- Mechanisms of Phloem Transport
- Flow from Source to Sink
- The Pressure Flow Hypothesis
Root PressureAs various ions from the soil are actively secreted into the root's vascular tissue water follows (its potential gradient) and osmotic pressure increases.
This osmotic pressure is called root pressure.
Root pressure is observable at night when evaporation is low and excess water collects in droplets around special openings near the tip of grass blades.
Such water loss in its liquid phase is known as guttation.
Note: the root endoderm because of a waxy layer of subrin has the ability to actively transport ions in one direction only.
Root pressure can only provide a modest push in the overall process of water transport. Its greatest contribution maybe to reestablish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
Water Potential and Vascular PlantsWhen a water potential gradient is established between two areas, water will spontaneously diffuse from the high end (soil) to the low end (air). This gradient is necessary for plants to transport water.
Water potential may be established by:
- increasing the concentration of solutes. Pure water has the highest potential while a saturated solution of ions etc. would have the lowest potential.
- converting water to a gas. Water potential is highest when water is a liquid and lowest when water is a gas in air.
Visit Biology 184 or an essay by Anne Bruce for a through explanation of Water Transport
See below for my explanation of water transport.
Leaves: Transpiration and Pulling of WaterPhotosynthesis requires water. The system of xylem vessels from root to leaf vein can supply the needed water.
What force does a plant use to move water molecules into the leaf parenchyma cells where they are needed? Read on!
Several forces combine to overcome the pull of gravity.
These combined forces culminate in a process called transpiration.
Ultimately water is pulled, molecule by molecule into the leaf. The pulling forces and energy needed involves:
- free energy of the water potential gradient
- free energy of evaporation
- force of surface tension
- force of hydrogen bonding between water molecules
Each force can be communicated to the next because water forms a strong continuous chain from root to leaf.
- Water moves in the direction it does (root to leaf) because of the water potential gradient. The gradient is highest in the water surrounding the roots and lowest in the air space within the spongy parenchyma of the leaf. (liquids have higher potential than gases and the purer the liquid the higher its potential)
- The energy of evaporation is needed to to pull molecules away from the film of water coating air spaces within the spongy parenchyma. (see diagram below)
- As more molecules evaporate from the film coating the air spaces the curvature of the meniscus increases which increases the surface tension. Water from surrounding cells and air spaces will then be pulled towards this area to reduce the tension.
- Finally these forces are communicated to water molecules within the xylem because each water molecule is bound to the next by hydrogen bonds.
Water Potential and the leaf
Evaporation from the leaf sets up a water potential gradient between the outside air and the leaf's air spaces. The gradient is transmitted into the photosynthetic cells and on to the water-filled xylem in the leaf vein.
Measurements reveal that the forces generated by transpiration can create pressures up to 12 atmospheres, sufficient to lift a xylem sized column of water over 350 feet high (130 meters).
Water Movement in Xylem through TACT Mechanism
Four important forces combine to transport water solutions from the roots, through the xylem elements, and into the leaves. These TACT forces are:
Transpiration involves the pulling of water up through the xylem of a plant utilizing the energy of evaporation and the tensile strength of water. The previous section describes transpiration more fully.
Adhesion is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.
Cohesion is the attractive force between molecules of the same substance. Water has an unusually high cohesive force again due to the 4 hydrogen bonds each water molecule potentially has with any other water molecule. It is estimated that water's cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter.
A combination of adhesion, cohesion, and surface tension (see below) allow water to climb the walls of small diameter tubes like xylem. This is called capillary action. The U shaped surface formed by water as it climbs the walls of the tube is called a meniscus.
Tension can be thought of as a stress placed on an object by a pulling force. This pulling force is created by the surface tension which develops in the leaf's air spaces.
As water molecules leave the surface film by evaporating into the air spaces the remaining film forms menisci (a, b, c) which become more and more concave. A meniscus has a tension that is inversely proportional to the radius of the curved water surface.
In other words, as the water surface becomes more curved tension increases. "Tension is a negative pressure -- a force that pulls water from locations where the water potential is greater." Campbell
The bulk flow of water to the top of a plant is driven by solar energy since evaporation from leaves is responsible for transpiration pull.
Water Transport in the RootThe flow of water and minerals from the soil to the cells of the root is accomplished by transpirational pull, active transport and a special layer of cells called the casparian strip.
Active transport establishes a lower water potential and helps the root hairs take in the necessary minerals dissolved in soil water. A lower water potential allows water to be drawn into the root cells by osmosis.
In order to regulate the quantity and type of minerals and ions reach the xylem, the root has a waxy layer between the endodermis and pericycle called the casparian strip. Water and mineral normally can travel through the porous cell walls of the root cortex -- this is the apoplastic route. But in order for water and minerals to reach the stele (xylem) the highly regulated (cytoplasmic) symplastic route must be taken. The apoplastic route is blocked by the casparian strip.
The symplastic route involves special openings between adjacent cell walls called plasmodesmata. (see below)
Paths that solutions can take through plant cells
Symplastic vs Apoplastic paths
Transpiration and photosynthesis - a compromiseAn actively photosynthesizing plant has an insatiable need for water. The efficiency of photosynthesis increases as the surface area for CO2, the second reactant, increases. This is the purpose of the spongy mesophyll which is honeycombed with air sacs. The spongy mesophyll has 10 to 30 times more surface area than the corresponding external surface of a leaf.
