Electrochemical Gradients

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Electrochemical Gradients

Introduction

In living cells, the plasma membrane or cell membrane is a selectively permeable barrier that allows selective substances to pass through it. Thus, it maintains different concentrations on both sides of the membrane. This gives rise to different electrical and chemical concentration gradients on the membrane surface which collectively form the electrochemical gradient.

What is an Electrochemical Gradient? 

It is defined as the difference in the charge and the chemical concentration across the plasma membrane due to its selective permeability. The combination of the concentration gradient and electrical charge gradient that affects the movement of a particular ion across the plasma membrane is known as a concentration gradient.

Simple concentration gradients are not so complex as they exist due to the differential concentration of a substance across a membrane. But in the case of living organisms, the gradients are not that simple. In addition to a concentration gradient, an electrical gradient is also present inside living cells because it’s not only the ions that move inside and outside the cells but cells’ intracellular space contain some proteins as well. Most of these proteins are negatively changed and don’t move outside. As a result of this, the inside of the membrane is more negatively charged which causes an electrical gradient to exist across the plasma membrane in addition to a concentration gradient due to ions. Both these electrical and concentration gradients are studied under an electrochemical gradient.

To understand this, consider the movement of sodium and potassium ions across the membrane. In addition to the negatively charged proteins present inside the cell, the cells have a higher concentration of potassium inside the cell and a higher concentration of sodium outside the cell. The concentration gradient pumps sodium inside the cell (from higher concentration to lower concentration) and the electrical gradient also drives sodium inside the cell due to the negatively charged interior of the cell. However, the situation is more complex for potassium. The electrical gradient of potassium (a positive ion) causes it to move inside the cell due to a negatively charged interior but the concentration gradient of potassium moves it outside the cell (due to a lower concentration of potassium outside). This process of movement due to concentration gradient and electrical charge are referred to as electrochemical gradient.

Components of Electrochemical Gradient

There are two components of an electrochemical gradient:

  1. Electrical component
  2. Chemical component

The electrical component results due to the difference in electrical charge across the plasma membrane. And the chemical component is due to the difference in concentration of ions across the membrane. The combination of these two predicts the thermodynamically favorable direction for the movement of ions through the selectively permeable plasma membrane.

Types of Active Transport Mechanisms in Electrochemical Gradient 

There are two types of active transport mechanisms for the movement of ions and substances:

  1. Primary active transport
  2. Secondary active transport

Primary active transport helps in the movement of ions across a membrane and establishes a difference in gradient which depends on ATP directly. While secondary active transport is for the movement of substances as a result of electrochemical gradient created by primary active transport and thus doesn’t depend upon ATP directly.

Moving Against Electrochemical Gradient 

The movement of substances against the electrochemical gradient occurs in the presence of energy. The energy comes from adenosine triphosphate (ATP) that is generated during cell metabolism. Active transport mechanisms, which are collectively known as pumps, help in the movement of substances against the electrochemical gradients. Many small substances continuously pass through the cell membrane. The concentration of ions and substances is maintained by active transport. Therefore, a major part of the cell’s metabolic energy is used to maintain these processes.

As these active transport mechanisms are dependent on the supply of energy by the cell’s metabolism, any poison that can interfere with the metabolism to stop the supply of ATP will affect these mechanisms.

Carrier Proteins for Active Transport

The active transport of substances across the membrane is facilitated by the presence of specific carrier proteins or pumps. Following three are the types of protein carriers or transporters that are present:

  1. Uniporters
  2. Symporters
  3. Antiporters

A uniporter is involved in the transport of one specific ion or molecule. A symporter transports two different ions or molecules and both in the same direction. An antiporter acts as a carrier protein for two or more different ions or molecules but in different directions. These protein carriers are also responsible for the transport of small, uncharged molecules such as glucose. These three carrier proteins also have a role in facilitated diffusion, but in that case, ATP is not needed. Some of these pumps or protein carriers for active transport are below:

Na+-K+ ATPase: It carries sodium and potassium ions

H+-K+ ATPase: It transports hydrogen and potassium ions

Ca+ ATPase: It transports only calcium ions

H+ ATPase: It transports only hydrogen ions

The first two from the above-mentioned pumps are antiporter carrier proteins.

Electrochemical Gradient of Sodium and Potassium Pump 

The electrochemical gradient Na+/K+ pump that is established by the active transport mechanism is an example of an electrochemical gradient in living cells.

Primary Active Transport 

Primary active transport creates an electrochemical gradient across the membrane by the transport of ions. The process is driven by using ATP. Sodium and potassium pump are one of the most important pumps in living organisms which maintains an electrochemical gradient across the membrane. This pump favors the movement of two potassium ions into the cell and three sodium ions outside the cell. Depending upon the orientation to the interior or exterior of the cell and affinity for both ions, the Na+-K+ ATPase (sodium and potassium pump) is present in two forms.

The process is completed in the following steps:

  • Initially, the carrier enzyme pump is oriented towards the interior of the cell. The carrier has a high affinity for sodium ions transport and three ions can bind to it at a time.
  • The protein carried catalyzes the hydrolysis of ATP and attaches a low energy phosphate group to it.
  • After phosphorylation, the shape of the carrier is changed and the orientation is shifted towards the exterior. As a result, the affinity for sodium is decreased and three sodium ions leave the pump.
  • The change in the shape of the carrier also favors the attachment of two potassium ions due to increased affinity for potassium ions. Due to this, the low-energy phosphate group leaves the carrier.  
  • After the removal of the phosphate group and attachment of potassium ions, the carrier protein changes position towards the interior of the cell.
  • Due to the changed configuration, the affinity for potassium decreases and it releases two ions into the intracellular space. Again, the protein in its initial state, has a greater affinity for sodium ions and the process starts again.

