Core Concepts
In this article, you will be able to predict the mode of transport a species takes, compare and contrast passive transport and active transport, describe the surface area to volume ratio, and explain the importance of biological transport.
Related Topics
- What is Electronegativity? Trends & Chart
- Polar vs. Non-Polar Bonds & Molecules
- What is a Polar Covalent Bond?
- Intermolecular Forces
- Lipid Metabolism
The Plasma Membrane: A Semipermeable Membrane
As you will recall in our discussion of organelles, one which deserves special attention is known as the plasma membrane (cell membrane). The plasma membrane acts as a physical barrier, allowing us to distinguish between the internal environment of the cell, also known as the intracellular space, and the external environment of the cell, also known as the extracellular space.

The plasma membrane is composed of phospholipids, with polar heads irradiating towards the extracellular space and the intracellular space, while nonpolar tails remain embedded within the membrane like a sandwich. Furthermore, the structure of the plasma membrane is what is known as semipermeable, allowing some substances to simply cross the membrane while not allowing others. The basis for this difference resides in our understanding of nonpolar molecules and polar molecules.

The Impact of Polarity
If you were to examine the plasma membrane closely, you would notice it tends to be more nonpolar than polar. Therefore, based on the principle of how “like-dissolves-like,” uncharged, nonpolar species have less challenges when crossing the membrane. However, polar, charged species have more of a challenge when traversing the membrane due the obstacle created from nonpolar tails. Thus, while nonpolar molecules can simply pass through the membrane, polar molecules cannot.
However, polar molecules, such as proteins and carbohydrates, are essential to life. Without them, cells would not be able to synthesize enzymes nor create energy in the form of ATP. Thus, cells must adapt to this inability to simply pass through the membrane and adopt new methods to account for this discrepancy, which leads us into a discussion on biological transport.

Types of Biological Transport
While we can acknowledge the role of polarity in the transport of materials across a semipermeable membrane, it is useful to classify specifically how the transport is carried out based on concentration and energy.
- Passive Transport: The movement of a species from a region of high concentration to a region of low concentration.
- Simple Diffusion: A species simply passing through a membrane from high concentration to low concentration.
- Facilitated Diffusion: A species requiring the assistance of a protein from high concentration to low concentration.
- Active Transport: The movement of a species from a region of low concentration to a region of high transportation using ATP.
- Primary Active Transport: ATP is directly used to move a species from low concentration to high concentration.
- Secondary Active Transport: ATP is indirectly used to move a species from low concentration to high concentration.
The Impact of Surface Area and Volume
While the ability to transport nonpolar species and polar species across the plasma membrane is an important facet of biological transport, the rate of transport is just as important. Biological transport is responsible for maintaining homeostasis, a constant internal balance, within a cell. It is involved in ensuring the cell has a sufficient supply of amino acids to repair proteins, of ions to mediate communication, of carbohydrates for energy production, of lipids for structural integrity, of nucleic acids for DNA replication, and so much more. Thus, the rate of biological transport is a critical feature of defining how a cell responds to environmental pressures, which is ultimately determined by the ratio of surface area to volume.
Generally, a cell with a high surface area to volume ratio is very efficient, while a cell with a low surface area to volume ratio is very inefficient.

Biological Transport Practice Problems
Problem 1
Calcium is a cation utilized by neurons to ensure effective and efficient communication in the form of neurotransmitters. Explain whether calcium would simply diffuse through the plasma membrane of a neuron or would require the assistance of a protein transporter.
Problem 2
A researcher is investigating how neurons communicate with each other. They determine various types of biological transport are responsible. One transporter moves sodium ions from a low concentration to a high concentration through the direct use of ATP. Identify the type of transport involved.
Problem 3
A researcher is investigating how neurons communicate with each other. They determine various types of biological transport are responsible. One transporter moves potassium ions from a low concentration to a high concentration through the direct use of ATP. Identify the type of transport involved.
Biological Transport Practice Problem Solutions
Problem 1
Calcium is an ion. Thus, it would most likely require a protein transporter.
Problem 2
A species is moving from low to high concentration with the direct use of ATP. Thus, it is primary active transport.
Problem 3
A species is moving from low to high concentration with the direct use of ATP. Thus, it is primary active transport.
Further Reading
If you are interested on the topic, we recommend you to read this article!