Cell Biology: Cell Structure, Cell Functions, Cell Division, Cell Theory and Cell Organelles

A cell is a functional basic unit of life discovered by Robert Hooke in Cork cells and is the smallest unit of life that is classified as a living thing, and is often called the building block of life. In the beginning of the 18th century, Antonie van Leeuwenhoek, a Dutch tradesman and scientist built a microscope and drew the protozoa from rainwater and bacteria from his own mouth. He is known as the “Father of Microbiology”.

In 1665 Robert Hooke discovered cells in cork, then in living plant tissue using an early microscope. He was the first person to use the term “cell”.

Largest and smallest cells

The organisms which have a single cell are unicellular and the organisms that have multiple cells are multicellular.  There are 1 trillion cells is a human body. The size of a typical cell is 10 micrometer and largest cells in human body are nerve cells called neurons. The largest known cells are unfertilized ostrich egg cells which weigh 3.3 pounds. Pleuropneumonia-Like Organisms (PPLO) which are now known as Mycoplasma are the smallest cells.

Cell Theory

Cell Theory was proposed by Scheilden and Schwann and this theory stated that:

• The body of all organisms is made up of cells
• New cells arise from the pre existing cells
• Cells are structural units of all organisms
• Cells are units of all biological functions.

Before the discovery of the cell, people were unaware that living organisms were made of building blocks like cells.

Prokaryotic and Eukaryotic cells

Prokaryotic cells are primitive cells in which there is no enclosed nucleus. Eukaryotic cells are those with a nucleus enclosed by a membrane. Bacteria and blue green algae are examples of prokaryotic cells. Algae, plants and animal cells are eukaryotic cells.

Cell Components

Cell Membrane / Plasma Membrane

The cell membrane or plasma membrane is the outer membrane of a cell.  Cell membrane is found around all cells and is selectively-permeable. Cell membrane encloses the cell itself, maintaining specific conditions for cellular function within the cell.

It controls the movement of substances in and out of cells. Main function of cell membrane is to protect the intracellular components from the extracellular environment. The cell membrane facilitates the transport of materials needed for survival. The movement of substances across the membrane can be active (with use of energy) or passive (diffusion without use of energy). Exocytosis and endocytosis are the processes by which the materials are taken in or out of a cell.  The cell membrane plays an important role in the respiration and electron transport chains.

Cell wall

Cell walls are found in plants, fungi and prokaryotic cells. They work like a bulwark or a pressure vessel, preventing over-expansion when water enters the cell. Cell walls are absent in animals and protozoa.

• Major components of the cell wall in plants are Cellulose, hemicelluloses and pectin. In the industrial uses, the cellulose is mainly obtained from wood pulp and cotton and used to produce the textiles and paper.
• Cell walls of Fungi are made of Chitin. Chitin is the same substance that makes the exoskeleton of arthropods (insects etc.)
• The cell walls of diatoms are composed of silicic acid.
• The Bacterial cell walls are made of peptidoglycan which is also called murein.

Nucleus

Nucleus is the master of a cell. It controls the cell functions such as metabolism, reproduction and development. It consists of Nuclear membrane, Nucleoplasm, Nucleolus and Chromatin. Kindly note that Mammalian red blood cells have no nucleus.

Nuclear Membrane

The nuclear membrane is a double membrane and the space between the two membranes is called pronuclear space. The outer membrane is continuous with the endoplasmic reticulum which indicates its firm position in the cell. During the cell division the membrane disintegrates and reappears once the division is almost complete.

Nucleoplasm

Nucleoplasm is a transparent and gel like matrix. It contains the nucleolus, chromatin threads and Ribosomes.

Nucleolus

Nucleolus also disappears in the later phase of cell division and reappears once the process is almost complete. It is made of RNA and protein and is the site of RNA synthesis.

Chromatin

Chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules attached to nuclear proteins called histones. Each DNA filament is a double helix of DNA and therefore a chromosome.

The Cytoplasm

Part of a cell that is enclosed within the cell membrane except the nucleus is cytoplasm. Cytoplasm contains organelles, such as mitochondria, Golgi bodies, Endoplasmic reticulum, Plastids etc. Cytoplasm is the site where most cellular activities occur, such as metabolism, glycolysis, cell division, protein synthesis etc.  It is divided into two parts, the inner, granular mass is called the endoplasm and the outer, clear and glassy layer is called the cell cortex or the ectoplasm. The cell membrane is the outermost layer of the cytoplasm.

