Chapter 12.5

*Plant Transport

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* http://www.biology.ualberta.ca/facilities/multimedia/uploads/alberta/transport.html

*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

*Activity…

*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