Biology Chapter 5

Why do ions have a difficult time getting through plasma membranes despite their small size?

34 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.

If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease?

A decrease in pH means an increase in positively charged H+ ions, and an increase in the electrical gradient across the membrane. The transport of amino acids into the cell will increase

Which plasma membrane component can be either found on its surface or embedded in the membrane structure? a. protein b. cholesterol c. carbohydrate d. phospholipid

A. protein

9. The principal force driving movement in diffusion is the __________. a. temperature b. particle size c. concentration gradient d. membrane surface area

C. concentration gradient

. Water moves via osmosis _________. a. throughout the cytoplasm b. from an area with a high concentration of other solutes to a lower one c. from an area with a high concentration of water to one of lower concentration d. from an area with a low concentration to an area of high concentration.

C. from an area with a high concentration of water to one of lower concentration

Injecting a potassium solution into a person's blood is lethal. This is how capital punishment and euthanasia subjects die. Why do you think a potassium solution injection is lethal?

Cells typically have a high concentration of potassium in the cytoplasm and are bathed in a high concentration of sodium. Injection of potassium dissipates this electrochemical gradient. In heart muscle, the sodium/potassium potential is responsible for transmitting the signal that causes the muscle to contract. When this potential is dissipated, the signal can't be transmitted, and the heart stops beating. Potassium injections are also used to stop the heart from beating during sugery.

Describe two ways that decreasing temperature would affect the rate of diffusion of molecules across a cell's plasma membrane.

Decreasing temperature will decrease the kinetic energy in the system. A lower temperature means less energy in the molecules, so they will move at a slower speed. Lowering temperature also decreases the kinetic energy of the molecules in the plasma membrane, compressing them together. This increases the density of the plasma membrane, which slows diffusion into the cell.

Discuss why the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled.

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.

A doctor injects a patient with what the doctor thinks is an isotonic saline solution. The patient dies, and an autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?

No, it must have been hypotonic as a hypotonic solution would cause water to enter the cells, thereby making them burst.

How can a cell use an extracellular peripheral protein as the receptor to transmit a signal into the cell?

Peripheral proteins can bind to other molecules in the extracellular space. However, they cannot directly transmit a signal to the inside of the cell since they do not have a transmembrane domain (region that goes through the plasma membrane to the inside of the cell). They must associate with integral membrane proteins in order to pass the signal to the inside of the cell.

The sodium/calcium exchanger (NCX) transports sodium into and calcium out of cardiac muscle cells. Describe why this transporter is classified as secondary active transport.

The NCX moves sodium down its electrochemical gradient into the cell. Since sodium's electrochemical gradient is created by the Na+/K+ pump, a transport pump that requires ATP hydrolysis to establish the gradient, the NCX is a secondary active transport process.

Where does the cell get energy for active transport processes?

The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps.

21. Why is it advantageous for the cell membrane to be fluid in nature?

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?

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.

Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?

The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and not bringing in anything else.

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

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.

Why does water move through a membrane?

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.

A cell develops a mutation in its potassium channels that prevents the ions from leaving the cell. If the cell's aquaporins are still active, what will happen to the cell? Be sure to describe the tonicity and osmolarity of the cell.

Without functional potassium channels, the potassium ions that are pumped into the cell will accumulate. This increases the osmolarity inside the cell, creating a hypotonic solution. Since the plasma membrane is still selectively permeable to water by the aquaporins, water will flow into the cell. If the potassium concentration is high enough, enough water will eventually flow into the cell to lyse it.

Which of the following organelles relies on exocytosis to complete its function? a. Golgi apparatus b. vacuole c. mitochondria d. endoplasmic reticulum

a. Golgi apparatus

What problem is faced by organisms that live in fresh water? a. Their bodies tend to take in too much water. b. They have no way of controlling their tonicity. c. Only salt water poses problems for animals that live in it. d. Their bodies tend to lose too much water to their environment.

a. Their bodies tend to take in too much water.

