Biology 2601A/B Lecture Notes - Surface Tension, Lignin, Aquaporin
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Organismal Physiology Lecture No. 11: Circulation In Plants
Tuesday October 16th, 2012
Requirement For A Circulatory System:
-The circulatory system found in plants is used for mainly for: Transporting the sugars from
photosynthesis in to leaves and other tissues where they will be used, sending hormones and signals
throughout the organism, and moving water and nutrients from the soil to the leaves.
Xylem & Phloem:
-Xylem is most often referred to as the water-transporting tissue in plants as it translocates water and
inorganic nutrients (like nitrogen, phosphorous and potassium) from the roots to the leaves. The other
component of vascular transport in plants is phloem whose main function is to translocate sugars,
proteins and signalling molecules from source tissues (like the leaves) to sink tissues (like the roots).
-Xylem is composed of tracheids (long, thin cells) and vessel elements (short, wide cells) that share some
key differences. Both of these cells are classified as dead cells that only act as pipes to transport water.
Water moves between the tracheids only through pits, while between vessel elements water moves
mostly through perforation plates (compound or simple) and subsidiary flow through pits. The xylem is
supported by fibres and other lignified cells (especially the case for trees) to ensure stability.
-Vessel elements are the primary vessel type in angiosperms (broad-leafed plant species), which also
contain tracheids. As vessel elements are stacked end-to-end by perforation plates, they appear in the
xylem as one continuous tube. Although pits connects the vessel elements laterally and do allow for
some lateral movement of water, this method of transport is quite slow compared to the vertical
movement of water.
-Although tracheids are found in angiosperms, they are the primary vessel type in gymnosperms (like
conifers and gingko trees), which only possess tracheids (do not contain vessel elements). Tracheids are
characterized by their permeable pit membranes that allow for the lateral movement of water to
provide long connected vessels.
Defining Circulatory Systems Without Pumps:
-Typically in animal circulatory systems, a pump (to push fluid through vessels), fluid (to trap O2 and
other nutrients) and vessels or space (to transport fluid to tissues) are required components. However,
in plant circulatory systems, no pump is required to transport water. This is mostly due to the pulling of
water and sometimes due to the root pressure pushing water into the xylem.
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