Ch.08  Cell : The Unit of Life

Unicellular organisms are capable of independent existence and they can perform the essential functions of life. Anything less than a complete cell does not ensure independent living. Hence, cell is called the fundamental structural and functional unit of life.

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CELL THEORY​

• The cell theory was first proposed by Matthias Schleiden (1838) and Theodore Schwann (1839). Rudolf Virchow (1855) later added the concept of formation of cells; to this theory. The cell theory is as follows:

• All living organisms are composed of cells and products of cells.

• All cells arise from pre-existing cells.

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AN OVERVIEW OF CELL​

• A typical animal cell is bound by a plasma membrane, while cell wall is also present in a typical plant cell. A dense membrane bound structure is present inside a typical cell. This structure is called nucleus. The nucleus contains the chromosomes. Chromosomes contain the genetic material DNA.

• Prokaryotic Cell: Membrane bound nucleus is absent in prokaryotic cell. Moreover, membrane bound organelles are also absent in prokaryotic cells. Bacteria are examples of prokaryotic cell.

• Eukaryotic Cell: Membrane bound nucleus is present in eukaryotic cell. Moreover, membrane bound organelles are present in eukaryotic cells. Cells of protista, fungi, plantae and Animalia are eukaryotic cells.

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STRUCTURE OF PROKARYOTIC CELL​

• Prokaryotic cells are usually smaller than eukaryotic cells. They multiply more rapidly than eukaryotic cells. The four basic shapes of bacteria are bacillus (rod-like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).

• Cell wall is present in all prokaryotic cells. The genetic material is naked, i.e. not enveloped by a nuclear membrane. Genomic DNA (single chromosome/circular DNA) mark the nuclear material. Additionally, there may be small circular DNA outside the genomic DNA in some bacteria. These smaller DNA are called plasmids. The plasmid DNA confers some unique phenotypic characters to such bacteria. Resistance to antibiotics is one of those characters.

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Cell Envelope: The cell envelope is chemically complex in most of the prokaryotes. The cell envelope is composed of three layers which are tightly bound. The outermost layer is glycocalyx, the middle layer is the cell wall and the innermost layer is the plasma membrane.

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Gram Positive and Gram Negative Bacteria:​

• Based on differences in the cell envelopes and their response to Gram stain; bacteria are divided into two categories, viz. gram positive and gram negative. The bacteria which take up the Gram stain are called gram positive bacteria. The bacteria which do not take up the Gram stain are called gram negative bacteria.

• Different bacteria have different types of glycocalyx; in terms of composition and thickness. It could be a loose sheath or could be thick and tough. The cell wall prevents the bacterium from bursting or collapsing. Plasma membrane in prokaryotes is similar as in eukaryotes.

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Mesosomes:​

• A special membranous structure is formed by the extensions of plasma membrane into the cell. This is called the mesosome. These extensions are in the form of vesicles, tubules and lamellae.

• The mesosomes help in cell wall formation, DNA replication and distribution to daughter cells.

• They also help in respiration, secretion process, to increase the surface area of the plasma membrane and enzymatic content.

• In some prokaryotes, there are other membranous extensions into the cytoplasm. These are called chromatophores and contain pigments, e.g. in cyanobacteria.

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Flagella:

Bacterial cells can be motile or non-motile. Thin filamentous extensions from cell wall are present in motile bacteria. These extensions are called flagella. The bacterial flagellum is composed of three parts; filament, hook and basal body.

Pili and fimbriae are also present on the surface of bacteria. Pili are elongated tubular structures made of special protein. Fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they help attach the bacteria to rocks in streams and to the host tissues.

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Ribosomes:​

Ribosomes are associated with the plasma membrane in prokaryotes. They are about 15 nm by 20 nm in size. They are made of two subunits; 50S and 30S. These two subunits together form 70S prokaryotic ribosomes. Ribosomes are the site of protein synthesis.

Inclusion bodies: Reserve materials in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. They lie free in cytoplasm, e.g. phosphate granules, cyanophycean granules and glycogen granules.

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EUKARYOTIC CELLS​

Following cell organelles are present in eukaryotic cells:

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Cell Membrane​

The cell membrane is composed of lipids which are arranged in a bilayer. The polar heads of lipids are towards the outer side and the hydrophobic ends are towards the inner side. This ensures protection of the non-polar tail of saturated hydrocarbons from the aqueous environment. Additionally, proteins and carbohydrates are also present in plasma membrane. The ratio of protein and lipids varies considerable in different types of cells. The peripheral proteins lie on the surface of the membrane, while the integral proteins lie partially of completely buried in the membrane.

