AL Bio notes Transport in Plants P.1
Transport in Plants
In
non-vascular plants - plants without vascular bundles, water, mineral salts and organic
substances circulate only by d______ from cell to cell. In ferns, gymnosperms and angiosperms
long distance transport is accomplished by m___ movement through vascular tissues.
Transport in higher plants occurs in three levels :
 Uptake of water and minerals by individual cells. E.g. Absorption of water and minerals from soil by
root hair
 Short-distance transport of substances from cell to cell or between tissues. E.g. loading of sugar from
mesophyll cells into phloem, passage of water and minerals from cells to cells in cortex.
 Long-distance transport of sap within xylem and phloem at the level of the whole plant.
I)
ABSORPTION & TRANSPORT OF WATER
A)
UPTAKE OF WATER BY THE ROOTS
a)
Pathways of Water Movement between cells and tissues
i) Apoplast pathway
Apoplast refers to the contin____ system of water-filled channels and spaces within a plant that
exist in the cellulose cell w___ and the intercellular sp____. It represents the non-living space
within the plant that allows mass flow and diffusion of water and solute to take place.
It is the most important pathway in terms of the variety and quantity of substances allowed.
E.g. In
leaves, carbon dioxide needs to be solubilized in the apoplast before it is taken up by chloroplasts. In the roots, ions diffuse into the
apoplast of the epidermis before being taken up into the symplast by specific ion channels and being pulled by the plant's
transpiration stream, which also occurs completely within the boundaries of the apoplast. Similarly, molecules emitted and received
by plants such as plant hormones must pass the apoplast.
ii) Symplast pathway
Water flows through the cytoplasm of each parenchyma cell in turn by os____, water may pass
from one cell to the next through the fine protoplasmic connections, plasmo________.
iii) Vacuolar pathway
Water is drawn from one vacuole to the next by osmosis, passing through cell surface mem_____,
cyto_____ and the tono____ of each cell in turn. It is the least important pathway. Why?
AL Bio notes Transport in Plants P.2
b)
Soil particles, Root hairs, and a large Surface Area of cortical cells
Most water absorption occurs in the root h___ zone, each root hair is a fine outgrowth from a cell of
the piliferous / epidermal layer of the root. They provide a very large surface area in close contact
with the soil particles.
Soil particles are coated with a film of soil w____ with dissolved min____.
The soil solution passes along the hydrophilic wall of epidermal cells and via the ap_____ into the
root c____. This exposes all the parenchyma cells of the cortex to the soil sol_____, providing a
much gr_____ absorption sur_____ than the area of the epidermis alone.
As the soil solution moves along cell walls, some water and minerals are taken up by the cells of
the epidermis and the cortex, switching pathways from the apo______ to the sym_______. It is this
crossing of membrane that enable mineral absorption sel________ .
A note for Mycorrhizae-- ‘Infected’ roots
Mycorrhizae refers to the symb_____ structures consisting of the plant’s roots intermingled with the hyp___
of fungus. The hyphae absorb water and selected minerals, transferring them to the host plants.
It is believed that many higher plants are associated with the fungal hyphae which endows the plants with an
mycelium – an enormous sur____ extending into the soil for water and mineral absorption.
c)
Role of Endodermis and the Casparian strip
In the endo______, the endodermal cells develop the C________ strip on their walls (radial and
tangential). It is a wa__ thickening of suberin which is impervious to w____ and minerals, ,
Minerals already in the symplast when they reach the endodermis continue through the
plasmo________ of the endodermal cells and pass to the vascular bundle.
Those minerals that reach the endodermis through the apoplast cannot continue unless they switch to
sy_______ through the e__________ cells. Thus the endodermis ensures no dissolved substances can
reach the vascular bundles without crossing living mem______ and thus subject to scre_____ or
sel_______.
Such control is necessary to control over water & salt movement into the xylem and may be a
protective measure against entry of toxic substances, fungal pathogens.
AL Bio notes Transport in Plants P.3
d)
Water Potential Gradient and Water Uptake
A water potential g________ exists from the s___ water, across the c____ of the root to the cells
adjacent to the xylem. The lowest potential being the cell adjacent to the xylem. O______ and
d_______ causes water to be absorbed and transported from soil to the xylem. This gradient is
maintained in two ways:

