Quiz 4: Cell Structure and Function
Cells without the presence of nucleus are called prokaryocytes (Greek: pro , before and karyon, nucleus). Cells which contain nucleus are called eukaryocytes. (Greek: eu , true and karyon, nucleus). Amoeba is a unicellular eukaryotic organism. Eukaryotic cells are larger than the prokaryotic cells, the volume of eukaryotic cells is thousand times larger than the prokaryotic cells but its volume is only hundred times larger. This low surface-to-volume ratio is compensated by compartmentalization of their activities into internal organelles. These organelles serve to meet all the metabolic activities needed for cell survival. As the size of the cell increase, its volume increases proportionately but more than its surface area. So, if there is more increase in surface area, it may help in more efficient take-up of materials from its surrounding environment, but this is not actually happening, because, the cells have low surface to volume ratio. Thus, if the amoeba increases to the size of two-ton weight, it fails to meet the cell demands to survive such as oxygen intake and excretion of carbon dioxide, etc. and it eventually dies.
The cell membranes are made of "phospholipids" and they are major kind of lipids found in all biological membranes. As the name indicates, the phospholipid contains phosphate and fatty acid molecules in its structure. Cell membrane also contains proteins, lipids and carbohydrates in its structure. The lipid end (fatty acid) of fatty acid is non polar (hydrophobic), and the phosphate attached alcohol serves as a polar head. In the structure of cell membrane, the outer and inner layers are exposed with these polar ends and the middle part contains lipid layer. Thus, the hydrophobic lipid layer is sand witched between two hydrophilic phosphate layers, so it is also called as "phospholipid bilayer (PLP, i.e. phosphate-lipid-phosphate)". Thus, due to the amphipathic nature (both polar and nonpolar) of phospholipids, they provide the nature of selectivity to the cell membrane. Thus, the correct option is (c) lipid bilayer and proteins Cell membrane contains lipid bilayer that forms a sheet like structure. Proteins and carbohydrates are embedded in this lipid bilayer. Thus, the options (a) and (b) are incorrect.
Cell is the basic structural unit of the body. Membrane is a structural component that separates intracellular and extracellular environment by means of plasma membrane. Inside the eukaryotic cell (nucleus having), numerous cell organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, ribosomes, nucleus, etc. are present, which serve various functions. Ribosomes are the particles (not organelles), which are present in both prokaryotic and eukaryotic cells. They are composed of two sub units, and each sub unit is a combination of protein and rRNA (ribosomal RNA). The main function of ribosomes is protein synthesis. They are not membrane bound structures, some are freely found in the cytoplasm and some are bound to the surface of rough endoplasmic reticulum. The polypeptide chain is formed in the ribosomes, and then transferred to the Golgi apparatus, where the final modifications are done. The Golgi apparatus then sorts the modified molecules and packages into new transport vesicles which reach their particular destinations. Endoplasmic reticulum is physically continuous with the outer membrane of nuclear membrane. Based upon the presence of granules, which are studded with ribosomes, it is of two types, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER ). RER contains ribosomes and hence capable to synthesise polypeptides (proteins). SER does not have attached ribosomes, it synthesises lipids (Eg: phospholipids and steroids). Mitochondria are the organelle present in the cell, which can be seen only through electron microscope. The main function of mitochondria is energy production or ATP (adenosine triphosphate) synthesis through the metabolism of nutrients. And so, because of this function, they often called as "power houses of the cell." Depend upon the energy needs; their occurrence varies from cell to cell. For example, skeletal muscles have more mitochondria; whereas red blood cells are completely lack of mitochondria. Thus, the Golgi bodies, RER, and mitochondria are the key cellular organelles that participate in cells vital functions such as protein synthesis and energy metabolism. Also they are essential to maintain the microenvironment of the cell in a homeostatic condition.