Video Transcript
Plant cells can take in large
amounts of water and swell up. What component of the plant cell
wall allows this change in shape to occur without letting the cell burst? (A) Chitin, (B) collagen, (C)
cellulose, or (D) fibrin.
The question is asking us about a
substance that can be deposited into plant cell walls that allows them to swell with
water but not burst. To work out what this substance is,
let’s first take a look at some of the structures we would be likely to find in a
typical plant cell like this one. Within each plant cell is a
structure called a vacuole. The large permanent vacuoles in
plant cells are filled with a substance called cell sap. Cell sap contains a lot of water
molecules, which we’ll represent in our diagram as blue dots. And it also contains dissolved
solutes and certain enzymes.
When the vacuole is especially
filled with water, it exerts a high turgor pressure upon the cell membrane that
surrounds the plant cell. As plant cells also contain a rigid
cell wall surrounding the cell membrane, a high turgor pressure allows these cells
to become turgid and swollen rather than bursting. The question mentions that there is
a component of plant cell walls that allow them some flexibility, strength, and
structure. This is describing a method of
structural support in plant cells.
Structural support is the permanent
deposition of strong polymers into the cell walls of certain plant cells to maintain
the shape of the plant and its cells. There are many different polymers
that can be deposited into different plant cell walls, depending on their location
and function, such as lignin, pectin, or cutin. But all plant cell walls have one
polymer in common, cellulose. Cellulose is a strong insoluble
polymer made of chains of thousands of glucose molecules joined together. These chains form a mesh within
plant cell walls that act as a supportive barrier. It is also what allows the plant
cells to change shape, for example, when the vacuoles are filled with water and are
exerting turgor pressure upon the cell wall.
A key difference between plant and
animal cells is that animal cells do not have cell walls, though they are surrounded
by a cell membrane. If a typical animal cell like this
one absorbed a lot of water, unlike the cell wall of a plant cell, the cell membrane
cannot withstand the turgor pressure that is exerted upon it by all of these water
molecules. This causes the animal cell to
burst as it does not have the cellulose cell wall that a plant cell would to help it
keep its shape.
Therefore, we’ve worked out that
the component of a plant cell wall that allows it to change in shape without
bursting is cellulose.