Download File

Introduction

Just like animal cells, plant cells are in constant need of oxygen and
nutrients. However, plants need different types of nutrients and gases, as
well as a different rate at which these are supplied.

Some of the requirements that are needed by plant cells are:
 Carbon dioxide – During daylight, plants are in constant need of CO2 for
photosynthesis.
 Oxygen – This is needed for respiration. However, most plant cells make enough
O2 during photosynthesis to supply their needs. Cells that are not undergoing
photosynthesis obtain O2 from their environment, but not at such high rate as
animal cells.
 Organic nutrients – Most of these nutrients are created during photosynthesis,
but if additional nutrients are needed or the plant cells do not photosynthesize,
they are obtained from their environment.
 Inorganic ions and water – These are taken up by the plant roots from the soil
and are then distributed to all of the regions of the plant.

Since the needs of plant cells are different, they have 2 transport systems.
One carries inorganic ions and water from the roots and the other carries
substances made by photosynthesis from the leaves.
Parts of a Plant
Transport of Water

Water is an important organic compound for plant cells.
 Water from the soil is taken up by the root hairs and then moves across the root
and into the xylem tissue.
 After it is inside the xylem vessels, it moves upwards through the root to the stem
and then into the leaves.

This movement of water is due to the
water potential gradient between the
soil, the plant, and the air. Since there is
a higher water potential in the soli, water
moves through the plant and escapes
into the air via transpiration.
Transport of Water
Sections of a Plant
From Soil to Root Hair

The tip of the root is covered by a tough, protective root cap that is not
permeable to water. Behind the tip some cells of the epidermis are drawn
out into long, thin extensions (root hairs) that reach into spaces between
the soil particles from where they absorb water.

Since the soil has inorganic ions in a dilute solution, it has a fairly high water
potential. On the other hand, the cytoplasm and the cell sap inside of the
root hair have a large amount of inorganic ions and organic substances,
causing a relatively low water potential. This potential gradient allows water
to diffuse from the soil, though the partially permeable membrane, and into
the cytoplasm and the vacuole of the root hair cell.

Since the root tip is covered by a large number of fine root hairs, it provides
a large surface area to come into contact with the soil water. This increases
the rate at which water is absorbed. However, the root hairs are delicate
and only function for a few days before they are replaced by new ones.

In addition, root hairs play a role in the absorption of mineral ions like
nitrate.
Mychorrhizas



Mychorrhizas are associations of fungi located in or on the roots of some
plants or trees. They serve a similar purpose to root hairs.
Mychorrhizas act like a mass of fine roots that absorb nutrients
(phosphate)from the soil and transport them into the plant.
The fungi and the tree, again, form a mutualistic (symbiotic) relationship.
The tree provides the fungi with some organic compounds, while the fungi
absorb nutrients and provide them to the plant.
Young Root
From Root Hair to Xylem

The water that was taken up by the root hairs now crosses the cortex and
enters the xylem of the root. The cells of the cortex (area between the
epidermis and the vascular tissues of the roots and stems) are surrounded
by cell walls made of many layers of cellulose fibres that cross one another.
Therefore, the water can now take 2 potential routes through the cortex.
 The apoplast pathway occurs when the water soaks into the cell walls and can
seep across the root from cell wall to cell wall without ever entering the cytoplasm
of the cortical cells.
 The symplast pathway
occurs when the water
moves into the cytoplasm
or vacuole of a cortical cell
and then into the adjacent
cells through the
interconnecting
plasmodesmata.
From Root Hair to Xylem - Continued

As water reaches the stele (central part of the root), the apoplast pathway is
prevented due to the endodermis cells, which have a thick, waterproof,
waxy band of suberin in their cell walls.
 The Casparian strip forms an impenetrable barrier
to water in the walls of the endodermis cells.
Therefore, the only way that water can cross the
endodermis is through the cytoplasm of the cells.
(Symplast plathway)
 Suberin deposits become more extensive as the endodermal cells get older,
except in passage cells. The passage cells allow water to continue to pass
freely. This arrangement gives the plant control over what inorganic ions
pass into the xylem vessels.

Once the water crosses the endodermis, it
will continue to move down the water
potential gradient across the pericycle
(outer part of the stele) and towards the
xylem vessels.
Water Movement from Root to Xylem
Vessel
Xylem Tissue

Xylem tissue contains different types of cells and has the functions of
support and transport. Some of the types of cells are:
 Vessel elements and trachelids are cells that are involved with the transport of
water.
○ Vessel elements are elongated cells that contain lignin within their cell walls.
Once lignin builds up, the contents of the cell die leaving a lumen inside. On
the other hand, in certain places of the cell wall (where plasmodesmata were)
no lignin was laid. These areas make up gaps (pits) that are crosses by
permeable, un-thickened cellulose cell wall.
○ Trachelids are dead cells with lignified walls but do not have open ends to
form vessels. Instead, they have tapering ends with pits to allow water to pass
from one cell to the other.
 Fibres are dead, elongated cells that have lignified walls that help with support.
 Parenchyma cells are known as “standard” plant cells. They have un-thickened
cellulose walls, are isodiametric, and contain all of the organelles that plant cells
should have, except for chloroplasts.

Once the water crosses the cortex, endodermis, and pericycle, it moves into
the xylem vessels through the pits. It then moves up the vessels towards
the leaves.
Xylem Vessels
Summary