Major Cell Organelles

There are two kinds of organelles in the cytoplasm viz. living and non living. The living organelles include the Plastids, Mitochondria, Endoplasmic reticulum, Golgi Bodies, Ribosome, lysosomes, Micro bodies such as peroxisomes, Microtubules, Centrosomes, Cilia and Flagella. The nonliving substances, called ergastic substances include the reserve products such as carbohydrates Fats, Oils and nitrogenous substances, Secretary products such as pigments, enzymes and nectar and execratory products such as tannins, resins, latex, alkaloids, essential oils, mineral crystals etc.

Plastids

Plastids are major organelles found in the cells of plants and algae. The term plastid was used by Schimper for the first time. Major function of the plastids includes photosynthesis, storage of products like starch. They are of 3 types:

• Leucoplasts: Colorless plastids,
• Chloroplasts: Green plastids.
• Chromoplasts: Colored plastids.

The plastids are of various shapes and have the ability interchange between the above forms & and many shapes. For example due to continuous absence of the sunlight the green chloroplast may turn to colourless leucoplasts.  In tomato, when it ripes, the chloroplasts change into Chromoplasts and this turns the color of tomato from green to red.

The leucoplasts don’t have any color. So they have no role in photosynthesis. Their major function is of storing. On the basis of the stored material they have been divided into 3 types viz. Amyloplats (which store the carbohydrates), Elaioplasts (which store the fats) and Aleuroplats (which store proteins).

Chloroplasts have a green pigment in them called Chlorophyll. They are responsible for photosynthesis. The number, shape and size of the chloroplasts vary from plants to plants. In higher plants they are biconvex in shape.

Each chloroplast is covered by a double membrane envelope. This envelope is made up of lipoproteins. The space between these two membranes is called periplastidial space. Inside these membranes are located the membrane-bound compartment called thylakoid which is basically a sac. This sac has stacks of disks referred to as “grana”, (singular: granum). Each grana is connected to other grana by intergrana or stroma thylakoid. The space enclosed by a thylakoid is called lumen. All lumens are collectively called thylakoid space. Each chloroplast has 40-60 grana. The inner side of the thylakoid membrane has some particles which are called quantasomes. Each quantasome has around 230-250 chlorophyll pigments.

Why Chlorophyll is green?

The chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion. But it is a poor absorber of green and near-green portions of the spectrum, hence the color of the tissues which contain chlorophyll is Green. The chlorophyll was first isolated by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.

What are Carotenoids and how they are related to Vitamin A?

There are two types of pigments Chlorophyll a and Chlorophyll b. Apart from these pigments, there are Carotenoids occurring in the chloroplasts and Chromoplasts. These Carotenoids are responsible for different colours. There are more than 600 known Carotenoids. Among them the most important are carotenes and Xanthophylls.  Carotenes are pure hydrocarbons, means they are basically made up of Carbon and Hydrogen. The Xanthophylls have oxygen too.

The Carotenoids absorb blue light of the spectrum generally.

Absorption of blue light serves a major purpose and that is they save the chloroplasts from the photo damage.

• Most fruits have Carotenoids. The Beta carotene is one example which gets converted into Vitamin A.
• Beta carotene is the precursor of Vitamin A.
• Vitamin A occurs in many forms. One form of Vitamin A is retinal, which is vitamin A aldehyde. The four kinds of Carotenoids viz. beta-carotene, alpha-carotene, gamma-carotene, and beta-cryptoxanthin can be converted in human beings in retinal.
• This retinal form of Vitamin A is a Chromophore and is responsible for its color, it absorbs certain wavelengths of visible light and transmits or reflects others.
• Retinal binds to some proteins called Opsins in the Eye’s retina. This Vitamin A + Opsins bond is the chemical basis of vision.

The Carotenoids also get converted to another type of Vitamin A called Retinol. Retinol is fat-soluble vitamin important in vision and bone growth. All Retinol, retinal (aldehyde form), retinoic acid (acid form) and retinyl esters (ester forms) are converted from the carotenes and thus important for Human vision.

Mitochondria

Mitochondria (singular: mitochondrion) are the power houses of the cells. They were discovered by Fleming; however the term was used by Benda & Meeves. Another name for mitochondria is Chondriosomes. They are absent in Prokaryotic cells.

Since they are the “Power houses of the Cells” the number of mitochondria in cells is directly proportional to the metabolic activity of the cells. This means that the more active a cells is metabolically, more is the number of mitochondria in that cell. This is the reason that number of mitochondria is maximum in muscular cells.

