Membrane Structure

Question 1

The plasma membrane is referred to as a “fluid mosaic” structure. Why?

Answer 1

The fluid aspect of the membrane is due to the lateral and rotational movement of phospholipids, and embedded proteins account for the mosaic aspect.

Question 2

Consider the currently accepted fluid mosaic model of the plasma membrane. Where in the plasma membrane would cholesterol most likely be found?

Answer 2

interior of the membrane

Question 3

Which of the following functions of membrane proteins is important in tissue formation during embryonic development in animals?

Answer 3

Membrane proteins with short sugar chains form identification tags that are recognized by other cells. Cell-cell recognition is important in tissue formation during embryogenesis.

Question 4

Where in the membrane would carbohydrates most likely be found?

Answer 4

outside (external) surface of the membrane

Question 5

Describe the aspects of the sidedness of the plasma membrane.

Answer 5

• The two lipid layers may differ in specific lipid composition.
• The asymmetrical distribution of membrane proteins, lipids, and carbohydrates across the plasma membrane is determined as the membrane is being constructed.
• Parts of proteins that are exposed on the cytoplasmic side of the endoplasmic reticulum are also exposed on the cytoplasmic side of the plasma membrane.
• Every integral membrane protein has a specific orientation in the plasma membrane.

Question 6

Which of the following molecules is most likely to passively diffuse across the plasma membrane?

Answer 6

carbon dioxide

Question 7

Which of the following would be LEAST likely to diffuse through a plasma membrane without the help of a transport protein?

Answer 7

a large, polar molecule

Question 8

Which of the following statements about passive transport is correct?

Answer 8

Passive transport permits the solute to move in either direction, but the net movement of solute molecules occurs down the concentration gradient of the molecule.

Question 9

A single plant cell is placed in an isotonic solution. Salt is then added to the solution. Which of the following would occur as a result of the salt addition?

Answer 9

Water would leave the cell by osmosis, causing the volume of the cytoplasm to decrease.

Question 10

Seawater is hypertonic to cytoplasm in vertebrate cells and in plant cells. If a red blood cell and a plant cell were placed in seawater, what would happen to the two types of cells?

Answer 10

Both cells would lose water; the red blood cell would shrivel, and the plant plasma membrane would pull away from the cell wall.

Question 11

What aspects describe facilitated diffusion?

Answer 11

Facilitated diffusion of solutes may occur through channel or transport proteins in the membrane.

Question 12

A selectively permeable membrane separates two solutions. Water is able to pass through this membrane; however, sucrose (a disaccharide) and glucose (a monosaccharide) cannot pass. The membrane separates a 0.2-molar sucrose solution from a 0.2-molar glucose solution. With time, how will the solutions change?

Answer 12

Nothing will happen, because the two solutions are isotonic to one another.

Osmotic pressure is produced by the concentration of dissolved substances and is not influenced by the relative sizes of the solutes.

Question 13

The concentration of solutes in a red blood cell is about 2%, but red blood cells contain almost no sucrose or urea. Sucrose cannot pass through the membrane, but water and urea can. Osmosis would cause red blood cells to shrink the most when immersed in which of the following solutions?

Answer 13

a hypertonic sucrose solution

It is not a urea solution because the membrane is permeable to urea. The urea will simply diffuse through the membrane until it reaches equilibrium. This will increase the solute concentration inside the cell relative to outside, and water will enter osmotically.

Question 14

Green olives may be preserved in brine, which is a 30% salt solution. How does this method of preservation prevent microorganisms from growing in the olives?

Answer 14

A 30% salt solution is hypertonic to the bacteria, so they lose too much water and undergo plasmolysis.

Question 15

Active transport requires an input of energy and can also generate voltages across membranes. Based on this information, what happens?

Answer 15

The sodium-potassium pump hydrolyzes ATP and results in a net positive change outside the cell membrane.

Question 16

Glucose can be moved into cells via an active transport mechanism when the concentration of glucose inside the cell is higher than the concentration of glucose outside of the cell. This active transport mechanism moves glucose and sodium into the cell at the same time. The glucose moves up its gradient and the sodium moves down its gradient. Which of the following statements about this mechanism is accurate?

Answer 16

To pump glucose up its concentration gradient, sodium moves down its concentration gradient, and the distribution of sodium ions across the membrane forms an electrochemical gradient that drives this mechanism.

Question 17

Which of the following is a correct difference between active transport and facilitated diffusion?

Answer 17

Active transport requires energy from ATP, and facilitated diffusion does not.

Question 18

A cell has a membrane potential of –100 mV (more negative inside than outside) and has 1,000 times more calcium ions outside the cell than inside. Describes the mechanism by which Ca2+ enters the cell.

Answer 18

facilitated diffusion of Ca2+ into the cell down its electrochemical gradient

Both the electrical and chemical (concentration) gradients contribute the energy to move Ca2+ into the cells by facilitated diffusion as long as there is a channel or carrier that is specific for Ca2+.

Question 19

Which of the following correctly describes a general property of all electrogenic pumps?

Answer 19

Electrogenic pumps create a voltage difference across the membrane. (membrane potential)

Question 20

What happens during the cotransport of solutes across a membrane?

Answer 20

Cotransport proteins allow a single ATP-powered pump to drive the active transport of many different solutes.

Question 21

Consider the transport of protons and sucrose into a plant cell by the sucrose-proton cotransport protein. Plant cells continuously produce a proton gradient by using the energy of ATP hydrolysis to pump protons out of the cell. Why, in the absence of sucrose, do protons not move back into the cell through the sucrose-proton cotransport protein?

Answer 21

The movement of protons through the cotransport protein cannot occur unless sucrose moves at the same time.

The obligate coupling of proton movement to sucrose movement prevents the energy of the proton gradient from being lost if sucrose is not present.

Question 22

Which of the following enables a cell to pick up and concentrate a specific kind of molecule?

Answer 22

receptor-mediated endocytosis

Question 23

Which of the following processes and organelles account for the replacement of lipids and proteins lost from the plasma membrane?

Answer 23

exocytosis and smooth ER and rough ER

In exocytosis, vesicles derived from the endomembrane system fuse with the plasma membrane, thus increasing the number of phospholipids in the plasma membrane and increasing its surface area.

Question 24

A nursing infant is able to obtain disease-fighting antibodies, which are large protein molecules, from its mother’s milk. These molecules probably enter the cells lining the baby’s digestive tract via which process?

Answer 24

endocytosis

Endocytosis is the procedure that cells use to import large molecules across their plasma membrane.

Question 25

Is this correct: “pinocytosis: the uptake of water and small solutes into the cell by formation of vesicles at the plasma membrane”

Answer 25

No. Pinocytosis is the uptake of liquid and the solutes dissolved in the liquid.

Question 26

What does amphipathic mean?

Answer 26

has hydrophobic and hydrophilic parts/properties

Question 27

What is the main function of the plasma membrane?

Answer 27

separate the inside and outside of the cell