Mass transport in plants

Transpiration

Water vapour diffuses from air spaces out of leaf through stomata down water vapour potential gradient
Water evaporates from mesophyll cells to air spaces
Water moves out of xylem to mesophyll cells down water potential gradient
This creates low hydrostatic pressure and tension in the xylem
Column of water is drawn up the xylem by mass flow under tension due to cohesion and adhesion of water molecules
Minerals actively transported into xylem - lowers water potential so water follows by osmosis

Xylem structure

Dead cells aligned end to end
Hollow - no end walls/ nucleus/ cytoplasm
Lignified walls (waterproof & prevent collapse)
Lignin spiral/ rings which prevents rigidity
Bordered pits to allow water to pass to adjacent vessel/ living tissue

Comparison of xylem and phloem

Substance transported
Direction
Dead/ living cells
Organelles
Hollow
Lignified
Companion cells

Phloem structure

Sieve tube elements and companion cells
Some cytoplasm & organelles, vacuole, no nucleus
Cross-walls at intervals - sieve plates which allow sap to flow
Plasmodesmata link cytoplasm of companion cells and sieve tube elements
Companion cells - many mitochondria, ribosomes, RER, transport proteins

Mass flow hypothesis

At source -
Sucrose actively loaded from companion cell into sieve tube element, reducing water potential
Water moves into sieve tube element by osmosis (from xylem/ living tissue) increasing hydrostatic pressure
At sink -
Sucrose leaves sieve tube element by facilitated diffusion/ active transport
This increases water potential in sieve tube element
Water moves down water potential gradient from sieve tube element to xylem by osmosis decreasing hydrostatic pressure
Water moves down sieve tube down hydrostatic pressure gradient

Evidence against mass flow

Sugar travels to many different sinks - not just the one with lowest hydrostatic pressure
Sieve plates create barrier - lots of pressure would be needed for solutes to get through at reasonable rate

Evidence for mass flow

Aphid stylets - pressure gradients
Ringing of trees - bulge above ringed section shows downwards movement of sugars
Radioactive tracers - radioactively labelled carbon > sugars in photosynthesis and incorporated into tissues around the plant
Metabolic inhibitors - ATP & active transport is involved

Translocation definition

Movement of assimilates e.g. sucrose from source to sink

Transpiration definition

Loss of water vapour from aerial parts of the plant

Importance of transpiration

Photosynthesis (metabolite)
Cells to grow and elongate
Turgidity
Carries useful mineral ions
Evaporation keeps plant cool

Factors that affect rate of transpiration

Number of leaves
SA of leaves
Waxy cuticle
Light intensity
Temperature
Relative humidity
Air movements
Water availability

Xerophytes

Smaller leaves
Thick waxy cuticle
Stomata close during day or when hot
Hairs on surface
Sunken pits
Rolling of leaves
Loss of leaves
High salt concentration in cells