9.2.2 Uptake of minerals from soil.
(a) The monocotyledon root has a fibrous highly branching structure which increases the surface area for the absorption of water.
(b) Dicotyledon root structure has a main tap root and often a surface branching root system for the absorption of surface run off.
Deeper in the soil the tap root branches to access deeper water and mineral.
Behind the apical meristem of the root there is a zone of differentiation of the epidermis called the root hair zone.
Root hairs increase the surface area available to the root cell to absorb water.
The extension of the cell wall increases the surface area for the absorption of water and minerals at the cellular level.
The root hair cell provides both an increase in the cell wall (apoplastic pathway) and the cytoplasmic route (symplastic pathway) for the movement of water.
Minerals move to the root system by the following pathways:
Diffusion which requires a concentration gradient (Note that in general minerals are in very low concentration in soil).
Fungal hyphae in mutualistic relationship with the plant root provides minerals such as nitrates. Fungal hyphae form a network (mycelium) that increases the surface area within the root to concentrate minerals.
Mass flow of soil water (minerals solutions).
Mass flow is a hypothesis to explain the movement of solute by means of a hydrostatic pressure gradient, not osmotic gradient.
Water is being taken up at the root which produces a negative pressure potential.
Minerals dissolved in water form hydrogen bonds with water such that the movement of water towards the root 'drags' the minerals with the water.
The mass flow of the solutions of mineral ions towards the root 'concentrates' them for absorption.
Any fertile soil contains at least some clay particles within its structure.
Clay particles carry a negative electrical charge to which the mineral ions (K+, Na+, Ca2+) attach.
This attachment effectively prevents the leaching of the mineral ions from the soil.
1. Unlike animal cell there are no potassium-sodium pumps in the cell membranes of plant cells. Rather there are proton pumps which pump protons ( H+) outside of the cell. This creates an electro-negative charge within the cell.
2. When the root cells secrete protons into the surrounding soil water the hydrogen ions displace the mineral ions from the clay particle, freeing them into solution.
3. The mineral ions in the soil water are free to be absorbed by various pathways.
Absorption of mineral ions.
The plasma membrane of the plant cell can bring about the absorption of mineral by two different energy demanding processes:
Indirect method in which proton pumps (hydrogen pumps) establish electrochemical gradients
Direct method in which membranes actively transport a particular mineral.
Proton (hydrogen) pumps in the plasma membrane pump out hydrogen ions (H+) this has a number of effects which are covered in the model below.
1. Hydrogen pumps of the plasma membrane actively pump hydrogen ions out into the soil water. This creates a membrane potential of -120 mV.
2. Hydrogen ions combine with anions such as Cl- in W or membrane carriers allow the uptake of the ion against the electrochemical gradient(3.).
4 H+ displace cations (e.g. K+) from the clay particles so that they are free to move down an electrochemical gradient by facilitated diffusion (5).
Note that both these pathways still rely on the initial active (needs energy) step of proton pumping.
Direct method of active mineral absorption:
The cations such as K+which are free and in solution in the soil water can be taken up actively by active transport membrane pumps.
Specific membrane pumps exist for the different cations.
Experiments that metabolically poison the root (stop ATP production) causes all mineral absorption to stop.
Students will be familiar with the methods of support in animals such as endoskeletons, exoskeletons and hydrostatic skeleton These various supports are characteristic of a mechanical structure which has evolved for movement. Plants are however more 'architectural' in their structure with adaptations which provide support for a static structure, much in the same way as seen in buildings.
Plants can support the mass of their tissues through:
Thickening of the cellulose cell wall.
Note the extra thickening of the cells towards the outer (lower) sections of this stem section.
Lignified xylem vessels
In the diagram to the far left the xylem shows a cylinder of cellulose cell wall with annular lignification in rings.
The photograph to the right show the thickening of the cellulose walls of the xylem.
Support in plants: All the example above contribute to support in plants. In the following diagram we see the spatial arrangement of these cells in various tissues.
The stem acts like a cylinder with extra thickened tissue of chollenchyma tissue just inside the stem epidermis(see first diagram in this section). Sometimes the cylinder principle is taken further by the degeneration of the pith tissue to create a hollow stem.
The vascular bundles allow the stem to flex under lateral pressure (wind) with the bundles dissipating the shearing forces. This principle is copied in the construction of buildings in which concrete is reinforced with steel rods.
Tissues like the sclerenchyma and xylem have additional thickening in the cell wall.The sclerenchyma tissue has so much thickening that the cytoplasm content of the cell is very much reduced.
