Nature of science:
Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.
Understandings:
Basic leaf structure
Water is lost in the form of a gas from the leaf through openings called stomata (singular stoma). Transpiration is the term given to the loss of water vapour from leaves and other aerial parts of the plant. The water lost from the plant’s upper structures must be replaced by water absorption.
Leaves are the main plant organs involved in photosynthesis. They vary greatly in form, but they generally consist of a flattened portion called the blade and a stalk called the petiole that attaches the blade to the stem.
Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.
Understandings:
- Transpiration is the inevitable consequence of gas exchange in the leaf.
- Plants transport water from the roots to the leaves to replace losses from transpiration.
- The cohesive property of water and the structure of the xylem vessels allow transport under tension.
- The adhesive property of water and evaporation generate tension forces in leaf cell walls.
- Active uptake of mineral ions in the roots causes absorption of water by osmosis.
- Application: Adaptations of plants in deserts and in saline soils for water conservation.
- Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
- Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
- Skill: Measurement of transpiration rates using potometers. (Practical 7)
- Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.
Basic leaf structure
Water is lost in the form of a gas from the leaf through openings called stomata (singular stoma). Transpiration is the term given to the loss of water vapour from leaves and other aerial parts of the plant. The water lost from the plant’s upper structures must be replaced by water absorption.
Leaves are the main plant organs involved in photosynthesis. They vary greatly in form, but they generally consist of a flattened portion called the blade and a stalk called the petiole that attaches the blade to the stem.
Many leaves have a layer of
wax called the cuticle as their outermost layer. This layer protects the plant against
water loss and insect invasion. If a cuticle is not present, the outermost layer is the epidermis that protects the plant. Like stems and roots, the leaves have vascular tissue that includes xylem and phloem. The xylem brings water to the leaves, while the phloem carries the products of photosynthesis to the rest of the plant. The xylem and phloem occur together in veins or vascular bundles. A densely packed region of cylindrical cells occurs in the upper portion of the leaf. This region is called the palisade mesophyll. The cells here contain large numbers of chloroplasts to carry out photosynthesis. The bottom portion of the leaf is composed of the spongy mesophyll. This consists of loosely packed cells with few chloroplasts. There are many air spaces in this area, which provide gas exchange surfaces. Stomata or stomatal pores occur on the bottom surface of leaves, and they allow oxygen and carbon dioxide exchange between the leaf and the surrounding environment. As oxygen and carbon dioxide are exchanged at the stomata, it is an inevitable consequence that water is lost from the plant. Specialized cells called guard cells control the opening and closing of the stomata.
It is important to note the functions of tissues in relation to their position in the leaf.
Plant water and mineral movement
Transpired water has to be replaced by the intake of water at the roots. There is a continuous stream of water from the roots to the upper parts of a plant.
This stream of water through the plant provides minerals to the plant as well as the water necessary to carry out photosynthesis. The water lost by transpiration is important in cooling sun-drenched leaves and stems.
There are many factors involved in the transport of water and minerals in plants. Given the height of some plants, the transport of water from the roots to the tree top can be a mammoth task. Xylem is involved in supporting the plant as well as being the specialized water-conducting tissue of terrestrial plants.
Xylem is actually a complex tissue composed of many cell types. The two cell types largely involved in water transport are tracheids and vessel elements. Tracheids are dead cells that taper at the ends and connect to one another to form a continuous column. Vessel elements (also called vessels) are the most important xylem cells involved in water transport. They are also dead cells, and have thick, lignified secondary walls. These secondary walls are often interrupted by areas of primary wall. These primary wall areas also include pits or pores that allow water to move laterally. The vessel elements are attached end to end to form continuous columns, like the tracheids. The ends of the vessel elements have perforations in them, allowing water to move freely up the plant.
Stomata and guard cells
Stomata can only be closed on a short-term basis. This is because carbon dioxide must enter the mesophyll region of the leaf so that photosynthesis can occur. The stomata open and close because of changes in the turgor pressure of the guard cells that surround them. These guard cells are cylindrical and their cell wall thickness is uneven
wax called the cuticle as their outermost layer. This layer protects the plant against
water loss and insect invasion. If a cuticle is not present, the outermost layer is the epidermis that protects the plant. Like stems and roots, the leaves have vascular tissue that includes xylem and phloem. The xylem brings water to the leaves, while the phloem carries the products of photosynthesis to the rest of the plant. The xylem and phloem occur together in veins or vascular bundles. A densely packed region of cylindrical cells occurs in the upper portion of the leaf. This region is called the palisade mesophyll. The cells here contain large numbers of chloroplasts to carry out photosynthesis. The bottom portion of the leaf is composed of the spongy mesophyll. This consists of loosely packed cells with few chloroplasts. There are many air spaces in this area, which provide gas exchange surfaces. Stomata or stomatal pores occur on the bottom surface of leaves, and they allow oxygen and carbon dioxide exchange between the leaf and the surrounding environment. As oxygen and carbon dioxide are exchanged at the stomata, it is an inevitable consequence that water is lost from the plant. Specialized cells called guard cells control the opening and closing of the stomata.
