TRANSPORT IN

ANIMALS AND PLANTS

THE PRESSURE–FLOW HYPOTHESIS

EXPLAINS

TRANSLOCATION IN PHLOEM

• Current experimental evidence supports the

translocation of dissolved sugar in phloem by the

pressure–flow hypothesis, which was first proposed

in 1926 by the German scientist Ernst Münch.

• The pressure–flow hypothesis states that solutes

(such as dissolved sugars) move in phloem by

means of a pressure gradient—that is, a difference

in pressure

• The pressure gradient exists between the source,

where the sugar is loaded into phloem, and the sink,

where the sugar is removed from phloem.

• According to the pressure flow hypothesis,

translocation which takes place in three stages

which involves a combination of active transport and

mass flow.

a) First stage occur at the leaves (source)

• Occurrence at leaves where photosynthesis is

carried out by mesophyll cells to produce organic

substances.

• Then, glucose is produced during photosynthesis

and condenses into sucrose which is a more

suitable soluble substance for transport.

• The leave ,which sucrose and other organic solutes

are actively loaded into sieve tubes.

• The loading is carried out by modified companion

cells called transfer cells.

• These modified companion cells contains numerous

mitochondria to provide Adenosine Triphosphate

(ATP) energy for the active loading and numerous

ingrowths (internal projections) of their cell wall to

increase surface area for more efficient loading.

• These transfer cells are used in active mechanism

to load into sieve tubes by cotransport

At source, Photosynthesis occurred in mesophyll cell

Glucose sucrose

Active transportation occurs from mesophyll cells to

companion cells

Loaded sugars to sieve tubes of phloem

Loading occurs by active transport. ATP required .

Transported by cotransport.

The Route of sugar

transport from

mesophyll cells to

sieve tube cells.

• At companion cells ,

sugars in cell walls are

actively loaded into

cytoplasm as known

Symplast route .

• The sugar then moves

into the adjoining sieve

tubes elements through

plasmodesmata.

• In some species, sugar

transport involves a

combination of symplast

and apoplast routes and

needs ATP energy to

actively load sugar into

sieve tube members.

SUGAR LOADING

• Sugar loading involves a chemiosmostic

mechanism that uses a cotransport.

• Chemiosmostic mechanism is involved in the

transport of sucrose from transfer cells into sieve

tube.

• Proton pumps hydrogen ions (H+) out of the cell

membrane resulting in a proton gradient across the

membrane.

• A cotransport protein then carries the hydrogen ions

(H+) down its concentration gradient back into the

cell and sugar is transported into the cell as well.

• The ATP supplies energy to pump protons out of the

sieve tube elements, producing a proton gradient that

drives the uptake of sugar through specific channels by

the cotransport of protons back into the sieve tube

elements.

• The sugar therefore accumulates in the sieve tube

element.

• The increase in dissolved sugars in the sieve tube

element at the source-a concentration that is 2 to 3

times as great as in surrounding cells-decreases

(makes more negative) the water potential of that cell.

• As a result, water moves by osmosis from the xylem

cells into the sieve tubes, increasing the turgor pressure

(hydrostatic pressure) inside them.

Thus, phloem loading at the source occurs as follows:

1. Proton pump moves

H out of sieve tube

element

2. Sugar is actively

transported into sieve

tube element.

3. Water diffuses from

xylem into sieve tube

element

4. Turgor pressure increases

within sieve tube

• At its destination (the sink), sugar is unloaded by various

mechanisms, both active and passive, from the sieve

tube elements.

• With the loss of sugar, the water potential in the sieve

tube elements at the sink increases (becomes less

negative).

• Therefore, water moves out of the sieve tubes by

osmosis and into surrounding cells where the water

potential is more negative.

• Most of this water diffuses back to the xylem to be

transported upward.

• This water movement decreases the turgor pressure

inside the sieve tubes at the sink.

• Thus, phloem unloading at the sink proceeds as follows:

Sugar is transported out

of sieve tube element

Water diffuses out of sieve tube element and into

xylem

Turgor pressure decreases

within sieve tube

The Pressure Flow-

Hypothesis

• Sugar is actively loaded into

the sieve tube element at the

source.

• As a result, water diffuses from

the xylem into the sieve tube

element.

