6. The Membrane

Simple and Facilitated Diffusion

What important assumption does parsimony make when assessing which phylogenetic tree is most accurate? Why was parsimony misleading in the case of the astragalus during the evolution of artiodactyls?

Animation: Facilitated Diffusion

Transport of an ion down its electrochemical gradient

Which of the following processes includes all of the others?

Channel – open to both sides of the membrane simultaneously.

Carrier – open to one side of the membrane at a time.

Carrier – requires a conformation change to complete function.

Channel – not selective for molecules that move through it.

Which of the following does not accurately describe a channel or a carrier?

 No. Even though large molecules move across membranes relatively slowly, there is a better answer. 

 Dissolved gases such as oxygen or carbon dioxide 

 No. Dissolved gases such as oxygen and carbon dioxide can cross membranes with ease. 

 No. Small nonpolar molecules can cross the bilayer membrane. 

 Any of the listed molecules would easily diffuse through the membrane. 

 No. There is definitely a category of molecule in the listed choices that cannot easily diffuse across a membrane. 

Consider the properties of molecules that would permit the molecules to move through a bilayer membrane.

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

The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora?

The researchers know that correlation does not equal causation. However, can the absence of a correlation enable you to reject a hypothesis of causation? How would the R2 values be different in a scenario where a single trait was important to diversification in many phyla versus a scenario where different traits were important to diversification in different phyla?

 Facilitated diffusion requires a concentration gradient. 

 No. This statement is true. Facilitated diffusion, like simple diffusion, requires a concentration gradient. 

 Facilitated diffusion can occur through protein channels. 

 No. This statement is true. Protein channels can be used to allow the movement of certain solutes across the membrane. 

 Facilitated diffusion can occur by means of transport proteins. 

 No. This statement is true. Facilitated diffusion can occur via transporters. 

 Facilitated diffusion can move ions across membranes. 

 No. This statement is true. There are many channels that allow ions to move into or out of the cell down their concentration gradients. 

 Facilitated diffusion requires the hydrolysis of ATP. 

Consider what the requirements are for facilitated diffusion to occur.

Which of the following is false in regard to facilitated diffusion?

 No. Glucose is a polar molecule and therefore is surrounded by water molecules; glucose requires a transport protein to cross a membrane. 

 No. Sodium has a positive charge and cannot cross a membrane without a transport protein. 

 No. DNA is a large, charged molecule and cannot cross a membrane easily. 

 No. Hemoglobin is a large protein molecule and does not cross membranes under normal circumstances. 

Consider the properties of the membrane that emerge as a consequence of its structure and also the properties a molecule would need to move freely through the membrane.

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

Transport Proteins of Facilitated Diffusion

 Facilitated diffusion is another name for osmosis. 

 No. Osmosis refers specifically to the movement of water across a selectively permeable membrane, but facilitated diffusion can involve a variety of substances. 

 Facilitated diffusion of solutes occurs through phospholipid pores in the membrane. 

 No. Facilitated diffusion occurs through integral membrane transport proteins. 

 Facilitated diffusion requires energy to drive a concentration gradient. 

 No. Facilitated diffusion is driven by a concentration gradient, so it does not require energy. 

 There is only one kind of protein pore for facilitated diffusion. 

 No. Each molecule or group of molecules requires its own specific pore. 

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

Consider what the requirements are for facilitated diffusion.

Which of these statements describes some aspect of facilitated diffusion?

A Level Biology Revision "Facilitated Diffusion"

Facilitated Diffusion Explained

Moves molecules against their concentration gradient.

Freely diffuses molecules against their concentration gradient.

The difference between simple and facilitated diffusion is that facilitated diffusion:

Which type(s) of molecules cannot enter/exit the cell via simple diffusion and require facilitated diffusion?

 A; the concentration gradient there is shallower 

 No. It is true that the concentration gradient is shallower in cell A, but that means that oxygen will diffuse more rapidly into cell B. 

 A; its membrane transport proteins will not be saturated 

 No. The diffusion of oxygen does not require membrane transport proteins. 

 B; the oxygen molecules inside cell B have a higher kinetic energy 

 No. Higher kinetic energy will make diffusion occur faster, but the higher metabolic rate in cell B does not give the oxygen higher kinetic energy. 

 B; the gradient of oxygen is oriented in the opposite direction compared to cell A 

 No. Oxygen is being consumed in both cells; therefore, the gradient is from outside to inside in both cells. 

 B; the diffusion gradient in cell B is steeper 

Consider how the consumption of oxygen would affect the concentration gradient.

Cells A and B are the same size, shape, and temperature, but cell A is metabolically less active than cell B, and cell B is actively converting oxygen to water in cellular respiration. Oxygen will diffuse more rapidly into cell __________ because __________.

Facilitated Diffusion vs Simple Diffusion

 It involves only the movement of water molecules. 

 No. A wide variety of molecules can move by diffusion, but the diffusion of water molecules across a selectively permeable membrane is called osmosis. 

 It requires expenditure of energy by the cell. 

 No. By definition, diffusion does not require the cell to expend energy. 

 It occurs when molecules move from a region of lower concentration to a region of higher concentration. 

 No. This would require the cell to expend energy. 

 It always requires integral proteins of the cell membrane. 

 No. Facilitated diffusion requires integral proteins, but passive diffusion does not. 

Consider how diffusion differs from active processes.

Which of the following statements about diffusion is true?

BioFlix: Passive Transport

Diffusion, Facilitated Diffusion & Active Transport: Movement across the Cell Membrane

Passive Transport in Cells: Simple and Facilitated Diffusion and Osmosis

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