What is the primary function of carbohydrates attached to the exterior of cell membranes? a. identification of the cell b. flexibility of the membrane c. strengthening the membrane d. channels through membrane

a. identification of the cell

A scientist compares the plasma membrane composition of an animal from the Mediterranean coast with one from the Mojave Desert. Which hypothesis is most likely to be correct? a. The cells from the Mediterranean coast animal will have more fluid plasma membranes. b. The cells from the Mojave Desert animal will have a higher cholesterol concentration in the plasma membranes. c. The cells' plasma membranes will be indistinguishable. d. The cells from the Mediterranean coast animal will have a higher glycoprotein content, while the cells from the Mojave Desert animal will have a higher lipoprotein content

b. The cells from the Mojave Desert animal will have a higher cholesterol concentration in the plasma membranes.

Imagine a cell can perform exocytosis, but only minimal endocytosis. What would happen to the cell? a. The cell would secrete all its intracellular proteins. b. The plasma membrane would increase in size over time. c. The cell would stop expressing integral receptor proteins in its plasma membrane. d. The cell would lyse.

b. The plasma membrane would increase in size over time.

Which transport mechanism can bring whole cells into a cell? a. pinocytosis b. phagocytosis c. facilitated transport d. primary active transport

b.. phagocytosis

In what important way does receptor-mediated endocytosis differ from phagocytosis? a. It transports only small amounts of fluid. b. It does not involve the pinching off of membrane. c. It brings in only a specifically targeted substance. d. It brings substances into the cell, while phagocytosis removes substances.

c. It brings in only a specifically targeted substance

18. Many viruses enter host cells through receptormediated endocytosis. What is an advantage of this entry strategy? a. The virus directly enters the cytoplasm of the cell. b. The virus is protected from recognition by white blood cells. c. The virus only enters its target host cell type. d. The virus can directly inject its genome into the cell's nucleus.

c. The virus only enters its target host cell type.

15. What happens to the membrane of a vesicle after exocytosis? a. It leaves the cell. b. It is disassembled by the cell. c. It fuses with and becomes part of the plasma membrane. d. It is used again in another exocytosis event

c. it fuses with and becomes part of the plasma membrane.

13. How does the sodium-potassium pump make the interior of the cell negatively charged? a. by expelling anions b. by pulling in anions c. by expelling more cations than are taken in d. by taking in and expelling an equal number of cations

c.. by expelling more cations than are taken in

In which situation would passive transport not use a transport protein for entry into a cell?a. water flowing into a hypertonic environment b. glucose being absorbed from the blood c. an ion flowing into a nerve cell to create an electrical potential d. oxygen moving into a cell after oxygen deprivation

d. Oxygen moving into a cell after oxygen deprivation

Active transport must function continuously because __________. a. plasma membranes wear out b. not all membranes are amphiphilic c. facilitated transport opposes active transport d. diffusion is constantly moving solutes in opposite directions

d. diffusion is constantly moving solutes in opposite directions

Which characteristic of a phospholipid contributes to the fluidity of the membrane? a. its head b. cholesterol c. a saturated fatty acid tail d. double bonds in the fatty acid tail

d. double bonds in the fatty acid tail

. What is the combination of an electrical gradient and a concentration gradient called? a. potential gradient b. electrical potential c. concentration potential d. electrochemical gradient

d. electrochemical gradient

Both of the regular intravenous solutions administered in medicine, normal saline and lactated Ringer's solution, are isotonic. Why is this important?

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.

31. Glucose from digested food enters intestinal epithelial cells by active transport. Why would intestinal cells use active transport when most body cells use facilitated diffusion?

intestinal epithelial cells use active transport to fulfill their specific role as the cells that transfer glucose from the digested food to the bloodstream. Intestinal cells are exposed to an environment with fluctuating glucose levels. Immediately after eating, glucose in the gut lumen will be high, and could accumulate in intestinal cells by diffusion. However, when the gut lumen is empty, glucose levels are higher in the intestinal cells. If glucose moved by facilitated diffusion, this would cause glucose to flow back out of the intestinal cells and into the gut. Active transport proteins ensure that glucose moves into the intestinal cells, and cannot move back into the gut. It also ensures that glucose transport continues to occur even if high levels of glucose are already present in the intestinal cells. This maximizes the amount of energy the body can harvest from food.