Sugar: From Source to Sink

“What are the main sugar sources and sugar sinks in a plant? How is it possible for some organs to be both a source and a sink at different times? How does sugar move through a plant from source to sink?”

When we hear the word “sap”, what we typically think of is maple syrup. But maple syrup is only a result of the sap from one type of plant—a tree. The maple tree. All plants have sap, we just don’t typically gather it to use like we do with the maple’s sap. A plant’s sap is the sugar of the plant, which contains energy needed to grow and sustain itself.

The main sugar source of a plant varies by season. While photosynthesis occurs, the leaves and such are creating the sugar, which then gets transported to the roots. The roots then store the sap. However, in the springtime, normally, when the plant has flowers, or fruit, the sugar is needed to grow these and the sap is transported upwards. The fruit/flower would now be called a “sink” and the roots the “source.” Before, the roots would be the “sink” and the leaves the “source.”

A girdled tree

A girdled tree

The ‘transport car’ of the sap is called the phloem. In a tree, the phloem is the innermost layer of the bark. It is made up of living cells that carry the sugar from one place to another within the plant. The phloem is essential to the life of a plant. For example, many farmers will cut the bark of a branch until they have cut through the phloem. They will also then cut of all fruits on that branch except for one. This prevents any sugars or nutrients that are created through photosynthesis to leave that part of the branch, which in turn causes all that sugar to go towards growing that one fruit. This makes the fruits bigger, helpful when trying to win a prize for the largest fruit, for example. This process is called girdling.

A plant’s sugar sources and sinks are equally important in the health of a plant, but most important is the means by which the sap gets transported—the phloem.


As an extra, I found this fun DIY “experiment” thing. I haven’t done it yet, but I plan to soon. It’s “How to extract the DNA from strawberries!” Below is the video and stuff you’ll need. I hope you enjoy! UPDATE: I did it and it’s quite cool, actually! (and super easy). 🙂

-You’ll need,

  • 2-3 strawberries
  • dish soap
  • ice cold rubbing alcohol
  • salt (any kind)
  • plastic sandwich bag
  • fork
  • cup-like clear container
  • tea strainer (or something similar)

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