Structure of the Nucleus, Endoplasmic Reticulum, Ribosomes, Mitochondria, Golgi Dody, and Lysosomes - Assignment Example

?Nucleus The cell nucleus is often referred to as the ‘brains’ of the cell, as it contains most of the genetic material needed for cell function. This genetic material (mostly DNA) is incredibly compact within the nucleus, as all the relevant information for any cell needs to fit within this small area (Campbell & Reece, 2005). The main function of the cell is to control the expression of proteins (as directed by DNA and RNA). The nucleus itself is a membrane-bound organelle, which means that it is slightly separated by a thin membrane from the rest of the cell components (Tortora & Derrickson, 2008). However, this poses a challenge as messages need to be carried from the enclosed DNA found in chromosomes to the rest of the cell, which is why there are nuclear pores to allow the passage of water-soluble molecules (Campbell & Reece, 2005). The chromosomes themselves are mostly found tightly-packed as chromatin (except during replication), and this function allows the huge amount of human DNA to be contained within each cell (Campbell & Reece, 2005). The structure of these chromosomes changes throughout the cell cycle, particularly relating to cell division. During mitosis or meiosis (cell division), chromatin structures become more condensed and as a result of this change in structure, transcription stops. During mitosis, structures known as microtubules form, anchoring the chromatid to the cell wall. This attachment is what allows the cell to divide, splitting sister chromatids in two to form two haploid daughter cells (Campbell & Reece, 2005). Additionally, the nucleus also contains a structure known as the nucleolus, which contains DNA which codes for ribosomal RNA. The ribosomes themselves have an important function in protein assembly (covered below), and thus their assembly within the nucleolus is essential for cell function. Endoplasmic Reticulum The endoplasmic reticulum (ER) is an organelle formed of a system of vesicles, tubules and cisternae. The cisternae are sac structures which are held together by the cellular cytoskeleton. Overall, this general structure of the ER cannot be linked to function, as the ER can vary greatly between cell type. However, this structure can change according to the needs of the cell, and the ER is generally always responsible for the synthesis of various proteins as required by the cell (Campbell & Reece, 2005). The ER itself is generally split into two sections, known as the rough endoplasmic reticulum and the smooth. The rough ER plays a part in the synthesis of proteins, which is why its surface is covered in ribosomes involved in the process (Tortora & Derrickson, 2008). The ribosomes themselves are bound to the rough ER on a transitory basis, depending on the signal from the nucleus (Tortora & Derrickson, 2008). In fact, the reason that the rough ER is so-called is because the fact that it is studded with these ribosomes makes it look rough under an electron microscope (Tortora & Derrickson, 2008). The ribosomes themselves only bind to the rough ER as required by the cell, and are associated with the initialization of protein-synthesis for part of a secretory pathway (Tortora & Derrickson, 2008). The smooth ER is involved in the synthesis of lipids required by the cell. It is the increased surface area created by the smooth ER that allows the action of enzymes involved in lipid and steroid synthesis. Each part of the ER thus has a distinct function in regulating cell behavior. Ribosomes Ribosomes are found within all living cells and are primarily involved in protein synthesis, also known as translation (Tortora & Derrickson, 2008). To do this, the ribosomes link together amino acids (found within the cell) to a template specified from messenger RNA (mRNA). This message, in turn, has come from the DNA code found within the nucleus. There are two subunits which comprise a ribosome; the small unit which reads the mRNA template, and the larger unit which anchors small amino acid units together to create a larger, fully formed protein (Campbell & Reece, 2005). In protein synthesis, the mRNA travels from the nucleus towards the ER, where the two parts of the ribosome begin to assemble around it. The ribosome itself moves along the mRNA message, and the larger subunit of the ribosome interacts with transfer RNA to attach and organize the amino acid subunits of the protein in the correct order and position (Tortora & Derrickson, 2008). The large ribosomal subunit has a central protuberance, a stalk and a ridge which serve the purpose of being aminoacyl binding sites during the translation process (Tortora & Derrickson, 2008). As translation occurs, the mRNA chain moves through the small subunit of the ribosome, ensuring that the amino acids are joined to the peptide chain in the appropriate order to ensure correct protein synthesis. Mitochondria The mitochondria are the energy supply for the cell (Tortora & Derrickson, 2008). They have a complex structure which is not unlike that of a prokaryotic cell (Tortora & Derrickson, 2008). Firstly, a mitochondrion has an inner and an outer membrane. The outer membrane is studded with proteins known as porins, which play a role in allowing small molecules to diffuse through the mitochondrial membrane. Larger molecules can also pass through the membrane if they have an appropriate signaling sequence which binds to the