• At the sink, the sugar is

actively or passively unloaded,

and water diffuses from the

sieve tube element into the

xylem.

• The pressure gradient within

the sieve tube, from source to

sink, causes translocation from

the area of higher turgor

pressure (the source) to the

area of lower turgor pressure

(the sink).

Sucrose

1.When high concentration of

sugar at sieve tubes

2.Water potential decreases

3.Water diffuse in from xylem to

sieve tube

4.IncreaseTurgor Pressure in Sieve

tube.

1.When low concentration of

sugar in sieve tubes.

2.Water potential increases

3.Water diffuse out of the sieve tube

to xylem

4.DecreaseTurgor pressure in sieve

tube.

b) Second stage (Translocated in the stem

from source to the sink by Mass Flow)

• Sink- is a area where organic substances translocate

from the source are used or stored.E.g.: stem tubes,

tap roots, fruits and seeds.

• The concentration of sucrose in the sink is lower than

that found in the source and sieve tubes as sugar is

continuously being consumed or converted into

starch to be stored .

• This allows sucrose to diffuse out of sieve tubes

down the concentration gradient and its continuous

flow from the source to the sink.

Mass Flow – Ernst Munch• Mass flow is a physical process demonstrated by

Ernst Munch in his famous osmometer experiment or

model in 1930.

• In mass flow, Munch’s model demonstrates that fluid flows

from region of high hydrostatic pressure to region of low

hydrostatic pressure.

• As fluid flow, it carried the whole mass of different substance.

• In osmometer A, concentrated sucrose solution (leaf) has

lower water potential. Water flows into it from a high water

potential region (xylem vessel) to a low water potential region

(leaf cells) by osmosis.

• This create high hydrostatic pressure in A and forces sucrose

solution to enter into the connecting tube (sieve tube) and

pass to B (root cell)

• As the flow of mass from osmometer A to osmometer B

continues, the sucrose solution is pushed along and finally

appears in B.

• In B, contain water / dilute sugar solution, water moves out

from a higher water potential region by the hydrostatic

pressure gradient produced and redistributed through

connecting tube (xylem vessels) between the two container.

• Mass flow continues until the concentration of sugar solution in

A and B are equal (balanced).

• In nature, equilibrium is not reached because solutes are

constantly synthesized at source A and utilized at the sink B.

C) The third stage of Pressure Flow.

• The sucrose and other organic substances are

actively unloaded at the sink (root cells) involving

companion cells and energy.

• Here, sucrose converted into insoluble starch, used

for cellular respiration and synthesis the cellulose of

cell wall.

• Now, the water potential in the cell sap of the root

cells is reduced. Water follows the organic solutes

from the sieve tube into the root cells by osmosis.

• This reduces the hydrostatic pressure at the sink.

• There exists a hydrostatic pressure gradient in the

sieve tube which causes the passive mass flow of

water and dissolved solutes from source to the sink

region due to the differences in water potential

between the leaves and roots.

• If sugar continues to be produces in the source and

converted to starch or to be oxidized at the sink , the

gradient will be maintained and the mass flow

continues.

• The return of excess water from the sink (root) to the

leaves (source) through xylem vessels is brought by

transpiration pull.

Supporting The Pressure Flow

Hypothesis

There are different pieces of evidences that support the

hypothesis.

• Firstly, there is an exudation of solution from the phloem

when the stem is cut or punctured by the mouthparts of an

aphid - A classical experiment demonstrating the

translocation function of phloem, indicating that the phloem

sap is under pressure

Phloem translocation is difficult to study in plants. Because

phloem cells are under pressure, cutting into phloem to

observe it releases the pressure and causes the contents of the

sieve tube elements (the phloem sap) to exude and mix with

the contents of other severed cells that are also unavoidably cut.

• In the 1950s, scientists developed a unique

research tool to avoid contaminating the phloem

sap: aphids, which are small insects that insert

their mouthparts into phloem sieve tubes for

feeding

• The pressure in the punctured phloem drives the

sugar solution through the aphid’s mouthpart into

its digestive system.

• When the aphid’s mouthpart is severed from its

body by a laser beam, the sugar solution

continues to flow through the mouthpart at a rate

proportional to the pressure in phloem.

• This rate can be measured, and the effects on

phloem transport.

• Secondly, concentration gradients of organic solutes are

proved to be present between the sink and the source.

