- 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. Cells are often called the ‘Building blocks’ of life.
- The study of cells is called cell biology or cellular biology.
- The cell consists cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acid.
- The number of cells in plants and animals varies from species to species. Humans contain around 40 trillion (4 × 1013) cells.
- Most plant and animal cells are visible only under a microscope, with dimensions between 1 to 100 micrometers.
- Cells were discovered by Robert Hooke in 1665 (detailed description in a book written by him-Micrographia).
- Cell theory was first developed in 1839, by Schleiden and Schwann. According to his theory, all organisms are made of one or more cell and all cells originate from pre-existing cells.
- Cells emerged on Earth at least 3.5 billion years ago.
On the basis of the absence or presence of a nucleus, the cells have been divided into two groups:-
I. Prokaryotic Cells :
- Prokaryotic cells are cells that do not have a true nucleus or most cell organelles. Organisms that have prokaryotic cells are unicellular and called prokaryotes.
- Prokaryotic cells do not have a true nucleus that contains their genetic material as eukaryotic cells do. Instead, prokaryotic cells have a nucleoid region, which is an irregularly shaped region that contains the DNA and is not surrounded by the membrane. The cell wall is present in prokaryotes.
- The ribosome is found in the cytoplasm.
- Histone protein is not annexed with the nucleic acid.
- Such type of cells is found in bacteria and blue-green algae. Prokaryotes include bacteria and archaea, two of the three domains of life. Their cells were the first form of life on Earth.
II. Eukaryotic Cells :
- Eukaryotic cells are cells that contain a nucleus and organelles, which are enclosed by a plasma membrane.
- Organisms that have eukaryotic cells include protozoa, algae, slime molds, fungi, plants, and animals. These organisms are grouped into the biological domain Eukaryota.
- Eukaryotic cells are large and more complex than prokaryotic cells. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume.
Eukaryotic cells contain a variety of structures called organelles, which perform various functions within the cell which are as follows –
(a) Cell Wall :
- The cell wall is found in the cells of plants and algae, fungi, and most chromalveolates, which is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism.
- Plant cell walls are made up of cellulose, hemicellulose, and pectin. The fungi cell wall is made up of chitin.
- Animal cells lack a cell wall.
(b) Plasma Membrane/Cell Membrane :
- The plasma membrane is a biological membrane that separates the interior of all cells from the outside environment, which protects the cell from its environment, It consists of a lipid bilayer with embedded protein.
- The cell membrane is a semipermeable or selectively permeable membrane which controls the movements of ions and organic molecules.
(c) Endoplasmic Reticulum (ER) :
- ER is a cellular organelle. It is the transport network for molecules going to specific places.
- ER is of two types-
(i) Rough Endoplasmic Reticulum (RER) – The ribosomes are found on its surface (which gives it a rough appearance) so it acts as a site for protein synthesis.
(ii) Smooth Endoplasmic Reticulum (SER) – It is without ribosome. It synthesizes lipids, phospholipids and steroids. Similar to the SER, Sarcoplasmic Reticulum (SR) is found only in muscle cells (myocytes). The SR stores and pumps calcium ions.
(d) Ribosome :
- The ribosome is a cell organelle. Ribosomes are found free in the cytoplasm or bound to the endoplasmic reticulum to form rough ER. It functions as a micromachine for making proteins.
(e) Mitochondria :
- Mitochondria are surrounded by two membranes and have their own genome.
- Mitochondria act as the power plants, which means that the maximum energy in the form of ATP is produced within the mitochondria, hence it is also known as the Power House of the cells.
- During cellular respiration, the initial process takes place in the cytoplasm and then in mitochondria.
- The most prominent roles of mitochondria are to produce the energy currency of the cell, ATP (adenosine triphosphate), respiration, and to regulate cellular metabolism. During cellular respiration, most ATP is formed in Kreb’s cycle (Citric acid cycle).
(f) Golgi body :
- The Golgi body is also known as the Golgi complex or Golgi apparatus. It was identified in 1897 by Camillo Golgi.
- The Golgi complex is responsible for manufacturing, warehousing, and shipping certain cellular products such as proteins and lipids that are synthesized by the cell. Its functional unit is called cisternae.
(g) Lysosomes :
- Lysosomes are also known as the suicidal bag of cells.
- These are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers-proteins, nucleic acids, carbohydrates, and lipids. It functions on the digestive system of the cell serving both to degrade material taken up from outside the cell and to digest absolute components of the cell itself.
- Lysosomes also take part in killing the viruses and bacteria that have entered the cell.
- The hydrolyzing enzymes act in an acidic medium.
- After the rupturing of the membrane of the lysosome, the enzyme came out, which hydrolyzes all components of the cell wall by the process of hydrolysis.
(h) Centrosome :
- It is the cytoskeleton organizer of the cell. It produces the microtubules of a cell – a key component of the cytoskeleton.
- A single centrosome is present in the animal cells. Plants and most fungi lack centrosomes.
- Vacuoles sequester waste products and in plant cells store water.
- The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.
Note: By scientific convention, the term lysosome is applied to these vesicular organelles only in animals, and the term vacuole is applied to those in plants, fungi, and algae (some animal cells also have vacuoles). However, some botanists argued that these vacuoles are lysosomes.
(j) Nucleus :
- The nucleus was discovered by Robert Brown in 1831.
- The nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus but a few cell types, such as mammalian R.B.C. have no nuclei and few others as Vaucheria and Cladophora have many nuclei.
- It is the controlling and coordinating organ of the nucleus due to the presence of DNA in it. It houses the cell’s chromosomes and is the place where almost all DNA replication and RNA synthesis occur.
- The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope.
- DNA is also found in mitochondria and chloroplast (found only in plant cells).
Programmed Cell Death (PCD) :
- Programmed cell death is the death of a cell in any form, mediated by an intracellular program, and is also referred to as cellular suicide.
- PCD is carried out in a biological process, which usually confers advantage during an organism’s life cycle. For example, the differentiation of fingers and toes in a developing human embryo occurs because cells between the fingers apoptosis; the result is that the digits are separate.
- PCD serves fundamental functions during both plant and animal tissue development.
- Apoptosis and autophagy, both are forms of programmed cell death.