translocase of the outer membrane. The inner membrane is also studded with proteins, which perform various functions. Some are involved in oxidative phosphorylation, which is the system in which energy is released by the oxidation of certain nutrients in the production of ATP (Tortora & Derrickson, 2008). ATP synthase can also be found in the inner membrane, which generates ATP. Additionally, the inner membrane is studded with transfer proteins which help to maintain a homeostatic state within the matrix and allows for regulation of metabolites into and out of the mitochondrial matrix (Tortora & Derrickson, 2008). The cristae of the mitochondrion play a role in increasing the surface area of the mitochondrion, allowing for greater production of ATP than a flat surface (Tortora & Derrickson, 2008). Between the inner and outer membranes, there is a matrix containing enzymes, certain specialized ribosomes, tRNA and mitochondrial DNA. miDNA is inherited through the maternal line. Mitochondrial DNA is important as it codes for certain proteins involved in the synthesis of ATP without the input of nuclear DNA (Tortora & Derrickson, 2008), hence why the mitochondria are sometimes referred to as being similar to prokaryotic cells (Tortora & Derrickson, 2008). Golgi Body The Golgi body is found within the cytoplasm of eukaryotic cells. The structure of the Golgi body is essentially stacked structures of cisternae, which are membrane-bound (Tortora & Derrickson, 2008). Within each cisterna, specialist Golgi enzymes can be found which play a role in the modification of cargo proteins moving within the body itself (Tortora & Derrickson, 2008). The Golgi body’s function is to modify and package macromolecules as part of cellular activity; these macromolecules are commonly used within the cell or secreted as part of certain pathways (Tortora & Derrickson, 2008). Additionally, the Golgi body plays a role in the synthesis of lysosomes by adding mannose-6-phosphate to certain proteins (Tortora & Derrickson, 2008). The Golgi body cisternae work by adding carbohydrates or phosphates to proteins as a way of modifying them for use. In certain cases, this modification works as a signal, allowing other parts of the cell to determine where the protein is needed (Tortora & Derrickson, 2008). All of these additions require energy in the form of ATP, so this substance is brought into the lumen of the Golgi apparatus when needed, then used by enzymes known as kinases (Tortora & Derrickson, 2008). The lumen of the Golgi body is also important as this is where the vesicles from the ER empty their contents into the structure for use in protein modification; some of these products may be the proteins which require modification itself. Thus, the lumen of the Golgi apparatus is perhaps one of the most integral parts of the structure for function. Lysosomes Eukaryotic cells also contain organelles known as lysosomes. Lysosomes themselves contain enzymes known as acid hydrolases which work to break down the waste from cell function, as well as other cellular debris (Tortora & Derrickson, 2008). This digestive function of the lysosome also allows them to digest macromolecules (such as those formed from phagocytosis), complete endocytosis (where certain receptor proteins can be broken down into their components for recycling into other proteins) and autophagy (where organelles from within the cell or useless proteins can be digested within the lysosome). To do this, the lysosome itself has an acid pH, usually around 4.8, which contrasts to the cytosol of the rest of the cell (which is alkaline). To protect the cytosol and the components of the cell, the lysosome has a membrane which allows the internal parts of the lysosome to retain their acidity to maintain their digestive function. Many of the enzymes involved in the function of lysosomal activity are also specific to the acidic environment, meaning that they will not function correctly in other areas of the cell, again working as a protective barrier (Tortora & Derrickson, 2008). Lysosomes also have the ability to fuse with vacuoles in the cell which may work as part of the phagocytic immune system. This means that the lysosome is able to digest macromolecules from invading pathogens and recycle them for use within the cell. The Cell Membrane The cell membrane is what separates each individual cell from its external surroundings. The cell membrane is based around the structure of the lipid bilayer (Campbell & Reece, 2005), which relies on chemical forces to keep a hydrophilic head region on each of two external surfaces with a hydrophobic tail region within the inner part of the membrane. The reason it is called the bilayer is that there are two layers of phospholipids, both arranged in the same way so that each surface of the membrane is similar (Campbell & Reece, 2005). This structure helps to prevent unwanted molecules from entering or leaving the cell, partly based on the polarity of the phospholipids. The membrane also has various elements embedded within it. One of these is the protein channel, through which slightly larger molecules can pass upon appropriate cell signaling. There are also carbohydrate molecules and surface proteins which play various roles in the cell membrane function (Tortora & Derrickson, 2008). References Campbell, N.A., Reece, J.B., 2005. Biology. Pearson Education. Tortora, G.J., Derrickson, B.H., 2008. Principles of Anatomy and Physiology. John Wiley & Sons. Read More