1. What is the lesson about?

2. Transpiration
Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants absorb soil water through their roots and this water can originate from deep in the soil. (For example, corn plants have roots that are 2.5 meters deep, while some desert plants have roots that extend 20 meters into the ground).
Plants pump the water up from the soil to deliver nutrients to their leaves. This pumping is driven by the evaporation of water through small pores called "stomata", which are found on the undersides of leaves.
Transpiration accounts for approximately 10% of all evaporating water.

3. Factors……….
Humidity - the air can only hold so much moisture. If the air is already humid then it is harder for water to evaporate than if the air was dry.
Wind speed - in still air, the air around the leave becomes humid and a micro-climate is set up which reduces the rate of evaporation. As the wind speed increases the humid air around the leave is replaced by drier air and so the micro-climate is removed. The faster the wind speed the quicker dry air can replace the humid air and so the greater the rate of evaporation.

4. Temperature - the hotter it is the faster the rate of evaporation and therefore transpiration.
Light Intensity - light causes the stomatal pores to open for photosynthesis. This also means that water can be lost. The pores close at night, so reducing water loss.

6. The xylem vessels are full of water and, as water leaves them, a tension is set up in the columns of water. This is transmitted down the stem all the way to root by cohesion of water (water molecules have high cohesion) and they also tend to stick to the vessel walls, a force called adhesion.

7. The Potometer
The main cause of water loss from a plant is through the stomata of the leaf. The stomata may be opened or closed in order to limit the amount of water-loss experienced by a plant. The amount of water lost by a plant may be investigated experimentally using a Potometer:
The movement of water through the plant is driven by evaporation of water from the leaf. Factors which affect the rate of evaporation will also affect the rate of transpiration.

8. Why do plants wilt?
The rate of water loss is higher than the intake the plant will start to wilt. This is because the cells of the plant lose their turgidity and go flaccid

9. Absorption of Water by Plants
Mechanism of Water Absorption
Water can be absorbed by two methods:
Active absorption
Passive absorption

10. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles.
As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis
Plants absorb water through the entire surface - roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone

14. Active Absorption
Water is absorbed due to activities going on in roots. Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root cells such as respiration. Absorption takes place against concentration gradient - even when the concentration of cell sap is lower than that of soil water.

15. Passive Absorption Passive absorption is by osmosis.
Passive absorption takes place along the concentration gradient - when the concentration of cell sap is higher than that of soil water.
Water is absorbed when transpiration rate is high or soil is dry. Due to high transpiration rate, water deficit is created in transpiring cells. Rapid transpiration removes water and reduces turgor pressure in living cells of root. The suction force thus developed is transmitted to root xylem. It pulls water from surrounding root cells to make up water deficit.

16. Water in the roots move by two pathways
Water in the roots move by two pathways. They can be classified as1) Apoplast pathway2) Symplast pathway
Apoplast pathway
In this pathway the movement of water occurs exclusively through cell wall without the involvement of any membranes. Majority of the amount of water goes through the apoplast pathway. The cortex of the root does not oppose such movement of the water.
For A level only
Symplast pathway
Here the movement of water molecules is from cell to cell through the plasmodesmata. The plasmodesmata forms a network of cytoplasm of all cells. The Casparian strip separates the cortex and the endodermis. It is composed of a wax like substance called suberin, which blocks water and solute molecules through the cell wall of the endodermis. Now the water is forced to go through the cell membranes of different cells leading to a transmembrane pathway.