13 Hierarchies and Levels of Organization for Living Things, Ecology

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Levels of biological organization refers to the hierarchy of complex biological structures and systems that are broke down into their simpler or more fundamental parts. By understanding how each part operates and interacts with others, scientists aim to depict how these complex systems functions as a whole. In biology, this hierarchy ranges from the simplest material, atom, to the most complex structures, biosphere. Each level in the hierarchy is built by previous lower one, and the more complex higher levels show some new characteristics that low levels do not have. There are 13 levels of organization in biology. In sequence, they are represent as atoms, molecules, bio-macromolecules, organelles, cells, tissues, organs, organ systems, organisms, population, community, ecosystem and biosphere.

Sometimes these 13 levels are classified more carefully. Acellular level or Pre-cellular level: atom, small molecule, biological macromolecules. Sub-cellular level: Organelles. Cells is a single level. Super-cellular level or Multicellular level: tissue, organ, organism system. Ecological levels: organism, population, community, ecosystem, biosphere.

Acellular level or Pre-cellular level

Atom and Small Molecule

Atoms consist of even smaller protons and neutrons that make up the nucleus, and they are surrounded by a cloud of electrons.The unchanged atoms are the smallest unit in chemical reactions. When they are joined together with chemical bonds, they form larger groups of atoms called molecules. The molecules in organisms are mainly consist of atoms like carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. For example, a water molecule is made of two hydrogen atoms and one oxygen atom. More complex molecules containing carbon atoms are called organic compounds. Phospholipids, fatty acids, amino acids, nucleotides and ATP are common small organic molecules in organisms.

Biological Macromolecules

The most notable biological macromolecules in organisms are proteins and DNA. Polypeptide chains made of amino acids fold into various shapes which are the basis for proteins to perform biological functions. Proteins execute life activities within organisms, such as material transport, signal recognition and catalysis for chemical reactions. DNA is a helical molecule made of nucleotides. The genetic information is stored in the sequence of these nucleotides. When this information is used by lives, DNA is transcribed into mRNA to guide the synthesis of proteins.

Sub-cellular level: Organelles

These membrane enclosed subcellular structures are unique to eukaryotic cells, except ribosomes. Organelles divide the cytoplasm into enclosed areas to perform different functions, so biochemical reactions proceed more orderly and rapidly by preventing interference with each other. Important organelles include ribosomes, endoplasmic reticulum, Golgi apparatus, Mitochondrion and Chloroplasts.

Cells is a single level

Cells are considered the basic unit of life. They perform a range of life activities independently, including predation, movement, metabolism, growth and reproduction. Cells reproduce through division, meaning the new cells can only arise from existing cells.

Super-cellular level or Multicellular level


Similar cells and their extracellular matrix form tissues that work together to perform specific functions. In animals, these tissues include muscle tissue (for movement), nerve tissue (for signal transmission), connective tissue (for support and protection), and epithelial tissue (for protection, absorption, secretion, etc.).


Organs are structures made of various tissues that work together to perform a specific set of functions. For example, the stomach is a digestive organ made of four types of tissue. The epithelial tissue in the stomach wall produces gastric acid and digestive enzymes. The blood vessels of the stomach are connective tissue. They nourish the stomach and transport absorbed nutrients. Muscle tissue allows the stomach to grind food. Additionally, the stomach is controlled by the nervous tissues. Some people experience stomach pain when they are too nervous or anxious. These tissues coordinate to grind, store and digest food partially.

Organ Systems

In multicellular animals, several different organs work together to perform tasks more complex than any single organ can do alone. The stomach can only break down proteins into peptides. To further break down peptides and other nutrients such as starch and fats, other organs' help is needed. The stomach, pancreas, intestines, liver and other organs combine together to make digestive system. It can break down various organic substances in food and absorb them into the bloodstream.

Ecological levels


An organism is an independent individual. They are divided into single-celled organisms and multicellular organisms. Yeast, used for baking and brewing, is a unicelluar organism. The largest land animal, the African elephant, is a multicellular organism.


A population refers to all individuals of the same species living in a specific area at the same time. For example, all the cattle within a closed reserve. The individuals in a herd do not simply gather statically; they cooperate with each other to fend off predators like lions and also compete among themselves for food and mates. They mate with each other to pass their genes to offspring. Since these cattle share a common gene pool, beneficial genetic mutations gradually accumulate over time to differentiate them from cattle in other locations. Therefore, a population is the smallest and fundamental unit of evolution.

Biological Community, biocenosis

A biotic community is the collection of all populations living in the same area. A biological community is not a random combination of species. The organisms within a biological community interact with each other and undergo natural selection. Predation, symbiosis, mutualism, parasitism, and competition are common within communities. Some biological communities also form distinct layers vertically, most notably in forest:

Canopy Layer: This is the highest layer of the forest. It consists of the tallest trees, such as pines and oaks.

Shrub Layer: Located beneath the canopy layer, this layer is made up of shorter shrubs or young trees, such as hawthorns, boxwoods and roses.

Herb Layer: Situated below the shrub layer, composed of herbaceous plants, such as grasses and wildflowers.

Moss Layer: The lowest layer, consisting of mosses, lichens, and other plants clinging to the ground.


An ecosystem comprises all living beings in a certain area and their environment, and it is more complex than a community. Its most noticeable feature: organisms not only interact with each other but also with their surrounding enviroment. Every ecosystem exhibits energy flow and material cycling.

Organisms are categorized as producers, consumers and decomposers according to their roles within the ecosystem. Producers produce sugars by photosynthesis and convert solar energy into chemical energy of organic matter. These sugars are stored in producers or used to make amino acids and lipids. Consumers feed on plants or animals, including herbivores, carnivores, omnivores and parasites. Decomposers break down the organisms and feces into inorganic substances for reuse of producers. Bacteria and fungi are common decomposers, but some animals also act as decomposers, such as earthworms.

Ecosystems are diverse, and categorized as natural ecosystems and artificial ecosystems. Artificial ecosystems include farmlands, orchards, and urban area. Natural ecosystems can be further divided into aquatic ecosystems and terrestrial ecosystems.


The biosphere consists of all ecosystems on Earth, from 10,000 meters above the sea level to the surface, the soil, underground caves, and the deepest parts of the ocean. Different ecosystems interact with each other, for example, snow on mountains comes from evaporated seawater. They serve as the source of rivers that nourish forests and grasslands through which they flow. The North Atlantic Drift warms European winter.

Life on Earth has evolved over more than 4 billion years to form the biosphere, the largest known living system. Humans have attempted to create a nother biosphere. Misunderstandings about material cycling and energy flow within the biosphere were the main reasons for the failure of Biosphere 2.

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Anec  > Biology > Introduction