Turgor Pressure: Support for plants generated by wall pressures
a) Water enters the cell by osmosis from the higher osmotic potential (solute potential) to the lower osmotic potential (solute potential).
b) The volume of the cell cytoplasm increases forcing the plasma membrane outwards against the cell wall. A pressure develops called the turgor pressure (pressure potential) , which is excerpted against the cell wall.
c) The outward pressure is matched by an inward pressure, equal in magnitude but opposite in direction
d) These pressures are called turgor pressures and provide mechanical support to the plant tissue. If a plant experiences a lack of water the cell becomes plasmolysed, wall pressure is lost and the plant wilts
Transpiration is the loss of water form the leaves and stem of plants.
summary: There is a gradient of negative pressure potential from the stomatal pore, through the leaf and down the xylem.
The leaf adsorbs light on it large surface area.
Heat is produced.
Water in the spongy mesophyll tissue enters the vapour phase.
There is a humidity gradient between the spongy mesophyll which is saturated with water vapour and the surrounding air.
Water evaporates through the stomatal pore down a humidity gradient.
The evaporation of water draws (pulls) more water by mass flow into the spongy mesophyll space.
Water molecules are held together cohesion due to hydrogen bonds between water molecules.
In turn this draws water from the end of the xylem by the same cohesion.
Water is therefore drawn up the stem by cohesion between water molecules and adhesion to the xylem vessel walls.
This transpiration 'pull or tension' extends all the way down the xylem to the root.
Uptake of water beginning in the roots:
Water enter the root by osmosis because the soil water has a lower solute concentration of minerals than the epidermal cell cytoplasm (there is a water potential gradient).
Water movement across the cortex cell is by two pathways both involving a water potential gradient. The cortex cell cytoplasm has a solute concentration gradient. This moves water symplastically from cell to cell by osmosis. The Apoplastic pathway moves water by capillary action of mass flow through the connecting cellulose cell wall.
The endodermis marks the beginning of the central stele of vascular tissue. Both minerals and water must pass through the plasma membrane of the endodermis.
(a) Water enters epidermal cell cytoplasm by osmosis. The solute concentration is lower than that of soil water due to the active transport of minerals from the soil water to the cytoplasm.
Symplastic Pathway (b) to (d): water moves along a solute concentration gradient. There are small cytoplasmic connections between plant cells called plasmodesmata. In effect making one large continuous cytoplasm.
Apoplastic Pathway(e) to (f):water moves by capillarity through the cellulose cell walls. Hydrogen bonding maintains a cohesion between water molecules which also adhere to the cellulose fibres.
(g) The endodermis is the outer tissue of the vascular root tissue.
The casparian strip of the endodermis is a barrier to the movement of water of minerals by the apoplastic pathway. All solute and water must move through the plasma membrane of the endodermal cells before entering the stele.
The cellulose cell wall contains a strip (casparian strip) of a waxy water repellant substance called suberin.
The suberin prevents water and dissolved minerals from passing into the xylem by the apoplastic pathway.
Therefore water solution must pass through the plasma membrane of the endodermis. The endodermis plasma membrane can then selectively control mineral uptake and rate of uptake.
Minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel. Pressure within the xylem increases forcing water upward (Root Pressure). This is probably not a major factor in transpiration of large plants.
Loading water into the xylem:
Minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel.
Chloride for example is actively pumped of pericycle (or endodermal) cells.
This creates a water potential gradient that moves water passively into the xylem.
Pressure within the xylem increases forcing water upward (Root Pressure). This is probably not a major factor in transpiration
Rather the pressure potential gradient (hydrostatic pressure) based on evaporation (tension) form the leaf is responsible for the upward movement of water in the xylem.
However, consider that many plants live in very humid environments where evaporation rates may not be that great. For small plants, the process of gutation has been observed in which water weeps from the stomatal pores in liquid rather than vapour phase.
Water in the Xylem:
Xylem vessels form a continuous pipe from the root up through the stem. along petioles to the leaf.
Xylem cells are produced from the division of the cambium and then differentiation into xylem
The cytoplasm full breaks down and the end wall break down to form the pipeline
To support the cell wall extra thickening take place. This often has characteristic patterns. Some spiral some annular (as here). This extra thickening resists the 'tension' created by the rate of evaporation
Water molecules are weakly attracted to each other by hydrogen bonds (Cohesion). This action extends down the xylem creating a 'suction' effect.
There is also adhesion between water molecules and the xylem vessels
The cohesion and adhesion act together to maintain the water column all the way up from the root to the stomata.
