10/14: Pre-Class Assignment #6

A red blood cell has been placed into three different solutions. One solution is isotonic to the cell, one solution is hypotonic to the cell, and one solution is hypertonic to the cell. Determine which type of solution is in each beaker based on the cell's reaction.

For a cell in an isotonic solution, water flows into the cell to the same extent that it flows out of the cell. If a cell is in a hypotonic solution, water flows into the cell, which causes it to swell and potentially burst. For a cell in a hypertonic solution, water flows out of the cell, which causes it to shrink.

This figure reviews the structure of a phospholipid. Drag the terms on the left to the appropriate blanks on the right to complete the sentences.

Phospholipids have a hydrophilic head and hydrophobic tails. The hydrophilic head includes a glycerol molecule attached to a single phosphate group, which is then attached to another small molecule. The phosphate group and its attachments are either polar or charged. Water molecules are polar and therefore are attracted to these charged regions of the phospholipid head. The hydrophobic tails are two fatty acids attached to the glycerol molecule of the head. Water molecules are not attracted to the tails because C-H bonds are relatively nonpolar and therefore do not result in charged regions in the tails.

Phospholipids form the main fabric of the plasma membrane. One feature of phospholipids is that when they are placed in an aqueous solution, they will self-assemble into a double layer (bilayer) that resembles the bilayer of the plasma membrane. This self-assembly occurs because phospholipids are hydrophilic at one end (the phospholipid head) and hydrophobic at the other end (the phospholipid tails).

Phospholipids make up the main fabric of the plasma membrane. In the plasma membrane, the phospholipids are found in a bilayer. The hydrophilic heads are exposed to the aqueous environments of the cytoplasm and extracellular fluid, and the hydrophobic tails are sandwiched within, sheltered from these aqueous environments. Other elements of the plasma membrane conform to the hydrophilic and hydrophobic regions established by the phospholipids. For example, membrane proteins have hydrophilic and hydrophobic regions that are found among the hydrophilic and hydrophobic portions of the plasma membrane, respectively. Cholesterol is a hydrophobic molecule and is found among the hydrophobic tails, which you can see in the figure below.

A semipermeable membrane is placed between the following solutions. Which solution will decrease in volume? -Solution A: 1.4% (m/v) starch -Solution B: 7.62% (m/v) starch

Solution A: 1.4% (m/v) starch The water molecules actually move in both directions, but they move to a greater extent toward Solution B because it has a higher solute concentration than Solution A. The net movement of water molecules causes Solution A to decrease in volume and Solution B to increase in volume.

A semipermeable membrane is placed between the following solutions. Which solution will increase in volume? -Solution C: 9% (m/v) NaCl -Solution D: 12.4% (m/v) NaCl

Solution D: 12.4% (m/v) NaCl The water molecules actually move in both directions, but they move to a greater extent toward Solution D because it has a higher solute concentration than Solution C. This net movement of water molecules causes Solution D to increase in volume and Solution C to decrease in volume.

Which of the following results is most likely result if a portion of an animal's blood is replaced with distilled water? -The animal's red blood cells will shrivel up because the blood has become hypotonic compared to the cells. -The animal's red blood cells will swell and possibly burst because the blood has become hypotonic compared to the cells. -The animal's red blood cells will burst because the blood has become hypertonic compared to the cells. -The animal's red blood cells will shrivel up because the blood has become hypertonic compared to the cells.

The animal's red blood cells will swell and possibly burst because the blood has become hypotonic compared to the cells.

A critical feature of the plasma membrane is that it is selectively permeable. This allows the plasma membrane to regulate transport across cellular boundaries--a function essential to any cell's existence. How does phospholipid structure prevent certain molecules from crossing the plasma membrane freely?

The structure of the plasma membrane makes it selectively permeable, enabling it to regulate the transport of substances into and out of the cell. Small, nonpolar molecules are hydrophobic, so they can easily cross the phospholipid bilayer of the plasma membrane. Polar molecules and ions are hydrophilic, so they cannot very easily cross the hydrophobic portion of the plasma membrane (formed by the phospholipid tails). Water is an unusual molecule because, despite the fact that it is polar, it is small enough to pass directly through the hydrophobic interior of the lipid bilayer, albeit slowly. Polar molecules and ions generally cross the plasma membrane with the help of transport proteins. For example, water crosses the bilayer rapidly via transport proteins called aquaporins.

