1. Chapter 9: Plant Biology
9.1 Transport in the Xylem of Plants

2. An Brief Intro to Plants
All living organisms require chemical energy (ATP) to run the various chemical reactions that sustain life.
In the process of cellular respiration, organisms convert simple sugars (i.e. glucose) into that chemical energy.
Animals such as ourselves obtain sugars from the food we eat.
But how do plants obtain the sugars required for cellular respiration to sustain life?

3. Photosynthesis!
The chemical process in which plants make sugar using light energy, water and carbon dioxide, making oxygen as a side product.
The sugar made during photosynthesis can then be used for cellular respiration.

4. Equation for Photosynthesis:
From the atmosphere
From the Sun
6 CO2 + 6 H2O + Light Energy → C6H12O6 + 6O2
From the surrounding
environment
A variety of simple sugars may be formed, though glucose (C6H12O6) is one of the most common.
If any of the reactants are lacking or are limiting, photosynthesis may not occur and the plant may die.

5. The Leaf The majority of photosynthesis in a plant occurs in the leaf.
Leaves are specialized for photosynthesis. They regulate the flow of gases and capture light energy for photosynthesis.
The structure and arrangement of leaves maximize the surface area exposed to sunlight and limits the distance gases need to travel.

6. Maple leaves are thin
and broad with a large
surface area
Pine leaves are thin and narrow. A single needle does not provide a sizable surface area, but a branch of needles do.

7. 3-D Cross-Section of a Leaf
A transparent colourless
layer that allows light to
pass through to the
mesophyll cells
Protects the leaf from excessive absorption
of light and evaporation of water
Where most of the photosynthesis takes place (abundant in chloroplasts).
A system of vessels that transport water, minerals, and carbohydrates within the plant.
Regulates the exchange of gases in the atmosphere
Photosynthetic epidermal cells that create microscopic openings called stomata.

8. Obtaining the Materials for Photosynthesis
Light is captured by the leaves – specifically by the chloroplasts of the mesophyll cells. (We will discuss this further in our photosynthesis unit next year).
Gas exchange happens via the stomata (stoma = singular form), which are small pores in the lower epidermis of the leaf
The stomata allows CO2 into the plant, and O2 out.
Water is absorbed through the roots, not the leaves.

9. Stomata

10. Stomata
Each stoma is surrounded by a pair of guard cells that control the control the size of a stoma by changing their shape in response to water movement by osmosis in the cells.
When water moves into guard cells, the cells become turgid (swollen) and the stoma opens.
When water move out of the guard cells, the guard cells become flaccid (limp), and the stoma closes.

11. O2
CO2

12. Cell Turgor Pressure
the pressure inside the cell that is exerted on the cell wall by the plasma membrane
created by water entering the cell via osmosis

13. Stomata Opening
In general, stomata are open in the daytime and closed at night.
When the Sun comes out in the morning, it activates receptors in the guard cell membranes, stimulating proton pumps that pump H+ out of the guard cells.
K+ move into the cells, followed by water (via osmosis)

14. Stomata Closing
Hormone absicis acid (ABA) causes the stomata to close.
Also, changes the particles in the guard cells of the stomata will cause the guard cells to lose water and become flaccid, closing the stomata.

15. H+ are pumped out of guard cells
K+ diffuses into guard cells
H2O diffuse into cells by osmosis
Guard cells swell and open
 CO2 enters stoma

16. Roots
Main function is mineral ion and water uptake for the plant.

17. Roots
Root hairs increase the surface area over which water and mineral ions may be absorbed.
The Root cap is important in protecting the apical meristem during primary growth of the root through the soil.

18. How do mineral ions and water move into the root?
WATER- must pass through the epidermis and cortex to get to the vascular tissue.
Water moves into the root hairs via osmosis.
There is a higher solute concentration and a lower water concentration than the surrounding soil.

19. How do mineral ions and water move into the root?
IONS (i.e. nitrates, ammonium, potassium, phosphates, calcium) enter through:
Diffusion
Fungal Hyphae
Active Transport

20. DIFFUSION – when the concentration of minerals is higher in the soil than in the root. They dissolve in water and then move into the root.
May also come in with water during MASS FLOW in which the plant takes in large volumes of water.

21. FUNGAL HYPHAE – some plant species have developed a symbiotic relationship (mutualism) with fungus to help absorb minerals.
They can grow into the plant roots and transport minerals to the roots that the plant cannot absorb without it. Also creates a larger surface area for absorption

22. ACTIVE TRANSPORT
Used when the concentration of minerals is higher inside the root than outside.
Requires energy and protein pumps, specific to certain mineral ions.
Mineral ions can only be absorbed by active transport if they make contact with the appropriate protein pump
Proton pump uses energy from ATP to pump H+ out of the cell.
Higher [ H+] outside the cell than inside  creating a negative charge inside the cell and an ELECTROCHEMICAL GRADIENT.
Now the positive ions can move into the cell via diffusion.

23. Water transport in the Plant
Once in the plant, water is transported by the vascular tissue known as xylem.
The other type of vascular tissue is the phloem

24. Xylem Long continuous hollow tubes.
Made of dead cells, responsible for transporting water.
Water flows in one direction (up!)
Reinforced by lignin.
Lignin is a highly branched polymer that strengthens the walls so they can withstand low pressure without collapsing
(Pressure in the xylem is usually much lower than in the atmosphere)

25. Xylem

26. Cross Section of a Stem (see page 411 on DRAWING XYLEM VESSELS)
Epidermis
Cortex
Phloem Vascular
Xylem Bundle
Cambium
Pith

27. Transpiration
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.)

28. Transpiration
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.

29. 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.

31. Mineral Uptake (long, detailed)
Water Uptake (long, detailed)
Uptake
Transpiration

32. 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

33. 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.

34. Using a Potometer

35. 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

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

37. 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

38. 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

39. 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

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

41. 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

42. 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)

43. 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.

44. 9.2 Transport in the Phloem of Plants
Plant Biology
9.2 Transport in the Phloem of Plants

45. Phloem
vessel transporting “food” or organic material (i.e. sucrose, amino acids) via TRANSLOCATION
Materials can move in either direction in the phloem
Phloem tissue is found throughout the plant (stem, roots, leaves)
It is composed of sieve tubes which are sieve tube cells separated by perforated walls called sieve plates
Sieve tube cells are closely associate with companion cells

46. Phloem

47. Phloem Sieve Tubes
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

48. 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.

51. Source and Sink
Sugars are made in photosynthetic organs (the leaves) and stored in the root.
“source” – where food is made or stored
Made: Green leaves, stems,
Stored: seeds, roots
“sink” – where food in used
Developing fruits, developing seeds, growing leaves, developing roots
Organic material moves through the phloem from source to sink

52. Phloem Loading

53. 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

54. 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

55. 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.

56. 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.

57. Translocation

58. 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

61. Cross section of a stem.
Vascular bundles are the coloured clusters
Larger openings xylem, smaller phloem

62. root of a buttercup (Ranunculus)

64. Homework Read Sections 9.1 and 9.2
Read “Experiments using aphid stylets” on page 417 and do DB Q on page 418
Read “Radioisotopes as important tools in studying translocation” on page 419 and do DB Q on same page