Translocation: Food glorious food!

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Last post we talked about transpiration, which involved the movement of water around the plant.
Now it's time to talk about the movement of other things in plants. By things I mean minerals and ions, such as glucose and other sugars.

So to start off we need to understand what sources and sinks are. A source is any part of the plant that produces more "food" (glucose usually) than it uses. Sources tend to be leaves and the stems of plants. It can also be roots and/or tubers if they are releasing their storage. A sink is any part of a plant that uses more glucose than it makes. This can be any growing part of the plant including the stem, leaves, roots, and/or tubers.

The process of translocation takes place in the phloem of a plant. This is made up of sieve tube members (elements) and companion cells. The sieve tube member is the structure that actually "does" the translocation. And beside each of these members resides a companion cell. These companion cells serve as, well, servants for these members. Sieve tube members lack a nucleus, and have very little cytoplasm. The companion cells support the sieve tubes, as they have a nucleus and have dense cytoplasm. The sieve tube members are all connected, and attach to each other "end-to-end". In-between each sieve tube is a sieve plate which is perforated (has holes, like Swiss cheese).

Alright now let us begin. We start off at a source. There a process called phloem loading occurs. This is essentially the process of dumping all the glucose from the source into the phloem. This requires active transport mechanisms, especially at the end when the concentration of glucose is much greater in the phloem than in the source. I should probably mention that the phloem and the xylem are very close to each other, only separated by the cambium. The increased concentration of solute in the phloem results in osmosis of water from the xylem to the phloem. The combination of the high concentration of glucose and water at the source area of the phloem creates high turgor pressure at the sink, and lower turgor pressure at the sink. As things move from an area of high pressure to low pressure we see a movement of the sap to the sink.

Once at the sink the carbohydrates are removed from the phloem, this could be partially passive, however, at least part of it will require active transportation. Now our phloem is back to having a low concentration of carbohydrates. The water remaining in the phloem will now diffuse (via osmosis) back to the xylem. Because now the ion concentration in the xylem is higher than the concentration of carbohydrates in the phloem.

And there we have it. You should all now be experts on the topic of transpiration and translocation.

Tea: Have yet to make one. I might have some lipton camomile later.