My AP Biology Thoughts  

Unit 2 Cell Structure and Function

Welcome to My AP Biology Thoughts podcast, my name is Helena and I am your host for episode #55 called Unit 2 Cell Structure and Function: Function of the Cell Wall. Today we will be discussing the cell wall and its importance to living organisms.

•  The cell wall is a unique structure to plants, fungi, and some prokaryotic organisms. This structure surrounds the cell’s plasma membrane and provides structural support and protection to the organelles and the cell against mechanical and osmotic stress. The cell wall provides strength, shape, and rigidity to the cell, it is responsible for transporting substances between the interior and exterior of the cell, it acts as a barrier, it functions as a storage unit by storing carbohydrates for use in plant growth, and it allows for turgor pressure which is exerted by fluid in the cell that presses the cell membrane against the cell wall. 

•  So now that you know what the cell wall does, we can take a closer look at what exactly it is made off. The plant cell wall is mainly made of cellulose which, fun fact, is the most abundant macromolecule on earth. Along with cellulose the cell wall is also composed of microfibrils, hemicellulose, pectin, lignin, and soluble protein. These components are arranged in layers. The first layer is the primary cell wall, and this is positioned closest to the inside of the cell. It mainly consists of pectic polysaccharides and structural proteins, and It is permeable and thinner than the other layers. This layer provides the strength and flexibility needed for cell growth. The next layer is the middle lamella which is the outermost layer. It primarily consists of pectic and it acts as an interface between the other neighboring cells. It also glues them together. The last layer is the secondary cell wall. It is formed inside the primary cell wall and it can consist of cellulose and lignin. It can provide additional rigidity and waterproofing. This layer also gives the cell the square/rectangular shape. It is the thickest layer and is permeable, along with the whole cell wall. In fungi the cell wall is primarily made of chitin. Fungi also have hydrophobins which gives the cell strength, helps it adhere to surfaces, and helps control the movement of water into the cell. In prokaryotes the cell wall is primarily composed of peptidoglycans. It contains an inner peptidoglycan layer and an outer layer composed of lipoproteins and lipopolysaccharides. A unique property of the cell wall is that it is fully permeable to smaller molecules while the membrane is selectively permeable.  

The cell wall is a vital component to plants, fungi, and some prokaryotic organisms. It provides unique properties that allow for these organisms to survive. For example, turgor pressure, as a I mentioned earlier, is the pressure of the cell contents against the cell wall. This is what allows for organisms to hold water and maintain a solid structure. Turgor pressure decreasing means that the plant has lost water. Visually, the loss of turgor pressure can be seen by a wilted flower or leaf. If a plant cell did not have a cell wall, turgor pressure would not be able to happen because the cell membrane cannot support a hypotonic environment, therefore the cell would burst. Another important property of the cell wall is that it provides strength and structure to the cell....