Transport in Multicellular Plants
Plant cell need a regular supply of oxygen and nutrients and their requirements differ from those of animals in several ways. Some particular requirements of plant cells are as follows:
Carbon Dioxide: Photosynthetic plant cells require a supply of carbon dioxide during daylight.
Oxygen: All plant cells require a supply of oxygen for respiration, but cells which are actively photosynthesizing produce more than enough oxygen for their own needs. Cells which are not photosynthesizing need to take in oxygen from their environment but they do not respire at such a high rate as mammals and therefore do not need such a rapid oxygen supply.
Organic Nutrients: Some plants cells make many of their own organic food materials, such as glucose, by photosynthesis. However, many plant cells do not photosynthesize and need to supplied with organic nutrients from photosynthetic cells.
Inorganic ions and water: All plant cells require a range of different inorganic ions, and also water. These are taken up from the soil, by roots, and are transported to all regions of the plant.
Energy Requirements
Plant cells needs a considerably less amount of energy and nutrients than mammalian cells.
A plant cell's respiration and transport system is much slower than the circulatory system of a mammal.
Photosynthesis and Transport Systems
Plants have large flat leaves that absorb sunlight for photosynthesis and allow gases to diffuse in and out of leaves. Because of this, gases are delivered to all cell through diffusion and plants don't have a transport system for gases.
Plants have two transport systems one for carrying water and inorganic ions and one for the transport of materials made during photosynthesis in the leaves to other areas.
Transport of Water
From Soil to Root Hair
Root hairs are the extension of the epidermis.
Soil water has a relatively high water potential because it contains few inorganic ions, while the water in the root hairs, which has large amount of inorganic ions and organic substances, has a low water potential.
The difference in water potential causes water from the soil to diffuse down the water potential gradient, through the partially permeable membrane, and into the cytoplasm and vacuole of the root hair cells (Mass Flow).
In some plants, fungi called mycorrhizas act like root hairs and absorbs nutrients.
From Root Hair to Xylem
The water passes through the cortex, a layer of cells under the epidermis, and into the xylem vessels. This happens because the water potential in the root is less than that of the root hair. The water can take two possible routes through the cortex: the Apoplast Pathway is when ater enters the cell walls of the cortex and seep through the root from cell wall to cell wall without entering the cytoplasm f the cortical cells and the Symplast Pathway is when water enters the vacuole or cytoplasm of the cll and moves to other cells through interconnecting plasmodesmata. Once water travels through the cortex and reaches the endodermis, the apoplast pathway abruptly stops. THe endodermis had deposits of suberin, a waxy water impermeably substance, in its cell walls. This band, the Casparian strip, forms an impenetrable barrier. THe only way for water to cross these cells is to pass through the cytoplasm of these cells. The system allows the roots to control which inorganic ions can enter in to the xylem vessels.
Xylem Tissues
Xylem tissues have the dual function of supporting the plant structure and transport. Xylem tissues contain vessel elements and tracheids, fibres, and Parenchyma cells. Vessel elements and tracheids are cells involved intransport. Fibres are elongated cells with lignified walls. Their sole purpose is to support the cell and they contain no living contents. Lignification occurs when lignin, a hard water impermeable substance, is laid down on the inside of the cell wall. Parenchyma cells are "standard" plant cells and contain all plant organelles except for chloroplasts because they are located within the cell.
Xylem Tissues and Tracheids
Vessels are made up of many elongated vessel elements arranged end to end. Vessel elements begin life as a normal cell in which lignification occurs. As the process occurs the cell dies leaving an empty space known as lumen inside the cell. The lignified walls contain pits where plasmodesmata had been These pits are covered in ta permeable cell membrane. The ends of the neighboring vessel elements break down and form long "pipes" throughout the plant. The product,a long non-living tube, is the xylem vessel. Tracheids are also lignified plant cells except their walls do not break down to form a continuous tube, instead the pits in the walls allow water to pass from tracheid to tracheid.
From Leaf to Atmosphere (Transpiration)
The loss of water vapor through the leaves is called transpiration. Cells in the mesophyll (in the center of the leaf) are not tightly packed and are saturated with water vapor. The air on inside of the leaf has direct contact with air outside the leaf through small pores called stomata. If there is a water potential gradient between air inside and outside the leaf, the water vapor will diffuse out of the leaf. Factors, sich as temperature, light intensity, humidity, and wind speed affect the rate of transpiration. You can measure the rate of transpiration of water vapor using a potometer. Xerophytes are plants that live in place with a low water supply. They have adapted to require less water to function.
From Xylem to Leaf
Water moves from the xylem to the leaf to replace water that has diffuses in to the air. The removal of water from the top of the xylem vessel reduces hydrostatic pressure (pressure exerted by liquid). When the pressure at the top of the vessel becomes lower then that at the bottom the pressure difference causes water to move up the xylem vessel. The movement of water is mass flow. This means all the water molecules move together. This process is facilitated by the attraction of water molecules to each other called cohesion and the attraction of water molecules to the xylem vessel walls is called adhesion. A plant can raise the pressure in the bottom of the root by secreting solutes into xylem of the root hair.
Translocation
Translocation of organic solutes, such as sucrose, occurs through living phloem sieve tubes. The phloem sap moves by mass flow, as a result of pressure differences produced by active loading od sucrose at sources such as photosynthesizing leaves.
Differences Between Sieve Elements and Xylem Vessels
Carbon Dioxide: Photosynthetic plant cells require a supply of carbon dioxide during daylight.
