introduction to plant transport · of the roots helps the plant? a. the plant gets its support from...
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• The algal ancestors of plants were completely immersed in
water and dissolved minerals.
• The adaptation to land involved the differentiation of the
plant body into roots, which absorb water and minerals from
the soil, and stems which are exposed to light and
atmospheric CO2.
• This morphological solution created a new problem: the need
to transport materials between roots and shoots.
• Roots and shoots are bridged by vascular tissues that
transport water and sap throughout the plant body.
Introduction to Plant Transport
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Items referring to
physiological processes
are limited to
photosynthesis, cellular
respiration, transpiration,
growth, and reproduction.
• Transport in plants occurs on three levels:
(1) the uptake and loss of water and solutes by individual
cells
(2) short-distance transport
of substances from cell to
cell at the level of tissues
or organs
(3) long-distance transport
of sap within xylem and
phloem at the level of
the whole plant.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 36.1
Know these for the EOC
• Roots, stems, leaves
• Flowers, fruits, cones
• Meristem, cambium, ground, dermal, vascular,
xylem, phloem
• Stomata, guard cells, seeds
• Diffusion in a solution is fairly efficient for transport
over distances of cellular dimensions (less than 100
microns).
• However, diffusion is much too slow for long-
distance transport within a plant - for example, the
movement of water and minerals from roots to
leaves.
7. Bulk flow functions in long-distance
transport
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Water and mineral salts from soil enter the plant
through the epidermis (dermal tissue) of roots,
cross the root cortex, pass into the stele, and then
flow up xylem vessels to the shoot system.
Introduction to Absorption by Roots
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In most land plants, the epidermis of leaves and
other aerial parts is coated with a cuticle of
polyesters and waxes.
• The cuticle protects the plant from microbial attack.
• The wax acts as
waterproofing to
prevent excessive
water loss.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 29.10
• Xylem sap flows upward to veins that branch
throughout each leaf, providing each with water.
• Plants lose an astonishing amount of water by
transpiration, the loss of water vapor from leaves
and other aerial parts of the plant.
• An average-sized maple tree losses more than 200 L of
water per hour during the summer.
• The flow of water transported up from the xylem
replaces the water lost in transpiration and also
carries minerals to the shoot system.
Introduction to Xylem Transport
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The water lost via the stomata is
replaced by water pulled out of the leaf
xylem. watch
• The transpirational pull on xylem sap is
transmitted all the way from the leaves to the root
tips and even into the soil solution.
• Cohesion of water due to hydrogen bonding makes it
possible to pull a column of sap from above without the
water separating.
• Helping to fight gravity is the strong adhesion of water
molecules to the hydrophilic walls of the xylem cells.
• The very small diameter of the tracheids and vessel
elements exposes a large proportion of the water to the
hydrophilic walls.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The plant expends none its own metabolic energy
to lift xylem sap up to the leaves by bulk flow.
• The absorption of sunlight drives transpiration by
causing water to evaporate from the moist walls of
mesophyll cells and by maintaining a high
humidity in the air spaces within a leaf.
• Thus, the ascent of xylem sap is ultimately solar
powered.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• A leaf may transpire more than its weight in water
each day. Let’s watch
• To keep the leaf from wilting, flows in xylem vessels may
reach 75 cm/min.
• Guard cells, by
controlling the size
of stomata, help balance
the plant’s need to
conserve water with
its requirements for
photosynthesis.
1. Guard cell mediate the photosynthesis-
transpiration compromise
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 36.12
• This is NOT on the EOC, but here goes…
• The phloem transports the organic products of photosynthesis
throughout the plant via a process called translocation. Let’s
watch
• In angiosperms, the specialized cells of the phloem that
function in translocation are the sieve-tube members.
• These are arranged end to end to form long sieve tubes with
porous cross-walls between cells along the tube.
• Phloem sap is an aqueous solution in which sugar, primarily the
disaccharide sucrose in most plants, is the most prevalent solute.
• It may also contain minerals, amino acids, and hormones.
Introduction to the Transport of Sugars
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
How would these affect transpiration?
• Heat.
• Wind.
• Light
• Humidity
• Let’s graph with some worksheets.
• In contrast to the unidirectional flow of xylem sap
from roots to leaves, the direction that phloem sap
travels is variable.
• In general, sieve tubes carry food from a sugar
source to a sugar sink.
• A sugar source is a plant organ (especially mature leaves)
in which sugar is being produced by either photosynthesis
or the breakdown of starch.
• A sugar sink is an organ (such as growing roots, shoots,
or fruit) that is a net consumer or storer of sugar.
1. Phloem translocates its sap from sugar
sources to sugar sinks
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Phloem sap flows from source to sink at rates as
great as 1 m/hr, faster than can be accounted for by
either diffusion or cytoplasmic streaming.
• Phloem sap moves by bulk flow driven by pressure.
• Higher levels of sugar at the source lowers the water
potential and causes water to flow into the tube.
• Removal of sugar at the sink increases the water potential
and causes water to flow out of the tube.
• The difference in hydrostatic pressure drives phloem sap
from the source to the sink
2. Pressure flow is the mechanism of
translocation in angiosperms
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Which of the structures in the diagram below
identifies this cell as a plant cell rather than an
animal cell?
Unlike animal cells, plant cells have a large central
vacuole. How does a vacuole support plant
structures?
A. It fills with liquid, creating pressure that helps to
support the cell.
B. It releases materials into the air that decrease the
weight of the cell.
C. It contains genetic information that controls the
activities of the plant.
D. It forms a rigid substance called cellulose that
supports the plant.
A sedge is a grasslike plant with fibrous roots, which are
small, shallow roots that branch out from the base of the
plant. Which of the following best explains how the structure
of the roots helps the plant? A. The plant gets its support from the roots, which
serve as a kind of anchor.
B. The roots transport water between the stems and
leaves of the plant.
C. The plant uses its roots to capture water that is
close to the surface of the soil.
D. The roots allow for the exchange of gases the
plant needs for photosynthesis.
Plants are composed of different organs, tissues and
cells. Which are found only in vascular plants?
A. Gametes and leaves
B. Xylem and phloem
C. Stomata and guard cells
D. Flowers and spores
The drawing below shows a cross section of a plant leaf. How
does the component marked “x” contribute to the survival of
the plant?
A. It allows the intake of minerals needed for plant growth.
B. It allows the intake of gases needed for photosynthesis.
C. It allows the intake of sunlight needed for ATP production
D. It allows the intake of sugars needed for plant reproduction
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