Cell and its constituent organelles, Biology tutorial

Introduction to Cell and its constituent organelles:

Human body has several different organs, like heart, lung, and kidney, with each organ doing different function. Cells also contain a set of little organs, known as organelles, these organelles are adapted and/or specialized for performing one or more vital functions. Both eukaryotic and prokaryotic cells contain organelles but organelles in eukaryotes are usually more complex and may be membrane bound. Organelles are found inside protoplasm of cell in cytosol (a gelatinous fluid which fills cell and surrounds organelles).

The cytoplasm:

Cytoplasm comprises of a gelatinous solution (cystosol) and contains microtubules (which serve as a cell's cytoskeleton) and organelles (literally referred to as "little organs'')

The Nucleus:

This is the cell's information center. Only Eukaryotic cells have nucleus within which is found DNA (deoxyribonucleic acid) in form of chromosomes plus nucleoli (within which ribosomes are created). Nucleus is lacking in prokaryotic cells. Nucleus is spherical and separated from cytoplasm by double membrane known as nuclear envelope. It is enveloped by the pair of membranes enclosing lumen which is continuous with that of endoplasmic reticulum. This membrane is known as nuclear envelop. The nuclear envelope isolates and protects cell's DNA from different molecules which could accidentally damage the structure or interfere with processing. In processing, DNA is transcribed, or copied in special RNA, known as messenger RNA (mRNA). Nucleus controls all cell activity. It does this by controlling enzymes present. Chromatin is made up of DNA.

Chromatin:

Nucleus has chromosomes of cell. Each chromosome comprises of single molecule of DNA complexed with the equal mass of proteins. Together, DNA of nucleus with its related proteins is known as chromatin. Chromatin also has small amounts of the wide variety of nonhistone proteins. The majority of protein in nucleus comprises of numerous copies of five types of histones.

The Nucleolus:

Nucleolus is the particular region within nucleus where ribosome subunits are collected. In prokaryotes, DNA processing occurs in cytoplasm. During period between cell divisions, when chromosomes are in their extended state, one or more of them (10 in human cells) contain loops extending into spherical mass known as nucleolus.

Nuclear Pore Complexes (NPCs):

Nuclear envelope is perforated with thousands of pores.

Each is constructed from a number (30 in yeast; probably around 50 in vertebrates) different protein called nucleoporins. The entire assembly forms an aqueous channel connecting the cytosol with the interior of the nucleus ("nucleoplasm"). When materials are to be transported through the pore, it opens up to form a channel some 25 nm wide - large enough to get such large assemblies as ribosomal subunits through. Transport through the nuclear pore complexes is active; that is, it requires

  • energy
  • many different carrier molecules each specialized to transport a particular cargo
  • docking molecules in the NPC (represented here as colored rods and disks).

Import into the nucleus:

Proteins are synthesized in the cytosol and those needed by the nucleus must be imported into it through the NPCs.

They include:

  • all the histones needed to make the nucleosomes
  • all the ribosomal proteins needed for the assembly of ribosomes
  • all the transcription factors (e.g., the steroid receptors) needed to turn genes on (and off)
  • all the splicing factors needed to process pre-mRNA into mature mRNA molecules; that is, to cut out intron regions and splice the exon regions.

Export from the nucleus:

  • Molecules and macromolecular assemblies exported from the nucleus include:
  • the ribosomal subunits containing both rRNA and proteins
  • messenger RNA (mRNA) molecules (accompanied by proteins)
  • transfer RNA (tRNA) molecules (also accompanied by proteins)
  • transcription factors that are returned to the cytosol to await reuse

Mitochondria and Chloroplasts:

Only found in eukaryotes. They are the power generators. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. They have a double-membrane: one outer membrane and a second highly convoluted inner membrane Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria 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 splitting in two. Respiration occurs in the cell mitochondria. Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through photosynthesis, which uses solar energy to generate carbohydrates and oxygen from carbon dioxide and water.