A red blood cell placed in a hypertonic solution will shrink in a process called crenation. A red blood cell placed in a hypotonic solution will swell and potentially burst in a process called hemolysis. To prevent crenation or hemolysis, a cell must be placed in an isotonic solution such as 0.9% (m/v) NaCl or 5.0% (m/v) glucose. This does not mean that a cell has a 5.0% (m/v) glucose concentration; it just means that 5.0% (m/v) glucose will exert the same osmotic pressure as the solution inside the cell, which contains several different solutes. A red blood cell is placed into each of the following solutions. Indicate whether crenation, hemolysis, or neither will occur. Solution A: 3.21% (m/v) NaCl Solution B: 1.65% (m/v) glucose Solution C: distilled H2O Solution D: 6.97% (m/v) glucose Solution E: 5.0% (m/v) glucose and 0.9%(m/v) NaCl

This activity shows why it is very important to use solutions that are isotonic to body fluids in intravenous solutions (IVs). If an IV solution were hypertonic to the body fluids, cells in the body would shrink. If an IV solution were hypotonic to the body fluids, cells in the body would swell.

What happens when two solutions separated by a selectively permeable membrane reach osmotic equilibrium?

Water molecules move between the two solutions, but there is no net movement of water across the membrane. Water molecules are constantly in motion and will continually move across the membrane. However, becuase the solutions are in equilibrium, there is not net movement across the membrane.

The movement of glucose into a cell against its concentration gradient is most likely to be accomplished by which of the following? -facilitated diffusion of the glucose using a carrier protein -passive diffusion of the glucose through the lipid bilayer -cotransport of the glucose with a proton or sodium ion that was pumped across the membrane using the energy of ATP hydrolysis -movement of glucose into the cell through a glucose channel -receptor-mediated endocytosis

cotransport of the glucose with a proton or sodium ion that was pumped across the membrane using the energy of ATP hydrolysis Movement of most solutes against their concentration gradients couples the movement of one solute down its concentration gradient to the movement of another (glucose in this case) against its concentration gradient.

Which of the following factors is a primary contributor underlying cause of familial hypercholesterolemia? -inhibition of the cholesterol active transport system in red blood cells -defective LDL receptors on the cell membranes -poor attachment of the cholesterol to the extracellular matrix of cells -a poorly formed lipid bilayer that cannot incorporate cholesterol into cell membranes

defective LDL receptors on the cell membranes

Endocytosis moves materials _____ a cell via _____.

into ... membranous vesicles The prefix "endo-" means "inward."

Diffusion of ions across membranes through specific ion channels is driven by ________.

ion electrochemical gradients

Which of the following membrane activities requires energy from ATP hydrolysis? -movement of carbon dioxide out of a paramecium -movement of glucose molecules into a bacterial cell from a medium containing a higher concentration of glucose than inside the cell -facilitated diffusion of chloride ions across the membrane through a chloride channel -movement of Na+ ions from a lower concentration in a mammalian cell to a higher concentration in the extracellular fluid

movement of Na+ ions from a lower concentration in a mammalian cell to a higher concentration in the extracellular fluid

Facilitated diffusion is a type of _______.

passive transport During facilitated diffusion, the cell is not expending energy to move the particles across the membrane; therefore, facilitated diffusion is a form of passive transport.

A white blood cell engulfing a bacterium is an example of _____.

phagocytosis Phagocytosis occurs when a cell engulfs a large particle.

Which of the following statements best describes the difference between pinocytosis and receptor-mediated endocytosis? -pinocytosis brings only water molecules into the cell, but receptor-mediated endocytosis brings in other molecules as well -pinocytosis can concentrate substances from the extracellular fluid, but receptor-mediated endocytosis cannot -pinocytosis is nonselective in the molecules it brings into the cell, whereas receptor-mediated endocytosis offers more selectivity -pinocytosis increases the surface area of the plasma membrane, whereas receptor-mediated endocytosis decreases the plasma membrane surface area

pinocytosis is nonselective in the molecules it brings into the cell, whereas receptor-mediated endocytosis offers more selectivity

Structure A is a _____.

solute A solute is crossing the plasma membrane.

You can recognize the process of pinocytosis when _____.

the cell is engulfing extracellular fluid Pinocytosis is "cell drinking."

Structure B is a _____.

transport protein The transport protein facilitates the movement of solute across the plasma membrane.