Cell structure
There are some major structures that somewhat get lost in the shuffle when we talk about organelles. They are not organelles, but are still extremely important components of the cellular system. They are the cell wall, the cell membrane and the cytoplasm.Cell wall
We know that not all living organisms have cell walls around their cells, most notably animals and many animal-like protists. The cell wall is a non-living and strong component of the cell, located outside the plasma membrane. It provides shape, strength, protection and support to the inner living matter (protoplasm) of the cell.Plant cells have a variety of chemicals in their cell walls. The outer layer of the plant cell wall is known as primary wall and cellulose is the most common chemical in it. Some plant cells, for example xylem cells, also have secondary walls on the inner side of the primary wall. It is much thicker and lignin and other chemicals are embedded in it.
In the walls of neighbouring cells there are cytoplasmic connections called Plasmodesmata. Through these connections, cells transfer chemicals among each other. Fungi and many protists have cell walls although they do not contain cellulose. Their cell walls are made of a variety of chemicals.
For example chitin is present in the cell wall of fungi. Prokaryotes have a cell wall composed of peptidoglycan that is a single large polymer of amino acids and sugar. All prokaryotic and eukaryotic cells have a thin and elastic cell membrane covering the cytoplasm. The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of chemicals inside the cell.
In this way the membrane maintains the internal composition of the cell to a constant or nearly constant level. In addition to this vital role, cell membrane can also sense chemical messages and can identify materials and other cells etc. Chemical analysis reveals that cell membrane is mainly composed of proteins and lipids with small quantities of carbohydrates.
Electron microscopic examinations of cell membranes have led to the development of the fluid-mosaic model of cell membrane . When we talk about all the membranes of a cell, we say them as cell membranes. When we talk about only the outer membrane of the cell, we say it as plasma membrane.
Cholesterol.
Proteins may be fully submerged in the lipid bilayer or some of them may "stick out" into the interior and outside of the cell. These proteins function as gateways that allow certain molecules to cross into and out of the cell. Small amounts of carbohydrates are also found in cell membranes.These are joined with proteins (in the form of glycoproteins) or with lipids (in the form of glycolipids). Both these forms act as fingerprints of the cell. Another lipid, cholesterol, is an important component of cell membranes embedded in e inner region of the lipid bilayer.
Most bacterial cell membranes do not contain cholesterol.cell many organelles e.g. mitochondria, chloroplasts, Golgi apparatus, and plasmic reticulum are also bounded by cell membranes. Cytoplasm Cytoplasm is the material between the plasma membrane (cell membrane) and the nuclear envelope.
It is a semi-viscous and semi-transparent substance. (proteins, carbohydrates, lipids) and inorganic salts are completely or partially dissolved. The cytoplasm of the cell provides space forthe properfunctioning of the organelles and also acts as the site for various biochemical (metabolic) reactions for example Glycolysis (breakdown of glucose during cellular respiration).
Cytoskeleton
The cytoskeleton is an important, complex and dynamic cell component. It is invisible under light microscope. It maintains the cell's shape, anchors organelles in place and moves parts of the cell in processes of growth and motility.Plasma membrane
There are many types of filaments that make up the cytoskeleton but the mostimportant ones are microtubules and microfilaments.. Microtubules are made of tubulin subunits and are often used by cells to hold their shape.Endoplasmic Microtubules
These are also the major reticu|um component of cilia and flagella. Ribosomes Microfilaments are made of actin subunits. These microfilaments are approximately one-third of the diameter of a microtubule and are often used by cells to change their Microfilaments shapes and to hold structures. Cell organelles Organelles are small structures within cells that perform dedicated functions.There are about a dozen types of organelles commonly found in eukaryotic cells. We will go* through the basic facts about important cell organelles. Nucleus Nucleus Nucleus A prominent nucleus occurs in eukaryotic cells. In animal cells it is present in the center while in mature-plant cells, due to the formation of large central vacuole, it is pushed to side. The nucleus is bounded by a double membrane known as nuclear envelope.
The nuclear envelope contains many small pores that enable it to act as a differentially- permeable membrane. Inside the nuclear envelope a granular matrix, the nucleoplasm, is present. The nucleoplasm contains one or two nucleoli (singular; nucleolus) and chromosomes .
Nucleolus
The nucleolus is usually visible as a dark spot and it is the site where ribosomal RNA are formed and assembled as ribosomes. Chromosomes are visible only during cell division while during interphase (non-dividing phase) of the cell they are in the form of fine thread-like structures known as chromatin.Chromosomes are composed of deoxyribonucleic acid (DNA) and histone proteins. Histone proteins provide structural support to DNAfor making the structure of chromosome. mRNA contains the message for the synthesis of specific proteins.
According to this message, a molecule of messenger Ribonucleic Acid (mRNA) is synthesized. In this way, message is handed over to mRNA, which carries it to ribosomes. Ribosomes manufacture specific protein according to the message present on mRNA.
In this way DNA controls all the activities of the cell and is also responsible for the transmission of characteristics to the next generation. That is why it is honoured as the hereditary material. The prokaryotic cells do not contain prominent nucleus rather their chromosome is made of DNAoniy and is submerged in the cytoplasm.
Ribosomes
Ribosomes are tiny granular structures that are either floating freely in the cytoplasm or are bound to the dopiasmic reticulum (ER). Each ribosome is made up of f eclual amounts of proteins and ribosomal RNA . Ribosomes are not bound by membranes and so jealso found in prokaryotes.Eukaryotic ribosomes are highly larger than prokaryotic ones. Ribosomes are the sites of protein synthesis, where the protein carried by mRNA is translated into protein. Animal cells have two centrioles located near the exterior surface of the nucleus . In cells that contain cilia and or flagella, centrioles are involved in the formation Vacuoles are fluid filled single-membrane bound organelles.
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