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

Membrane permeability is regulated by the insertion or removal of carrier proteins, which facilitate the transport of specific molecules across the cell membrane. In the case of glucose, the insulin-sensitive cells use GLUT4 proteins to regulate permeability. When insulin is present, GLUT4 proteins are inserted into the cell membrane, increasing glucose uptake. In the absence of insulin, GLUT4 proteins are removed, reducing glucose permeability. Water permeability is regulated through aquaporin-2 proteins in kidney's collecting duct cells. When blood osmolality increases, the antidiuretic hormone (ADH) triggers the insertion of aquaporin-2 proteins into the cell membrane, increasing water permeability and reabsorption. In the absence of ADH, aquaporin-2 proteins are removed, decreasing water permeability and promoting urine production.

Step-by-step solution

Introduction to Membrane Permeability

Membrane permeability refers to the ability of a substance to pass through the cell membrane. The cell membrane is a selectively permeable barrier composed of lipids, proteins, and carbohydrates. This selective permeability allows the cell to maintain a stable internal environment by controlling the entrance and exit of substances.

Role of Carrier Proteins

Carrier proteins are membrane proteins that facilitate the transport of specific molecules across the cell membrane. They bind to the molecule on one side of the membrane, undergo a conformational change, and release the molecule on the other side. Since carrier proteins are selective for the molecules they transport, they play a crucial role in maintaining membrane permeability to essential molecules such as glucose and water.

Regulation of Membrane Permeability by Insertion or Removal of Carrier Proteins

The insertion or removal of carrier proteins can regulate the permeability of a membrane. By increasing or decreasing the number of carrier proteins in the membrane, cells can control the rate at which glucose and water pass through the membrane. When the number of carrier proteins increases, the membrane becomes more permeable to the specific molecule, allowing for higher rates of transport. Conversely, when the number of carrier proteins decreases, membrane permeability is reduced, and the transport rate of the specific molecule decreases.

Example: Regulation of Glucose Permeability

One example of glucose permeability regulation is in the insulin-sensitive cells (such as muscle and adipose tissue) which employ GLUT4 (glucose transporter 4) proteins. GLUT4 is a specific carrier protein for glucose. When the insulin hormone is present, it activates a signaling pathway that results in the insertion of GLUT4 proteins into the cell membrane. This increase in GLUT4 proteins enhances the cell's glucose uptake and storage capacity. In the absence of insulin, GLUT4 proteins are removed from the cell membrane, reducing the cell's permeability to glucose.

Example: Regulation of Water Permeability

The regulation of water permeability can be observed in the kidney's collecting duct cells, which employ aquaporin-2 water channel proteins. Aquaporin-2 is a specific carrier protein for water. In response to an increase in blood osmolality (high concentration of solutes), the antidiuretic hormone (ADH) is released, signaling the insertion of aquaporin-2 proteins into the cell membrane. This leads to an increase in water permeability and water reabsorption, helping to maintain the body's water balance. In the absence of ADH, aquaporin-2 proteins are removed from the membrane, reducing the cell's permeability to water, resulting in decreased water reabsorption and increased urine production.