Chapter 6: Membrane

Question 1

The Na+/K+ ATPase pumps Na+ from the cell into the lumen of the intestine. T/F

Answer 1

False.
The Na+/K+ ATPase pump does not pump sodium ions from the cell into the lumen of the intestine. Its primary function is to transport sodium ions out of the cell.

Question 2

The K+ channel is gated closed to Na+ and only opens when its “senses” K+. T/F

Answer 2

False
K+ channels are selective for potassium ions (K+), but they are not completely closed to sodium ions (Na+). While K+ channels do preferentially allow the passage of K+ ions, they can also allow the passage of Na+ ions to some extent, although at a lower rate compared to K+ ions.
So, in summary, the statement is false. While K+ channels preferentially allow the transport of K+ ions, they are not completely closed to Na+ ions and can permit the passage of Na+ ions to a limited extent.

Question 3

K+ ions have a smaller hydration shell than Na+ ions, allowing the passage of hydrated K+, but not Na+, through the selectivity filter of the K+ channel. T/F

Answer 3

False

K+ ions and Na+ ions have different characteristics when it comes to their hydration shells. Na+ ions are smaller in size compared to K+ ions, and they have a stronger affinity for water molecules. As a result, Na+ ions have a smaller and more tightly bound hydration shell, while K+ ions have a larger and less tightly bound hydration shell.

Question 4

The action of the Na+/K+ ATPase pumps maintains am excess of Na+ ions outside the cells and an excess of K+ ions inside the cell. The K+ ion channels are unidirectional and only allow the transport of ion out of the cell. T/F

Answer 4

True
The Na+/K+ ATPase pumps do maintain an excess of Na+ ions outside the cells and an excess of K+ ions inside the cell. This is accomplished by actively pumping three Na+ ions out of the cell for every two K+ ions brought into the cell.

Regarding K+ ion channels, they are primarily responsible for the movement of K+ ions out of the cell, which makes the statement “K+ ion channels are unidirectional and only allow the transport of ions out of the cell” true.

To summarize:

True: The Na+/K+ ATPase pumps maintain an excess of Na+ ions outside the cells and an excess of K+ ions inside the cell.
True: K+ ion channels are unidirectional and primarily allow the transport of ions out of the cell.

Question 5

The permease (transporter) allows glucose and Na+ into the cell requires ATP. T/F

Answer 5

False
- Permeases or transporters involved in the facilitated diffusion of glucose and the passive movement of Na+ ions do not require ATP.
- ATP is utilized in active transport processes, such as the Na+/K+ ATPase pump, which actively transports ions against their concentration gradients.

Glucose and Na+ are transported across the cell membrane through specific transporter proteins known as glucose transporters (GLUTs) and sodium-glucose co-transporters (SGLTs), respectively. These transporters utilize the concentration gradient of glucose and Na+ to facilitate their movement into the cell, and they do not directly require ATP for their function.

Question 6

The permease (transporter) pumps glucose from the cell into the blood requires ATP. T/F

Answer 6

False.
The statement is incorrect. The permease (transporter) responsible for glucose transport does not pump glucose from the cell into the blood, and it also does not require ATP. Glucose transporters (GLUTs) are responsible for facilitating the movement of glucose across the cell membrane. These transporters use facilitated diffusion, meaning they allow glucose to passively move down its concentration gradient without the need for ATP. In the case of glucose transport, GLUTs primarily function to transport glucose into the cell from the blood, not the other way around.

Question 7

The Na+/K+ ATPase pumps Na+ from the cell into the blood, maintaining low Na+ levels in the cell. T/F

Answer 7

True.
The Na+/K+ ATPase, also known as the sodium-potassium pump, is responsible for pumping sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. This process requires ATP (adenosine triphosphate) to actively transport the ions against their concentration gradients. By pumping Na+ out of the cell, the Na+/K+ ATPase helps to maintain low Na+ levels inside the cell, which is essential for various cellular processes and maintaining cell volume.

Question 8

What types of transport would increase at a linear rate (no saturation) proportional to the concentration gradient?

