All About a Cell


This blog post is basically about cell working at the basic level. This post is a bit lengthy and might be a bit difficult for all readers to grasp. This post talks more about molecular biology and uses a lot of biological terms/words .I request the readers to kindly bear with me as the basic cell working of great interest & relevance to us because the Pranayama yoga acts on cellular level.We shall now see how Pranayama makes a true difference on the physical and mental plane of the human body.

Cells are the structural and functional units of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multi cellular, or have many cells of the order of 100,000,000,000,000 cells.  Each cell can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as and when necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

There are two general categories of cells.  They are

  • The Prokaryotes  
  • The Eukaryotes. 

We shall now discuss first the prokaryotes and then the eukaryotes. All the animals including human beings have eukaryote cells.

Prokaryotic Organisms

It is said that life arose on earth about 4 billion years ago. The simplest of cells and the first types of cells to evolve were Prokaryotic cells. These are organisms that lack a nuclear membrane which is the membrane that surrounds the nucleus of a cell. Bacteria are the best known and most studied form of Prokaryotic organisms.  However, the recent discovery of a second group of Prokaryotes, called archaea, has provided evidence of a third cellular domain of life and probably, new insights into the origin of life itself

Prokaryotes are unicellular organisms that do not develop or differentiate into multi cellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells, Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies. Prokaryotes are distinguished from Eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of Eukaryotic cells.

Eukaryotic Organisms

Eukaryotes include fungi, animals including human beings, and plants as well as some unicellular organisms. Eukaryotic cells are about 10 times the size of a Prokaryote and can be as much as 1000 times greater in volume.

The major and extremely significant difference between Prokaryotes and Eukaryotes is that Eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the Eukaryotic cell’s DNA. It is this nucleus that gives the Eukaryote—literally, true nucleus—its name.

The human beings contain cells which are Eukaryotes. Eukaryotic organisms also have other specialized structures, called organelles, which are small structures within cells that perform dedicated functions which we shall describe later in this section. As the name implies, the organelles are extremely small organs. They play a very important role at a molecular level in the cell. This fact is of great interest & relevance to us because the Pranayama yoga acts on cellular level. The figure shown below illustrates a typical human cell . The internal structures of Eukaryotic cells have   the nucleus, the nucleolus, mitochondria and ribosomes

The Cell Structures

Plasma Membrane:

The plasma membrane provides a protective coat to the cell to provide the structural integrity.

The outer lining of a Eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids (fat-like molecules). Embedded within this membrane are a variety of other molecules that act as channels and pumps, which move different molecules in and out of the cell.

Cytoskeleton

The cytoskeleton is an important dynamic components complex of the cell. It organizes and maintains the cell’s structural integrity during vibrations of the cells. It is a skeleton structure that anchors organelles in place especially during the uptake of external materials by a cell. It maintains the structural integrity of the parts of the cell during the processes of its growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.

Cytoplasm

There is a large fluid-filled space, called the cytoplasm (also called the cytosol), inside the cell. In human cells, the cytosol is the “soup” within which all of the cell’s organelles reside. It is also the home of the cytoskeleton. The cytosol containing dissolved nutrients helps break down waste products, and moves material around the cell through a process called cytoplasmic streaming. The living cells during their entire life span are in a constant state of vibrations. The nucleus, during these vibrations, often moves with the cytoplasm, changing its shape as it vibrates. The cytoplasm also contains many salts which make it as an excellent conductor of electricity. This enables creation of a perfect environment for the cellular electro-chemical processes and the appropriate mechanisms required for the cellular activities. The function of the cytoplasm, and the organelles which reside in it, are thus extremely critical for a cell’s survival.

Genetic Material

Two different kinds of genetic materials exist in the cell:

Deoxyribonucleic Acid (DNA) and Ribonucleic acid (RNA).
The biological information contained in an organism is encoded in its DNA or RNA sequence. Human cell genetic material is very complex and is divided into discrete units called genes. Human genetic material is made up of two distinct components: the nuclear genome and the mitochondrial genome. The nuclear genome is divided into 24 linear DNA molecules, each contained in a different chromosome. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some very important proteins.

Organelles

Similar to the whole human body, which contains many organs, such as the heart, lung, and kidney etc, with each organ performing a different function, cells also have a set of “little organs”, called organelles, that are specialized for carrying out one or more of  vital functions. Organelles are found only in Eukaryotes and are always surrounded by a protective membrane. It is important to know some basic facts about the organelles. Following are the most important organelles.

(1)The nucleus

The nucleus is the most prominent organelle found in a Eukaryotic cell. It houses the cell’s chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus has a spherical shape and is separated from the cytoplasm by a 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 synthesized, into a special RNA, called the mRNA (messenger RNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule.

(2)Ribosomes

 The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA’s genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell. Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the endoplasmic reticulum. Ribosomes are composed of one large and one small subunit, each having a different function during protein synthesis.

(2)Mitochondria

They are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all Eukaryotic cells. Mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.

Mitochondria play a very critical role in generating energy in the Eukaryotic cell, and this process involves a number of complex pathways.

Some of the best energy-supplying foods that we eat contain complex sugars. These complex sugars are broken down into a less chemically complex sugar molecule called glucose.  Once this happens, glucose can then enter the cell through special molecules found in the membrane, called glucose transporters. We discussed about this in details when we discussed diabetes. Once inside the cell, glucose is broken down to make Adenosine Triphosphate (ATP), a form of energy, via two different pathways namely glycolysis and Krebs cycle or citric acid cycle.

