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Eukaryotes are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ and κάρυον. The domain Eukaryota makes up one of the domains of life in the three-domain system; the two other domains are Bacteria and Archaea. Eukaryotes represent a tiny minority of the number of living organisms; however, due to their generally much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved approximately 1.6–2.1 billion years ago, during the Proterozoic eon.
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Respiratory system and circulatory system.
The respiratory system allow living things to obtain oxygen from the air while the circulatory system allows oxygen to be carried around the body.
C
The respiratory system removes the waste carbon dioxide produced from aerobic respiration in the body.
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Key points
Neurons communicate with one another at junctions called synapses. At a synapse, one neuron sends a message to a target neuron—another cell.
Most synapses are chemical; these synapses communicate using chemical messengers. Other synapses are electrical; in these synapses, ions flow directly between cells.
At a chemical synapse, an action potential triggers the presynaptic neuron to release neurotransmitters. These molecules bind to receptors on the postsynaptic cell and make it more or less likely to fire an action potential.
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