Fluid Mosaic Model: This model was proposed by Singer and Nicolson (1972) and is the most widely accepted model of plasma membrane. Per this model, lipids are present as quasi-fluid. Lateral movement of proteins within the bilayer is possible because of the quasi-fluid nature of the lipid bilayer. The fluid nature of the membrane is also important from many aspects; like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.

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Functions of Plasma Membrane: Transport of molecules across it is one of the most important functions of plasma membrane. Plasma membrane is selectively permeable. Many molecules can move across the membrane without any requirement of energy. The movement without the expense of energy is called passive transport. Passive transport takes place through diffusion and osmosis. Polar molecules need a carrier protein of the membrane to be transported across against concentration gradient. This type of transport is dependent on energy and is called active transport.

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Cell Wall​

• The cell wall is a non-living rigid structure and forms an outer covering for the plasma membrane of fungi and plants. The cell wall in algae is made up of cellulose, galactans, mannans and minerals; like calcium carbonate. The cell wall of plants is made up of cellulose, hemicelluloses, pectins and proteins. The cell wall of a young plant cell is called the primary wall and is capable of growth. When the cell matures, this capability diminishes and a secondary wall is formed on the inner side of the cell.

• The middle lamella is a layer mainly composed of calcium pectate. This holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata. Plasmodesmata connect the cytoplasm of neighbouring cells.

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Endomembrane System​

The endomembrane system is composed of endoplasmic reticulum, golgi complex, lysosomes and vacuoles. Functions of these cell organelles are coordinated with each other and hence they form the endomembrane system. Mitochondria, chloroplast and peroxisomes are not a part of the endomembrane system.

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The Endoplasmic Reticulum (ER)​

• The Endoplasmic Reticulum is a network of tiny tubular structures which are scattered in the cytoplasm. The ER divides the intracellular space into two distinct compartments, viz. luminal and extra-luminal. The compartment inside the ER is called luminal compartment, while the compartment in the cytoplasm is called extra-luminal compartment.

• Smooth ER: Ribosomes are absent on the surface of smooth ER. Smooth ER are the major sites of lipid synthesis.

• Rough ER: Ribosomes are present on the surface of rough ER. Rough ER is quite common in those cells which are actively involved in protein synthesis. They are extensive and continuous with the outer membrane of the nucleus.

Golgi apparatus​

• Camillo Golgi (1898) was the first to observe densely stained reticular structures near the nucleus. These structures were later named after him. The Golgi Complex consists of many flat, disc-shaped sacs or cisternae. The cisternae are 0.5m to 1.0m in diameter. The cisternae are stacked parallel to each other and are concentrically arranged near the nucleus. The convex face of cisternae is called cis and it is the forming face. The concave face of cisternae is called trans and is the maturing face. The cis and trans faces are interconnected.

• Functions of Golgi Complex: Packaging of materials is the main function of Golgi Complex. The materials are then delivered to the intracellular targets or secreted outside the cell. The materials come from the ER; in the form of vesicles; and fuse with the cis face. Then they move towards the trans face.

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Lysosomes​

• The membrane bound vesicles formed by the process of packaging in the Golgi apparatus are called lysosomes. Lysosomes usually contain various hydrolytic enzymes. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

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Vacuoles​

• Membrane bound space found in the cytoplasm is called vacuole. A vacuole contains water, sap, excretory product and other useless materials. The vacuole is bound by a single membrane; called tonoplast. The vacuole can occupy up to 90% of the volume in a plant cell. In plant cells, the tonoplast facilitates the transport of several ions and other materials against concentration gradient into the vacuole. Hence, their concentration is significantly higher in the vacuole than in the cytoplasm.

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Mitochondria​

• Mitochondrion is sausage-shaped or cylindrical structure. The diameter of mitochondria is 0.2-1.0m (average 0.5m) and length is 1.0-4.1m. A mitochondrion is bound by two membranes. The two membranes divide the lumen of mitochondria into outer and inner compartments. The inner compartment is called the matrix. The inner membrane forms numerous finger-like infoldings; called cristae. The cristae increase the surface area.

• Functions of Mitochondria: Aerobic respiration takes place in mitochondria. Energy is produced in the form of ATP and stored in the mitochondria. The matrix of mitochondria also has single circular DNA molecule, a few RNA molecules, ribosomes and the components needed for the synthesis of proteins. Mitochondria can replicate itself by fission.

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Plastids​

• Plastids are found in all plant cells and in Euglenoids. Plastids bear some specific pigments and hence impart characteristic colours. Based on the type of pigments, plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

• Chloroplast: They contain chlorophyll and carotenoid pigments. These pigments are responsible for trapping light energy which is required for photosynthesis.

