WATER and MINERAL UPTAKE IN PLANTS

Transport of Water in Plant Water enters a plant through its ROOT HAIR CELLS. Root hairs increase the surface area and speed up the water uptake into the plant.  Due to osmosis, water moves from higher water potential to lower water potential. Water is absorbed in root hair cells by osmosis where there is lower water potential than soil. Soil around the roots has higher water potential than leaves. Since the water evaporates from the leaves by TRANSPIRATION.  Water moves from roots to leaves. Furthermore, plants transport ions such as nitrates to their root hair cells by active transport, this lowers the water potential in the plant.

ROOT STRUCTURE Endodermis is a single layer of tightly-packed cells.It contains waterproof layer called casparian strip. This prevents the movement of water between the cells. Root tissues: Epidermis is a single layer of cell with long extentions called Root hairs. Cortex is a thick layer of packing cells. It often contains stored starch.

When the water is absorbed into the root hairs, it gets through two root tissues that are: Cortex Endodermis When the water gets across the root through the root tissues, it needs to reach xylem. Xylem: it takes water up the plant. The water is carried in tubes called Xylem Vessels. They have very narrow diameters.  The water moves only one way in the xylem.  No ends between the walls that prevent the water flow.

The Pathways of water can take through the ROOT 1.APOPLAST PATHWAY 2.SYMPLAST PATHWAY

APOPLAST PATHWAY: Water goes through the non-living parts of the root;cell wall. Cell wall is very absorbent, thick and open so the water can easily diffuse through it. Water has to stop at the endodermis cells that have bands of impermeable suberin known as Casparian Strip. The casparian strip prevents the passage of water into the xylem via this pathway As the water is stopped by casparian strip, it has to get across the cell membrane and then take another pathway.  Cell membrane controls the substances in water that get through it. That’s why the apoplast pathway is main route.

SYPMLAST PATHWAY Water goes through the living cytoplasm of the cells. Water diffuses from the epidermis through the root to the xylem down a water potential gradient. (by osmosis) The cytoplasm of all the cells in the root are connected by plasmodesmata that are little strands extending through the holes in the cell walls. When the water passes the endodermis, it moves into xylem.

Xylem transports water There are 3 ways that xylem transports water against the force of gravity: 1.Cohesion-tension theory 2.Root pressure 3.Capillarity

1.Cohesion-tension theory It determines how water moves from roots to leaves. As the water evaporates from the leaves through the transpiration at the top of the xylem, tension is created. It is needed to pull water through the xylem. This tension develops at the surface of the cell walls in the leaf that are in contact with the air. Thus more water is pulled into the leaf. Water molecules stick together by hydrogen bonds and cohesion is created. When some are pulled into the leaf others follow. The whole column of the water in the xylem moves upwards by mass flow. (from the leaves to the roots)

2.Root Pressure R oot creates pressure and tends to shove water in the xylem further upwards. So the r oot pressure is a force that helps to push up the water to the xylem. This pressure is weak, water is not reached to top of the bigger plants by itself. This pressure helps young, small plants whose leaves are still developing. Root pressure causes guttation that is water loss in its liquid phase. The highest root pressures occur in the spring when the sap is strongly hypertonic to soil water, but the rate of transpiration ishypertonic low.In summer, when transpiration is high and water is moving rapidly through the xylem, often no root pressure can be detected.

3.Capillarity It helps to suck water to the leaves. Water moves in a thin tube. Water molecules also stick to the sides of the vessel which helps to speed this mechanism up and then they climb up slowly. However this mechanism does not account for the Greater distance that water can travel in trees. This is known as adhesion. It is a reason of the capillarity.

Potometer-is used to measure the rate of water uptake

Potometer is an apparatus to measure the RATE of WATER UPTAKE. It has;  a part that reservoirs water  a tap under reservoir of water that prevents water flow and it is shut off during experiment  a capillary tube with scale to measure the rate of water uptake by timing how long the water level takes to move a certain distance along the scale. When the water evaporates from leaf surfaces, plant starts to take up water and the water moves along the scale, thus the water uptake is measured by scale.

Potometer can also be used to investigate how the environmental factors affect the rate of transpiration such as temperature, humidity, light and air movement.  High temperature increases the rate of water evaporation, so the transpiration increases.  High humidity means that the water potential in the air is high, so there is less difference between water potential, thus transpiration decreases.  Light is needed in photosynthesis, it stimulates the stomata and present more photosynthesis, so transpiration increases.  Water transpires from a leaf, the water saturates the air surrounding the leaf, and wind blows them away, and so the air is replaced by drier air. Thus it increases the transpiration.

Transport of Mineral Ions in Plant Plants absorb dissolved mineral ions from soil by passive and active transport  As the concentration of ions inside the cell are higher than outside the root cell, ions move up their concentration gradient by active transport.  On the other hand if the respiratory inhibitors are applied to livings root, the ions uptake is reduced by passive transport since there is no ATP being made to drive membrane pumps.

 travels of mineral ions: Ions diffuse down their concentration gradient from the epidermis to the xylem by passive transport with water. As the water is pulled up the stem, mineral ions travel up the xylem by mass flow. In the leaves they are selectively absorbed into the surrounding cells by membrane pumps.  Also plant can pump out the ions which are not needed.

Scientist know that the mineral ions have moved up the plant with water in the xylem by performing radioactivity tracers experiment. It is used to trace the ions and also sugars and water. In this experiment radioactive forms of ions are used, and thus radioactivity can be detected. Then it can be determined where the ions have moved to.

Functions of Mineral Ions in plant Nitrate: It is needed to make amino acids for protein synthesis. It is also needed to make amino acids for organic bases and nucleic acid. Phosphate: It is needed in photosynthesis and respiration reactions. There are phosphate group in ATP and NADP. It is also needed for phospholipids and in the backbones of DNA and RNA. Magnesium: it is needed to make chlorophyll.

THE END Prepared by AYSE HACIARIF