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7.4 The transport of water - Coggle Diagram
7.4 The transport of water
Leaf
Spongy mesophyll cells have many air spaces
Lots of water can evaporate into these
More water vapour inside the leaf than outside, so water diffuses out through stomata
Stomata are (in most plants) open at day and closed at night
Transpiration occurs
Water moves up xylem to replace water lost from leaf
Xerophytes
Cacti
Thick waxy cuticle
Reduced surface area - needles as leaves
Sunken stomata
Extensive roots - able to draw up more water
Marram grass
Sunken stomata
Rolled leaves
Trichomes
Conifers
Reduced surface area for transpiration - needles as leaves
Adaptations to reduce transpiration
Thick waxy cuticle - reduces evaporation in upper epidermis
Small surface area - reduces area of possible evaporation
Low stomata density - reduces amount of water able to leave the leaf during transpiration
Sunken stomata - forms pockets of moist air, reduces concentration gradient between environment and air inside leaf. Also increases diffusion distance, reducing the rate
Trichomes - network of hairs reduces air movement, concentration gradient between environment and inside leaf is lower
Rolled leaves - reduces air movement/less water vapour blown away, reduces concentration gradient
Structure of xylem
Fibres
Long narrow cells surrounding vessel elements
Structural support
Vessel elements
Made of dead cells, empty tube forming lumen, no cytoplasm, reduces resistance to flow of water
Impregnated with lignin, thickened strong cell wall, waterproof and tension resistant
Lumen has wide diameter
Perforated end-plates reduces resistance to flow of water
Pits in side walls, allows water to flow between xylem vessels in case of a blockage or air bubble
Extra cellulose and lignin is arranged in spirals or rings so vessels can elongate without cellulose/lignin breaking
Movement of water
Cohesion
Attraction of water molecules to each other
Causes water molecules to move upwards and replace those lost by transpiration
Adhesion
Attraction of water molecules to surfaces
Water molecules attracted to hydrophilic cell walls in xylem vessels, keeps them as a continuous stream
From root hairs to xylem
Apoplast pathway
Water moves via diffusion
Water moves through cell walls between cells
Symplast pathway
Water moves via osmosis
Water moves into cytoplasm or vacuole
Water moves between cells via plasmodesmata
Casparian strip
Thick band of waterproof suberin
Blocks apoplast pathway - all water must enter xylem via symplast pathway
Present in cell walls of endodermis cells
All water must pass through cell surface membrane - gives control over which ions enter the xylem
In older plants, suberin deposits may be so large that even the symplast pathway is blocked in endodermis cells
Passage cells keep symplast pathway open
From soil to root hairs
Root hairs - extensions of root epidermal cells. Able to reach in between individual soil particles
Cytoplasm of root epidermis cells has higher concentration of ions, sugars, and proteins
Lower water potential in root epidermis cells than soil means water moves into cells via osmosis
High number of hairs = higher surface area for absorption
Passive process