UNIT – I
BASIC CELL BIOLOGY
Cell Structure and Functions
Cell:
Thecell is the basic structural, functional, and biological unit of all knownliving organisms. Cells are the smallest unit of life that canreplicateindependently, and are often called the "building blocks of life". The cell was first discovered by Robert Hooke in 1665.The cells are capable of surviving through self-regulatory mechanisms such as metabolism, reproducing and coping with the external environment. It also possesses the capabilities to move, respond to external and internal stimuli, grow and can develop to adjust to the environment in which they live in. The study of cells is calledcell biology. Cells consist ofcytoplasmenclosed within amembrane, which contains manybiomoleculessuch asproteinsandnucleic acids. Organisms can be classified asunicellular(consisting of a single cell; includingbacteria) ormulti-cellular(includingplantsand animals).
Cell theory:
The cell was discovered byRobert Hookein 1665. Cell theory was first developed in 1839 byMatthias Jakob SchleidenandTheodor Schwann. The cell theory is as follows,
1. Organisms are composed of one cell (Prokaryotes - Bacteria) or more cells (Eukaryotes – including plants, animals and humans).
2. Cells are the fundamental basic unit of structure and function in all living organisms.
3. All cells are arising from preexisting cells by division.
4. All cells contain thegenetic informationthat are necessary for regulating cell functions and for transmitting information to the next generation of cells.
Prokaryotic Cells:
Aprokaryoteis asingle-celledorganismthat lacks amembrane boundnucleus,mitochondria, or any other membrane-boundorganelle. In the prokaryotes all the intracellular water-soluble components (proteins,DNAandmetabolites) are located together in thecytoplasmenclosed by thecell membrane, rather than in separatecellular compartments. Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed as “nucleoid” in which a single chromosomal, circular, double-standard DNA molecule is located. Prokaryotes can be divided into twomajor domains,ArchaeaandBacteria.
Archaea:cell membrane contains ether linkages; cell wall lacks peptidoglycan; and have replaced with fatty acids of bacterial membranes with isoprene.
Bacteria:cell membrane contains ester bonds; cell wall made of peptidoglycan; have only one RNA polymerase; react to antibiotics in a different way than archea do.
There are four main structures shared by all prokaryotic cells, bacterial or Archaean:
ü The plasma membrane
ü Cytoplasm
ü Ribosomes
ü Genetic material (DNA and RNA)
Some prokaryotic cells also have other structures like thecell wall, pili(singular pillus), andflagella(singular flagellum). Each of these structures and cellular components plays a critical role in the growth, survival, and reproduction of prokaryotic cells.
Prokaryotic internal cell structure:
1. Plasma Membrane:
The cell membrane is a double-layer of phospholipidswith associatedproteinsand other molecules. It is essentially the “bag” that holds all of the intracellular material and regulates the movement of materials into and out of the cell. As in all cells, the plasma membrane in prokaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane (poor fellas) also aid prokaryotic cells in communicating with the surrounding environment. Among other things, this communication can include sending and receiving chemical signals from other bacteria and interacting with the cells of eukaryotic organisms during the process of infection.
2. Cytoplasm:
This is the gel-like fluid that the cell is filled with, inside the plasma membrane; liquid with all of the cellular organelles suspended within. It is very similar to the eukaryotic cytoplasm, except that it doesnotcontain organelles.
Cytoskeleton:
It's only recently been discovered that rod-shaped bacteria and Archaea possess cytoskeletal proteins that function in a similar way to the cytoskeleton of eukaryotic cells. This scaffolding provides structural support to the cell and plays a role incell division. The cytoskeleton helps prokaryotic cells divide and helps the cell maintain its round shape.
3. Ribosomes:
All cells, bothprokaryotic and eukaryotic, have multiple ribosomes within. Ribosomes are the tiny protein-making machines of the cell. The ribosomes found in prokaryotic cells are much smaller than the eukaryotic ribosomes. The the ribosomes in prokaryotic cells are 70S type (S - Svedberg units (sedimentation coefficients)). Prokaryotic ribosomesare smaller and have a slightly different shape and composition than those found in eukaryotic cells.
4. Genetic Material of Prokaryotes
Nucleoid:
The nucleoid is the region of the prokaryotic cytoplasm that contains the genome—the main genetic material (DNA) of the cell. Bacteria and Archaeans typically have a single, circular chromosome. All prokaryotic cells contain large quantities ofgenetic materialin the form ofDNA andRNA. Because prokaryotic cells, by definition, donothave a nucleus, the single large circular strand of DNA containing most of the genes needed for cell growth, survival, and reproduction is found in the cytoplasm.
The DNA tends to look like a mess of string in the middle of the cell:
Usually, the DNA is spread throughout the entire cell, where it is readily accessible to be transcribed intomessenger RNA (mRNA)that is immediately translated by ribosomesinto protein. Sometimes, when biologists prepare prokaryotic cells for viewing under a microscope, the DNA will condense in one part of the cell producing a darkened area called anucleoid.
