Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling.
A chemosensor, also known as chemoreceptor, is a sensory receptor that transduces a chemical signal into an action potential. Or, more generally, a chemosensor detects certain chemical stimuli in the environment.
In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell, changing the shape of the chemoreceptor. All chemoreceptors therefore have some degree of specificity, in that they only bind to specific molecules or specific classes of molecules.
In amplification, the cell uses energy to transform the shape change of the chemoreceptor into biochemical or electrical signals within the cell. In many cases, amplification is mediated by formation of cAMP (cyclic adenosine monophosphate), which increases the cell's permeability to sodium ions and alters the electrical potential of the cell membrane.
In signaling, the amplified signal is transformed into a physiological or behavioral response. In higher animals the nervous system does the signaling, while in single-celled organisms signaling is intracellular, which may be manifested as chemotaxis, a directional movement in response to a chemical stimulus.
Detection, amplification, and signaling are often connected by feedback pathways. Feedback pathways allow adjustment of the sensitivity of the chemoreceptive system to different concentration ranges of the elicitor molecule. Thus, the sensitivity decreases as the background concentration of the molecule increases; the sensitivity increases as the background concentration of the molecule decreases.
Chemoreceptive systems detect chemical changes within an organism (interoreception) or outside an organism (exteroreception). The most familiar examples of exteroreception in humans are the senses of taste and smell.
Humans have chemoreceptor cells for taste in taste buds, most of which are on the upper surfaces of the tongue. Each human has about 10,000 taste buds and each taste bud consists of about 50 cells. An individual taste bud is specialized for detection of a sweet, sour, salty, or bitter taste. The sense of smell is important in discriminating among more subtle differences in taste.
Many chemoreception systems also collect information about the internal environment of multicellular organisms. For example, the carotid body in the carotid artery of humans has chemoreceptive cells which respond to changes in the pH and oxygen levels in the blood. As the amount of dissolved oxygen in the blood decreases.
Chemical gradients are sensed through multiple transmembrane receptors, called methyl accepting chemotaxis proteins (MCPs), which vary in the molecules that they detect. These receptors may bind attractants or repellents directly or indirectly through interaction with proteins of periplasmatic space. The signals from these receptors are transmitted across the plasma membrane into the cytosol, where Che proteins are activated. The Che proteins alter the tumbling frequency, and alter the receptors.
The proteins CheW and CheA bind to the receptor. The activation of the receptor by an external stimulus causes autophosphorylation in the histidine kinase, CheA, at a single highly conserved histidine residue. CheA in turn transfers phosphoryl groups to conserved aspartate residues in the response regulators CheB and CheY [ note: CheA is a histidine kinase and it does not actively transfer the phosphoryl group. The response regulator CheB takes the phosphoryl group from CheA]. This mechanism of signal transduction is called a two-component system and is a common form of signal transduction in bacteria. CheY induces tumbling by interacting with the flagellar switch protein FliM, inducing a change from counter-clockwise to clockwise rotation of the flagellum. Change in the rotation state of a single flagellum can disrupt the entire flagella bundle and cause a tumble.
 Receptor regulation
CheB, when activated by CheA, acts as a methylesterase, removing methyl groups from glutamate residues on the cytosolic side of the receptor. It works antagonistically with CheR, a methyltransferase, which adds methyl residues to the same glutamate residues. The more methyl residues are attached to the receptor, the more sensitive the receptor. As the signal from the receptor induces demethylation of the receptor in a feedback loop, the system is continuously adjusted to environmental chemical levels, remaining sensitive for small changes even under extreme chemical concentrations. This regulation allows the bacterium to 'remember' chemical concentrations from the recent past, a few seconds, and compare them to those it is currently experiencing, thus 'know' whether it is traveling up or down a gradient. However, the methylation system alone cannot account for the wide range of sensitivity that bacteria have to chemical gradients. Additional regulatory mechanisms such as receptor clustering and receptor-receptor interactions also modulate the signalling pathway.