Question

Explain how the permeability of a membrane to glucose and to water can be regulated by the insertion or removal of carrier proteins, and give examples.

Short answer

The permeability of a membrane to glucose and water can be regulated by the insertion or removal of carrier proteins, such as glucose transporters (GLUTs) and aquaporins. For glucose regulation, an example is the muscle cells inserting GLUT4 proteins into the membrane to increase glucose uptake during exercise and removing them when demand decreases. For water regulation, aquaporins in the kidney's collecting duct cells control water reabsorption by increasing or decreasing their number in response to the antidiuretic hormone (ADH).

Step-by-step solution

1. Definition of Membrane Permeability

Membrane permeability is the ability of a substance to pass through a biological membrane, such as the cell membrane. This process is crucial for the transportation of essential molecules (e.g., nutrients, waste products) in and out of cells.

2. Definition of Carrier Proteins

Carrier proteins are a type of membrane proteins that facilitate the transport of specific molecules across cell membranes. They are essential for the passage of molecules that cannot easily diffuse through the lipid bilayer, such as glucose and ions. Carrier proteins work by binding to the molecule they transport, undergoing a conformational change, and releasing the molecule on the other side of the membrane.

3. Regulation of Membrane Permeability through Carrier Proteins

The permeability of a membrane to a specific molecule can be regulated by modifying the number or activity of the carrier proteins. Inserting carrier proteins into the membrane will increase the membrane's permeability, as more molecules will be able to enter or leave the cell. Conversely, removing carrier proteins will decrease the membrane permeability, making it harder for the molecules to cross the membrane.

4. Example of Glucose Regulation

The regulation of glucose transportation is primarily done through glucose transporters called GLUTs. An example of this regulation can be seen in muscle cells, where the glucose transporter GLUT4 is stored inside the cell in vesicles. When the cell needs more glucose, such as during exercise, these vesicles fuse with the cell membrane, inserting GLUT4 proteins and increasing the membrane permeability to glucose. Once the demand for glucose decreases, the GLUT4 proteins are removed from the membrane and stored back in vesicles to decrease the glucose uptake.

5. Example of Water Regulation

Water movement across cell membranes is regulated by a type of carrier protein called aquaporins. Aquaporins are specialized water channels that allow water molecules to pass through the membrane rapidly. The number of aquaporins present in the cell membrane can be regulated to control the amount of water entering or leaving the cell. For instance, in the cells of the kidney's collecting duct, insertion of more aquaporins in response to the antidiuretic hormone (ADH) increases the membrane permeability to water, allowing more water to be reabsorbed and ultimately concentrating the urine.

Key concepts

Carrier Proteins

Carrier proteins are crucial in helping specific molecules cross cell membranes, which can otherwise act as barriers. Not everything can effortlessly pass through the lipid bilayer, the essential structure of the cell membrane. This is where carrier proteins step in, providing necessary assistance.
These proteins work by:

• Binding to the target molecule.
• Undergoing a conformational change which allows transportation across the membrane.
• Releasing the molecule on the opposite side.

Carrier proteins are specific, meaning each type is designed to transport particular molecules, like glucose or ions. By increasing or decreasing the number of these proteins, cells can regulate their permeability, affecting which substances enter or exit the cell. For example, more carrier proteins mean higher permeability and vice versa. This regulatory ability is vital for maintaining the correct internal conditions within the cell.

Glucose Transport Regulation

Glucose transport regulation is a great example of how cells control their permeability to meet their energy needs. Glucose transporters, specifically GLUTs, play a crucial role in this process. These transporters are carrier proteins that manage how glucose molecules move inside the cells.
One common mechanism of regulation involves storing certain types of GLUTs, like GLUT4, within the cell. When the cell requires more glucose, such as during physical activity, these GLUT4 proteins are moved to the cell membrane to allow glucose uptake.
Once the need for glucose is reduced, these transporters are retracted back inside the cell. To recap:

• GLUT4 storage and activation adjust glucose transport based on cellular demand.
• Energy needs directly influence GLUT4 presence on the membrane.

This system ensures that cells have adequate glucose without wasting energy.

Aquaporins

Aquaporins are specialized proteins that create channels in the cell membrane specifically for water, making them vital for efficient water transport. Without aquaporins, water would move much slower across the cell membrane.
The presence of aquaporins is tightly regulated, so cells can control water balance:

• Inserting more aquaporins increases permeability to water, as seen in kidney cells when retaining water is necessary.
• Removing them decreases permeability, reducing water movement.

A notable instance is in the kidneys, where the body's need to conserve water leads to an increased number of aquaporins. This response is mainly triggered by the antidiuretic hormone (ADH), aiding in water reabsorption during periods when the body needs to concentrate urine.
Managing water transport efficiently is essential for maintaining cell volume and overall fluid balance within the body.