Golgi apparatus:

Only present in eukaryotes. Golgi apparatus consists of a series of flattened sacs (or cisternae). It has a structure that is made up of cisternae, which are flattened stacks of membrane usually found in a series of five to eight. The function of the Golgi apparatus is to modify, sort, and package proteins and other materials from the endoplasmic reticulum for storage in the cell or secretion outside the cell. The main function of the Golgi apparatus is to be responsible for handling the macromolecules that are required for proper cell functioning. It processes and packages these macromolecules for use within the cell or for secretion. Primarily, the Golgi apparatus modifies proteins that it receives from the rough endoplasmic reticulum, however, it also transports lipids to vital parts of the cell and creates lysosomes. These cisternae help proteins travel from different points in the cell using enzymes.

Ribosomes:

Ribosome is component of the biological cell which develops proteins from all amino acids and RNA representing protein. Ribosome translates genetic information from RNA in proteins. Ribosomes perform this by binding to mRNA and using it as the template for correct sequence of amino acids in particular protein. Amino acids are attached to transfer RNA (tRNA) molecules that enter one part of ribosome and bind to messenger RNA sequence. Attached amino acids are then joined together by another part of ribosome. Ribosome moves along mRNA, reading its sequence and generating chain of amino acids. Ribosomes are prepared from complexes of RNAs and proteins. Ribosomes are separated in two subunits. Smaller subunit binds to mRNA, while larger subunit binds to tRNA and amino acids. When the ribosome finishes reading mRNA, these 2 subunits split apart.

Lysosomes and Peroxisomes:

Only present in eukaryotes. Lysosomes have digestive enzymes (acid hydrolases). They digest surplus or worn-out organelles, food particles, and swallowed up viruses or bacteria. Peroxisomes contain enzymes that rid cell of toxic peroxides. Cell couldn't house the destructive enzymes if they weren't contained in membrane-bound system.

Centrosome-cytoskeleton organizer:

Centrosome generates microtubules of cell - a main component of cytoskeleton. It directs transport through ER and Golgi apparatus. Centrosomes are made up of 2 centrioles that separate in cell division and help in formation of mitotic spindle.

Vacuoles:

Vacuoles store food and waste. Few vacuoles store additional water. They are frequently explained as liquid filled space and are enclosed by membrane. Few cells, particularly Amoeba, contain contractile vacuoles that can pump water out of cell if there is too much water.

Centrioles:

Paired cylindrical structures situated near nucleus. Play the significant role in cell division

Endoplasmic reticulum (smooth and rough):

Endoplasmic reticulum (ER) is the eukaryotic organelle which forms the interconnected network of tubules, vesicles, and cisternae inside cells. Rough endoplasmic reticula synthesize proteins, whereas smooth endoplasmic reticula metabolize carbohydrates and steroids (but not lipids), synthesize lipids and steroids, and regulate calcium concentration, drug metabolism.

i) Structure of Endoplasmic Membrane:

General structure of endoplasmic reticulum is the extensive membrane network of cisternae (sac-like structures) held together by cytoskeleton. Phospholipid membrane encloses a space, cisternal space (or lumen), that is continuous with perinuclear space but separate from cytosol. Functions of endoplasmic reticulum differ greatly depending on exact kind of endoplasmic reticulum and kind of cell in which it resides.

The three varieties are known as rough endoplasmic reticulum, smooth endoplasmic reticulum and sarcoplasmic reticulum.

ii) Rough and Smooth:

There are 2 basic kinds of ER. Both rough ER and smooth ER have same kinds of membranes but they have various shapes. Rough ER looks like sheets or disks of bumpy membranes whereas smooth ER looks more like tubes. Rough ER is known as rough as it contains ribosomes attached to surface.

Double membranes of rough and smooth ER form sacs known as cisternae. Protein molecules are synthesized and gathered in cisternal space/lumen. When sufficient proteins have been synthesized, they gather and are pinched off in vesicles. Vesicles frequently move to Golgi apparatus for extra protein packaging and distribution.

iii) Sarcoplasmic reticulum (SR):

It is the variation of SER. It is capable to store several ions in solution that cell will require at later time. When the cell requires doing something immediately, it does not make sense to search environment for additional ions which may or may not be floating around. It is simpler to have them stored in pack for easy use.

