Video Transcript
Complete the following: The folding
and opening of mimosa leaflets are dependent on changes in turgor.
Let’s approach this question by
addressing what we already know about the word turgor and how it is involved in the
folding and opening of mimosa leaflets. All plant cells, like the one
that’s shown in the simple diagram here, contain water. The pressure that is exerted by
water on the plasma membrane of each cell helps to maintain cell shape and
rigidity. This enables the plant to remain
upright and the leaflets on a mimosa plant to stay open. The pressure that is exerted on the
cell membrane by water is called turgor pressure, and it allows certain cells in
this leaflet to remain turgid.
When a leaflet is touched, this
mechanical stimulation is recognized by the plants and converted into an electrical
signal. Each distinct, smaller, leaf-like
structure on a leaflet is called a pinnule. Let’s magnify two pinnules and
where they connect to see how turgor pressure in certain cells might change to cause
these leaflets to close. In the diagram on the left, you can
see two pinnules from the base of the leaflets and pulvini, which form swollen,
joint-like structures at the base of each pinnule. Changes in the turgor pressure of
certain cells at the pulvini are what allow the leaflets to open and close.
So let’s erase some of these labels
so we can look at this more closely. The cells in the upper half of each
pulvinus are called extensor cells, labeled here in pink. And the cells of the bottom of each
pulvinus are called flexor cells, which have been labeled here in blue. You might have noticed that the
flexor cells appear less turgid than the extensor cells. They have a slightly lower water
potential, and so a slightly lower turgor pressure. This holds the leaflets open.
When an electrical signal arrives
at the pulvini, however, ions flow out of the extensor cells. This increases the water potential
of the extensor cells, so water then flows out of the extensor cells and into the
flexor cells, which have a comparatively lower water potential. This increases the turgor pressure
in the flexor cells but decreases the turgor pressure in the extensor cells, as they
contain less water. The changes in the turgor pressure
of the flexor cells and of the extensor cells causes their shape to change. So the flexor cells become larger
and more turgid, while the extensor cells shrink and become less turgid. This causes the pinnules to fold
upwards and the leaflet to temporarily close.
Therefore, the folding and opening
of mimosa leaflets are dependent on changes in turgor pressure.