The shape of the mitochondria may be spherical, filamentous or even rod shaped. Like the chloroplasts, they are also bound by double unit membranes.  The space between these two membranes is called perimitochondrial space. The liquid inside these membranes is called matrix. The matrix contains the enzymes. Apart from the enzymes matrix contains ribosomes, double stranded DNA and RNA.

Due to presence of double stranded DNA along with the RNA and Ribosome, the mitochondria are called semiautonomous structures. Both chloroplasts and mitochondria are semiautonomous structures.

Role of Mitochondria in Krebs cycle

Mitochondria are the sites of oxidation of food material. This oxidation is called aerobic respiration. It is carried out by Krebs cycle or TCA cycle. The Krebs cycle is also known as Citric Acid Cycle and is basically a series of enzyme-catalyzed chemical reactions.  The raw material in the Krebs cycle is carbohydrates, fats and proteins and the final products are Carbon Dioxide and Water and Energy.  The usable energy which is produced by the Krebs cycle is in the form of ATP which is Adenosine triphosphate.  The correct name of ATP is Adenosine-5′-triphosphate.

Endoplasmic Reticulum

The interconnected network of tubules, vesicles, and cisternae within cells is called “Endoplasmic reticulum”. The term was coined by Keith R. Porter in 1945. The tubules are narrow long structures, vesicles are round structures and cisternae are long, flat unbranched structures which are parallel to each   other. They are of two types, Rough endoplasmic reticulum (appears rough because it has ribosomes on it) which synthesize proteins and the smooth endoplasmic reticulum which synthesize lipids and steroids, metabolize carbohydrates and steroids, and regulate calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins.

Another function of the endoplasmic reticulum is that it provides the mechanical support to the cytoplasm and provides larger surface area for exchange of materials and transportation.

During the cell division, the endoplasmic reticulum organizes the nuclear envelope at the telophase stage of cell division.

Golgi Apparatus

These are named after Camillo Golgi who identified them in 1898. The size of the Golgi body changes as per the metabolic activity of the cells and they are bigger in young cells and metabolically active cells. Function of the Golgi apparatus is to process and package proteins, polysaccharides and lipids. During the cell division they provide a cell plate. At the end of the cell division (telophase) the Golgi vesicles fuse and make the new plasma membrane.  The Lysosomes which digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria etc. are formed by the Golgi body. Golgi Bodies, unlike the Chloroplasts and Mitochondria are bound by the single membranes.

Lysosomes

Lysosomes are very small sacs with irregular shapes. These are bags of Hydrolytic or digestive enzymes and so also called Suicide Bags. The major function is the autolysis of a cell by release of the enzymes within the cells. It also helps in the intracellular digestion of dead, injured or defective cells. Intracellular digestion of the material taken from the endocytosis.

Ribosome

Ribosomes were discovered by Palade in 1955. They are not enclosed by any unit membrane. They are made up of RNA and proteins.

Peroxisomes

These are also sac like structures bound with single membranes. They have enzymes and take part in the metabolism of fatty acids, respiration and many other metabolic processes.

Glyoxisomes

They are mainly found in plants particularly in plants the fat storage tissues of germinating seeds such as castor seed. The major function is in the conversion of the fatty acids in Carbohydrates.

Spherosomes

Spherosomes are present in the endosperm and cotyledons of seeds. They have the enzymes which are necessary in synthesis of oils and fats.

Centrioles

Centrioles are present in animal cells mostly and not in higher plants. They organize the spindle fibers in cell division.

Cilia and Flagella

Both Cilia and Flagella are present in the motile cells. Both help in cell mobility. Both are made up of fibrils. When they cut in a section, they show 9+2 arrangement which shows that they have 9 pairs of fibrils on the circumference and 2 pairs of fibrils at the centre.

Prokaryotic and Eukaryotic cells

There are two groups of cells. All cells are either prokaryotic or eukaryotic. Prokaryotic cells are primitive and don’t possess a well defined nucleus. Eukaryotic cells have a nucleus.

In prokaryotic cells DNA material remains scattered in the Cytoplasm only. Further, same compartment is used in the Prokaryotic cells for synthesis of RNA and protein while in the Eukaryotic cells the RNA is synthesized in the Nucleus while the protein in the cytoplasm. There is no sexual reproduction in Prokaryotic cells and only genetic recombination is present in the name of sexual reproduction while in eukaryotic cells, the true sexual reproduction is present.