The rapid loss of water from the leaf pulls the water column stressing the cohesion and adhesion between water molecules. This creates 'tension' within the xylem vessel sufficient to cause the walls of the xylem to be bent inwards.
In large trees this tension can be so great that the diameter of the tree can decrease during the summer months with peak rate of transpiration
The movement of water is an example of mass flow due to a negative pressure potential.
Water movement through the leaf:
Some of the light energy absorbed by the large surface area of the leaf is changed to heat.
The heat raises the temperature of the leaf and water in the spongy mesophyll tissue is changed into water vapour.
There is 100% saturation of the sub stomatal air space (b) which contrast with the very low % saturation of air with water vapour.
With the stomatal pore open this gradient operates only over one cell thickness.
Water evaporates into the air
The water loss from the leaf draws new water vapour from the spongy mesophyll (symplastic & apoplastic movement) into the sub stomatal air space.
In turn the water molecules of the mesophyll space draw water molecules from the end of the xylem (a).
Stomata ( singular Stoma ) are pores in the lower epidermis.
Each stomata is formed by two specialised Guard Cells.
The epidermis and its waxy cuticle is impermeable to carbon dioxide and water.
During the day the pore opens to allow carbon dioxide to enter for photosynthesis. However the plant will experience water loss. If the water loss is too severe the stoma will close.
During the night plants cannot photosynthesis and so the plant closes the pores thereby conserving water.
Plants have a mechanism which closes the stoma at night. However when a plant is suffering water stress (lack of water) there is another mechanism to close the stoma.
(a) dehydrated (low water potential) of the mesophyll cell causes them to release abscisic acid.
(b) Abscisic acid stimulates the stoma to close.
The greater the rate of transpiration the greater the waterloss from a plant. Most of this waterloss occurs through the stoma with other surfaces adapted to reduce such losses.
Brown & Escombe(1900) showed that although the stomatal pore is small the sum of the circumferences of the pores in a leaf effectively mean that with the pores open, there is no lower epidermis!
The rate of evaporation is proportional to the diameter of the pore not the surface area of the pore. In the diagram the sum of the transpiration of water vapour through the small pores (B) is greater than the larger pore.
At the edges the molecules of water vapour are able to fan out in many directions and avoid the gradient of water vapour concentration in the vertical plane.
Abiotic Factors affecting transpiration:
This is a measure of the water vapour in air and is normally expressed as a percentage.
Light absorbed by the leaf warms the water within the mesophyll tissue and it enters the vapour phase in the space above the pore called the sub stomatal air space (SSAS).
With the pore open water vapour diffuses out down a step water vapour gradient.
The water vapour forms diffusion shells of changing % humidity.
The steepest gradient is found at the edge of the pore where effectively most waterloss occurs.
The significance of Brown & Escombe's research now becomes evident
In high humidity the diffusion gradient is not as steep and the rate of diffusion is less.
In high humidity the diffusion shells of water vapour from one pore to the next joint and the steep gradient associated with the edge of the pore is lost.
The formation of this boundary layer of high humidity reduces the rate of transpiration.
Moving air reduces the external water vapour concentration such that the gradient between the sub stomatal air space and the surrounding air increases.
Still air allows the build up of boundary layers as shown above and so reduces the rate of transpiration.
The leaf absorbs light and some of the light energy is lost as heat. Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C. The effective evaporation is in the sub stomatal air space and increases the gradient of water vapour between the sub stomatal air space and the surrounding air.
The rate of plants transpire is faster in the light than in the dark. This is because light stimulates the opening of the stomata and warming of the leaf. The mechanism of pore opening relates to the Guard cell becoming turgid. The mechanism:
(a) The guard cell absorbs light and produces ATP in the light dependent reaction.
(b) The ATP is used to drive proton pumps that pump out H+ . The inside of the cell becomes more negative.
(c) Potassium ions enter the cell which increases the solute concentration.
(d) Water moves from the surrounding tissue by osmosis.
The turgid cell increases the pressure potential and the cell expands.
The cells have asymmetric thickening with lignin on the stoma-side wall.
The cells expand more on the outside wall.
The pore opens in the middle as each cell bends.
Plants adapted to dry environments are called Xerophytes.
Xerophytes are plants that have adaptations to reduce water loss or indeed to conserve water. They occupy habitats in which there is some kind of water stress. Examples of such water stress habitats include:
Desert (high temp, low precipitation)
High Altitude & High Latitude ( low precipitation or water locked up as snow or ice)
Rapid drainage (sand dunes)
This plant was photographed in SE Asia where it was growing very rigorously.
The temperatures in this environment has an average over 30 centigrade with a high potential for water loss.