Transpiration causes water moving up the xylem, setting up tension / negative p________ in the xylem
and thus lowering the water p_______ of its sap, as water is drawn from cells adjacent to the xylem, it is
replaced from cells farther away and in this way lowering the cells’ water potential, thus a water
potential g_______ is set up across the root; the lowest potential being the cells immediately ad______ to
the xylem.
AL Bio notes Transport in Plants P.4

Root hair sap contains sugars and other solutes and is more con________ than soil water, water is
taken up by os______ from the soil; at the other side, mineral salts are actively secreted into the xylem
sap from the endodermis, creating an osmotic potential causing water to move from the cortex into the
xylem.
B)
THE PASSAGE OF WATER UP THE STEM
a)
Evidences of Water Transport in Xylem / and NOT via phloem

Stems which have been k____, either by taking up poisons such as picric acid (2,4,6-trinitrophenol (TNP),
or by heating encased in a steam jacket, are still capable of transpiring. This indicates that the movement
of water up the stem occurs through non-l______ cells.

Cut stems immersed in a solution of d__, such as eosin, absorb the dye and subsequent sectioning and
microscopic examination reveals that the x_____ higher up the stem has been stained.

'Ringing' experiments. An outer ring of bark, including phloem, is removed and, in the short term, this
does not affect the upward movement of water.
b)
Xylem -- Structure as related to Functions
The vessels and tracheids of the xylem offer little resistance to water movement. Both have lig_____
walls and no liv____ contents when mature.
i)
Vessels are composed of a series of cells called vessel elements arranged end to end. As they mature :
 Cellulose cell walls become impregnated with impermeable l______
 Their living contents d___ and disint_________.
 Their e___ wall between vessel elements break down, leaving a continuous t___ and
at numerous points in their walls are thinner regions called p__. Here, there is only the cellulose
pri_______ wall through which water can easily pass from one cell to another.
ii) Tracheids offer much more res________ to water flow because they retain their e___ walls which taper
and overlap each other. The oblique end walls and also the lateral walls have p__s.
Angiosperm xylem contains both vessels and tracheids, but gymnosperms have only t________.
AL Bio notes Transport in Plants P.5
Xylem may also contain supporting f______ (sclerenchyma) and xylem parenchyma, which acts as
a food store in which starch accumulates towards the end of the growing season, and is used up by
cambial activity the next spring.
Vascular bundle of a dicot plant
Vascular bundle of a monocot plant
FEATURES OF XYLEM AS RELATED TO FUNCTION
Functions / Adapted for
1 Long distance transport
2 Reduce resistance
3 Strength to withstand high
negative pressure
4 Impermeability
5 Lateral transport in case of
blockage
6 Capillary action
Related structural features
AL Bio notes Transport in Plants P.6
c)
Mechanism of Water Movement up the Xylem
Q. Californian redwoods with over 100m in height are probably the tallest tree in the world. The mechanism by
which sap moves up the plants to such heights has long puzzled plant physiologists. The Three hypotheses
have been put forward to account, in whole or in part, for this movement. They are:
1 The Cohesion-tension theory.
Water lost in t_________ leads to the movement of water from one c__ to another. The
coh______ of water molecules prevents the water column from br_______ when subjected
to the transpiration p__. As a consequence of this pull, the water in the xylem is dra____ up
the stem under a tension or n_______ pressure (relative to the atmospheric pressure).
2 Root pressure
The pressure exerted by the actively resp______ roots taking in water from the soil also
causes it to be forced upwards because of a hy_________ pressure (p_______ relative to the
