Question

Choose the total number of correct statements from the following: (1) Some channels in membrane are always open. (2) Porins allow the passage of molecule of size up to small protein. (3) Water channel made up of eight different type of aquaporins. (4) Facilitated diffusion is very specific. (a) 1 (b) 2 (c) 3 (d) 4

Short answer

Based on the analytical steps, the number of correct statements is 3 (answers 1, 2 and 4). Therefore, the correct answer choice would be (c) 3.

Step-by-step solution

Analyzing Statement 1

(1) Some channels in membrane are always open. True, some channels including leaky channels are always open.

Analyzing Statement 2

(2) Porins allow the passage of molecule of size up to small protein. True, porins are proteins that create large pores in the outer membrane of the cells, allowing the passage of molecules up to a certain size, including small proteins.

Analyzing Statement 3

(3) Water channel made up of eight different type of aquaporins. False, not all cells have eight different types of aquaporins. The type and number of aquaporins differ in various cell types.

Analyzing Statement 4

(4) Facilitated diffusion is very specific. True, facilitated diffusion is carried out by carrier proteins that are specific for the molecule they transport.

Key concepts

Leaky Channels

Leaky channels are a fascinating feature of cell membranes that allow ions and molecules to pass through the membrane continuously. Unlike most channels, which open or close based on specific signals, leaky channels remain open all the time.
This perpetual openness means they consistently permit specific ions, like potassium (K+), to move across the cell membrane. This movement helps maintain essential cell functions such as the resting membrane potential.

• They contribute to the regulation of ion concentrations inside and outside cells.
• They play a crucial role in cell signaling and homeostasis.

Leaky channels are essential for maintaining a stable internal environment, and any disruption in their function can significantly disrupt cellular activities.

Porins

Porins are large molecules that form channels in the outer membranes of certain types of cells, such as Gram-negative bacteria and mitochondria. These channels are vital for allowing the movement of small molecules and ions across the membrane without hindrance.
Porins create wide, non-specific passageways that enable the passage of molecules up to the size of small proteins. This makes them unique compared to other channels in the cell membrane, which are usually highly selective.

• Porins facilitate rapid diffusion of nutrients and waste products.
• They are essential for the survival of cells in hostile environments.

Through porins, cells ensure an efficient exchange of materials necessary for their various processes.

Aquaporins

Aquaporins are specialized membrane proteins dedicated to water transport. They form channels that specifically allow water molecules to pass through the cell membrane at a high rate, playing a vital role in water balance within the body.
These channels are highly selective for water and do not allow ions or other solutes to pass.

• Aquaporins are found in various tissues, including kidneys and plant roots.
• They facilitate processes like urine concentration and water uptake in plants.

While aquaporins in some species may number eight or more distinct types, their variety and abundance differ across organisms, challenging the notion that all cells contain the same amount.

Facilitated Diffusion

Facilitated diffusion is a crucial process allowing specific molecules to move across cell membranes using protein carriers. Unlike simple diffusion, this process does not require energy input because it occurs along the concentration gradient.
It differs from active transport, which uses energy to move molecules against the gradient.
Facilitated diffusion is highly specific, with carrier proteins recognizing and binding to particular molecules, then assisting their passage through the membrane.

• This process is essential for transporting glucose and amino acids into cells.
• It allows cells to efficiently uptake necessary nutrients while maintaining internal balance.

Overall, facilitated diffusion exemplifies the cell's precise regulation of its internal environment, enabling appropriate cellular function.