CH 7: Mastering Biology

Which of these cannot rapidly pass directly through the phospholipids of the plasma membrane?

-Ions, such as hydrogen ions, and hydrophilic molecules, such as water and glucose, cannot rapidly pass directly through the phospholipids of a plasma membrane. To move rapidly through the membrane, they must pass through membrane transport proteins.

If the concentration of phosphate in the cytosol is 2.0 mM and the concentration of phosphate in the surrounding fluid is 0.1 mM, how could the cell increase the concentration of phosphate in the cytosol?

Active Transport.For the cell to accumulate phosphate, it must move calcium against the concentration gradient. This process requires energy.

Lactose transport by membrane proteins occurs under conditions in which the concentration of lactose inside the cell is higher than the concentration outside the cell. What type of transport is used to move lactose into the cell?

Active Transport.Under these conditions, lactose moves against its concentration gradient, so its transport will require energy.

Active and passive transport of solutes across a membrane typically differ in which of the following ways?

Active transport always involves the utilization of cellular energy, whereas passive transport does not require cellular energy.Active and passive transport can be distinguished by whether or not they use cellular energy.

Which of the following transporters does not use passive transport to move molecules across a membrane?

Sodium-Potassium Pump. This pump uses the energy provided by ATP hydrolysis to transport sodium and potassium ions across the membrane.

Which of the following is least likely to be important in holding the components of a biological membrane together?

-Covalent interactions between the phospholipid and protein components of the membrane. This is not an important interaction because there are rarely covalent bonds between the phospholipids and integral membrane proteins.

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 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

-Solution A: 3.21% (m/v) NaCl (Crenation) -Solution B: 1.65% (m/v) glucose (Hemolysis) -Solution C: distilled H2O (Hemolysis) -Solution D: 6.97% (m/v) glucose (Crenation) -Solution E: 5.0% (m/v) glucose and 0.9%(m/v) NaCl (Crenation) ***Remember this is a red blood cell, not animal cell.

Which of the following statements correctly describe(s) the driving forces for diffusion of Na+ and K+ ions through their respective channels?

-The diffusion of Na+ ions into the cell is facilitated by the Na+ concentration gradient across the plasma membrane. -The diffusion of K+ ions out of the cell is impeded by the electrical gradient across the plasma membrane. -The electrochemical gradient is larger for Na+ than for K+. Explanation: The concentration gradient of Na+ ions across the membrane (higher Na+ concentration outside) facilitates the diffusion of Na+ into the cell. At the same time, the electrical gradient across the membrane (excess positive charge outside) drives Na+ into the cell. The concentration gradient of K+ ions across the membrane (higher K+ concentration inside) facilitates the diffusion of K+ out of the cell. However, the electrical gradient across the membrane (excess positive charge outside) impedes the diffusion of K+ out of the cell. The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and the electrical gradient (charge difference) across the membrane. For Na+ ions, diffusion through the Na+ channel is driven by both the concentration gradient and the electrical gradient. But for K+ ions, the electrical gradient opposes the concentration gradient. Therefore, the electrochemical gradient for Na+ is greater than the electrochemical gradient for K+.

Which of the following statements about a typical plasma membrane is correct?

-The two sides of the plasma membrane have different lipid and protein composition. Because the membrane serves different functions on the cytoplasmic and exterior surfaces, the structure and composition of the surfaces must be different

The permeability of a biological membrane to a specific polar solute may depend on which of the following?

-The types of transport proteins in the membrane. The lipid bilayer will be impermeable or very poorly permeable to polar or charged solutes. The presence of the correct transport protein will determine the permeability.

Which of the following best describes the structure of a biological membrane?

-Two layers of phospholipids with proteins either crossing the layers or on the surface of the layers. The membrane proteins can be found either embedded in or attached to the surface of the phospholipid bilayer.

Some solutes are able to pass directly through the lipid bilayer of a plasma membrane, whereas other solutes require a transport protein or other mechanism to cross between the inside and the outside of a cell. The fact that the plasma membrane is permeable to some solutes but not others is what is referred to as selective permeability.

-Water, Carbon Dioxide, Oxygen, and Lipids

Active transport by the sodium-potassium pump follows this cycle:

1.) Three Na+ ions from the cytosol bind to the pump. 2.)The binding of Na+ stimulates the phosphorylation of the pump protein by ATP. 3.) Phosphorylation causes a conformational change in the pump that moves the three Na+ ions against their concentration gradient and releases them outside the cell. 4.) The release of the Na+ ions permits two K+ ions from outside the cell to bind to the pump, and the phosphate group is released. 5.) Release of the phosphate group causes another conformational change in the pump. 6.) The conformational change in the pump moves the two K+ ions against their concentration gradient and releases them into the cytosol.

