*Overview…
*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
*Movement of water…
*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).
*Transport into the root…
*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
*The flow…
*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
*Transport through the
stem…
*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
*Transport to the leaves…
*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.
*Wilting…
*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
*Transport of sugars…
*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
*3 stages…
*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.
*From Source to Phloem…
*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
*Translocation
*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
*From Phloem to sink…
*Once they reach the sink, the sugar leaves the phloem*Passive transport*After sugar exits, water exits back to the
xylem