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Cell Structure And Function

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Cell Structure and Function PDF Notes, Important Questions and Formulas

 

Cell Structure and Function 

INTRODUCTION

All living organisms are made up of the same elements and compounds. If we perform an analysis of a plant tissue, animal tissue or a microbial paste, carbon, hydrogen, oxygen and several other elements are obtained. The same analysis made on a non-living matter like a piece of earth's crust, gives a list of similar chemicals. A close examination reveals that the relative abundance of carbon and hydrogen with respect to other elements is higher in living beings than in earth's crust.

2.     HOW TO ANALYSE CHEMICAL COMPOSITION

  • Various present in a living tissue (like a vegetable or a piece of liver) can be studied by their chemical analysis.
  • Take a living tissue and grind it in trichloroacetic acid (CI3CCOOH) using a mortar and pestle. We obtain thick slurry.
  • When we strain this slurry through cheese cloth or cotton, it gives two fractions.
  • One is called filtrate or acid soluble pool having thousands of organic compounds.
  • Other fraction is called retentate or acid insoluble pool containing proteins, nucleic acid, polysaccharides etc.
  • The acid soluble pool contains chemicals with small molecular mass of 18-800 daltons approximately. They are called micromolecules or biomicromolecules. They include amino acids, sugars, nucleotides etc.
  • The acid-insoluble fraction contains organic compounds that have molecular weights in the range of ten thousand daltons and above. They are known as macromolecules or biomacromolecules. They include polysaccharides, proteins, and nucleic acids.
  • Lipids are not strictly macromolecules. their molecular weight do not exceed 800 Da, but they come under the macromolecular fraction because when we grind a tissue, cell membrane and other membranes are broken into pieces and form vesicles which are not water soluble (lipids are also present in structures like cell membrane and other membranes).
  • The acid-soluble fraction represents roughly the cytoplasmic composition (without organelles), while the acid insoluble fraction represents the macromolecules of the cytoplasm and cell organelles. The two fractions together represent the entire chemical composition of living tissues or organisms.

Bimolecular: All the carbon compound that present in living tissue.

Special note: Lipids are micromolecules but obtained under macromolecular fraction due to their insoluble nature in aqueous medium of a cell.

Table: Average composition of cells

Component

% of the total cellular mass

Water

70-90

Proteins

10-15

Carbohydrates

3

Lipids

2

Nucleic acids

5-7

Ions

1


Analytical techniques, when applied to the compound give us an idea of molecular formula and the probable structure of the compound.

  • All carbon compounds that we get from living tissue can be called -Biomolecules.
  • Inorganic elements and compounds are also present in the living organisms which can be known with the help of 'ash' analysis technique
  • A small amount of a living tissue (e .g. Leaf or liver and this is called wet weight is weighed and dried. All the water evaporates.
  • When the tissue is fully burnt, the carbon compounds are oxidised to gaseous form like CO2, water vapour are removed and the remnant is called 'ash'. This ash contains many inorganic elements like calcium, magnesium etc.
  • In the acid-soluble fraction -inorganic compounds like sulphates, phosphates etc are also present.
  • Elemental analysis gives composition of living tissue in the form of O. C. H, N etc.
  • Analysis of compounds gives an idea of the kind of organic and inorganic constituents as mentioned in the table.

Table: A comparison of elements present in non-living and living matter

Elements                                               % weight of

   

 

Earth’s crust

Human body

Hydrogen (H)

0.14

10.5

Carbon (C)

0.03

18.5

Oxygen (O)

46.6

65.0

Nitrogen (N)

Very little

3.3

Sulphur (S)

0.03

0.3

Sodium (Na)

2.8

0.2

Calcium (Ca)

3.6

1.5

Magnesium (Mg)

2.1

0.1

Silicon (Si)

27.7

Negligible

 3.     COMPOUNDS OF PROTOPLSM

Although some elements occur in protoplasm as free ions but mostly two or more elopements are variously combined to form different kinds of compounds.

Inorganic compounds:

  1. Water                                                       = 70-90%
  2. Salts, acids, bases, gases                           =1-3%

Organic Compounds

  1. Proteins                                                   =7-14%
  2. Lipids                                                       =1-3%
  3. Carbohydrates                                          =1-2%
  4. Nucleic acids, enzymes and other               =1-3%

 

CELL CYCLE AND CELL DIVISION

INTRODUCTION

Rudolf Virchow proposed the cell lineage theory. Cell lineage theory states "monies cellula e- cellula" i.e. new cells arise from pre-existing cells. Rudolf Virchow failed to prove the theory.

Karl Nageli: - New cells arise by division of pre-existing cells.