atmospheric pressure)
3 Capillarity
Because of its high surface tension, water will rise in fine c________ tubes e.g. xylem to an
appreciable height.
Below are given a number of pieces of observation / evidence. For each say which, if any, of the three
hypotheses it supports or disproves, giving your reasons for doing so.
OBSERVATION / EVIDENCE
1
When the stem of a transpiring plant is cut below the
surface of a dye solution, the dye rushes in, going
both up and down the stem in the xylem. Water is
absorbed when poured onto the root stump of a
newly felled tree.
2
If the stem mentioned in (1) is first defoliated the dye
does not enter the stem.
3
The upward movement of indicator materials such as
dyes or radioactive isotopes can usually be related to
the rate of transpiration.
4
The cohesive or tensile strength of extracted sap
measured in the laboratory varies between about 130
and 210 atmospheres (atm). A cohesive force of one
atm should support a 9.07 m vertical column of water
under ideal conditions. The tallest trees of about 90
m would therefore require sap with a cohesive
strength of about 10 atm only. If one also takes into
account the forces of resistance and other factors,
the minimum cohesive strength theoretically required
becomes more like 40atm. (One atm is the amount of
pressure exerted by the air at sea level, and is 105
N/m2)
5
Water evaporating from a porous pot will cause a 100
cm long column of mercury to be drawn up from a
reservoir below. A leafy twig may be used instead of
the porous pot with the same effect.
6
The diameter of the trunk of a tree changes over 24
hrs, being smallest at midday and largest at midnight.
7
In the morning water begins to move in the small
branches of a tree before it does in the trunk. The
reverse is true in the evening.
8
Using the radioactive isotope of phosphorus (32P) it is
possible to trace the flow of sap up a tree. When
Support
/ reasons
Disprove
/ reasons
AL Bio notes Transport in Plants P.7
deep cuts are made into the wood directly in the path
of the ascending isotope, the pathway taken avoids
the cuts by moving laterally.
9
Bubbles of air appear in the xylem vessels of some
trees from time to time. Even though this possibly
break the continuous column required for cohesiontension, the tree seem not affected
10
The injection of tree trunks with metabolic inhibitors
and poisons causes little change in the speed at
which xylem sap flows.
11
Pressures of 3 to 5 atm have been recorded in
investigations of root pressure. Roots in an
atmosphere deficient in oxygen, or roots treated with
KCN, have a very much lower pressure than normal.
The root pressure is also reduced by lowering the
temperature.
12
Root pressure is frequently absent in actively
transpiring plants (see also point 1)
13
The diameter of xylem elements varies from 0.01 mm
(some gymnosperm tracheids) to 0.2 mm (largest
vessels). It is known that capillarity can cause water
to rise by 3m in a 0.01mm diameter glass tube.
1)
Cohesion-Tension Theory

According to this theory, e___________ of water from the cells of a l____ is responsible for
raising w____ from the roots. Evaporation results in a red_____ water potential in the cells
next to the X____ in leaves.

Water therefore enters these cells from the xylem sap which has a relatively h_____ water
pot_______.

as water ascends the xylem vessel a t______ is set up in the col______ of water in the xylem.
This is transmitted back down all the way to the root by coh_______ of w______ molecules.
AL Bio notes Transport in Plants P.8

The high c_______ of water molecules means that a relatively
large ten____ is required to break a column of water, that is a
water column has a high tensile strength.

The tension in the xylem vessels builds up to a f______
capable of pull____ the whole col_____ of water upwards
by means of mass flow, and water enters the base of the
columns in the roots from neighbouring root cells.

