3.2: The Basic Structural and Functional Unit of Life- The Cell

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Learning Objectives

• Diagram the components of a cell.
• Describe the organization of the human body.

What distinguishes a living 

organism

 from an inanimate object? A living 

organism

 conducts self-sustaining biological processes. A cell is the smallest and most basic form of life. Robert Hooke, one of the first scientists to use a light microscope, discovered the cell in 1665. In all life forms, including bacteria, plants, animals, and humans, the cell was defined as the most basic structural and functional unit. Based on scientific observations over the next 150 years, scientists formulated the 

cell theory

, which is used for all living organisms no matter how simple or complex. The 

cell theory

 incorporates three principles:

• Cells are the most basic building units of life.
• All living things are composed of cells.
• New cells are made from preexisting cells, which divide into two.

Who you are has been determined because of two cells that came together inside your mother’s womb. The two cells containing all of your genetic information (DNA) united to begin making new life. Cells divided and differentiated into other cells with specific roles that led to the formation of the body’s numerous body 

organs

, systems, blood, blood vessels, bone, tissue, and skin. As an adult, you are comprised of trillions of cells. Each of your individual cells is a compact and efficient form of life—self-sufficient, yet interdependent upon the other cells within your body to supply its needs.

One cell divides into two, which begins the creation of millions of more cells that ultimately become you. (Public Domain; National Institutes of Health).

Independent single-celled organisms must conduct all the basic processes of life: it must take in 

nutrients

 (

energy

 capture), excrete wastes, detect and respond to its environment, move, breathe, grow, and reproduce. Even a one-celled 

organism

 must be organized to perform these essential processes. All cells are organized from the atomic level to all its larger forms. Oxygen and hydrogen 

atoms

 combine to make the molecule water (H2O). Molecules bond together to make bigger macromolecules. The carbon atom is often referred to as the backbone of life because it can readily bond with four other elements to form long chains and more complex macromolecules. Four macromolecules—

carbohydrates

, 

lipids

, 

proteins

, and nucleic acids—make up all of the structural and functional units of cells.

Figure 3.2.1$\mathrm{3.2.}1$: The cell is structurally and functionally complex.

Although we defined the cell as the “most basic” unit of life, it is structurally and functionally complex (Figure 3.2.1$\mathrm{3.2.}1$). 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.

Video 3.1: Discovery Video: Cells

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 (Table 3.2.1$\mathrm{3.2.}1$).
• Organism
  . An 
  organism
   is the complete living system capable of conducting all of life’s biological processes.

Table 3.2.1$\mathrm{3.2.}1$: The Eleven 

Organ Systems

 in the Human Body and Their Major Functions

Organ System	Organ Components	Major Function
Circulatory	heart, blood/lymph vessels, blood, lymph	Transport  nutrients  and waste products
Digestive	mouth, esophagus, stomach, intestines	Digestion and absorption
Endocrine	all glands (thyroid, ovaries, pancreas)	Produce and release  hormones
Immune	white blood cells, lymphatic tissue, marrow	Defend against foreign invaders
Integumentary	skin, nails, hair, sweat glands	Protective, body temperature regulation
Muscular	skeletal, smooth, and cardiac muscle	Body movement
Nervous	brain, spinal cord, nerves	Interprets and responds to stimuli
Reproductive	gonads, genitals	Reproduction and sexual characteristics
Respiratory	lungs, nose, mouth, throat, trachea	Gas exchange
Skeletal	bones, tendons, ligaments, joints	Structure and support
Urinary	kidneys, bladder, ureters	Waste excretion, water balance

Figure 3.2.2$\mathrm{3.2.}2$: 

Organ Systems

 in the Human Body © Networkgraphics

An 

Organism

 Requires 

Energy

 and Nutrient Input

Energy

 is required in order to build molecules into larger macromolecules, and to turn macromolecules into 

organelles

 and cells, and then turn those into 

tissues

, 

organs

, and 

organ systems

, and finally into an 

organism

. Proper nutrition provides the necessary 

nutrients

 to make the 

energy

 that supports life’s processes. Your body builds new macromolecules from the 

nutrients

 in food.

Nutrient and 

Energy

 Flow

Energy

 is stored in a nutrient’s chemical bonds. 

Energy

 comes from sunlight, which plants then capture and, via photosynthesis, use it to transform carbon dioxide in the air into the molecule, glucose. When the glucose bonds are broken, 

energy

 is released. Bacteria, plants, and animals (including humans) harvest the 

energy

 in glucose via a biological process called 

cellular respiration

. In this process the 

chemical energy

 of glucose is transformed into cellular 

energy

 in the form of the molecule, adenosine triphosphate (ATP). 

Cellular respiration

 requires oxygen (aerobic) and it is provided as a waste product of photosynthesis. The waste products of 

cellular respiration

 are carbon dioxide (CO2) and water, which plants use to conduct photosynthesis again. Thus, 

energy

 is constantly cycling between plants and animals. As 

energy

 is consumed 

nutrients

 are recycled within it.

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.

Discussion Starter

1. Discuss the importance of organization in the human body. If the body becomes disorganized by a 
   disease
    or disorder what happens to its function? Can you think of a good example (what about leg fracture and movement)?