Cell Wall

Cell Wall Definition

Outside of the cell membrane, a cell wall is an outer layer that surrounds certain cells. Most bacteria, archaea, and plants have cells without cell walls, but all have cell membranes. In addition to providing strength and structural support to the cell, the cell wall also controls to some extent what types and concentrations of molecules enter and leave the cell. 

Cell walls are composed of different materials depending on the organism. Different organisms have evolved their cell walls at different times.

Cell Wall Functions

There are a few different functions that the cell wall serves. It is flexible, yet strong, which helps protect the cell against physical damage. The cytoskeleton gives the cell its shape and allows the organism to maintain a certain shape. 

Besides providing protection against pathogens, the cell wall can also prevent the invasion of bacteria. Small molecules can pass through the cell wall, but larger molecules that could harm the cell cannot.

Cell Wall Structure

Plant Cell Walls

Plant cell walls are composed primarily of cellulose, a carbohydrate that forms long fibers and gives them their rigidity. As cellulose fibers group together, they form bundles called microfibrils. Aside from hemicellulose and pectin, liginin and hemicellulose are also important carbohydrates. 

Together with structural proteins, these carbohydrates form the cell wall. Plant cells in the process of growing have thin primary cell walls. Secondary cell walls are formed once the cells have fully grown. On the inside of the primary cell wall, there is a thick layer called the secondary cell wall. 

It is this layer that is commonly referred to as a plant’s cell wall. Also in between plant cells, there is a layer called the middle lamella, which is pectin-rich and helps plant cells stick together.

Plant cells’ cell walls maintain turgor pressure, or the pressure of the cell membrane pressing against the cell wall. It is ideal for plants to have a lot of water within their cells, resulting in a high level of turgidity. 

In contrast to an animal cell without a cell wall, which can swell and burst when excessive water diffuses into it, plants need hypotonic solutions to maintain turgor pressure and their structural shape (more water inside than outside, leading to lots of water entering the cell). So that the cell does not burst, the cell wall efficiently holds water inside. 

A wilting plant occurs when turgor pressure is lost. The turgor pressure gives plant cells their characteristic square shape; the cells are full of water, so they fill up the space available and press against each other.

Algae Cell Walls

The diversity of algae’s cell walls reflects their diversity as a group. The cell walls of some algae, such as green algae, are similar to those of plants. Among other polysaccharides or fibrils, brown algae and red algae contain cellulose. 

The cell walls of diatoms are made of silicic acid. Mannans, xylans, and alginic acid are also important molecules in algal cell walls.

Fungi Cell Walls

It is a glucose derivative that has a similar structure to cellulose that makes up the cell walls of fungi. Insects and crustaceans have rigid exoskeletons made of chitin; this molecule is found in the layers of their exoskeletons. Along with lipids and proteins, glucans, another glucose polymer, are also found in the fungal cell wall. 

Hydrophobins are proteins found in the cell walls of fungi. Hydrophobins are found only in fungi, and they help the cells adhere to surfaces and control water movement. In fungi, the cell wall surrounds the cell membrane and is the most external layer.

Bacteria and Archaea Cell Walls

Bacteria’s cell walls contain polysaccharide peptidoglycan, which lets small molecules pass through. A cell envelope consists of both a cell membrane and a cell wall. For many bacteria, the cell wall is essential to survival. Bacteria, which are single-celled, benefit from mechanical structure and are protected from internal turgor pressure. 

The cell wall of bacteria prevents water from rushing into the cell due to a higher concentration of molecules such as proteins within them than in the environment. It is also possible to stain cells differently based on the thickness of their walls. Generally, bacteria with thick cell walls are gram-positive, while bacteria with thinner cell walls are gram-negative. Gram staining is used to identify bacteria.

Although archaea and bacteria share many similarities, very few archaeal walls contain peptidoglycan. Archaea have several types of cell walls. Pseudopeptidoglycan is found in some animals, polysaccharides in others, glycoproteins in others, and surface-layer proteins in bacteria.

Related Biology Terms

  • Cell membrane – A membrane found on the outside of all cells that separates them from the outside environment.
  • Turgor pressure – Water pressure inside cells.
  • Chitin – A polysaccharide that is a main component of fungal cell walls and also of the exoskeletons of certain animals like insects.

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