It is important to note the functions of tissues in relation to their position in the leaf.
- The palisade mesophyll is located in the upper portion of the leaf, where light is most available. The cells of this region are chloroplast rich, to allow maximum photosynthesis.
- Veins are distributed throughout the leaf to transport raw materials and products of photosynthesis. The veins occur roughly in the middle of the leaf so that they are near all the leaf cells.
- The spongy mesophyll is located just superior to the stomata, to allow continuous channels for gas exchange.
- The stomatal pores are on the bottom surface of the leaf. This area receives less light, so as a result the temperature here is lower than on the upper surface. The lower temperature minimizes water loss from the pores and the plant, so the lower epidermis usually has a thinner cuticle than the upper epidermis. The positioning of the epidermis is such that the remaining structures of the leaf are protected and supported.
Plant water and mineral movement
Transpired water has to be replaced by the intake of water at the roots. There is a continuous stream of water from the roots to the upper parts of a plant.
This stream of water through the plant provides minerals to the plant as well as the water necessary to carry out photosynthesis. The water lost by transpiration is important in cooling sun-drenched leaves and stems.
There are many factors involved in the transport of water and minerals in plants. Given the height of some plants, the transport of water from the roots to the tree top can be a mammoth task. Xylem is involved in supporting the plant as well as being the specialized water-conducting tissue of terrestrial plants.
Xylem is actually a complex tissue composed of many cell types. The two cell types largely involved in water transport are tracheids and vessel elements. Tracheids are dead cells that taper at the ends and connect to one another to form a continuous column. Vessel elements (also called vessels) are the most important xylem cells involved in water transport. They are also dead cells, and have thick, lignified secondary walls. These secondary walls are often interrupted by areas of primary wall. These primary wall areas also include pits or pores that allow water to move laterally. The vessel elements are attached end to end to form continuous columns, like the tracheids. The ends of the vessel elements have perforations in them, allowing water to move freely up the plant.
Stomata and guard cells
Stomata can only be closed on a short-term basis. This is because carbon dioxide must enter the mesophyll region of the leaf so that photosynthesis can occur. The stomata open and close because of changes in the turgor pressure of the guard cells that surround them. These guard cells are cylindrical and their cell wall thickness is uneven
The thickened area of the guard cell wall is oriented towards the stoma. Thus when the cells take in water and swell, they bulge more to the outside. This opens the stoma. When the guard cells lose water, they sag towards each other and close the stoma.
The gain and loss of water in the guard cells is largely because of the transport of potassium ions. Light from the blue part of the light spectrum triggers the activity of adenosine triphosphate (ATP)-powered proton pumps in the plasma membrane of guard cells. This triggers the active transport of potassium into the cell. The higher solute concentration within the guard cells causes inward water movement by osmosis.
When potassium ions passively leave the cells, water also leaves. The plant hormone abscisic acid causes potassium ions to diffuse rapidly out of the guard cells. The result is stomatal closure. This hormone is produced in the roots during times of water deficiency.
Other factors, such as carbon dioxide levels and even circadian rhythms (the basic 24- hour biological clock) within plants, affect stomata opening and closing.
The cohesion–tension theory of plant fluid movement
The gain and loss of water in the guard cells is largely because of the transport of potassium ions. Light from the blue part of the light spectrum triggers the activity of adenosine triphosphate (ATP)-powered proton pumps in the plasma membrane of guard cells. This triggers the active transport of potassium into the cell. The higher solute concentration within the guard cells causes inward water movement by osmosis.
When potassium ions passively leave the cells, water also leaves. The plant hormone abscisic acid causes potassium ions to diffuse rapidly out of the guard cells. The result is stomatal closure. This hormone is produced in the roots during times of water deficiency.
Other factors, such as carbon dioxide levels and even circadian rhythms (the basic 24- hour biological clock) within plants, affect stomata opening and closing.
The cohesion–tension theory of plant fluid movement
Roots and fluid movement in plants
The main function of roots is to provide mineral ion and water uptake for the plant. Roots are efficient at this because of an extensive branching pattern, and because of some specialized epidermal structures called root hairs.
Root hairs increase the surface area over which water and mineral ions can be absorbed by a factor of nearly three. The root cap is very important in protecting the apical meristem during primary growth of the root through the soil. The three root zones indicate regions of cell development.
