A cell wall is located outside of the cell membrane as the outermost layer of plant, bacteria, algae and fungi cells. It has several functions and is an important component of every cell. It helps to keep the cell rigid and intact, and it protects the entire cell from infection by not allowing pathogens to enter the cell. It also allows transportation of materials both in and out of a cell. Human and animal cells do not include a cell wall, but they do have a cell membrane.
What Is the Composition of the Cell Wall?
The cell wall is structured differently depending on the type of cell. In plant cell walls, it is composed of strong fibers of a carbohydrate polymer called cellulose. Carbohydrates are produced in plant cells during photosynthesis and are the main source of energy for plant cells. In bacterial cells, the cell walls are a compound made of a sugar and an amino acid polymer called peptidoglycan.
What Is the Structure of a Plant Cell Wall?
The plant cell wall has up to three sections in layers, know as the middle lamella, the primary cell wall and the secondary cell wall.
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The middle lamella is the outermost layer. It contains polysaccharides known as pectins. The purpose of this layer is to aid in cell adhesion by helping adjacent cells bind to each other.
The primary cell wall is in the center layer of the plant's cell wall between the middle lamella and the cell membrane. It is found in plants that are actively growing and is made mainly of cellulose microfibrils that are contained inside of a matrix of hemicellulose fibers and pectin polysaccharides. This portion of the cell wall provides all of the strength and flexibility that is pertinent to cell growth.
The secondary cell wall is formed between the primary cell wall and the plasma membrane in some plant cells, but not all. After the primary cell wall stops dividing and growing, it can thicken to form the secondary cell wall. This is a rigid layer in the cell wall that strengthens and supports the cell's shape. It can contain lignin in addition to the cellulose and hemicellulose that were in the primary cell wall. Lignin has the important task of strengthening the cell walls in plants and aiding in water conductivity in the plant's vascular tissues, which allow water transport both in and out of a cell.
What Are the Functions of a Plant Cell Wall?
A cell wall in plants has many functions that enable the plant to grow. When a seedling sprouts after receiving the right amount of water and sunlight, in the right season and temperature, two leaves emerge from the seed. Those two original leaves open in opposite directions so that they can get an adequate amount of sunlight and grow through the process of photosynthesis. The cell wall supports the plant so it can stand upright and give it the mechanical strength to hold the smaller leaves in that position. The plant cell wall also prevents the cell membrane from pushing on the cell and changing its shape while resisting water pressure. Plant cell walls control cell growth by sending messages to tell a cell when to begin or stop cell division.
A plant cell wall contains many small openings in it that allow substances to enter or leave the cell; however, it allows only very small substances to enter and leave while it provides a physical barrier to larger substances such as pathogens that can damage the plant. The plant cell wall stores the food as carbohydrates that are needed for metabolic processes within the plant and are needed for the plant to grow and stay healthy.
What Is the Composition of the Cell Wall of Bacteria?
The bacterial cell wall is composed of peptidoglycan, a unique molecule that is found only in the cell walls of bacteria. Peptidoglycan is a polymer made of double sugars and amino acids that give the cell rigidity and maintain the shape of the bacteria. These molecules form sheets to enclose and protect the plasma membrane of the bacterial cells.
The cell wall in gram-positive bacteria has several layers of peptidoglycan to increase its thickness. Gram-negative bacteria have a thinner cell wall with a much lower percentage of peptidoglycan, and the wall includes lipopolysaccarides as an outer layer. This layer acts as a poison for pathogenic bacteria that cause diseases. The lipopolysaccarides layer also protects dangerous disease-causing bacteria from some antibiotics, including that of penicillin, increasing the survival of the bacteria's cells.
What Are the Differences Between the Cell Wall and the Cell Membrane?
Cell walls and cell membranes both serve as the most external portion of a cell. The role of each is very much alike in terms of transportation and inter-cell communication. Both the cell membrane and the cell wall exist from the formation of a cell through division until the cell's death.
There are 10 distinct differences between a cell membrane and a cell wall. The cell wall is present only in the cells of plant and bacteria cells; animal cells do not have a cell wall but instead have only a cell membrane.
The cell wall encloses a plasma membrane and provides the cell with enough rigidity to retain its shape. The cell membrane of an animal cell, however, is more flexible. The cell in an animal still has a shape, but the membrane is not nearly as rigid as a cell wall would be.
The cell wall in a plant is composed of pectin, chitin and lignin, while in bacterial cells it is composed of glycolipids and glycoproteins. A cell membrane is a multilayered lipid made of lipid proteins.
The cell wall is rigid and also has a fixed and distinct shape; pressure applied to a cell wall can break it. The cell membrane is flexible and has the ability to contract to change its shape as needed and move away from force.
The cell wall does not have any receptors, but a cell membrane has receptors that are used to receive signals from external chemical messages of other cells.
The cell wall includes flagella and pilli as small hairlike items on the exterior. In the cell membrane of a bacterial cell, it has flagella to initiate movement and pilli to help in the reproductive functions of cell division.
The cell wall actually grows in thickness over time and is present from the time the cell is developed through cell division until the cell dies. The cell membrane stays the same thickness from the time of creation and all throughout its lifetime in an organism.
The cell wall is semi-permeable to allow the passage of substances both in and out of the cell. The cell membrane is also permeable and it controls the movement of substances in and out of the cell, but it is selective about the substances it lets through.
The cell wall protects the cell from forces in the external harsh environment while the cell membrane has the functions of permeability, receiving chemical signals, conduction of nerve cells, cell division and sexually reproducing.
The cell wall does not need nutrition except if it becomes damaged and it needs to repair itself. The cell membrane requires constant nutrition from the cell because it can shrink due to loss of nutrition and water.
What Is Cell Diffusion?
Cell diffusion is the process by which substances such as ions, molecules and water enter and leave cells to support various cellular processes. Molecules that are in a highly concentrated area move to an area of lesser concentration. The process of diffusion usually occurs until molecules on both sides of the membrane reach a state of equilibrium.
What Is Passive Transport Diffusion?
The passive transport diffusion process includes substances that move across a cell membrane without any use of cellular energy. It is a naturally occurring process in which they move from a higher-concentrated area to a lower-concentrated area. Passive transport is a spontaneous process that is directly related to the permeability of the cell membrane. All cells allow some items in and keep others out of the cell. This allows some substances to move across a cell membrane easily than others. Since plant cells contain a cell wall, they are thicker and have less permeability than other types of molecules. Many cellular processes involve the use of water, and as such, water can easily move through cell membranes, compared to other molecules.
What Is Facilitated Diffusion?
This type of diffusion process is still considered passive in nature. Several types of molecules, such as glucose, are not able to go through the pores of a cellular membrane to diffuse properly on their own. This is when a carrier protein can bind to the glucose and carry the molecule through the cell membrane and out of the cell. It is still considered a passive process because it doesn't use cellular energy for it to occur.
What Is Active Transport Diffusion?
Active transport diffusion is the opposite of passive diffusion. In this process, molecules move across the membrane to move to an area where there is a higher level of concentration, instead of moving to a lower-concentrated area to reach equilibrium.
Primary active transport is the process that uses metabolic energy to transport the molecules across a cell membrane. There is another form of active transport called secondary active transport, which also uses energy but is not using metabolic energy in the form of ATP. This method occurs when entropy and pumping ions of the cell have a direct effect on the electrochemical potential difference. Common items that transfer between cells are ions, amino acids and glucose, which are needed in large amounts for the cell's processes.