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LIFE. HIERARCHY OF LIFE.

 

Observe the following diagram. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

All living things consist of smaller parts that are organized in a hierarchical way.

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Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. In this course, the smallest level we will examine is the atom, which is the basic unit of matter. The atom consists of a dense nucleus surrounded by electrons. Atoms join together to form molecules. A molecule is a chemical structure consisting of at least two atoms held together by a chemical bond. Macromolecules are biologically important molecules, and they are technically polymers. Polymers are made by combining smaller units called monomers, which are simpler macromolecules. An example of a macromolecule is the genetic molecule deoxyribonucleic acid (DNA) that contains the instructions for the development of all living organisms. DNA is built of four kinds of monomers (nucleotides). They are strung together, or polymerized, in a sequence that codes for the structure of proteins and other biological molecules. The DNA in one of your microscopic cells contains a sequence of almost three billion nucleotides.

When macromolecules are used as building blocks to form a membrane-bound sphere, you have a cell, which is the fundamental unit of life. A cell is essentially a tiny droplet of water and other molecules enclosed by a fluid “skin” or membrane. The cell is the smallest and simplest entity that possesses all the characteristics of life. There are two main types of cells: prokaryotes and eukaryotes. The cells of prokaryotes are relatively small and simple; they do not have any clearly defined compartments inside of them. The cells of eukaryotes, by contrast, include membrane-bound organelles: compartments inside the cell that contain specific groups of macromolecules and carry out specific cellular functions. One of these organelles is the nucleus; it encloses the DNA within the cell.

Some organisms consist of just one cell and include unicellular organisms such as bacteria and protists. Single-celled life forms are typically referred to as microorganisms. Other organisms consist of many cells working together. These multicellular organisms include animals, land plants, and most fungi. Most multicellular organisms have cells that are specialized to carry out specific functions. Tissues are formed when many different kinds of cells work together to fulfill the same detailed function. Organs are collections of tissues that work together to carry out a common general function. Organs are present not only in higher level animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For example, the circulatory system transports blood through the body and includes organs such as the heart and blood vessels. Organisms are individual living entities that survive and reproduce as a unit. For example, each tree in a forest is usually an individual organism.

Consider this example to help clarify the nature of the levels between a cell and an organism. A human is an organism which has a circulatory system (organ system) that transports blood through the body. It is made up of organs such as the heart and blood vessels. Each of the organs, in turn, is made of more specific tissues. Your heart, for example, has muscle tissue for pumping and nerve tissue that helps coordinate each heartbeat.

The hierarchical organization of living systems continues beyond single organisms. A population consists of all the individuals of a species living within a specific area. For example, a forest may include many pine trees. All those pine trees represent the population of pine trees in that forest. As you know, many different populations can live in any specific area. All of these populations can interact with each other in positive and negative ways, and together they form a community. Continuing with our example, the forest with pine trees includes populations of flowers, mammals, birds, insects, fungi, and bacteria, all of which can interact. These interacting populations make up a community. An ecosystem consists of all the living things in a particular area together with the abiotic, or nonliving, parts of that environment. The pine forest ecosystem includes not just plants, animals, and microbes but also rocks, water, temperature changes, air chemistry, and other abiotic factors that interact with living organisms in the area. Finally, the highest level of organization in living systems is the biosphere, which is the collection of all ecosystems on Earth. The biosphere includes all habitable zones on the planet, including land, soil, and rocks to a great depth in the Earth’s crust; water and ice; and the atmosphere to a great height.

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