short answers only , basic bio 1-Can you explain how a plasma membrane, solely c
ID: 87846 • Letter: S
Question
short answers only , basic bio
1-Can you explain how a plasma membrane, solely constituted of phospholipids, is inherently semipermeable?
2-What two things are required for anything to cross the plasma membrane?
3-Can you explain how distance, size of solute, steepness of gradient, temperature and electrical forces can influence the rate of diffusion?
4-What is the primary mechanism by which water crosses the lipid bilayer? What are a few good examples of molecules that freely can pass the lipid bilayer?
5-What is the difference between active and passive transport? What is the difference between simple diffusion and facilitated diffusion?
6-A cell is hypertonic too its environment. Will the cell crenate or swell? Which has a lower osmotic potential, the cytosol of the cell or the interstitial fluid?
7-Is a glucose transporter (GLUT) a uniporter, symporter or antiporter? How does glucose enter your body cells? How does it enter the cells lining your small intestine?
8-What is the stoichiometry (i.e., numbers and direction of ions transported) of the Na+/K+ pump? Can you provide a detailed explanation of secondary-active transport?
9-The pump-leak leak model is an extraordinarily important process in the human body. Can you explain the pump-leak model and its fundamental utility in the human body?
10-Can you explain the process by which cholesterol enters your body cells?
Explanation / Answer
1. Biological membranes are inherently semipermeable. They are permeable to water and other molecules, but they are impermeable to a whole host of solutes, including all charged molecules (ions). Thus biological membranes are impermeable to all those inorganic ions found in the extracellular and intracellular fluids, such as Na+, K+, Cl- and Ca++, just to name a few. These ions, as well as other inorganic ions, organic ions and large polar molecules that cannot penetrate biolgical membranes, all contribute to the osmotic pressure of the extracellular and intracellular fluids.
2.There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used. Passive mechanisms like diffusion use no energy, while active transport requires energy to get done.
3. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:
4. Water transport across cell membranes occurs by diffusion and osmosis. The two main pathways for plasmamembrane water transport are the lipid bilayer and waterselective pores (aquaporins). Aquaporins are a large family of water pores; some isoforms are waterselective whereas others are permeable to small solutes. Aquaporin 1 (AQP1), the best studied isoform, is present in the membrane as a tetramer; each monomer has a water pore. The pore of AQP1 is long and narrow (c. 2.8Å diameter); water molecules lose their hydrogen bonds and permeate in single file. AQPmediated water permeability appears to be regulated mostly by controlling the number of molecules present in the membrane. Regulation of the ‘opening’ of the pores has also been proposed for some isoforms.
The most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules like O2 and CO2 can freely pass through the membrane, but charged ionsand large molecules such as proteins and sugars are barred passage.
5. The main difference between active and passive transport is energy consumption. Active transport require energy while passive transport does not require energy.
In facilitated diffusion, molecules only move with the aid of a protein in the membrane. In simple diffusion, molecules move down the concentration gradient but in facilitated diffusion molecules move up the concentration gradient.
6. The cell will swell because of osmotic potential. Osmotic Potential is the potential of water molecules to move from a hypotonic solution (more water, less solutes) to a hypertonic solution (less water, more solutes) across a semi permeable membrane. Hence cytosol will have lower osmotic potential.
7. GLUT is a uniporter because it works by binding to one molecule of substrate at a time. Whether a cell uses facilitated diffusion or active transport for glucose transport depends on the specific needs of the cell. For example, the sugar glucose is transported by active transport from the gut into intestinal epithelial cells, but by facilitated diffusion across the membrane of red blood cells.
8. Three sodium ions out of the cell for every two potassium ions. Secondary active transport is a form of active transport across a biological membrane in which a transporter protein couples the movement of an ion (typically Na+ or H+) down its electrochemical gradient to the uphill movement of another molecule or ion against a concentration/ electrochemical gradient.
9.The pump leak model is a mechanism for the reabsorption of sodium in the kidney nephrons, in order to achieve and maintain the fluid-balanced euvolemic state within the human body. The "pump" is the active transport of sodium from the tubular cell into the interstitial space at the basolateral membrane, via carrier proteins, i.e. Na+/K+ ATPase. The "leak" results from the action of active transport (of the Na+/K+ ATPase), which creates a diffusion gradient that opposes its own action by favouring back-diffusion. Sodium is passively transported from the tubular fluid into the tubular cells at the apical membrane (via ion channels), and then actively pumped out of the tubular cells at the basolateral membrane (via Na+/K+ ATPase), which then enters the blood. The pump-leak model explains the relationship between the transport of ions and cellular metabolism.
10.Most cholesterol is transported in the blood as cholesteryl esters in the form of lipid-protein particles known as low-density lipoproteins (LDL) (Figure 13-43). When acell needs cholesterol for membrane synthesis, it makes transmembrane receptor proteins for LDL and inserts them into its plasma membrane.
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