Difference between Plant cells and Animal cells

The animal cells don’t contain the cell wall and the outer boundary of the animal cells is cell membrane. In Plant cells the cell wall is present which is made up of mostly cellulose, is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism.

In bacteria the cell wall is made of peptidoglycan. There are no plastids in animal cells. There is no photosynthesis in animal cells. Cytokinesis which is a process by which cytoplasm of a single eukaryotic cell is divided to form two daughter cells, is by equatorial furrowing from periphery to the centre in animal cells and by disk formation in plant cells.

In animal cells the ribosome are of 55S and 80S types while in the plant cells they are of 70s and 80S types.

Cell Division

The cell division is of two types viz. Mitosis and Meiosis.

Mitosis

In mitosis the mother cell divides into two daughter cells which are genetically identical to each other and to the parent cell. In mitosis:

• The number of the Chromosomes in Parent and daughter cells remains constant
• The parent and daughter cells are similar in all respects.
• The parent and daughter cells are genetically identical
• The purpose of Mitosis is growth by increasing number of cells.
• In most plants and animals the regeneration of the lost parts and vegetative propagation in some plant species takes place by Mitosis.

Meisis

In Meiosis, the number of chromosomes is divided into half in this process.  Meiosis is required to create the Gametes in animals and Spores in other organisms. Meiosis is a prerequisite for sexual reproduction in organisms with Eukaryotic cells.

Significance of Meiosis

The cell division in the reproductive cells takes place by Meiosis. In meiosis the number of the chromosomes is reduced to half of that in the parent cells.  Meiosis maintains the number of Chromosomes constant in all sexually reproducing organisms.

Programmed cell death

Apoptosis, or programmed cell death is a process by which cells deliberately destroy themselves. The process follows a sequence of events controlled by nuclear genes. In this process, the chromosomal DNA breaks into fragments, and this is followed by breakdown of the nucleus. The cell then shrinks and breaks up into vesicles that are phagocytosed by macrophages and neighbouring cells.

Significance of Apoptosis

Apoptosis plays an important role in maintaining the life and health of organisms. During human embryonic development apoptosis removes the webbing between the fingers and toes; it is also vital to the development and organization of both the immune and nervous systems.

How cells become Cancerous?

Cancer is caused by the unrestrained growth of cells. Cells that do not “follow the rules” of normal cell cycling may eventually become cancerous. This means that the cells reproduce more often than normal, creating tumors. Usually this happens over an extended period of time and begins with changes at the molecular level. Our body has trillions of cells and all cells replicate in normal fashion. However, some agents may change the way genes carry the information regarding the cell division and thus cells become cancerous. Such genes are called Oncogenes and such agents are called Carcinogens.

In normal cells, there are have types of genes that are important in determining whether or not cancerous tissue can form. These genes control the production of proteins that affect the cell cycle. Proto-oncogenes are DNA sequences that promote normal cell division. By mutation, these genes may be converted into oncogenes, which promote the overproduction of cells. Another class of genes, known as tumor-suppressor genes prevents excess reproduction of cells. Mutation in these genes can also allow cells to become cancerous.

How Cyanide kills cells?

Cyanide acts by inhibiting the enzymes cells need for oxygen utilization. Without these enzymes, a cell cannot produce ATP and will die. Very small amounts of cyanide naturally occur in some foods and plants. For example. cyanide is present in cigarettes and in the smoke produced by burning plastics.

How carbon monoxide kills people using heating appliances using fossil Fuels?

Because of its molecular similarity to oxygen, haemoglobin can bind to carbon monoxide instead of oxygen, and this subsequently disrupts haemoglobin’s efficiency as an oxygen carrier. Carbon monoxide in fact has a much greater affinity (about 300 times more!) for haemoglobin than oxygen. When carbon monoxide replaces oxygen, this causes cell respiration to stop, leading to death. The particular danger of carbon monoxide poisoning lies in the fact that a person exposed to high levels of this toxin cannot be saved by being transporting to an environment free of the poison and rich with oxygen. Since the haemoglobin remains blocked, artificial respiration with over pressurized pure oxygen must first be performed to return the haemoglobin to its original function and the body to normal cell respiration.

What is impact of Coffee on Cellular level?

Caffeine affects cells by stimulating lipid metabolism and slowing the use of glycogen as an energy source. As a whole, the body responds to caffeine by extending endurance, allowing us to stay awake for longer periods of time or perform extra activities. Adverse effects of excess caffeine intake include stomach upset, headaches, irritability, and diarrhoea.