ApplyIt & Critical thinking | CELL STRUCTURES AND THEIR FUNCTIONS
Secretory vesicles fuse with the cell membrane to release their contents to the outside of the cell. In this process, the membrane of the secretory vesicle becomes part of the cell membrane. Because small pieces of membrane are continually added to the cell membrane, we would expect the cell membrane to become larger and larger as secretion continues. However, the cell membrane stays the same size. Explain how this happens.
One must conclude that some process is removing small pieces of membrane at the same rate they are added. The balance of adding new and removing old components of the cell membrane would allow the cell to remain the same size.
Predict the effect of cystic fibrosis on the concentration of Cl- inside and outside the cell.
Cystic fibrosis is the result of defective chloride ion channels that fail to transport Cl- out of cells. This description allows us to easily predict the effect on Cl- concentrations inside and outside the cell, because ion channels allow ions to diffuse down their concentration gradients. The concentration of Cl- would remain higher inside the cell and lower outside the cell.
List the organelles that are plentiful in cells that (a) synthesize and secrete proteins, (b) actively transport substances into cells, and (c) ingest foreign substances by endocytosis. Explain the function of each organelle you list.
(a) Cells that synthesize and secrete proteins would require organelles involved in the manufacturing, packaging, and releasing of proteins from the cell. The rough endoplasmic reticulum, with the attached ribosomes, carries out the synthesis of proteins that will be released from the cell. The Golgi apparatus is also involved in the packaging of cellular materials that are secreted by packaging the proteins into secretory vesicles that move to the plasma membrane. (b) Active transport requires ATP to move materials across the cell membrane, so we would expect the cell to have many mitochondria to produce ample ATP. (c) Cells that ingest foreign substances by endocytosis would require the enzymes needed to break down those substances. We learned that lysosomes are vesicles of digestive enzymes that break down materials brought into the cell.
The body of a man was found floating in the water of Grand Pacific Bay, which has a salt concentration slightly greater than that of body fluids. When examined during an autopsy, the cells in his lung tissue were clearly swollen. Choose the most logical conclusion and explain your choice. a. He probably drowned in the bay b. He may have been murdered elsewhere c. He did not drown
Grand Pacific Bay has a salt concentration slightly greater than that of human body fluids.
Explain how changing one nucleotide within a DNA molecule of a cell can change the structure of a protein produced by that cell.
Recall from "Gene Expression" that a gene is a sequence of DNA nucleotides that provides the instructions for making a specific protein. The sequence of DNA nucleotides determines the sequence of RNA nucleotides of an mRNA, which then determines the sequence of amino acids of a protein in the processes of transcription and translation. The information in mRNA is carried in three nucleotide groups called codons, which specify a particular amino acid or signal the end of translation. So, if one nucleotide in a DNA molecule is changed, that one codon in the mRNA molecule will also be changed. This can lead to several different outcomes. Since an amino acid can often be specified by more than one codon, the changed nucleotide may not change the amino acid and hence the protein structure will not be affected. Alternatively, the changed nucleotide may change the codon to specify a different amino acid. This single amino acid change could in turn change the protein structure. Depending on the changed amino acid, the structural change could be subtle or severe. Finally, the changed nucleotide could create a new stop codon or eliminate an existing stop codon. These changes would very likely cause dramatic changes in the protein structure since a premature stop codon would cut the protein short and removal of a stop codon would add extra amino acids not normally in the protein.
The proteins (hemoglobin) in red blood cells normally organize relative to one another, forming "stacks" of proteins that are, in part, responsible for the normal shape of red blood cells. In sickle-cell anemia, proteins inside red blood cells do not stack normally. Consequently, the red blood cells become sickle-shaped and plug up small blood vessels. Sickle-cell anemia is hereditary and results from changing one nucleotide for a different nucleotide within the gene that is responsible for producing the protein. Explain how this change results in an abnormally functioning protein.
