Every star starts with an initial mass of hydrogen. Each atom of hydrogen has one proton and one neutron. How are the total number of protons and neutrons conserved as a star ages and undergoes continuous nucleosynthesis?

sexual reproduction produces offspring with novel combinations of genes. species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.

oceans

indian ocean

pacific ocean

atlantic ocean

arctic ocean

southern ocean

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.