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CELL: THE UNIT OF LIFE: All organisms are composed of cells. Organisms that are composed of a single cell and are called unicellular organisms while organisms that are composed of many cells, are called multicellular organisms.
What is a Cell:
Cell is the fundamental structural and functional unit of all living organisms.
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Cell Theory:
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.
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
An Overview of Cell:
- Cells differ greatly in size, shape and functions.
- Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA.
- Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic.
- 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 and vacuoles.
PROKARYOTIC CELLS:
- The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
- All prokaryotes have a cell wall surrounding the cell membrane.
- Prokaryotic cells are generally smaller and multiply more rapidly than the eukaryotic cells.
- Prokaryotic cells don’t have well-defined nucleus.
- In addition to the exposed genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These small circular DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters such as resistance to antibiotics to bacteria.
- A specialised differentiated form of cell membrane called mesosome (extension of cell membrane) is the characteristic of prokaryotes.
Cell Envelope and its Modifications:
- Most prokaryotic cells have a complex cell envelope that consists three layers glycocalyx (outermost), the cell wall (middle) and the plasma membrane (innermost). The cell wall determines the shape of the cell and provides structural support to prevent the bacterium from bursting or collapsing. The plasma membrane is semi-permeable in nature and interacts with the outside world.
- Bacterias that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria.
- Mesosome is a special membraneous structure formed by the extensions of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae and helps cell wall formation, DNA replication and distribution to daughter cells.
- Bacterial cells may be motile or non-motile. Motile cells have thin filamentous extensions from their cell wall called flagella. Bacterial flagellum is composed of three parts – filament, hook and basal body.
- Besides flagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. They help attach the bacteria to rocks in streams and also to the host tissues.
Ribosomes and Inclusion Bodies:
Ribosomes:
Inclusion bodies:
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EUKARYOTIC CELLS:
- The eukaryotes include all the protists, plants, animals and fungi.
- Eukaryotic cells have compartmentalized cytoplasm through the presence of membrane bound organelles.
- Even though both plant and animal cells are eukaryotic cells, they are not identical. Differences between plant and animal cells are given below:
Cell Membrane:
- The cell membrane is composed of lipids that are arranged in a bilayer. The lipid component of the membrane mainly consists of phosphoglycerides.
- Cell membranes also possess protein and carbohydrate surrounded by lipids. These membrane proteins can be classified as integral or peripheral. Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane.
- One of the most important functions of the plasma membrane is the transport of the molecules across it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport. 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.
Cell Wall:
Non-living rigid structure that forms an outer covering for the plasma membrane of plants and fungi.
Function of cell wall:
Function of cell wall:
- Gives shape to the cell
- Protects the cell from mechanical damage and infection
- Helps in cell-to-cell interaction
- Provides barrier to undesirable macromolecules.
Endomembrane System:
- The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.
- It is called endomembrane system because functions of endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles are coordinated.
- Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.
The Endoplasmic Reticulum (ER):
- SER (Smooth Endoplasmic Reticulum): The endoplasmic reticulum with no ribosomes on their surface(SER), appears smooth (Figure 1). Function: helps in lipid synthesis.
- RER (Rough Endoplasmic Reticulum): The endoplasmic reticulum bearing ribosomes on their surface, appears rough (Figure 1). Function: helps in protein synthesis and secretion.
Golgi apparatus:
- They consist of many flat, disc-shaped sacs or cisternae (Figure 2 above).
- Functions: (i) Packaging materials (proteins) to be delivered either to the intra-cellular targets or secreted outside the cell (ii) Important site of formation of glycoproteins and glycolipids.
Lysosomes:
- Membrane bound vesicular structures formed by the process of packaging in the golgi apparatus.
- Very rich in almost all types of hydrolytic enzymes (hydrolases) capable of digesting carbohydrates, proteins, lipids and nucleic acids.
- Function: Intracellular digestion.
Vacuoles:
- Membrane-bound space found in the cytoplasm. The membrane is called tonoplast.
- Function: In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.
Following organelles discussed below are not in endomembrane system.
Mitochondria:
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- Mitochondria are the sites of aerobic respiration. They produce cellular energy capsules ATP (hence they are called ‘power houses’ of the cell).
Plastids:
- Plastids are found in all plant cells.
- Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
Chloroplasts:
structure of chloroplast ( Figure 4):
- Found in the mesophyll cells of the leaves.
- The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
structure of chloroplast ( Figure 4):
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Chromoplasts:
Leucoplasts:
- In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour.
Leucoplasts:
- The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.
Ribosomes:
- They are composed of ribonucleic acid (RNA) and proteins.
- The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
- ‘S’ stands for the sedimentation coefficient; it indirectly is a measure of density and size.
Cytoskeleton:
- Network of filamentous proteinaceous structures present in the cytoplasm.
- Involved in mechanical support, motility, maintenance of the shape of the cell.
Cilia and Flagella:
- Cilia and flagella are hair-like outgrowths of the cell membrane.
- Cilia are small structures that causes the movement of either the cell or the surrounding fluid.
- Flagella are comparatively longer and responsible for cell movement.
Structure (Figure 5):
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Centrosome and Centrioles:
Centrosome is an organelle usually containing two cylindrical structures called centrioles.
Nucleus:
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Microbodies:
Membrane bound minute vesicles that contain various enzymes.