1
Water and Sugar Transport in Plants
(Chapter 35)
As mentioned in a previous lecture, land plants face two major challenges:
1) they must maintain water balance, and
2) they must maintain structural support to facilitate upright growth.
Regarding the maintenance of water balance… the vascular system of vascular
plants facilitates water balance by being an efficient means of water transport.
Here it’s important to understand that land plants typically use a lot of water when
they are actively carrying out photosynthesis.
The reason for this is because as the plant acquires CO2 to use in the
photosynthesis reaction, and as it releases the O2 that is a by-product of
photosynthesis, the plant also looses water via a process called transpiration.
Of course, all of this takes place through the stomata and for the most part, the
great bulk of a typical plant’s stomata occur in the leaves; hence, this is an
indication that the leaves are the principal site of photosynthesis (and water loss)
for most land plants.
In fact, a plant can loose so much water through transpiration that on a hot
summer day, a large maple tree can loose enough water to fill several 55-gallon
drums.
Bottom line here is that to keep from becoming desiccated, land plants must be
able to effectively replace the water that is typically lost during the process of
photosynthesis.
Regarding structural support… in vascular plants that exhibit secondary growth,
vascular tissue provides structural support, primarily in the form of woody growth.
, 2
In herbaceous plants (plants that grow without any appreciable secondary growth),
water balance itself can provide structural support by making cells turgid.
Of course, specialized cells such as collenchyma cells can also provide support in
plants that lack the tissues required for appreciable secondary growth (i.e., lateral
meristems).
Water and mineral movement through plants
The water potential in and about a plant regulates water movement into and
throughout the plant.
Water in a plant moves from the soil into the roots (via the root hairs), on to the
stem or trunk, on to the leaves, and then it is lost to the atmosphere from the
leaves via transpiration (i.e., the evaporation of water vapor from the leaves into
the atmosphere).
Water along this pathway forms a gradient regarding water potential.
The water potential in the soil about the roots can be close to zero.
The water potential becomes successively more negative along the pathway from
the soil to the roots, to the stem, to the leaves, and then into the atmosphere.
Water moves toward regions possessing more negative water potentials.
On the surface of leaves and other plant organs, water loss (via transpiration)
creates the negative water pressure that establishes the aforementioned regime
of water potentials that wicks water upwards from the roots.
And so, the negative pressure generated by transpiration is largely responsible for
the upward movement of water through a plant’s xylem.
Water and Sugar Transport in Plants
(Chapter 35)
As mentioned in a previous lecture, land plants face two major challenges:
1) they must maintain water balance, and
2) they must maintain structural support to facilitate upright growth.
Regarding the maintenance of water balance… the vascular system of vascular
plants facilitates water balance by being an efficient means of water transport.
Here it’s important to understand that land plants typically use a lot of water when
they are actively carrying out photosynthesis.
The reason for this is because as the plant acquires CO2 to use in the
photosynthesis reaction, and as it releases the O2 that is a by-product of
photosynthesis, the plant also looses water via a process called transpiration.
Of course, all of this takes place through the stomata and for the most part, the
great bulk of a typical plant’s stomata occur in the leaves; hence, this is an
indication that the leaves are the principal site of photosynthesis (and water loss)
for most land plants.
In fact, a plant can loose so much water through transpiration that on a hot
summer day, a large maple tree can loose enough water to fill several 55-gallon
drums.
Bottom line here is that to keep from becoming desiccated, land plants must be
able to effectively replace the water that is typically lost during the process of
photosynthesis.
Regarding structural support… in vascular plants that exhibit secondary growth,
vascular tissue provides structural support, primarily in the form of woody growth.
, 2
In herbaceous plants (plants that grow without any appreciable secondary growth),
water balance itself can provide structural support by making cells turgid.
Of course, specialized cells such as collenchyma cells can also provide support in
plants that lack the tissues required for appreciable secondary growth (i.e., lateral
meristems).
Water and mineral movement through plants
The water potential in and about a plant regulates water movement into and
throughout the plant.
Water in a plant moves from the soil into the roots (via the root hairs), on to the
stem or trunk, on to the leaves, and then it is lost to the atmosphere from the
leaves via transpiration (i.e., the evaporation of water vapor from the leaves into
the atmosphere).
Water along this pathway forms a gradient regarding water potential.
The water potential in the soil about the roots can be close to zero.
The water potential becomes successively more negative along the pathway from
the soil to the roots, to the stem, to the leaves, and then into the atmosphere.
Water moves toward regions possessing more negative water potentials.
On the surface of leaves and other plant organs, water loss (via transpiration)
creates the negative water pressure that establishes the aforementioned regime
of water potentials that wicks water upwards from the roots.
And so, the negative pressure generated by transpiration is largely responsible for
the upward movement of water through a plant’s xylem.
