What evidence shows that water moves by root pressure in a plant?
- When the stem of a plant is cut water continues to exude from the xylem vessels of the plant stem.
- The continuous exudation of water from the xylem vessels of the cut stem is due to root pressure because the leafy shoot is cut off, meaning that water not only moves upwards by transpiration pull, but also due to pressure and other forces.
What is a shortcoming of root pressure?
Not sufficient to push water to the leaves
Describe the transpiration pull (cohesion-tension theory) mechanism
- Water is removed from the plant leaves by transpiration which creates a tension within the leaf xylem vessels that pulls water in the xylem tubes upwards in a single unbroken column or string held together by the cohesive forces of attraction between water molecules.
- According to the cohesion-tension theory, evaporation of water from the mesophyll cells of the leaf to the sub-stomatal air chamber and eventually to the atmosphere via the stomata by transpiration, is responsible for the rising of water from the roots to the leaves.
- This is because the evaporated water molecules get replaced by neighbouring water molecules which in turn attract their other neighbours and this attraction continues until the root is reached.
- Evaporation of water results in a reduced water potential in the cells next to the leaf xylem.
- Water therefore enters these mesophyll cells by osmosis from the xylem sap which has the higher water potential.
- Once in the mesophyll cells water moves using the three pathways namely; apoplast, Symplast and vacuolar pathways from one cell to another by osmosis across a water gradient.
- When water leaves the leaf xylem to the mesophyll cells by osmosis, a tension is developed within the xylem tubes of water which is transmitted to the roots by cohesive forces of water molecules.
- The tension develops in the xylem vessels and builds up to a force capable of pulling the whole column of water molecules upwards by means of mass flow and water enters the base of these columns from neighbouring root cells.
- Because such a force is due to water loss by osmosis by transpiration, it is referred to as transpiration pull.
- The upward movement of water through the xylem tissue from the roots to leaves is also facilitated by the cohesive forces of attraction which holds the water molecules firmly together, due to the hydrogen bonds which exist between them.
- This enables water to have a high tensile strength which enables it to move upwards in a continuous stream without breaking.
- In addition, the upward movement of water from roots to leaves is also facilitated by adhesive forces which hold the water molecules on the xylem walls so that it continues moving upwards.
Which mechanism offers an explanation for the continuous flow of water upwards through the xylem of the plant?
Transpiration pull
Describe the capillarity mechanism of water movement up thw xylem
- Since the water rises upwards through narrow tubes, it is also facilitated by capillarity through the stem.
- This is because the xylem vessels are too narrow and the flow of water is maintained without breaking by both the cohesive and adhesive forces.
What is a shortcoming of capillarity?
- Maximum height water can travel is only 3 meters even in the tallest of trees
Differentiate between cohesion and adhesion forces
Adhesion is the force of attraction between molecules of different substances while cohesion is the force of attraction between molecules of the same substance.
Define translocation
Translocation is the movement of mineral salts and chemical compounds within a plant.
What are the two main processes of translocation?
a. The uptake of soluble minerals from the soil and their passage upwards from the roots to the various organs via the xylem tubes.
b. The transfer of organic compounds synthesized by the leaves both upwards and downwards to various organs via the phloem tubes
Describe the mechanism of mineral ion uptake
Minerals such as nitrates, phosphates, sulphates e.t.c. may be absorbed either actively or passively.
- Active absorption of minerals
- Most minerals are absorbed from the soil solution having the less mineral concentration into the root hairs with the higher mineral concentration, selectively by using active transport which uses a lot of energy.
- The rate of active absorption of minerals into the root hairs depends on the rate of root respiration.
Factors such as oxygen supply and temperature will affect the rate of ion uptake. The addition of respiratory poison has shown to inhibit uptake of mineral ions.
- Passive absorption
- If the concentration of a mineral in a soil solution is greater than its concentration in the root hair cell, the mineral may enter the root hair cell by diffusion.
- Mass flow or diffusion occurs once the minerals are absorbed by the root hairs so that they move along cell walls (apoplast pathway).
- In mass flow, the mineral ions are carried along in solution by water being pulled upwards in the plant in the transpiration stream, due to the transpiration pull i.e. the mineral ions dissolve in water and move within the water columns being pulled upwards.