RER is also attached to nuclear envelope which surrounds nucleus. This direct connection between perinuclear space and lumen of ER permits for movement of molecules by both membranes. Procedure of protein synthesis begins when mRNA moves from nucleus to ribosome on surface of RER. As ribosome builds amino acid chain, chain is pressed in cisternal space of RER. When proteins are complete, they gather and RER pinches off the vesicle. That vesicle, small membrane bubble, can move to cell membrane or Golgi apparatus.

iv) Function of Endoplasmic reticulum:

It serves several general functions, comprising;

i) Insertion of proteins in endoplasmic reticulum membrane: Integral membrane proteins are inserted in endoplasmic reticulum membrane as they are being synthesized. Insertion into endoplasmic reticulum membrane needs correct topogenic signal sequences in protein.

ii) Glycosylation: Glycosylation involves attachment of oligosaccharides.

iii) Disulfide bond formation and rearrangement: Disulfide bonds stabilize tertiary and quaternary structure of several proteins.

iv) Drug metabolism: Smooth ER is the site at which some drugs are altered by microsomal enzymes that include cytochrome P450 enzymes.

Protoplasm:

Protoplasm is living content of cell that is enclosed by plasma membrane (cell membrane). Protoplasm is made up of mixture of;

i) Small molecules such as: ions, amino acids, monosaccharides and water,

ii) Macromolecules like: lipids, nucleic acids, proteins, and polysaccharides. In eukaryotes protoplasm surrounding the cell nucleus is called as cytoplasm and that inside nucleus is nucleoplasm. In prokaryotes the material inside plasma membrane is bacterial cytoplasm, whereas in gram-negative bacteria region outside plasma membrane but inside outer membrane is periplasm.

i) Physical properties of protoplasm:

It is transparent and jelly-like material, constancy varying from more liquid, slightly gelatinous white of fresh egg to that of semi-solidified gelatin of jelly. If protoplasm is more liquid it is termed as sol, if more gelatinous, a gel.

ii) Chemical properties of protoplasm:

Chemical properties of protoplasm can be separated in inorganic and organic substances. Inorganic Substances: Inorganic substances are water, that make up 90% of protoplasm, mineral salts, like NaCl-salt, and gases like oxygen and carbon dioxide. Organic Substances: Organic substances comprise proteins, lipids, carbohydrates, nucleic acids and enzymes.

iii) Functions of the Protoplasm:

a) Reproduction Cells divide to form identical daughter cells; function of the nucleus of the protoplasm, e.g. the meristematic region of angiosperms.

b) Irritability The living protoplasm responds to stimuli, e.g. retinal cells in the eye respond to light.

c) Chemical All these functions are carried out inside the cell, e.g. respiration in the mitochondria;

d) Excretion Cells must get rid of excretory wastes; they usually diffuse out of the cell through the cell membrane.

e) Movement: Movement is exhibited by certain cells, e.g. unicells; the protoplasm of these cells has a contractile ability.

Cytoplasm:

Cytoplasm is thick liquid residing between cell membrane holding all organelles, except for nucleus. All contents of cells of prokaryote organisms (that lack cell nucleus) are contained within cytoplasm. Within cells of eukaryote organisms contents of cell nucleus are divided from cytoplasm, and are then known as nucleoplasm.

Constituents of cytoplasm: Cytoplasm has 3 major elements; cytosol, organelles and inclusions.

Cytosol: Cytosol is portion not within membrane-bound organelles. Cytosol is translucent fluid in which plasmic elements are suspended. Cytosol makes up approx 70% of cell volume and is made up of water, salts and organic molecules.

Organelles: Organelles are membrane-bound organs inside cell which have specific functions. Few major organelles which are suspended in cytosol are mitochondria, endoplasmic reticulum, Golgi apparatus.

Cytoplasmic inclusions: Inclusions are small particles of insoluble substances suspended in cytosol. Huge range of inclusions exists in different cell kinds, and range from crystals of calcium oxalate or silicon dioxide in plants, to granules of energy-storage materials like starch, glycogen, or polyhydroxybutyrate.

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