Plasmids:
In addition to the bacterialchromosome, bacteria may also contain one or more plasmids. A plasmid is a non-essential circular piece ofDNAthat confers an advantage to the bacteria, such as antibiotic resistance, virulence (the ability to cause disease) andconjugation(a bacterium’s ability to share its plasmids with other bacteria).
(A) Prokaryotic external cell structure (outside of plasma membrane):
1. Cell Wall:
Nearly all prokaryotes have a protectivecell wallthat prevents them from bursting in a hypotonic environment (an aqueous environment with a lower concentration of solutes than are found within the cell).The composition of cell walls vary depending on the type of organisms, but most cell walls contain a combination of themajor organic molecules—proteins, carbohydratesandlipids. Bacteria have a unique molecule called peptidoglycan in their cell wall.But Archaean cell walls do not contain peptidoglycon molecule. Based on the cell wall composition, the bacteria are classified intoGram-positiveorGram-negative.
2. Glycocalyx:
Theglycocalyxis a layer present in some bacteria, and located outside of the cell wall. There are two types of glycocalyces: slime layers and capsules. Slime layers help bacteria stick to things and protect them from drying out, particularly in hypertonic environments. Capsules also allow bacteria to stick to host cell during infection, and also have the added benefit of helping encapsulated bacteria hide from the hosts immune system.
3. Cell Extensions:
There are several different types of cell extensions associated with bacteria, all a made of delicate protein strands. Bacterial cell extensions include:
3.1. Flagella:
Llong whip-like extensions that help bacteria move about the environment.
3.2. Axial filaments (endoflagella):
Flagella that are wrapped around corkscrew-shaped bacteria, and which move in waves that make the bacteria spin.
3.3. Fimbriae (Pili):
Fimbriae or pili allow the bacteria to adhere to target host cells, so play a major role in bacterial virulence.
Prokaryotic cell structure
Morphology of prokaryotic cells:
The prokaryotic bacteria have various shapes as follows,
Cocci– spherical Bacilli– rod-shaped
Spirochaete– spiral-shape Vibrio– comma-shaped
The prokaryotic archeal organism Haloquadratumhas flat square-shaped cells.
Archea - Haloquadratum
Comparison Between Prokaryotic and Eukaryotic CellsCharacteristic / Prokaryotes / Eukaryotes
Size of cell / Typically 0.2-2.0mm in diameter / Typically 10-100mm in diameter
Nucleus / No nuclear membrane or nucleoli (nucleoid) / True nucleus, consisting of nuclear membrane & nucleoli
Membrane-enclosed organelles / Absent / Present;
Eg - lysosomes, Golgi complex, endoplasmic reticulum, mitochondria & chloroplasts
Flagella / Consist of two protein building blocks / Complex; consist of multiple microtubules
Glycocalyx / Present as a capsule or slime layer / Present in some cells that lack a cell wall
Cell wall / Usually present; chemically complex (typical bacterial cell wall includes peptidoglycan) / When present, chemically simple
Plasma membrane / No carbohydrates and generally lacks sterols / Sterols and carbohydrates that serve as receptors present
Cytoplasm / No cytosketeton or cytoplasmic streaming / Cytoskeleton; cytoplasmic streaming
Ribosomes / Smaller size (70S) / Larger size (80S); smaller size (70S) in organelles
Chromosome (DNA) arrangement / Single circular chromosome; lacks histones / Multiple linear chromosomes with histones
Cell division / Binary fission / Mitosis
Sexual reproduction / No meiosis; transfer of DNA fragments only (conjugation) / Involves meiosis
Eukaryotic cell:
Eukaryotic cells contain a defined nucleus. The chromatin bodies are enclosed by a layer called nucleus membrane. In eukaryotes the other cell organelles are also present within the membrane. The presence of a nucleus gives eukaryotes as their name, which comes from theGreek(eu, "well") and(karyon, "nut" or "kernel").Eukaryotic cells also contain other membrane-boundorganellessuch as mitochondriaand theGolgi apparatus.
In addition,plantsandalgaecontainchloroplasts. Manyunicellular organismsare eukaryotes, such asprotozoa. Allmulti-cellular organismsare eukaryotes. Bothasexual reproductionthroughmitosisandsexual reproductionthroughmeiosisare found in eukaryoticcell division. In mitosis, one cell divides to produce two genetically identical cells. In meiosis onediploidcell (gamete) from each parent, undergoes recombination, and creating azygote. There are then two stages of cell division, resulting in fourhaploidcells. Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parentalchromosomes.
