What is the Mass Flow Hypothesis? (biological terms)?

The mass flow hypothesis in terms of plants, is the following:

The accepted mechanism needed for the translocation of sugars from source to sink is called the pressure flow hypothesis.

As glucose is made at the source (by photosynthesis for example) it is converted to sucrose (a dissacharide). The sugar is then moved into companion cells and into the living phloem sieve tubes by active transport. This process of loading at the source produces a hypertonic condition in the phloem.

Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds the phloem sap will move to areas of lower pressure.

At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the pholem sap and into the cells which will use the sugar -- converting it into energy, starch, or cellulose. As sugars are removed osmotic pressure decreases and water moves out of the phloem.

So, the following is true and perhaps a more concise version, if you are learning it for an exam:

The movement of sugars in the phloem begins at the source, where (a) sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the movement of water into the dense phloem sap from the neighboring xylem (b). As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins (c), and the sap moves through the phloem. Meanwhile, at the sink (d), incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out (e), returning eventually to the xylem.

Hope this helps !

Mass Flow Hypothesis

Pressure Flow Hypothesis

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RE:

What is the Mass Flow Hypothesis? (biological terms)?

In terms of plants.

If humans were as natural as the ancestors we share with chimpanzes then raping someone would not bring about the same negative consequences; being beaten up or killed by the victims friends and family or being arrested and prosecuted. While some of those ancestors developed their brains, others developed their strength (if our brains and bodies were the same size, we would be 20 times more intelligent than chimps and they would be 20 times stronger than us). Humans gave up strength for intelligence and as a result, the sex hormones in our bodies are at a much lower level. So humans have a naturally lower desire to rape, and our higher intelligence just so happens to have developed a society where rape is generally not acceptable. If we didn't develop such a society then rape might be a natural thing, but I think, a little less common than in other animals. Some animals mate for life and if we were as natural as other primates, I think we would be a species that is divided (some will mate for life, others will have sex with as many people as possible).

rh