Photosynthesis is limited by available water which can be swiftly depleted by transpiration. It has been estimated that for every gram of CO2 fixed by the Calvin cycle, anywhere from 300 to 600 grams of water escape through transpiration. The humidity of rainforests is due in large part to this vast cycling of water from root to leaf to atmosphere and back to the soil.
Transpiration has more that one purpose however, it:
- supplies water for photosynthesis
- transports minerals from the soil to all parts of the plant
- cools leaf surfaces some 10 to 15 degrees by evaporative cooling
- maintains the plant's shape and structure by keeping cells turgid
Guard Cells and Water Transport
- The physical structure of guard cells
A stoma is a physical gap between two special epidermal cells called guard cells. When the pair of guard cells are turgid -- full of water -- they bow in such a way as to increase the gap -- stoma -- between them.
If the plant experiences water deprivation it will wilt. To compensate the guard cells become flaccid and the stoma is closed.
The structure of guard cells explains why they bow apart when turgid.
- The two guard cells are fused at their ends.
- The inner cell walls which form the stoma are thicker than the outer walls.
- Cellulose microfibrils are oriented radially rather than longitudinally.
- The sequence of events which result in stomatal opening
The immediate cause is an increase in turgor pressure - water enters the central vacuole by osmosis
Turgor pressure increases because of a negative water potential due to an influx of potassium ions (K+). The cell becomes hypertonic to its environment
The reversible uptake of K+ ions takes place because of the membrane potential created when H+ are actively pumped out of the cell - consuming ATP. The cell's interior becomes negative compared to the surroundings.
The stoma is closed at night when the large
central vacuole is isotonic, even hypotonic to
surrounding fluids. K+ ions are outside
of the cell, and H+ ions by and large
remain attached to the weak organic acids within
the cell. Blue light is absorbed by a membrane protein
which somehow causes an increase in the activity of
proton pumps which use ATP to transport
H+ out of the cell. With H+ on the outside K+
readily diffuse into the cell to compensate for the
negative electrical potential. The hypertonic
conditions within the cell attract water molecules
and the stoma opens as turgor pressure
The stoma is closed at night when the large central vacuole is isotonic, even hypotonic to surrounding fluids. K+ ions are outside of the cell, and H+ ions by and large remain attached to the weak organic acids within the cell.
Blue light is absorbed by a membrane protein which somehow causes an increase in the activity of proton pumps which use ATP to transport H+ out of the cell.
With H+ on the outside K+ readily diffuse into the cell to compensate for the negative electrical potential. The hypertonic conditions within the cell attract water molecules and the stoma opens as turgor pressure increases.
- What factors trigger the change of shape in guard cells
- increase in blue light at dawn - a blue light sensitive receptor activates proton pumps and turgor pressure increases. Light also stimulates the photosynthetic production of ATP
- Absence of CO2
- Circadian rhythms - All eukaryotic cells have chemical based metabolic clocks entrained to the day-night cycle. A common 24 hour biological clock is responsible for circadian rhythms (circa, about - dies, day)
Strategies for maximizing the availability of CO2 while minimizing water loss have evolved in land plants.
- C4 Photosynthesis
C4 plants are twice as efficient as C3 varieties in terms of fixing carbon (making sugar). A C4 plant will lose "only" 300 grams of water by evaporation for every gram of CO2 fixed by photosynthesis whereas C3 plants lose 600 grams of water for the same grams of CO2 fixed.
- CAM Photosynthesis
A CAM plant is adapted to the hot dry conditions prevalent in desert climes. These plants have a unique strategy in which their stomata remain closed most of the day when water loss is highest, but can maintain a healthy rate of photosynthesis even though CO2 is not supplied by gas exchange through these pores. How can this be?
These plants absorb and store CO2 at night when stomata are open. Water loss is reduced and the acids which are used to sequester the CO2 readily release it during the day as needed.
Mechanisms of Phloem Transport - Links
- Aphid Investigation
- Techniques to measure plant cell water and solute relations
- Biology 11 - Lecture 21 Fertilizers, Soil, Water
Flow from Source to SinkFood, primarily sucrose is transported by the vascular tissue called phloem from a source to a sink.
Unlike transpiration's one-way flow of water sap, food in phloem sap can be transported in any direction needed so long as there is a source of sugar and a sink able to use, store or remove the sugar.
The source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilized to become a source of food in the early spring when the buds of trees, the sink, need energy for growth and development of the photosynthetic apparatus.
Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported. The movement of such substances in the plant is called translocation.
The Pressure Flow or Mass Flow HypothesisThe accepted mechanism needed for the translocation of sugars from source to sink is called the pressure flow hypothesis. (see diagram below)
As glucose is made at the source (by photosynthesis for example) it is converted to sucrose (a dissacharide). The sugar is then moved into companion cells and into the living phloem sieve tubes by active transport. This process of loading at the source produces a hypertonic condition in the phloem.
Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds the phloem sap will move to areas of lower pressure.
At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the pholem sap and into the cells which will use the sugar -- converting it into energy, starch, or cellulose. As sugars are removed osmotic pressure decreases and water moves out of the phloem.
The movement of sugars in the phloem begins at the source, where (a) sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the movement of water into the dense phloem sap from the neighboring xylem (b). As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins (c), and the sap moves through the phloem. Meanwhile, at the sink (d), incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out (e), returning eventually to the xylem.
Updated June 20, 2002