Biology, SS 2 Week: 1

Topic: What is a Cell?

Introduction – What is a Cell?

The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

Cell can be defined as the structural and functional unit of life. It could also be describe as the simplest and basic unit of life in which are living organism are made of cells. The cell (from Latin cella, meaning “small room”) is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the “building blocks of life”. The study of cells is called cell biology. A cell is the smallest unit of life that can replicate independently, and cells are often called the “building blocks of life”. The study of cells is called cell biology.

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (1012) cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.

Classification of Living Organism Based on the Number of Cells

This Includes

  1. Unicellular Organism: These are organisms which consist of only one cell e.g. Amoeba, Paramecium.
  2. Multi-cellular Organism: These are organisms which consist of two or many cells e.g. flowering plant, Bird, Hydra.

History of Cell

Many scientist have contributed to the distribution of history of cell which are listed below;

  1. Robert Hooke: He is regarded as the father of cell. He was the first human being to discover the honey comb structure of the cell in 1665. In his book, Micrographic, he described his observation of a slice of a cork of an oak tree. He established that the cork is made up of thin components or rooms which he later named the components of cells.
  2. Felix Dujardin: He was a French biologist in 1835, which he discovered that the cell was made up of living substance. He however named the living substance protoplasm.
  3. Matthias Schledien: He is also a German botanist in 1838 revealed that the bodies of plants are made of cells which were described as units of life.
  4. Theodore Schwann: He was also a German zoologist in 1839; He discovered that the bodies of all animals are composed of cells.
  5. Rudolf Von Virchow: He was also a German biologist in 1855, concluded in his research that all cells come from previously existing cells.

The Cell Theory

The cell theory is stated below;

  1. The cell is the structural and functional unit of life.
  2. All living organisms are made of cells
  3. All cells come from previously existing cells.
  4. There is no life apart from the life of cells.
  5. All living things are either single cells or multicellular cell.


Cell Structure

  • Cell nucleus: A cell’s information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell’s chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell’s DNA from various molecules that could accidentally damage its structure or interfere with its processing. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.
  • Mitochondria and Chloroplasts: They generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Respiration occurs in the cell mitochondria, which generate the cell’s energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun’s energy to make carbohydrates through photosynthesis.
  • Chloroplast: They are the site of photosynthesis, which are present in chlorophyll bacteria, blue-green algae.
  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell.
  • Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.
  • Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.
  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes)

Forms In Which Living Cells Exist

These include;

  1. As Independent or Single and Free Living Organism: This means that organism that possesses only one cell and is capable of living freely on their own. e.g. Amoeba, Paramecium.
  2. As a Colony: This means that some organism are made of many similar cells which are joined or massed together  but they cannot be differentiated from each other .e .g Volvox, Pandorina, etc.
  3. As a Filament: This simply that certain cells are organised into filaments. Each cells functions as an independent living cell. E.g. Spirogyra, Zygnema.
  4. Cells as Part of a Living Organism: This means that in a multi-cellular organism, a group of numerous, similar cells are arranged together to perform specific functions and this is called a tissue. A group of similar tissue forming a layer in an organism which performs a specific function is called an organ. A group of organs which work together to perform specific functions is called a system.