Although we defined the cell as the “most basic” unit of life, it is structurally and functionally complex (see Figure 3.1). A cell can be thought of as a mini-organism consisting of tiny organs called organelles. The organelles are structural and functional units constructed from several macromolecules bonded together. A typical animal cell contains the following organelles: the nucleus (which houses the genetic material DNA), mitochondria (which generate energy), ribosomes (which produce protein), the endoplasmic reticulum (which is a packaging and transport facility), and the golgi apparatus (which distributes macromolecules). In addition, animal cells contain little digestive pouches, called lysosomes and peroxisomes, which break down macromolecules and destroy foreign invaders. All of the organelles are anchored in the cell’s cytoplasm via a cytoskeleton. The cell’s organelles are isolated from the surrounding environment by a plasma membrane.
Figure 3.1
The cell is structurally and functionally complex.
Video 3.1
Discovery Video: Cells
(click to see video)
This video describes the importance of cells in the human body.
Tissues, Organs, Organ Systems, and Organisms
Unicellular (single-celled) organisms can function independently, but the cells of multicellular organisms are dependent upon each other and are organized into five different levels in order to coordinate their specific functions and carry out all of life’s biological processes.
Cells. Cells are the basic structural and functional unit of all life. Examples include red blood cells and nerve cells.
Tissues. Tissues are groups of cells that share a common structure and function and work together. There are four types of human tissues: connective, which connects tissues; epithelial, which lines and protects organs; muscle, which contracts for movement and support; and nerve, which responds and reacts to signals in the environment.
Organs. Organs are a group of tissues arranged in a specific manner to support a common physiological function. Examples include the brain, liver, and heart.
Organ systems. Organ systems are two or more organs that support a specific physiological function. Examples include the digestive system and central nervous system. There are eleven organ systems in the human body (see Table 3.1 "The Eleven Organ Systems in the Human Body and Their Major Functions").
Organism. An organism is the complete living system capable of conducting all of life’s biological processes.
Table 3.1 The Eleven Organ Systems in the Human Body and Their Major Functions
Organ SystemOrgan ComponentsMajor Function
Circulatoryheart, blood/lymph vessels, blood, lymphTransport nutrients and waste products
Digestivemouth, esophagus, stomach, intestinesDigestion and absorption
Endocrineall glands (thyroid, ovaries, pancreas)Produce and release hormones
Immunewhite blood cells, lymphatic tissue, marrowDefend against
Plants harvest energy from the sun and capture it in the molecule, glucose. Humans harvest the energy in glucose and capture it into the molecule, ATP.
In this section, we have learned that all life is composed of cells capable of transforming small organic molecules into energy. How do complex organisms such as humans convert the large macromolecules in the foods that we eat into molecules that can be used by cells to make cellular energy? In the next section, we will discuss the physiological process of digestion to answer this question.
KEY TAKEAWAYS
The cell is the basic structural and functional unit of life. Cells are independent, single-celled organisms that take in nutrients, excrete wastes, detect and respond to their environment, move, breathe, grow, and reproduce. The macromolecules carbohydrates, proteins, lipids, and nucleic acids make up all of the structural and functional units of cells.
In complex organisms, cells are organized into five levels so that an organism can conduct all basic processes associated with life.
There are eleven organ systems in the human body that work together to support life, all of which require nutrient input.
Energy is constantly cycling between plants and animals. As energy is consumed nutrients are recycled within it.
Explanation:
