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The Levels of Organization of Living Things Including Organelles

At a basic level, living things are made of atoms and combinations of atoms called molecules. Atoms and molecules form other structures, such as organelles and cells. Unicellular organisms perform all their life functions with a single cell. Multicellular organisms, including humans, are more complex, as cells work together to form the tissues and organs that characterize multicellular organisms.

Organelles and Cells
- Cells contain organelles with specific functions.
  
  Organelles are considered "tiny organs" within a cell. Organelles are made of molecules, and molecules are made of atoms. Organelles have specific functions within living cells.
  
  Types of organelles include cytoplasm, cell membrane, endoplasmic reticulum, golgi apparatus, lysosomes, mitochondria, nucleus, nucleolus and ribosomes. Organelles specific to plant cells are the cell wall, chloroplasts and vacuoles.
  
  The next-highest level of organism is the cell, which encases all of the organelles. The cell membrane organelle protects the cell.
Tissues
- At the next-highest level of complexity are collections of specialized cells grouped together, called tissues. The four basic types of tissue are epithelial, connective, muscle and neural. Epithelial tissue's functions include absorption, excretion, protection, secretion and transportation. Connective tissue provides the body's structural framework, protects organs, stores energy, defends the body, transports fluids and supports other types of tissue. Muscle tissue contracts in response to stimuli. Muscle tissues include cardiac, skeletal and smooth muscle. Neural tissue is responsible for electrical impulses and cell-to-cell communication.
Organs
- The lungs are an organ inside some organisms.
  
  Organs, at yet a higher level of organization, are made of tissue, which is made of specialized cells. Organs perform specific functions within an organism. Organs consist of at least two types of tissues that work together to serve a common purpose. Examples include the heart, kidneys and liver. Skin is the largest organ in the body. The skin consists of three layers: the epidermis, dermis and subcutaneous layer. The epidermis is epithelial tissue, and the dermis and subcutaneous layers are connective tissue. Organs working toward a common function form organ systems.
Multicellular Organism
- A human being is a multicellular organism and occupies the highest rung on the ladder of complexity and organization.
  
  Multicellular organisms are complex life forms made of cells, tissues and organs. At this highest level of organization of living things, cells, tissues and organs work together to form physiological systems, including the muscular, circulatory and skeletal systems of humans. Plants and animals are also multicellular organisms. A multicellular organism experiences turnover of chemicals through metabolism. It responds to stimuli and grows. It can reproduce through sexual or asexual means. In addition, a multicellular organism has the ability to evolve over time.

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