Pressure Flow or Mass Flow Hypothesis Video Lecture - NEET

Ans. The pressure flow hypothesis is a theory that explains the movement of sugars in plants. According to this hypothesis, sugars are transported from the source (where they are produced, such as leaves) to the sink (where they are utilized, such as fruits or roots) through a pressure-driven mechanism. This mechanism involves the active loading of sugars into the phloem at the source, creating a high concentration of sugars. As a result, water enters the phloem through osmosis, resulting in an increased pressure. This pressure gradient then drives the movement of sugars towards the sink, where they are actively unloaded.

Ans. The mass flow hypothesis is another theory that explains the movement of sugars in plants. According to this hypothesis, sugars are transported from the source to the sink by mass flow. Unlike the pressure flow hypothesis, the mass flow hypothesis suggests that sugars move passively along a concentration gradient. The high concentration of sugars at the source creates a pressure gradient, which pushes the sugars towards the sink. This mass flow of sugars is facilitated by the continuous supply of water from the xylem, which maintains the necessary pressure gradient.

Ans. The pressure flow hypothesis and the mass flow hypothesis differ in their explanations of how sugars are transported in plants. The pressure flow hypothesis suggests that sugars are actively loaded into the phloem at the source, creating a high concentration of sugars and an increased pressure. This pressure gradient then drives the movement of sugars towards the sink. On the other hand, the mass flow hypothesis suggests that sugars move passively along a concentration gradient. The high concentration of sugars at the source creates a pressure gradient, which pushes the sugars towards the sink. Unlike the pressure flow hypothesis, the mass flow hypothesis does not involve active loading and unloading of sugars.

Ans. Several lines of evidence support the pressure flow hypothesis. One piece of evidence is the observation that the rate of sugar movement in phloem correlates with the rate of transpiration. This suggests that the pressure generated by transpiration helps drive the flow of sugars. Additionally, experiments have shown that when the phloem is punctured, the flow of sugars stops, supporting the idea that pressure is necessary for sugar transport. Furthermore, studies using radioactive tracers have demonstrated that sugars move in the phloem in the direction predicted by the pressure flow hypothesis, from source to sink.

Ans. The pressure flow hypothesis explains the movement of sugars in both upward and downward directions in plants through the combination of active loading and unloading at different locations. At the source, sugars are actively loaded into the phloem, creating a high concentration and increasing pressure. This pressure pushes the sugars towards the sink, where they are actively unloaded and utilized. However, in certain situations, sugars can also move in the opposite direction. This can occur when the sink becomes a source, such as during storage of sugars in roots or tubers. In such cases, sugars are actively loaded into the phloem at the sink, creating a high concentration and pressure that drives the movement of sugars in the upward direction.