Zusammenfassung der Ressource
Adaptations for Transport in Plants
- Roots
- Roots absorb water through root hairs. Then transported
through plant to xylem. In leaves, either used in
photosynthesis or evaporates away in transpiration
- Water moves into root hair cell (large SA:V,
through thin cell wall, freely permeable) from
soil down a water potential gradient by osmosis.
Soil very dilute and high water potential.
Cytoplasm, with many diddolved substances eg.
sugars had a low water potential
- Ions absorbed into root hair cell by active
transport, also lowers water potential
- After entering root hair cell, water
moves through the cortex
- Apoplast pathway through cortex moving from cell wall to cell wall
- Symplast pathway through membranes/cytoplasm/plasmodesmata by osmosis
- Vacuolar pathway: water can also travel through the
cell cytoplasm and vaculoes
- Endodermis has waterproof band of
suberin called the casparian strip
which blocks the apoplast pathway
so water moves into symplast
pathway
- Selective active transport of ions into pericycle eg. nitrate by endodermal cells
into cytoplasm (symplast pathway) bypassing the casparian strip and lowering
water potential in xylem. Water follows by osmosis. Water and minerals then
move into xylem. This is root pressure (positive hydrostatic pressure).
- Xylem
- Consists of dead, lignified tracheids and
vessels with pits, supporting fibres and
living parenchyma
- Tracheids and vessels form a continuous
system of channels for water transport
- Tracheids have tapered ends that fit together
- Columns of water in xylem are held up by the cohesive force
between water molecules and the adhesive forces between the
water molecules and the hydrophilic lining of the xylem vessles
- Also capillarity may help as water rises up thin tubes
- Structure: hollow with no cell contents
- Function: Less resistance to water
- Lack of end walls
- Creates a continuous column of water
- Large lumen (0.1-0.2mm)
- Less resistance/ wide to carry plenty of water
- Thick walls
with lignin
- Strengthens vessel so stops inward collapse when water sucked along it.
Waterproofing: stops water entering/leaving. Hydrophilic lining helps water
adhesion (stick) to the walls and so move upwards
- Pits
- Allows transfer of water
between cells
- Stems
- Water passes through the root to the
xylem, up through the stem to the
leaves where most evaporates and
moves out of the stomata
- Transpiration
- The loss of water vapour by
evaporation from the leaves
through stomata, powered by
sunlight
- Water follows a water potential gradient
- Gives rise to the transpiration
stream, pulling water up
- The continued removal of water molecules
from the top of the xylem vessels results in
a tension causing an upward force on the
xylem water column
- So water moves by 'cohesion
tension'
- Factors affecting rate of transpiration
- Temperature
- More kinetic energy, more evaporation
from mesophyll cells, more diffusion of
water vapour through stomata. A
higher air temperature can hold more
moisture so it has lower water
potential, so gradient is steeper
- Humidity
- Air inside leaf is saturated, whereas external
air has lower water potential. Increases in
external humidity would lower gradient
- Air movement
- Wind removes the layer of saturated air
just outside the stomata which reduces
the water potential so water potential
gradient steeper, transpiration increases
- Light
intensity
- Light opens the stomata, so
transpiration increases
- Plant adaptations to the amount
of available water:
- Plants must allow the exchange of gases, so must open stomata. But this
means water is lost through transpiration. Plants can be classified on the
bases of structure in relation to the prevailing water supply
- Translocation
- Plants must transport the
organic products of
photosynthesis:
- Transported as soluble sucrose
- From the leaves source
- To other parts where they are used for energy requiring
processes such as growth eg. ATP for active transport or
storage 'sink'
- Phloem consists of sieve tubes and companion cells linked by plasmodesmata,
with fibre cells and parenchyma cells also present
- Sieve tube element:
- Structure/property: living cell
- Allows active processes
- Few organelles at the edge
- More space for
transport with little
resistance
- Elongated
- Less resistance
- Sieve plate with pores
- Connects elements and lets
material through
- Joined end to end
- For continuous long
distance transport
- Bi-directional flow
- So sugar can move up and down
- Mass flow theory of translocation:
- Passive mass flow of sugars from the leaf
where they are produced and exported
(source, high concentration) to growing
tissues where the sugars are used (sink,
lower concentration)
- But this does not explain the presence of sieve plates which act as barriers to this
- Sucrose and amino acids have been observed to move at different
rates and in different directions in the same tissue at the same time
- Phloem tissue has a high rate of oxygen
consumption, companion cells with many
mitochondira and translocation is slowedby
respiratory inhibitors so maybe not a passive
process
- Companion cell:
- Structure/property: many mitochondira
- Metabolically active to provide ATP energy
- Nucleus
- Controls the functions of both cells
- Plasmodesmata
- Allows exchange with sieve tube element in
cytoplasmic strands
- Hydrogen pump
- Allows co-transport for sucrose loading
- Other theories for translocation:
- Diffusion
- Cytoplasmic streaming