Recall that a protein is composed of amino acids. The sequence of those amino acids is determined by a gene sequence. If a nucleotide is changed in the gene that specifies the protein, this could cause a change in the amino acid sequence as well. During transcription, the nucleotide sequence of a gene is transcribed into an mRNA molecule. Changing a nucleotide in the gene would also cause a change in the nucleotide sequence of the mRNA. During translation, codons of the mRNA determine the amino acid sequence of the protein. Changing the nucleotide sequence of the mRNA changed one of the codons, and therefore changed one of the amino acids. As a result of the nucleotide changed in DNA, there was a change in the nucleotide sequence in the mRNA and a subsequent change in the amino acid sequence of the protein, ultimately changing the protein shape.
Cancer cells divide continuously. The normal mechanisms that regulate whether cell division occurs or ceases do not function properly in cancer cells, resulting in accelerated cell cycles. Cancer cells, such as breast cancer cells, do not look like normal, mature cells. Explain.
Recall that after cell division, the new cells undergo the process of differentiation, which is when the cell develops the specialized structures and functions of a mature cell. As a result, cells of different tissues do not all look the same. If cancer cells are continuously dividing, they do not undergo this differentiation process and therefore do not appear the same as a mature cell of the specific tissue.
The transport of glucose into most cells occurs by facilitated diffusion. Because diffusion occurs from a higher to a lower concentration, glucose cannot accumulate within these cells at a higher concentration than exists outside the cell. Once glucose enters a cell, it is rapidly converted to other molecules, such as glucose phosphate or glycogen. What effects does this conversion have on the cell's ability to transport glucose?
Remember that diffusion, whether simple or facilitated, is the movement of a substance down its concentration gradient. That means that the glucose concentration gradient between the extracellular fluid and the cytoplasm depends on the amount of glucose molecules only and is not affected by other molecules, even if they are similar. If glucose is converted to other molecules inside the cell, the concentration gradient is maintained and the cell can continue to take up more glucose.
Suppose that a cell has the following characteristics: many mitochondria, well-developed rough ER, well-developed Golgi apparatus, and numerous vesicles. Predict the major function of the cell. Explain how each characteristics supports your prediction.
The major characteristics we need to consider are these: First, the mitochondria are the site of ATP production, so we can assume the function has a high energy demand. Rough ER is the site of protein synthesis, and the Golgi apparatus is involved in modifying and packaging proteins produced at the rough ER. We can conclude that this cell is producing high levels of proteins that are then packaged into vesicles by the Golgi apparatuses. Many of the vesicles produced at the Golgi apparatus are secretory vesicles, so we can predict that this cell produces and secretes proteins at a high level. The formation of proteins, packaging of vesicles and the process of exocytosis are all energy requiring process, explaining the high mitochondria number.
Patient with kidney failure can be kept alive by dialysis, which removes toxic waste products from the blood. In a dialysis machine, blood flows past one side of a selectively permeable dialysis membrane, and dialysis fluid flows on the other side of the membrane. Small substances, such as ions, glucose, and urea, can pass through the dialysis membrane, but larger substances, such as proteins, cannot. If you wanted to use a dialysis machine to remove only the toxic waste product urea from blood, what could you use for the dialysis fluid? Explain your choice. a. a solution that is isotonic and contains only protein b. a solution that is isotonic and contains the same concentration of substances as blood, except for having no urea in it c. distilled water d. blood
The purpose is to ensure that only urea is crossing the dialysis membrane moving from the blood into the dialysis fluid. We do not want any water movement across the dialysis membrane, so we know we need an isotonic solution. This eliminates choice C. Choice D is eliminated because we need something different from blood, otherwise there would be no net movement of substances out of the blood. We can conclude that B is the correct answer. By having a solution that differs from the blood only by the absence of urea, this sets up a urea concentration gradient that allows urea to diffuse out of the blood.
Urea is a toxic waste produced inside liver cells. It diffuses from those cells into the blood and is eliminated from the body by the kidneys. What would happen to the intracellular and extracellular concentrations of urea if the kidneys stopped functioning?
Urea diffuses from liver cells, which is the intracellular region, to the blood, which is the extracellular region. This also defines the direction of urea diffusion, from the area of higher urea concentration inside the cells to the area of lower urea concentration in the blood. The kidneys remove the urea from the blood; therefore, if the kidneys stopped functioning, the concentration of urea in the blood would increase. Eventually, this would eliminate the urea concentration gradient or even reverse it. Urea would remain in the cells and increase to toxic levels that could damage or even kill the cells.
