Endocrine signals are signals from cells that move using the bloodstream to signal to distant cells, 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). Beta cell in the pancreas produce insulin, a hormone, which is secreted into the bloodstream.

 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.

Pyruvate decarboxylation is an oxidative decarboxylation reaction, or an oxidation reaction where a carboxylate group is removed. This reaction converts pyruvate which was produced through glycolysis to acetyl CoA to be used in the Citric Acid Cycle.

The pyruvate dehydrogenase complex is an enzyme complex that consists of 3 enzymes, which work together to catalyze the pyruvate decarboxylation reaction, where pyruvate is converted to acetyl CoA.

Pyruvate decarboxylation occurs in the mitochondrial matrix. The acetyl CoA produced from the pyruvate decarboxylation reaction will undergo the Citric Acid cycle also in the mitochondrial matrix.

During glycolysis, for each molecule of glucose, two molecules of pyruvate are produced ( glucose+ NAD+ + 2 ADP + 2Pi-> 2 pyruvate+ 2 ATP + 2NADH+. These 2 molecules of pyruvate then undergo the pyruvate decarboxylation reaction: 2(pyruvate+ CoA-SH+ NAD+ -> NADH+ CO2+ acetyl CoA).

During the pyruvate decarboxylation reaction , a thioester bond links the acetyl group of pyruvate with coenzyme A to produce acetyl CoA.

The pyruvate decarboxylation reaction is pyruvate+ CoA-SH+ NAD+ -> NADH+ CO2+ acetyl CoA.

Desmosomes are the primary junction that helps keep tissues that are under a lot of stress, like skin, intact. Although tight junctions typically accompany desmosomes, their primary function is to prevent substances from passing between cells. Gap junctions are used for communication between cells, and intercalated discs are only found in cardiac muscle tissue.

The lysosome is an organelle that is used to digest broken cellular machinery or foreign particles. It maintains an acidic environment inside by pumping hydrogen ions in, not out. This environment helps denature the things it needs to digest, and is the most effective pH for the digestive enzymes that are inside.

The other structures do not maintain acidic environments, and would need the capability of removing protons.