1. Plant Transport

2. Overview
a/facilities/multimedia/uploads /alberta/transport.html

3. Water from the environment (lakes, rivers, soil) is actually a solution of dissolved substances, including nutrients. Will be referred to as “soil water”
Sugars are exclusively carried by the phloem
Nutrients in the soil water are carried by the xylem
Overview…

4. Xylem
Xylem- long tube, thickened walls (lignin), can withstand low pressure without collapsing (cavitation)
Formed from files of cells- arranged end to end
Non-living at maturity
The pressure inside the xylem is less than atmospheric pressure
Water moves through xylem at low pressure causing water to be pulled upward due to transpiration- called transpiration-pull

6. Epidermis Cortex Phloem Vascular Xylem Bundle Cambium Pith
(see page 411 on DRAWING XYLEM VESSELS)

7. The loss of water vapour from leaves through the stomata.
Often leaves are exposed to direct sunlight.
They have a large surface area to capture light for photosynthesis but also creates a large surface for water to be evaporated out.
(A medium sized tree can evaporate +1000L on a hot, dry day.)
Transpiration

8. When water evaporates from the surface of the wall in a leaf, adhesion causes water to be drawn through the cell wall from the nearest available supply to replace the lost water.
The nearest available water supply is the xylem vessels in the veins of the leaf.
Transpiration

9. Transpiration
The water that is lost by transpiration is replaced by the intake of water in the roots.
TRANSPIRATION PULL is a continuous stream of water against gravity from the roots to the upper parts of the plant, aided by cohesion and adhesion.
COHESION: H bonds between water molecules
ADHESION: H bonds between water molecules and the sides of the vessels – it counter acts gravity.

10. Factors that affect Transpiration
Light – warm leaf and open stomata
Humidity- decrease in humidity increases transpiration
Wind – increases rate – because humid air near the stomata is carried away
Temperature – increases – because more evaporation
Factors that affect Transpiration

11. Soil water – if intake of water by the roots does not keep up with transpiration, cells lose turgor pressure and stomata close.
Carbon Dioxide – high levels around the plant cause guard cells to lose turgor and the stomata close.

12. http://www. passmyexams. co. uk/GCSE/biology/measuring- transpiration
transpiration.html
Using a Potometer

13. Using a Potometer A device used to measure transpiration rates.
Consists of:
A leafy shoot in a tube
A reservoir
Graduated capillary tube with a bubble marking zero

14. As the plant takes up water, the bubble will move along the capillary tube
Time to move along the tube can be measure

15. Adaptations for Water Conservation
XEROPHYTES
Plants that can tolerate dry conditions (such as deserts)
Adapted to increase rate of water uptake and reduce water loss
Less competition in these environments
Adaptations for Water Conservation

16. Xerophyte Adaptations
Reduced leaves – smaller surface area reduces transpiration
Rolled Leaves – reduces stoma exposure to air and sun thus reduces transpiration
Spines – decrease in surface area

17. Xerophyte Adaptations
Thickened waxy cuticle – less water can escape
Low growth form – closer to the ground and thus less wind exposure
Fleshy stems – with water stored from rainy seasons

18. Xerophyte Adaptations
Reduced number of stomata
Sunken stomata in pits surrounded by hairs – the water vapour stays in the pit reducing the concentration gradient.
Xerophyte Adaptations

19. Xerophyte Adaptations
Hair like cell on leaf surface – trap a layer of water vapour maintaining a higher humidity
Shedding leaves in driest months
CAM photosynthesis – stomata are open at night when it is cooler so less water loss.
C4 photosynthesis
- involves a specialized leaf
structure to maximize
photosynthesis
Xerophyte Adaptations

20. Adaptations for Water Conservation
Halophytes
Plants that live in saline soils (high salt concentrations)
They require adaptations for water conservation (otherwise water loss will occur because of osmosis)
Adaptations for Water Conservation

21. Halophyte Adaptations
Reduced leaves or spines
Shedding of leaves when water is scarce (and then stem takes over photosynthesis)
Water storage structures in leaves (away from saline root environment)
Thick cuticle; multiple epidermal layers
Sunken stomata
Long roots to search for water
Structures to remove salt build up.
Halophyte Adaptations

22. Water and nutrient transport involves three stages: (i) from the soil into the roots, (ii) from the roots to the stem, and (iii) from the stem to the leaves.
Water also returns to the environment, mostly from the leaves (transpiration).
Movement of water…

23. Transport into the root…
Water enters via osmosis (high to low water molecule concentration) –less water in cells than in soil or more solute in cells
Nutrients enter via active transport (low to higher concentration)- more nutrients in plant cells than in the soil and the process requires energy
Transport into the root…

