Organismal Physiology Lecture No. 11: Circulation In Plants
Tuesday October 16 , 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 O2and
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. Pulling Fluids Through Tubes:
-As the plant’s leaves transpire, the evaporation creates a low pressure system at its extremities. The
establishment of this low pressure area forces water to be drawn up from the roots (a less negative,
higher pressure system) and pulled up through the plant’s vascular tubing.
Capillary Action & Capillarity:
- Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in
opposition to external forces like gravity. Plant capillaries have what are known as “wettable” walls,
where there is strong adhesion between the liquid and the walls of the vessel. There is also surface
tension between water molecules, which allows for further adhesion. Generally, a decrease in diameter
increases the height of the water column supported. Hydrogen bonds cause elasticity and wettable
walls. As xylems possess all of these features, capillary action can account for a few extra metres in
-The Cohesion-Tension Theory is used to explain how water moves up a plant. Evaporation at the leaves
first causes negative pressure. As water moves down the pressure gradient from (positive to negative),
the cohesive properties of the water molecules transfer this tension through the length of the water
column. This requires a continuous water column so as not to break the tension from the transfer of
water. Do to the enormous amount of tension, dense wood is used to prevent the xylem from imploding
(not very common in plants) and the danger of cavitation is always a concern (occurs quite frequently).
Water & Pressure Gradients:
-Pores known as aquaporins will selectively permit access to water, but is water cannot be moved
actively (pumped) as there are no known active water transporters in plants. The reality is that water
moves down a pressure gradient in which it is referred to as a water potential (Ψ) and measured in MPa
(a lot of potential). The water potential of a given cell is calculated as: Ψ = Ψ W Ψ + s whepe Ψ ig s
the osmotic (or solute) potential, Ψ is phe pressure potential and Ψ is the gravitational potential.
Moving Water (Osmosis):
-In determining the osmotic potential (Ψ ), is is calculated as follows: Ψ = -Rsc wheresR is the gas
constant, T is the temperature and c is tse concentration of the solute. Increasing solute concentration
lowers water potential. By osmosis, water will move through a selectively-permeable membrane in the
direction of the highest solute concentration. Th