Question

An aquaporin is a. a channel protein that allows the influx of K+ into cells, causing water to also flow in between the phospholipids of the plasma membrane. b. a type of blue-colored pore in the epidermal surfaces of plants. c. a protein channel in plasma membranes that facilitates the diffusion of water. d. a transport protein, or transporter, in plasma membranes that uses protons to cotransport water. e. none of the above.

Short answer

The correct answer is: c. a protein channel in plasma membranes that facilitates the diffusion of water.

Step-by-step solution

Understand Key Terms

Aquaporins are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane. They are a major class of water channel proteins that are widely distributed in all kingdoms of life. The mechanism of water transport across cell membranes through aquaporin plays an important role in many physiological processes.

Evaluate all options

Go through all the given options and match them with the definition of aquaporin gathered. Eliminate the options that do not correspond to the definition of aquaporins.

Choose the correct option

After evaluating all the options, it is evident that the answer is option c. 'a protein channel in plasma membranes that facilitates the diffusion of water.' This definition accurately describes the role and function of aquaporins. So, option c is the correct answer.

Key concepts

Water channel proteins

Water channel proteins, often referred to as aquaporins, are crucial for maintaining water balance in cells. These proteins form pores in the cell membrane, allowing water molecules to pass through efficiently. Without water channels, water movement across cell membranes would be highly inefficient.
The importance of water channel proteins lies in their ability to regulate water flow, ensuring that cells stay hydrated and maintain their shape. They are not specific to a single type of organism, and are found in many diverse environments. Different types are tailor-made for specific conditions, allowing for adaptability across various species.
Aquaporins can be classified into different groups:

• Those that selectively transport water,
• And those that also facilitate the movement of other small molecules, like glycerol.

This ability to function specifically and effectively keeps processes like sweating, kidney filtration, and plant hydration running smoothly. The specificity of these channels ensures that water, and only intended molecules, are moved efficiently across membranes.

Membrane transport

Membrane transport is a complex and vital process for all cells, involving various methods for moving substances in and out of cells. This process can be passive, using no energy, or active, requiring energy input.
Aquaporins facilitate a type of passive transport known as facilitated diffusion. This method allows water to move along its concentration gradient, from areas of higher concentration to lower, without the need for cellular energy. It stands in contrast to active transport, which moves substances against their concentration gradient.
The role of aquaporins in membrane transport is primarily about ensuring the rapid and precise movement of water molecules. This is essential for maintaining cellular functions and responses to environmental changes. In summary, membrane transport, driven by water channels, plays a key function in cellular hydration and molecular balance.

Diffusion of water

The diffusion of water, or osmosis, is a fundamental process in biology where water moves across a semipermeable membrane. This happens from areas where water is more concentrated to areas where it is less so.
Aquaporins significantly enhance this process by providing a channel specifically designed for water. They ensure the fast rate of water diffusion, which is crucial for processes like nutrient absorption in the gut or water balance in kidneys.
Without aquaporins, the process of osmosis would still occur, but at a much slower pace. The existence of these proteins shows evolutionary efficiency, allowing organisms to adapt to various water demands. By fundamentally increasing the rate of water diffusion, aquaporins contribute to maintaining homeostasis within organisms.

Integral membrane proteins

Integral membrane proteins are permanent components of the cell membrane that span from the inside to the outside of the cell. They are crucial for a variety of cellular processes, including signaling, transport, and cellular structure maintenance.
Aquaporins, as integral membrane proteins, are specifically dedicated to facilitating the movement of water. Their structure allows them to embed into the lipid bilayer tightly, forming a selective filter that permits only water molecules to pass while blocking ions and other solutes.
These proteins are characterized by their transmembrane alpha-helices, which anchor them securely in the membrane. Aquaporins demonstrate how integral membrane proteins can specialize to meet specific cellular needs, reflecting nature's ability to optimize function at the molecular level.