Cell theory is one of the core concepts of biology, a scientific theory that defines cells as the basic composition of all living organisms.
Our understanding of cells first began in the mid-1600s, and cell theory itself was proposed in 1839, when the continued improvement of microscopes made understanding cellular structure easier.
Developing microscope sensitivity has enhanced our understanding of the cell, but the principles at the heart of cell theory have remained.
What Is Cell Theory?
Cell theory is a scientific principle developed by Matthias Schleiden and Thomas Schwann. Their observations were built from previous discoveries by scientists such as Robert Hooke, who first proposed the cell. At the core of cell theory are three tenets:
- All living organisms are made of one or more cells.
- A cell is the fundamental unit of structure in all living things.
- All cells come from pre-existing cells, and are created through cell division.
While these three principles were the basis of the originally proposed cell theory, the continued evolution of science has led to some additions to the theory. Modern versions of cell theory also include:
- The activity of independent cells determines the activity of an organism.
- Energy flow occurs within cells.
- Cells are able to pass hereditary information (DNA) between themselves.
- The basic chemical composition is the same across all cells.
The concepts of cell theory might seem basic to us now, but before the development of the compound microscope, these structures were unknown. The proposal of cell theory had an incredible effect on all areas of science.
The development of cell theory proved that there are fundamental similarities between all living things.
It confirmed the discovery of similarities between cells at an elemental level, while further observations noted differences between cells, relating to their genetics and function.
About Robert Hooke
Robert Hooke (1635 – 1703) was an English scientist, credited with the first observations and descriptions of cells. His work Micrographia, published in 1665, was a fundamental work in the development of cell theory.
It was the cork tree that led to Hooke proposing the idea of the cell. Observing thin layers of cork under the microscope, Hooke was able to note the box like structures that formed the organism.
He named them ‘cells’, after the small rooms that monks lived in, and the structure of the honeycomb (cellulae).
Although Hooke was able to create detailed illustrations of the multitude of cells that made up the slice of cork, he was unable to observe any further details due to the relatively poor magnification.
He didn’t understand the function of cells, and was unable to see the organelles found in nearly all living cells. Instead, he believed the empty cell walls might transport liquids around an organism.
While these early observations lacked the understanding to come later, Hooke’s work had an undeniable impact on cell theory. Hooke also helped to popularize the use of the microscope in science.
Although the earliest compound microscopes were thought to have been invented around the late 1500s or early 1600s, they remained a novelty.
The work detailed in Robert Hooke’s Micrographia, particularly his large and impressive illustrations, helped increase interest in the uses of the microscope.
One popular illustration of a louse folded out to four times the size of the original book, and captured the interest of numerous scientists.
It wasn’t just cells that interested Hooke. A polymath and architect, he also observed light, planetary bodies, and fossils.
He also discovered ‘Hooke’s Law’, a law of elasticity describing the scale of force needed to extend or compress a spring. One of his spring inventions led to the development of the pocket watch.
Hooke died in 1703, over a century before the development of cell theory. However, his early experiments had a profound effect on the theory.
A Timeline Of Cell Theory
Although cell theory is credited to Matthias Schleiden and Theodor Schwann, the work of many scientists went into its development. Below is a timeline of some of the key scientists involved in the development of the tenets of cell theory.
- Early 1600s. The first compound microscope is created.
- 1665. Robert Hooke published his Micrographia, including observations of cork under a microscope, and the first use of the word ‘cell’. This was the first observation of the units that are considered fundamental to life.
- 1668. After experimenting with covered and uncovered spoiled meat, Francesco Redi begins the work of disproving the theory of spontaneous generation. This leads the way to the discovery that cells only develop from other cells.
- 1676. Anton van Leeuwenhoek wrote to the Royal Society describing his observations of cells under his own improved microscopes. Considered the ‘Father of Microbiology’, he is recognized as the first scientist to identify unicellular organisms. His observations of sperm cells further disprove the idea of spontaneous generation.
- 1765-67. Lazzaro Spallanzani experimented with microbes in boiling broth, paving the way for an end to the idea of spontaneous generation. By boiling and sealing the broth, Spallanzani observed that the microbes were killed, and failed to grow in sealed containers.
- 1802. Franz Bauer observed and illustrated the nucleus of the cell, which was later named by Robert Brown.
- 1804. Karl Rudolhi and J. H. F. Link proved that cells have independent walls, rather than shared walls for passing fluids.
- 1824. Henri Dutrochet theorized the link between cells in plants and animals, and the fundamental units of life. He also stated that ‘the cell is the fundamental element of organization’.
- 1839. Matthias Schleiden suggested that every part of a plant is made of cells, or has developed from cells. He also proposed the theory that cells are created within other cells by the crystallization process, although this is no longer accepted. In the same year, Theodor Schwann proposed that plants and animals were composed of cells, or the products of cells. These observations formed the first two proponents of cell theory: all living things are composed of cells, and cells are the basic unit of life.
- 1855. Rudolf Virchow, along with other scientists, refuted the theory of crystallization process. Instead, he proposed that all cells arise only from pre-existing cells, creating the third tenet of cell theory.
