The levels of biological organization are the hierarchy of living organisms from simplest to most complex: atoms to molecules, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystems, and, finally, the biosphere.
The human body maintains its life processes at different levels of structural organization. Depending on who you talk to, the exact number of categories may vary, as some people prefer to categorize systems of the body differently. In general, though, scientists have placed the structural levels into six different categories: chemical processes (organelles), cells, tissues, organs, organ systems, and the whole organism.
Chemical Processes, Macromolecules, and Organelles
The first level of structural organization can be thought of in terms of chemical processes and molecules. The atom is a fundamental constituent of matter, consisting of a nucleus that is orbited by electrons. When one or more atoms joined together, they form a chemical structure called a molecule, with one or more chemical bonds holding the molecule together. A large amount of the molecules that are important to living things are called macromolecules. The term “macromolecules” refers to large molecules formed through the process of polymerization. One of the most famous macromolecules is deoxyribonucleic acid or DNA, which functions as the blueprints for all known life.
“The human body is the most complex system ever created. The more we learn about it, the more appreciation we have about what a rich system it is.” — Bill Gates
Macromolecules can join together into organelles, which are tiny structures that exist within the bodies of cells. Examples of organelles are mitochondria, which creates energy to enable the cell to carry out its functions, and chloroplasts which let plants use solar energy to make sugar.
Cells: The Building Blocks of the Body
After organelles and macromolecules, cells are the most basic part of the human body, and they are the foundation of structure and function in the body. Every person has around 100 trillion cells, by the time that they grow into adulthood. Every cell will carry out a special function that allows the body to survive, but they won’t all carry out the same function. Cells are specialized in function and in their form, with the different types of cells carrying out different roles.
Nerve cells have long projections called axons, which carry electrical signals throughout the body and to other cells. Meanwhile, muscle cells have many mitochondria so that they can generate the energy the body needs to move. Epithelial cells, also known as skin cells, serve as a protective barrier between interior organs and the outside world, helping to regulate body temperature and prevent infections. Sex cells, or gametes, exist in two different varieties: sperm cells and eggs. These sex cells are what allow reproduction and the passing on of genetic material.
Tissues Are Task-Specific
Cells work together and form multicellular groups to carry out specific tasks. These groups of specific cells are referred to as tissues, and the human body is made up of four different types of tissue. The four different tissue types are connective tissue, nervous tissue, muscle tissue, and epithelial tissue.
“If any thing is sacred, the human body is sacred.” — Walt Whitman
Connective tissue refers to cells whose purpose is to support and connect groups of tissue to other groups of tissue. These connective tissues are usually found suspended in an “extracellular matrix.” The extracellular matrix is frequently composed of protein fibers such as collagen and it exists in either a liquid or a solid state.
The nervous tissue’s role is to sense stimuli and then process and transmit the relevant information. There are two main types of cells in all the nervous tissue, the aforementioned nerve cells and glial cells. Muscle tissue is necessary for the body to maintain its posture and to move, as well as to move food through the digestive tract and pump blood. Subgroups of muscle tissue include cardiac muscle, skeletal muscle, and smooth muscle.
Epithelial tissue consists of tight bundles of cells that cover a large amount of surface area, including the entire outside of your body. Epithelial tissue is made up of polarized cells that have a top and bottom side, and they are densely packed so that they can function as barriers to prevent the infiltration of harmful microbes and fluids.
Organs Carry Out Vital FunctionsThe term organ is used to refer to two or more types of tissue that work together to carry out a particular function. These include the heart, the kidneys, the skin, the liver, and the lungs. Most organs in the body are made up of all four tissue types. Epithelial tissue protects the organ, muscle tissue and nerve networks provide the information and mechanism to carry out the organ’s respective action, and connective tissue fills in any gaps.
“The human body is a machine which winds its own springs.” — Julien Offroy de la Mettrie
The human body has five vital organs that are absolutely essential for its survival. These vital organs are the kidneys, liver, lungs, heart, and of course the brain. The brain is the control center of the body which regulates and sends messages to the body with the nervous system. The heart is responsible for pumping blood which delivers oxygen to the entire body, while the lungs take in this oxygen. The kidneys remove waste products from the blood, and the liver has a variety of functions including breaking down harmful chemicals, filtering the blood, and secreting bile.
Organ Systems and the Whole Organism
Organ systems are made up of groups of organs. There are ten major organ systems within the body, which all work together to make up the human body itself. The ten organ systems are the circulatory system, the digestive system, the endocrine system, the integumentary system, the muscular system, the nervous system, the reproductive system, the respiratory system, the skeletal system, and the urinary or excretory system.
The circulatory system transports nutrients to the tissues of the body, through the circulation of the blood. It can be subdivided into the cardiovascular system and the lymphatic system. The circulatory system includes the heart, the lymph nodes, the thymus, and the spleen. The digestive system is responsible for breaking down food into energy, and it consists of the mouth, intestines, and stomach. The endocrine system is responsible for regulating processes like metabolism and homeostasis and is made up of organs like the pineal gland, the thymus, the pituitary gland, and the thyroid gland.
The integumentary system consists of the skin and protects the body from damage, while the muscular system enables the contraction of the muscles, leading to movement. The reproductive system consists of sex organs, and the respiratory system provides the body with oxygen through the lungs and nose. The excretory system removes waste products from the body and is made up of the bladder and kidneys. The skeletal system protects and gives form to the body, and the nervous system controls everything through the brain and nerves.
All of these systems together make up the entire organism, the human body. Our bodies are made up of complex systems that go all the way down to the atomic level. While it is difficult to draw lines that perfectly separate the components of the body into discrete categories, it is necessary for our ability to discuss and research the science of the body.
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