Active transport requires energy to work, and it is how a cell moves molecules. Transporting materials into and out of the cells is essential for overall function. Active transport and passive transport are the two main ways that cells move substances. Unlike active transport, passive transport does not require any energy. The easier and cheaper way is passive transport; however, most cells have to rely on active transport to stay alive.
Why Use Active Transport?
Cells often have to use active transport because there is no other choice. Sometimes, diffusion does not work for cells. Active transport uses energy like adenosine triphosphate (ATP) to move molecules against their concentration gradients. Usually, the process involves a protein carrier that helps the transfer by moving the molecules into the cell's interior.
For instance, a cell may want to move sugar molecules inside, but the concentration gradient may not allow passive transport. If there is a lower concentration of sugar inside the cell and a higher concentration outside the cell, then active transport can move the molecules against the gradient. Cells use a large portion of the energy they create for active transport. In fact, in some organisms, the majority of the generated ATP goes toward active transport and maintaining certain levels of molecules inside the cells.
Electrochemical gradients have different charges and chemical concentrations. They exist across a membrane because some atoms and molecules have electrical charges. This means that there is an electrical potential difference or membrane potential.
Sometimes, the cell needs to bring in more compounds and move against the electrochemical gradient. This requires energy but pays off in better overall cell function. It is required for some processes, such as the maintenance of sodium and potassium gradients in the cells. Cells usually have less sodium and more potassium inside, so sodium tends to enter the cell while potassium leaves. Active transport lets the cell move them against their usual concentration gradients.
Primary Active Transport
Primary active transport uses ATP as a source of energy for movement. It moves ions across the plasma membrane, which creates a charge difference. Often, a molecule enters the cell as another type of molecule leaves the cell. This creates both concentration and charge differences across the cell's membrane.
The sodium-potassium pump is a crucial part of many cells. The pump moves sodium out of the cell while moving potassium inside. The hydrolysis of ATP gives the cell the energy it needs during the process. The sodium-potassium pump is a P-type pump that moves three sodium ions to the outside and brings two potassium ions inside.
The sodium-potassium pump binds ATP and the three sodium ions. Then, phosphorylation happens at the pump so that it changes its shape. This allows the sodium to leave the cell, and the potassium ions to be picked up. Next, the phosphorylation reverses, which again changes the shape of the pump, so potassium enters the cell. This pump is important for overall nerve function and benefits the organism.
Types of Primary Active Transporters
There are different types of primary active transporters. P-type ATPase, such as the sodium-potassium pump, exists in eukaryotes, bacteria and archaea. You can see P-type ATPase in ion pumps like proton pumps, sodium-potassium pumps and calcium pumps. F-type ATPase exists in mitochondria, chloroplasts and bacteria. V-type ATPase exists in eukaryotes, and the ABC transporter (ABC means "ATP-binding cassette") exists in both prokaryotes and eukaryotes.
Secondary Active Transport
Secondary active transport uses electrochemical gradients to transport substances with the help of a cotransporter. It allows the carried substances to move up their gradients thanks to the cotransporter, while the main substrate moves down its gradient. Essentially, secondary active transport uses the energy from the electrochemical gradients that primary active transport creates. This allows the cell to get other molecules, like glucose, inside. Secondary active transport is important for overall cell function.
However, secondary active transport can also make energy like ATP through the hydrogen ion gradient in the mitochondria. For example, the energy that accumulates in the hydrogen ions can be used when the ions pass through the channel protein ATP synthase. This allows the cell to convert ADP to ATP.
Carrier proteins or pumps are a crucial part of active transport. They help transport materials in the cell. There are three major types of carrier proteins: uniporters, symporters and antiporters. Uniporters carry only one type of ion or molecule, but symporters can carry two ions or molecules in the same direction. Antiporters can carry two ions or molecules in different directions.
It is important to note that carrier proteins appear in active and passive transport. Some do not need energy to work. However, the carrier proteins used in active transport do need energy to function. ATP allows them to make shape changes. An example of an antiporter carrier protein is Na+-K+ATPase, which can move potassium and sodium ions in the cell.
Endocytosis and Exocytosis
Endocytosis and exocytosis are also examples of active transport in the cell. They allow for bulk transport movement into and out of cells via vesicles, so cells can transfer large molecules. Sometimes cells need a big protein or another substance that does not fit through the plasma membrane or transport channels. For these macromolecules, endocytosis and exocytosis are the best options. Since they use active transport, they both need energy to work. These processes are important for humans because they have roles in nerve function and immune system function.
During endocytosis, the cell consumes a large molecule outside of its plasma membrane. The cell uses its membrane to surround and eat the molecule by folding over it. This creates a vesicle, which is a sac surrounded by a membrane, that contains the molecule. Then, the vesicle comes off the plasma membrane and moves the molecule into the interior of the cell.
