THE CELL Click for (Audio file)
Notes Developed by :-Guddu Bajpai
INTRODUCTION *ALL THE LIVING BEINGS ARE MADE UP OF LIVING CELLS CAPABLE OF PERFORMING ALL SORTS OF ACTIVITY THAT AN OGANISM CAN FERFORM SUCH AS REPRODUCTION, RESPIRATION, EXCRETION ETC. *THEY ARE THE BUILDING BLOCKS OUR BODY, SOME ORGANISMS LIKE Amoeba ARE MADE UP OF SINGLE CELL (UNICELLULAR) WHILE OTHERS MADE UP OF MANY CELLS (MULTICELLULAR). *THE WORD CELL COMES FROM LATIN Cellula MEANING, A SMALL ROOM AND CELL WORD WAS COINED BY ROBERT HOOKE IN 1665. *LIVING CELLS VARY GREATLY IN SHAPE & SIZE THAT DEPEND ON THEIR SPECIFIC FUNCTION, GENERALLY CELLS ARE ROUGHLY ROUND, CUBOIDAL, POLYGONAL, THREAD LIKE etc. GENERALLY THEIR SIZE VARY BETWEEN 1micrometer TO 10icrometer EXCEPT OSTRICH EGG (the largest cell in living world). *THE CELLS ARE OF TWO TYPES PRIMITIVE SIMPLE TERMED AS PROKARYOTES (unicellular organisms) AND DEVELOPED COMPLEX ONES CALLED AS EUKARYOTES (multicellular organisms). *A TYPICAL CELL IS BASICALLY DIVIDED INTO 3 PARTS PLASMA/CELL MEMBRANE, CYTOPLASM, AND CENTRALY LOCATED NUCLEUS, ALL THE LIVING MATTER INSIDE THE CELL CALLED AS PROTOPLASM (cytoplasm + nucleoplasm), IT IS SEMIFLUID, TRANSPARENT & RESPONSIVE IN NATURE. CENTRAL NUCLEUS IS SURROUNDED BY NUCLEAR MEMBRANE SO THE PART BETWEEN NUCLEAR MEMBRANE & CELL MEMBRANE IS REGARDED AS CYTOPLASM. -->
PARTS OF A TYPICAL CELL
0-CYTOSKELETON
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1-CELL/PLASMA MEMBRANE
FUNCTIONS – 1. To provide shape & rigidity. 2.- Regulate inflow & outflow of substances as per requirement of the cell. 3.-Protection from physical & chemical shocks.
2-CELL WALL
CAPSULE
FLAGILLA/CILLIA
FIMBRIAE (PILI)
3- CYTOPLASM
FUNCTION—Cytoplasm is the centre of all chemical reactions occurring inside the cell required for life process.
4-PROTOPLASM
5-NUCLEUS
Function—Nucleus is the boss of the cell, it controls all the activity of cell. It also protects genetic material and plays important role in protein biosynthesis.
1-MITOCHONDRIA (POWER HOUSE in Eukaryotes) –
Mitochondria are self-replicating organelles that occur in various numbers, shapes(roughly rectangular), and sizes in the cytoplasm of all eukaryotic cells. Mitochondria generate the cell's energy by the process of oxidative phosphorylation, utilizing oxygen to release energy stored in cellular nutrient glucose, in the form of phosphorous rich molecules called ADENOSINE TRI-PHOSPHATE (ATP).
Mitochondria is the main site of aerobic cellular respiration, it is a double layered membranous structure whose inner layer is folded inside forming finger like projections called CRISTAE while the remaining dense jelly like part is called matrix which contains required enzymes for Krebs cycle. Each mitochondrion contains DNA, RNA, ribosomes and granules. Cristae surface bears numerous cup-shaped ELEMENTARY/F-1 particles (Oxysomes) filled with required enzymes for energy production.
FUNCTION—It is the main site for cellular respiration.
2-PLASTIDS/CHLOROPLAST (food factory in plant cell only)----
A plant cell contains many round shaped structures covered by double layered membrane floating in cytoplasm called plastids which are further classified based on their functions as follows:-
1-Leucoplast (colorless)—These are the colourless plastids found in fruits & vegetables, its main function is to store proteins, starch & lipids.
2-Chromoplast (other than green)--These pigments are of any color other then green and can be categorised as Xanthophylls (Yellow group colors), Carotene (Orange group) and Anthocyanins(violet & purple group). They imparts variety of colors to flowers and fruits.
3-Chloroplast (green)-- The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space.
