Cell Wall – Definition, Function, Structure, Location, Vs. Cell Membrane

The cell wall is the outermost layer of cells. It provides structural support and protects the cell from damage or infection.

The cell membrane is the thin layer surrounding the cytoplasm, which contains the organelles and other structures within the cell.

The cell wall consists of polysaccharides (carbohydrates) such as cellulose, hemicelluloses, pectins, and glycoproteins.

The cell membrane is composed of phospholipids, proteins, and cholesterol.

In this article, we will be exploring the differences between a cell wall and cell membrane, their functions, structure, location, and how they are related to each other.

What Is The Cell Wall?

A cell wall is an organic material that surrounds plant cells. It has three main components: cellulose, hemicellulase, and pectin.

Cellulose is a long-chain polymer made up of glucose units linked together by β(1→4) bonds.

Hemicellulose is another type of carbohydrate polymer found in plants.

It is a branched heteropolymer with xylose, arabinose, mannose, galactose, glucuronic acid, fucose, rhamnose, and many others.

Pectic substances are linear homogalacturonan chains containing α-(1→4)-linked D-galacturonic acid residues.

They can also contain L-rhamnose, D-xylose, and/or D-arabinose. These molecules form a meshwork around the cell wall.

This meshwork helps maintain the shape of the cell.

Cell walls are mainly located on the outside of the plasma membrane. In some cases, it may extend into the cytoplasm.

The cell membranes are generally not considered part of the cell wall. However, there are exceptions where the cell membrane extends into the cell wall.

For example, the root cap is a thickened region of the endodermis that forms at the tip of roots. It is a modified cell wall that serves as a barrier against pathogens.

Cell Wall Definition

The term “cell wall” refers to the extracellular matrix that surrounds all eukaryotic cells.

It is primarily composed of polysaccharide biopolymers, including cellulose, hemicellulose, pectin, and glycoprotein.

The cell wall plays an essential role in maintaining the physical integrity of the cell and protecting the cell from external stresses.

There are two types of cell walls: primary and secondary.

Primary cell walls surround most plant cells and are mostly composed of cellulose microfibrils embedded in a matrix of hemicellulose and pectin.

Secondary cell walls are present in specialized cells, such as tracheary elements (wood), pollen tubes, fungal hyphae, and epidermal cells.

They have a different composition than primary cell walls. They consist of more lignin and less cellulose.

Functions Of The Cell Wall

The cell wall acts as a mechanical barrier to protect the cell from environmental stressors. It also prevents water loss through osmosis.

When the cell wall is damaged, it allows for the entry of harmful organisms.

The cell wall also plays an important role in signaling. Plant hormones regulate growth and development by acting directly on the cell wall.

Hormones like auxins, cytokinins, gibberellins, abscisic acid, ethylene, brassinosteroids, jasmonates, and salicylic acid affect the synthesis or degradation of cell wall components.

The cell wall provides structural support for the cell. It also helps determine the size of the cell. The thickness of the cell wall varies between tissues and species.

For example, the cell walls of leaves are much thicker than those of roots.

In addition to this, the cell wall protects the cell from damage caused by other cellular processes.

The cell wall contains enzymes that help break down proteins, carbohydrates, lipids, and nucleic acids.

The cell wall also contains proteases that degrade proteins. This helps to prevent protein aggregation inside the cell, which could lead to cell death.

If the cell wall becomes too rigid, it will inhibit the normal function of the cell. If the cell wall is too weak, it cannot withstand the forces exerted by neighboring cells.

Structure Of The Cell Wall

Plant cell walls are complex structures composed of three main layers: the outermost layer, called the cuticle; the middle layer, called the epidermis; and the innermost layer, called the cortex.

Cuticle

The cuticle is the first protective layer of the cell wall. It consists of a thin layer of waxes, sterols, and cutin. Cutin is a polymer of hydroxy fatty acids with ester linkages.

The cutin molecules are held together by hydrogen bonds. Waxes are long-chain alcohols that contain many hydroxyl groups.

Sterols are derivatives of cholesterol. This layer is usually about 1 micron thick.

Epidermis

The epidermis is the second layer of the cell wall and is mainly made up of cellulose microfibers. These fibers are arranged into sheets and form a dense network.

In some cases, there may be additional layers of cellulose in the epidermis. This layer is usually 2-5 microns thick.

