The cell (from Latin ‘cella’, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. Cells are the smallest unit of life that can replicate independently, and are often called the "building blocks of life". A cell is generally made of different organelles each with their own individual function.
Bone marrow and cancer cells under a microscope
Plant cells under microscope
Standard stained onion skin cells under microscope
Cheek cells under microscope
Unicellular or Multicellular?
The number of cells in the body varies with different organisms. in some lower forms of life (e.g. Amoeba, Euglena) The body is formed of a single cell; these organisms are described as unicellular. In a unicellular organism, all life activities are carried out by itself with it internal structures. The intracellular structures are called organelles. In the vast majority of plants and animals, the body is made up of numerous cells. They are called multicellular organisms. This is actually an aggregation of cells. Here a group of cells function in the same way to form a tissue or an organ (e.g. Xylem vessels, cork cells, skin cells). Multicellular organisms have a greater capacity to survive that unicellular organisms.
Eukaryotes and Prokaryotes
Prokaryotes are organisms made up of cells
that lack a cell nucleus or any membrane-encased organelles. This means the
genetic material DNA in prokaryotes is not bound within a nucleus.
Additionally, the DNA is less structured in prokaryotes than in eukaryotes. In
prokaryotes, DNA is a single loop. In Eukaryotes, DNA is organized into
chromosomes. Most prokaryotes are made up of just a single cell (unicellular)
but there are a few that are made of collections of cells (multicellular).
Scientists have divided the prokaryotes into two groups, the Bacteria and the
Archaea.
Eukaryotes are organisms made up of cells that
possess a membrane-bound nucleus (that holds genetic material) as well as
membrane-bound organelles. Genetic material in eukaryotes is contained within a
nucleus within the cell and DNA is organized into chromosomes. Eukaryotic
organisms may be multicellular or single-celled organisms. All animals are
eukaryotes. Other eukaryotes include plants, fungi, and protists.
Difference in Prokaryotes and Eukaryotes
The distinction between prokaryotes and eukaryotes is considered to be the most important distinction among groups of organisms. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotic cells do not. Differences in cellular structure of prokaryotes and eukaryotes include the presence of mitochondria and chloroplasts, the cell wall, and the structure of chromosomal DNA.
The discovery of cells
English Scientist and Microscopist Robert Hooke described a honeycomb-like network of cellulae (Latin for little storage rooms) in cork slice using his primitive compound microscope. Robert Hooke used the term cells to describe units in plant tissue (thick cell walls could be observed). Of course he saw only cell walls because cork cells are dead and without protoplasm. He drew the cells he saw and also coined the word cell. The word cell is derived from the latin word cellula which means small compartment. Hooke published his findings in his famous work, Micrographia. When Hooke viewed a thin cutting of cork he discovered empty spaces contained by walls, and termed them pores, or cells. The term cells stuck and Hooke gained credit for discovering the building blocks of all life. Hooke calculated the number of cells in a cubic inch to be 1,259,712,000, and while he couldn't grasp the full effect of his discovery, he did at least appreciate the sheer number of these cells. The first person to witness a live cell under a microscope was Anton van Leeuwenhoek, who saw and described the algae Spirogyra and bacteria.
Hooke's Drawing of his Microscope from Micrographia.
Hooke viewed a thin cutting of cork and discovered empty spaces contained by walls which he termed cells.
Specialised cells (Multicellular)
Cells may be specialised for a particular function. Their structure will allow them to carry this function out. Here are some examples:
Cells must be shaped to perform specific functions in the context of the organism. If there was no cellular specialization, every cell in your body would be the same Cells in your stomach are specialized to secrete mucus that protects your stomach wall from the extremely acidic stomach contents
Organelle
An organelle is a specialized part of the cell that serves a specific purpose within the cell. There are many organelles present within a cell and they also vary depending on what kind of cell we are talking about, whether it be a plant or animal cell. Here are a few of the organelles and their functions:
nucleus-this would be like the brain of the cell and gives direction to the rest of the cell
cell membrane-this provides protection for the cell and allows material to enter and leave the cell
vacuoles-these are used for storage of materials such as water
mitochondria-this is the powerhouse of the cell-it has ATP (energy)
endoplasmic reticulum-this is sort of like circulation for the cell-its use is for transportation