Answer 8

The types of transport that would increase at a linear rate (no saturation) proportional to the concentration gradient are simple diffusion and facilitated diffusion.

In simple diffusion, molecules move from an area of high concentration to an area of low concentration, driven solely by the concentration gradient. The rate of diffusion is directly proportional to the concentration gradient, and it increases linearly as the gradient becomes steeper.

Passive transport (at least until the gradient is destroyed). Note: Active transport cannot, and any receptor mediated transport essentially would become saturated (reach a maximum transport level.

Question 9

Co-transport of nutrients across the intestinal cell membranes is an active process that can move glucose against a concentration gradient. What is the energy requiring step for co-transport?

Answer 9

Co-transport of glucose across intestinal cell membranes is an active process that moves glucose against its concentration gradient. The energy-requiring step in co-transport involves the movement of sodium ions. The sodium-potassium pump actively pumps sodium ions out of the cell, creating a high concentration of sodium outside and a low concentration inside the cell. The sodium-glucose co-transporter (SGLT) uses the energy from this sodium concentration gradient to transport glucose into the cell against its concentration gradient. This process allows for the absorption of glucose from the intestinal lumen into the body.

Question 10

Potassium ion (K+) channels are very selective for K+, although the ion sodium (Na+) has the same charge as K+ and is even smaller. What feature of the K+ ion channel explains this selectivity?

Answer 10

The selectivity of potassium ion (K+) channels for K+ over sodium (Na+) ions is due to a feature called the selectivity filter. This filter is a narrow region in the channel that interacts with ions. The selectivity filter of K+ channels is designed to accommodate and stabilize K+ ions but not Na+ ions effectively. It has the right size and shape to fit K+ ions, while excluding smaller Na+ ions. This size and charge selectivity in the selectivity filter allow K+ ions to pass through the channel while restricting the movement of Na+ ions. This selective flow of K+ ions contributes to important cellular processes and the establishment of the membrane potential.

K+ channels have a selectivity filter that is specifically designed to accommodate the larger K+ ions while excluding smaller Na+ ions. The selectivity filter consists of carbonyl oxygen atoms that interact favorably with the K+ ions, forming stable interactions. The size of the selectivity filter is critical in determining the selectivity of K+ channels.

While it is true that dehydrated Na+ ions are smaller than hydrated Na+ ions, and dehydration of ions generally requires energy, the selectivity of K+ channels is primarily determined by the size and fit of the ions within the selectivity filter, rather than the energetic favorability of dehydration.

Question 11

Membrane proteins called _________ channels open to allow ions to flow in and out of the cell when the concentration of ions nearby is changed.

Answer 11

Ion channels

Question 12

The central cavity of the K+ channel can only accommodate hydrated K+ but not
hydrated Na+. T/F

Answer 12

False
The hydrated Na+ ions (or any other small ion) can indeed enter the central cavity of the K+ channel. The central cavity is large enough to accommodate hydrated ions of similar size, including hydrated Na+ ions. However, it is at the selectivity filter, which is located deeper within the channel, where the selectivity between K+ and Na+ ions occurs.

In summary, the central cavity of the K+ channel can accommodate hydrated Na+ ions, but it is at the selectivity filter where the selectivity between K+ and Na+ ions occurs.

Question 13

Specific amino acids that line the selectivity filter of the K+ channel can dehydrate K+ and allow passage of that ion but cannot coordinate the dehydration of Na+. T/F

Answer 13

True.
The selectivity filter of the K+ channel contains specific amino acids that can dehydrate K+ ions, removing water molecules and allowing the passage of dehydrated K+ ions through the channel. However, the selectivity filter is less effective in coordinating the dehydration of Na+ ions due to their smaller size. This selectivity allows K+ ions to pass through the channel while restricting the passage of hydrated Na+ ions.

Question 14

Concerning ion transport, how can passive carriers be distinguished from active transporters (pumps)?

Answer 14

Passive Carriers:
- Operate without energy input.
- Allow ion movement down their concentration gradients.
- Exhibit saturation kinetics.
- Generally exhibit broad specificity for similar ions.