- 1862. Experiments by Louis Pasteur ended the idea of spontaneous generation, confirming the third tenet of cell theory.
- 1882. Experiments by German scientists including August Weissmann trace the ability of the cell to pass on genetic information, leading to additional aspects now central to cell theory.
The first two principles of cell theory were proposed by Schwann and Schleiden in 1839, based on the observations and experiments of many scientists before them.
The theory continued to grow, with the improvement of the microscope helping us to learn more about what exists within the cell, and the implication this has for cell theory.
The core proponents of cell theory have been questioned, disproved, and validated by many scientists around the world.
Cell Theory And The Microscope
The importance of the microscope in cell theory can’t be overstated. It wasn’t until improvements to the compound microscope made it possible to view the minute details of plant matter in the mid-1600s that cells were first proposed.
Improved magnification discovered by Anton van Leeuwenhoek allowed the scientist to better observe the interior of cells, laying the groundwork for the eventual abandonment of spontaneous gestation theory.
It took until the 1800s, when microscope function was again improved, for much of cell theory to be observed and confirmed.
At its foundation, cell theory proposes that all living things are made of cells. The study of cells considers their structure and function, and how cells work both individually and alone.
Cells are considered the basic unit of life, and they carry out numerous functions within all living organisms.
To understand much of life, it’s important to understand the function of cells. Cells play a crucial role in our genetics and diseases, as well as helping us to understand the behaviors of living organisms.
Cells can be broadly divided into two categories: prokaryotic and eukaryotic cells.
Prokaryotic cells are single-celled organisms that don’t contain a nucleus, but do have a nucleoid region. Prokaryotic cells include bacteria and archaea, which make up two of the three domains of life.
Small, simple, and without the nucleus and other membrane-bound organelles, prokaryotic cells were the first forms of life on Earth.
There are three regions to the prokaryotic cell. First, the cell envelope. This is typically plasma membrane covered by the cell wall. This protects the cell from damage, and helps it to maintain form and structure.
Second is the cytoplasmic region. This is where the genetic material is found, including the DNA. The DNA in a prokaryotic cell is condensed into a nucleoid, but there is no nucleus.
Third, outside the prokaryotic cell are flagella and pili. Made of proteins and projecting from the cell surface, these enable movement and communication. Not all prokaryotic cells have these.
Prokaryotic cells reproduce asexually, typically by binary fission.
Eukaryotic cells make up the third domain of life: plants, animals, algae, protozoa etc. are all formed of eukaryotic cells. Larger and more complex than prokaryotic cells, eukaryotic cells contain membrane-bound organelles in which specialized activities take place.
Importantly, eukaryotic cells also contain a nucleus, storing the DNA of the cell. Unlike single-celled prokaryotes, eukaryotic cells form colonies and multicellular organisms (although some are single-celled).
Eukaryotic cells typically reproduce asexually via mitosis, with daughter cells receiving a single copy of each chromosome.
Common Organelles In Eukaryotic Cells
Contained within the eukaryotic cells of plants and animals are membrane-bound organelles. These are specialized structures, able to carry out a specific job or function within the cell. Some common organelles in eukaryotic cells are:
The role of the mitochondria is to provide energy to the cell, and they can be found in nearly all types of eukaryotic cells. Due to its role in providing energy, the mitochondria is often referred to as the ‘powerhouse of the cell’.
It oxidizes either sugar or fats, then releases energy in the form of Adenosine triphosphate (ATP). Mitochondria contain their own DNA, similar in structure to that of bacterial DNA.
The Golgi apparatus is found in the endomembrane system of most eukaryotic cells. It takes proteins and sorts them into membrane-bound vesicles. From here, the vesicles can be passed on to their destinations.
The Golgi apparatus is crucial to passing proteins around the body.
The endoplasmic reticulum, or ER, is the transportation system for the eukaryotic cell. It takes protein molecules and folds them into sacs known as the cisternae, which are then passed on to the Golgi apparatus.
Within the cell, ribosomes connect amino acids together, forming polypeptide chains as instructed by the codons of the messenger RNA. By binding together the amino acids, ribosomes are able to generate proteins as determined by the RNA.
The role of the lysosomes is to break down the biomolecules found in cytoplasm using digestive enzymes. They’re sometimes described as the waste disposal unit of the cell, digesting the used materials that are found in the cytoplasm.
Vacuoles are organelles that act as closed compartments within the cell. They might contain inorganic or organic molecules, and in some cases even contain solids. The exact function of a vacuole varies depending on the needs of the cell.
There are many more organelles found in animal cells, assisting in the cell function and keeping the cell alive.
Through cell theory and the development of the microscope, we’ve been able to gain a much greater understanding of the role of the cell.
Seen as the fundamental unit of life, by understanding the function and structure of the cell, we can understand disease, growth, and even behavior.
The development of cell theory has been a long road, from Robert Hooke’s first discovery, through to Schleiden and Schwann initial proposals. Continued improvement of the microscope and scientific technique is leading to developments in cell theory even now.
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