WHAT IS A CELL?
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👉🏿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.Hense,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..jpeg)
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Matthias Jakob Schleiden
(1804-1881)
Nationality :- German
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 plants tissues,that the presence of cell wall is a unique character of the plant cells.
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Theodore Schwann
(1810-1882)
Nationality :- British
👉🏿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 understand 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 organelles like the endoplasmic reticulum (ER), the Golgicomplex,lysosome,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.
👉🏿Animal cells contain another non-membrane bound organelles called centriole which helps in cell division.
SIZE AND SHAPE OF Cell
Size :
• Cell differ greatly in size,shape and activities.
•Mycoplasma (Smallest cells) : Only 0.3 mm in length {PPLO(Pleuro Pneumonia like organisms) is an example of mycoplasma having the size about 0.1 mm.}
• Bacteria = 3 to 5 mm
• Largest isolated single cell = egg of an ostrich.
• Human red blood cell = 7.0 mm 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.
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Diagram showing different shapes of the cells
PROKARYOTIC CELLS
• The prokaryotic cells are represented by bacteria,blue-green algae, mycoplasma or PPLO(Pleuro Pneumonia like organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells.
EUKARYOTIC CELLS
• The prokaryotic cells include all the protists, plants, animals and fungai. In eukaryotic cells there is an extensive 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 cytoskeleton 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 almost all higher plants cells.
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BIOMEMBRANE OR CELL MEMBRANE
👉🏿 Biochemical investigation clearly revealed that the cell membranes possess lipid, protein and carbohydrates. The ratio of protein and lipid varies considerably different cell types. In human beings, the membrane of the erythrocytes has approximately 52 per cent protein and 40 per cent lipids.
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 BIOMEMBRANE
(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 20A° thick and bilayer of phospholipid is 35A° thick. Thus total thickness is 75A°(PPLO - structure, range 75-100A°)
👉🏿 Phospholipid molecule called as amphipathic molecule due to presence of two type of parts ( hydrophilic) 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.
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(2) Unit membrane model :
By Robertson (1959)
👉🏿According to this model all the cellular and organelles 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, specially 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. This membrane is termed as "protein iceberg in a sea of phospholipid "or "Gulab Jamun(protein) in a concentrated solution (phospholipid) of sugar".
(A) Phospholipid:
👉🏿 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 unsaturated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides (Phospholipid).
👉🏿 phospholipid layer provides fluidity to plasma membrane because phospholipids are rich in unsaturated fatty acid which are liquid in nature.
👉🏿The Quasifluid nature of lipid enable lateral movement of protein with in the overall bilayer. This ability to move within the membrane is measured as its, fluidity.
👉🏿The fluid nature of the membrane is also important in various function like cell growth, formation of intercellular junction, endocytosis,secretion,cell division etc.
👉🏿 Cholesterol is also present in plasma membrane. Cholestrol are more rigid than phospholipid. So it helps in stability of membrane structure.
👉🏿 Cholestrol is absent in membrane of prokaryote. Thus Hopanoids(Pentacyclic sterol) provides stability to prokaryotic cell membrane.
(B) Proteins:
👉🏿Two types of protein are present in plasma membrane. (On the basis of ease of extraction)
(a) Integral or intrinsic protein
👉🏿These protein are tightly binds with phospholipid. Thus, they can not easily removed from membrane.
👉🏿Integral proteins are of two types :
(i) Partially buried (ii) Totally buried
👉🏿 Some integral proteins which are totally buried through the complete thickness of membrane. These type of protein are called as tunnel (Channel) protein which provide a passage for movement of water soluble material across the membrane.
(b) Peripheral or extrinsic protein
👉🏿 These are superficially arranged on the surface of lipid layer and can be separated easily. These protein have enzymatic activity.
👉🏿 Oligosachharides (sugar) of the glycolipids & glycoproteins on the outer surface of plasma membranes are involved in Cell to cell recognition mechanism. Best example of cell recognition is fertilization, (where sperm & egg recognize to each other) and blood - Antigens.
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Fluid mosaic model of plasma membrane
Fluid mosaic model of plasma membrane
• One of the most important functions of the plasma membrane is the transport of molecule across it. The membrane is selectively permeable to some molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
• Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower. Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
• As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
• A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g.,Na+/K+ pump.
(i) Endocytosis (Bulk transport)
(a) Pinocytosis or cell Drinking:-Ingestion of liquid material by plasmalemma in the form of vesicles or bag like structure (pinosome) is called pinocytosis.
(b) Phagocytosis or cell eating:-
Ingestion of solid complex materials by membranes in the form of vasicles (Phagosome) is called phagocytosis.
(ii) Exocytosis/Emiocytosis/Cell vomitting:- Egestion of waste materials from cell through plasma membrane.
CELL WALL
👉 A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of Bacteria, Fungi, Algae And plants.
👉 Algae have cell wall, made of cellulose, galactans, Mannans and minerals like calcium carbonate.
👉 In other plants cell wall consists of cellulose, hemicellulose, pectins and proteins.
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👉 Middle lamella is consist of Ca & Mg pectates (plant cement). Amount of Ca is more.
👉 Cellulose is a main constituent of cell wall but addition to cellulose- Hemicellulose, Cutin, Pectin, Lignin, Suberin are also presents in cell wall.
👉 Cell wall walked as frame or protective layer of cell.( Exoskeleton of cell).
👉 Cellulose microfibrils and microfibrils are arranged in layers to form skeleton of cell wall. In between these layers other substances like pectin, hemicellulose may be present. These from matrix of cell wall.
👉 Cell wall materials (hemicellulose, pectin, lignin) are synthesized in plant golgibodies or dictyosomes. formation of cellulose occurs in cell membrane.
👉PLASMODESMATA:- Name proposed by strasburger (1901). These are cytoplasmic connections between two adjacent plant cells. Plasmodesmata are characteristic of multicellular plants. E.R. tubules (Desmotubules) help to maintain continuity of cytoplasm.
👉 FUNCTIONS OF CELL WALL:-
(i) Cell wall gives shape to the cell
(ii)It protects the cell from mechanical damage and infection
(iii)It also helps in cell-to-cell interaction
(iv)It provides barrier to undesirable macromolecules.
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