Section 3B

Biology 1002 - Flowering Plants Transport - Part B Notes

Water and minerals enter the plant through:
- Root epidermis
- Cross root cortex
- Pass into vascular cylinder
Xylem sap: Composed of water and dissolved minerals, transported from roots to leaves
Bulk flow: Movement of a fluid due to a difference in pressure between two locations
Transport of xylem sap involves:
- Transpiration: The evaporation of water from a plant’s surface
- The transpired water is replaced as it travels up from the roots
Potential Exam Note: Understanding the mechanisms of bulk flow transport

At night, when there is no transpiration:
- Root cells continue pumping mineral ions into the xylem
- Water flows in from the root cortex
This generates root pressure, pushing xylem sap
Positive root pressure: A minor mechanism of xylem bulk flow; can sometimes cause more water to enter leaves than is transpired
This can lead to guttation: The exudation of water droplets at the tips or edges of leaves
- Illustration: Root pressure causing excess water from a strawberry leaf as depicted in Figure 36.10

According to the cohesion-tension hypothesis:
- Transpiration provides the pull for the ascent of xylem sap
- The cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots
Process:
- Water vapor diffuses out of the leaf via stomata into drier outside air
- This water is compensated by surface tension created by water lining mesophyll cell surfaces
- The pull is generated, drawing water from xylem into the leaf
Transpirational pull: The mechanism that transmits this pull from leaves to roots

Key Factors:
- Adhesion: Water molecules are attracted to cellulose in xylem cell walls
- Cohesion: Water molecules are attracted to one another through hydrogen bonds
Mechanism:
- Cohesive forces allow for the pulling of a column of xylem sap
- Exiting water molecules pull on adjacent ones, relaying the action down the column
Structural Integrity: Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure
- Illustrated in Figure 36.12

Stoma Characteristics:
- Leaves have large surface areas and high surface-to-volume ratios
- These traits increase photosynthesis but also water loss via stomata
- Guard cells control stoma diameter by shaping changes
Stomatal Function:
- Changes in turgor pressure open and close stomata
- When turgid (swollen), guard cells bow outward and open the pore
- When flaccid (deflated), guard cells flatten and close the pore
- Illustrated in Figure 36.14

Mechanism of Turgor Pressure Changes:
- Result from the reversible uptake and loss of potassium ions (K+) by guard cells
- Active transport of H+ ions out generates a membrane potential driving K+ into the cell through specific channels
- Following K+ influx, water enters via osmosis, making cells turgid and opening stomata
- Stomatal closure occurs with the exit of K+, resulting in water loss
- Illustrated in Figure 36.13b

Generally, stomata open during the day and close at night to minimize water loss
Triggers for Stomatal Opening at Dawn:
- Light
- CO2 depletion
- Internal “clock” within guard cells
All eukaryotic organisms possess internal clocks regulating cyclic processes, known as circadian rhythms
- Figure 36.14 illustrates mechanisms of stomatal opening and closing

Xerophytes: Plants adapted to arid climates, with adaptations such as:
- Completing life cycles during the rainy season
- Fleshy stems for water storage
- Cacti have reduced leaves, carrying out photosynthesis primarily through stems
- Crassulacean Acid Metabolism (CAM): A specialized photosynthesis in succulents where stomata open at night to take in CO2, remaining closed during the day
Examples of Xerophytic Adaptations:
- Ocotillo: Leafless most of the year, produces small leaves only after heavy rain
- Oleander: Possesses a thick cuticle, multilayered epidermis, stomata in crypts which reduces transpiration
- Old Man Cactus: Long hairlike bristles reflect sunlight
- Illustrated in Figure 36.15

Concept 36.5: Sugars are transported from sources to sinks via phloem
- The products of photosynthesis are transported by a process known as translocation
- In angiosperms, sieve-tube elements serve as conduits for translocation

Phloem sap: An aqueous solution high in sucrose, travels from sugar sources to sugar sinks
Definitions:
- Sugar Source: Organs that net produce sugar (e.g., mature leaves)
- Sugar Sink: Organs that are net consumers/storers of sugar (e.g., growing roots, buds, stems, fruits)
Storage Organs: E.g., tubers, bulbs - can act as both sources and sinks under different conditions

Process Requirements: Sugar must be loaded into sieve-tube elements before being exported to sinks
Depending on species, sugar may move via:
- Symplastic route: Directly through cytoplasm
- Apoplastic route: Through cell walls and intercellular spaces
- Companion cells: Enhance solute movement between apoplast and symplast
- Illustrated in Figure 36.16

Active Transport Mechanism: Many plants require active transport for sugar movement into phloem:
- Sucrose is more concentrated in sieve-tube elements and companion cells than in mesophyll cells
- Mechanism: Proton pumping and cotransport of sucrose and H+ ions facilitate accumulation of sugars into companion cells and sieve-tube elements
- Illustrated in Figure 36.15

Bulk flow by Positive Pressure: The mechanism for moving phloem sap through sieve tubes:
- Known as pressure flow
- At the source, sugars are loaded into sieve tubes, and water follows by osmosis
- The resulting uptake of water generates positive pressure that pushes sap through the sieve tube
- At the sink, sugar diffuses into sink tissues by facilitated diffusion
- The pressure gradient (build-up at source and reduction at sink) drives sap from source to sink
- In leaf-to-root translocation, xylem recycles water from sink back to the source
- Depicted in Figure 36.18

Question #6: Which route is used for short distance transport in plant tissue that takes a non-living pathway through cell walls and spaces?
- A) Apoplast route
- B) Symplast route
- C) Transmembrane route
- D) Plasmodesmata route