Main difference - cell membrane vs. cell wall
Cell wall and cell membrane are two types of extreme boundaries found in cells. The cell wall is the outermost limit of bacteria, archaea, fungi and plant cells. The cell membrane is the outermost limit of animal cells . Cell membranes can be identified on the inside of the cell wall, in cells that own the cell wall. The main difference between cell membrane and cell wall is that the cell membrane is a universal feature of all living cells, while the 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 that separates the inside of the cell from the outside environment. The cell membrane is also known as the plasma membrane and the cytoplasmic membrane . It is selectively permeable to substances such as ions and organic molecules. The cell membrane maintains a constant environment within the protoplasm by controlling the passage of substances in and out of the cell. It also protects the cell from its surroundings.
Structure of the cell membrane
The structure of the membrane is described by the fluid mosaic model. The cell membrane consists of a lipid bilayer with proteins embedded in it. The lipid bilayer is viewed as a two-dimensional liquid in which the lipid and protein molecules diffuse more or less easily. It arises from the self-organization of lipid molecules. These lipids are amphipathic phospholipids. Their hydrophobic “tail” regions are hidden from the surrounding water or the hydrophilic environment by the double-layer structure. Thus, hydrophilic heads interact with either intracellular / cytosolic or extracellular surfaces. This forms a continuous, spherical lipid bilayer. Therefore, hydrophobic interactions are considered to be the main driving forces behind lipid bilayer formation.
The lipid bilayer structure prevents polar solutes from entering the cell. However, the 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. Phosphatidylserine is concentrated on the membrane to create an additional barrier for charged molecules.
Membrane structures such as podosome, caveola, focal adhesion, invadopodium, and various types of cell junctions are present in the membrane. These are called " supramembrane " structures that enable communication, cell adhesion, exocytosis, and endocytosis. The cytoskeleton is located in the cytoplasm below the cell membrane. The cytoskeleton provides a scaffold to anchor the membrane proteins. A detailed diagram of the cell membrane is shown in Figure 1 .
Composition of the cell membrane
The cell membrane consists mainly of lipids and proteins . There are three classes of amphipathic lipids in the cell membrane: phospholipids, glycolipids, and sterols. Phospholipids are the most common lipids. Cholesterol is found in animal cells distributed across the membrane.
Liposomes are the lipid vesicles that reside in the cell membrane; they are circular pockets enclosed by a lipid bilayer. Carbohydrates can be found as glycoproteins and glycolipids. 50% of the cell membrane consists of proteins. Proteins can be found on the membrane in three ways: integral or transmembrane proteins, lipid-anchored proteins, and peripheral proteins.
Function of the cell membrane
The cell membrane physically separates the cytoplasm from its extracellular environment. It also anchors the cytoskeleton and provides the shape of the cell. On the other hand, the cell membrane is bound to the other cells in the tissue and provides the cell with mechanical support.
The cell membrane is selectively permeable and regulates a constant internal environment for the cell to function. The movement through the cell membrane can take place 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 such as sugars, amino acids and metabolites move passively through transmembrane protein channels. Aquaporins are a type of protein channel that transport water through facilitated diffusion. The uptake of molecules into the cell by enveloping them is called endocytosis. Solid particles are engulfed by phagocytosis and small molecules and ions are engulfed by pinocytosis. Some undigested residue is removed from the cell by invagination and formation of a vesicle. This process is known as exocytosis.
What is a cell wall?
The cell wall is a rigid, outermost structural layer that occurs in bacterial, archeal, fungal and plant cells. Structural support and protection is provided by the cell wall. It also acts as a pressure vessel that prevents the cell from overstretching. Structure and composition vary between species.
Structure and composition of the cell wall
Plant cell wall
The plant cell wall is made up 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 rich in pectin. The primary cell wall is made of wood and contains cellulose, hemicellulose and pectin-like carbohydrates. The secondary cell wall includes cellulose, xylan, lignin, and some structural proteins. The secondary cell wall in the xylem contains lignin. During cytokinesis, a central lamella is formed in the cell plate. The structure of the plant cell wall is shown in Figure 2 .
- The cell wall of mushrooms consists of three main components: chitin, glucans and proteins.