The main function of roots is to provide mineral ion and water uptake for the plant. Roots are efficient at this because of an extensive branching pattern, and because of some specialized epidermal structures called root hairs.
Root hairs increase the surface area over which water and mineral ions can be absorbed by a factor of nearly three. The root cap is very important in protecting the apical meristem during primary growth of the root through the soil. The three root zones indicate regions of cell development.
- The zone of cell division is where new undifferentiated cells are forming, corresponding with the M phase of the cell cycle
- The zone of elongation is where cells are enlarging in size, corresponding with the G1 phase of the cell cycle.
- The zone of maturation is where cells become a functional part of the plant.
Most of the water entering a plant comes in through the root hairs by osmosis. Once in the root, water moves to the vascular cylinder, which contains the xylem and phloem.
Mineral ions
It is essential that mineral ions move into the root as well as water. There are three major processes that allow mineral ions to pass from the soil to the root:
• diffusion of mineral ions and mass flow of water in the soil that carries these ions
• the action of fungal hyphae
• active transport.
When there is a higher concentration of a mineral dissolved in water outside the root than inside, the mineral ions may move passively, without cell ATP expenditure, into the root cells. This is an example of diffusion. Also, some of the minerals dissolved in the water will move into the roots as water moves into the outer root cells via osmosis. This passive flow of water and the minerals dissolved in it is referred to as bulk flow or mass flow.
Often there is a higher concentration of various mineral ions inside the plant than outside. In this situation, the passive means of transport mentioned so far are not useful. If the plant is to absorb these minerals, active transport is needed. This requires energy.
Another very common reason why a plant’s roots may have to expend energy for a particular mineral ion to pass into it, is because the ion cannot cross the lipid bilayer of the membranes. In this instance, the ions must pass through a transport protein in the membrane. These transport proteins are specific for certain ions. They bind to the ion on one side of the membrane and then release it on the other side. This requires energy. Potassium ions move through specialized transport proteins called potassium channels.
The result of the active uptake of mineral ions is a high solute (hypertonic) concentration within the root. Because of this, the amount of water absorbed from the soil by the root through the process of osmosis is increased.
Plant adaptations for water conservation
Plants that survive in desert and high saline environments have adaptations that allow their survival. These adaptations allow water conservation.
Xerophytes are plants adapted to arid climates. They have an impressive list of adaptations to reduce transpirational water loss.
Halophytes are plants adapted to grow in water with high levels of salinity. Some of these plants are being studied for use as the next generation of biofuel. They are a promising sure of biofuel because they do not compete with food corps for resources. Halophytes adaptations:
One final adaptation of both halophytes and xerophytes to reduce water loss is to simply close the stomata using the action of guard cells.
Xerophytes are plants adapted to arid climates. They have an impressive list of adaptations to reduce transpirational water loss.
- Small, thick leaves reduce water loss by decreasing the surface area of the leaves.
- A reduced number of stomata decreases the number of openings through which
water loss may occur. - Stomata are located in crypts or pits on the leaf surface. This causes higher humidity
- near the stomata.
- A thickened, waxy cuticle reduces water loss by acting as an impenetrable barrier to
water.
- Hair-like cells on the leaf surface trap a layer of water vapour, thus maintaining a
higher humidity near the stomata.
- Many desert plants shed their leaves and/or become dormant in the driest months.
- Cacti exist on water that the plant stores in fleshy, watery stems. Plants of this type are called succulents. This stored water is obtained in the rainy season.
- Xerophytes can use alternative photosynthetic processes. It is known as the C3 photosynthetic pathway. There are two alternative processes, called CAM photosynthesis and C4 photosynthesis. CAM plants close stomata during the day and incorporate carbon dioxide during the night. C4 plants have stomata that open during the day but take in carbon dioxide more rapidly than non-specialized plants
Halophytes are plants adapted to grow in water with high levels of salinity. Some of these plants are being studied for use as the next generation of biofuel. They are a promising sure of biofuel because they do not compete with food corps for resources. Halophytes adaptations:
- Many become succulent by storing water, thus diluting the salt concentrations.
- Several species, for example the mangrove, secrete salt through salt glands.
- Some species are able to compartmentalize Na+ and Cl– in the vacuoles of their cells,
thereby preventing NaCl toxicity.
- Sunken stomata on thickened leaves reduce water loss by creating a higher humidity
near the stomata. The thickened leaves often include a more developed cuticle to
minimize water loss.
- The surface area of the leaves is reduced.
One final adaptation of both halophytes and xerophytes to reduce water loss is to simply close the stomata using the action of guard cells.