Many changes occur as a result of this process. At this position, sodium ions are in a higher concentration outside the cell than inside and potassium ions are more in the intracellular space of the cell. As a result of two potassium ions moving inside the cell, three potassium ions move outside. This makes the interior of the cell slightly more negative than the exterior. This difference is responsible for creating the necessary conditions for the secondary mechanism. The sodium-potassium pump thus functions as an Electrochemical pump and contributes to membrane potential by establishing an electrical imbalance.

Secondary Active Transport

In the secondary active transport process, for one molecule that moves down the electrochemical gradient, another molecule moves up its concentration gradient. In this process, ATP is not directly attached to the carrier protein. Instead, the molecule or ion moves against its concentration gradient which establishes an electrochemical gradient. The required molecule then moves down the electrochemical gradient. ATP is used in this process as well for generating gradient and energy is not used for the movement of a molecule across the membrane. That’s why it is known as secondary active transport.

Antiporters and symporters are involved in secondary active transport. This process is responsible for the movement of sodium and some other substances into the cell. The other substances include many amino acids and glucose as well. It is also responsible for maintaining a high hydrogen ions concentration in the mitochondria of plants and animals for generating ATP.

Role of Electrochemical Gradient in Biological Process 

Electrochemical gradient determines the direction of movement of substances in biological processes by diffusion and active transport. The diffusion and active transport generate an electrochemical potential across the membrane. The electrochemical potential is due to:

  1. Ion Gradient
  2. Proton Gradient

Ion Gradient

The electrochemical potential as a result of the electrochemical gradient determines the ability of ions to cross the membrane. The membrane can be of cell or organelle or any other sub-cellar entity. This potential is generated basically due to the difference in concentration of ions inside and outside the membrane, the charge present on ions or molecules, and the voltage difference that exists across the membrane.

Transmembrane ATPases are often responsible for maintaining ions gradients. The Sodium and potassium ion gradient are maintained by Na+/K+ ATPase.

Proton Gradient

The proton gradient is established by active transport by proton pumps. This proton electrochemical gradient is responsible for generating chemiosmotic potential (proton motive force) in photosynthesis and cellular respiration. The proton gradient is also responsible to store energy for producing heat and rotation of flagella.

This proton gradient is formed during the electron transport chain in mitochondria or chloroplast by the pumping of protons across the membrane by an active transport mechanism.

Electrochemical gradient in Bacteriorhodopsin 

Electrochemical gradient causes the generation of the proton gradient in Bacteriorhodopsin. By the absorption of photons at a wavelength of 568nm, a proton pump is activated which causes the movement of hydrogen ions from a higher concentration to a lower concentration. After the complete process of proton pumping due to the conformational shift in the retinal, Bacteriorhodopsin restores the initial resting state.

Electrochemical Gradient in Phosphorylation

The electrochemical gradient is also helpful in generating a proton gradient during the process of phosphorylation in mitochondria. In this process, protons are transported from the mitochondrial matrix to the transmembrane space. The protons, that are transferred, include I, III, and IV protons. For generating an electrochemical potential, a total of ten protons are transported from the matrix to the transmembrane space. The electrochemical potential is important for the generation of ATP in presence of ATP synthase. Without the proton electrochemical gradient, energy production doesn’t occur in mitochondria.

Electrochemical Gradient in Photophosphorylation

Photophosphorylation, Cyclic, and Non-cyclic, involves the conversion of ADP to ATP in the presence of sunlight by activation of PSII. The proton gradient is generated due to the absorption of the photon as in the case of Bacteriorhodopsin. Electrons move in the electron transport chain and ATP is formed in the presence of ATP synthase. The electrons are transported from high energy molecules to low energy molecules in the electron transport chain. In Photophosphorylation, a transmembrane electrochemical potential gradient is established by the movement of protons from stroma to thylakoid space.

Importance of Electrochemical Gradient

The importance of the electrochemical gradient is highlighted by the following points:

  1. Adenosine triphosphate, or ATP, is known as the primary energy source in living cells. However, in addition to the ATP energy is also stored in the electrochemical gradient of a molecule or ion across the cell membrane which helps to drive processes of living organisms.
  2. Some of the major biological processes that are due to the electrochemical potential or gradient include nerve impulse conduction, muscle contraction, hormone secretion, and some sensory processes.
  3. The process of oxidative phosphorylation in mitochondria is due to the proton gradient which is a result of an electrochemical gradient. The photon electrochemical gradient is essential for the production of energy in mitochondria.

In plants, during the light-dependent reactions of photosynthesis, a proton electrochemical gradient is established. This is crucial for the completion of the process. In both mitochondria and chloroplast, the proton electrochemical gradient generates chemiosmotic potential which is also known as the proton motive force. This potential energy is involved in the synthesis of ATP by oxidative phosphorylation and photophosphorylation.

References

  1.  Lodish, H; Berk, A; Kaiser, C; Krieger, M; Bretscher, A; Ploegh, H; Amon, A (2000). Molecular Cell Biology (7th ed.). New York, NY: W. H. Freeman and Company. p. 695.
  2. Marieb, E. N., & Hoehn, K. (2014). Human anatomy & physiology. San Francisco, CA: Pearson Education Inc.