- The algae cell wall consists of cellulose and other glycoproteins.
- The bacterial cell wall consists of peptidoglycans such as murein.
- The cell wall of the archaea consists of psedomureins or glycoproteins.
Functions of the cell wall
The cell wall gives the cell rigidity and strength. It also protects the cell from mechanical stress. The cell wall can flex with considerable tensile strength. Secondary cell wall components such as lignin and cellulose give plants rigidity. The hydraulic turgor pressure leads to rigidity in the cell. The cell wall enables the cell to have a certain shape. The secondary cell wall is also waterproof.
On the other hand, the cell wall is a completely permeable structure. However, it prevents large molecules such as toxins from entering the cell. In most plants, the primary cell wall is completely permeable to small molecules. The cell wall creates a stable osmotic environment as it prevents osmotic lysis and helps retain water.
Difference between cell membrane and cell wall
Cell membrane : The cell membrane is a universal characteristic of all living cells.
Cell wall : The cell wall is present in bacteria, archaea, fungi and plant cells and is absent in animal cells.
Cell membrane : The cell membrane is a thin, delicate structure with a width of 5-10 nm.
Cell wall : The cell wall is a thick, rigid structure with a width of 4-20 µm.
Cell membrane : The cell membrane can be observed under the electron microscope.
Cell wall : The cell wall can be observed under the light microscope.
Cell membrane : The cell membrane is the outermost layer of animal cells.
Cell wall : The cell wall is the outermost layer of bacteria, archaea, fungi and plant cells.
Cell membrane : The cell membrane acts as a protective covering for the protoplasm and maintains a constant environment in the protoplasm.
Cell wall : The cell wall acts as a protective covering for the cell membrane and maintains the shape of the cell.
Shape of the cell
Cell membrane : The cell membrane gives the cell a round, flexible shape.
Cell wall : The cell wall gives the cell a solid shape.
Cell membrane : The cell membrane consists of lipids, proteins and carbohydrates.
Cell wall : The cell wall consists of peptidoglycan in bacteria, chitin in fungi and cellulose in plants.
Cell membrane : The cell membrane is selectively permeable so that selected molecules can move through it.
Cell wall : The cell wall is completely permeable to macromolecules.
Cell membrane : The cell membrane is alive and metabolically active.
Cell wall : The cell wall is inanimate and metabolically inactive.
Cell membrane : Receptors on the cell membrane allow the cell to receive signals from the external environment.
Cell wall : The cell wall lacks receptors.
Flagella and pili
Cell membrane : The cell membrane leads to flagella and pili, which support the movement or attachment of the cell.
Cell wall : The cell wall facilitates flagella and pili through small openings.
Cell membrane : The cell membrane maintains the same thickness throughout its life.
Cell wall : The cell wall thickens over time and takes up the entire cell, causing cell death, especially in plant cells.
Cell membrane : The cell membrane requires food from the cell and shrinks when it is dry.
Cell wall : Since the cell wall is a mere deposit of substances, it does not require any nourishment from the cell.
Cell membrane and cell wall can be identified as the outermost cell layers. The cell wall is the outer layer of most cells, including plants, bacteria, and fungi. The cell membrane forms the outer layer of animal cells because they do not have a cell wall. The cell wall is completely permeable to substances and does not contain any receptors. The cell membrane is semi-permeable to substances and maintains a constant environment in the protoplasm. The cell membrane also contains receptors that enable cells to respond to external environmental changes. A regular shape can be maintained in the cell through the cell wall rather than the cell membrane. The main difference between cell membrane and cell wall is their universality as a characteristic of a particular cell.
Reference: 1. "Cell membrane". Wikipedia, the free encyclopedia. 2016. Accessed March 01, 20172. “Cell wall”. Wikipedia, the free encyclopedia. 2017. Retrieved March 01, 2017
Image courtesy of: 1. “Cell membrane detailed diagram de“ By LadyofHats Mariana Ruiz - Own work. Image renamed from file: Zellmembran detaileddiagram.svg (Public Domain) via Commons Wikimedia 2. “Plant cell wall diagram-en” By LadyofHats - Own work (Public Domain) via Commons Wikimedia