It is essential that the xylem walls should also are str___ and
rigid if they are not to coll____ under high negative pressure.
L____ and cell_____ is thought to provide this strength.
Critics of the theory point to the fact that any break in a column of sap should stop its flow, the vessel tending
to fill with air and water vapour. Shaking, bending and shortage of water can all induce column break.
However, studies showed that breaks in water columns do not greatly affect water flow rates.
The explanation may be that water flows from one vessel to another, or by-passes air-locks by moving
through neighbouring parenchyma cells and their walls.
Also, it is calculated that only a small proportion of the vessels need be functional at any one time to account
for the observed flow rates. In some trees and shrubs water moves only through the younger outer wood,
which is therefore called sapwood. New vessels are added throughout the growing season, mostly early in
the season when flow rates are higher.
2)
Root pressure

Root p_______ can be observed and measured when a freshly cut root stump continues to
exude s__ from its xylem vessels.

The process is inhibited by respiratory inh______ such as cyanide, lack of o_______ and low
t__________. The mechanism probably depends on a_____ secretion of salts or other
solutes into the xylem sap, thus l_________ its water potential. Water then moves into the
xylem by osmosis from neighbouring root cells. This builds up a p________ pressure pushing
water up the xylem from the root.

Under most conditions root pressure alone is not sufficient to account for water movement up
the xylem but it is no doubt a contributing factor in many plants. It can be sufficient, however,
in sl_____ transpiring herbaceous plants, when it can cause guttation. It might be important
in conditions of high h________ and well-watered plants or in spring before l______ have
been formed.
Guttation is the loss of water as drops of
l______ from the surface of a plant (as
opposed to vapour in transpiration).
It is favoured by the same conditions that
favour l__ transpiration rates, including
dim light and high humidity. It is common
in many rain forest species and is
frequently seen at the tips of the leaves of
young grass seedlings.
AL Bio notes Transport in Plants P.9
A summary of the pathway of water from the soil through a tree to the atmosphere
ATMOSPHERE
Stomata
Decreasing
Substomatal chamber
Water
Diffusion
Water vapour in intercellular space in leaf
Potential
EVAPORATION
Leaf cytoplasm
Leaf cell wall
(SYMPLAST)
(APOLPLAST)
ASCENT
Leaf stalk
OF
up the trunk
WATER
XYLEM
Endodermis (Carsparian Strip)
(VACUOLAR)
(SYMPLAST)
(APOPLAST)
Root Cells’
Vacuole
Root Cells’
Cytoplasm
SOIL
Root Cells’
Cell Wall
AL Bio notes Transport in Plants P.10
C)
STRUCTURE AND ACTION OF STOMATA
There are generally more stomata on the l_____ epidermis of leaves than on the u_____ epidermis
and fewer still in st___.
Density of stomata (per mm2)
Upper Epidermis
Lower Epidermis
Apple leaves
Nil
400
Bean leaves
40
250
Stomata occur in all the plant groups except the Algae and Fungi and their structure is remarkably
constant. Each consists of two modified epidermal cells called g_____ cells, enclosing a pore.
The opening and closing of stomata involves changes in w______ content of the guard cells, which
have th_______ cell walls next to the pore than next to the surrounding epidermal cells.
When they are tur___, their thin walls str_____ more than the thick ones, causing the two cells to
cu____ away from each other and open the pore. As they lose water and their turgor pressure drops,
the guard cells become straightened and close the pore.
Unlike other cells of the epidermis, guard cells contain ch_________ , photosynthesise and response
to change in light i_________.
AL Bio notes Transport in Plants P.11
D)
FACTORS AFFECTING TRANSPIRATION RATES
External factors
1 Vapour pressure differences
The greater the d_________ between water vapour pressure inside
and outside, the greater the rate.
2 Temperature / humidity
At constant temperature, decrease in humidity increases the rate; at
constant humidity, increased temperature increases the rate.
Internal factors
1 Stomatal control
Especially near
opening and closing
times.
2 the ability of cells to
retain water
3 Wind velocity
Wind blows away dif_____ shells and flexes leaves and stems.
4 Light intensity
Light causes st______ to open and therefore increases rate.
5 Soil water content
De________ causes stomata closure.
E)
EFFECTS OF TRANSPIRATION
Before the absorption of salts was shown to be mainly an active process, transpiration was believed to
be responsible for it. Now it is generally agreed that transpiration is merely, responsible for the
dist________ of salts along with the water. Similarly, there is no clear evidence that transpiration
prevents the overheating of plants.
High rates of transpiration and water deficit cause wilt___ and, in extreme cases, the death of the
plant. Protein synthesis is also inhibited by water deficiency. Transpiration appears to be a
ne_______ evil for land plants, the disadvantages balancing the benefits.
II)
A)
UPTAKE & TRANSPORT OF IONS
UPTAKE AND TRANSPORT OF MINERALS ACROSS ROOT CORTEX
In green plants minerals are take up from the soil or surrounding water by roots. Uptake is greatest
in the region of the root hairs. Mineral elements exist in the form of i___ in salts. In attempting to
explain the uptake and movement of mineral ions, the following facts must be taken into account.