Starsburger: - New Nuclei arise by division of pre-existing nuclei.

Cell division: – 3 main types.

  1. Mitosis
  2. Meiosis
  3. Amitosis

MITOSIS

Mitosis name proposed by Flemming & its detail study was given by A. Schneider. Mitosis produced genetically identical cells, which are similar to mother cell.

Cause of mitosis:-

  1. Kern plasm theory: Hertwig proposed kern plasm theory. According to this theory mitosis is due to disturbance in Karyoplasmic Index (KI) of cell.
  2. Surface-volume Ratio :
  1. Surface-volume ratio of a cell plays an important role in starting cell division.
  2. A cell draws all the materials needed for its maintenance & growth from its surface. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes.
  3. Insufficient to draw the material. At such critical stage, division of cell started. 

Cell Cycle

  • Complete life cycle of a cell is called as cell cycle. A typical eukaryotic cell is illustrated by human cells in culture. These cells divide once in approximately every 24 hours.
  • Yeast can progress through the cell cycle in only about 90 minutes. The time period of cell cycle is varied from organist to organism.

 

Cell cycle involves two stages.

  1. Interphase
  2. Division phase/M-phase

Interphase: - This is middle stage of cell cycle, because there occurs one interphase between two division phase. In interphase cell grows in size and prepares itself for next division. Interphase is most active phase of cell cycle.

  • In interphase, metabolism of cell increases. A series of metabolic changes occurs during interphase in cell. These changes are not visible under microscope, so some scientist termed interphase as resting phase. But now it's known that it is the most active phase of cell cycle.
  • Howard and Pelc classified interphase into three sub stages :-

 

(a) G1 – phase or Pre DNA synthesis phase (Ist  Gap phase) (Longest phase of cell cycle (12 hr))

During G1-number of cell organelles increases in cell and cell rapidly synthesizes different types of RNA and proteins. Due to availability of protein, synthesis of new protoplasm takes place in cell and it starts growing in size. Cell grows maximum in G1 stage.

 

(b)S – phase (DNA Synthesis phase (6 – 10 hr)) :

  • Replication of nuclear DNA and synthesis of histone protein takes place in s-phase. Replication of cytoplasmic DNA may occur in any stage of cell cycle. Centriole replicates in late s-phase.

 

(c) G2 – phase (2nd Gap phase) or Post DNA synthesis phase (3 –12 hrs): (Pre mitosis phase)

  • Number of cell organelles increases in cell. Actual preparation (Final preparation) of cell division occurs during this phase. Special materials required for cell division are synthesized in G2 phase. eg. Tubulin protein. (Required for formation of spindle fibres).
  • Cell division involves enormous expenditure of energy thus cell stores ATP in G2 phase.
  • After G2 phase cell enters in division or M–phase.

 

Regulation of Cell Cycle

Cell cycle is running by a group of special proteins "Cyclins and Cdks (MPF). (Nurse, T.Hunt & Hartwell 2001 studies on saccharomyces (Baker yeast))

  • A cell reproduces by performing an orderly set sequences of irreversible events, in which it duplicates its contents & then divides into two, these events are known as cell cycle.
  • Molecular biologists, have made remarkable progress in identifying the biomolecules, that control or drive the cell cycle, many biologists, some of whom worked with invertebrate or frog egg's others with yeast cell or cell culture. Scientists concluded that the activity of enzymes, known as cyclin dependant kinases. (Cdk's) regulates the cell cycle. Kinase is an enzyme that removes a phosphate group from ATP & add to another protein. The kinases involved in the cell cycle are called Cdks because they are activated when they combined with key protein called cyclin.

CELL: THE UNIT OF LIFE

INTRODUCTION -

All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms. While others, like us, composed of many cells, are called multicellular organisms. Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life.

Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms. First cell discovered by -Robert Hook in Cork

Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus. The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

 2.  CELL THEORY

In 1838, Matthias Schleiden, a german botanist, examined a large number of plants and obseved that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, Theodore. Schwann (1839), British zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the 'plasma membrane'. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.

Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.

Cell theory as understood today is:

  1. All living organisms are composed of cells and products of cells.
  2. All cells arise from pre-existing cells.

3.  AN OVERVIEW OF CELL

The onion cell which is a typical plant cell, has a distinct cell wall as its outer boundary and just within it is the cell membrane. Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell.

The cytoplasm is the main arena (zone) of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the 'living state'. Besides the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes. Mitochondria, microbodies. The prokaryotic cells lack such membrane bound organelles. Ribosomes are non-membrane bound organelles found in all cells -both eukaryotic as well as prokaryotic cell. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles - chloroplasts (in plants) and mitochondria and on rough ER.

 

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