Video Transcript
The vacuole of the cell contains many sugars. The soil contains a lower concentration of sugars. Water moves across the semipermeable membrane to the area with the higher
concentration of sugars, as shown in the diagram. What name is given to this process? (A) Endocytosis, (B) exocytosis, (C) osmosis, or (D) active transport.
This question is asking about the movement of molecules across a cell membrane. Each of the given answer options describes a different biological process. So which describes the movement of water to areas of higher sugar concentrations? Well, let’s look at the answers and narrow down our options.
It can be confusing when we talk about the movement of water molecules between
different solute concentrations. We need to remember that a weak solution means there is a high concentration of water
molecules, a high water potential. And a strong solution means there are more solute molecules and less water molecules,
a lower water potential. This question is therefore asking about the process by which water molecules move
from an area where they are in a higher concentration to an area where they are in a
lower concentration.
Now, let’s look at the biological processes given in the answer options.
Active transport is the movement of molecules across a cell membrane from where they
are in a low concentration to where they are in a higher concentration. It can be likened to a car driving up a hill. Just like the car needs an engine to move it, active transport requires transport
proteins in the cell membrane to move the molecules. The engine requires fuel to drive it, just like the transport proteins require
energy. An example of where active transport is used is the movement of mineral ions from the
soil, where they are in a low concentration, into a root hair cell, where they are
in a higher concentration. This means option (D) is not correct as the question is asking about water molecules
passing from where they are in a higher concentration to an area where they are in a
lower concentration.
Exocytosis is a process by which cells transport molecules out of the cells. Exo- means outside and cyto- means cell. Secretory vesicles inside the cell move towards the cell membrane and fuse with it,
releasing their contents into the surroundings. Again it is a process that requires energy. An example of exocytosis is when a neuron releases neurotransmitters at a
synapse. So we can rule out option (B) because in the question it is asking about the
transport of the water molecules into the cell, not out. And exocytosis does not rely on concentration gradients.
Now, let’s look at the opposite of exocytosis, which is endocytosis. Endocytosis is a process by which cells transport molecules into the cell, endo-
meaning inside. The cell membrane invaginates, in other words it bends inwards, to form a
vesicle. This, again, requires energy. An example of this is when a phagocyte, a type of white blood cell, engulfs a
bacterium. Although endocytosis is used to move molecules into the cell, it is not reliant on a
difference in concentration and the molecules are not moving across the
membrane. So again, we can eliminate option (A).
This leaves us with osmosis. Osmosis can be defined as the movement of water molecules from a higher water
potential to a lower water potential across a semipermeable, also known as a
selectively permeable, membrane. As we saw earlier, this also means the water molecules are moving from a low solute
concentration to a high solute concentration. This is a passive process; in other words, it requires no energy.
Therefore, the correct answer to our question is (C). The name given to the process whereby water moves across the semipermeable membrane
from the soil, which contains a lower concentration of sugars, into the cell with
the higher concentration of sugars, is osmosis.