Cell membranes, including the cell surface membrane and tonoplast, are not truly semipermeable but dif_________ permeable, selectively allowing the passage of different
substances such as ions.

Active transport can occur across cell membranes. This requires en______ in the form of ATP,
made during respiration, and can lead to an accumulation of ions along or against a
concentration gradient.

There is a continuous system of cell walls, the apoplast, extending inwards from the
piliferous layer of the root to the endodermis. Water, and any solutes it contains, enters the
system from the soil by m____ flow and to a lesser extent by dif_______.

Water moves through the apoplast as part of the transp_______ stream.
Q. Figure below shows the uptake of potassium ions by young cereal roots which had previously been thoroughly
AL Bio notes Transport in Plants P.12
washed in pure water. After 90 min the respiratory inhibitor potassium cyanide was added to the solutions.
a. The Figure also shows that the rate of respiration of carrot discs increases when they are transferred from pure
water to potassium chloride solution. From the results shown, account for this increase.
b. Why does the rise in potassium chloride content stop when KCN is added?
c. In an experiment similar to that described in figure above, but involving phosphate uptake, 16% of the phosphate
taken up by barley roots over a short period could be washed out after transferring to pure water again. Explain.
d.
Could ions reach the xylem entirely by means of the apoplast pathway?
Lateral movement of minerals and water in roots
Minerals are absorbed with the s___ solution by the root surface, especially by root hairs and
mycorrhizae. The water and minerals then move across the root c______ to the vascular cylinder by
a combination of ap________ and sym_______ routes.
Each cell of the root cortex is bathed in a solution (in the intercellular space) similar in composition
to that of the soil solution as a result of passive uptake—through the apoplast. Thus there is a large
surface area for ion uptake.(c.f. P.2).
The transit of minerals from apoplast to s________ occurs in the root epidermis and through the
cortex. This involves the crossing of me________ which is mostly selective and energy dependent –
a_____ transport.
Ions moving in the apoplast can only reach the end__________, where the C_________ strip
prevents further progress. To cross the endodermis, ions must pass by diffusion or active transport
through the cell m_________ of endodermal cells, entering their cytoplasm (sy______) and
possibly their vacuoles (va________). Thus the plant monitors and con____ which types of ions
eventually reach the xylem.
AL Bio notes Transport in Plants P.13
1. Uptake from of water 
and minerals from
soil

2. Across the cortex and 
from apoplast to
symplast

Endodermal and parenchyma cells within the stele discharge water
and minerals into their walls, which, as part of the apoplast, are
continuous with the xylem vessels.
The apoplast extends along the cell walls and intercellular spaces,
water and minerals can soak throughout the cortex along the cell walls.
As soil solution moves along the apoplast, some water and minerals are
transported through membranes into cells of the epidermis and
cortex and then move inwards via the symplast.
3. Selective absorption
into the stele by
membranes


The uptake of soil solution by hydrophilic walls of the epidermis
provides access to the apoplast.
Minerals and water that cross the plasma membranes of root hairs enter
the symplast.