Oxygen: All plant cells require a supply of oxygen for respiration, but cells which are actively photosynthesizing produce more than enough oxygen for their own needs. Cells which are not photosynthesizing need to take in oxygen from their environment but they do not respire at such a high rate as mammals and therefore do not need such a rapid oxygen supply.
Organic Nutrients: Some plants cells make many of their own organic food materials, such as glucose, by photosynthesis. However, many plant cells do not photosynthesize and need to supplied with organic nutrients from photosynthetic cells.
Inorganic ions and water: All plant cells require a range of different inorganic ions, and also water. These are taken up from the soil, by roots, and are transported to all regions of the plant.
Energy Requirements
Plant cells needs a considerably less amount of energy and nutrients than mammalian cells.
A plant cell's respiration and transport system is much slower than the circulatory system of a mammal.
Photosynthesis and Transport Systems
Plants have large flat leaves that absorb sunlight for photosynthesis and allow gases to diffuse in and out of leaves. Because of this, gases are delivered to all cell through diffusion and plants don't have a transport system for gases.
Plants have two transport systems one for carrying water and inorganic ions and one for the transport of materials made during photosynthesis in the leaves to other areas.
Transport of Water
From Soil to Root Hair
Root hairs are the extension of the epidermis.
Soil water has a relatively high water potential because it contains few inorganic ions, while the water in the root hairs, which has large amount of inorganic ions and organic substances, has a low water potential.
The difference in water potential causes water from the soil to diffuse down the water potential gradient, through the partially permeable membrane, and into the cytoplasm and vacuole of the root hair cells (Mass Flow).
In some plants, fungi called mycorrhizas act like root hairs and absorbs nutrients.
From Root Hair to Xylem
The water passes through the cortex, a layer of cells under the epidermis, and into the xylem vessels. This happens because the water potential in the root is less than that of the root hair. The water can take two possible routes through the cortex: the Apoplast Pathway is when ater enters the cell walls of the cortex and seep through the root from cell wall to cell wall without entering the cytoplasm f the cortical cells and the Symplast Pathway is when water enters the vacuole or cytoplasm of the cll and moves to other cells through interconnecting plasmodesmata. Once water travels through the cortex and reaches the endodermis, the apoplast pathway abruptly stops. THe endodermis had deposits of suberin, a waxy water impermeably substance, in its cell walls. This band, the Casparian strip, forms an impenetrable barrier. THe only way for water to cross these cells is to pass through the cytoplasm of these cells. The system allows the roots to control which inorganic ions can enter in to the xylem vessels.
Xylem Tissues
Xylem tissues have the dual function of supporting the plant structure and transport. Xylem tissues contain vessel elements and tracheids, fibres, and Parenchyma cells. Vessel elements and tracheids are cells involved intransport. Fibres are elongated cells with lignified walls. Their sole purpose is to support the cell and they contain no living contents. Lignification occurs when lignin, a hard water impermeable substance, is laid down on the inside of the cell wall. Parenchyma cells are "standard" plant cells and contain all plant organelles except for chloroplasts because they are located within the cell.
Xylem Tissues and Tracheids
Vessels are made up of many elongated vessel elements arranged end to end. Vessel elements begin life as a normal cell in which lignification occurs. As the process occurs the cell dies leaving an empty space known as lumen inside the cell. The lignified walls contain pits where plasmodesmata had been These pits are covered in ta permeable cell membrane. The ends of the neighboring vessel elements break down and form long "pipes" throughout the plant. The product,a long non-living tube, is the xylem vessel. Tracheids are also lignified plant cells except their walls do not break down to form a continuous tube, instead the pits in the walls allow water to pass from tracheid to tracheid.
From Leaf to Atmosphere (Transpiration)
The loss of water vapor through the leaves is called transpiration. Cells in the mesophyll (in the center of the leaf) are not tightly packed and are saturated with water vapor. The air on inside of the leaf has direct contact with air outside the leaf through small pores called stomata. If there is a water potential gradient between air inside and outside the leaf, the water vapor will diffuse out of the leaf. Factors, sich as temperature, light intensity, humidity, and wind speed affect the rate of transpiration. You can measure the rate of transpiration of water vapor using a potometer. Xerophytes are plants that live in place with a low water supply. They have adapted to require less water to function.
From Xylem to Leaf
Water moves from the xylem to the leaf to replace water that has diffuses in to the air. The removal of water from the top of the xylem vessel reduces hydrostatic pressure (pressure exerted by liquid). When the pressure at the top of the vessel becomes lower then that at the bottom the pressure difference causes water to move up the xylem vessel. The movement of water is mass flow. This means all the water molecules move together. This process is facilitated by the attraction of water molecules to each other called cohesion and the attraction of water molecules to the xylem vessel walls is called adhesion. A plant can raise the pressure in the bottom of the root by secreting solutes into xylem of the root hair.
Translocation
Translocation of organic solutes, such as sucrose, occurs through living phloem sieve tubes. The phloem sap moves by mass flow, as a result of pressure differences produced by active loading od sucrose at sources such as photosynthesizing leaves.
Differences Between Sieve Elements and Xylem Vessels
- Xylem vessels move water and nutrients without energy and can do so with dead xylem cells
- Xylem cells have lignified walls because it prevents water from moving across the walls of the xylem. Xylem vessels does not need water because they are dead.
- The wall between xylem vessels disappear completely, whereas phloem elements have sieve plates with open pores. Sieve plates also help support the phloem and can seal themselves to prevent damage. Xylem vessels lignified walls support themselves.