- The mineral ions can also move from one cell of the root to another against the concentration gradient by using energy inform of ATP.
- The mineral ions can also move through the Symplast pathway i.e. from one cell cytoplasm to another.
- When the minerals reach the endodermis of the root, the Casparian strip prevents their further movement along the cell walls (apoplast pathway).
- Instead the mineral ions enter the cytoplasm of the cell (Symplast pathway) where they are mainly pumped by active transport into the xylem tissues and also by diffusion to the xylem tissues.
- Once in the xylem, the minerals are carried up the plant by means of the transpiration stream.
- From the xylem tissues, minerals reach the places where they are utilised called sinks by diffusion and active transport i.e. the minerals move laterally (sideways) through pits in the xylem tissue to the sinks by diffusion and active transport.
What evidence shows that most mineral ions are absorbed actively by the root hairs?
a. Increase in temperature around the plant increases the rate of mineral ion uptake from the soil as it increases respiration that can provide energy for active transport
b. Treating the root with respiratory inhibitors such as potassium cyanide prevents active mineral ion uptake leaving only absorption by diffusion. This is because the rate of mineral ion uptake greatly reduces when potassium cyanide is applied to the plant.
c. Depriving the root hairs of oxygen prevents active uptake of minerals by the roots and as a result very few ions enter the plant by diffusion.
What is the evidence supporting the role of the xylem in transporting minerals?
a. The presence of mineral ions in the xylem sap i.e. many mineral ions have been found to be present in the xylem sap.
b. There’s a similarity between the rate of mineral ion transport and the rate of transpiration i.e. if there’s no transpiration, then there’s no mineral ion transport and if transpiration increases, the rate of mineral ion transport also increases.
c. There’s evidence that other solutes e.g. the dye, eosin, when applied to the plant roots, it is carried in the xylem vessels
d. By using radioactive tracers e.g. phosphorous- 32. When a plant is grown into a culture solution containing radioactive phosphorous-32, phosphorous -32 is found to have reached all the xylem vessels but not the phloem tubes. (The interpretation of these elements is that where lateral transfer of minerals can take place minerals pass from the xylem to the phloem and where lateral transfer is prevented, the transport of minerals takes place in the xylem)
What evidence supports that organic molecules of photosynthesis are transported in the phloem?
a. When the phloem is cut, the sap which exudes out of it is rich in organic food materials especially sucrose and amino acids.
b. The sugar content of the phloem varies in relation to environmental conditions. When the conditions favor photosynthesis, the concentration of the sugar in the phloem increases and when they not favor photosynthesis and concentration of the sugar in the phloem reduces.
c. Removal of a complete ring of phloem around the phloem causes an accumulation of sugar around the ring, which results into the swelling of the stem above the ring. This indicates that the downward movement of the sugars has been interrupted and results into the part below the ring failing to grow and may dry out. This is called the ringing experiment.
d. The use of radioactive tracers. If radioactive carbon dioxide-14 is given to plants as a photosynthetic substrate, the sugars later found in the phloem contain carbon-14. When the phloem and the xylem are separated by waxed paper, the carbon-14 is found to be almost entirely in the phloem.
e. Aphids have needle like proboscis with which they penetrate the phloem so as to suck the sugars. If a feeding aphid is anaesthetized using carbon dioxide or any other chemical e.g. chloroform and then its mouth parts cut from the main body, some tiny tubes called the proboscis remain fixed within the phloem sieve tubes from which samples of the phloem content exudes.
State the three theories of mechanisms of translocation of organic materials in the phloem
1. The mass flow or pressure flow hypothesis (i.e. Much’s hypothesis)
2. Electro-osmosis
3. Cytoplasmic streaming
What is mass flow?
Mass flow is the movement of large quantities of water and solutes in the same directions.
Under what conditions does mass flow occur?
- Solute addition at the source and solute removal at the sink
- Hydrostatic pressure gradient between the two points
- Continuous pathway for flow of solutes and water
Is mass flow active or passive?
Passive
Describe the mass flow or pressure flow hypothesis
- Leaves make sucrose during photosynthesis which accumulates and lowers the water potential of the photosynthesizing cells in the leaf
- Sucrose is actively pumped into the phloem sieve cells of the leaf.
