: “AROUND”
When H2O soaks through the cell walls and then seeps across the root from cell wall to cell wall and through the spaces between cells
Symplast pathway: “CYTOPLASM”
When H2O enters the cell walls and moves from cell to cell by osmosis
Or through strands of cytoplasm that makes direct connection between adjacent cells- plasmodesmata

15. When water reaches the stele the apoplast pathway is blocked.
Endodermis cells (stele) have suberin (waterproof)

16. Casparian strip: belt of waxy material, allows only minerals in the symplast to pass into the vascular cylinder through the plasma membrane of endodermal cells.
Cells in the vascular cylinder transport water and minerals throughout the plant.

20. Xylem Long narrow cells Xylem elements
Start as living cells (nucleus, cell wall)
Then differentiated into specialised structures and died
No living material
Just empty shells

21. Stem
Root cross section

22. Leaf
Cross
section

23. Leaf cross section

24. Leaf tissue:
Palisade and spongy mesophyll cells have very large internal surface for gas exchange (CO2-
As the carbon dioxide concentration in the air is so low (0.04%), the surfaces are large so that enough can be absorbed for photosynthesis.

26. The air inside leaves is always fully saturated with water vapour.
Usually, the air outside is less saturated and so a concentration gradient for water vapour exists between the air spaces and the outside.
Therefore, water vapour diffuses down this humidity gradient. (High to Low H2O potential)
The pathway with the least resistance is through the stomata.

27. How does the water goes up?
Water properties???
Capillary Action
Cohesion-Adhesion tension theory
Bonding
Animation

28. Transpiration drives the movement of water in plants
The loss of water from leaves by transpiration causes water to travel upwards through the plant by mass flow.
The mechanism is called ‘cohesion-tension’ and it works as follows:

29. Cohesion-tension theory
Water loss caused by transpiration
Causes a pulling force
Negative pressure produced
Transpiration pull

30. Cohesion-tension theory
Root
Absorption through osmosis
Endodermal cells actively secrete mineral salts
( Why?- To keep the water potential in the xylem lower)
Causing water to be drawn through the endodermis
“pulling” of water caused by cortex cells produce positive hydrostatic pressure inside the xylem , forcing water upwards
Root pressure

31. How does the water goes up?
Transpiration pull (negative pressure)
Root pressure (positive pressure)
2 important factors of the water:
Cohesion: H2O molecules tend to stick together
Adhesion: H2O molecules tend to stick to the inside of the xylem

32. Transpiration
Spongy mesophyll cells are not tightly packed (air spaces are in direct contact with the air outside the leaf, through stomata)
If air outside the leaf contains less H2O vapour then inside
There is a H2O potential gradient from the air spaces inside the leaf to the outside

34. Potometer Measures the water absorption
Estimate the rate of transpiration
Air/water tight
Water transpired
Water entering to xylem

35. Factors affecting rate of transpiration
Light intensity:
Affects the opening and closing of the stomata
ROT
Indirect effect

36. Factors affecting rate of transpiration
Humidity:
Humid atmosphere
Contains a lot of H2O molecules
Reduction of the water potential gradient between the air spaces and atmosphere
ROT decreases
Low humidity increases ROT

37. Factors affecting rate of transpiration
Temperature:
Temperature
kinetic energy
Rate of diffusion through the stomata pores
Air is able to hold more water molecules at higher temperatures
ROT

38. Factors affecting rate of transpiration
Wind speed:
Still air makes the H2O molecules to accumulate around the stomata pores (leaves)
Reduces the H2O potential gradient and slows the ROT
Wind disperse H2O molecules
gradient in H2O potential ROT

39. Xerophytes A plant adapted to live in dry conditions
They have a range of adaptations to reduce the loss of water vapour by transpiration.
A plant adapted for survival in soil with a limited supply of water. Capillary water is absent from the surface horizons of the soil for extended periods of time. Some xerophytes, such as cacti and succulents, store water in their stems to survive extended periods of extreme drought. Cactus spines are modified leaves which provide protection against browsing animals. Because photosynthetic leaves are absent from most mature cactus plants, photosynthesis occurs within chloroplasts in the stems. In addition, cactus plants typically inhabit well-drained alluvial slopes and have shallow, surface roots to absorb the scant rainfall.

40. xerophytes Leaves Small to reduce the surface area
Thick to reduce surface area: volumes ratio

41. xerophytes
Sunken Stomata

42. xerophytes
Stomata
Set deep inside the leaf so that they are at the base of a depression full of water vapour
Some plants open their stomata at night to store and absorb CO2

43. xerophyte
Thick waxy cuticles
reduce water loss through the epidermis

44. Xerophytes Rolling up of leaves
Lower surface faces inside and traps humid air next to the stomata
Varies with conditions

45. Xerophytes Leaf hairs Trap damp air Reduces air movement
Cut down transpiration

46. Transport in the Phloem
Most photosynthesis occurs in the leaves.
The reactions take place in the chloroplasts.
The compounds that the plant makes are called assimilates.
Animation
Video notes
Many of these are exported form the leaves to the rest of the plant in the phloem.

48. Sources and Sinks
The transport of these assimilates is called translocation (means ‘from place to place’)
Assimilates are loaded in the phloem in the leaves, they are often called sources.
They are transported to other parts of the plant, such as roots, stems, flowers, fruits and seeds. These are called sinks.

51. Movement in the Phloem in an active transport
The transport of these assimilates is called translocation
Sucrose and other assimilates travel throughout a plant in phloem sieve tubes.
These are made from cells called sieve elements.

52. Sieve tube Made of sieve elements Living cells No nucleous
Ribosomes or tonoplast
Diameter um
End walls: sieve plates
Large pores

53. Alongside sieve tubes are companion cells.
Mesophyll cells in the leaf are close to veins containing sieve tubes.
Sucrose travels to the phloem companion cells in two ways.

54. From cell to cell through the plasmodesmata.
Along cell walls in the mesophyll.
Carrier proteins in the cell surface membranes of companion cells actively pump sucrose into the cytoplasm.
From here it passes through plasmodesmata into a sieve element.

55. The accumulation of sucrose and other solutes, such as amino acids, in sieve elements lowers the water potential so that water diffuses in by osmosis from adjacent cells and form the xylem.
This creates pressure in the sieve elements causing the liquid (phloem sap) to flow out of the leaf.

56. Phloem sieve elements are adapted for transport as it has:
End walls that have sieve pores allowing sap to flow freely.
Little cytoplasm to impede the flow of sap.
Plasmodesmata to allow assimilates to flow in from companion cells.

57. Phloem cells vs. xylem cells:
Sieve elements differ form xylem vessels because they are alive.
Sieve cells have some cytoplasm with organelles.
Sieve cells are not lignified, as they do not need to withstand the same forces as exist in the xylem.

58. Sucrose is unloaded at sinks.
This is taken up by the cells and is respired or stored as starch.
This reduces the concentration of phloem sap and lowers the pressure, so helping to maintain a pressure gradient form source to sink so the sap keeps flowing in the phloem.

61. REVIEW! Could you answer all objectives thoroughly??
Explain the need for transport systems in multicellular plants
Describe the distribution of xylem and phloem tissue in roots, stems and leaves
Explain the absorption process in roots
Describe transport mechanisms (various ways of movement; both water, food and assimilates- compounds that the plant makes)
List factors that affects rate transpiration
Describe xerophyte properties
List the series of events that leads to translocation