Active Transporters (Pumps):
- Require energy, usually ATP.
- Can move ions against their concentration gradients.
- Often do not exhibit saturation kinetics.
- Display high selectivity for specific ions.

By considering the energy requirement, direction of ion movement, kinetics, and specificity, one can differentiate between passive carriers and active transporters involved in ion transport.

(Anytime ATP is required, or an ion is moving from low concentration to high concentration, it is active transport. Passive transport will always move ions down the concentration gradient).

Question 15

Explain the mechanism of a voltage-gated channel.

Answer 15

Mechanism of Voltage-Gated Channels:

1- Closed State: In the absence of a voltage stimulus, the channel is closed, and an activation gate blocks ion passage.

2- Activation: When the membrane potential reaches a threshold, voltage-sensing regions in the channel undergo conformational changes in response to the electric field.

3- Opening: Conformational changes in the voltage-sensing regions cause the activation gate to open, allowing ions to pass through the channel.

4- Ion Conduction: Open channel allows ions (Na+, K+, Ca2+) to flow down their electrochemical gradient across the membrane.

5- Inactivation: After a period of time or specific membrane potential, an inactivation gate closes within the channel, blocking ion passage.

6- Reset: The channel returns to its closed state, ready to respond to subsequent changes in voltage.

Voltage-gated channels play a critical role in electrical signaling of cells, allowing selective ion passage in response to changes in membrane potential.

Question 16

What is an antiporter?

Answer 16

• Definition: A type of membrane protein involved in facilitated transport across the cell membrane.
• Function: Simultaneously transports two different molecules or ions in opposite directions.
• Operation: Movement of one molecule/ion into the cell is coupled with the movement of another molecule/ion out of the cell.
• Energy Requirement: Relies on concentration gradients and does not require direct ATP energy input.
• Example: Sodium-calcium exchanger, which exchanges one Ca2+ for three Na+ ions across the membrane.
• Role: Contributes to ion homeostasis, pH regulation, and nutrient absorption.

Antiporters facilitate the simultaneous transport of different molecules or ions in opposite directions, playing important roles in maintaining cellular balance and functionality.

Antiporters are secondary active transporter proteins that move two different ions (or other molecules) in opposite directions. One moves into the cell while one moves out of the cell.

Question 17

The Na+/K+ transporter is well-studied example of ________ type of transport.

Answer 17

Primary active, antiporters

Question 18

What is facilitated diffusion.

Answer 18

• Definition: Passive transport mechanism using specific transport proteins to move molecules/ions across cell membranes.
• Direction: Moves substances down the concentration gradient, from high to low concentration.
• Energy Requirement: No energy input required.
• Transport Proteins: Channel proteins form pores for passage, while carrier proteins bind and undergo conformational changes.
• Types: Channel proteins and carrier proteins facilitate transport.
  Specificity: Proteins are selective for certain molecules/ions based on size, charge, or other properties.

Also: is a passive transport process that move molecules down their concentration gradient that are too large to pass through the cell membrane, so they use transport proteins.

Question 19

Cells can continue to import glucose molecules even when the cytoplasmic
concentration is very high. This would be an example of _________ type of transport.

Answer 19

Active transport. (usually secondary active transport for glucose)

Question 20

What are the differences between passive transport and simple diffusion

Answer 20

Simple diffusion is the movement of particles from an area of high concentration to an area of low concentration, driven solely by the concentration gradient. It occurs directly through the lipid bilayer of the cell membrane. Simple diffusion does not require the use of proteins or any specific transporters.

On the other hand, passive transport is a broader term that encompasses various mechanisms of substance movement down their concentration gradient, including simple diffusion. Passive transport can involve the use of transport proteins or channels to facilitate the movement of molecules or ions across the membrane. While some molecules may passively diffuse through the lipid bilayer, others may require the assistance of specific transport proteins to cross the membrane.

To summarize:

Simple diffusion is a specific type of passive transport where particles move from high to low concentration directly through the lipid bilayer without the involvement of proteins.
Passive transport refers to the movement of molecules down their concentration gradient, which can occur through simple diffusion or with the assistance of transport proteins or channels, depending on the specific molecules and the characteristics of the cell membrane.

