In 1655, the English scientist Robert Hooke made an observation that would change basic biological theory and research forever. While examining a dried section of cork tree with a crude light microscope, he observed small chambers and named them cells. Within a decade, researchers had determined that cells were not empty but instead were filled with a watery substance called cytoplasm.
Over the next 175 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, this theorem has four basic parts:
- The cell is the basic structural and functional unit of life; all organisms are composed of cells.
- All cells are produced by the division of preexisting cells (in other words, through reproduction). Each cell contains genetic material that is passed down during this process.
- All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.
- The activities of cells depends on the activities of subcellular structures within the cell (these subcellular structures include organelles, the plasma membrane, and, if present, the nucleus).
The cell theory leads to two very important generalities about cells and life as a whole:
- Cells are alive. The individual cells of your organs are just as "alive" as you are, even though they cannot live independently. This means cells can take energy (which, depending on the cell type, can be in the form of light, sugar, or other compounds) and building materials (proteins, carbohydrates and fats), and use these to repair themselves and make new generations of cells (reproduction).
- The characteristics and needs of an organism are in reality the characteristics and needs of the cells that make up the organism. For example, you need water because your cells need water.
What's in a cell?
Most of the activities of a cell (repair, reproduction, etc.) are carried out via the production of proteins. Proteins are large molecules that are made by specific organelles within the cell using the instructions contained within its genetic material (see our Fats and Proteins module).
Cytology is the study of cells, and cytologists are scientists that study cells. Cytologists have discovered that all cells are similar. They are all composed chiefly of molecules containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Although many nonliving structures also contain these elements, cells are different in their organization and maintenance of a boundary, their ability to regulate their own activity, and their controlled metabolism.
All cells contain three basic features:
- A plasma membrane consisting of a phospholipid bilayer, which is a fatty membrane that houses the cell. This membrane contains several structures that allow the cell to perform necessary tasks - for example, channels that allow substances to move in and out of the cell, antigens that allow the cell to be recognized by other cells, and proteins that allow cells to attach to each other.
- A cytoplasm containing cytosol and organelles. Cytosol is a fluid consisting mostly of water and dissolved nutrients, wastes, ions, proteins, and other molecules. Organelles are small structures suspended in the cytosol. The organelles carry out the basic functions of the cell, including reproduction, metabolism, and protein synthesis.
- Genetic material (DNA and RNA), which carries the instructions for the production of proteins.
Apart from these three similarities, cell structure and form are very diverse and are therefore difficult to generalize. Some cells are single, independent units and spend their entire existence as individual cells (these are the single-celled organisms such as amoebas and bacteria). Other cells are part of multicellular organisms and cannot survive alone.
One major difference among cells is the presence or absence of a nucleus, which is a subcellular structure that contains the genetic material. Prokaryotic cells (which include bacteria) lack a nucleus, whereas eukaryotic cells (which include protozoans, animal and plant cells) contain a nucleus.
There are other major differences in cell structure and function between different types of organisms. For example:
- The cells of autotrophic organisms (most plants and some protozoans), which can produce their own food, contain an organelle called the chloroplast that contains chlorophyll and allows the cell to produce glucose using light energy in the process known as photosynthesis.
- The cells of plants, protists, and fungi are surrounded by a cell wall composed mostly of the carbohydrate cellulose; the cell wall helps these cells maintain their shape. Animal cells lack a cell wall but instead have a cytoskeleton, a network of long fibrous protein strands that attach to the inner surface of the plasma membrane and help them maintain shape.
Cell differences within an organism
There are even major differences in cells within the same organism, reflecting the different functions the cells serve within the organism. For example, the human body consists of trillions of cells, including some 200 different cell types that vary greatly in size, shape, and function. The smallest human cells, sperm cells, are a few micrometers wide (1/12,000 of an inch) whereas the longest cells, the neurons that run from the tip of the big toe to the spinal cord, are over a meter long in an average adult!
Human cells also vary significantly in structure and function. For example:
- Only muscle cells contain myofilaments, protein-containing structures that allow the cells to contract (shorten) and therefore cause movement.
- Specialized cells called photoreceptors within the eye have the ability to detect light. These cells contain special chemicals called pigments that can absorb light, and special organelles that can then turn the absorbed light into electrical current that is sent to the brain and is perceived as vision.
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