A typical parenchyma cell contains about 10 to 100 chloroplasts. The material within the chloroplast is called the stroma, corresponding to the cytosol of the original bacterium, and contains one or more molecules of small circular DNA. Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of photosynthesis, the thylakoids are arranged in stacks called GRANA. Photosynthetic light reaction occurs in grana part whereas dark reaction occurs in stroma part of chloroplast.
Plant appears green because of a green pigment present inside chloroplast called chlorophyll, the only chemical on earth able to convert solar energy into chemical energy
FUNCTION—Kitchen of plant, chloroplast are the site of PHOTOSYNTHESIS here trapped solar energy is used to form starch with the help of carbon dioxide & water though a series of chemical reaction that occurs in two parts—LIGHT reaction & DARK reaction.
Photosynthesis Chemical Reaction
6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2
3-ENDOPLASMIC RETICULUM (only in Eukaryotes)—
The endoplasmic reticulum (ER) is the double membranous transport network of sacs spread throughout the cytoplasm connecting nuclear membrane to cell membrane that manufactures, processes, and transports chemical molecules targeted for certain modifications and specific destinations for use inside & outside of the cell, as compared to molecules that floats freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface to secretes proteins into the cytoplasm, and the smooth ER lacking ribosomes, situated on either side of nucleus.
FUNCTION—To communicate chemically between nucleus and extracellular fluid (ECF) directly.
5-RIBOSOME (protein factory)— Present in all living cells, ribosome is a single walled dense spherical body contain large complex of RNA (60%) and protein (40%) molecules. They each consist of two subunits, and act as main site for protein synthesis where RNA from the nucleus is used to synthesize proteins from amino acids. Ribosome are found either floating freely or bound to a membrane (the rough endoplasmic reticulum in eukaryotes, or the cell membrane in prokaryotes).
FUNCTION—They are the site of protien biosynthesis.
6-LYSOSOME— Discovered by Christiande Duve (1955), These are round 0.4-0.8 microM surrounded by single memebrane enclosing dense matrixcontaining variety of Enzymes such as Nucleases, phosphatases, proteases, peptriases,glycosides lipases etc. They originate directly from ER or golgi complex.
FUNCTION—They are called Suisidal bags as they contain hydrolytic enzymes to distroy the cell (Autophagic –self cell death). They are are the cells' garbage disposal system and distroys foreign particles by Phagocytosis (process to kill micro-organisms by engulfing them).
7-CENTROSOME (only in animal cell)— The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
FUNCTION—They play key role in cell division.
8-VACOULES--Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane TONOPLAST. Some cells, most notably Amoeba, have contractile vacuoles, which are able to pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usually larger in those of plants than animals.
FUNCTION—They secretes various substances required by cell for life process.
NOTE--Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, thus strongly supporting the evolutionary theory of endosymbiosis; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.
ENERGY PRODUCTION IN CELL
Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically-complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways.
1-The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.
2-The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
CELLULAR RESPIRATION
Cellular respiration is the series of reactions that make adenosine triphosphate (ATP) by completely breaking down glucose into inorganic molecules of carbon dioxide and water. These reactions happen in several stages and include:
* glycolysis
* synthesis of Acetyl-CoA
* Krebs cycle
* the electron transport chain
Glycolysis
The first step, glycolysis, occurs in cytoplasm of most cells, and the word itself describes the process—‘glyco’ = sugar and ‘lysis’ = breaking down. Glycolysis involves the splitting of a six-carbon glucose into two three-carbon molecules of pyruvic acid, and results in a net production of two molecules of ATP.
Synthesis of Acetyl-CoA
Pyruvic acid is then transformed into the molecule acetyl-CoA. This is one of the cellular respiration reactions that produces CO2, the gas that we breathe out when we exhale. In addition to acetyl-CoA and CO2 waste, two molecules of the electron carrier NADH are produced. The energy of electron carriers will be used later, during electron transport.
Kreb's Cycle
Also known as the Citric Acid Cycle, this complex series of reactions transfers much of the energy left in the bonds of acetyl-CoA to more electron carriers (NAD+ and FAD). The reactions of Krebs Cycle occur in the mitochondria of eukaryotes and result in two more molecules of ATP, two molecules of FADH2, six molecules of NADH, and more CO2 waste.
Electron Transport Chain
The most significant production of ATP occurs through a stepwise release of energy from the series of oxidation-reduction reactions in the electron transport chain.
CELL DIVISION
* glycolysis
* synthesis of Acetyl-CoA
* Krebs cycle
* the electron transport chain
Glycolysis
The first step, glycolysis, occurs in cytoplasm of most cells, and the word itself describes the process—‘glyco’ = sugar and ‘lysis’ = breaking down. Glycolysis involves the splitting of a six-carbon glucose into two three-carbon molecules of pyruvic acid, and results in a net production of two molecules of ATP.
