Difference Between Cell Membrane and Cell Wall

Main Difference – Cell Membrane vs Cell Wall

Cell wall and cell membrane are two types of outermost boundaries found in cells. Cell wall is the outermost boundary of bacteria, archaea, fungi and plant cells. Cell membrane is the outermost boundary of animal cells. Cell membrane can be identified on the inner side of the cell wall, in cells which possess the cell wall. The main difference between cell membrane and cell wall is that cell membrane is a universal feature of all living cells whereas cell wall is absent in animal cells.

This article explains, 

1. What is a Cell Membrane
      – Structure, Composition, Function
2. What is a Cell Wall
      – Structure, Composition, Function
3. What is the Difference between Cell Membrane and Cell Wall

What is a Cell Membrane

The cell membrane is a biological membrane which separates the cell interior from the outside environment. Cell membrane is also called the plasma membrane and cytoplasmic membrane. It is selectively-permeable to substances like ions and organic molecules. Cell membrane maintains a constant environment inside the protoplasm by controlling the passage of substances in and out of the cell. It also protects the cell from its surroundings.

Structure of Cell Membrane

The structure of the membrane is described by the fluid mosaic model. Cell membrane consists of a lipid bilayer with embedded proteins in it. The lipid bilayer is considered as a two-dimensional liquid, where the lipid and protein molecules are diffused more or less easily in it. It is formed during self-assembly of lipid molecules. These lipids are amphipathic phospholipids. Their hydrophobic “tail” regions are hidden from the surrounding water or the hydrophilic environment by the bilayer structure. Thus, hydrophilic heads interact with either intracellular/cytosolic or extracellular faces. By this, a continuous, spherical lipid bilayer is formed. Therefore, hydrophobic interactions are considered as major driving forces for the lipid bilayer formation. 

Lipid bilayer structure prevents the entry of polar solutes into the cell. But, passive diffusion of non-polar molecules is allowed. Therefore, transmembrane proteins function either as pores, channels or gates for the diffusion of the polar solutes. Phosphatidyl serine is concentrated on the membrane, in order to create an extra barrier to charged molecules.

Membrane structures like podosome, caveola, focal adhesion, invadopodium and different types of cell junctions are present in the membrane. These are called “supramembrane” structures which allow communication, cell adhesion, exocytosis and endocytosis. Underlying the cell membrane, the cytoskeleton is found in the cytoplasm. Cytoskeleton provides a scaffolding to anchor the membrane proteins. A detailed diagram of the cell membrane is shown in figure 1. 

Figure 1: A detailed diagram of the cell membrane

Composition of Cell Membrane

Cell membrane is mostly composed of lipids and proteins. Three classes of amphipathic lipids can be found in the cell membrane: phospholipids, glycolipids and sterols. Phospholipids are the most abundant type of lipids among them. Cholesterol is found dispersed throughout the membrane in animal cells.

Liposomes are the lipid vesicles found in the cell membrane; they are enclosed circular pockets by a lipid bilayer. Carbohydrates can be found as glycoproteins and glycolipids. 50% of the cell membrane consists of proteins. Proteins can be found in three types on the membrane: integral or transmembrane proteins, lipid anchored proteins and peripheral proteins.

Function of Cell Membrane

Cell membrane physically separates the cytoplasm from its extracellular environment. It also anchors the cytoskeleton, providing the shape of the cell. On the other hand, cell membrane is attached to the other cells in the tissue, providing the mechanical support to the cell.

Cell membrane is selectively permeable, regulating a constant internal environment for the functioning of the cell. The movement across the cell membrane can occur either in passive or active diffusion. Four transport mechanisms can be identified in the cell membrane. Small molecules like carbon dioxide, oxygen and ions move across the membrane by passive osmosis and diffusion. Nutrients like sugar, amino acids and metabolites move across passively, through transmembrane protein channels. Aquaporins are a kind of protein channels which transport water by facilitated diffusion. Absorption of molecules into the cell by engulfing them is referred to as the endocytosis. Solid particles are engulfed by phagocytosis and small molecules and ions are engulfed by pinocytosis. Some undigested residues are removed from the cell by invagination and formation of a vesicle. This process is referred to as the exocytosis. 

What is a Cell Wall

The cell wall is a rigid, outermost, structural layer found in bacterial, archeal, fungal and plant cells. Structural support and the protection is provided by the cell wall. It also acts as a pressure vessel which prevents the over-expansion of the cell. Structure and composition varies between species.

Structure and Composition of Cell Wall

Plant Cell Wall

Plant cell wall consists of three layers, the primary cell wall, which is a thin, flexible layer, the secondary cell wall, which is a thick layer and the middle lamella, which is a rich in pectin. The primary cell wall is composed of wood and includes cellulose, hemicellulose and pectin like carbohydrates. The secondary cell wall includes cellulose, xylan, lignin and some structural proteins. Secondary cell wall in the xylem contains lignin. During cytokinesis, middle lamella is formed in the cell plate. The structure of the plant cell wall is shown in figure 2.

