The diagram provided shows how
cells can increase their turgidity. By what process does water move
into the vacuole of the cell?
Turgidity refers to the point at
which the cell’s plasma membrane pushes against the cell wall, making the cell
swollen and firm. In a plant cell, a state of high
turgidity occurs when the vacuole is full of cell sap, which is a mix of water and
dissolved solutes, like sugars and enzymes. The question asks us to give the
process by which water moves into the vacuole. And for this to happen, water must
first move into the cell.
Plants use a variety of mechanisms
to transport materials in and out of their cells. And these mechanisms fall into two
major categories: active transport and passive transport.
Passive transport moves substances
from areas of higher concentration to lower concentration and does not require the
input of cellular energy. An example of passive transport is
diffusion, in which dissolved substances, or solutes, move across the plasma
membrane from areas of higher concentration to areas of lower concentration. In the absence of active transport,
diffusion will continue until both areas have approximately equal concentrations of
the solute, a state known as equilibrium.
Sometimes the cell may need to
maintain a higher solute concentration inside the cell than outside of the cell. This means the molecule in question
has to move against its concentration gradient and against the natural tendency
towards equilibrium. There are also times when molecules
must be moved in or out of the cell but they are too large to cross the plasma
membrane. In these cases, active transport
mechanisms, which require cellular energy, will be used.
One example of active transport,
called primary active transport, uses transmembrane enzymes powered by ATP to pump
ions in and out of the cell against their concentration gradient. Luckily for plants and all living
organisms, water does not require cellular energy to be moved in and out of
cells. Instead, it is transported through
a process called osmosis, which is similar to diffusion.
In osmosis, water molecules move
from areas of lower solute concentration and higher water concentration to areas
with higher solute concentration and lower water concentration. In other words, during osmosis,
water molecules move towards equilibrium, just as nature likes it.
Plant cells can store large amounts
of water in their vacuoles. So, when the concentration of water
molecules is higher outside of the cell than inside of the cell, water will move
through the process of osmosis into the vacuole of the cell.