Seeing as peptide hormones are generally large, water-soluble molecules, they cannot transverse the phospholipid membrane. Instead, they must act through a membrane-bound protein receptor. Steroid hormones are generally small, fat-soluble organic molecules that can easily travel through the phospholipid membrane and the nuclear membrane. They can then act on transcription factors or interact directly with DNA. Both peptide and steroid hormones initiate changes within the cell; they simply do so by different mechanisms.

After a ligand binds to receptor tyrosine kinases, the receptors need to form a dimer to foster activation of their tyrosine kinase activity. After dimerization, a phosphate is transferred from ATP to the amino acid tyrosine at specific sites on the cytoplasmic region of receptor. The phosphorylation of the tyrosines provides a site where other cellular proteins can bind and further relay the signal from the receptor to the cell.

Intracellular receptors are found in the cytoplasm of the cell. Ligands for intracellular receptors are usually small molecules that can pass through the cell membrane, and include substances such as steroid hormones. Upon binding and activation, intracellular receptors bind specific DNA motifs in the nucleus and function as transcription factors, directly changing expression of genes.

In contrast, transmembrane receptors are embedded in the plasma membrane and bind extracellular ligands to mediate intracellular responses. Ligand binding to transmembrane receptors often initiates a signal cascade or mediates channel activity within the membrane of the cell.

Calcium is a widely used second messenger of signal transduction. Calcium ions can function as a second messenger because its concentration within cell cytosol is much lower than outside the cell, and it is actively transported out of the cell by protein pumps. Modulation in calcium levels is used to transmit signals from both G protein and receptor tyrosine kinase signaling cascades and is involved in such functions as muscle contractions and synaptic signaling.

The other common second messenger molecule is cAMP.

Tyrosine kinase receptors are fully activated when they bind to an extracellular ligand, dimerize, and then autophosphorylate at tyrosine residues on the C terminus. The signal is not transduced until relay proteins are phosphorylated by the tyrosine kinases. These relay proteins can then stimulate a phosphorylation cascade that initiates signaling pathways, which regulate nuclear gene transcription

When inactive, ion gated receptors are closed. When a ligand binds, the channel undergoes a conformational change and opens: creating a tunnel. This conformational change does not last for a long period of time; the ligand soon dissociates and the ion channel closes.

Cell signaling is the process used by cells to communicate and control cellular activities. It can occur both within and between cells. The correct sequence of events that takes place during cell signaling is as follows: reception, transduction, and response. The reception stage is the detection of a signal, typically by a receptor on the cell surface. Next, transduction is characterized by the transmission of signals from the cell’s exterior to its interior by way of proteins. Finally, the response is the subsequent cellular reaction to the signaling. Cell signaling is critically important in normal cell function and widely diversified. 

G protein-coupled receptors are part of a large class of receptors involved in intercellular signaling. Structurally, G protein-coupled receptors have an extracellular N terminus, seven transmembrane helices, three intracellular loops, three extracellular loops, and an intracellular C terminus. The ligand-binding domain is within the transmembrane helices.

G protein-coupled receptors are only found in eukaryotes including yeast cells and animal cells. Bacteria cells are prokaryotes, and therefore do not contain G protein-coupled receptors. Even though yeast cells are single-celled, they possess all the characteristics of eukaryotic cells.

G proteins are a class of protein signaling molecules that are activated by G protein-coupled receptors (GPCRs). When a ligand binds to the transmembrane domain of GPCRs, the GPCR undergoes a conformational change. This conformational change activates the G protein, which binds to GTP rather than lower energy GDP. The active G protein can then dissociate and transmit the signal by interacting with other proteins.