Transport Processes
Diffusion, Osmosis, and Active Transport
Water Potential
Short-Distance Intercellular Transport
Long-Distance Transport: Phloem
Long-Distance Transport: Xylem
Cells and Water Movement
Guard Cells
Motor Cells
Transfer Cells
Properties of Water
Water Transport Through Xylem
Control of Water Transport by Guard Cells
Diffusion is when the random movement of particles in solution causes them to move from areas of high concentration to areas of low concentration.
Diffusion through a membrane is known as osmosis
Three types of membranes:
Completely Impermeable
Differentially or Selectively Permeable
Freely Permeable
Allow all solutes to diffuse through them and have little biological significance.
Do not allow anything to pass through and occur as isolation barriers
Allow only certain substances to pass though
All lipid/protein cell membranes are differentially permeable.
Water molecules pass through all membranes, but they diffuse faster if the membrane has protein channels called aquaporins
Active Transport is when a membrane uses membrane-bound molecular pumps that use energy of ATP to force molecules across the membrane.
Water has free energy, for most chemicals this is called chemical potential.
In botany it is called water potential
Pronounced "Sigh"
Pressure potential is the effect that pressure has on water potential
Megapascals (MPa) or bars, is the units used to measure pressure
"Sigh Pie" is the osmotic potential, which is the effect that solutes have on water potential.
Matric potential is water's adhesion to nondissolved structures such as:
Cell Walls
Membranes
Soil Particles
Adhesion only decreases water's free energy. Matric potential is always negative.
Lysis
Animal cells that burst if placed in pure water.
Plant cells can never burst, the cell grows rather than bursts.
Incipient Plasmolysis is the point at which the protoplast has lost just enough water to pull slightly away from the wall.
If the cell has not reached equilibrium at the point of incipient plasmolysis and continues to lose water the protoplast will pull away from the wall and shrink.
The cell has become plasmolyzed.
Symplast is when all of the protoplasm of one plant can be considered one continuous mass
Apoplast is wall and the intercellular spaces of the plant
The opening and closing of stomatal pores are based on short-distance intercellular transport.
At night, guard cells are somewhat shrunken and have little internal pressure, except in CAM plants.
Can either accumulate or expel potassium and thus adjust their water potential and turgidity
Are located along the midrib, when they are shrunken pressure in other midrib cells causes the two halves of the blade to be appressed, the trap is closed.
In certain transfer cells, the walls are smooth on the outer surface but have numerous finger-like and ridge-like outgrowths on the inner surface.
Transfer cells are found in areas where rapid short-distance transport is expected to occur:
In regions where sugar is loaded into or out of phloem
In glands that secrete salt
In areas that pass nutrients to embryos
Pressure flow hypothesis
Membrane-bound molecular pumps and active transport are postulated to be the important driving forces.
Sources are the sites from which water and nutrients are transported
Sinks are sites that receive transported phloem sap, and they are extremely diverse.
Within sources of many species, sugars are actively transported into sieve elements, sieve tube members in angiosperms and sieve cells in other plants besides angiosperms
In other species, phloem is loaded by the polymer trap mechanism:
Conducting-cell plasma membranes are permeable to monosaccharides and disaccharides but not to polysaccharides.
STM/CC complex is the common way to think of the functional unit as consisting of both a conducting cell and one or more companion cells.
The actual amount of sugars and other nutrients, besides water, transported by phloem per hour is called mass transfer.
Mass transfer can be divided by the cross-sectional area of phloem to obtain the specific mass transfer.
Two mechanisms seal broken sieve elements opened by chewing insects and larger animals.
P-protein
Within uninjured phloem there is another polymer called callose.
Found as a fine network adjacent to the plasma membrane inner surface of uninjured sieve elements.
When it is carried to a sieve are or sieve plate, the P-protein mass is too large to pass through and forms a P-protein plug.
It is present in all eudicots and many but not all monocots, conifers do not produce P-protein.
Water molecules interact strongly with other water molecules, behaving as if weakly bound together, when frozen the molecules are strongly bound to each other.
Because of this liquid water is said to be cohesive, and any force acting on one molecule acts on all neighboring ones as well.
Another property of water is that its molecules interact with many other substances, it is adhesive.
Another property of water is that it is heavy, and lifting it tot the top of a tree requires a great deal of energy.
Water also adheres firmly to soil particles.
The cohesion-tension hypothesis is the most widely accepted model of the process.
When stomatal pores are open, they allow water loss
This water loss is called transstomatal transpiration.
Water lost through the cuticle is known as transcuticular transpiration
Having a body water content that changes with habitat moisture is called poikilohydry
Many desert-adapted mosses that have no capacity to store water are able to change with their habitat
Also in some species of selaginella
Also occurs in in some liverworts
The tiny cactus Blossfeldia
And a few other vascular plants
Lichens are arguably the best examples
Bulk water movement through xylem is influenced and powered primarily by water loss to the atmosphere
Numerous mechanisms have evolved that control stomatal opening and closing.
If the leaf has an adequate moisture content, light and carbon dioxide are the normal controlling factors.
Abscisic acid immediately causes guard cells to close the stomatal pore even in blue light and low concentrations of carbon dioxide