Cells need boundaries. Those boundaries help keep the cell’s contents together, regulate what enters and leaves, and protect the cell from its surroundings. But not all cellular boundaries do the same job.
The cell membrane is a thin, flexible layer found in all cells. It controls movement of substances into and out of the cell and helps the cell communicate with its environment. The cell wall, by contrast, is a more rigid outer layer found in plants, fungi, bacteria, and some other organisms. It provides structural support and protection.
The simplest distinction is this: the cell membrane regulates the cell’s internal environment, while the cell wall mainly provides strength and shape.
What is a cell membrane?
The cell membrane, also called the plasma membrane, is the thin boundary that surrounds every cell. It separates the cell’s interior from its external environment.
The membrane is made primarily of phospholipids, a type of fat molecule, arranged in a double layer called a phospholipid bilayer. Embedded in this layer are proteins that perform many functions, including transporting substances, receiving signals, and helping cells interact with one another.
The membrane is described as selectively permeable. That means some substances can cross it easily, while others require assistance from membrane proteins or are prevented from crossing.
This selectivity is essential. A cell must obtain nutrients and other useful substances while limiting the entry of harmful materials and controlling the loss of important molecules. The membrane also helps maintain stable internal conditions, a process known as homeostasis.
Because the membrane is flexible rather than rigid, it can change shape and accommodate movement. In animal cells, it forms the cell’s outer boundary because animal cells do not have a cell wall.
What is a cell wall?
A cell wall is a relatively rigid structure outside the cell membrane. Its composition varies among organisms.
In plants, the cell wall is composed mainly of cellulose, a complex carbohydrate made of linked glucose molecules. Plant cell walls also contain other substances, including hemicelluloses and pectin.
In fungi, the main structural material is chitin, rather than cellulose. Many bacteria also have cell walls, but their walls are made primarily of peptidoglycan, a material consisting of sugars and short chains of amino acids.
The cell wall gives cells mechanical strength. In plants, it helps cells maintain their shape and allows tissues to withstand pressure. It also helps prevent plant cells from taking in so much water that they burst.
Unlike the cell membrane, the cell wall is not the cell’s primary system for controlling which individual molecules enter and leave. That job belongs to the membrane underneath it.
Cell wall vs. cell membrane at a glance
| Feature | Cell wall | Cell membrane |
|---|---|---|
| Main role | Support, protection, and shape | Controls movement of substances and communication |
| Location | Outside the cell membrane | Forms the cell’s immediate boundary |
| Structure | Relatively rigid | Thin and flexible |
| Selective permeability | Generally not the main control point for entry and exit | Selectively permeable |
| Present in | Plants, fungi, bacteria, and some other organisms | All cells |
| Main composition | Varies by organism; cellulose in plants, chitin in fungi, peptidoglycan in many bacteria | Mainly phospholipids and proteins |
| Flexibility | Relatively rigid | Flexible |
| Controls what enters and leaves the cell? | No, not primarily | Yes |
The two structures can occur together
A plant cell has both a cell wall and a cell membrane, but they are not redundant.
Moving from the outside toward the interior, a typical plant cell has the cell wall first, followed by the cell membrane. The wall provides structural reinforcement, while the membrane provides the controlled boundary between the cell and its surroundings.
This arrangement is especially important when water moves into a plant cell. The cell membrane regulates water movement, while the rigid cell wall helps resist the resulting internal pressure. The pressure exerted by the cell contents against the wall contributes to turgor, which helps keep many plant tissues firm.
Bacterial cells can likewise have both structures. Their cell wall provides mechanical protection, while their plasma membrane carries out selective transport and many other essential cellular functions.
Why animal cells have a membrane but no cell wall
Animal cells have a plasma membrane but generally lack a cell wall. Their cells therefore rely on other structures for support and organization, including the cytoskeleton, a network of protein filaments inside the cell.
The absence of a rigid wall also gives animal cells considerable flexibility. Cells can change shape, move, engulf material, and interact dynamically with other cells and their surroundings.
This difference helps explain why plant and animal cells have different physical properties. A plant cell is reinforced by a rigid external wall, whereas an animal cell is bounded primarily by a flexible membrane.
The biggest difference is function
It can be tempting to think of the cell wall and cell membrane as two versions of the same structure because both surround cells. Their functions, however, are fundamentally different.
The cell membrane is an active gatekeeper. Its proteins help move ions and molecules across the membrane, receive chemical signals, and communicate information about the cell’s environment. The membrane can also participate in processes such as cell recognition and the movement of materials into and out of the cell through vesicles.
The cell wall is primarily a structural framework. It strengthens the cell, helps determine its shape, and protects it from mechanical stresses and, in many organisms, certain environmental challenges.
A useful way to remember the distinction is that the membrane is mainly about regulation, whereas the wall is mainly about support.
Why the distinction matters
The differences between these structures are important beyond basic cell biology because they affect how organisms grow, survive, and interact with their environments.
For example, plant growth requires changes to the cell wall as cells expand. Bacterial cell walls are also biologically important because they are structurally distinct from animal cells. Some antibacterial drugs work by interfering with bacterial cell-wall construction, targeting a feature that human cells do not have.
At the same time, the plasma membrane is universal to cells and performs essential functions in both bacteria and eukaryotic organisms. Without a functional membrane, a cell cannot maintain the controlled internal environment required for life.
The cell wall and cell membrane therefore solve different problems. The wall gives certain cells physical strength; the membrane gives every cell a controlled interface with its surroundings.
