Primary Active Transport

The Primary Active Transport Definition

Active Transport requires energy because the molecules must 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, chloride molecules are higher inside than outside the cell, chloride needs to go into the cell to go against its concentration gradient.

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

Molecules that go with their concentration gradient participate in passive transport. Some common types of passive transport are simple diffusion, facilitated diffusion, Osmosis, and filtration. Simple diffusion is passive diffusion that doesn't require help from transport Proteins.

Carrier Proteins are a type of transport protein that helps with the movement of molecules in and out of the cells.

Channel proteins are transport proteins that do the same thing as carrier proteins, except they diffuse molecules faster because they don't have to change shape.

Proteins are organic compounds that have a variety of functions, including acting as enzymes, antibodies, and structural components for our cells. Enzymes speed up our chemical reactions, while antibodies defend our bodies against foreign materials or antigens.

Molecules that need help from transport proteins do so because they are either polar or charged, too big to diffuse through the cell or both. This occurs because of the cell membrane's arrangement.

The cell membrane is a structure that acts as a barrier, letting certain things travel in and out of the cell. Transport proteins are usually embedded into the cell membrane to help molecules that need to go into the cell but can't diffuse into the cell directly.

The cell membrane is made out of phospholipids that have a hydrophilic head and hydrophobic tail. Hydrophilic means water-loving, and hydrophobic means water-hating. Water-loving molecules are polar, while water-hating molecules are non-polar. The phospholipid heads face the outside while the tails face the inside of the cell, which is why hydrophilic molecules can't go in without help from proteins. A detailed image of the cell membrane's structure is shown in Figure 1 below.

Figure 1: Cell membrane structure. Wikimedia, LadyofHats.

Facilitated diffusion is passive transport that requires help from transport proteins such as carriers or channels.

Osmosis is the passive transport of liquids, such as water, through the cell's selectively permeable membrane.

Filtration involves physical pressure forcing fluid through a selectively permeable membrane like the ones our cells have. Filtration usually refers to Blood being pushed through the walls of capillaries.

Primary active transport example

Primary active transport is a type of active transport that involves ATP directly. The sodium-potassium pump (Fig. 2) is one of the most famous examples of primary active transport.

The sodium-potassium (Na⁺/K⁺) pump is integral to our bodies as it drives Nerve Impulses. Nerve impulses send messages from the various parts of the body to the spinal cord and brain. It allows your body to communicate with your brain giving it helpful information regarding your environment. For instance, if you touch a hot stove, you will feel a sense of pain, allowing you to withdraw your hand before you damage your hand any further.

The sodium-potassium pump works as follows:

1. A carrier protein gets three sodium ions bound to it.

2. ATP hydrolysis results in ADP due to the release of one phosphate group. This one phosphate group binds to the sodium-potassium pump and supplies the energy, causing a conformational change in the carrier protein.

3. The pump or carrier protein undergoes a conformational change allowing the sodium \((Na^+)\) ions to cross the membrane and exit the cell.

4. The carrier protein's conformational shape allows two potassium \((K^+)\) to bind to it.

5. The phosphate group is released from the pump, resulting in the carrier protein returning to its original shape.

6. This change to the original shape lets two potassium \((K^+)\) move across the membrane and enter the cell.

Figure 2: Sodium-potassium pump illustrated. Wikimedia, LadyofHats.

Primary vs. secondary active transport

The sodium-potassium pump is an example of primary active transport because ATP directly attaches to the pump. Secondary active transport is another type of active transport. Unlike primary active transport, secondary active transport doesn't use ATP directly.

Instead, secondary active transport couples or pairs the transport proteins to the movement of ions or charged molecules down their concentration or electrochemical gradient to another molecule moving against its concentration or electrochemical gradient.

The sodium-glucose pump is an example of a secondary active transport:

• Cellslike to maintain a higher sodium concentration outside and a higher potassium concentration within the cell. The sodium-glucose pump works through a carrier protein attaching to glucose and two sodium ions simultaneously. This is because neither glucose nor sodium wants to go against their gradient, resulting in sodium wishing to enter the cell while glucose doesn't.