Cortex

The third layer of the cell wall is the cortex. It is made up of sclerenchyma, which is a type of fiber that has been modified so that its ends become swollen.

Sclerenchyma is often found in stems, branches, and roots. It can also be found in the bark of trees. Sclerenchyma are formed when the cell wall is stretched during cell elongation.

This causes the pectin side chains to stretch out and create a gel-like substance. Pectin molecules are polysaccharides that have acidic groups attached to them.

They bind to calcium ions, creating a gel-like structure.

Plant cell walls consist of three major components: cellulose, hemicelluloses, and lignins. Each component contributes to the overall structural integrity of the cell wall.

Cellulose

Cellulose is a linear polymer of glucose units linked together by β-1,4 glycosidic bonds. There are two types of cellulose: crystalline cellulose and amorphous cellulose.

Crystalline cellulose is more rigid than amorphous cellulose because it contains fewer water molecules. Therefore, it provides greater tensile strength.

Amorphous cellulose is less stiff than crystalline cellulose but is much stronger.

Hemicelluloses

Hemicelluloses are complex carbohydrates that contain many sugars.

Some hemicelluloses include xyloglucan, arabinoxylan, glucomannan, mannan, galactomannan, glucuronoxylan, and xylans.

These compounds contribute to the overall mechanical properties of the cell wall.

Xyloglucan is an important hemicellulose that makes up about 10% of the total dry weight of the cell wall.

It is considered the most abundant hemicellulose in dicots. In monocots, xyloglucan is replaced with xylans.

Lignin

Lignin is a phenolic compound that is produced from shikimate acid.

Lignin consists primarily of guaiacyl propane (G) and syringyl propane (S). G and S are joined together by ether linkages.

Lignin is one of the main contributors to the stiffness of the cell wall. Because lignin is so strong, it does not dissolve easily when treated with solvents.

Consequently, it cannot be removed from the cell wall without destroying the structure of the plant cell.

The Function Of The Cell Wall

The primary function of the cell wall is to provide strength and rigidity to the plant cell.

The secondary function of the cell wall includes protecting against pathogens and insects.

Protection Against Pathogens

The cell wall acts as a barrier to protect plants against pathogen attacks.

When a pathogen invades the host plant, it must penetrate through the cell wall before reaching the cytoplasm.

To do this, the pathogen produces enzymes that can digest the cell wall. Once the pathogen reaches the cytoplasm, it uses these same enzymes to destroy the plant’s cells.

Protection Against Insects

Insects use their mouthparts to puncture holes in the plant cell wall. Then they inject saliva containing digestive enzymes into the plant tissue.

Digestive enzymes break down the cell wall. After the insect ingests the plant tissue, it moves on to another part of the plant.

Location Of The Cell Wall

Most of the cell wall is located inside the plasma membrane. However, some parts of the cell wall are exposed to the outside environment.

For example, the cuticle is exposed to the atmosphere while the epidermal cell wall is not.

The pectins in the middle lamella are also exposed to the outside environment and are involved in intercellular adhesion.

This type of adhesion allows the plant cell to stay connected to other cells.

What Is The Cell-Membrane?

A cell membrane is made up of phospholipids, proteins, and polysaccharides. Phospholipids consist of fatty acids attached to phosphate groups.

Proteins are composed of amino acids and lipids. Polysaccharides are large molecules that are built from sugar units. They may have branches or side chains.

Types Of Lipid Structures

Phosphatidylethanolamine (PE)

This lipid has two fatty acids attached to a phosphate group. PE is found in the inner leaflet of the plasma membrane.

Phosphatidylcholine (PC)

This lipid has two fatty acids linked to a phosphate group. PC is found in both leaflets of the plasma membrane. It is often used for signaling between cells.

Sphingomyelin (SM)

SM has long-chain fatty acids attached to a sphingosine backbone. SM is found in the outer leaflet of the plasma membranes.

Cholesterol (CHOL)

Cholesterol is an important component of biological membranes. It is usually found in high concentrations in the plasma membrane.

Proteins

There are many different types of proteins. Some are soluble and others are insoluble.

Most proteins contain amino acids and lipids, but there are also carbohydrates and nucleic acids.

Nucleic Acids

DNA contains deoxyribonucleotides and ribonucleotides. RNA contains only ribonucleotides and does not have a 5′ cap structure.

DNA is packaged with histones and forms chromatin. Chromatin is tightly packed around the DNA. Nucleosomes are the basic building blocks of chromatin.