Figure 2: Plant cell wall

• Fungal cell wall is composed of three major components: chitin, glucans and proteins.
• Algal cell wall is composed of cellulose and other glycoproteins.
• Bacterial cell wall is composed of peptidoglycans like murein.
• Archeal cell wall is composed of psedomureins or glycoproteins.

Functions of Cell Wall

Cell wall gives rigidity and the strength to the cell. It also protects the cell against mechanical stress. Cell wall can bend with a considerable tensile strength. Secondary cell wall components such as lignin and cellulose give rigidity to plants. Hydraulic turgor pressure gives rise to rigidity in the cell. Cell wall permits a definite shape to the cell. Secondary cell wall is also waterproof.

On the other hand, cell wall is a completely permeable structure. But, it prevents the entry of large molecules into the cell like toxins. In most plants, primary cell wall is completely permeable to small molecules. Cell wall creates a stable osmotic environment since it prevents the osmotic lysis and helps to retain water.

Difference Between Cell Membrane and Cell Wall

Presence

Cell Membrane: Cell membrane is a universal feature of all living cells.

Cell Wall: The cell wall is present in bacteria, archaea, fungi and plant cells and absent in animal cells.

Structure

Cell Membrane: Cell membrane is a thin, delicate structure, 5-10 nm wide. 

Cell Wall: Cell wall is a thick, rigid structure, 4-20 µm wide.

Observation

Cell Membrane: Cell membrane can be observed under electron microscope.

Cell Wall: Cell wall can be observed under the light microscope.

Outermost Layer

Cell Membrane: Cell membrane is the outermost layer of animal cells.

Cell Wall: Cell wall is the outermost layer of bacteria, archaea, fungi and plant cells.

Function

Cell Membrane: Cell membrane functions as protective covering to the protoplasm and maintains a constant environment in protoplasm.

Cell Wall: Cell wall functions as a protective covering to the cell membrane and maintains the shape of the cell.

Shape of the Cell

Cell Membrane: Cell membrane provides a round, flexible shape to the cell.

Cell Wall: Cell wall provides a fixed shape to the cell.

Composition

Cell Membrane: Cell membrane is made up of lipids, proteins and carbohydrates.

Cell Wall: Cell wall is made up of peptidoglycan in bacteria, chitin in fungi and cellulose in plants.

Permeability

Cell Membrane: Cell membrane is selectively permeable, allowing selected molecules to move across it.

Cell Wall: Cell wall is completely permeable to macromolecules.

Living Status

Cell Membrane: Cell membrane is alive and metabolically active.

Cell Wall: Cell wall is non-living and it is metabolically inactive.

Receptors

Cell Membrane: Receptors on the cell membrane allow the cell to receive signals from the external environments.

Cell Wall: Cell wall lacks receptors.

Flagella and Pili

Cell Membrane: Cell membrane gives rise to flagella and pili which help the movement and attachment of the cell, respectively.

Cell Wall: Cell wall facilitates flagella and pili through small apertures.

Thickness

Cell Membrane: Cell membrane maintains the same thickness throughout the lifetime.

Cell Wall: Cell wall increases its thickness over time and occupies the whole cell, causing the cell death, especially in plant cells.

Nutritional Requirements

Cell Membrane: Cell membrane requires nutrition from the cell and it shrinks during drought conditions.

Cell Wall: Since the cell wall is a mere deposit of substances, it requires no nutrition from the cell.

Conclusion

Cell membrane and cell wall can be identified as the outermost layers of cells. Cell wall is the outer layer of most cells including plants, bacteria and fungi. Cell membrane forms the outer layer of animal cells since they do not possess a cell wall. Cell wall is completely permeable for substances and do not contain receptors. Cell membrane is semi-permeable to substances, maintaining a constant environment in the protoplasm. Cell membrane also contains receptors, allowing the cells to respond to external environmental changes. A regular shape can be maintained in the cell, by the cell wall rather than the cell membrane. The main difference between cell membrane and cell wall is their universality as a feature of a particular cell.

Reference:
1.“Cell membrane”. Wikipedia, the free encyclopedia. 2016. Accessed 01 March 2017
2.“Cell wall”. Wikipedia, the free encyclopedia. 2017. Accessed 01 March 2017

Image Courtesy:
1.”Cell membrane detailed diagram en” By LadyofHats Mariana Ruiz – Own work. Image renamed from File: Cell membrane detailed diagram.svg (Public Domain) via Commons Wikimedia
2. “Plant cell wall diagram-en” By LadyofHats – Own work (Public Domain) via Commons Wikimedia