Video Transcript
The transport of sugars around the
plant can go in more than one direction. What term is given to this
process? (A) Polydirectional, (B)
tridirectional, (C) didirectional, or (D) bidirectional.
Let’s start by reviewing some key
terms. Translocation is the movement of a
sugar called sucrose in a plant. But where does this sucrose come
from in the first place? Photosynthesis is the process by
which plants use light energy to turn carbon dioxide and water into oxygen and
glucose, which is also a type of sugar. Glucose is very important for all
living things because it can react with oxygen in a process called cellular
respiration to release energy that cells can use.
As we know, translocation moves
sucrose around, not glucose. This is because sucrose, unlike
glucose, will not react with oxygen in cellular respiration as it is
translocated. The place where photosynthetic
products like glucose are made is called a source, which is usually in the leaves
and stems in plants, as this is where most photosynthesis will occur when light
intensity is high enough. The photosynthetic products
themselves are called assimilates, and they move through specialized tissues called
the phloem to target locations called sinks. Typically, the sinks will be
locations in the plant that require a lot of energy to function but cannot provide
this energy themselves, such as in the roots, developing fruits, or underground
tubers that store these sugars as starch, such as in potato plants.
When light intensity is too low for
photosynthesis to occur, the location of the sources and sinks in a plant can
change. In these conditions, the sugar that
is stored in the roots or tubers as starch could be converted back into sucrose and
transported to the leaves. In this case, the roots or tubers
become the source, as this is where the sugars will be transported from. And the leaves and stem become the
sinks, as these are the regions which require energy that the sugars will be
transported to.
Luckily, the phloem is capable of
moving sugars both up and down the plant to account for these changes in sugar
requirements. We describe this two-way movement
of assimilates as bidirectional. So the correct answer is option
(D). The transport of sugars in more
than one direction is called bidirectional.
