IB Bio: Plant Biology Topic 9
9.1: Transport in xylem How and why does water move up a plant? How do plants conserve water? 9.2: Transport in phloem How and why and where does food move in a plant? 9.3: Plant Growth How is plant growth different from animal growth? What controls plant growth? 9.4: Plant Reproduction What controls flowering? What structures are used in plant reproduction? What are the seed structures in monocots and dicots?
9.1: Transport in xylem water potential & adhesion/cohesion allow water to move up TRANSPIRATION = escape of water from leaves (stomata) First, the water has to enter a root (next slide)
Roots - absorption most absorption is through the root hairs: = thin extensions of epidermal cells increase surface area 2014 Pearson Education, Inc.
Water Transport
Structure of a root Roots have different compartments through which water must move: epidermis cortex endodermis pericycle xylem
Mechanics of Water Movement Water moves through a root by two different pathways Apoplast--through cell walls without ever entering the cells Symplast--through the plasmodesmata between cells
Mechanics of Water Movement The pathway through symplast involves the living portion of the cells. It moves from the cytoplasm of one cell to the cytoplasm of another cell through plasmodesmata
Mechanics of Water Movement When water reaches the endodermis, it can continue to the vascular cylinder ONLY thorough the symplast pathway.
Mechanics of Water Movement Why? The suberin that permeates the Casparian strip blocks the path.
Stems
Evaporation Some light energy is converted to heat; water evaporates in spongy mesophyll vapor diffuses out stomata between guard cells negative pressure pulls water from the xylem
Xylem adaptations Xylem consists of two cell types: tracheids (all plants) joined by pits vessel elements (also in angiosperms) ends fuse and are continuous tube Xylem cells are dead at maturity Lignin in plant cells; strong, hydrophobic
STRUCTURE OF A XYLEM CELL Vessel element = continuous tube Tracheids = linked tapered cells Remnants of fused wall = indents Xylem includes pits Lignin = spirals or rings
Regulating water loss guard cells change shape to regulate water vapor loss stomata open when full of water; closed when less water regulated by abscissic acid, which causes cells to lose potassium water loss influenced by:
Reducing Water Loss Sometimes guarding stomata isn t enough. Adaptations to low water conditions: Smaller or rolled leaves Deep roots Thick cuticle Stomata in pits w/ hairs Water storage tissue C4 physiology CAM physiology Reversed CO2 timing (take in CO2 at night) Xerophytic plants
Xerophyte Adaptations That Reduce Transpiration Xerophytes Adapted to arid climates Leaf mods limit water loss:
Adaptations of Hydrophytes Cuticle Stomata Flat leaves, with air sacs Roots
Please sit next to a partner you are an A or a B
Concept Check If you are a B, explain to your partner the type of transport(s) necessary for water to gain access to xylem cells.
Concept Check If you are an A, name to your partner the structure that makes symplastic transport mandatory to get through the endodermis
Phloem Structure Recognition in slides In roots:
Phloem Structure Recognition in slides In stems:
9.2: Transport in phloem Translocation Organic compounds (sap) usually = sucrose Source = where the sugars are made Sink = where sugars delivered for use or storage
Two cell types: Sieve elements Companion cells Phloem Structure Structure-function Connected w/ sieve plates at ends Alive, but no nuclei & few organelles Thick, rigid walls Metabolic support to sieve elements Plasmodesmata for symplastic connection
Concept Check If you are a B, explain to your partner why companion cells are needed.
Concept Check If you are an A, explain the function of sieve element cells thick walls
Phloem Cells ID top cell: Companion cell how did you know? ID bottom cell: Sieve tube element Name of gaps that join the cells? plasmodesmata
Phloem: Pressure Flow Mechanism Sap pushed through a sieve tube by bulk flow driven by positive pressure Requires active loading and unloading (green arrows) (H+ co-transport Sugar into cells, H+ pumped back out) (uncompressible) H2O pushes flow to sink. H2O gradient at sink: H2O uses osmosis to cross to xylem what factors influence rate of flow? how could we measure rate?
Calculating Rate of Flow Use aphids:
Calculating Rate of Flow Use aphids plants grown with radioactive CO2 aphids encouraged to feed along stem anesthetized with CO2 and then stylets cut time until radioactive sap detected Colony 1 2 3 d from Start (cm) 20 40 60 t until radioactivity (min) 75 144 228 rate of transport (cm min -1 )
How do plants maximize uptake? Root hairs surface area Mycorrhizal fungi symbiosis mutualism