3.3: A Cell is the Smallest Unit of Life

Levels of Biological Organization

Living things are highly organized and structured, following a hierarchy of scale from small to large (Figure \(\PageIndex{1}\)). The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms combine to form molecules, which are chemical structures consisting of at least two atoms held together by a chemical bond. In plants, animals, and many other types of organisms, molecules come together in specific ways to create structures called organelles. Organelles are small structures that exist within cells and perform specialized functions. As discussed in more detail below, all living things are made of one or more cells.

Figure \(\PageIndex{1}\). From an atom to the entire Earth, biology examines all aspects of life. (credit “molecule”: modification of work by Jane Whitney; credit “organelles”: modification of work by Louisa Howard; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer, National Cancer Institute; credit “tissue”: modification of work by “Kilbad”/Wikimedia Commons; credit “organs”: modification of work by Mariana Ruiz Villareal, Joaquim Alves Gaspar; credit “organisms”: modification of work by Peter Dutton; credit “ecosystem”: modification of work by “gigi4791″/Flickr; credit “biosphere”: modification of work by NASA)

In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function. Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism.

All the individuals of a species living within a specific area are collectively called a population. A community is the set of different populations inhabiting a common area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization, the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.

Cell Theory

Close your eyes and picture a brick wall. What is the basic building block of that wall? It is a single brick, of course. Like a brick wall, your body is composed of basic building blocks, and the building blocks of your body are cells. Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is made from many cell types. For example, bone cells help to support and protect the body. Cells of the immune system fight invading bacteria and red blood cells carry oxygen throughout the body. Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, all cells share certain fundamental characteristics.

Important historical advances in understanding cells:

• mid-1600s: Antony van Leeuwenhoek used simple lenses to observe movements of single-celled organisms and sperm, which he collectively termed “animalcules.”
• 1665: Robert Hooke coined the term “cell” (from the Latin cella, meaning “small room”) for the box-like structures he observed when viewing cork tissue.
• 1670s: van Leeuwenhoek discovered bacteria and protozoa with more advanced lenses.
• late 1830s, Matthias Schleiden and Theodor Schwann observed tissues with better microscopes and proposed the unified cell theory, which states that
  • all living things are composed of one or more cells,
  • that the cell is the basic unit of life, and
  • that all new cells arise from existing cells.

These principles still stand today.

There are many types of cells, and all are grouped into one of two broad categories: prokaryotic (bacteria and archaea) and eukaryotic (animals, plants, fungi, and protists). 

All cells share four common components:

1. a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment;
2. cytoplasm, consisting of a jelly-like region within the cell in which other cellular components are found;
3. DNA, the genetic material of the cell; and
4. ribosomes, particles that synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

Prokaryotic Cells

A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{2}\). This figure shows the generalized structure of a prokaryotic cell.

Bacteria and Archaea are different in parts of their cellular structures. Both are very different from eukaryotes and are critical in the functioning of all life. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are protein appendages used by bacteria to attach to other cells.

Eukaryotic Cells

A eukaryotic cell is a cell that has a membrane-bound nucleus and other membrane-bound compartments called organelles. There are many different types of organelles, each with a highly specialized function (see Figure \(\PageIndex{3}\)). 

Cell Size

At 0.1–5.0 µm in diameter, most prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10–100 µm (Figure \(\PageIndex{3}\)). The small size of prokaryotes allows ions and organic molecules that enter them to spread rapidly to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly move out. However, larger eukaryotic cells have evolved different structural adaptations to enhance cellular transport. Indeed, the large size of these cells would not be possible without these adaptations. 

Figure \(\PageIndex{3}\). This figure indicates the relative sizes of various cells and cellular components. An adult human is shown for comparison.

Animal Cells versus Plant Cells

Figure \(\PageIndex{4}\). An example of a typical animal cell.

Figure \(\PageIndex{5}\). An example of a typical plant cell.

Despite their fundamental similarities, there are some striking differences between animal and plant cells (Figure \(\PageIndex{3}\)). Animal cells have centrioles, centrosomes, and lysosomes, whereas plant cells do not. Plant cells have a rigid cell wall that is external to the plasma membrane, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not.

Chloroplasts

From an ecological perspective, chloroplasts are a very important type of organelle because they perform photosynthesis. Photosynthesis forms the foundation of food chains in most ecosystems. Chloroplasts are only found in eukaryotic cells such as plants and algae. During photosynthesis, carbon dioxide, water, and light energy are used to make glucose and molecular oxygen. One major difference between algae/plants and animals is that plants/algae can make their own food, like glucose, whereas animals must obtain food by consuming other organisms.

Contributors and Attributions

• “Essentials of Environmental Science” by Kamala Doršner is licensed under CC BY 4.0. “Levels of Organization of Living Things” by Open Stax is licensed under CC BY 4.0. Modified from the originals by Matthew R. Fisher.