The membrane of the cell is a phospholipid bilayer, which allows hydrophobic molecules to diffuse through it. Small, hydrophobic ligands are able to diffuse through the plasma membrane. Nitric oxide is lipophilic, readily dissolving in lipids, and can diffuse across the plasma membrane. Steroid hormones are hydrophobic, and can thus diffuse across the plasma membrane. Water-soluble ligands cannot diffuse across the plasma membrane to enter a cell.

In animal cells, hormones mediate long distance cell signaling. Hormones are chemical messengers secreted by cells that travel through the circulatory system to the target cell receptors. Hormones communicate between diverse cell types and initiate diverse transduction pathways. Hormones are used for long distance cell signaling in both plant and animal cells.

The membrane of the cell is a phospholipid bilayer, which allows hydrophobic molecules to diffuse through it. Small, hydrophobic ligands are able to diffuse through the plasma membrane. Nitric oxide is lipophilic, readily dissolving in lipids, and can diffuse across the plasma membrane. Steroid hormones are hydrophobic, and can thus diffuse across the plasma membrane. Water-soluble ligands cannot diffuse across the plasma membrane to enter a cell.

In animal cells, hormones mediate long distance cell signaling. Hormones are chemical messengers secreted by cells that travel through the circulatory system to the target cell receptors. Hormones communicate between diverse cell types and initiate diverse transduction pathways. Hormones are used for long distance cell signaling in both plant and animal cells.

Second messenger systems begin with an extracellular ligand that binds to a receptor on the cell surface. The receptor then activates intracellular primary effectors (proteins that transduce the signal from the plasma membrane to the cytosol). In the cytosol, effectors activate second messenger molecules, which regulate intracellular process including transcription, neurotransmitter release, and enzyme activation. Second messengers have several common characteristics: they are localized, are easily synthesized and degraded, and are intracellular. These systems are responsible for diverse cellular processes and are able to amplify signals through kinase cascades. Common second messenger systems are the cAMP system and the tyrosine kinase system.

Second messenger systems begin with an extracellular ligand that binds to a receptor on the cell surface. The receptor then activates intracellular primary effectors (proteins that transduce the signal from the plasma membrane to the cytosol). In the cytosol, effectors activate second messenger molecules, which regulate intracellular process including transcription, neurotransmitter release, and enzyme activation. Second messengers have several common characteristics: they are localized, are easily synthesized and degraded, and are intracellular. These systems are responsible for diverse cellular processes and are able to amplify signals through kinase cascades. Common second messenger systems are the cAMP system and the tyrosine kinase system.

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, autocrine signals are signals that are received by the same cell in which the signal originated, and direct signaling occurs across gap junctions through the movement of small molecules (such as Calcium ions).

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, autocrine signals are signals that are received by the same cell in which the signal originated, and direct signaling occurs across gap junctions through the movement of small molecules (such as Calcium ions).

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, and autocrine signals are signals that are received by the same cell in which the signal originated. Paracrine signals are signal are short-lasting, whereas endocrine signals are long-lasting.

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, and autocrine signals are signals that are received by the same cell in which the signal originated. Paracrine signals are signal are short-lasting, whereas endocrine signals are long-lasting.