Adaptation For Transport In Plants Flashcards
Xylem
Vessels
•Tracheids
•Fibres
•Parenchyma
Xylem are dead cells that transport water and minerals up the plant and provide mechanical strength and support as they are strengthened by waterproof lignin
Phloem
Phloem sieve tubes carry sucrose and amino acids. Sieve elements end in sieve plates containing pores through which cytoplasmic filaments extend linking cells. No other organelles are in the sieve elements. Companion cells contain many mitochondria for ATP and the organelles for protein synthesis. Proteins and ATP are passed to the sieve elements through plasmodesmata.
Transpiration
is the loss of water as water vapour, by evaporation and
diffusion out of the open stomata, from the leaves of plants. It leads to the transpiration stream.
Transpiration stream
– water moves into the root and enters the xylem
(root pressure). Cohesive forces between water molecules and adhesive forces between water molecules and the hydrophilic lining of the xylem create a transpiration pull as the water leaving the xylem into the leaf cells pulls on molecules below. This is cohesion–tension theory
Factors increasing transpiration
High temperature
Low humidity
High wind speed
High light intensity
Hydrophyte
Water plants, e.g
water lilies
Adaptation of hydrophyte
Little/no waxy cuticle as no need to conserve water.
Stomata on upper surface as lower surface submerged.
Poorly developed xylem as no need to transport water.
Large air spaces (aerenchyma) provide buoyancy and act as
reservoirs of gas
Mesophyte
live with adequate water
Adaptation of mesophyte
Close stomata at night to decrease water loss.
Shed leaves in unfavourable conditions, e.g. winter.
Underground organs and dormant seeds survive winte
Xerophyte
water is scarce,
e.g marram grass
Adaptation of xerophyte
Thick waxy cuticle reducing water loss by evaporation from
epidermal tissue.
Sunken stomata increasing humidity in an air chamber
above the stomata, reducing diffusion gradient and
therefore water loss.
Rolled leaves - reduces area of leaf exposed directly to air.
Stiff interlocking hairs trap water vapour inside rolled leaf,
reducing water potential gradient and therefore water lo
Translocation
The phloem transports the products of photosynthesis from the source (the leaf) to the sink (area of use or storage). This is called translocation. There is evidence to show that this is bidirectional through the phloem.
Experimental evidence of translocation
- Ringing experiments (removal of phloem) show accumulation of sucrose products on leaf side of the ring but none on root side. Movement of sucrose was blocked by removal of phloem.
Therefore, phloem is the route of transport. - Using aphids to sample sap from the phloem. An aphid stylus extends into sieve tube elements. If a laser is used to remove the stylus from the body, the stylus then becomes a micropipette
and sap drips out. This can be analysed to show that sucrose and
amino acids are carried in the phloem, both above and below leaves. - Radioactive labelling of carbon dioxide which will become
incorporated into sucrose can be used in conjunction with the above technique to determine the rate of transport in the phloem. - Sources and sinks can be determined by autoradiography
using radioactively labelled carbon dioxide
Theory of mass flow-FOR
Sucrose made at source lowers water potential. Water enters cells and sucrose is forced into phloem (loading). This increases hydrostatic pressure and therefore mass flow occurs along the phloem to the root where sucrose is stored as starch, water potential is less negative and water flows into xylem
Against mass flow
Sieve plates impede flow.
Translocation is faster than expected with diffusion.
This theory does not explain bidirectional flow or different rates of flow of sucrose and amino acids.
Does not explain companion cell mitochondria, high O2 intake or stopping of translocation by cyanide.