Synthesis of Acetyl-CoA
Pyruvic acid is then transformed into the molecule acetyl-CoA. This is one of the cellular respiration reactions that produces CO2, the gas that we breathe out when we exhale. In addition to acetyl-CoA and CO2 waste, two molecules of the electron carrier NADH are produced. The energy of electron carriers will be used later, during electron transport.
Kreb's Cycle
Also known as the Citric Acid Cycle, this complex series of reactions transfers much of the energy left in the bonds of acetyl-CoA to more electron carriers (NAD+ and FAD). The reactions of Krebs Cycle occur in the mitochondria of eukaryotes and result in two more molecules of ATP, two molecules of FADH2, six molecules of NADH, and more CO2 waste.
Electron Transport Chain
The most significant production of ATP occurs through a stepwise release of energy from the series of oxidation-reduction reactions in the electron transport chain.
CELL DIVISION
Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary fission. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization.
MITOSIS CELL DIVISION (For Growth)
During the cell cycle, somatic cells (non reproductive cells) of eukaryotic organisms grow and divide. It is the process of a single cell (‘parent cell’) splitting into two identical ‘daughter cells’. The daughter cells are clones of the parent, and have the same number of chromosomes as does the parent cell.
There are two major phases to the cell cycle:
- Interphase (3 substages)
- Mitosis (4 substages)
Interphase
During interphase, the cell is not dividing, but is going about the everyday business of being a cell. The DNA is constantly being read and the genetic instructions translated into polypeptides, so the DNA exists in long strands called chromatin.
Interphase consists of 3 stages:
During interphase, the cell is not dividing, but is going about the everyday business of being a cell. The DNA is constantly being read and the genetic instructions translated into polypeptides, so the DNA exists in long strands called chromatin.
Interphase consists of 3 stages:
- G1 phase: cell grows in size
- S phase: DNA is copied (replicated) in preparation for cell division
- G2 phase: cell competes preparations for division (Absent in meosis)
Mitotic Cell Division consists of 2 major processes:
- Mitosis: Nuclear division (separation of the duplicated genetic material)
- Cytokinesis: cytoplasmic division (cell divides into two daughter cells)
Mitosis has 4 basic subphases:
- Prophase – Chromatin strands condense into chromosomes. The chromosome consists duplicated, condensed strands of DNA, each copy called a sister chromatid.
- Metaphase – Duplicated chromosomes align at the cell’s equatorial plane.
- Anaphase – Sister chromatids separate and migrate to opposite poles of the cell.
- Telophase – Chromosomes revert to their extended state (chromatin). Nuclear envelope reforms around each of the 2 groups of genetic material. Cytokinesis begins.
Cytokinesis
The cytoplasm and its contents are then divided by a process called cytokinesis. In animal cells a cleavage furrow forms that essentially pinches the cell into two daughter cells.
The cytoplasm and its contents are then divided by a process called cytokinesis. In animal cells a cleavage furrow forms that essentially pinches the cell into two daughter cells.
MEIOSIS (Reduction Division for Reproduction)
MEIOSIS-I----Meiosis I separates homologous chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division. The phase of meiosis are as follows-
Prophase-1
During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata THIS PHASE DIVIDED UNDER FOLLOWING HEADINGS
LEPTOTENE---The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads”, During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
ZYGOTENE----The zygotene stage, also known as zygonema, from Greek words meaning "paired threads", it occurs as the chromosomes approximately line up with each other into homologous chromosomes. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place.
PACHYTENE---The pachytene stage, also known as pachynema, from Greek words meaning "thick threads". Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. (The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.)
DIPLOTENE-- During the diplotene stage, also known as diplonema, from Greek words meaning "two threads",the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred.
DIAKINESIS--Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through".This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
METAPHASE-1
The homologous chromosomes align along an equatorial plane that bisects the spindle fibers joining centrioles located at opposites poles of cell.
ANAPHASE-1
Whole chromosomes are pulled toward opposing poles by shortening of spindle fibres, forming two haploid sets. Each chromosome still contains a pair of sister chromatids.
TELOPHASE-1
The last meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I.
MEIOSIS-II
Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis. The end result is production of four haploid cells (23 CHROMOSOMES=GAMETE)
Prophase II
In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
Metaphase II,
The centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
Anaphase II,
The centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
NOTE- The kinetochore (pronounced /kɪˈnɛtəkɔər/) is the protein structure on chromosomes where the spindle fibers attach during cell division to pull the chromosomes apart.
Telophase II
This phase similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.
SIGNIFICANCE OF MEIOSIS
1- Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.
2-Facilitates sexual reproduction, as it maintains total number of chromosomes in an organism.
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