Question 21

What is resting potential?

Answer 21

Resting potential is the normal electrical potential that exists across a cell membrane due to a more positive charge on the outside of the cell and a less positive (considered negative) charge inside the cell caused by the relative concentrations of Na and K. The resting potential is -70 mV

• Definition: Normal electrical potential across a cell membrane, resulting from a difference in charge between the inside and outside of the cell.
• Value: Typically -70 millivolts (mV) inside the cell relative to the outside.
• Cause: Arises from a higher concentration of positively charged ions (such as Na+) outside the cell and a higher concentration of negatively charged ions (such as K+) inside the cell.
• Ion Concentrations: The concentration gradients of Na+ and K+ ions contribute to the establishment of the resting potential.
• Electrical State: The inside of the cell is considered more negative (less positive) relative to the outside.
• Importance: Essential for proper cellular functioning, signal transmission, and the ability to generate action potentials.

The resting potential reflects the electrical balance maintained by the distribution of ions across the cell membrane. It is a critical aspect of cellular physiology and plays a key role in processes such as nerve impulse transmission and muscle contraction. The standard value for resting potential is approximately -70 mV, indicating the negative charge inside the cell relative to the outside.

Question 22

By changing the proteins within its membrane, our cells can be made to function very differently. T/F

Answer 22

True

Membrane proteins play crucial roles in various cellular processes such as transport of molecules, cell signaling, enzymatic reactions, cell adhesion, and structural support. Modulating the types or quantities of specific proteins in the cell membrane can alter its permeability, signaling capabilities, and overall functionality. This adaptability allows cells to adjust their behavior and respond to different physiological or environmental conditions.

Question 23

A _________ is a membrane protein structure that allows ions to flow freely in or out through an opening of a particular size

Answer 23

Channel

Channel proteins form pore-like structures in the cell membrane, creating a passageway for specific ions to traverse the lipid bilayer. These channels are often selective for certain ions based on their size, charge, and other properties. The opening or pore of the channel protein provides a pathway for ions to move across the membrane, facilitating their rapid and specific transport. Channel proteins play a crucial role in processes such as nerve impulse transmission, muscle contraction, and the maintenance of ion balance in cells.

Question 24

What are the difference between channel and carrier?

Answer 24

Channels are open at both ends, like a tunnel and allow for much more rapid transport. Carriers are always closed at one end and open at the other. Carriers are also much slower than channels.

Channels:
-Structure: Open pathway resembling a tunnel with openings at both ends.
- Transport Mechanism: Facilitates rapid passive transport through direct diffusion.
- Specificity: Highly selective for specific ions or molecules.
- Rate of Transport: Allows for rapid transport due to the open pathway.

Carriers:
- Structure: Binding site with one closed end and one open end.
- Transport Mechanism: Undergoes conformational changes to transport molecules across the membrane.
- Specificity: Specific for certain molecules and can transport related substances.
- Rate of Transport: Slower transport rate due to conformational changes involved.

In summary, channels provide rapid, selective transport through open pathways, while carriers transport molecules through conformational changes, have specificity for certain molecules, and operate at a slower rate.

Question 25

Phospholipids containing _______ are always found exclusively on the exterior side of a membrane.

Answer 25

Glycolipids (carbohydrate chains) The carbohydrate chains of glycolipids are hydrophilic (water-loving), making them more suitable for interactions with the extracellular environment. This localization helps to maintain the asymmetry of the lipid bilayer and plays important roles in cell recognition, cell-cell interactions, and signaling processes.

Question 26

Passive carriers dissipate concentration gradients, activate transporters build concentration gradients. T/F

Answer 26

True Passive carriers, also known as passive transporters or facilitative transporters, do not require energy input and do not actively create concentration gradients. Instead, they facilitate the movement of molecules down their concentration gradients through facilitated diffusion, allowing molecules to passively diffuse from an area of higher concentration to an area of lower concentration. Active transporters, on the other hand, do require energy input (usually in the form of ATP) to actively transport molecules against their concentration gradients, creating and maintaining concentration gradients across the membrane. In summary, passive carriers do not require energy and facilitate the movement of molecules down their concentration gradients, while active transporters require energy and actively transport molecules against their concentration gradients, contributing to the establishment and maintenance of concentration gradients. True. Passive creating a concentration gradient requires energy, while diffusion does not.

