The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

Ubiquitin ligases add ubiquitin to their ligands. The addition of ubiquitin acts as a signal that a protein has become ineffective and is ready for degradation. When multiple ubiquitin residues have been added to a protein molecule, it is transported to the lysosome in the cell to be digested.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

Several different types of proteins can change the structure of a ligand, such as isomerases.

When secretory proteins are synthesized they localize to the endoplasmic reticulum (ER), specifically the rough ER, for modification. Following modification there, secretory proteins are then packaged in secretory vesicles which go on to interact with the Golgi body, and are then finally released from the the plasma membrane.

Local cell signaling in eukaryotic cells refers to the communication between nearby cells. This is done through direct contact between cells, namely via cell junctions and cell-cell recognition. Gap junctions are intercellular connections that allow cytoplasmic transfer in animal cells. The counterpart in plant cells is the plasmodesmata, which are channels penetrating the cell walls of cells, allowing communication. Cell-cell recognition is the ligand-receptor binding between two cells that elicits receptor cell response. Methods of local cell signaling allow nearby cells to communicate with each other and coordinate cellular responses and activities.

As the name implies, when activated and induced to undergo a conformational change by a ligand, ions are able to flow through the channel and into the cell. This allows the charge across the membrane tobe manipulated by the cell. 

The binding of ligands causes the activation and conformational change in the ion channel to open it. Then, ions are able to flow into the cell. After a short time, the ligand dissociates from the ion gated channel. This causes the channel to deactivate and close. 

Phagocytosis is a form of endocytosis in which a cell takes up solid material through the invagination of the plasma membrane to form intracellular vesicles. In multicellular organisms, the process of phagocytosis is utilized in nutrient uptake, immune system response, and in cell recycling. Cells perform phagocytosis to uptake solid nutrients into the cell. The immune system uses phagocytosis to consume foreign material for eventual degradation. In the continual process of cell recycling, old and dead cell material is taken up and reused by cells.

In clathrin-mediated endocytosis, the binding of a specific ligand to a receptor triggers intracellular protein recruitment, which includes clathrin. These proteins stabilize the invagination and allow the clathrin pit to pull away from the plasma membrane. After it has separated from the membrane, the proteins and clathrin dissociate from the vesicle, which then fuses with an endosome. The invagination and vesicle formation from the plasma membrane includes the internalization of both the receptor and ligand. Over time the uptake by clathrin-mediated endocytosis decreases as the number of receptors on the cell’s surface. In other words, particle uptake declines due to the internalization of recptors as a result of clathrin-mediated endocytosis.

In clathrin-mediated endocytosis, the binding of a ligand to a receptor triggers plasma membrane invagination and protein recruitment. The recruited proteins, including clathrin, bind to the intracellular domain of the receptor. Clathrin facilitates invagination and endocytosis of the ligand by stabilizing the plasma membrane curvature. Clathrin and other recruited proteins dissociate from the endocytic vesicle once it fully invaginates and pinches off from the plasma membrane. 

 Exocytosis is the release of the contents of vesicles into extracellular space through the fusion of the vesicles with the plasma membrane. The process of exocytosis is a method through which the cell is able to secrete proteins into extracellular space. An example of this is the release of neurotransmitters from a neuron. During the process of exocytosis, intracellular vesicles originating from the Golgi apparatus fuse with the plasma membrane. This fusion increases the surface area of the plasma membrane and incorporates any proteins within the vesicle membrane into the cell’s plasma membrane.