The leaves of this plant have waxy cuticle on both the upper and lower epidermis
The waxy repels water loss through the upper and lower epidermal cells. If an epidermal cell has no cuticle water will rapidly be lost as the cellulose cell wall is not a barrier to water loss.
Notice that in the background there are a variety of other plants all of which have this xerophytic adaptation of a waxy cuticle
Rolled Leaves / Stomatal Pits / Hairs on epidermis:
This is a cross section of marram grass leaf. (click image for photograph of marram grass). This species of grass occupies much of the sand dunes habitat. They are able to tolerate the poor water retention of the soil (sand) and the drying effects of wind by limiting water loss.
Note the Thick waxy upper epidermis extends all all the way around as the leaf rolls up. This places the stomata in an enclosed space not exposed to the wind.
Note that the stomata are in pits which allows boundary layer of humidity to build up which also reduces water loss by evaporation.
The hairs on the inner surface also allow water vapour to be retained which reduces water loss through the pores.
The groove formed by the rolled leaf also acts as a channel for rain water to drain directly to the specific root of the grass stem
Needles as leaves (Firs and Pines):
Conifer distribution is often associated with the northern taiga forests of North America, Scandinavia and Russia. Like a rolled leaf, study of the internal structure shows it has effectively no lower epidermis.
This adaptation is required as northern climates have long periods in which water is actually frozen and not available for transpiration. Plants in effect experience water availability more typical of desert environments.
This type of adaptation means that confers have their distribution extended beyond the the northern forests to a variety of water stress climates. e.g. SE Asian Mountains, Mid Atlas and High Atlas mountains and very many other regions of the world.
The leaves have been reduced to needles to reduce transpiration.
The stem is fleshy in which the water is stored.
The stem becomes the main photosynthetic tissue.
CAM and C4(closing pore during day reducing water loss):
CAM: Crassulacean acid metabolism (CAM) in plants like the Stonecrop reduces water loss by opening pores at night but closing them during the day. This is a Time based alteration of biochemistry.
At night carbon dioxide is combined with phosphoenol pyruvic acid (C3) to form Oxoloacetic acid (C4).
Oxoloacetic acid is changed to malic acid or aspartic acid. This stores the carbon dioxide until required for photosynthesis during the day.
During the day the pore is closed and the malic acid degenerates to PEP (C3) and carbon dioxide.
Carbon dioxide is then used in photosynthesis.
C4: A C4 compound is temporarily stored in the spongy mesophyll (which lack Rubisco for carbon fixation)
carbon dioxide is stored in the mesophyll layer by combining with the PEP (C3) to form oxoloacetic acid and malic acid as seen before in the CAM plants.
This breaks down to provide the palisade layer (RUBISCO) with more carbon dioxide.
Therefore pores can remain open for a reduced time.
1.Translocation moves the organic molecules (sugars, amino acids) from their source through the tube system of the phloem to the sink. Phloem vessels still have cross walls called sieve plates that contain pores.
2. Companion cells actively load sucrose (soluble, not metabolically active) into the phloem.
3. Water follows the high solute in the phloem by osmosis. A positive pressure potential develops moving the mass of phloem sap forward.
4. The sap must cross the sieve plate. Current hypothesis do not account for this feature.
5. The phloem still contains a small amount of cytoplasm along the walls but the organelle content is greatly reduced.
6. Companion cells actively unload (ATP used) the organic molecules
7. Organic molecules are stored (sucrose as starch, insoluble) at the sink. Water is released and recycled in xylem.
Sources and Sinks:
Plants will not transport glucose as it is used directly in respiration and is metabolically active. Sucrose is soluble and transportable but not metabolically active in respiration. At the sink it is necessary to have the transported molecule insoluble (no osmotic effect) and inactive ( no respiration effect).
Sources could be a photosynthesising leaf or a storage region at the beginning of a growing season (Tuber)
Sinks could be the growth point on a stem or the storage area at the end of a growing season (tuber)
The hypothesis does not account for how the direction of sap travel could be reversed nor does it explain how the resistance of the sieve plate could be overcome.
Combined models of translocation and transpiration:
1. Source produces organic molecules
2. Glucose from photosynthesis produced
3.Glucose converted to sucrose for transport
4. Companion cell actively loads the sucrose
5. Water follows from xylem by osmosis
6. Sap volume and pressure increased to give Mass flow
7. Unload the organic molecules by the companion cell
8. Sucrose stored as the insoluble and unreactive starch
9. Water that is released is picked up by the xylem
10. water recycles as part of transpiration to re supply the sucrose loading
9.2.2 Uptake of minerals from soil.