Question

Proteins in the membrane, responsible for facilitated diffusion and active transport, show some common characteristic like. (1) Being highly selective (2) Bcing liable to saturate (3) Responding to inhibitor (4) Being regulated by hormones (a) 1 and 2 only (b) 2 and 3 only (c) 3 and 4 only (d) All of these

Short answer

Based on the above analysis and understanding of all the given features, (d) All of these is the correct answer.

Step-by-step solution

Analyze Feature 1

Feature 1 is about proteins being highly selective. It is known that proteins involved in facilitated diffusion and active transport are indeed highly selective. They allow only specific molecules to pass through the membrane. So this feature is correct.

Analyze Feature 2

Feature 2 refers to the proteins being liable to saturate. This is also true. Both facilitated diffusion and active transport processes can be saturated when all the proteins in the membrane are occupied by molecules. Thus, this feature is also correct.

Analyze Feature 3

Feature 3 refers to proteins responding to inhibitors. Both types of proteins can be inhibited by specific molecules, indicating that the proteins' activity could be regulated or stopped. Therefore, this feature is also applicable.

Analyze Feature 4

Feature 4 says the proteins are regulated by hormones. This feature is also valid, as certain hormones can influence the actions of these transport proteins, altering their activity.

Key concepts

Facilitated Diffusion

Facilitated diffusion is a passive transport mechanism used by cells to move molecules across the cell membrane with the assistance of membrane proteins. Unlike simple diffusion, which occurs freely across the cell membrane’s phospholipid bilayer, facilitated diffusion requires specific transport proteins to move substances that are either too large or not soluble in lipids.

One key characteristic of facilitated diffusion is its protein selectivity. The transport proteins, often called carriers or channels, are designed to recognize and bind to a specific molecule or ion. For instance, glucose transporters will only allow glucose to pass through and will exclude other sugars.

Moreover, facilitated diffusion exhibits saturation kinetics. Once all the transport proteins are occupied, an increase in the concentration of the substance will not increase the rate of transport. This is because the proteins are fully saturated and cannot work any faster, similar to a busy highway at peak traffic hours where adding more cars doesn't increase the flow of traffic.

Active Transport

On the other hand, active transport is an energy-requiring process that moves molecules across a cell membrane from a region of lower concentration to one of higher concentration, against the concentration gradient. This process is fundamental in maintaining cellular homeostasis and allows cells to import nutrients and export toxins effectively.

Active transport is also tightly dependent on protein selectivity. These transport proteins, known as pumps, are specific to the molecules they move. An example is the sodium-potassium pump which actively translocates Na⁺ and K⁺ ions in opposite directions across the membrane.

Moreover, active transport also shows saturation kinetics. The rate of transport will level off even if substrate concentration increases once all the protein pumps are engaged. Additionally, this process can be influenced by cellular energy levels, since active transport requires ATP to function.

Protein Selectivity

Protein selectivity refers to the preference of transport proteins for specific substrates over others. This selectivity is vital for maintaining proper cellular function by ensuring that only the right molecules enter or exit the cell at the right times.

Both facilitated diffusion and active transport proteins exhibit high specificity, meaning they are designed to recognize and move particular molecules. This is analogous to a lock and key model, where the lock (transport protein) will only open for the right key (molecule). Factors that can affect protein selectivity include the shape and charge of the molecule and its chemical compatibility with the binding site on the protein.

Regulated by factors such as inhibitors and hormones, transport proteins can have their activity increased or decreased, which further illustrates the precise control cells have over what enters and exits their boundaries.

Saturation Kinetics

Saturation kinetics is a characteristic of many enzymes and transport proteins that denotes a plateau in the rate of a process despite further increases in the concentration of substrates or ligands. In the context of membrane transport proteins, once all the proteins are occupied by their specific molecules, the rate of transport will not increase, even if the concentration of those molecules is increased.

This is the result of all available binding sites being filled, rendering additional substrate molecules ineffective in increasing the rate of transport. This concept can be visualized when graphing the rate of transport against substrate concentration, resulting in a sigmoidal or hyperbolic curve that eventually plateaus. Saturation kinetics ensures that a cell can regulate nutrient intake and waste output efficiently but also implies that there is a limit to how quickly a cell can respond to changes in environmental conditions.