1. Plant biology
Topic 9

2. Transport in the xylem of plants
Topic 9.1

3. U: Transpiration is the inevitable consequence of gas exchange in the leaf.
Leaf = primary organ of photosyn.
Gas exchange in leaf = CO2 in, O2 out
Occurs through stoma (stomata, pl) = pores in epidermis
With gas exchange comes H2O loss
Transpiration = loss of H2O vapor from leaves & stems
Guard cells = minimize H2O loss by controlling aperture of stoma
Liverworts = group of plants without stomata

4. How Stomata work

6. App: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots & capillary tubing
In capillary tubes, water fills tube because of adhesion of H2O to glass, the cohesion of H2O causes water to be drawn up.
Same thing happens in porous clay pots & filter paper.
Adhesion & cohesion are not properties of mercury.

7. Water transport demo

8. Skill: Measurement of transpiration rates using potometers (Practical 7)
Rate of transpiration difficult to measure directly
Rate of water uptake can be measured instead, using a potometer
Potometer = measures water uptake in plant

9.  Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant
Light
Temperature
Increasing light intensity increases rate of transpiration
Light stimulates the opening of stomata (gas exchange required for photosynthesis to occur) 
Some of the light energy absorbed by leaves is converted into heat, which increases the rate of water evaporation
Increasing temp increases rate of transpiration
Higher temps cause increase in water vaporisation in spongy mesophyll and increase in evaporation from leaf surface
This leads to an increase in diffusion of water vapour out of leaf which increases rate of transpiration

10. wind
Humidity
Greater air flow around leaf surface increases the rate of transpiration
 Wind removes water vapour (lower concentration of vapour on leaf surface), increasing rate of diffusion from within spongy mesophyll
Increasing humidity decreases rate of transpiration
Humidity is water vapour in air, thus high humidity means there is high conc of water vapour in air
This reduces rate of diffusion of water vapour from inside leaf (concentration gradient is smaller resulting in less net flow)

11. Skill: design of an experiment to test hypotheses about the effect of temp or humidity on transpiration rates
Questions to consider…
How will you measure the rate of transpiration in your investigation?
What biotic or abiotic factor will you investigate?
How will you vary the level of this factor?
How many results do you need, at each level of the factor that you are varying?
How will you keep other factors constant, so that they do not affect the rate of transpiration?

12. U: The cohesive property of water & the structure of the xylem vessels allow transport under tension
Structure of xylem vessels allows them to transport water inside plants efficiently:
Long continuous tubes that transport water
Walls thickened
Reinforced & strengthened with spiral bands of lignin
Lignin strengthens walls so they can withstand low pressures w/out collapsing
Spiral bands allow xylem vessels to elongate & grow lengthwise
Xylem cells are nonliving, so water flow is passive

13. Three ways by which terrestrial plants may support themselves are:
Thickened cellulose:  Thickening of the cell wall provides extra structural support
Cell turgor:  Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells turgid
Lignified xylem:  Xylem vessels run the length of the stem and branches, lignification of these vessels provides extra support

14. Xylem vessels are formed from files of cells arranged end-to-end
Xylem cells are dead! so water flow is passive
Xylem cells are hollow & consist only of cell wall
Cohesion = attraction of H2O molecules to each other due to polarity
Adhesion = attraction of H2O molecules to hydrophilic parts of xylem cell walls
Transpiration stream = H2O is pulled up from the xylem in a continuous stream

15. U: The adhesive property of water & evaporation generate tension forces in leaf cell walls
First step of transpiration = evaporation
Evaporation of one molecule of water moves another molecule with it due to adhesion & cohesion
Transpiration pull = pulling force that is transmitted through H2O in xylem down stem to roots (by low pressure)
Transpiration pull allows water to move up, against gravity
Passive process
Energy provided by thermal energy (heat) that causes transpiration
Cavitation = breaking of column of a liquid
Rarely happens with H2O

16. U: Active uptake of mineral ions in the roots causes absorption of water by osmosis
Explain the process of mineral ion absorption from the soil into roots by active transport
Minerals that need to be taken up from the soil include K+, Na+, Ca2+, NH4+, PO43- and NO3-
Fertile soil contains (-) charged clay particles to which (+) charged minerals attach
Root cells contain proton pumps that actively pump H+ ions into soil, which displaces (+) charged minerals allowing for their absorption
(-) charged minerals may bind to the H+ ions and be reabsorbed with H+

17. Continued…U: Active uptake of mineral ions in the roots causes absorption of water by osmosis
H2O is absorbed into root cells by osmosis
Solute conc. inside the root cells is greater than that in the water in the soil
Most solutes are mineral ions
Conc gradients established by active transport (protein pumps)
Separate pumps for each type of ion

18. U: Plants transport water from roots to leaves to replace losses from transpiration
Movement of water from roots to leaves:
Water leaves stomata by transpiration
Replaced by water from xylem
Water in xylem climbs stem through transpiration pull, cohesion, adhesion
Water moves from soil to roots by osmosis due to active transport of minerals into roots
Water in root travels to xylem through cell walls (apoplast pathway) and through cytoplasm (symplast pathway)

20. App: Adaptation of plants in deserts & in saline soils for water conservation
Xerophytes = plants adapted to growing in dry habitat
Increase rate of water uptake from soil
Reduce transpiration
Ephemeral = short life cycle during rainy season, then go dormant
Perennial = more than 2 years, rely on storage of water in specialized leaves, stems or roots

21. Adaptations of xerophytes: Cactus
Reduced leaves (spines)
Stems point straight up
Water storage in swollen stems
Thick waxy cuticle
CAM physiology (Crassulacean acid metabolism)
Stomata in stems that open at night
CO2 absorbed at night and stored as 4-C malic acid (C4 physiology)
CO2 released during day, allowing photosyn during day when stomata closed

22. Adaptations of xerophytes: Merram Grass
Rolled leaf to hold in water vapor
Stomata sit in small pits within curls, making them less likely to open and lose water
Hairs on inside of fold, to slow or stop air movement

23. Adaptations of halophytes:
Reduced leaves (small scales or spines)
Leaves are shed when low water, stem becomes green and takes over photosyn
Water storage structures develop in leaves
Thick cuticle and multiple layered epidermis
Sunken stomata
Long roots which search for water
Structures that remove salt build-up

24. Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images

25. Draw stem & leaf!