4. Water and minerals
absorbed from soil
are now ready for
upward transport into
the shoot system

Water and minerals that move all the way to the endodermis along the
cell walls cannot continue into the stele via the apoplastic route
because of the casparian strip.
Only mineral that are already in the symplast or enter pathway by
crossing the plasma membrane of an endodermis cell can get around
the casparian strip and pass into the stele.
Q. How could you demonstrate, using a radioactive ion and autoradiography, that the endodermis is a barrier to the
movement of ions through cell walls?
B)
TRANSLOCATION OF MINERAL SALTS THROUGH PLANTS
Once in the xylem, they are distributed throughout the plant by the transpiration st______, in which
they move by m___ flow. Movement of the mineral elements in the xylem can be demonstrated by
r_______ experiments like those already described, in which removal of tissues external to the xylem,
such as phloem, has no effect on upward movement of ions.
The chief sinks, that is sites of utilisation, for mineral elements are the g_______ regions of the plant,
such as the apical and lateral mer_____, y_____ leaves, developing fr____ and fl______, and st_____
organs.
Q. The pathway of movement for inorganic solutes in the plant
AL Bio notes Transport in Plants P.14
Stout and Hoagland in 1939 investigated the uptake of radioactive potassium ( 42K)
in solution by small plants of cotton, geranium and willow.
The phloem and xylem in the experimental branches were separated for a length of
23 cm by delicately inserting strips of impervious wax paper between them, and
then binding the branch up with paper.
After five hours of uptake the distribution of 42K was determined by measuring the
amount of radioactivity in the experimental region, and above and below it. The
results are given below.
Distribution of 42K in willow
stem
Above stripped section
Stripped section:
top
middle
bottom
Below stripped section
Branch with xylem and phloem
separated (ppm 42K)
phloem
xylem
53
47
11.6
119
0.7
112
20
113
84
58
Control branch
(ppm 42K)
phloem
xylem
64
56
87
69
74
67
1
The control branch had been treated in order to act as a satisfactory control for the experiment. What do you think
had been done to it?
2
What do the figures for the waxed strip area of the experimental branch suggest about the tissue responsible for the
upward translocation of 42K ?
3
One would not have reached the same conclusion by an analysis of the 42K in the xylem and phloem of an intact
branch. One would instead have assumed that both tissues were equally responsible for the transport of
potassium. In the light of the information gained from the experimental branch, how do you account for the figures
for the control branch and also those above and below the waxed strip in the experimental branch?
Note : Analysis of the xylem sap also reveals that although some of the nitrogen traveI as inorganic nitrate or
ammonium ions, much of it is carried in the organic form of amino acids and related compounds. Some
conversion of these ions to amino acids must therefore take place in the roots. Similarly small amounts of
phosphorus and sulphur are carried as organic compounds.
Thus, although xylem and phloem are traditionally regarded as conducting inorganic and organic materials
respectively, the distinction is not clear-cut. In addition, a small amount of exchange between xylem and
phloem is common and phloem carries significant quantities of mineral elements.
III)
TRANSLOCATION OF ORGANIC SOLUTES
The green leaves and stems are the photosynthetic sites of the plant, where food is manufactured. It
must then be transported or translocated in solution to all parts of the plant through the phloem,
providing a source of energy, materials for growth and in some places a food store.
The direction of translocation is mainly downward from the leaves to the roots but may also be
up_____ and la_____ to growing leaf buds, flowers and fruits. The direction may also vary with
the time of year and age of the plant.
AL Bio notes Transport in Plants P.15
A)
STRUCTURE OF PHLOEM
In an angiosperm, phloem consists of si
tubes and com
cells and varying amounts of
parenchyma and fibres.
Sieve tubes develop from a linear series of cells called sieve tube elements. They have cellulose cell
walls and the end walls between adjacent elements develop p____ in groups called sieve pl_____,
with fine cyto________ strands passing through the p_____ from one element to the next.
Beside each sieve tube element is a companion cell with a prominent nucleus and dense cytoplasm,
strands of which (plasmo_________) pass through into the adjacent sieve element.
Sieve tube elements and companion cells are developed side by side as a result of longitudinal
division of a meristematic cell.
At maturity the n_____, contents of the sieve tube elements disintegrate, leaving only cytoplasm and
nucleoli. The companion cells keep their nuclei and also have many mitochondria, so the
companion cell and sieve tube element may form a single fun______ unit in which en____ and
nuclear con____ are provided by the companion cell.
AL Bio notes Transport in Plants P.16
Phloem parenchyma generally stores starch but probably also provides energy for the active
secretion of nutrients into the sieve tubes at the source (photosynthetic site) and their removal at the
sink.
B)
EVIDENCE FOR TRANSPORT IN THE PHLOEM
Q. The pathway of movement for organic materials
The transport of the radioactive isotope of phosphorus ( 32P) was studied in a way similar to that outlined in the Problem
above where the xylem and phloem were separated by strips of impervious wax paper. 32P was introduced into a leaf
and its presence in the xylem and phloem of the stem later determined by measuring the amount of radioactivity.
The amounts of radioactive phosphorus present in experimental and control branches are shown in the table.
Distribution of 32P in stem of plant
Branch with xylem and
phloem separated (% of 32P)
Top of strip
Bottom of strip
phloem
12
7
13
5
xylem
1
trace
0
1
Control branch
(% of 32P)
phloem
15
10
5
3
xylem
5
6
2
1
a. Are these results in accord with the hypothesis that the phloem transports organic materials produced by the leaf?
Give reasons for your answer.
b. Suggest an explanation for the fact that fruit on branches which have been ringed develop extraordinarily large.
Experimental Evidence for Transport in Phloem

