Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid

Phospholipids can self-assemble into a bilayer when placed in water.

Think about why this predictable structure results, and discuss with a partner or in a group.

Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid

Phospholipids can self-assemble into a bilayer when placed in water.

This predictable structure results because:

a. individual phospholipids have a hydrophilic “head” and a longer, hydrophobic “tail.”

b. they assort themselves so that the hydrophilic ends don’t contact the water.

c. they assort themselves so that the hydrophilic heads face inwards, interacting with each other.

d. they assort themselves so that the hydrophobic ends face inwards, avoiding contact with water.

e. Both a and d

Concept 5.2 Some Substances Can Cross the Membrane by Diffusion, Part 1

Discuss the following scenarios with a partner or with a group:

• A cucumber placed in a solution of brine (very salty water), over time turning into a pickle

• A person sitting in a tapwater bath

Osmosis of water by diffusion is the only process occurring in these two scenarios.

Discuss the direction of the gradient of water flow, and determine the correct term (e.g., isotonic, hypertonic, etc.) for the starting condition inside of the cells (plant cells of the cucumber, and skin cells of the human in the bath).

Concept 5.2 Some Substances Can Cross the Membrane by Diffusion, Part 1

Consider the process of osmosis for:• A cucumber placed in a solution of brine (very salty

water), over time turning into a pickle• A person sitting in a tapwater bath

Which of the following correctly describes the scenarios at the starting condition?

a. The cucumber is hypotonic with respect to the brine.b. The human skin cells are hypertonic with respect to

the bathwater.c. Water begins to flow by osmosis into the cucumber

and out of the human skin cells.d. Both a and be. All of the above

Concept 5.2 Some Substances Can Cross the Membrane by Diffusion, Part 2

Type 2 diabetes, often associated with obesity, results in an increase in blood-glucose levels. It occurs in humans as well as in animals, including domestic dogs and cats.

One early sign that a person or animal might be developing type 2 diabetes is excessive thirst. Discuss a possible reason for this symptom based on what you have learned in this chapter about tonicity in cells.

Concept 5.2 Some Substances Can Cross the Membrane by Diffusion, Part 2

Type 2 diabetes, often associated with obesity, results in an increase in blood-glucose levels. It occurs in humans as well as in animals, including domestic dogs and cats.

One early sign that a person or animal might be developing type 2 diabetes is excessive thirst. Consider the following possible explanation:

Elevated glucose levels in blood would result in water leaving cells, causing dehydration.

a. True

b. False

c. I don’t know.

Concept 5.3 Some Substances Require Energy to Cross the Membrane

Consider the following statements, and discuss whether each is correct:

• Simple or facilitated diffusion requires cellular energy.

• Diffusion results in a net flow of solute down a concentration gradient, while active transport works against the concentration gradient.

• Secondary active transport uses energy to set up a concentration gradient which then allows some passive diffusion to occur, which recaptures some of the energy expended.

• Membrane proteins are required for all membrane transport into and out of cells.

Concept 5.3 Some Substances Require Energy to Cross the Membrane

Which of the following statements is/are correct?

a. Simple or facilitated diffusion requires cellular energy.

b. Diffusion results in a net flow of solute down a concentration gradient, while active transport works against the concentration gradient.

c. Secondary active transport uses energy to set up a concentration gradient which then allows some passive diffusion to occur, which recaptures some of the energy expended.

d. Membrane proteins are required for all membrane transport into and out of cells.

e. Both b and c

Concept 5.4 Large Molecules Cross the Membrane via Vesicles

Discuss the validity of the following statement with a partner or group:

Endocytosis effectively decreases the length of the plasma membrane of the cell engulfing the large particle or cell, while exocytosis effectively increases the length of the plasma membrane of the cell releasing the large molecules or particles.

Concept 5.4 Large Molecules Cross the Membrane via Vesicles

Endocytosis effectively decreases the length of the plasma membrane of the cell engulfing the large particle or cell, while exocytosis effectively increases the length of the plasma membrane of the cell releasing the large molecules or particles.

This statement is:

a. True

b. False

c. I don’t know.

Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals

Intercellular signaling is somewhat analogous to a lock and key system. The lock won’t open unless the right key is used, analogous to the cell needing to have the appropriate receptor in order to respond to a signal.

Discuss in groups whether this statement is true or false.

Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals

Intercellular signaling is somewhat analogous to a lock and key system. The lock won’t open unless the right key is used, analogous to the cell needing to have the appropriate receptor in order to respond to a signal.

This statement is:

a. True

b. False

c. I don’t know.

Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment

Think about each of the following statements and discuss whether they are true with respect to the signaling cascade stimulated by epinephrine (the G protein–mediated protein kinase pathway).

• In liver cells, each molecule of epinephrine results in the liberation of one molecule of blood glucose.

• With respect to epinephrine, the same signal-receptor binding event always produces the same cellular response in muscles throughout the body.

• The signaling cascade set off by epinephrine involves the activation of some enzymes as well as the inactivation of others.

Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment

Which of the following statements are true with respect to the signaling cascade stimulated by epinephrine (the G protein–mediated protein kinase pathway).

a. In liver cells, each molecule of epinephrine results in the liberation of one molecule of blood glucose.

b. With respect to epinephrine, the same signal-receptor binding event always produces the same cellular response in muscles throughout the body.

c. The signaling cascade set off by epinephrine involves the activation of some enzymes as well as the inactivation of others.

d. Both a and b

e. All of the above