• The difference in energy gradient resulting from sodium wanting to enter the cell drives the glucose into the cell with it. The cell has to use the sodium-potassium pump mentioned earlier to move sodium ions back out.

• The sodium-glucose pump is a secondary active transport because it couples the movement of sodium down its concentration gradient with the movement of glucose, which, unlike sodium, is going against its concentration gradient. This is shown graphically in Figure 3.

Figure 3: Sodium-glucose pump illustrated. Wikimedia, LadyofHats.

Primary active transport diagram

Active transports can be classified based on the direction molecules travel in and how many there are. The types of transporters are antiporters, uniporters, and symporters, as shown in Figure 4.

• Uniporters are transporters that only move one type of molecule.

• Symporters transport two types of molecules in the same direction. An example of a symporter is the sodium-glucose pump.

• Antiporters transport two types of molecules but in opposite directions. An example of an antiporter is the sodium-potassium pump.

Figure 4: Types of transporters illustrated. Wikimedia, Lupask.

The electrochemical gradient is a gradient that consists of a chemical and electrical aspect, hence the name. The chemical gradient is created by the difference in molecule concentration inside and outside the membrane. In contrast, the electrical gradient is created by the difference in charge inside and outside the membrane. Since the electrochemical gradient deals with charges, it occurs when ions or polar molecules are involved.

Electrochemical gradients are essential in processes like Photosynthesis and cellular Respiration. This is because both Biological Processes need to generate ATP. For Photosynthesis, ATP is generated by light-dependent reactions in chloroplasts. While in cellular Respiration, it occurs in the final step called the electron transport chain (ETC). The process of ETC is illustrated in Figure 5 and deals with protein complexes that create an electrochemical gradient to power ATP synthase and create ATP.

Other uses of electrochemical gradients are Nerve Impulses through the sodium-potassium pump, hormone secretion, and even Muscle Contraction. The sodium-potassium pump uses an electrochemical gradient because the ions' concentration differs inside and outside the cell. Since the sodium-potassium pump deals with ions, the charges are also different.

Figure 5: Electron transport chain illustrated. Wikimedia, LadyofHats.

Primary active transport types

The types of primary active transports are:

• P-type ATPase

  • They are ion and lipid pumps found in eukaryotes and prokaryotes. Examples of these types of transporters are the sodium-potassium pump.

  • Prokaryotic organisms are single-celled organisms with no membrane-bound organelles, such as Archaea and Bacteria.

  • Eukaryotes are multicellular or single-celled organisms that have membrane-bound organelles. They consist of Animals, Plants, Fungi, and protists.

  • The P-type stands for the fact that these transporters can autophosphorylate. Autophosphorylation refers to the kinase adding a phosphate group to itself. In the sodium-potassium pump, the carrier protein changes shape after its phosphorylated.

  • Within humans, these ATPases control nerve impulses, muscle relaxation, and other Biological Processes.

• F-ATPase

  • These types of transporters are found in the Mitochondria and Chloroplasts.

  • They make ATPs by allowing protons to go down their electrochemical gradient.

• ABC Transporter

  • ABC stands for ATP binding cassette. This type of transporter couples phosphate groups or energy we get through ATP hydrolysis with molecule movements across membranes.

  • ABC transporters are found in both prokaryotic and eukaryotic organisms.

  • ABC transporters are involved in Signal Transduction, secreting proteins, antibiotic resistance, etc.

  • These transporters can be involved in diseases such as cystic fibrosis. Cystic fibrosis is a genetic disease that results in many lung infections that make it hard for the afflicted to breathe.

• V-ATPase

  • V-ATPase stands for vacuolar proton-translocating ATPases and is located in eukaryotic organisms.

  • These transporters are found in many cells, such as sperm and kidneys. V-ATPases help with coupled transport and keeping the pH stable. For instance, they acidify the sperm to help it get through the egg's cell membrane.