• Chromoplast: They contain fat soluble carotenoid pigments; like carotene, xanthophylls and others. Different plant parts get bright colours; like yellow, orange or red; because of the pigments in chromoplast.

• Leucoplast: These are colourless plastids. They store various nutrients. The leucoplast which stores carbohydrates is called amyloplast. The leucoplast which stores oil and fats is called elaioplast. The leucoplast which stores proteins is called aleuroplast.

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Structure of Chloroplast​

• Chloroplasts are lens-shaped, oval, spherical, discoid or even ribbon-like. Their number can vary from 1 per cell (as in Chlamydomonas) to 20 – 40 (as in the mesophyll of leaves).

• Chloroplast is bound by two membranes. The inner membrane of chloroplast is less permeable. The space within the inner membrane of chloroplast is called the stroma. Several organized flattened membranous sacs are present in the stroma. These flattened sacs are called thylakoids. The thylakoids are arranged in stacks called grana. There are flat membranous tubules connecting the thylakoids of the different grana. The membrane of the thylakoids encloses a space called a lumen.

• The stroma contains enzymes required for the synthesis of carbohydrates and proteins. The stroma of chloroplast also contains double stranded DNA molecules and ribosomes. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

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Ribosomes​

• Ribosomes were first observed under electron microscope by George Palade (1953). Ribosomes are composed of RNA and proteins. The eukoryotic ribosomes are 80S, while the prokaryotic ribosomes are 70S. In this case, ‘S’ stands for sedimentation coefficient or Svedberg’s Unit. It is an indirect measure of density and size of ribosome. Both 70S and 80S ribosomes are composed of two subunits.

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Cytoskeleton​

• The cytoskeleton is composed of an elaborate network of filamentous proteinaceous structures in cytoplasm. Cytoskeleton is involved in many functions; like mechanical support, motility, maintenance of the shape, etc.

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Cilia and Flagella

• Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are smaller than flagella. Cilia work like oars and facilitate movement of either the cell or the surrounding fluid. Flagella are responsible for cell movement. Flagella in prokaryotes are structurally different from those in eukaryotes.

• A flagellum or a cilium is covered with plasma membrane. Their core is called the axoneme. The axoneme has several microtubules running parallel to the long axis. There are usually nine pairs of radially arranged peripheral microtubules and a pair of centrally located microtubules. Such an arrangement of microtubules is called the 9 + 2 array. The central tubules are connected by bridges and are also enclosed by a central sheath. The central sheath is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Cilium and flagellum emerge from basal bodies; which are centriole-like structures.

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Centrosome and Centrioles​

• Centrosome usually contains two cylindrical structures which are called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other. They are made up of nine evenly spaced peripheral fibrins of tubulin protein. Each peripheral fibril is a triplet. Each is linked to the adjacent triplets. The central part of the proximal region of the centriole is also made up of protein. The central part is called the hub. The hub relates to tubules of the peripheral triplets by radial spokes. The radial spokes are made up of protein. Centrioles form the basal bodies of cilia and flagella and spindle fibres. The centrioles give rise to spindle apparatus during cell division in animal cells.

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Nucleus​

• Nucleus was first described by Robert Brown in 1831. Nucleus is enclosed by a double-membrane nuclear envelope. The space between the two membranes is called the perinuclear space. The perinuclear space forms a barrier between the nucleic materials and cytoplasmic materials. The outer membrane is usually continuous with the endoplasmic reticulum. Ribosomes are present on the outer membrane of nuclear envelope.

• The nuclear membrane is interrupted by minute pores at various places. These pores provide passage to RNA and protein molecules. Usually, there is only one nucleus in a cell, but some variations can also be observed. Some mature cells even lack nucleus.

• The fluid inside the nucleus is called nucleoplasm or nuclear matrix. The nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures. Nucleolus is not a membrane bound structure. Synthesis of ribosomal RNA takes place in the nucleolus.

• Nucleus also contains chromatin fibres; which are distinct during some stages of cell division. The chromatin contains DNA and some basic proteins; called histones; some non-histones and RNA.

• Chromosome: A chromosome has a primary constriction called centromere. Disc-shaped structures; called kinetochores; are present on the sides of the centromere. Based on position of the centromere, chromosomes can of four types, viz. metacentric, sub-metacentric, acrocentric and telocentric.

• When the centromere divides the chromosomes into two identical arms, it is called metacentric chromosome. When the centromere is slightly away from the middle, it is called sub-metacentric chromosome. When the centromere divides the chromosome into a smaller and another much larger arm, it is called acrocentric chromosome. When the centromere is at the tail, it is called telocentric chromosome.

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Microbodies​

Many membrane bound minute vesicles are present in both plant and animal cells. They contain various enzymes and are called microbodies.