Flashcards
Module: Adaptation for transport in plants
Q: Describe the distribution of vascular tissue in roots, in stems and in leaves?
1. In roots, the xylem is central and star-shaped with phloem between groups of xylem
cells. This arrangement resists vertical stress and anchors the plant in the soil.
2. In stems, vascular bundles are in a ring at the periphery, with xylem towards the
centre and phloem towards the outside. This gives flexible support and resists
bending.
3. In leaves the vascular tissue is in the midrib and in a network of veins, giving flexible
strength and resistance to tearing.
Q: Describe the structure of xylem?
Main type of cells vessels and tracheids.
Tracheid’s occur only in ferns, conifers and angiosperms, but not in mosses.
Vessels occur only in angiosperms. As lignin builds up in their cell walls, the contents
die, leaving an empty space, the lumen. As the tissue develops, the end walls of the
cells break down, leaving a long hollow tube through which water climbs straight up
the plant.
Two functions; transport of water and minerals and to provide mechanical strength
and support.
Q: Describe the water uptake in roots:
The region of greatest uptake of water is the root hair zone, where the surface area
of the root is enormously increased by the presence of root hairs and uptake is
enhanced by their thin cell walls.
Soil water contains a very dilute solution of mineral salts and has a high-water
potential. The vacuole and cytoplasm of the root hair cell contain a concentrated
solution of solutes and have a lower, more negative, water potential. Water passes
down its water potential gradient by osmosis.
Q: Describe the three ways water can move through the root and into the xylem.
1. The Apoplast pathway = water moves in the cell walls. Cellulose fibres in the cell wall
are separated by through which the water moves. Fastest root.
2. The symplast pathway = water moves through the cytoplasm and plasmodesmata.
Plasmodesmata are strands of cytoplasm through pits in the cell wall joining
adjacent cells so the symplast is a continual pathway across the root cortex.
3. The vacuole pathway = water moves from vacuole to vacuole.
Q: Describe the adaptations of mesophytes:
Many shed their leaves before winter, so that they do not lose water by
transpiration, when liquid water may be scarce.
The aerial parts of many non-woody planta die off in water, so they are not exposed
to frost or cold winds.
, Most annual mesophytes over-winter are dormant seeds, with such a low metabolic
rate that almost no water is required.
Q: Describe the structure and role of the endodermis:
Water cannot enter the xylem from the apoplast pathway because lignin makes
xylem walls waterproof. Water can only pass into the xylem from the symplast and
vacuole pathways, so it must leave the apoplast pathway. The vascular tissue, in the
centre of the root, is surrounded by a region called the pericycle. The pericycle is
surrounded by a single layer of cells, the endodermis. Their cell walls are
impregnated with a waxy material, suberin, forming a distinctive band on the radial
and tangential walls, called the casparian strip. Suberin is waterproof so the
casparian strip prevents water moving further in the apoplast and drives it into the
cytoplasm.
Water moves from the root endodermis into the xylem by osmosis across the
endodermal cell membranes. The water in the xylem is more negative than in the
endodermal cells which is achieved by:
1. The water potential of the endodermal cells is raised by water being driven ibn by
the casparian strip.
2. The water potential of the xylem is decreased by active transport of mineral salts,
mainly sodium ions, from the endodermis and pericycle into the xylem.
Q: Summarise the movement of water from the soil into the xylem:
Water passes from root hair cells across the root cortex down a water potential
gradient by osmosis.
The water passes along two main pathways, the apoplast and symplast pathways.
On reaching the endodermis a band of suberin, known as the casparian strip,
prevents the use of the apoplast pathway and water is diverted into the cytoplasm of
the endodermis.
Q: State the three main mechanisms through which water moves from roots to the leaves:
1. Cohesion-tension & adhesion
2. Capillarity
3. Root pressure
Q: Describe the mechanisms through which water moves from roots to the leaves:
1. Cohesion-tension. Water evaporates from leaf cells into the air spaces and diffuses
out through the stomata into the atmosphere. This draws water across the cells of
the leaf in the apoplast, symplast and vacuolar pathway, from the xylem. As water
molecules leave xylem cells in the leaf they pull up other water molecules behind
them in the xylem. They water molecules move because they show cohesion. This
continuous pull produces tension in the water column. The charges on the water
molecules also cause attraction to the hydrophilic lining of the vessels.
2. Capillarity which is the movement of water up narrow tubes.
