What is the simplest structure of life?

As previously mentioned, a cell's cytoplasm is home to numerous functional and structural elements. These elements exist in the form of molecules and organelles — picture them as the tools, appliances, and inner rooms of the cell. Major classes of intracellular organic molecules include nucleic acids, proteins, carbohydrates, and lipids, all of which are essential to the cell's functions.

Nucleic acids are the molecules that contain and help express a cell's genetic code. There are two major classes of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the molecule that contains all of the information required to build and maintain the cell; RNA has several roles associated with expression of the information stored in DNA. Of course, nucleic acids alone aren't responsible for the preservation and expression of genetic material: Cells also use proteins to help replicate the genome and accomplish the profound structural changes that underlie cell division.

Proteins are a second type of intracellular organic molecule. These substances are made from chains of smaller molecules called amino acids, and they serve a variety of functions in the cell, both catalytic and structural. For example, proteins called enzymes convert cellular molecules (whether proteins, carbohydrates, lipids, or nucleic acids) into other forms that might help a cell meet its energy needs, build support structures, or pump out wastes.

Carbohydrates, the starches and sugars in cells, are another important type of organic molecule. Simple carbohydrates are used for the cell's immediate energy demands, whereas complex carbohydrates serve as intracellular energy stores. Complex carbohydrates are also found on a cell's surface, where they play a crucial role in cell recognition.

Finally, lipids or fat molecules are components of cell membranes — both the plasma membrane and various intracellular membranes. They are also involved in energy storage, as well as relaying signals within cells and from the bloodstream to a cell's interior (Figure 2).

Some cells also feature orderly arrangements of molecules called organelles. Similar to the rooms in a house, these structures are partitioned off from the rest of a cell's interior by their own intracellular membrane. Organelles contain highly technical equipment required for specific jobs within the cell. One example is the mitochondrion — commonly known as the cell's "power plant" — which is the organelle that holds and maintains the machinery involved in energy-producing chemical reactions (Figure 3).

Figure 2: The composition of a bacterial cell

Most of a cell is water (70%). The remaining 30% contains varying proportions of structural and functional molecules.

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Figure 3: The relative scale of biological molecules and structures

Cells can vary between 1 micrometer (μm) and hundreds of micrometers in diameter. Within a cell, a DNA double helix is approximately 10 nanometers (nm) wide, whereas the cellular organelle called a nucleus that encloses this DNA can be approximately 1000 times bigger (about 10 μm). See how cells compare along a relative scale axis with other molecules, tissues, and biological structures (blue arrow at bottom). Note that a micrometer (μm) is also known as a micron.

With millions of different kinds of organisms in the world, scientists must find order in all of this diversity. Scientists group living organisms into one or more of a few major categories as part discipline known as taxonomy. The bodies of organisms are organized into functional systems—cells are organized into tissues, and tissues are organized into organs. Body systems carry out critical functions, such as locomotion, reproduction, digestion, and circulation. All living things on Earth are composed of the same carbon-based, molecular building blocks.

Watch this presentation for a general Introduction to Organisms. The accompanying slide set provides additional notes for each slide.

Subtopics:

Characteristics of Organisms from Major Taxonomic Groups

The beginning teacher describes characteristics of organisms from the major taxonomic groups.

Key Concepts:

• Modern taxonomy, using molecular analyses, divides all living organisms into three domains: Bacteria (includes the Kingdom Eubacteria), Archaea (includes the Kingdom Archaebacteria), and Eukarya (includes the Kingdoms Protista, Fungi, Plantae, and Animalia).
• The hierarchical system of classification originally conceived by Linnaeus includes seven levels or taxons. These levels are kingdom, phylum, class, order, family, genus, and species; representing general to specific organism attributes.
• Species names of organisms always are written as two words, consisting of the genus and species designation.

Resources:

Introduction to Biological Classification. This presentation and slide set from BioEd Online provides an overview of how living organisms are classified by biologists.

Classifying Living Things. Dr. Michael McDarby explains how living organisms are classified as part of the Online Introduction to the Biology of Animals and Plants

The Three Domain System. Dr. Gary Kaiser compares the three domains of cellular organisms: Archaea, Bacteria (Eubacteria) and Eukarya.

