Cell Parts PDF

Cell Walls Cell walls (or outer membrane) are found in bacteria, plants, fungi, and algae and help to form the shape of the cell. The cell wall is an additional layer surrounding the cell membrane; it is rigid and for the most part cannot change its shape. As the outermost layer, the cell wall plays...

Cell Walls Cell walls (or outer membrane) are found in bacteria, plants, fungi, and algae and help to form the shape of the cell. The cell wall is an additional layer surrounding the cell membrane; it is rigid and for the most part cannot change its shape. As the outermost layer, the cell wall plays an important role in providing support, structure, and protection. The combination of the cell membrane and the outer membrane (cell wall) are referred to as the cell envelope. The composition of cell walls is also very diverse. The bacterial cell wall contains peptidoglycan, a peptide-polysaccharide matrix, which can vary in thickness. The peptidoglycan layer is essential for bacterial survival as it protects the cell from potentially damaging environmental stresses. Interestingly, despite the harsh conditions in which they are found, organisms in the class Archaea lack peptidoglycan in their cell walls. Instead, the cell wall is composed primarily of surface-layer proteins (called an S-layer) that provide both a physical and chemical protective barrier to the harsh surrounding environments in which they are often found. The specific type of protein comprising the S-layer varies from organism to organism. Diversity also exists among the various subgroups of eukaryotic cells. Some cell walls may contain polysaccharides, like cellulose and pectin, whereas fungi have cell walls containing cellulose and chitin. Yeasts have cell walls composed of mannoproteins — highly glycosylated polypeptides called mannan and mannose. Diatoms, a type of algae, have thick cell walls containing silica (the main component found in glass), making them very rigid. Cytoplasm The cytoplasm constitutes the open volume within a cell. It is comprised mostly of water containing dissolved substances important to the functioning of the cell. Within the cytoplasm are organelles, membrane-enclosed structures that perform specific functions, as described below. All of these organelles, with the exception of the ribosomes, are exclusive to eukaryotic cells. Nucleus The nucleus is the defining organelle of all eukaryotic cells and is defined by a porous double lipid bilayer that contains the genomic content of the cell (DNA). The nucleus is thus the command center of the cell. In contrast, the genomic material of prokaryotic cells is found within the cytoplasm. Within the nucleus is the nucleolus, the site of ribosome synthesis. Ribosomes Ribosomes are responsible for protein synthesis. They are located in the cytoplasm of prokaryotic cells and are found attached to the endoplasmic reticulum in eukaryotic cells. Endoplasmic Reticulum (ER) The endoplasmic reticulum (ER) is a series of membrane-enclosed sacs and interconnected tubes (lumen). The membrane of the endoplasmic reticulum is continuous with the nuclear membrane. The endoplasmic reticulum is composed of the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER). The rough appearance is due to the presence of ribosomes that stud the ER membrane surface. Under a microscope, this particular section appears rough in texture. Golgi Complex The Golgi complex is series of interconnected flattened sacs (vesicles) located between the nucleus and cytoplasmic membrane. The Golgi complex serves as the distribution center of the cell. Here, the proteins may undergo modifications or be packaged before being distributed. Lysosomes Lysosomes are membrane-enclosed organelles that contain hydrolytic enzymes capable of degrading unwanted cellular debris, such as proteins, lipids, nucleic acids, and polysaccharides. Lysosomes serve as the waste disposal system of the cell. Mitochondria Mitochondria are double membrane-enclosed organelles primarily responsible for generating ATP, a chemical source of energy within the cell. Thus, the mitochondria can be considered the power house of the cell. To maximize its power output, mitochondria contain tightly packed folded internal membranes called cristae to drastically increase the surface area where chemical reactions occur. The number of mitochondria may vary between cell types ranging from zero in red blood cells to more than 2000 in liver cells. Chloroplasts Chloroplasts are double membrane-enclosed organelles specific to algae and plants, and they house the photosynthetic pigment chlorophyll. Important to note, animal cells do not contain chloroplasts. Chloroplasts are the site of photosynthesis—the process of capturing sunlight and converting it into the usable energy sources ATP and NADPH.