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Title: Transport in Plants - GCSE level
Description: Entry of water into the plants, movement of water across the root, transpiration-tension-cohesion theory, root pressure, control of water loss, factors affecting transpiration, leaf modifications, translocation of organic solutes in plants, evidence that organic solutes travel in the phloem, the pressure flow model of translocation. Images included.
Description: Entry of water into the plants, movement of water across the root, transpiration-tension-cohesion theory, root pressure, control of water loss, factors affecting transpiration, leaf modifications, translocation of organic solutes in plants, evidence that organic solutes travel in the phloem, the pressure flow model of translocation. Images included.
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Transport in Plants
Some animals have a large surface area to volume ratio; therefore they rely on
diffusion for the exchange of nutrients and wastes
...
As organisms increase in size and complexity;
1
...
Surface area to volume ratio decreases
3
...
Mass flow involves the bulk transport of materials from one point to another as a
result of a pressure difference between the two points
...
A circulatory fluid; transport medium (e
...
xylem/ phloem, blood/lymphatic)
b
...
g
...
A force; circulates the medium (e
...
transpiration and root pressure/active
transport and osmosis, heart/muscular activity)
In plants: vascular tissues are called xylem and phloem (movement of solutes in the
phloem -> translocation)
...
Water enters by osmosis
...
They are taken up by facilitated diffusion along a concentration gradient, or
actively against a concentration gradient (selective)
...
A water potential gradient exists
across the root
...
Water moves through:
1
...
Symplast pathway: cytoplasm of cells linked with plasmodesmata
3
...
transmembrane pathway: membranes, cytoplasms and tonoplasts
When water moving through the apoplast pathway reaches the endodermis its flow
is stopped by suberin (waterproof) which is deposited in bands; casparian strips
...
Therefore cells of the endodermis regulate when solutes reach the xylem
...
Transpiration-tension-cohesion theory
2 | P a g e
Water will rise in fine capillary tubes due to its high surface tension (capillarity)
...
Water escaping through the stomata comes from substomatal air spaces which come
from spongy mesophyll cells
...
Water molecules in the xylem are bound to each other by H bonds
– cohesion
...
This forms the transpiration stream
...
Adhesion prevents the molecules from sliding back (stick to the walls of the xylem)
...
Lignin is present in xylem vessels to prevent it from collapsing inwards,
Transpiration can also be:
1
...
2
...
Root pressure
This is when mineral salts are actively transported into the xylem from cells
...
Guttation: water is forced out through openings in leaves
...
The mesophyll cells release abscisic acid which makes stomata close
...
3 | P a g e
§
When guard cells inflate with water, cells only increase in length
§
Guard cell walls are unevenly thickened therefore wall next to the pore is
less elastic
...
But proof shows K+ ions are causing this
...
Blue light absorbed by a pigment in the guard cell
activates a proton pump which transports H+ out of guard cells
...
Factors affecting transpiration
ü Light: stomata open in light; transpiration rate increases
ü Humidity: low humidity favours transpiration (diffusion gradient is steeper)
...
High altitude plants show
adaptations to lower transpiration rate
...
Air movements sweep away
this layer and increase steepness of gradient
...
4 | P a g e
Leaf modifications
ü Xerophytes: dry habitats and subjected to drought therefore reduce water
loss
...
g
...
They reflect radiation and do not heat up
§
CAM plants: open stomata at night and store CO2 as carboxyl groups
...
They are also shiny – reflect radiation
...
g
...
§
Shallow root system: (desert plants) root grows in upper soil layer where
moisture accumulates
§
Long tap roots: reach water supplies underground
ü Mesophytes: normal adequate water supply
§
Mechanisms to open and close stomata
§
Cuticle
§
Woody perennial shed their leaves
§
Herbaceous perennials lose shoot system and survive as underground
organs
§
Survive winter as dormant seeds
5 | P a g e
ü Hydrophytes: live submerged in freshwater
They have a lot of air spaces (aerenchyma) which provides bupyance and
stores gases
ü Halophytes: high salinity e
...
estuaries and salt marshes
They accumulate Na+ and Cl- in their root cells making Ψ negative
...
Succulence offers a reserve of water for the
plant during maximum salinity
...
Sink: an organ that consumes sugars
Phloem sugars are transported as disaccharide sucrose
1
...
Inactive: ideal transport sugar
It carries nitrogen and sulphur as amino acids, phosphorus as inorganic phosphate
ions and sugar phosphates and K+, vitamins and growth substances
...
Autoradiography:
Radioactive
14
C introduced into a plant as
14
CO2
...
Aphids:
6 | P a g e
They have fine stylets which penetrate through the tissues and feed on
translocating sugars
...
Other experiments show that:
-
Translocation stops if the phloem is killed
-
Translocation proceeds in both directions
-
Translocation is inhibited by components that inhibit respiration and
production of ATP
...
A high H+ concentration builds up
outside the cells, then they diffuse rapidly back using specific carrier proteins that
only function if they co-transport sucrose (negative Ψ) – secondary active transport
...
The sucrose travels from the sieve
tubes through plasmodesmata into sieve tubes (decrease pressure)
...
7 | P a g e
Sugars are used for respiration and synthesis of cellulose
...
8 | P a g e
Title: Transport in Plants - GCSE level
Description: Entry of water into the plants, movement of water across the root, transpiration-tension-cohesion theory, root pressure, control of water loss, factors affecting transpiration, leaf modifications, translocation of organic solutes in plants, evidence that organic solutes travel in the phloem, the pressure flow model of translocation. Images included.
Description: Entry of water into the plants, movement of water across the root, transpiration-tension-cohesion theory, root pressure, control of water loss, factors affecting transpiration, leaf modifications, translocation of organic solutes in plants, evidence that organic solutes travel in the phloem, the pressure flow model of translocation. Images included.