 The first pathway is glycolysis which does not require oxygen (anaerobic activity). The glycolysis occurs in the cytoplasm outside the mitochondria. During glycolysis, glucose is broken down into a molecule called Pyruvate. Each of these reactions produces some H ions that can be used for making the ATP. However, only four ATP molecules can be made from one molecule of glucose in this pathway. The majority of ATP is formed in the Krebs cycle which occurs in the mitochondria. Increasing evidence shows that mitochondrial dysfunction due to the oxidation of lipids, proteins, and nucleic acids plays an important role in brain aging and neurodegenerative diseases such as Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and Huntington disease. Mitochondria provide energy for basic metabolic processes, produce oxidants as inevitable by-products, and decay with age, impairing cellular metabolism and leading to cellular decline. Before we discuss about Krebs cycle, it is necessary to discuss some important aspects of ATP.

(4) ATP

The physical human body is a biochemical unit. We know that all biological machines must have many well-engineered parts (organs) to work such as the liver, kidney, and heart etc. These extremely complex life units are made from still smaller parts called cells which in turn are constructed from yet smaller (micro) machines known as organelles.  The cell organelles include mitochondria, Golgi complexes, microtubules etc. Even below this level are other parts so small that they are formally classified as macromolecules (large molecules).

A critically important macromolecule which can be considered as the second most important molecule after DNA is ATP. ATP is a complex nanomachine that serves as the primary energy currency of the cell and thereby of the whole body. A nanomachine is a complex precision microscopic-sized machine that fits the standard definition of a machine. ATP is the most widely distributed high-energy compound within the human body. This ubiquitous molecule is used to build complex molecules, contract muscles, generate electricity in nerves, and light fireflies. All fuel sources of nature, all foodstuffs of living things, produce ATP, which in turn powers virtually every activity of the cell and organism.

ATP is an abbreviation for Adenosine Triphosphate, a complex molecule that contains the nucleoside adenosine and a tail consisting of three phosphates. Figure shown below shows   a simple structural formula and a space filled model of ATP (Jerry Bergman). As far as known, all organisms from the simplest bacteria to humans use ATP as their primary energy currency. The energy level it carries is just the right amount for most biological reactions. Nutrients contain energy in low-energy covalent bonds which are not very useful to do most of kinds of work in the cells.

These low energy bonds must be translated to high energy bonds, and this is a role played by ATP. A steady supply of ATP is therefore extremely critical.

Energy is usually liberated from the ATP molecule to do work in the cell by a reaction that removes one of the phosphate-oxygen groups, leaving adenosine diphosphate (ADP).  When the ATP converts to ADP, the ATP is said to be spent. Then the ADP is usually immediately recycled in the mitochondria where it is recharged and comes out again as ATP.  This recycling of phosphate is the most critical event in the body’s energy management programme.

ATP is manufactured as a result of several cell processes including fermentation (glycolysis), respiration and photosynthesis (in plants). Most commonly, the cells use ADP as a precursor molecule and then add phosphorus to it. In Eukaryotes this can occur either in the soluble portion of the cytoplasm (cytosol) or in special energy-producing structures called mitochondria. Charging ADP to form ATP in the mitochondria is called chemiosmotic phosphorylation. This process occurs in specially constructed chambers located in the mitochondrion’s inner membranes.

The mitochondrion itself functions to produce an electrical chemical gradient—somewhat like a battery—by accumulating hydrogen ions in the space between the inner and outer membrane. This energy comes from the estimated 10,000 enzyme chains in the membranous sacks on the mitochondrial walls. Most of the food energy for most organisms is produced by the electron transport chain. Cellular oxidation in the Krebs cycle causes an electron build-up that is used to push H+ ions outward across the inner mitochondrial membrane (Hickman et al., 1997, p. 71). We shall now discuss the second pathway of energy generation i.e. Krebs cycle.

The second pathway, called the Krebs’s cycle, or the citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions. Once again, the cycle begins with a glucose molecule, which, during the process of glycolysis is stripped of some of its hydrogen atoms, transforming the glucose into two molecules of pyruvic acid. Next, pyruvic acid is altered by the removal of one carbon atom and two oxygen atoms, which go on to form carbon dioxide. When the carbon dioxide is removed, energy is given off, and a molecule called NAD+ is converted into the higher energy form, NADH.  Another molecule, coenzyme A (CoA), then attaches to the remaining acetyl unit, forming acetyl CoA

Acetyl CoA enters the Krebs’s cycle by joining to a four-carbon molecule called oxaloacetate. Once the two molecules are joined, they make a six-carbon molecule called citric acid. Citric acid is then broken down and modified in a stepwise fashion. As this happens, hydrogen ions and carbon molecules are released. The carbon molecules are used to make more carbon dioxide. The hydrogen ions are picked up by NAD and another molecule called flavin-adenine dinucleotide (FAD). Eventually, the process produces the four-carbon oxaloacetate again, ending up where it started off. All in all, the Krebs’s cycle is capable of generating from 24 to 28 ATP molecules from one molecule of glucose converted to Pyruvate. Therefore, it is easy to see how much more energy we can get from a molecule of glucose if our mitochondria are working properly and if we have enough oxygen

I do understand that all that has been discussed is too complex for the common man to understand but, unfortunately, it is rather unavoidable. The sum substance of this discussion is that the cells must receive adequate quantity of oxygen and the nutrients.The oxygen deficiency at cell/tissue level, also called the cellular hypoxia/tissue hypoxia is mother of all ailments. We, therefore, shall discuss this at length later in other posts

Leave a Reply

Your e-mail address will not be published. Required fields are marked *