Tree ringing experiments : b____ and outer tissues were removed down to the xylem. Organic
compounds were found to accumulate slowly above the ring causing a bulge. If the tree did not
sprout leaves below the ring, it eventually died from r___ starvation.

Investigations with radioactive tracers Carbon isotope (14C) administered in carbon dioxide,
showed that the radioactivity was incorporated into the end products of photosynthesis and
travelled down the stem. If photographic film were exposed to cut sections of stem, the
radioactivity could be seen to correspond to the position of the phloem. However, if a short
section of the stem was killed by treatment with hot wax, the transport of organic solutes ceased
at that point, indicating that liv
cells were required for their transport. Meanwhile phosphates
containing radioactive phosphorus 32P were still conducted upwards in the non-living xylem.

Cooling and respiratory poisons were found to in_____ the flow, whereas supplying additional
ATP to leaves accelerated it. Indicating that translocation requires living cells.

Aphids have been used to investigate the contents and rates of flow in sieve
tubes. The aphid inserts its mouthparts into a sieve tube to feed and if it is then
cut away the sieve tube contents continue to exude from the mouthparts and
can be collected. Analysis of these exudates has shown mainly su____, a few
oligosaccharides, a_____ acids, amides and a few minerals, varying little with
species or age.
AL Bio notes Transport in Plants P.17
C)
FACTORS AFFECTING THE RATE OF TRANSLOCATION

Temperature. It is difficult to separate temperature effects from their influence on other
physiological processes but it seems that increased temperature increases translocation rates up
to a maximum of about 35° C, probably by affecting the enzymes involved in the secretion and
removal of sucrose from the tubes.

Light Translocation to the roots is greatly enhanced in the dark in those plants studied.

Metabolic inhibitors Hydrogen cyanide, 2-4 dinitrophenol (DNP) etc., inhibit carbohydrate
translocation.