Eukaryotic cell
Examples of eukaryotes:
Animals
Plants
Fungi
Algae
Protozoa – unicellular
All eukaryotic cells have anucleus, genetic material,plasma membrane, ribosomes, cytoplasm and including thecytoskeleton. Eukaryotic cells also have other membrane-bound internal structures called organelles. It includes Mitochondria, Golgi bodies, Lysosomes, Endoplasmic reticulum and Vesicles.
The few major differences between animal, plant, fungal, and protistan cells as follows
Allplant cells have acell wallmade of cellulose and a large centralvacuole and Chloroplasts.
Ø Some animal and protistan cells have flagella, cilia. Allanimal cells have centrioles
Ø All fungal cells have acell wall whichmade of chitin.
The eukaryotic cell is formed by three main components. They are the following:
1. Plasma membrane
2. Cytoplasm
3. Nuclus
The cell is surrounded by a plasma membrane. It is filled with colloidal substances called cytoplasm. The cytoplasm contains a spherical body called nucleus. The cytoplasm and nucleus are together called as protoplasm. The protoplasm with its components constitutes the protoplast. In plant cells, there is a thick firm layer called cell wall around the plasma membrane.
I. Eukaryotic Plasma Membrane:
The plasma membrane in eukaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane help the cell communicate with the surrounding environment. Plasma membrane is made up of thin membrane (Phospolipids) and it gives a mechanical support with shape. Among other things, this communication can include Sending and receiving chemical signals from other eukaryotic cells. Interacting with the cells of prokaryotic organisms during the process of infection. The plasma membrane is universal to all cells, prokaryotic and eukaryotic.
Cell wall:
The animal cells have no cell wall. The plant cells have an outer covering of firm layer called cell wall. It is found exterior to plasma membrane. The cell wall is differentiated into three layers, namely primary wall, secondary wall and tertiary wall. It composed of cellulose, hemicelluloses and pectic substances. It provides the mechanical support to the cell. The cell wall of adjacent cells are cemented by middle lamella.
II. Cytoplasm:
The mass of protoplasm lying outside the nucleus is called the cytoplasm. The aqueous phase of cytoplasm excluding cell organelles is called as cytosol. It is colorless, homogenous, translucent and colloidal fluid. The cytoplasm is composed of various molecules such as water, salts, metals and organic compounds such as carbohydrates, lipids, proteins, nucleoproteins, nucleic acids and enzymes.
The peripheral part of cytosol is normally non-granular and clear and known as ectoplasm. The inner portion is granular and less viscous and known as endoplasm.
Cell
Plasma membrane Protoplasm
Cytoplasm Nucleus
Cytosol Organelles
Cell Organelles:
Cell organelles are organized structure of cytoplasm capable of grow and in some cases multiplication. They do specific functions. The following are the cell organelles:
1.Endoplasmic reticulum
2.Ribosomes
3.Mitochondria
4.Golgi
5.Lysosomes
6.Centrosome
7.Basal granules
8.Plastids
1. Endoplasmic Reticulum:
The cytoplasm contains a network of membrane bound tubules, vesicles and sac. It is called endoplasmic reticulum. It is also called the cytoskeleton. Endoplasmic reticulum is a hallow system. It is a continuous system connected on one side to the plasma membrane and other side with nuclear membrane. It functions as a circulatory system of the cytoplasm because it transports many substances.
There are two types of endoplasmic reticulum (ER) in eukaryotic cells: Smooth ER (SER) and Rough ER (RER). The rough type contains ribosomes attached on the surface. The smooth type does not contain ribosomes. The endoplasmic reticulum gives mechanical support to the cytoplasmic matrix and function as cytoskeleton. Synthesis of lipids, cholesterol, glycerides, glycogen occurs in it.
2. Eukaryotic Ribosomes:
Ribosomes are minute spherical structure. They either attached with the membrane of endoplasmic reticulum or scattered freely in the cytoplasm. Ribosomes are small cellular machines made of proteins and ribonucleic acid (RNA).
Eukaryotic ribosomes are larger and have a slightly different shape and composition than those found in prokaryotic cells. Each eukaryotic ribosome is composed of two sub-units, a smaller unit known as 40S and a large unit known as 60S. They play important role in protein synthesis because it translates the mRNA into protein. During protein synthesis, many ribosomes are linked together to form polyribosome.
3. Mitochondria:
Mitochondria are rod shaped or filamentous with double membrane bound structure occurring in the cytoplasm. The outer membrane is complete and forms a bag-like structure. The inner membrane gives out many finger-like projections into the matrix of mitochondria. These projections known as cristae. All cells need energy to grow, reproduce, and function. One of the most important types of cellular food is a molecule calledglucose, which is a type of sugar and a carbohydrate. Mitochondria contain many oxidative enzymes and co-enzymes. These enzymes carry out the oxidation of food substances and thus liberates the energy. Eukaryotic cells take in glucose through proteins that cross theplasma membraneand then transport it through thecytoskeletonto the mitochondriain the cytoplasm. The mitochondrion is often called the cell's powerhouse.