9.2.1 Root systems for the uptake of water and minerals.

roots

(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.

 

 

roothair

 

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.

 

 

 

Root hairs:

 

root hairroothair

 

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.

 

 

 

 

 

top

 

 

9.2.2 Movement of minerals to the root.

Minerals move to the root system by the following pathways:

  1. Diffusion which requires a concentration gradient (Note that in general minerals are in very low concentration in soil).

  2. 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.

  3. Mass flow of soil water (minerals solutions).

top

 

 

9.2.3 Processes of mineral absorption from the soil by active transport.

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.

mineral absobtion in root

 

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 processes:

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.

 

mineral absorption

 

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:

potassium absorption in root

 

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.

 

 

 

top

9.2.4 Plant support.

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:

cell wall

Note the extra thickening of the cells towards the outer (lower) sections of this stem section.

 

 

 

 

 

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.

stem structure support

 

 

 

turgid

Turgor Pressure: Support for plants generated by wall pressures

top

 

 

 

9.2.5 Definition of Transpiration

Transpiration is the loss of water form the leaves and stem of plants.

transpiration

 

top

9.2.6 Cohesion tension theory

summary: There is a gradient of negative pressure potential from the stomatal pore, through the leaf and down the xylem.

transpiration

 

top

Uptake of water beginning in the roots:

 

 

apoplastic pathway

(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.

caspariam strip

 

 

 

 

Loading water into the xylem:

pericycle root

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

 

 

 

 

cohesion tension

 

 

 

Water movement through the leaf:

evaporation from the leaf

 

top

9.2.7 Guard cells and transpiration regulation.

GUARD CELLSStomata ( 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.

 

top

 

9.2.8 Abscisic hormone and guard cells.

aba

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.

 

 

 

top

 

 

 

9.2.9 Abiotic factors and the rate of transpiration.

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.

stoma area

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:

1. Humidity

This is a measure of the water vapour in air and is normally expressed as a percentage.

 

humidity transpiration

 

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

 

boundary layer

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.

 

 

 

2. Wind:

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.

3. Temperature:

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.

 

4. Light:

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:

light stoma open

(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.

 

 

 

top

 

 

9.2.10 Xerophytic adaptation to reduce transpiration.

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:

 

 

Waxy Leaves:

xerophyte waxy cuticle

 

 

 

 

 

 

 

 

Rolled Leaves / Stomatal Pits / Hairs on epidermis:

 

marram grass pits

 

Needles as leaves (Firs and Pines):

pine needleConifer 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.

 

Succulent:

cacti

 

 

 

 

 

 

 

 

CAM and C4(closing pore during day reducing water loss):

CAM

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 plantsC4: 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.

 

 

top

9.2.11 Phloem translocation

translocation 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:

 

Combined models of translocation and transpiration:

dual plnat transport model

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

 

 

top

Click4Biology: Topic 9.2 Plant Science

Transport in angiospermophytes

9.2.1 Root systems for the uptake of water and minerals.

9.2.2 Uptake of minerals from soil.

9.2.3 Processes of mineral absorption from the soil by active transport.

9.2.4 Plant support.

9.2.5 Definition of Transpiration

9.2.6 Cohesion tension theory

9.2.7 Guard cells and transpiration regulation.

9.2.8 Abscisic hormone and guard cells.

9.2.9 Abiotic factors and transpiration.

9.2.10 Xerophytic adaptation to reduce transpiration.

9.2.11 Phloem translocation.