Mineral Uptake:
Mineral uptake can be explained by another method. This time in the presence of water.
Mass flow describes the movement of a mineral solution or soil water up through the plant. Because the osmotic nature of water, it has a tendency to move from an area of high pressure to low pressure. This allows water to diffuse into the plant, carrying the dissolved minerals towards the root as well. As water evaporates from the leaves, there develops a negative pressure in the xylem. Water, also due to its polar and therefore adhesive characteristics, will follow this negative pressure up into the roots and xylem. The minerals that have been dragged towards the roots will have built up a concentration ready for absorption.
Active transport is used when there is a low concentration of minerals or water outside of the root.
The direct method for absorbing minerals into the root is through specific membrane protein pumps that require ATP.
An indirect method of active transport is called ion exchange. Cations such as calcium or potassium go through ion exchange to enter the cell. A proton pump will pump positive hydrogen ions out of the root using ATP. Because the clay particles in the soil are negative, it is easy to pump these hydrogen ions out. The now negative interior will then accept positive cations that diffuse from the soil into the plant through an ion channel (aka facilitated diffusion).
The indirect version of active transport used is called symport. Symport is meant for anions by combing positive hydrogen ions with the negative anions. Hydrogen ions are first pumped out of the root using a proton pump and ATP. Since the root and the anions are both negative, diffusion across the membrane into the root is not possible. Together, with the energy from the Hydrogen ion, they are pumped back into the root. Symport means 'pumped together.'
Water Uptake:
Water mainly enters the plant through the epidermis of the root hairs. Then water will travel through or between the cells of the cortex until it reaches the vascular cylinder. This is where the xylem is located.
The high concentration of solutes (low concentration of water) in the epidermal cells of the root hairs causes water to go towards the area of low concentration. Therefore it travels from the soil to the root hairs of the plant. Once inside the plant, water can then go through the symplastic pathway or the apoplastic pathway until it reaches the endodermis (outer tissue of vascular bundle) then the xylem.
The symplastic pathway is the movement of water along the solute concentration gradient within the epidermal cells. Bridging the cells are small cytoplasmic connects called plasmodesmata.
The apoplastic pathway is water's movement between the plant cells using capillary action. Hydrogen bonding between the water and the cellulose of the cell walls helps the water move. This is the more common method. Any water, or even minerals, that reach the endodermis will be repelled by the casparian strip which is coated with a waxy water repellant called suberin. So the water and minerals must pass through the plasma membrane of the endodermis to be selectively taken up by the xylem. Minerals will be actively loaded into the xylem causing water to follow and increasing the upward root pressure within the xylem. The active transport of minerals into the root hairs is one of the reason why water enters the roots.
Transpiration - Loss of water from the leaf's stomata caused by heating.
- Water travels to leaf through xylem
- Water in leaf's mesophyll heated by sunlight to became vapour that can escape through stomata
- Negative pressure caused by the loss of water leads to transpiration pull of water molecules from lower parts of xylem because of its cohesiveness
- Forms transpiration stream/pull of water up xylem
Water's unique cohesive and adhesive properties allows this to occur. This is also called the cohesion theory which explains how water can 'climb' up the plant roots. Cohesion occurs between water molecules that hydrogen bond to each other. Adhesion occurs when the water molecules attach to the xylem vessels. This forms a column all the way up the xylem. When water is lost out through the leaf, it creates tension (negative pressure) along the xylem which causes its walls to bend inwards to allow more adhesion and cohesion between water molecules and also with the xylem.
Stoma: Pores in the lower epidermis formed by two guard cells.
Allow for the entrance of carbon dioxide used in photosynthesis but also exit of water when open..
Stoma is open during the day so photosynthesis can occur, but will close during the night when photosynthesis is not happening or when water loss is too great
The opening and closing of the stoma is triggered by the blue light of the upcoming daylight. The receptors on the guard cells will sense the blue light and begin the pumping of hydrogen protons out of the guard cell. This leaves a negative charge in the cell and for positive potassium ions to enter the guard cell through channels in the membrane. The entrance of positive ions lowers the osmotic pressure causing water to move towards the high concentration of potassium ions in the guard cells. The large volume of water increases the hydrostatic pressure (turgor pressure) of the cell changing its shape and opening the stoma. To close the stomata, the reverse process occurs.The potassium ions move out of the guard cell with water following. The loss of water causes the guard cell to become flaccid and the stomata to close. In extreme cases when the water level is low, the mesophyll cells can also release abscisic acid that triggers the stoma to close.
Abiotic Factors affecting Transpiration:
Light: blue wavelengths in daylight signals receptors to open the stoma
Humidity: the boundary surrounding the leaf creates the sub stomatal air space (SSAS). In high humidity there is a greater amount of water vapour in the SSAS so less water vapour will escape form the stomata. This is because there is little concentration difference of water between the SSAS and the leaf.
Wind: wind can blow away the water layer in the SSAS to increased the water vapour gradient between the leaf and SSAS. Transpiration increases with more wind
Heat: Liquid water turns into vapour with the rise in heat. High temperatures increase transpiration.
Adaptations:
Plants can make adaptations in response to their environments.
Plants that make respond to dry environments are called Xerophytes. These plants will make adjustments in order to reduce water loss to combat the high temperatures and low levels of precipitation or in areas of high altitudes/latitudes where there is low precipitation or water is locked as ice or snow.
Adaptatations of Xerophytes:
Waxy leaves: control water loss from epidermis of leaf in hot temperatures.
Rolled leaves: usually found in sand dunes habitat. It's difficult for soils to retain water so by rolling up the leaves the upper waxy epidermis is on the outside so the lower epidermis is enclosed. This allows humidity to build up on the inside and prevents wind, reduces evaporation and maintains the SSAS. The root hairs of the lower epidermis and the groove of the leaf will guide water to the stem/roots
Needle leaves: No lower epidermis. Entirely the waxy upper epidermis
Succulents (Cactus):leaves reduced to needles and stem is enlarged for water storage
Crassulacean acid Metabolism (CAM plants): plants with CAM opens their stomata at night rather than day to reduce water loss. Carbon dioxide can be combined with a C3 acid to be stored as a C4 acid until daytime where it is broken down into a C3 acid and carbon dioxide to be used. The stomata can be closed in the day.
Stomata in pits: stomata is surrounded by hairs and the pits to maintain SSAS layer of water vapour
Low growth: plants that grow closer to the ground will experience less wind or more shade.
Phloem:
transports sugars within plant from the source to a sink (storage area) or from sinks to other parts of plants.This movement is multidirectional and called TRANSLOCATION.
Companion cells not only provide energy for the sieve cell of the phloem, but they also load sucrose into the phloem until the phloem becomes hypertonic. The energy from the companion cells facilitates active transport of the sucrose into the phloem. Water follows the increased concentration of sucrose and the turgor pressure increases. The water and sucrose they create a sap. When water follows the movement of the organic nutrients, it creates a pressure within the plant that explains the multidirectional nature of the phloem.
Support of Terrestrial plants:
Thickening of cellulose walls: the cells and xylem can be thickened to further support the plant.
Lignin Rings: xylem also has lignin rings for support
Turgor pressure: high concentration of water in plant cells increase support.
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