1. Chapter 36 Resource Acquisition and Transport in Plants

2. Xylem transports water and minerals from roots to shoots
The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis
Xylem transports water and minerals from roots to shoots
Phloem transports photosynthetic products from sources to sinks .

3. Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
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4. Shoot Architecture and Light Capture
Stems serve as conduits for water and nutrients and as supporting structures for leaves
There is generally a positive correlation between water availability and leaf size

5. Phyllotaxy, the arrangement of leaves on a stem, is specific to each species
Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral
The angle between leaves is 137.5 and likely minimizes shading of lower leaves

6. Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows
Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize

7. Ground area covered by plant
Figure 36.4
Ground area covered by plant
Figure 36.4 Leaf area index.
Plant A Leaf area  40% of ground area (leaf area index  0.4)
Plant B Leaf area  80% of ground area (leaf area index  0.8)

8. Leaf orientation affects light absorption
In low-light conditions, horizontal leaves capture more sunlight
In sunny conditions, vertical leaves are less damaged by sun and allow light to reach lower leaves

9. Root Architecture and Acquisition of Water and Minerals
Soil is a resource mined by the root system
Taproot systems anchor plants and are characteristic of gymnosperms and eudicots
Root growth can adjust to local conditions
For example, roots branch more in a pocket of high nitrate than low nitrate

10. Short-Distance Transport of Water Across Plasma Membranes
To survive, plants must balance water uptake and loss
Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure

11. Water potential is a measurement that combines the effects of solute concentration and pressure
Water potential determines the direction of movement of water
Water flows from regions of higher water potential to regions of lower water potential
Potential refers to water’s capacity to perform work

12. Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa)
Ψ = 0 MPa for pure water at sea level and at room temperature

13. How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration affect water potential
This is expressed by the water potential equation: Ψ  ΨS  ΨP
The solute potential (ΨS) of a solution is directly proportional to its molarity

14. Pressure potential (ΨP) is the physical pressure on a solution
Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast
The protoplast is the living part of the cell

15. If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid
Turgor loss in plants causes wilting, which can be reversed when the plant is watered

16. Concept 36.3: Transpiration drives the transport of water and minerals from roots to shoots via the xylem
Plants can move a large volume of water from their roots to shoots

17. Absorption of Water and Minerals by Root Cells
Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water
Root hairs account for much of the surface area of roots
After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

18. Microfibrils in cell wall of mesophyll cell Mesophyll Air space
Figure 36.12
Cuticle
Xylem
Upper epidermis
Microfibrils in cell wall of mesophyll cell
Mesophyll
Air space
Figure Generation of transpirational pull.
Lower epidermis
Cuticle
Stoma
Microfibril (cross section)
Water film
Air-water interface

19. Adhesion and Cohesion in the Ascent of Xylem Sap
Water molecules are attracted to cellulose in xylem cell walls through adhesion
Adhesion of water molecules to xylem cell walls helps offset the force of gravity

20. Water molecules are attracted to each other through cohesion
Cohesion makes it possible to pull a column of xylem sap
Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure
Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules

21. Concept 36.4: The rate of transpiration is regulated by stomata
Leaves generally have broad surface areas and high surface-to-volume ratios
These characteristics increase photosynthesis and increase water loss through stomata
Guard cells help balance water conservation with gas exchange for photosynthesis

22. Stomata: Major Pathways for Water Loss
About 95% of the water a plant loses escapes through stomata
Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape
Stomatal density is under genetic and environmental control

23. Mechanisms of Stomatal Opening and Closing
Changes in turgor pressure open and close stomata
When turgid, guard cells bow outward and the pore between them opens
When flaccid, guard cells become less bowed and the pore closes

24. Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed
Figure 36.15
Guard cells turgid/ Stoma open
Guard cells flaccid/ Stoma closed
Radially oriented cellulose microfibrils
Cell wall
Vacuole
Guard cell
(a)
Changes in guard cell shape and stomatal opening and closing (surface view)
H2O
H2O
H2O
H2O
Figure Mechanisms of stomatal opening and closing.
H2O K
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing

25. This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells

26. Stimuli for Stomatal Opening and Closing
Generally, stomata open during the day and close at night to minimize water loss
Stomatal opening at dawn is triggered by
Light
CO2 depletion
An internal “clock” in guard cells
All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

27. Drought, high temperature, and wind can cause stomata to close during the daytime
The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata

28. Effects of Transpiration on Wilting and Leaf Temperature
Plants lose a large amount of water by transpiration
If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt
Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

29. Adaptations That Reduce Evaporative Water Loss
Xerophytes are plants adapted to arid climates

30. Some desert plants complete their life cycle during the rainy season
Others have leaf modifications that reduce the rate of transpiration
Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night

31. Concept 36.5: Sugars are transported from sources to sinks via the phloem
The products of photosynthesis are transported through phloem by the process of translocation

32. Movement from Sugar Sources to Sugar Sinks
In angiosperms, sieve-tube elements are the conduits for translocation
Phloem sap is an aqueous solution that is high in sucrose
It travels from a sugar source to a sugar sink
A sugar source is an organ that is a net producer of sugar, such as mature leaves
A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb