Active Transport: Definition & Examples

You were told to take lunch to your grandmother. You get on your bike and set off. It’s downhill to your grandmother’s house. On the way you don’t have to use any energy to get ahead. After lunch together, we head back home. As a thank you, you will receive a piece of cake. This time it’s uphill and you have to pedal hard to get home.

On the way there, you transported lunch without spending any energy, but on the way back, you had to expend energy to bring the piece of cake home. In cells, there are comparable principles of transport. If additional energy has to be provided for mass transport, this is referred to as active transport.

Active transport definition

In living beings, individual cells and cell organelles are separated from their surroundings by biomembranes. For the exchange of substances and the communication between cells and their environment is a mass transport across the biomembrane essential.

If the mass transport takes place against the concentration gradient or against the electrochemical gradient with the provision of energy, it is a matter of a active transport.

Active transport describes the transport of substances through a biomembrane with the provision and consumption of energy. Active transport enables the transport of molecules against the concentration gradient or the transport of charged ions against the electrochemical gradient.

A transport against the concentration gradient corresponds to a molecular movement from the place of lower concentration to the place of higher concentration.

The active transport of substances through the biomembrane can only take place with the help of transport proteins, which are stored in the biomembrane. These are so-called carrier proteins.

In the case of charged ions, not only the concentration gradient but also the electrical gradient plays a role. The electrical gradient causes positively charged particles to move to the negative charge and negatively charged particles to move to the positive charge.

The so-called electrochemical gradient of ions results from the concentration gradient and the electrical gradient. Active transport is required for transport against the electrical gradient.

Difference between active and passive transport

Passive mass transport is opposed to active transport. No additional energy is required for passive transport through a biomembrane. Passive transport of molecules takes place along the concentration gradient or the electrochemical gradient. Passive transport can occur without the help of proteins (diffusion) or with the help of proteins (facilitated diffusion).

You can find more detailed information on passive transport, as well as on diffusion and facilitated diffusion, in the corresponding explanations on the platform.

The table below and the figure that follows illustrate the key differences between active and passive transport.

Passive transportActive transportNo additional energyProvision of energy (e.g. ATP)transport along the concentration gradienttransport against the concentration gradient passive transport can take place with or without the help of transport proteins active transport can only take place with the help of transport proteins

Figure 1: Representation of passive and active mass transport.

Active transport – energy supply

As you have already learned, the provision of energy is required for active transport. Therefore, the active transport of substances is always at a exogenous reaction coupled.

Exogenous reactions are chemical reactions that release energy.

Depending on where the required energy comes from, between primary active transport and secondary active transport be distinguished.

Primary active transport

In primary-active transport, the energy required for transport comes from the cleavage of adenosine triphosphate (ATP).

ATP (adenosine triphosphate) is a high-energy molecule, which is equipped with three phosphate residues. The elimination of a phosphate residue releases energy. This can be used for a wide variety of processes. ATP is something like the fuel of living cells and plays an important role in providing energy in organisms.

Primary-active transport takes place with the help of so-called Transport ATPases instead of. Transport ATPases are carrier proteins (transmembrane proteins) which pass through a organicmembrane form. They are able to split ATP and use the released energy to transport particles through a membrane against the electrochemical gradient.

proton pumps

Proton pumps are a group of transport ATPases, which transport positively charged hydrogen molecules (protons) against their electrochemical gradients through biomembranes while splitting ATP.

The so-called V-type ATPase is a proton pump that transports single protons. Among other things, it is used to increase the proton concentration within lysosomes and thus lower the pH value in the organelles. This promotes the enzymatic digestion that takes place within the lysosomes.

Secondary active transport

Carrier proteins can transport individual molecules, but also coupled multiple molecules. Carrier proteins, which transport several molecules in a coupled manner, are also cotransporter called. Depending on the type of transport, a distinction is made between the following (Fig. 2):

  • uniport: A molecule is passed through a biomembrane transported.
  • symport: Molecules are transported coupled in the same direction across a membrane.
  • antiport: Molecules are transported in the opposite direction.

A co-transport (symport or antiport) of two or more molecules is a prerequisite for secondary active transport. The transport of molecules along the electrochemical gradient is coupled to the transport of molecules against the electrochemical gradient.

The energy required for active transport is obtained from molecular motion along the electrochemical gradient of coupled transport.

A closer look at the secondary-active transport reveals further prerequisites. So that the electrochemical gradient can be used by the cotransporter, it has to be generated beforehand.

As a rule, this gradient can only be built up by primary active transport using ATP. The electrochemical gradient established by primary active transport stores energy. This energy can in turn be used by the cotransporters for secondary active transport.

Without primary-active transport, molecules would always strive for concentration equalization or charge equalization and the build-up of an electrochemical gradient would not be possible.

Sodium/glucose cotransporter

The so-called sodium-glucose co-transporter (SGLT-1) can be found in the small intestine and is responsible for absorbing the glucose obtained through digestion. It is a secondary active symport in which sodium ions (Na+) are transported along the electrochemical gradient coupled with glucose molecules.

The required electrochemical gradient of the sodium ions (N / A+) is maintained by the primarily active transport of a sodium-potassium ion pump (transport ATPase) with the splitting of ATP.

Figure 2: Representation of uniport, symport and antiport.

Danger! Not every antiport or symport is a secondary active transport. There are cotransporters that can be assigned to the transport ATPases. These transport molecules or ions coupled with the splitting of ATP against the electrochemical gradient. This includes the sodium-potassium ion pump mentioned in the previous example.

Tertiary active transport

In the case of tertiary-active transport, the electrochemical gradient is used, which was built up by the secondary-active transport. This is again a co-transport, which uses the energy from transport along the electrochemical gradient to transport molecules counter to the electrochemical gradient.

Active transport by carriers

Active transport of substances through a biomembrane is always dependent on transport proteins. These are carrier proteins.

carrier proteins are transmembrane transport proteins. They form locks in the cell membrane and enable passive transport (facilitated Diffusion) or the active transport of molecules or ions.

You have learned that carrier proteins can transport single molecules or multiple molecules coupled. Depending on the type, it is a uniport, symport or antiport.

Depending on the type of carrier protein, they can be involved in passive transport, primary active transport or secondary active transport. However, the functional principle of the carrier proteins is always the same.

Cotransporters, which are involved in active transport, are often referred to as pumps or ion pumps (e.g. sodium-potassium ion pump).

Properties of carrier proteins

carrier proteinschange their shape (conformational change) to transport molecules or charged ions from one side of the membrane to the other side.

Due to the change in conformation, molecules and ions are biomembrane smuggled. The conformational change is usually in response to the binding of their target molecule, which change moves the molecule to the opposite side.

Due to a specific binding site, carrier proteins are substrate-specific. This means that only a certain type of molecule, more precisely a group of structurally related molecules, can pass through the membrane through the carrier proteins.

Active Transport – The Most Important

  • Active transport describes the transport of substances through a biomembrane with the provision and consumption of energy.
  • Active transport enables the transport of molecules against the concentration gradient or the transport of charged ions against the electrochemical gradient.
  • The active transport of substances through the biomembrane can only take place with the help of transport proteins, which are stored in the biomembrane. These are so-called carrier proteins.
  • Carrier proteins can transport single molecules or several molecules coupled. Depending on the type, it is a uniport, symport or antiport.

  • In primary-active transport, the energy required for transport comes from the cleavage of adenosine triphosphate (ATP).

  • A co-transport (symport or antiport) of two or more molecules is a prerequisite for secondary active transport. The transport of molecules along the electrochemical gradient is coupled to the transport of molecules against their electrochemical gradient.

proof

  1. Chris (2012). Green row – metabolic physiology. Schroedel.
  2. Karp (2005). Molecular Cell Biology. jumper.