9.2 – Transport in Angiospermophytes
9.2.1 – Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs
The roots of plants have numerous branches and hairs to increase their surface area. This means that they are in greater contact with the soil. Water and minerals are absorbed from the soil solution through these root hairs.
The monocotyledon roots are highly fibrous and branching structures. This increases their surface area for water absorption. Dicotyledon plants have a min tap root, with additional root branching off it. This enables them to access deeper water and minerals. They have shallow roots close to the surface, and then the tap root penetrates deeper into the ground.
Root hairs are found behind the growing tip of each root, and are an extension of individual epidermal cells. They are used to increase the surface area for water absorption.
The cells with hairs have a greater cell wall size for increased nutrients and water absorption.
9.2.2 – List ways in which mineral ions in the soil move to the root
Diffusion This requires a concentration gradient. The minerals are generally in low concentration in the soil. The move through the route called the symplast.
This requires a concentration gradient. The minerals are generally in low concentration in the soil. The move through the route called the symplast.
Fungal Hyphae
This is a form of mutualism. The fungi provide minerals in a form that the plants can use, such as nitrates. They form networks called mycelium to increase surface area within the root to concentrate the minerals. In return, fungi receive sugar from the plants.
Mass Flow of Water
Ions are carried through the apoplast, through spaces in the cellulose wall. This way, the water actually does not go near the living content of the cells. There is a hydrostatic pressure gradient instead of an osmotic gradient, and negative pressure potential.
9.2.3 – Explain the process of mineral ion absorption from the soil into the roots by active transport
This process requires metabolic energy for transport.
The process occurs against a concentration gradient. The ions move from a region of high concentration to a region of lower concentration. The cells tend to hoard essential ions.
This is also a selective process, in which specific ions can be absorbed based on the needs of the plant
It also uses protein pumps which select specific ion to be transported to the other side of the membrane. The process requires ATP. If a certain pump is not present, then substance will not be transported.
All these ions are found in the soil solution
9.2.4 – State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem
Thickened Cellulose
The cellulose is found on the cells located on the outer sections of the stem. These may also be called collenchyma cells.
Cell Turgor
Turgidity exerts pressure on the surrounding cells. When water enters by osmosis, this increases the volume of the cells. More pressure, the turgor pressure, is exerted on the cell walls, providing mechanical support for the tissue. When the plant wilts, it is because there is not enough water to create this cell turgor.
Lignified Xylem
The xylem tissue is strengthened by extra cellulose, and hardened further with lignin. This is a chemical substance which increases the strength of the xylem and provides most of the support in woody stems.
9.2.5 – Define transpiration
Transpiration is the loss of water vapour from the leaves and stems of plants
9.2.6 – Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation
Structure of Xylem Vessels
Xylem cells develop into long, hollow tubes. The inside of the lateral walls of the vessel has cellulose deposited in it, and is hardened by lignin. This is a very tough tissue so it can resist negative pressure, or suction. Water moves up through the xylem vessel. It is a passive process, requiring no energy.
Transpiration Pull
Transpiration is the evaporation of water vapour through stomata of green plant leaves. As this water is lost, the osmotic pressure increases, causing suction for water uptake.
Cohesion
This is based on the hydrogen bonding of water. It means that the column of water does not break under tension, such as negative pressure or suction. The column of water is a continuous stream from the roots to the leaves. The water molecules are attracted through hydrogen bonding, which results in the pull.
Adhesion
Water molecules adhere to the xylem vessel walls as a result of the properties of water. Combined with cohesion, the water column is able to run the entire length of the xylem at a continuous rate. The water replaces that lost in transpiration.
