Endocytosis

Endocytosis moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are three types of endocytosis. Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. Pinocytosis is a variation of endocytosis that means “cellular drinking”. This process takes in solutes that the cell needs from the extracellular fluid. Receptor-mediated endocytosis is a targeted variation of endocytosis that employs binding proteins in the plasma membrane that are specific for certain substances. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. The following diagram outlines the three types of endocytosis described above. It can help you understand these forms of membrane transport by visualizing each process.

The following activity tests the reader on their understanding of the three types of endocytosis. Three real-life examples are provided for each type and the reader must drag the examples to the correct endocytic mechanism. By providing real-life examples of phagocytosis, pinocytosis, and receptor-mediated endocytosis, the reader gains a better understanding of when and where these mechanisms are used in an organism. This lays foundational knowledge that students must comprehend in order to look at larger systems.

Diffusion

Diffusion is the passive (doesn’t require energy expenditure) movement of substances across a plasma membrane. Molecules such as oxygen and carbon dioxide are transported via simple diffusion because they are small and uncharged. Lipids can also be transported by simple diffusion because of their hydrophobic nature. Diffusion is suitable for transporting substances across small distances and is driven by concentration gradients and electrical gradients. 

Concentration gradients move substances from areas of higher concentration to areas of lower concentration, while electrical gradients move substances based on differences in charge. The combined gradient that affects a substance is called its electrochemical gradient, and it is especially important to muscle and nerve cells.

The following image provides an example of an electrochemical gradient involving sodium ions (Na+), potassium ions (K⁺), chloride ions (Cl^–), and negatively charged proteins that are stuck inside the cytoplasm (A^–). This diagram is a strong example as it clearly shows the net charge in both the extracellular fluid and the cytoplasm.

The following video provides a clear explanation of how concentration gradients and electrical gradients work. As well as how these two different types of gradients work together to form an electrochemical gradient. An electrochemical gradient can be a difficult topic for students to conceptualize. For visual-based learners, this video will enhance understanding. Chemical gradients are shown at 0:00-1:45, electrical gradients from 1:45-6:10, and electrochemical gradient and potential difference is shown at 6:10-9:40.

Protein Channels

The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided. Therefore, charged particles, which are hydrophilic by definition, and large particles use protein channels to travel across the cell membrane. Protein channels show specificity based on particle size or charge, and the number and types of channels are not fixed per cell. There are two types of protein channels.

Voltage-dependent channels are a form of protein channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane. Na⁺ and K⁺ channels in nerves and skeletal muscles are examples of voltage-dependent protein channels. The following figure illustrates the opening of a voltage-gated channel, and the movement of substances through it.

Ligand-dependent channels are a form of protein channel that opens in response to a signaling molecule, a ligand, which binds to an active site on the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge. Nicotinic acetylcholine channels in the neuromuscular junction are examples of ligand-dependent protein channels. The following figure shows the mechanism behind opening a ligand channel and the movement of ions through the channel. This figure contrasts figure 3, which shows the mechanism behind voltage-gated channels.

Active Transport

Active transport is the movement of substances from an area of low concentration to high concentration (ie. against the concentration gradient) through the use of energy such as ATP. Active transport is carried out by a specific type of transmembrane proteins called carrier proteins. There are two types of active transport.

Primary (1^o) Active Transport: The movement of substances across the plasma membrane, creating a difference in charge across the membrane. This type of active transport is directly dependent on ATP.
The sodium-potassium pump is a notable example of primary active transport. The pump is one of the most important in the cell, as it maintains the electrochemical gradient (and the correct concentrations of Na⁺ and K⁺). The sodium-potassium pump moves K⁺ into the cell while moving Na⁺ out at the same time, at a ratio of three Na⁺ for every two K⁺ ions moved in. The Na⁺-K⁺ ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. 

The video below shows the sodium-potassium pump in action. The way that the video shows shapes within the pump that correspond to the shapes of Na⁺ and K⁺ is helpful for remembering that carrier proteins are specific for certain ions or molecules.

The diagram below shows the process of primary active transport. This example shows a sodium-potassium pump (Na⁺-K⁺ ATPase), which is one of the most important pumps in animal cells. The diagram also clearly shows that the ions are moving against their concentration gradients (from low to high concentration).

Secondary (2^o) Active Transport: The movement of substances across the membrane using the electrochemical gradient established by primary active transport. This type of active transport does not directly require ATP.

The following diagram shows an example of secondary active transport using sodium ions (Na⁺) and glucose. The transporter moves Na⁺ with its concentration gradient (from high concentration to low concentration) and at the same time couples glucose movement against its concentration gradient (from low concentration to high concentration).

Still a bit confused about the difference between passive and active transport? Check out Biology: Cell Transport which provides a nice visualization of the two mechanisms.

Want some more information about secondary active transport? Check out Detailed Animation of Secondary Active Transport which helps explain how secondary active transport works by giving multiple examples.

Carrier Proteins for Active Transport

A carrier protein is a glycoprotein embedded in the plasma membrane. When a specific substance binds to a carrier protein, the protein undergoes a conformational change, resulting in the movement of the bound substance across the cell membrane. Unlike ion channels, carrier proteins use active transport. There are three types of carrier proteins. A uniporter carries one specific molecule. A pb_glossary id=”3784″]symporter[/pb_glossary] carries two different molecules in the same direction. Lastly, an pb_glossary id=”3785″]antiporter[/pb_glossary]: carries two different molecules in opposite directions.

Did you know there is a third type of active transport? It uses photon energy (from light) to move ions and molecules across cell membranes. These light-driven pumps are commonly found in bacteria.