Question

A student is trying to determine the type of membrane transport occurring in a cell. She finds that the molecule to be transported is very large and polar, and when transported across the membrane, no energy is required. Which of the following is the most likely mechanism of transport? (A) Active transport (B) Simple diffusion (C) Facilitated diffusion (D) Exocytosis

Short answer

The most likely mechanism of transport is facilitated diffusion (C).

Step-by-step solution

Analyze the molecule's properties

The molecule in question is very large and polar. This property is important because large and polar molecules typically cannot pass through the cell membrane via simple diffusion.

Determine if energy is required

It is stated that the transport process does not require energy. This means the process cannot be active transport or exocytosis, as both require energy (ATP).

Eliminate irrelevant options

Since active transport (A) and exocytosis (D) require energy, they can be eliminated from the choices.

Distinguish between remaining options

The remaining options are simple diffusion (B) and facilitated diffusion (C). Given the molecule is large and polar, it cannot pass through the membrane without assistance, thus simple diffusion is not feasible.

Identify the correct mechanism

Facilitated diffusion (C) is the transport of molecules across a cell membrane through protein channels or carriers, without utilizing energy. This fits the criteria described.

Key concepts

Facilitated Diffusion

In facilitated diffusion, molecules move across the cell membrane with the help of protein channels or carriers.
Unlike simple diffusion, this process is necessary for large or polar molecules that cannot easily pass through the membrane.
Since no energy is required, facilitated diffusion is a type of passive transport.
Specific proteins provide pathways for these molecules to traverse the membrane.
For instance, glucose, a large polar molecule, often relies on facilitated diffusion to enter cells.
These proteins are highly specific, binding temporarily to the molecule they transport and allowing it to move from an area of higher concentration to an area of lower concentration.

Passive Transport

Passive transport is the movement of substances across the cell membrane without the use of energy.
It relies on the concentration gradient, moving substances from an area of higher concentration to an area of lower concentration.
There are several types of passive transport: simple diffusion, osmosis, and facilitated diffusion.
In simple diffusion, small and nonpolar molecules pass directly through the membrane. In osmosis, water molecules move through a semipermeable membrane.
In facilitated diffusion, specific carrier or channel proteins aid larger or polar molecules, as discussed earlier.
It's essential to note that passive transport is driven purely by the natural random motion of molecules, not by cellular energy.

Cell Membrane

The cell membrane, also known as the plasma membrane, is a crucial structure that surrounds the cell and separates it from the external environment.
This membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Its primary function is to regulate what enters and exits the cell, ensuring the internal environment remains stable.
The bilayer's hydrophobic interior prevents large and polar molecules from passing through directly.
However, membrane proteins, such as channels and carriers, provide selective pathways for these molecules.
Thus, the cell membrane plays a vital role in both active and passive transport mechanisms.