Title CELL BIOLOGY.pdf ✅

CELL BIOLOGY 139 Jr|11th NEET|BOTANY:VOL-II WHAT IS A CELL? 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 func- tions 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 Hooke 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. CELL THEORY In 1838, Matthias Schleiden, a botanist, examined a large number of plants and observed 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), 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: i) all living organisms are composed of cells and products of cells. ii) all cells arise from pre-existing cells. 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 or- Cell Biology
140 CELL BIOLOGY Jr|11th NEET|BOTANY:VOL-II ganelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, micro- bodies. 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. Animal cells contain another non-membrane bound organelle called centriole which helps in cell division. SIZE AND SHAPE OF CELL Size : Cell differ greatly in size, shape and activites. Mycoplasma (Smallest cells): Only 0.3 m in length [PPLO (pleuro Pneumonia like organisms) is an example of Mycoplasma having the size about 0.1 ( m)]. Bacteria = 3 to 5 m Largest isolated single cell = egg of an ostrich. Human red blood cell = 7.0  m in diameter, Nerve cell = longest cell Shape: The shape of the cell may vary with the function they perform. They may be disc-like, polygonal, columnar, cuboid, thread like or even irregular. Diagram showing different shapes of the cells PROKARYOTIC CELLS The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma or PPLO (Pleuro Pneu- monia Like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells. EUKARYOTIC CELLS The eukaryotes include all the protists, plants, animals and fungi. In eukaryotic cells there is an exten- sive compartmentalisation of cytoplasm through the presence of membrane bound organelles. Eukaryotic cells possess an organised nucleus with a nuclear envelope. In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes. All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in higher plant cells.
CELL BIOLOGY 141 Jr|11th NEET|BOTANY:VOL-II Plant cell Animal cell BIOMEMBRANES OR CELL-MEMBRANE Biochemical investigation clearly revealed that the cell membranes possess lipid, protein and carbo- hydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 percent protein and 40 per cent lipids
142 CELL BIOLOGY Jr|11th NEET|BOTANY:VOL-II Lipids = 40% (Phospholipid, Cholestrol, Glycolipids) Proteins = 58-59% (Arginine, Lysine rich) Carbohydrates = 1-2% It is a universal structure and structurally cell membrane of prokaryotes is similar to the eukaryotes. STRUCTURE OF BIOMEMBRANES : 1) Sandwitch or Trilamellar model : By Davson & Danielli (1935). According to this model, the plasma-membrane is made up of three layers in which a bimolecular layer of lipid is sandwitched between two single layers of proteins. According to this model each protein layer is 20 0 A thick and bilayer of phospholipid is 35 0 A thick. Thus total thickness is 75 (PLLP - structure, range 75-100 0 A ) Phospholipid molecule called as amphipathic molecule due to presence of two type of parts (hydrophillic head and hydrophobic tail). Hydrophilic head of the phospholipid binds with protein layer by hydrogen and ionic bonds. Hydrophobic tail of phospholipid are attached to each other by vanderwal force. Polar hyrophilic head Nonpolar hydrophobic tail Bio-m olecular la yer of p h osp h olipids (3 5 A) Pores (0.35nm) Protein layer (20 )0 A Protein layer (20 )0 A 2) Unit membrane model : By Robertson 1959. According to this model all the cellular and organeller membranes are structurally & functionally similar (difference in chemically & size). Both of the above models are rejected because they fails to explain the selective permeability of plasmalemma. The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane. 3) Fluid mosaic model : By Singer & Nicolson (1972) This is latest & most widely accepted model for the structure of plasmalemma. According to fluid mosaic model proteins are arranged in phospholipid layer as mosaic pattern. Thus membrane is termed as "protein iceberg in a sea of phospholipid" or "Gulab Jamun (protein) in a concenterated solution (phospholipid) of sugar". 1) Phospholipids: Phospholipid is the main component of cell membrane because it forms continuous structural frame of cell membrane. The studies showed that the cell membrane is composed of lipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.This ensures that the nonpolar tail of saturated and unsatur- ated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides (phospholipids). Phospholipid layer provides fluidity to plasma membrane because phospholipids are rich in unsatur- ated fatty acid which are liquid in nature. The Quasifluid nature of lipid enable lateral movement of protein with in the overall bilayer. This