cell- Robert Hooke 1st discovered basic structural and functional unit of life

The cell is a basic structural, functional, and biological unit of all living organisms. A cell is the smallest unit of life. Cells are also called the building blocks of the body. 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 the basis of cells, such as 1 - unicellular (consisting of a single cell,e.g- bacteria). 2 - multicellular e.g- plants and animals.

The average size of a cell

The human body contains more than 10 trillion (1013) cells. cells are visible only under a microscope, with dimensions between 1 and 100 micrometers.

Robert Hooke 1st discovered Cell

Cells were discovered by Robert Hooke in 1665 while examining a thin slice of cork, Robert hook saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Robert hook called these boxes cells. the cell is a Latin word for a little room. 
Cell theory, In 1839 by Matthias Jakob Schleiden and Theodor Schwann was first described, 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, and that all cells come from pre-existing cells.

Shape and size of a cell 

Every multicellular organism has come from a single cell. cells divide and produce their own kind of cells. All cells come from pre-existing cells.
The human body has different organs for different functions. Like, hands to hold something, feet to move from one place to another. Similarly, the cell also varies in size, shape, and type. Such as brain cells are different from heart cells. Skin cell - different from a blood cell.
Each cell has specific components known as cell organelles. The cell organelle performs a defined and specific function, such as creating new material in the cell, cleaning the waste material from the cell. The organelles of a cell combine together to form a basic unit called the cell. Similar organelles are found in all cells.

Types of cells: There are two types of cells,

  • Prokaryotic cell, 
  • and Eukaryotic cells.

Prokaryotes cell 

In some organisms such as bacteria, the nuclear area of ​​a cell can be poorly defined due to the absence of a nuclear membrane. The undefined Nucleus containing only one nucleic acid is called a nucleoid.
Such organisms lack a nuclear membrane called prokaryotes. (Pro (means) - primitive or primary, and eukaryote (means) -nucleus).
Prokaryotic cells also lack other cytoplasmic organelles. The DNA of a prokaryotic cell contains a single chromosome that is in direct contact with the cytoplasm. Most prokaryotes are the smallest of all organisms with diameters ranging from 0.5 to 2.0 μm.

Eukaryotic cell 

cells having a nuclear membrane are called eukaryotes. Such organisms that have eukaryote cells are called eukaryotic organisms. such as plants, animals, fungi, slime molds, protozoa, and algae are all eukaryotic organisms. 
These cells are about fifteen times wider than a normal prokaryote.
Cell walls may or may not exist. Eukaryotic DNA is organized into one or more linear molecules, called chromosomes, which are associated with histone proteins.
All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane called the nuclear membrane.

Main types of cells in the human body

  • Smooth muscle cell.
  • Blood cell-
  • Nerve cell-
  • Bone cell.
  • Sperm.
  • Ovum.
  • Fat cell.

 Animal Cell organelles

Organelles are parts of the cell that are adapted and specialized for carrying out one or more vital functions, analogous to the organs of the human body, (such as the heart, lung, and kidney, with each organ performing a different function). 
Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.
There are several organelles in a cell. such as the nucleus and Golgi apparatus are typically solitary, while others such as mitochondria, chloroplasts, peroxisomes,, and lysosomes can be numerous (hundreds to thousands). 
The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles. Three features in almost every cell have found.

  • Plasma membrane.
  • Nucleus.
  • And Cytoplasm.

Plasma membrane

This is the outermost covering of the cell that separates the contents of the cell from its external environment. also known as the cell membrane.
While in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment.

It is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane.

Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.
The membrane is semi-permeable, selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all.

Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

The cell nucleus is the information center of the cell,  it is the most conspicuous organelle found in a eukaryotic cell. the nucleus is the house of 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-layer 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. 

During processing, DNA is transcribed or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. 

The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.

Mitochondria is the powerhouse of the cell. Mitochondria 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 is complete in the mitochondria. It generates the energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. 

In eukaryotic cells, the huge majority of ATP synthesis occurs in the mitochondria in a process called oxidative phosphorylation.
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.

Endoplasmic reticulumThe 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 transporting, processing, and packaging the macromolecules such as proteins and lipids into the vesicle for delivery to targeted destinations. 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 organizer: 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.

Cell wall: Many types of prokaryotic and eukaryotic cells have a cell wall.  The cell wall acts to protect the cell mechanically and chemically from its environment and is an additional layer of protection to the cell membrane.
Different types of cells have cell walls made up of different materials; plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin, and bacteria cell walls are made up of peptidoglycan.
when a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall this phenomenon is known as plasmolysis.

Diffusion: The plasma membrane is also called the selectively permeable membrane. some substances like carbon dioxide and oxygen can move across the cell membrane by a process called diffusion. 
Diffusion is the process where the movement of the substances from higher concentration to low lower concentration.

Osmosis: water obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis.

The cytoskeleton: The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. 

The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments, and microtubules. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. 

The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity, and cytokinesis. 

The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. 

But some of the subunit proteins of intermediate filaments include vimentin, desmin, lamin(lamins A, B, and C), keratin (multiple acidic and basic keratin's), neurofilament proteins (NF–L, NF–M).

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