Chemoreceptor

A chemoreceptor, also known as chemosensor, is a specialized sensory receptor cell which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemoreceptor is a neuron, or in the form of a neurotransmitter that can activate a nerve fiber if the chemoreceptor is a specialized cell, such as taste receptors, or an internal peripheral chemoreceptor, such as the carotid bodies. In physiology, a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of carbon dioxide (hypercapnia) or a decrease in blood levels of oxygen (hypoxia), and transmits that information to the central nervous system which engages body responses to restore homeostasis.
Plant chemoreceptors.
Plants have various mechanisms to perceive danger in their environment. Plants are able to detect pathogens and microbes through surface level receptor kinases (PRK). Additionally, receptor-like proteins (RLPs) containing ligand binding receptor domains capture pathogen-associated molecular patterns (PAMPS) and damage-associated molecular patterns (DAMPS) which consequently initiates the plant's innate immunity for a defense response.
Plant receptor kinases are also used for growth and hormone induction among other important biochemical processes. These reactions are triggered by a series of signaling pathways which are initiated by plant chemically sensitive receptors. Plant hormone receptors can either be integrated in plant cells or situate outside the cell, in order to facilitate chemical structure and composition. There are 5 major categories of hormones that are unique to plants which once bound to the receptor, will trigger a response in target cells. These include auxin, abscisic acid, gibberellin, cytokinin, and ethylene. Once bound, hormones can induce, inhibit, or maintain function of the target response.
Classes.
There are two main classes of chemoreceptor: direct and distance. 
Sensory organs.
When inputs from the environment are significant to the survival of the organism, the input must be detected. As all life processes are ultimately based on chemistry it is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals.
Chemoreception is important for the detection of food, habitat, conspecifics including mates, and predators. For example, the emissions of a predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been. Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. It passes in either chemical or electrochemical form to the central processor, the brain or spinal cord. The resulting output from the CNS (central nervous system) makes body actions that will engage the food and enhance survival.
Physiology.
Control of breathing.
Particular chemoreceptors, called ASICs, detect the levels of carbon dioxide in the blood. To do this, they monitor the concentration of hydrogen ions in the blood, which decrease the pH of the blood. This can be a direct consequence of an increase in carbon dioxide concentration, because aqueous carbon dioxide in the presence of carbonic anhydrase reacts to form a proton and a bicarbonate ion.
The response is that the respiratory centre (in the medulla), sends nervous impulses to the external intercostal muscles and the diaphragm, via the intercostal nerve and the phrenic nerve, respectively, to increase breathing rate and the volume of the lungs during inhalation.
Chemoreceptors that regulate the depth and rhythm of breathing are broken down into two categories.
Heart rate.
The response to stimulation of chemoreceptors on the heart rate is complicated. Chemoreceptors in the heart or nearby large arteries, as well as chemoreceptors in the lungs, can affect heart rate. Activation of these peripheral chemoreceptors from sensing decreased O2, increased CO2 and a decreased pH is relayed to cardiac centers by the vagus and glossopharyngeal nerves to the medulla of the brainstem. This increases the sympathetic nervous stimulation on the heart and a corresponding increase in heart rate and contractility in most cases. These factors include activation of stretch receptors due to increased ventilation and the release of circulating catecholamines.
However, if respiratory activity is arrested (e.g. in a patient with a high cervical spinal cord injury), then the primary cardiac reflex to transient hypercapnia and hypoxia is a profound bradycardia and coronary vasodilation through vagal stimulation and systemic vasoconstriction by sympathetic stimulation. In normal cases, if there is reflexive increase in respiratory activity in response to chemoreceptor activation, the increased sympathetic activity on the cardiovascular system would act to increase heart rate and contractility.