Question 27

Passive carriers consume ATP energy when they transport ions, active carriers create ATP. T/F

Answer 27

False. Passive carriers do not consume ATP energy when they transport ions. They facilitate the movement of ions or molecules down their concentration gradients through facilitated diffusion, and this process does not require the expenditure of ATP. Passive carriers do not use ATP. Active carriers hydrolyze ATP for energy (like gas in a car).

Question 28

Passive carriers contain beta sheets in their transmembrane domains, active carriers contain alpha helices. T/F

Answer 28

False Both alpha helices and beta sheets can be found in both passive and active carriers. The specific secondary structure elements within the transmembrane domains of carriers can vary depending on the protein and its function. The arrangement of these secondary structure elements is determined by the sequence of amino acids in the protein. Therefore, it is not accurate to associate alpha helices exclusively with active carriers or beta sheets exclusively with passive carriers. The presence of alpha helices and beta sheets in carrier proteins can vary, and both types of secondary structures can be found in different carrier proteins, irrespective of their mode of transport.

Question 29

Passive carriers transport sugars, active carriers transport ions. T/F

Answer 29

False Passive carriers, also known as facilitative transporters, facilitate the movement of molecules down their concentration gradients without consuming energy. While some passive carriers may transport sugars, they can also transport a variety of other molecules such as amino acids, nucleotides, or even ions. Active carriers, on the other hand, actively transport molecules against their concentration gradients, requiring the input of energy (usually in the form of ATP). Active carriers can transport a wide range of molecules, including ions, sugars, amino acids, and other organic molecules. In summary, the specificity of carriers, both passive and active, is diverse and not limited to sugars or ions. They can transport a variety of molecules depending on their specific structure and function.

Question 30

The definition of ________ is a measure of ionic imbalance across the plasma membrane, caused by unequal concentrations of ions in the cytosol vs. the extracellular space; measured in millivolts.

Answer 30

Membrane Potential

Question 31

GLUTs are uniporters that use ATP hydrolysis for energy. T/F

Answer 31

False. GLUTs (glucose transporters) are not uniporters that use ATP hydrolysis for energy. They are passive facilitative transporters that facilitate the facilitated diffusion of glucose across the cell membrane. GLUTs do not require ATP hydrolysis for their transport function. GLUTs utilize the concentration gradient of glucose to transport molecules from an area of higher concentration to an area of lower concentration. They undergo conformational changes to allow glucose to bind and be transported across the membrane. This process is passive and does not involve ATP hydrolysis. In summary, GLUTs are not uniporters that use ATP hydrolysis for energy. They are passive facilitative transporters that rely on concentration gradients for glucose transport.

Question 32

GLUT1 transports glucose across the blood brain barrier. T/F

Answer 32

True. GLUT1 is a glucose transporter protein that is responsible for transporting glucose across the blood-brain barrier. The blood-brain barrier is a specialized barrier formed by the endothelial cells of the brain capillaries, which tightly regulate the passage of substances from the bloodstream into the brain. GLUT1 is highly expressed in the endothelial cells of the blood-brain barrier and plays a crucial role in facilitating the transport of glucose from the blood into the brain to meet its energy demands. Therefore, GLUT1 is indeed involved in transporting glucose across the blood-brain barrier, ensuring a sufficient supply of glucose for the brain's metabolic needs.

Question 33

GLUT2 initially transports glucose into the pancreas after eating, allowing insulin to be released into the bloodstream. T/F

Answer 33

False. GLUT2 exports glucose out of glucose producing organs like the liver. GLUT2 is primarily involved in exporting glucose out of glucose-producing organs like the liver, rather than transporting glucose into the pancreas. GLUT2 is located in the plasma membrane of hepatocytes (liver cells) and plays a key role in glucose homeostasis. In the liver, GLUT2 allows for the transport of glucose from hepatocytes into the bloodstream, ensuring that glucose produced by the liver is released into circulation and can be utilized by other tissues in the body. GLUT2 has a high glucose transport capacity and helps maintain blood glucose levels within a normal range.

