Secondary Active Transport

Secondary Active Transport Definition

Active transport is a type of transport that moves against its concentration gradient. This means that active transport requires energy either in the form of ATP or the electrochemical gradient, which we will go over in greater depth later.

Active transport needs the energy to transport materials because the molecules have to go against their concentration gradient. When a molecule goes against its concentration gradient, it goes from low to high concentration. This means if, let's say, carbon dioxide molecules are higher inside than outside the cell, then carbon dioxide needs to go into the cell to go against its concentration gradient.

Active transport needs energy because all molecules want to go with their concentration gradient, not against it.

When a molecule goes with its concentration gradient, it goes from high to low concentration. This type of transport is called passive transport and does not require energy in the form of ATP.

Some essential passive transport types are simple diffusion and facilitated diffusion. Simple diffusion is passive diffusion that doesn't require help from any transport Proteins. In contrast, facilitated diffusion is passive transport that needs help from transport Proteins such as carriers or channels.

Transport proteins are proteins that move materials throughout our bodies and Cells. Specifically, transport proteins usually work to move molecules and substances across biological membranes such as the cell membrane.

The most common types of transport proteins are carrier and channel proteins. Channel proteins remain open and don't change shape compared to Carrier Proteins. This results in channel proteins diffusing molecules at faster rates than Carrier Proteins.

The molecules that need help to travel across the membrane are either large hydrophobic molecules, hydrophilic molecules, or ions. Hydrophobic molecules are water-hating, while hydrophilic molecules are water-loving. Meanwhile, ions are charged molecules. This is due to the cell membrane's arrangement.

The two types of active transport are primary and secondary active transport, which we will cover in the following sections.

Secondary Active Transport Examples

Secondary active transport couples the transport proteins to the movement of ions or charged molecules down their concentration gradient to another molecule moving against its concentration.

The sodium-glucose pump is the most common example of a secondary active transport and is illustrated in Figure 1:

• Our cells have a higher sodium concentration outside than inside the cell. And have a higher potassium concentration within than outside the cell.

• The sodium-glucose pump uses a carrier protein to bind simultaneously to two sodium atoms and glucose. This is because all molecules, including sodium and glucose, do NOT want to go against their gradient. This means sodium intends to enter the cell, while glucose does not.

• Since sodium wants to enter the cell, it drives the glucose into the cell with it.

• The sodium-glucose pump uses secondary active transport because it combines the transport of sodium down its concentration gradient with the transport of glucose against its concentration gradient.

• This type of transport uses a carrier protein that's considered a cotransporter. A cotransporter is a carrier protein that lets two kinds of molecules be transported simultaneously.

Figure 1: Sodium-glucose pump illustrated. Wikimedia, LadyofHats (Public Domain).

Primary vs. Secondary Active Transport

Primary active transport is a type of active transport that utilizes ATP directly. In contrast, secondary active transport does NOT utilize ATP directly. Instead, it uses one ion going down its concentration gradient to drive the movement of another ion going against its concentration gradient.

An example of a famous primary active transport is the sodium-potassium pump. The sodium-potassium pump causes nerve impulses to fire. Nerve impulses are electrical signals that relay messages from the many parts of the body to the spinal cord and brain. In other words, our body communicates with our brain giving it vital information regarding our environment. For example, if you touch a hot stove, you will feel a sense of pain, allowing you to withdraw your hand before you burn it severely.

The sodium-potassium pump works as follows and is illustrated in Figure 2.

1. Carrier proteins bind to three sodium ions.

2. ATP hydrolyses one phosphate group leading to ADP.

• The hydrolyzed phosphate group attaches to the sodium-potassium pump and provides the energy, resulting in a change in the shape of the carrier protein.

3. The carrier protein's change in shape leads the sodium ions to cross the membrane and be able to exit the cell.

4. The carrier protein's change in shape allows two potassium ions to attach to it.

5. The phosphate group is freed from the pump, allowing the carrier protein to return to its original conformation.

6. This change in shape allows the two potassium to travel across the membrane and enter the cell.

Figure 2: Sodium-potassium pump illustrated. Wikimedia, LadyofHats (Public Domain).

Secondary Active Transport Types

When we talk about secondary active transport, we usually refer to two types: cotransporters and counter-transports.

Cotransporters can also be referred to as symports. This occurs when two molecules bind to the transporter at the same time. After this, the cotransporter can undergo conformational change resulting in one of the ions being able to go down its concentration gradient.

As this one ion goes down its concentration gradient, it drags the second ion against its concentration gradient. Usually, this second ion would not want to go against its concentration gradient, but it can do so because of the coupling with the first ion.

An example of a cotransporter is the sodium-glucose pump we went over earlier. Cotransporters, or symporters, move both ions in the same direction. An illustration of how symporters work is illustrated in red in Figure 3.

In comparison, counter-transport, or antiport, occurs when two molecules or ions travel across the membrane in opposite directions. Like cotransporters, two molecules are transported simultaneously except in opposite directions.

One molecule is transported down its concentration gradient, usually sodium, while the other molecule or ion is transported to the other side. One famous example of a counter-transport is the sodium-calcium counter-transport. The sodium-calcium counter-transport works by allowing three sodium ions to flow down its concentration gradient and into the cell. At the same time, calcium ions are moved out of the cell. For every three sodium ions that are moved into the cell, one calcium ion is moved out of the cell.

The sodium-calcium counter-transport is vital as it's used to keep calcium levels low within cells after an action potential. Action potentials are essential for cell-to-cell communication as they help nerves with saltatory conduction so messages can be relayed throughout our bodies. Action potentials open calcium channels, letting an influx of calcium in. This means the sodium-calcium antiport must send calcium back out to restore membrane potential. An image of how antiporters work is illustrated in blue in Figure 3.

Figure 3: Types of transporters illustrated. Wikimedia, Lupask (Public Domain).

Secondary active transport energy source

Unlike primary active transport, secondary active transport does not use ATP directly; its energy source is the electrochemical gradient that pumps ions into and out of the cells.

Electrochemical gradients are vital in biological processes, like photosynthesis and cellular respiration. The electron transport chain (ETC) is used in both photosynthesis and cellular respiration.

The method of ETC works like water running through a dam to generate power. This involves protein complexes that create an electrochemical gradient by having electrons pass down a cascade from one molecule to another. As electrons go down, they lose energy. This lost energy pumps hydrogen ions across the membrane to create an electrochemical gradient. This electrochemical gradient allows cells to use ATP synthase to make ATP.

Other uses of electrochemical gradients are muscle contraction, cell-cell communication, etc. For instance, the sodium-potassium pump uses an electrochemical gradient to send nerve impulses because the concentration of ions differs within the cell's internal and external environment. The sodium-potassium pump also has ions of different charges.