- As a result, water brought to the leaf via the xylem enters by osmosis which increases the pressure potential of the leaf cells.
- The food solution in the sieve tubes then moves by mass flow from a region of higher water potential in the leaves to that of lower water potential in the sink such as roots following a hydrostatic pressure gradient.
- At the other parts of the plant which form the sink sucrose is either being utilized as a respiratory substrate
- The soluble content of the sink cells therefore is low and this gives them a higher water potential and consequently lower pressure potential exists between the source (leaves) and the sink such as roots and other tissues
- The sink and the source are linked by the phloem sieve tubes and as a result the solution flows from the leaves to other tissues (sinks) along the sieve tube elements.
What evidence supports the mass flow theory?
1. When the phloem is cut, the sap exudes out of it by mass flow
2. There’s rapid and confirmed exudation of the phloem’s sap from the cut mouth parts of the aphids which shows that the content of the sieve tubes move out at high pressure.
3. Most researchers have observed mass flow in microscopic sections of the sieve tube elements.
4. There’s some evidence of concentration gradient of sucrose and other materials with high concentration in the leaves and lower concentration in the roots.
5. Any process that can reduce the rate of
photosynthesis indirectly reduces the rate of translocation of food.
6. Certain viruses are removed from the phloem in the phloem translocation stream indicating that mass flow rather than diffusion, since the virus is incapable of locomotion.
What are the criticisms of mass flow?
1. By this method all organic solutes would be expected to move in the same direction and at the same speed. It was however observed that the organic solutes move in different directions and at different speeds.
2. The phloem has a relatively high rate of oxygen consumption which this theory does not explain.
3. When a metabolic poison such as potassium cyanide enters the phloem, the rate of translocation is greatly reduced, implying that translocation is not a passive process, but an active one.
4. The mass flow hypothesis does not mention any translocation of solutes with influence of transfer cells and Indole Acetic Acid (IAA) hormone that loads the sugars or solutes into the sieve tubes and also unload it into the cells of the sink.
5. The sieve plates offer a resistance which is greater than what could be overcome by the pressure potential of the phloem sap. This implies that the pressure would sweep away the sieve plates during this transport.
6. Higher pressure potential is required to squeeze the sap through the partially blocked pores in the sieve plates than the pressure which has been found in the sieve tubes
Define Electro-Osmosis
This is the passage of water across a charged membrane.
Describe electro-osmosis mechanism of movement of organic compounds in plants
- The sieve plate is charged because positively charged ions e.g. K+ , actively pumped by the companion cells across the sieve plate into the sieve tube element using energy from ATP of the companion cells.
- Potassium ions accumulate on the upper side of the sieve plate thereby making it positively charged.
- Negatively charged ions accumulate on the lower sides of the sieve plate thereby making it negatively charged.
- The positive potential above the sieve plate is further increased by hydrogen ions, actively pumped from the wall to the upper sieve tube element into its cytoplasm.
- Organic solutes such as sucrose are transported across the sieve plates due to an electrical potential difference between the upper and the lower side of the sieve plate whereby the lower side is more negative than the upper side
- solutes move from the upper sieve tube element which is positively charged to the lower sieve element which is negatively charged.
- The electrical potential difference is maintained across the plate by active pumping of positive ions, mainly potassium ions, in an upward direction.
- The energy used is produced by the companion cells.
- The movement of K+ ions through the pores of the sieve plates rapidly draws molecules of water and dissolved solutes through the sieve pores, to enter the lower cell.
What evidence supports the electro-osmosis theory?
1. K+ ions stimulate the loading of the phloem in the leaves with sugars during photosynthesis.
2. Numerous mitochondria produce a lot of energy for translocation, an indicator that translocation is an active process. If however, the phloem tissues are treated with a metabolic poison, the rate of translocation reduces.
3. Presence of a sieve plate where a potential difference can be developed across the plate
Describe the cytoplasmic streaming theory
- This suggests that the protoplasm circulates using energy from sieve tubes elements or companion cells through the sieve tube elements from cell to cell via the sieve pores of the sieve plates.
- As the protoplasm circulates, it carries the whole range of the transported organic materials with it. - The solutes are moved in both directions along the trans-cellular strands by peristaltic waves of contraction, such that they move from one sieve tube element to another using energy in from of ATP.