• Thirdly, when viruses or growth chemicals are applied to a

well-illuminated (actively photosynthesizing) leaf, they are

translocated downwards to the roots. Yet, when applied to

shaded leaves, such downward translocation of chemicals

does not occur, hence showing that diffusion is not a possible

process involved in translocation.

Against The Pressure Flow Hypothesis

• Some argue that mass flow is a passive process while sieve

tube vessels are supported by companion cells. Hence, the

hypothesis neglects the living nature of phloem.

• It is difficult to make measurements of transporting in the

phloem due to disruptions caused to the phloem.

• In this hypothesis, substances cannot flow opposite

directions in the same sieve tubes.

• In actual fact, phloem contains many sieve tubes and

different solutes could travel in opposite directions at the

same time in different sieve tube with different sources and

sinks.

The Electro-osmosis Hypothesis

• This mechanism is proposed by Spanner

• Electroosmosis is the movement of ions in an electrical field

through a fixed porous which is electrically charged by

carrying water and any dissolved solutes.

• The sieve plates and phloem protein are normally negatively

charged, thus forming a fixed porous surface with an electrical

charge

• As mass flow occur through the negatively charged sieve

plates, anions will be repelled but cations will be able to pass

through.

• When mass flow occur downwards through the phloem, the

repulsed anions will accumulates above the sieve plates so

that the cell above the sieve plate will become negative.

• The sieve plate will now be a fixed porous surface within a

electrical field, such as is needed for electroosmosis to occur.

• When a critical potential difference across the sieve plate is

reached , protons (H+ ions) surge from the wall of upper cell

into its cytoplasm by lowering its pH and making the

cytoplasm above the sieve tube to positively charged.

• The increased positive charged generated by the H+ surge

pushes other cations mainly ( K+) by electrical repulsion,

through the sieve plate from the upper to the lower cell and

therefore, electroosmosis occurs.

• The Potassium ions are then secreted on the other side of the

sieve plates.

• The presence of Positive K+ ions on the other side of the sieve

plate induces negative charges on the other side of the sieve

creating a potential different across the sieve plates.

• This surge the hydrated Potassium ions carries water

molecules and dissolved solutes like sucrose across the sieve

plates.

• With this energy causes an electro-osmosis flow of polar water

molecules and dissolved solutes through the sieve pores to

the adjacent sieve tube element.

Supporting Electro-Osmosis

• The energy derived from potential difference

shows that translocation is an active

transport.

• The presence of porous sieve plates in all

sieve tubes of plants.

• High concentration of K+ ions have been

found in phloem sap.

Against Electro-Osmosis

• There is a lack of evidence and no

detailed mechanism of how substance or

solutes move between the sieve plates.

The Cytoplasmic Streaming

Hypothesis

• Was proposed by Thaine in 1962.

• The cytoplasm of plant cells is often observed to move

around within the cell, a process called Streaming

• It has been proposed that solutes might be carried from one

end of a sieve tube element to the other by streaming and

then transferred across sieve plates by active transport.

• Both streaming and transfer through the sieve plates would

be energy-dependent, explaining high turnover of ATP in

phloem cells

• Different solutes move through the sieve pore at different

rates due to their different molecular characteristics and level

of impermeability of sieve plates to different solutes.

Supporting C.S.H

• Upwards and downwards movement of

solutes that occur within the confines of the

sieve tube explains the two way flow

substance.

• Low temperature and metabolic poison affect

cytoplasmic streaming indicating an active

process is involved.

Against C.S.H

• Cytoplasmic streaming is not enough to

account for the rate of translocation

observed in phloem.

• Cytoplasmic streaming has been observed

in immature sieve tube only.

Peristalsis Waves• Transcellular strands contain contractile protein which are

present in some phloem sieve tubes.

• sieve tube is filled with fine cytoplasmic filaments

• continuous from sieve tube to the next

• thru pores of sieve plate.

• contain phloem sap tube constrict + relax alternately

• pushing sap from one sieve tube to the next.

• constriction + relaxation/peristaltic movement form a pattern of wave = peristaltic wave

• can be at diff speed + in opposite direction (in sieve tube)

• depends on metabolic energy/ATP.• Their rhythmic contraction produces peristalsis waves that

facilitate the long-distance transport of solutes in phloem.