Each nucleosome consists of 147 base pairs of DNA wrapped around eight histone octamers.

Protein synthesis occurs when messenger RNAs attach to transfer RNAs. Transfer RNAs carry amino acids to the ribosome where protein synthesis takes place.

Ribosomes are complex structures that contain three subunits: small, medium, and large. Small and large subunits form a central cavity that holds mRNA.

A large subunit binds tRNA and initiates translation. The medium subunit attaches to the small subunit and helps stabilize the large subunit during translation.

Cell-Membrane Definition

The cell membrane is the boundary separating the cytoplasm from the extracellular space.

It is made up of various components including phospholipids, cholesterol, and glycoproteins.

The composition of the cell membrane varies depending on what part of the cell it is.

In animal cells, the cell membrane is mostly made up of phospholipoproteins and cholesterol. Cholesterol is needed for the proper functioning of the cell membrane.

Functions Of The Cell-Membrane

The main function of the cell membrane is to separate the interior of the cell from the exterior.

The cell membrane prevents substances from entering the cell from the surrounding environment and keeps substances out of the cell.

The cell membrane also provides nutrients to the cell by allowing water to enter the cell through pores called aquaporins.

The cell membrane also transports ions into and out of the cell.

Additionally, the cell membrane regulates the movement of molecules across the membrane.

Molecules can move freely across the cell membrane if they are smaller than about 1 nanometer (nm).

However, larger molecules cannot cross the cell membrane unless channels or pores are present. These channels allow certain molecules to pass through the membrane.

Structure Of The Cell-Membrane

The cell membrane is composed of two layers. The inner layer is called the lipid bilayer. This layer is mainly composed of phospholipids.

Phospholipids consist of a hydrophilic head group and a hydrophobic tail. A single molecule of phospholipid has both polar and nonpolar regions.

The polar region interacts with other molecules while the nonpolar region remains in contact with itself. The outer layer of the cell membrane is called the glycocalyx.

The glycocalyx is a meshwork of carbohydrate chains attached to the plasma membrane. The glycocalyx allows for the attachment of receptors to the cell surface.

Location Of The Cell-Membrane

The cell membrane is located between the nucleus and the cytoskeleton. The nucleus is inside the cell and the cytoskeleton is outside the cell.

The cell membranes of different parts of the body have different functions. For example, red blood cells have only one type of cell membrane.

White blood cells have multiple types of cell membranes. The cell membrane of neurons contains many proteins involved in neurotransmission.

Neurons communicate with each other using electrical signals.

Cell Wall Vs Cell-Membrane

A cell wall is an extracellular structure that surrounds the entire cell. The cell wall consists of polysaccharides such as cellulose, chitin, and pectic acid.

Unlike the cell membrane, which is found only at the surface of the cell, the cell wall is found throughout the cell.

The cell wall helps protect the cell against mechanical damage and infection.

Cell Membrane Components

Phospholipids: Phospholipids are amphipathic molecules consisting of a polar head group and a fatty acid tail. They form the major component of biological membranes.

There are three classes of phospholipids: phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. Each class has its unique chemical properties.

Glycosphingolipids: Glycosphingolips are sugar lipids containing ceramide. Ceramides are sphingolipids with a long chain base and a short-chain base.

The long-chain base may be saturated or unsaturated. The short-chain base may be saturated, monounsaturated, or polyunsaturated.

The most common glycosphingolipids are cerebrosides, gangliosides, sulfatides, and globotriaosylceramides.

Proteins: Proteins are macromolecules made up of amino acids.

Proteins perform various important functions in the cell including structural support, enzymatic activity, signal transduction, and transport.

Proteoglycan: Proteoglycans are large glycoprotein complexes. They contain glycosaminoglycan side chains that bind water-soluble molecules.

Proteoglycans help maintain the shape of tissues by binding to extracellular matrix components.

Lipids: Lipids include fats, waxes, sterols, and pigments. Fatty acids are carbon chains with carboxylic groups on their ends.

Sterols are cholesterol derivatives. Waxes are esters of alcohol and fatty acids. Pigments are organic compounds that absorb light.

Cell Wall Components

Polysaccharides: Polysaccharides are complex carbohydrates. They can be linear or branched. Linear polysaccharides do not branch.

Branched polysaccharides branch into smaller units.

Examples of linear polysaccharides are glucose, fructose, mannose, galactose, xylose, arabinose, N-acetylglucosamine, N-acetylgalactosamine, and glucuronate.

