Download Plant Transport Overview

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Chapter 12.5
*Overview
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* Water from the environment (lakes, rivers,
soil) is actually a solution of dissolved
substances, including nutrients. Will be
referred to as “soil water”
* Sugars are exclusively carried by the phloem
* Nutrients in the soil water are carried by the
xylem
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* Water and nutrient transport involves three stages: (i) from
the soil into the roots, (ii) from the roots to the stem,
and (iii) from the stem to the leaves.
* Water also returns to the environment, mostly from the
leaves (transpiration).
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* Water enters via osmosis (high to low water
molecule concentration) –less water in cells
than in soil
* Nutrients enter via active transport (low to
higher concentration)- more nutrients in plant
cells than in the soil and the process requires
energy
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* 1. With one of the above methods, water and
nutrients enter into the root hairs and epidermis
cells (travel between cell spaces or in cell)
* 2. They then diffuse into the cortex toward the
endodermis through interconnecting cytoplasm
between cells
* 3. At the endodermis they encounter the Casparian
strip. The key role of the Casparian strip is to
prevent substances from leaking back into the
cortex.
* Then the soil water moves into the xylem by active
transport
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* Once passed the Casparian strip, the nutrients and
water form a liquid called xylem sap
* As more water enters, root pressure builds that
helps push the sap up
* Capillary action, which is the tendency of a liquid in
a narrow tube to rise or fall contributes to the rise
of the xylem sap.
* The liquid has cohesion or attractive forces
between molecules (cling together by H bonding)
and adhesion to the sides of the wall. The water
molecules in the xylem sap stick to each otherand
are also drawn up the sides of the xylem tubes.
* Can move between xylem tubes to surrounding
tissue from pits
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* Water is often lost through the stomata of leaves in
a process called transpiration (evaporation)
* As a water molecule exits the leaf, the attractive
forces between water molecules causes the
following water molecule to pulled after it.
* The second molecule pulls the one behind it, and
the “pull” continues down the length of the xylem.
* If a plant does not transpire, the water column will
not move.
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* If a plant does not have enough water it may
wilt
* Water is stored in central vacuoles, which
exerts pressure, known as “turgor” pressure to
a plant cell wall
* When water is not available, water moves from
the vacuoles
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* Source: a plant cell with a high concentration of
sugars and other solutes, such as a leaf cell
* Sink: a plant cell with a low concentration of
sugars; sugars may be converted to starch for
storage or used rapidly for energy or as building
blocks of other carbohydrates
* Sugars can move up or down
* Source and sinks may change upon season; i.e. leaf
growth becomes a sink in the spring and root and
stem cells are sources.
* Developing seeds are a sink
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* We can divide the process of sugar transport
into three general stages:
* (i) transport of sugars from source cells to
phloem cells,
* (ii) transport through the phloem, and
* (iii) transport from phloem cells to sink cells.
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* Concentration of sugar is lower in the source cells
compared to phloem cells
* Active transport is required to move sugar from the
source to the phloem
* In angiosperms, companion cells transport sugars
from source cells to the sieve tube elements. In
gymnosperms, sugars are transported from source
cells directly into sieve cells, since this is the only
cell type in the phloem.
* As sugar increases in the phloem, water enters from
the xylem. This drives movement of the phloem sap
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* Long distance movement of sap through the
phloem is known as translocation
* Sap does not move through hollow tubes like
with xylem, but through living cells
* However, sugar molecules can travel more
quickly between phloem cells than between
other living cell types.
* Thought to be driven by a difference in turgor
pressure near source cells and sink cells
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* Once they reach the sink, the sugar leaves the
phloem
* Passive transport
* After sugar exits, water exits back to the
xylem
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* In groups of 4….
* Create a flow chart that outlines the steps of how
transport happens for each of the following;
* A) water
* B) sugar
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