Question 34

After insulin signaling, GLUT4 is transported to the membrane of various cells to increase glucose transport in tissues (But not if you have Type II Diabetes). T/F

Answer 34

True. GLUT4 is added to the membranes of adipose and muscle cells to increase glucose uptake in response to insulin signals after eating. In type II diabetes, the transport of GLUT4 to the membrane is inhibited. After insulin signaling, GLUT4 transporters are translocated from intracellular vesicles to the plasma membrane of various cells, including adipocytes and muscle cells. This translocation is a key step in increasing glucose transport into tissues. Insulin binds to its receptor on the cell surface, triggering a signaling cascade that leads to the activation of intracellular processes. One of the effects of insulin signaling is the translocation of GLUT4 transporters from intracellular storage vesicles to the plasma membrane. By increasing the number of GLUT4 transporters on the cell surface, glucose uptake into the cells is enhanced. This process plays a vital role in regulating blood glucose levels by promoting glucose utilization in insulin-sensitive tissues. However, in individuals with Type II Diabetes, there is a dysfunction in GLUT4 translocation, leading to impaired glucose uptake by tissues. This results in persistent hyperglycemia, a characteristic feature of Type II Diabetes. In summary, GLUT4 translocation refers to the insulin-mediated process of moving GLUT4 transporters from intracellular vesicles to the plasma membrane, increasing glucose transport into tissues and helping to regulate blood glucose levels.

Question 35

If the plasma membrane of an animal cell was made entirely permeable to Na+, the Na+/K+ pump would continue to pump ions and hydrolyse ATP, but not build a Na+ gradient. T/F

Answer 35

True. The Na/K ATPase would continue to pump the ions, but since the Na is able to pass right back through the membrane the gradient would be destroyed at the same instant that it was created. If the plasma membrane of an animal cell was made entirely permeable to Na+, the Na+/K+ pump would still continue to actively pump ions and hydrolyze ATP. However, since Na+ could freely cross the membrane, it would not be able to build a concentration gradient of Na+. The Na+/K+ pump functions by actively transporting three Na+ ions out of the cell and two K+ ions into the cell for every ATP molecule hydrolyzed. This process helps maintain the concentration gradient of Na+ and K+, with higher Na+ levels outside the cell and higher K+ levels inside the cell. If the plasma membrane became permeable to Na+, the passive movement of Na+ into the cell would counteract the active transport of the Na+/K+ pump. As a result, the pump would continue to consume ATP but would be unable to establish or maintain a significant Na+ concentration gradient. In summary, if the plasma membrane became permeable to Na+, the Na+/K+ pump would still function and consume ATP, but it would not be able to build a Na+ gradient due to the unrestricted movement of Na+ ions across the membrane.

Question 36

Passive carriers are localized to the basolateral membrane of epithelial cells, activate transporters are localized to the apical membrane. T/F

Answer 36

True In epithelial cells, passive carriers or facilitated transporters are indeed localized to the basolateral membrane, while active transporters are localized to the apical membrane. The apical membrane of epithelial cells faces the lumen or the external environment, such as the lumen of the intestine. Substances need to be actively transported from the lumen into the cell through specialized transporters located on the apical membrane. This active transport process requires energy. Once inside the cell, substances can be transported across the basolateral membrane, which faces the underlying tissue or bloodstream. This movement can occur passively through facilitated transporters or passive carriers, allowing substances to move down their concentration gradient out of the cell and into the blood or surrounding tissue. This segregation of transporters between the apical and basolateral membranes is crucial for maintaining the directional movement of substances and ensuring efficient absorption and secretion across epithelial cell layers. In summary, passive carriers are localized to the basolateral membrane, which is oriented away from the lumen, while active transporters are localized to the apical membrane, which is oriented toward the lumen.