Phloem tissue, which transports sugars and other organic compounds, is made of one type of cell and its buddy.
sieve-tube member
Are alive at maturity, but lack nuclei and ribosomes.
Pores at the end of the cell create a sieve plate.
These allow the passage of fluid from cell to cell.
Companion cell
Each sieve-tube member has this cell as a buddy.
Connected to the sieve-tube member via plasmodesmata.
Provides physiological support for sieve-tube member.
Figure 35.11, Purves's Life: The Science of Biology, 7th Edition
Water Movement in Plants: Cohesion–Tension Theory
Transpiration
The evaporative loss of water from the leaves of plants through their stomata
This transpiration generates negative pressure, a pulling force—tension—on the water in the leaves.
Cohesion
The molecular attraction between like substances
In water, hydrogen bonding between water molecules is a cohesive force.
Water in a column of xylem tissue acts like a long polymer, since it is held together by the cohesive force of the hydrogen bonding between individual water molecules.
Bulk flow
Bulk flow, in this instance, is movement of water in “bulk.”
It occurs as a result of transpiration.
As a water molecule exits the leaf, it pulls up an entire column of water molecules.
Thus, this process is ultimately driven by the sun.
Wind can also increase transpiration and thus move water up the xylem.
Figure 36.8, Purves's Life: The Science of Biology, 7th Edition
Translocation of Sugars: Pressure–Flow Hypothesis
Translocation: the movement of carbohydrates through phloem from a source, such as leaves, to a sink, such as a root, a place where the carbohydrates are used.
Sugar enters the sieve-tube members.
Sugar moves from the site of production (the mesophyll cells in the leaves) to the sieve-tube members by active transport.
The concentration of dissolved sugars in the sieve-tube members at the source is now higher than at the sink.
Water enters the sieve-tube members.
Because of the movement of sugars into the sieve-tube members, the concentration of water becomes less inside the cell than in the area outside the cell.
As a result, water diffuses into the cells, moving down its concentration gradient (osmosis).
Pressure in the sieve-tube members at the source moves water and sugars through the sieve-tubes towards the sieve-tube members at the sink.
Pressure builds up as water enters the sieve-tube members in the leaves (or any other source).
As a result, water and sugars move by bulk flow through the sieve-tubes.
Pressure is reduced in sieve-tube members at the sink when sugars are removed for utilization by nearby cells.
As water and sugars move by bulk flow from source to sink, pressure begins to build at the sink.
Sugars are removed from the sieve-tube members by active transport, which increases the concentration of water within the sieve-tube members.
Water then diffuses out of the cell, moving down its concentration gradient (osmosis) and relieving the pressure.