Evaporation
Water evaporates through the stomatal pore down a humidity gradient. This pulls more water by mass flow into the spongy mesophyll space. There is a gradient of negative pressure potential from the stomatal pore, through the leaf and down the xylem
The leaf absorbs light on its large surface area and heat is produced. The water in the spongy mesophyll tissue enters the vapour phase and evaporates through the stomata. This is then replaced by water from the xylem, drawing it up using transpiration pull. The water enters at the root. The plant will actively pump minerals into the xylem to create osmotic pressure
Uptake in the roots
The water enters through osmosis, as the soil has a lower solute concentration of minerals than the cytoplasm of the epidermal cells. Water moves across the cortex using the water potential gradient. Symplastic movement is a form of osmosis through the plasmodesma. Apoplastic movement is a form of capillary action, moving through the connecting cell walls. At the end of the endodermis, there is a casparian strip, which acts as a barrier to the movement of water into the xylem by the apoplastic pathway. Instead, solute and water must go through the plasma membrane before it can enter the stele. As a result, the uptake of minerals is controlled by the plasma membrane.
ABA and Water Stress
ABA [abscisic acid] is a growth regular in the stems, fruits and leaves of plants. It is important in the leaves during physiological stress, such as drought, where it causes the stomata to close to maintain water levels. The main factors that decrease the availability of water to the plant
The main factors that decrease the availability of water to the plant are:
- Light, as this causes the stomata to open, causing more water loss, as well as raising the temperature of the plant and turning the water into vapour.
- Temperature, causes more water to evaporate and raises the level of transpiration
- Wind increases the rate of transpiration because it increases the concentration gradient between the inside and outside of the leaf
- Humidity decreases the rate of transpiration as it lowers the concentration gradient between the inside and the outside of the leaf
Organic Solute Transport in the Stem
Called translocation, which is when organic foods, including sugars and amino acids, move through the phloem. The sugars are produced in the leaves during photosynthesis, which is used to assist growth or storage. Amino acids are made in the root tips from nitrates in the soil, then taken to where they are needed for photosynthesis. Other chemicals may also move through translocation.
Any plant that has many pores connected to the phloem in called a sieve plant. The sieve tubes carry the solutes from the pores to the phloem using ATP, then they use hydrostatic pressure to be taken away for storage. Remember that the phloem itself is living, undergoing metabolism. The phloem allows for transport in any direction by mass flow.
Movement Through the Leaf
Leaves have tiny pores on them called stomata, to allow gas exchange. There are also some in the stems. They are mainly found in the lower epidermis of dicot leaves. Stomata are made of two, elongated guard cells that are joined to the other cells and each
Stomata are made of two, elongated guard cells that are joined to the other cells and each other, but they can separate to make a pore. When the cells increase in turgor pressure because of water uptake, they open, and then close again when the water is lost. In general, the guard cells stay closed when it is dark, but open when it is light.
In general, the guard cells stay closed when it is dark, but open when it is light. Alternatively, if the plant is wilting from lack of water, they will close. The function of the stomata is to control transpiration to stop too much water loss.
Since leaves have such a large surface area, they are able to absorb more light, which is consequently converted into heat energy. The temperature of the leaf rises, turning water in the spongy mesophyll into vapour. Guard cells then open to allow this vapour to be released, and cooling the leaf. The space is then filled with more vapour through symplastic and apoplastic movement. This in turn draws the water through the xylem from the roots.
Plant Example that you Need to Know: Xerophytes
Xerophytes are plants [such as cacti] which live in permanently dry and arid conditions.
Xerophytes have evolved to have many features that allow them to retain water under their conditions.
These include:
- Thick cuticle on leaf and stem epidermis – to prevent water loss
- Hairs on the epidermis – traps moisture to reduce diffusion
- Fewer Stomata – Less opportunity for water escape
- Stomata in pits or groves – these trap water to slow diffusion
- Rolled or folded leaves – this occurs when the leaf becomes flaccid due to lack of water to reduce the rate of transpiration
- Superficial roots – absorbs condensation from the surface at night
- Deep roots – absorbs water from the deep water table
- Alternative photosynthesis pathway – called the C4 pathway, which takes places as well as the C3 pathway to enhance carbon fixation so that the stomata can be closed
- CAM metabolism – stores CO2 in the form of acids during the night for fixation during the light. Called crassulacean acid metabolism.