Chapter 28 13-03-03 1:59 PM
Membrane potential: an electrical voltage that measures the potential
inside a cell membrane relative to the fluid just outside; it is negative under
resting conditions and is positive during an action potential.
Water Potential: the potential energy of water, representing the difference
in free energy between pure water and water in cells an solutions; it is the
driving force for osmosis.
Solute Potential: the effect of dissolved solutes on water potential.
Osmosis: the passive transport of water across a selectively permeable
membrane in response to solute concentration or pressure gradients.
Tonoplast: the membrane that surround the central vacuole in a plant cell.
Guttation: the exudation of water from leaves as a result of strong root
Translocation: the long distance transport of substances by xylem and
Passive transport: substances move with their concentration gradient– the
cells do not need to expend energy. (simple and facilitated diffusion)
Active Transport: moves ions and large molecules across membranes via
transport proteins against the concentration gradients– the cells must and
do expend energy.
Long-distance transport: water and dissolved minerals and products of
photosynthesis travel in the xylem and phloem travel from roots to shoots,
and from leaves to roots and other structures.
Short-distance transport: mechanisms move water and solutes in and out
of specific cells in roots leaves and stems. Water potential is measured in megapascals– pure water has a water
potential of zero
- determinants of water potential: solute concentration.
• The addition of solutes to water disrupts some hydrogen bonds..
water bonds to the solutes… decreases the free energy of the water
molecules… decreases the water potential.
• More solute –> v water potential
• Less solute –> ^ (less negative) water potential.
• OSMOSIS: water moves from areas of higher water potential to
areas of lower water potential.
o Water potential tends to be lower inside cells (contains more
solute) .˚. water is drawn into the roots
- determinants of water potential: pressure.
• The more pressure there is on the water… the more free energy it
has… it is under positive pressure and will have positive water
• If water is under negative pressure (tension)… water has less
potential energy and will have a negative water potential.
Water potential= solute potential + pressure potential
Turgor pressure: the pressure exerted on the cell wall of plants (because
the cell wall keeps plant cells turgid).
Proton pump: pump that moves hydrogen ions across membranes and
pushes hydrogen ions across the plasma membrane from the cytoplasm to
the cell exterior .
Symport: the transport of two molecules in the same direction across a
Antiport: a secondary active transport mechanism in which a molecule
moves through a membrane channel into a cell and powers the active
transport of a second molecule out of the cell.(exchange diffusion)
The cell regularly pumps and moves molecules and solutes and ions in an
out of the cell… As the solutes are moved into the cell (and decrease the water potential) the
water from outside the cell with follow.
Symplastic Pathway: water passes into and through living cells. After
being taken up into root hairs , water diffuses through the cytoplasm and
passes from one living cell to the next through plasmodemata
Apoplastic pathway: water moves through the nonliving regions – the
continuous network of adjoining cell walls and tissue air spaces (in-between
the cell wall and plasma membrane); must pass into the cytoplasm to cross
• The endodermis is the inner most layer of the cortex… endodermal
cells are tightly packed and also has a ribbon like Casparian strip
in its radial and transverse walls, positioned somewhat like a ribbon
of packing tape around a rectangular package.
• The Casparian strip is impermeable to water… all water is foces to
take a symplastic pathway through the cytoplasm.
• Casparian strip helps control which substances enter and leave the
plant vascular tissue.
Root epidermal cells actively transport ions inward via transport proteins
Moves through the cortex via either symplastic or apoplastic pathway…
-once the ion reaches the stele, it diffuses from cell to cell until it is “loaded”
into the xylem (via apoplastic pathway)
28.3 Long Distance Transport of Water and Minerals in the Xylem
Bulk Flow: the mass movement of molecules in response to difference in
pressure between two locations.
⋅ Water and minerals (xylem sap) move up the xylem by bulk flow (cells