What property of dishwashing liquid (detergent) makes it useful to wash grease from pans?

Amphipathic Nature. Detergents form micelles around the grease, which are then washed away because the polar head groups facing outward on the micelle are water-soluble.

The movement of glucose into a cell against a concentration gradient is most likely to be accomplished by which of the following?

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 gradient couples the movement of one solute down its concentration gradient to the movement of another (glucose in this case) against its concentration gradient.

True or false? Active transport differs from passive transport in that active transport does not require an input of energy.

False.Active transport requires the input of energy, whereas passive transport is primarily driven by the gradient.Active transport requires energy, whereas passive transport does not.

True or false? During active transport through the sodium-potassium pump, ATP hydrolysis provides the energy to pump three sodium ions into the cell for every two potassium ions pumped out of the cell.

False.The sodium-potassium pump maintains a state in which the concentration of sodium is low inside the cell relative to the outside and the concentration of potassium is high inside the cell relative to the outside; thus, three sodium ions are pumped out of the cell and two potassium ions are pumped into the cell against the electrochemical gradient.

If a red blood cell is placed in a salt solution and bursts, what is the tonicity of the solution relative to the interior of the cell?

Hypotonic.The salt concentration in the solution is lower than it is in the cell, so water enters the cell, causing it to burst.

In many animal cells, the uptake of glucose into the cell occurs by a cotransport mechanism, in which glucose is cotransported with Na+ ions.

In cotransport, the energy required to move one solute against its concentration or electrochemical gradient is provided by an ion moving into the cell down its electrochemical gradient. The ion that moves into the cell down its gradient is usually the same ion that is pumped out of the cell by an active transport pump: for example, Na+ in animal cells using the sodium-potassium pump, or H+ in plants and prokaryotes using the proton pump. In the case of the glucose-sodium cotransporter in animals, Na+ moves back into the cell down its electrochemical gradient, providing the energy for glucose to move into the cell against its concentration gradient. The energy for glucose transport into the cell is supplied indirectly by the sodium-potassium pump's hydrolysis of ATP, and directly by the Na+ electrochemical gradient created by the pump.

Endocytosis && Exocytosis

In exocytosis, substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell. In endocytosis, substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle. Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell.

How can a lipid be distinguished from a sugar?

Lipids are mostly non polar, whereas sugars are polar.

What distinguishes facilitated diffusion from simple diffusion?

Membrane proteins help move molecules across the membrane. Membrane proteins facilitate the movement of molecules across the membrane.

Which of the following particles could diffuse easily through a cell membrane?

Oxygen (O2). Small nonpolar molecules such as oxygen can diffuse across cell membranes.

Which of the following statements is true about the net movement of an ion across a membrane by passive diffusion through a membrane channel?

The charge on the other side of the membrane is generally the opposite of the ion's charge. Ions tend to move toward regions of unlike charge.

Which of the following factors does not affect membrane permeability?

The polarity of membrane phospholipids. Phospholipids contain both a polar head and a nonpolar hydrocarbon tail, both of which are necessary for their ability to form membrane bilayers.

Which of the following statements about osmosis is correct?

The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis. Aquaporins facilitate water movement across membranes and thus speed up the process of osmosis.

Which factors affect the rate of osmotic movement of water?

The rate of osmosis increases with increasing differences in solute concentrations between two solutions separated by a selectively permeable membrane. The rate of osmosis varies with a number of factors, including temperature, pressure, and the difference in solute concentrations between two solutions separated by a selectively permeable membrane.

In facilitated diffusion, what is the role of the transport protein?

Transport proteins provide a hydrophilic route for the solute to cross the membrane. This is the most general description of facilitated diffusion by membrane transport proteins.

Which membrane protein would be used to move glucose across a membrane from a region of higher concentration to a region of lower concentration?

Transporter.The GLUT-1 transporter assists the diffusion of glucose through the membrane.

True or false? Osmosis is a type of diffusion.

True. Osmosis is the diffusion of water across a selectively permeable membrane.

True or false? The water-soluble portion of a phospholipid is the polar head, which generally consists of a glycerol molecule linked to a phosphate group.

True.The hydrophilic, or water-loving, portion of a phospholipid is the polar head, whereas the hydrophobic portion is the nonpolar tail.

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.

Which of the following molecular movements is due to diffusion or osmosis?

When a plant cell is placed in concentrated salt water, water moves out of the cell. If the salt water is hypertonic to the plant cell's contents, water moves out of the cell by osmosis, a form of diffusion.