Domains. Mary Poffenroth explains the three biological domains in this presenation from Mahalo Biology.

The Taxonomic Hierarchy. Mary Poffenroth looks at the biological classification system in a video from Mahalo Biology.

How Structure Complements Function in Cells

The beginning teacher analyzes how structure complements function in cells.

Key Concepts:

• All organisms are composed of cells. Organisms can be single-celled or multicellular.
• In multicellular organisms, the shape of the cell helps determine its function. For example, red blood cells are donut-shaped to easily exchange oxygen and freely pass through narrow blood vessels while nerve cells are long so when connected to other nerve cells they can span long distances in the organism.
• Modern cell theory states that all organisms are composed of one or more cells; cells are the basic units of all organisms; and cells arise only through division of a previously existing cell.
• All cells have three major features: a nucleoid or nucleus (central portion of the cell containing genetic material), cytoplasm (semifluid matrix or gel, which fills the interior of the cell), and plasma membrane (phospholipid bilayer embedded with proteins surrounding the cell).
• Prokaryotes are the simplest organisms. There are two main types of prokaryotes: archaebacteria and bacteria. Most prokaryotes have a strong cell wall outside of the plasma membrane. The internal organization of prokaryotic cells is simple, with few internal compartments and no subunits (organelles surrounded by a membrane). Prokaryotic cells do not have a true nucleus surrounded by a membrane. Instead, their genetic material is present as a simple circle of DNA.
• Eukaryotes, which comprise the Domain, Eukarya, contain membrane-bounded organelles that carry out specialized functions and a nucleus that is surrounded by a double membrane (nuclear envelope). DNA inside the nucleus is organized into chromosomes.

Resources:

View the following videos to get a better understanding of size and shape of cells as it relates to their specific function.

Cell Size Shape and Form. Tutorvista provides a simple explanation of different kinds of cells.

Structure and Function of Nucleic Acids. Dr. Rae Lynn Alford provides an introduction to the molecules import for heredity in living organisms in this presentation and annotated slide set.

Cell Structure. Biology4Kids reviews the different components of cells. Click on the side menu for information about different organelles and structures.

Cell Function. Biology4Kids provides an easy-to-follow summary of the many jobs of cells.

How Structure Complements Function in Tissues, Organs, Organ Systems, and Organisms

The beginning teacher analyzes how structure complements function in tissues, organs, organ systems, and organisms.

Key Concepts:

• The cell is the basic unit of all forms of life. However, there are several levels of cell organization within many multicellular organisms. Cells are arranged in hierarchical levels of organization.
• The lowest form of organization is the cell followed by tissues, organs, and organ systems.
• A tissue is a group of similar cells in an organism working together to perform a particular function (e.g. nerve or muscle tissue).
• An organ is a collection of different tissues that carries out a particular function (e.g. liver).
• An organ system is a collection of organs that functions to carry out a specific task in the organism (e.g. digestive system).
• At each level of organization, the structure helps determine the function.
• The highest level of organization in multicellular living forms is the whole organism.

Resources:

Watch this video to understand organization of tissues, organs, and organ systems.

Tissues and Organs. This video from Brightstorm describes how animal cells are arranged into tissues, organs and organ systems.

What Is a System? Review concepts related to the organization of cells, tissues, organs and organ systems with Biology4Kids. Click on the Animal Systems menu to learn about each body system.

Human Body Systems and their Functions

The beginning teacher identifies human body systems and describes their functions.

Key Concepts:

• Organization of the body’s cells into specialized tissues, organs, and organ systems helps create a division of labor in the body and makes multicellular life possible.
• The human body has several organ systems. These systems include the skeletal, muscular, circulatory/cardiovascular, nervous/sensory, respiratory, digestive, excretory, endocrine, reproductive, and immune/lymphatic.

Resources:

Basic Anatomy: Tissues & Organs. Read this web page for a quick overview of the body systems, including organs, and the major role of each.

