Answer:
Oxygen
Oxygen is the most abundant element contained within living organisms, composing about 65% of the human body. Oxygen is also the most abundant element in the Earth's crust, and in the air that is essential for most life on Earth. Oxygen's presence in the body is largely in the form of water, which is used to produce the energy within the body needed to sustain life.
Carbon
Carbon forms the basis for all life on Earth; indeed, life forms on Earth are referred to as carbon-based life forms, emphasizing the importance of this element for life. Carbon atoms readily bond to other atomic elements, such as oxygen and nitrogen. Since carbon can so readily bond to other elements, long chains of bonds can form and provide the physical and chemical structure needed for the complex processes and structures that occur within living organisms, such as structural proteins and genetic information in the form of nucleic acids.
Hydrogen
Hydrogen is the simplest element, as its atom contains only a single proton and a single neutron. As a result of this simplicity, hydrogen readily bonds with other elements, making it an important component for the formation of living organisms. Hydrogen is the other element (along with oxygen) which forms water, a crucial component for most life forms on Earth. Hydrogen is also a byproduct in many biological reactions, including photosynthesis and metabolism.
Nitrogen
Nitrogen is one of the most abundant elements on Earth, composing approximately 80% of the air on Earth. Nitrogen is an important element in the development of plant life, as compounds containing these elements are readily absorbed and used by plants. Nitrogen is also an important component of many proteins and deoxyribonucleic acids (DNA), which is crucial for genetic material to be passed on to subsequent generations of life.
Sulfur
Sulfur is a major component of two essential amino acids used by living organisms: cysteine and methionine. These amino acids, like all amino acids, are crucial for the construction of proteins that are used for structural stability and repair of living organisms. For example, the structural integrity of hair and feathers can be attributed to these amino acids. Sulfur is also used as a source of energy and is metabolized by some species of bacteria and other lower life forms.
Phosphorus
Phosphorus is used in the formation of phospholipids, a type of molecule that is a major component of the cell membrane of all living cells. Without this cell membrane, cells would not be able to develop and would not have the structural stability to form in the first place. This protective layer of phospholipids holds all the internal components of cells in place, allowing for the processes that maintain the life of the cell to take place. The phospholipid layer also protects the cell by keeping any unwanted or potentially destructive materials outside of the cell.
(via sciencing.com)
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