Transpiration in Plants

Pressure Flow Hypothesis STEP 2: STEP 1: Pressure Flow Hypothesis STEP 3: STEP 4: STEP 5:

Pressure Flow Hypothesis Translocation Movement of organic molecules in plants What do you think are some of these molecules? Water, sugar, carbon dioxide, amino acids, mRNA Based on SINK AND SOURCE dynamics Sink: the stored sugar and water in plant stems. Source: where you get the nutrients and water. Analogy: Faucet Leaves versus Stems: Which is the sink and which is the source ? Leaves are the source, stem are the sinks. How do you think their roles change with seasons? In the winter, the stem and the whole plant acts as the source to access stored food.

STEP 2: Water from xylem is transferred to the phloem to help break down/dissolve food in the phloem. The water pressure is kept at low. STEP 1: Food and water enters phloem of the stem through diffusion and osmosis. STEP 3: Water is being transferred in the xylem (going up) to keep balanced pressure. STEP 4: water is being transferred from the phloem – to maintain pressure in the phloem due to excess water from xylem from step 2. STEP 5: food is stored in the sink cell.

Movement of water in plants Process Explanation/details 1. Water moves down concentration gradients Water moves through osmosis from an area of higher to lower concentration. 2. Water lost by transpiration is replaced by water from vessels Water goes from a high water potential to area of lower water potential. (analogy: straw) 3. Vessel water column is maintained due to cohesion and adhesion Charges in water molecules and xylem attract and repel each other to defy gravity. 4. Tension occurs in the columns of water in the xylem A lot of water retained in xylem. Charges in water molecules and xylem attract and repel each other to defy gravity. 5. Water is pulled from the root cortex into xylem cells This is caused by capillary action – this is when something is transport throughout the system through concentration difference. 6. Water is pulled from the soil into the roots