Examples of branched polysaccharides are starch and amylopectin.

Chitin: Chitin is a linear homopolymer of β-(1→4)-linked D-N-acetylglucoseamines. It is insoluble in water but soluble in dilute alkali solutions.

Chitin occurs naturally in the exoskeletons of crustaceans, insects, mollusks, and fungi.

Pectin: Pectin is a highly methylated hetero-polysaccharide composed of α(1→5) linked rhamnogalacturonan I (RG-I), RG-II, and HG regions.

Pectin is one of the main constituents of plant walls. In plants, pectin is also present in the middle lamella between epidermal cells.

Hemicelluloses: Hemicelluloses are non cellulosic polysaccharides. They consist of pentoses, hexoses, uronic acids, acetylated sugars, and ferulic acid.

Hemicelluloses provide strength to plant cell walls.

Waxes: Waxes are hydrocarbons that have been modified by oxidation. They occur as waxy solid substances at room temperature.

Waxes are used for waterproofing, lubrication, insulation, and flame retardant applications.

Sterols: Sterols are lipid molecules that are derived from squalene. Squalene is an oxidized form of lanosterol. Lanosterol is produced by the enzyme lanosterol synthase.

Sugar Chains: Sugar chains are linear polymers of monosaccharides joined together through phosphodiester bonds.

A single monosaccharide unit has four possible stereoisomers; therefore, there are 16 different stereoisomer combinations. Each combination is called a diastereomer.

There are two types of sugar chains: Oligosaccharide chains and glycoconjugates. An oligosaccharide chain consists of three or more monosaccharide units.

Glycoconjugates are formed when a protein binds to another molecule.

Glycogen: Glycogen is a polymer of glucose. The structure of glycogen depends upon its level of polymerization.

When it is low in polymerization, it is referred to as alpha-glycogen. High levels of polymerization produce beta-glycogen.

Alpha-glycogen is found in the liver, muscle, brain, heart, and other tissues. Beta-glycogen is stored in the liver and pancreas.

What Is The Difference Between The Cell Wall And The Cell Membrane?

The cell wall is located outside the plasma membrane. The cell membrane is located inside the plasma membrane.

The difference between these two structures is important because they have very different functions. The cell wall provides support for the cell.

The cell membrane allows nutrients to enter and wastes to leave the cell.

How Does The Cell Wall Support Cells?

The cell wall contains hemicellulose, cellulose, pectins, chitin, and waxes. These components work together to give the cell wall its structural integrity.

The cell wall is made up of layers. The outermost layer of the cell wall is the primary cell wall. This layer gives the cell shape and protects the cell from damage.

The next layer is the secondary cell wall. It surrounds the primary cell wall. The secondary cell wall strengthens the primary cell wall.

The third layer is the tertiary cell wall. This layer is made up of complex carbohydrates such as xyloglucans. Xyloglucans help hold the cell wall together.

The innermost layer of the cell is the endoplasmic reticulum. The endoplasmic reticular system helps create proteins, which are then transported into the Golgi apparatus.

The Golgi apparatus sorts out proteins so that only those needed for certain functions can be released.

How Does The Cell Membrane Allow Nutrients Into And Wastes Out Of The Cell?

The cell membrane is composed of lipids (fatty compounds) and proteins. Lipids are hydrophobic (water-repelling). Proteins are hydrophilic (water-attracting).

Therefore, the lipid portion of the cell membrane repels water while the protein portion attracts water. Water cannot pass through the lipid portion of the membrane.

However, some molecules can pass through the lipid part of the membrane. For example, ions can pass through the lipid portion.

In addition, small molecules such as oxygen, carbon dioxide, and amino acids can pass through the lipid side of the membrane.

Proteins form channels on the cell surface. Channels allow nutrients to enter the cell. They also permit waste products to exit the cell.

Some channels are large enough to let larger molecules pass through. Others are smaller and allow only specific molecules to pass through.

Final Thoughts

The cell wall is an essential component of plant cells. It provides strength to the cell and also serves as a barrier against pathogens.

The cell membrane allows the cell to take in nutrients and expel waste.

This article discussed the cell wall and how it supports cells. You also learned about the cell membrane and how it allows nutrients to enter and waste products to leave the cell.

Now that you know what the cell walls and membranes look like, you will be able to identify them on your microscope slides!

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