Plant Transport Flashcards
Structure of Xylem
- Two types of cells -> tracheids and vessel elements
- Dead, containing no cytoplasm
- Cells have secondary cell wall made out of lignin and cellulose microfibrils
Features of Tracheids
- Long, slender
- Tapered ends
- Lignified secondary cell wall -> main function of structural support
- Pits in cell wall and ends
Features of Vessel Elements
- Short and wide
- Pits in cell wall
- Perforated ends
- Transports most water
- Also has lignified secondary cell wall
Function of Xylem
- Transports water and minerals and ions dissolved in water
- By simple diffusion/osmosis
- From roots to transpiring leaves
Structure of Phloem
- Two types of cells -> sieve-tube elements and companion cells
- Alive at maturity, no 2nd cell wall
Features of sieve-tube elements
- Long
- Lack cellular content to conduct nutrients effectively
- Perforated ends called ‘sieve plates’
- Conducts all nutrients
Features of companion cells
- Provides material & energy to sieve-tube
- Contains cellular content -> does not conduct nutrients
Function of Phloem
- Transports water dissolved solutes (eg. sucrose)
- By active transport
- From sources (mature leaves) to sinks (growing leaves or fruits, storage roots)
Similarities of Xylem and Phloem
- Both transport substances throughout the plant
- Both transport substances dissolved in water
What is a vascular bundle?
- Consists of xylem, phloem and cambium
- Surrounded by bundle sheath cells
What are the differences in Xylem and Phloem in stems/leaves/roots?
Xylem -> Larger, on the inside/top
Phloem -> Smaller, on the outside/bottom
Routes of water movement into root
- Symplastic Route
- Transmembrane Route
- Apoplastic Route
What is the symplastic route?
Moves along cell cytosol through plasmodesma channels
What is the transmembrane route?
Water moves through water channels in the cell walls
What is the apoplastic route?
- Water moves along cell walls
- Stopped by Casparian strip at endodermis of vascular bundle -> joins other paths
How does root pressure cause movement of water?
- Root hair cells absorb ions from soil
- Ions pumped into xylem by active transport -> lowers water potential of xylem
- Water moves from higher water potential root cells into xylem via osmosis
How does capillary action cause water movement?
- Works through three forces -> adhesion, cohesion and surface tension
- Adhesion -> attraction between water and cell walls of xylem
- Cohesion -> attraction between water molecules
- Surface tension -> force betwen water molecules at an air surface
What is transpiration?
Loss of water as water vapour from aerial parts of plant:
1. Water from xylem moves through spongy mesophyll cells -> evaporates as water vapour into air spaces
2. Diffuses from air spaces through stomata to surroundings
3. Loss of water vapour drives greater evaporation
What is transpiration pull?
- Evaporation of water in leaves -> creates tension trasmitted from roots to leaves
- Tension pulls water from roots into xylem and up xylem into leaves
How are sugars transported?
- Movement of sugars by bulk flow from source to sink
- Source -> sugar enters phloem
- Sink -> sugar exits phloem
- Sources and sink may vary during seasons
What is the bulk flow at source?
- Surcrose from source pumped into phloem by mainly active transport or simple diffusion
- High solute concentration in phloem sap -> lower water potential
- Water moves from xylem into phloem -> increases pressure
What is bulk flow at sink
- Sucrose from phloem moves into sink cells by active transport/simple diffusion
- Low solute concentration -> high water potential
- Water moves from phloem into xylem -> decreases pressure
How does translocation occur?
Difference in pressure at source at sink drives sucrose in phloem from source to sink by bulk flow