In addition to consuming large molecules, the cell can eat other cells or parts of them. The two main types of endocytosis are phagocytosis and pinocytosis. Phagocytosis is how a cell eats a large molecule. Pinocytosis is how a cell drinks liquids such as extracellular fluid. Some cells constantly use pinocytosis to pick up small nutrients from their surroundings. Cells can hold the nutrients in small vesicles once they are inside.
Examples of Phagocytes
Phagocytes are cells that use phagocytosis to consume things. Some examples of phagocytes in the human body are white blood cells, such as neutrophils and monocytes. Neutrophils combat invading bacteria through phagocytosis and help prevent the bacteria from hurting you by surrounding the bacteria, consuming it and thus destroying it. Monocytes are bigger than neutrophils. However, they also use phagocytosis to consume bacteria or dead cells.
Your lungs also have phagocytes called macrophages. When you inhale dust, some of it reaches your lungs and goes into the air sacs called alveoli. Then, the macrophages can attack the dust and surround it. They essentially swallow the dust to keep your lungs healthy. Although the human body has a strong defense system, it sometimes does not work well. For example, macrophages that swallow silica particles can die and emit toxic substances. This can cause scar tissue to form.
Amoebas are single-celled and rely on phagocytosis to eat. They look for nutrients and surround them; then, they engulf the food and form a food vacuole. Next, the food vacuole joins a lysosome inside the amoebas to break down the nutrients. The lysosome has enzymes that help the process.
Receptor-mediated endocytosis allows the cells to consume specific types of molecules that they need. Receptor proteins help this process by binding to these molecules so that the cell can make a vesicle. This allows the specific molecules to enter the cell.
Usually, receptor-mediated endocytosis works in the cell's favor and allows it to capture important molecules it needs. However, viruses can exploit the process to enter the cell and infect it. After a virus attaches to a cell, it has to find a way to get inside the cell. Viruses accomplish this by binding to receptor proteins and getting inside in the vesicles.
During exocytosis, vesicles inside the cell join the plasma membrane and release their contents; the contents spill out, outside of the cell. This can happen when a cell wants to move or get rid of a molecule. Protein is a common molecule that cells want to transfer this way. Essentially, exocytosis is the opposite of endocytosis.
The process starts with a vesicle fusing to the plasma membrane. Next, the vesicle opens and releases the molecules inside. Its contents enter the extracellular space so that other cells can use them or destroy them.
Cells use exocytosis for many processes, such as secreting proteins or enzymes. They may also use it for antibodies or peptide hormones. Some cells even use exocytosis to move neurotransmitters and plasma membrane proteins.
Examples of Exocytosis
There are two types of exocytosis: calcium-dependent exocytosis and calcium-independent exocytosis. As you can guess from the name, calcium affects calcium-dependent exocytosis. In calcium-independent exocytosis, calcium is not important.
Many organisms use an organelle called the Golgi complex or Golgi apparatus to create the vesicles that will be exported out of the cells. The Golgi complex can modify and process both proteins and lipids. It packages them in secretory vesicles that leave the complex.
In regulated exocytosis, the cell needs extracellular signals to move materials out. This is usually reserved for specific cell types like secretory cells. They may make neurotransmitters or other molecules that the organism needs at certain times in certain amounts. The organism may not need these substances on a constant basis, so regulating their secretion is necessary. In general, the secretory vesicles do not stick to the plasma membrane for long. They deliver the molecules and remove themselves.
An example of this is a neuron that secretes neurotransmitters. The process starts with a neuron cell in your body creating a vesicle filled with neurotransmitters. Then, these vesicles travel to the plasma membrane of the cell and wait. Next, they receive a signal, which involves calcium ions, and the vesicles go to the pre-synaptic membrane. A second signal of calcium ions tells the vesicles to attach to the membrane and fuse with it. This allows the neurotransmitters to be released.
Active transport is an important process for cells. Both prokaryotes and eukaryotes can use it to move molecules in and out of their cells. Active transport must have energy, like ATP, to work, and sometimes it is the only way a cell can function. Cells rely on active transport because diffusion may not get them what they want. Active transport can move molecules against their concentration gradients, so cells can capture nutrients like sugar or proteins. Protein carriers play an important role during these processes.
- Georgia State University: HyperPhysics: Active Transport Across Cell Membranes
- Brooklyn College: Active Transport
- University of Nebraska – Lincoln: Plant and Soil Sciences eLibrary: Active Absorption — General Concepts
- Lumen: Active Transport
- Quantitative Human Physiology: Active Transport: Pumps and Exchangers
- An Introduction to Biological Membranes: Chapter 19 – Membrane Transport