- The proteins in the strands contract in a wave form, pushing the solutes from one sieve tube element to another, using energy in form of ATP.
What evidence supports the cytoplasmic streaming theory?
- It has been found that the solute materials move in both directions in the phloem tissue
- The theory explains the existence of the trans- cellular strands in the phloem tissue as well as many mitochondria in the companion cells
What are the criticisms of the Cytoplasmic Streaming Theory?
1. Cytoplasmic streaming has not been reported in mature sieve tube elements but only in young sieve tubes.
2. The rate at which the protoplasm streams is far slower than the rate of translocation
What is the significance of the casparian strip?
- It actively pump salts (ions) from the cytoplasm of the endodermal cells into the xylem vessels which creates a high solute concentration in the xylem, thereby greatly lowering the water potential in the xylem than in the endodermis.
- This makes the water potential of the xylem vessels more negative (very low) and results into rapid osmotic flow of water from the endodermal cells to the xylem vessels, due to the steep water potential gradient between the endodermal cells and the xylem vessels.
- The casparian strip facilitates the pushing of water upwards through the xylem vessels by root pressure up to the leaves due to its active pumping of the salts.
- In addition, this active pumping of the salts into the xylem vessels prevents leakage of slats (ions) out of the xylem vessels so as to maintain a low water potential in this vessel.
What is the main transport carbohydrate in plants?
Sucrose
What are the adaptations of the xylem to their function?
- Vessels have narrow lumen; increasing capillarity forces
- (Vessels)Bordered pits; lateral flow of water
- (Tracheids) bordered pits; reduce resistance to flow of water where end walls are present
- Joined end to end; continuous flow of water
- (Vessels) End walls broken down; uninterrupted flow of water
- (Tracheids & vessels) Long; transport of water over long distances
- Lignified; prevent escape/ loss of water
- Lignified; rigid to prevent collapsing under tension forces
- Xylem vessels hollow; minimize resistance as water moves
What are the three patterns of thickening?
- Annular
- Spiral
- Reticulate
What are the adaptations of the phloem to their function?
- Sieve plates; perforated to enable continuous movement of food
- Plasmodesmata; lateral movement of organic substances in sieve tubes
- (Companion cells) Numerous mitochondria; energy for loading food onto phloem
- (Mature sieve tubes) No nucleus; space for translocation
- Elongated; transport food over long distances
- Fibres; mechanical support
Give similarities between xylem and phloem
- Both conducting tissues
- Both elongated
- Both have parenchyma
- Perforated
- Fibres
- Tubular
What is a bordered pit?
- An area in the walls of xylem vessels and tracheids that is unlignified and has numerous plasmodesmata
Describe the structure of a bordered pit
- Many plasmodesmata
- Unlignified
- Perforated
What is the use of the casparian strip?
- Prevents movement of water by apoplast pathway
- Prevents mineral salts from leaking into the endodermis
Describe the process of water uptake in root hair cells
- Active transport
- Water potential of root hair cell sap lowers below that of the soil solution
- Water moves by osmosis
- Water entering the root hair cells increases the water potential of root hair cell sap above that of the cells of the cortex
- Water moves into the cortex by apoplast, symplast and vacuolar pathways
- Water entering the cortex cells increases the water potential of cortex cell sap above that of the cells of the endodermal cells
- Water moves into the endodermis by the same three means
- Endodermal cells secrete mineral salts into the xylem by active transport thus water potential of xylem cells lowers below that of the endodermal cells
- Endodermal cells have suberin deposits that form the casperian strip which prevents apoplast as well as diffusion of mineral salts back into endodermal cells
- Therefore water moves into xylem by symplast and vacuolar only
What are the adaptations of xerophytes to reducing transpiration?
- Waxy cuticle (prevents water loss)
- Shiny cuticle (reflects light)
- Few stomata on upper epidermis(reduced exposure to direct light)
- Sunken stomata(high humidity build up to reduce water vapor pressure)
- Reversal of stomatal opening( at night)
- Hairy leaves (trap moisture around leaves)
- Curled leaves (reduce surface area for transpiration)
- Absence of leaves (reduced surface area )
- Leaves reduced/ modified to thorns
- Shedding off leaves
- High abscissic acid concentration in leaves
- Tolerance of tissues to high temperatures
