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transport in the xylem of plants
Essential idea: Structure and function are correlated in the xylem of plants.
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:
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:
- 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 photometers. (Practical 7)
- Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.
transpiration
Transpiration is the inevitable consequence of gas exchange in the leaf
The loss of water vapour from the leaves and stems of plants is called transpiration. Plants minimize water losses through stomata using guard cells. These are the cells that are found in pairs, one on either side of a stoma. Guard cells control the aperture of the stoma and adjust from wide open to fully closed. Stomata are found in nearly all groups of land plants for at least part of the plants life cycle. The exception is a group called liveworts.
Stomata: are pores through the epidermis that help in the process of photosynthesis.
The figure bellow shows that the problem for plants is that if stomata allow carbon dioxide to be absorbed, they will usually also allow water vapor to escape.
The loss of water vapour from the leaves and stems of plants is called transpiration. Plants minimize water losses through stomata using guard cells. These are the cells that are found in pairs, one on either side of a stoma. Guard cells control the aperture of the stoma and adjust from wide open to fully closed. Stomata are found in nearly all groups of land plants for at least part of the plants life cycle. The exception is a group called liveworts.
Stomata: are pores through the epidermis that help in the process of photosynthesis.
The figure bellow shows that the problem for plants is that if stomata allow carbon dioxide to be absorbed, they will usually also allow water vapor to escape.
photometer
The photometer is a device used to measure water uptake in plants. Consists of leafy shoot in a tube, a reservoir, and a graduated capillary tube. A bubble in the capillary tube marks the zero point. As the plant takes up water through its roots, the bubble will move along the capillary tube. The progress of the bubble is being timed here, along with noting the distance travelled. The tap bellow the reservoir allows the bubble to be reset to carry out new measurements. Mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure of plant tissues.
xylem structure helps withstand low pressure
The cohesive property of water and the structure of the xylem vessels allow transport under tension
The structure of xylem vessels allows them to transport water inside plants very efficiently. Xylem vessels are long continuous tubes. Their walls are thickened, and the thickenings are impregnated with a polymer called lignin. This strengthens the walls, so that they can withstand very low pressures without collapsing.
Xylem vessels are formed from files of cells, arranged end-to-end. In flowering plants the cell wall material in some areas between adjacent cells in the file is largely removed and the plasma membrane and contents of the cells break down. When mature the xylem cells are nonliving, so the flow of water along them must be a passive process. The pressure inside xylem vessels is usually much lower than atmospheric pressure but he rigid structure prevents the xylem vessels from collapsing.
Water molecules are polar and the partial negative charge o the oxygen atom in one water molecule attracts the hydrogen atom in a neighbouring water molecule. This is called cohesion. Water is also attracted to hydrophilic parts of the cell walls of the xylem. This is called adhesion.
The structure of xylem vessels allows them to transport water inside plants very efficiently. Xylem vessels are long continuous tubes. Their walls are thickened, and the thickenings are impregnated with a polymer called lignin. This strengthens the walls, so that they can withstand very low pressures without collapsing.
Xylem vessels are formed from files of cells, arranged end-to-end. In flowering plants the cell wall material in some areas between adjacent cells in the file is largely removed and the plasma membrane and contents of the cells break down. When mature the xylem cells are nonliving, so the flow of water along them must be a passive process. The pressure inside xylem vessels is usually much lower than atmospheric pressure but he rigid structure prevents the xylem vessels from collapsing.
Water molecules are polar and the partial negative charge o the oxygen atom in one water molecule attracts the hydrogen atom in a neighbouring water molecule. This is called cohesion. Water is also attracted to hydrophilic parts of the cell walls of the xylem. This is called adhesion.
tension in leaf cell walls maintains the transpiration stream
When water evaporates from teh surface of the wall in a leaf, adhesion causes water to be drawn through the cell wall from the nearest available supply to replace the water lost by evaporation and the nearest supply is the xylem vessels in the veins of the leaf. Even if the pressure in the xylem is already low, the force of adhesion between the water and the cell walls in the leaf is strong enough to suck water out of the xylem, reducing the pressure. This creates low pressure and it generates a pulling force that is transmitted though the water in the xylem vessels down the stem and to the ends of the xylem in the roots. This is called transpiration-pull and is strong enough to move water upwards against the force of gravity. In plants it is a passive process and all the energy comes from the thermal energy (heat) that causes transpiration.
Cavitation: when many liquids are unable to resist the very low pressures in xylem vessels and the column of liquid breaks.
Cavitation: when many liquids are unable to resist the very low pressures in xylem vessels and the column of liquid breaks.
active transport of minerals in the roots.
Active uptake of minerals ions in the roots causes absorption of water by osmosis.
Water is absorbed into root cells by osmosis. This happens because the solute concentration inside the roots cells is greater than that in the water in the soil. Most of the solutes in both the roots cells and the soil are mineral ions. The concentration of mineral ions in the root can be 100 or more times higher than those in the soil. These concentration gradients are established by active transport, using protein pumps in the plasma membrane of the root cells.
There are separate pumps for each type of ion that the plant requires. Mineral ions can only be absorbed by active transport if they make contact with an appropriate pump protein. This occurs by diffusion or by mass flow when the water carrying the ions drains through the soil.
Some ions move through the soil very slowly because the ions bind to the surface of soil particles. To overcome this problem certain plants have developed a mutualistic relationship with a fungus.
Water is absorbed into root cells by osmosis. This happens because the solute concentration inside the roots cells is greater than that in the water in the soil. Most of the solutes in both the roots cells and the soil are mineral ions. The concentration of mineral ions in the root can be 100 or more times higher than those in the soil. These concentration gradients are established by active transport, using protein pumps in the plasma membrane of the root cells.
There are separate pumps for each type of ion that the plant requires. Mineral ions can only be absorbed by active transport if they make contact with an appropriate pump protein. This occurs by diffusion or by mass flow when the water carrying the ions drains through the soil.
Some ions move through the soil very slowly because the ions bind to the surface of soil particles. To overcome this problem certain plants have developed a mutualistic relationship with a fungus.
replacing losses from transpiration
Plants transport water from roots to leaves to replace losses from transpiration.
The movement of water from roots to leaves is summarized here:
The movement of water from roots to leaves is summarized here:
Water leaving through stomata by transpiration is replaced by water from xylem. Water in the xylem climbs the stem through the pull of transpiration combined with the forces adhesion and cohesion. Water moves from the soil into roots by osmosis due to the active transport of minerals into the roots. Once the water is in the roots it travels to the xylem through cell walls and through the cytoplasm