Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of
extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound. Which plasma membrane component can be either found on its surface or embedded in the membrane structure? Which characteristic of a phospholipid contributes to the fluidity of the membrane?
What is the primary function of carbohydrates attached to the exterior of cell membranes?
Free ResponseWhy is it advantageous for the cell membrane to be fluid in nature? Answer The fluid characteristic of the cell membrane allows greater flexibility to the cell than it would if the membrane were rigid. It also allows the motion of membrane components, required for some types of membrane transport. Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane? Answer The hydrophobic, nonpolar regions must align with each other in order for the structure to have minimal potential energy and, consequently, higher stability. The fatty acid tails of the phospholipids cannot mix with water, but the phosphate “head” of the molecule can. Thus, the head orients to water, and the tail to other lipids. 5.2: Passive TransportPlasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells. Review Questions
Water moves via osmosis _________.
The principal force driving movement in diffusion is the __________.
What problem is faced by organisms that live in fresh water?
Free ResponseDiscuss why the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled. Answer Heavy molecules move more slowly than lighter ones. It takes more energy in the medium to move them along. Increasing or decreasing temperature increases or decreases the energy in the medium, affecting molecular movement. The denser a solution is, the harder it is for molecules to move through it, causing diffusion to slow down due to friction. Living cells require a steady supply of nutrients and a steady rate of waste removal. If the distance these substances need to travel is too great, diffusion cannot move nutrients and waste materials efficiently to sustain life. Why does water move through a membrane? Answer Water moves through a membrane in osmosis because there is a concentration gradient across the membrane of solute and solvent. The solute cannot effectively move to balance the concentration on both sides of the membrane, so water moves to achieve this balance. Both of the regular intravenous solutions administered in medicine, normal saline and lactated Ringer’s solution, are isotonic. Why is this important? Answer Injection of isotonic solutions ensures that there will be no perturbation of the osmotic balance, and no water taken from tissues or added to them from the blood. 5.3: Active TransportActive transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Review QuestionsActive transport must function continuously because __________.
How does the sodium-potassium pump make the interior of the cell negatively charged?
What is the combination of an electrical gradient and a concentration gradient called?
Free ResponseWhere does the cell get energy for active transport processes? Answer The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps. How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell? Answer The sodium-potassium pump forces out three (positive) Na+ ions for every two (positive) K+ ions it pumps in, thus the cell loses a positive charge at every cycle of the pump. 5.4: Bulk TransportIn addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see Table 5.4.1 for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell. Review QuestionsWhat happens to the membrane of a vesicle after exocytosis?
Which transport mechanism can bring whole cells into a cell?
In what important way does receptor-mediated endocytosis differ from phagocytosis?
Free ResponseWhy is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell? Answer The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and not bringing in anything else. Why do ions have a difficult time getting through plasma membranes despite their small size? Answer Ions are charged, and consequently, they are hydrophilic and cannot associate with the lipid portion of the membrane. Ions must be transported by carrier proteins or ion channels. What substance should not be found in filtrate?Answer and Explanation: The substances that are normally found in the filtrate of nephrons are plasma components (such as water, nutrients, dissolved gases, electrolytes, and metabolic wastes) excluding plasma proteins and blood cells.
What substances are too large to cross the filtration membrane?Under normal conditions, high molecular weight proteins in the plasma (e.g., albumin and globulin) cannot pass through the filtration membrane due to the effects of the size barrier and charge barrier of the glomerular capillary filtration membrane.
Which of the following substances are not normally found in the urine?Usually, glucose, ketones, protein, and bilirubin are not detectable in urine.
Which of the following substances would not be found in glomerular filtrate?Platelets are not present in the initial filtrate of nephron.
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