Animal Tissues and Organs. To review organ systems and their functions, select the appropriate links on this site to watch a short video.

Human Body Systems. These slides and notes provide an overview of tissues in the human body.

How Organisms Obtain and Use Energy and Matter

The beginning teacher describes how organisms obtain and use energy and matter.

Key Concepts:

• All living organisms depend on a source of energy to survive.
• Energy is the ability to perform work.
• Adenine triphosphate (ATP) is the chemical that stores and releases energy to drive reactions in each cell.
• Autotrophs (e.g. plants) use light energy from the sun to produce chemical energy (stored as chemical bonds in glucose) and ultimately structural components for the body of the organism, as well as energy for work.
• Producing chemical energy from the light energy (electromagnetic radiation) is called photosynthesis.
• Heterotrophs obtain energy by ingesting food sources, including plants or other animals (plant consumer only = herbivore; animal consumer only = carnivore; plant and animal consumer = omnivore; absorbs chemical energy from the environment = saprobe or saprotroph).
• The chemical energy and molecular building blocks (nutrients) obtained from these food sources are used by heterotrophs for new body structures or are converted to energy for work.
• Metabolism is all of the chemical reactions in an organism that occur in order to manage its material and energy resources.

Resources:

Autotrophs vs. Heterotrophs. Watch this video to learn about autotrophs and heterotrophs.

Energy, Ecosystems and the Atmosphere. Dr. Nancy Moreno discusses the flow of energy from the sun through producers and consumers.

Cycling through the Food Web. Scientists from the Bigelow Laboratory for Ocean Scientists explain the cycling of matter and energy flow.

ATP and Energy Storage. Interactive animation of how ATP stores energy from Dr. Saul’s Biology in Motion.

Structure and Function of Basic Chemical Components of Living Things

The beginning teacher applies chemical principles to describe the structure and function of the basic chemical components (e.g. proteins, carbohydrates, lipids, nucleic acids) of living things.

Key Concepts:

• All living organisms on earth are made up of chemicals based mostly on the element carbon. Carbon can form covalent bonds with up to four atoms. This characteristic allows carbon to form many diverse molecules.
• Most biological molecules consist of carbon atoms bonded to other carbon atoms or to atoms of oxygen, nitrogen, sulfur or hydrogen. Molecules containing carbon can form chains, branches or rings.
• Some biological molecules, such as sugars, are relatively small. Other biological molecules are large and complex, and are referred to as macromolecules. In many cases, the macromolecules are polymers, which are long chains of similar, linked subunits. Complex carbohydrates, such as starch, proteins and nucleic acids all are polymers.
• Carbohydrates store energy and provide building materials. Carbohydrates are a large group of molecules that contain carbon, hydrogen and oxygen. Simple sugars (monosaccharides) such as glucose, double sugars (disaccharides) such as sucrose and lactose, and starches (polysaccharides) are carbohydrates.
• Lipids (fats and oils) make up membranes and store energy. Lipids will not dissolve in water. Lipids have long sections of non-polar carbon-hydrogen bonds. When placed in water, lipid molecules form clusters with any polar sections facing toward the exterior and non-polar portions toward the interior (away from water). Lipids include triglycerides (dense energy storage formed in fat cells), phospholipids (structural component of the cell membrane phospholipid bilayer), steroids (e.g. cholesterol), and waxes (waterproofing).
• Proteins perform the chemistry of cells. Proteins consist of chains of amino acids that may interact and fold over each other to form different shapes and structures. Protein functions include structural support, enzymes for reactions, transport of other molecules, storage, signaling, movement in the organism, and immune defense.
• Nucleic acids store and transfer genetic information. Nucleic acids are composed of a sugar, a phosphate group, and a nitrogen base. The two major types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA sequences are a code for the inherited traits of an organism.

Resources:

The following short videos describe the structures and functions of the basic chemical components of cells.

The Chemistry of Life. Spark Notes provides a brief introduction to the four elements that comprise 98% of all living matter.

The Molecules of Life. The most important classes of chemical compounds in living organisms are reviewed in this Spark Notes article.

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What is the smallest structure of life?