Concentration gradients Carbohydrate always seems to move from regions of higher
concentrations to regions of lower concentration.

Hormones. Cytokinins, IAA and gibberellins appear to at least partially control translocation,
probably by their effects on metabolic rates at the source and sink.
D)
MECHANISMS OF TRANSLOCATION
The following statements sum up the results of a number of observations about the translocation of
organic materials in the phloem.
a
The contents of the sieve tubes vary in relation to changes in the environmental conditions around
the plant.
b
Any change in the sugar content of the leaves is followed by similar changes in the sieve tube
contents.
c
Transport of materials stops when the phloem is killed. The rate of transport increases with
increasing temperature, reaching a maximum at about 25 ° C, and then falling off.
d
Stems treated with a substance which inhibits respiration, or placed in an environment with no
oxygen, do not transport materials in the phloem.
e
Sugars can be transported both up and down the plant: young leaves, roots, and growing fruits will
import sugars.
f
Aphids (greenfly) feed by inserting their stylets into the phloem sieve tubes. It is possible to cut the
anaesthetized body of the aphid from its stylet and collect from the cut end of the stylet the contents
of the phloem (called honey-dew) on which the insect was feeding. To do this no suction need be
applied. The drops of solution exude easily and quickly from the severed stylet.
a. What process in the plant seems to be responsible for the formation of the contents of the sieve tube ? Say which
pieces of evidence support your answer.
b. Is translocation in the phloem a biological process or a physical process ? Say which pieces of evidence support
your answer, and why they do.
It is now generally agreed that solutes enter and leave sieve tubes by a
transport, but it is by
no means certain how they travel through the tubes. One suggestion is the Mass Flow hypothesis.
AL Bio notes Transport in Plants P.18
The Mass Flow Hypothesis

It postulates a pass___ flow of solutes along a t______ pressure gradient from a region of h____
concentration of solutes in the leaves to a region of lower concentration in the roots.

Figure shows apparatus for demonstrating mass flow. Osmometer A contains a higher
concentration of solute and therefore it will develop a higher t_____ pressure as water flows
into it from the reservoir. This will cause water to flow from A to B, passively carrying the solute
with it.

The hydrostatic pressure built up in B will force w____ out of it back into the reservoir from
which it could again enter A. If we had some way of keeping up the concentration in A then the
circulation would continue indefinitely.
In the living plant A is represented by the
l___ cells supplying sugars and B by the
plant cells receiving them.
The connecting link C is the p______
carrying water and solutes while the recirculation of water through the reservoir is
represented by the x_____.
Continuous flow would be possible as long
as the leaves continued to produce
s______.
AL Bio notes Transport in Plants P.19
Criticism of the Mass Flow hypothesis
This Theory explains why liquid exudes under slight p________ from phloem when bark is cut or
from aphid mouthparts. There is also good evidence for concentration gr_______ of sugars in the
phloem especially in trees. Estimates of the pressure within phloem also tends to support the high
turgor pressure required to cause flow through the sieve plates.
It does not, however, show why l______ cells are necessary for
translocation or explain why factors affecting m_________ rate
also affect translocation speeds, unless one assumes that these only
alter the rate of entry and exit to the sieve tubes and not flow
through them.
One objection to the mass flow theory is that b_-directional
movement of solutes through the phloem has been shown. It is
possible however, that these occur through separate sieve tubes and
along different concentration gradients to separate sinks e.g.
from higher leaves to growing buds and from lower leaves to
developing roots.
The other major theory is that of protoplasmic streaming, occurring
round each sieve tube element and through the pores from one
element to the next. This would account for the requirement for
living contents and would allow bi-directional flow, even in the same
sieve tube. Protoplasmic streaming has been observed in some sieve
tubes but not all.
No one theory fits all the known facts of phloem transport but combinations of active transport,
mass flow and protoplasmic streaming explain most of the observations.
End