Question

All of the following are correct about an ionophore except it A. requires the input of metabolic energy for mediated transport of an ion. B. may diffuse back and forth across a membrane. C. may form a channel across a membrane through which an ion may diffuse. D. may catalyze electrogenic-mediated transport of an ion. E. will have specificity for the ion it moves.

Short answer

A) An ionophore requires the input of metabolic energy for the mediated transport of an ion. B) An ionophore may diffuse back and forth across a membrane. C) An ionophore may form a channel across a membrane through which an ion may diffuse. D) An ionophore may catalyze electrogenic-mediated transport of an ion. E) An ionophore will have specificity for the ion it moves. Answer: A) An ionophore requires the input of metabolic energy for the mediated transport of an ion.

Step-by-step solution

Analyzing Statement A

Statement A says that an ionophore requires the input of metabolic energy for the mediated transport of an ion. Ionophores are molecules that, indeed, facilitate the transport of ions across a cell membrane. However, most ionophores do not require energy for this process, as they do not employ active transport mechanisms such as carrier proteins. Instead, they generally enable passive transport of ions. Thus, statement A is incorrect about ionophores.

Analyzing Statement B

Statement B says that an ionophore may diffuse back and forth across a membrane. Ionophores, like other molecules, can pass through a cell membrane depending on their chemical properties, size, and the permeability of the membrane. So statement B is a correct description of ionophores.

Analyzing Statement C

Statement C states that an ionophore may form a channel across a membrane through which an ion may diffuse. There are indeed two types of ionophores: the ones forming ion channels, and the others called mobile ion carriers. The ion channel-forming ionophores create a path for ions to pass through the membrane by forming a hydrophilic channel. Thus, statement C is correct.

Analyzing Statement D

Statement D says that an ionophore may catalyze electrogenic-mediated transport of an ion. Electrogenic transport means that an ion is transported across the membrane, leading to a change in the electrical potential of the membrane. Some ionophores, like valinomycin, can facilitate this type of transportation, making statement D correct.

Analyzing Statement E

Statement E states that an ionophore will have specificity for the ion it moves. Ionophores can indeed have specificity for the ions they transport, like valinomycin for potassium ions or nonactin for ammonium ions. So statement E is a correct description of ionophores. In conclusion, statement A is the only incorrect statement about ionophores, as most ionophores do not require metabolic energy input for the transport of ions. The correct answer is A.

Key concepts

Passive Transport

Passive transport is the movement of molecules or ions across biological membranes without the need for energy input from the cell. It relies on the natural motion of particles, which tend to move from areas of high concentration to areas of low concentration, a principle known as the concentration gradient.

Ionophores are a perfect example of facilitators of passive transport. They allow specific ions to pass through the membrane without utilizing cellular energy, such as adenosine triphosphate (ATP). Instead, they utilize the existing concentration gradient to enable ions to move into or out of the cell. An important point to remember is that passive transport, facilitated by ionophores or not, does not go against the concentration gradient; it simply allows substances to 'flow downhill' along this gradient in a process known as diffusion.

Ion Transport Across Membrane

Ion transport across membranes is a vital process for maintaining the electrochemical balance essential for cell function. Membrane proteins, such as ion channels or ion carriers, often regulate the passage of ions; however, ionophores can also facilitate this transport.

There are two primary mechanisms by which ionophores operate: some integrate into the lipid bilayer to form channels, while others bind specific ions and shuttle them across the membrane. The common goal is transporting ions like sodium, potassium, calcium, and chloride, each critical for processes like nerve impulse transmission and muscle contraction. Although ionophores mediate ion passageway, this action is usually selective, meaning that an ionophore will typically transport certain ions more effectively than others, providing an additional layer of control over the intracellular and extracellular ionic concentrations.

Ion Channel

Ion channels are integral proteins that span cell membranes and provide a pathway for ions to flow into or out of a cell. These channels are highly selective, often permitting only one type or a few types of ions to pass. This specificity is determined by the size and charge distribution within the channel.

Ionophores that act as ion channels are passive transport mechanisms. They align with the lipid bilayer of the membrane, opening a hydrophilic route for ions to move with the concentration gradient. Unlike transporter proteins, these ion channels do not bind with the ions but rather provide a conduit for their passage, akin to a tunnel through the mountain. Their opening and closing can be regulated by various factors including voltage changes across the membrane, binding of molecules, or mechanical forces.

Electrogenic-Mediated Transport

Electrogenic-mediated transport refers to the movement of charged particles, such as ions, across a membrane that results in a change in the electrical potential across that membrane. This can be contrasted with electroneutral transport, where no net charge is moved. When ionophores facilitate the transport of ions, the process can become electrogenic if more positive or negative charges are moved in one direction.

This movement of charges contributes to the cell's membrane potential—a critical factor in the conduction of nerve impulses, muscle contractions, and the general excitability of cells. Some ionophores can conduct such transport selectively, influencing the membrane potential and thus playing a crucial role in cellular signaling and homeostasis.