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The process by which organic molecules and some mineral ions
are transported from one part of a plant to another is translocation
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Their end walls
are perforated to form sieve plates
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As sinks can be anywhere in the
plant (sometimes above or below the source) it follows that translocation of molecules in phloem can be in either direction
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The phloem also transports inorganic ions such as potassium chloride, phosphate and magnesium ions
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Mass flow of sucrose through sieve tube elements:
Transport of organic substances in the phloem

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Mass flow is the bulk movement of a substance
through a given channel or area in a specified time
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This causes the sieve tubes to have a lower
(more negative) water potential
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At the respiring cells (sink), sucrose is either
used up during respiration or converted to starch
for storage
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Due to this lowered water potential, water also
moves into these respiring cells, from the sieve
tubes by osmosis
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As a result of water entering the sieve tube elements at the source and leaving at the sink, there is
a high hydrostatic pressure at the source and a low
one at the sink
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What is in doubt is the precise mechanism by which
translocation is achieved
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Transfer of sucrose into sieve elements from photosynthesising tissue:

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Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
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Sucrose molecules are transported along the with the hydrogen ions (co-transport)
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There is therefore a mass flow of sucrose solution
down this hydrostatic gradient in the sieve tubes
...
Therefore
the process as a whole is active which is why its affected by temperature for example
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would seem to hinder mass flow (suggested they may
have a structural function helping to prevent tubes
from bursting under pressure)
The concentration of sucrose is higher in leaves
(source) than in roots (sink)

Not all solutes move at the same speed—they should
do so if movement is by mass flow

Downward flow in the phloem occurs in daylight but
ceases when leaves are in shade or at night

Sucrose is delivered at more or less the same rate to all
regions rather than going quickest to those with the
less sucrose concentration, which the mass flow theory
would suggest

Increases in sucrose levels in the leaf are followed by
similar increases in sucrose levels in the phloem a little
later
Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem
Companion cells possess many mitochondria and readily produce ATP



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