24. 1. Water enters passively through osmosis and nutrients enter actively into the root hairs and epidermis cells
Can travel between cell spaces (cytoplasm)- symplast
or through the cell wall apoplast
2. They then diffuse into the cortex toward the endodermis through interconnecting cytoplasm between cells
3. At the endodermis they encounter the Casparian strip. The key role of the Casparian strip is to prevent substances from leaking back into the cortex.
The flow

26. Transport through the stem to leaves…
Once passed the Casparian strip, the nutrients and water form a liquid called xylem sap
As more water enters, root pressure builds that helps push the sap up
Capillary action, which is the tendency of a liquid in a narrow tube to rise or fall contributes to the rise of the xylem sap.
The liquid has cohesion or attractive forces between molecules (cling together by H bonding) and adhesion to the sides of the wall. The water molecules in the xylem sap stick to each other are also drawn up the sides of the xylem tubes.
Can move between xylem tubes to surrounding tissue from pits
Transport through the stem to leaves…

28. Phloem Composed of sieve tubes- which are made up of sieve tube cells
Individual sieve tubes are separated by perforated walls called sieve plates
Sieve tube cells are closely associated with companion cells
Transport organic molecules (sugar/amino acids)- called translocation

29. The sieve tubes are composed of columns of specialized cells
Remember the cells that make up the xylem are dead.
These cells are living (though no nucleus) because they need to be able to undergo active transport to transport materials in and out of the phloem
The sieve plates are remnants of cells walls that separated the adjacent sieve tube cells
Phloem Sieve Tubes

30. Phloem Sieve Tube Cells
Sieve tube cells are closely associated with companion cells. (They are daughter cells from a mitotic division of one same parent cell)
The companion cell performs many of the genetic and metabolic functions to support the sieve tube cell.
They are abundant in mitochondria for this purpose.
Plasmodesmata connect companion cells with sieve tube cells.
Phloem Sieve Tube Cells

31. Transport of sugars… Called translocation
Source: a plant cell with a high concentration of sugars and other solutes, such as a leaf cell
Sink: a plant cell with a low concentration of sugars; sugars may be converted to starch for storage or used rapidly for energy or as building blocks of other carbohydrates
Sugars can move up or down
Source and sinks may change upon season; i.e. leaf growth becomes a sink in the spring and root and stem cells are sources.
Transport of sugars…

32. Examples Sources Sinks Mature photosynthesizing leaves Growing roots
Green stems
Developing seeds/fruits
Storage tissues in germinating seeds
Growing leaves/developing roots or tubers
Examples

33. Phloem Loading
Ex: Sugar is made in the leaves during photosynthesis. However, it is required throughout the plant for cellular respiration. In many plants, excess sugar is stored in the roots as longer carbohydrates.
How is sugar made in the leaves moved to the roots?
Answer: Translocation via the phloem – using the Pressure Flow Hypothesis
Source= leaves Sink = roots
Remember: 1)materials move from source to sink
2) molecules move from high pressure to low pressure

34. Pressure Flow Hypothesis
At the source, sugar is brought into the phloem by active transport
Water follows, moving into the phloem (from the adjacent xylem) via osmosis (remember H2O follows solutes) to produce sap
 High pressure created in this area of the phloem
The sap will be pushed to a lower pressure area, a sink
Pressure Flow Hypothesis

35. Pressure Flow Hypothesis
At the sink, the presence of sap now creates a high pressure situation. Phloem cells move the sugar out.
Water will also move out of phloem following osmotic gradient (H2O will move back into xylem)
 Low pressure recreated in the sink, resulting in
more sap flowing to the area.
Pressure Flow Hypothesis

36. Later in the life of the plant, the plant may require this stored sugar from the roots, for example to grow a fruit.
In this new scenario, now the roots will be the source and the developing fruit would be the sink and the sap would move against gravity up the stem.

37. Active transport for phloem loading
1) Proton pump uses energy from ATP to pump H+ out of the cell. 2) Higher [ H+] outside the cell than inside  creating a negative charge inside the cell and an ELECTROCHEMICAL GRADIENT. 3) Now the sugars can move into the cell via diffusion.
Active transport for phloem loading

39. Translocation
_view0/chapter38/animation_-_phloem_loading.html

40. Identifying Xylem and Phloem
Clues:
Xylem larger than phloem
Within one vascular bundle, phloem cells are closer to the outside of the plant in stems and roots.
See page
Identifying Xylem and Phloem