The bond between two oxygen atoms (a molecule of oxygen air) has two characteristics. what are they? ᔕeᒪeᑕt ᗩᒪᒪ tᕼᗩt ᗩᑭᑭᒪ! a. an ionic bond is formed. b. a triple covalent bond is formed. c. valence electrons are shared between oxygen atoms. d. a double covalent bond is formed.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.