Cell: The Unit of Life PDF

1080CHo h e d CHAPTER 8 CELL: THE UNIT OF LrE 8.1 What is a Cell? When you look around, you see both living and non-living things. You 8.2...

1080CHo h e d CHAPTER 8 CELL: THE UNIT OF LrE 8.1 What is a Cell? When you look around, you see both living and non-living things. You 8.2 Cell Theory must have wondered and asked yourself- what is it that makes an 8.3 An Overview of organisn living, or what is it that an inanimate thing does not have which Cell a living thing has' ? The answer to this is the presence of the basic unit of life- the cell in all living organisms. 8.4 Prokaryotic Cells All organisms are composed of cells. Some are composed of a single 8.5 Eukuryotic Cells cell and are called unicellular organisms while others, like us, composed of many cells, are called multicellular organisms. 8.1 WHAT IS A CELL? Unicellular organisms are capable of (i) independent existence and () 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. Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus. The invention of the mieroscope and its improvement leading to the electron microscope revealed all the structural details of the cell. 8.2 CELL THEORY In 1838, Matthias Schleiden, a German 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 2024-25  Schwann (1839), a British Zoologist, studied different types of animal cells and reported that cells hada 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. On the basis of this, 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. RudolfVirchow (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: () all living organisms are composed of cells and products of cells. () all cells arise from pre-existing cells. 8.3 AN OVERVIEw oF CELL You have earlier observed cells in an onion peel and/or human cheek cells under the microscope. Let us recollect their structure. 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. The cells of the human cheek have an outer membrane as the delimiting structure of the cel. 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. In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the living state. SBesidesthe 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. The prokaryotic cells lack such membrane bound organelles. Ribosomes are non-membrane bound organelles found in all cells - both eukaryotic as well as prokaryotic. 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 centrosome which helps in cell division. Cells differ greatly in size, shape and activities (Figure 8.1). For example, Mycoplasmas, the smallest cells, are ony 0.3 um in length while bacteria 2024-25 CEL: THE UNIT OF LIFE 89 White blood cells Columnar epithelial cells Red blood cells (amoeboid) (long and narrow) (round and biconcave) NCERT Nerve cell (Branched and long) Mesophyl cells A tracheid (round and oval) (elongated) Figure 8.1 Diagram showing different shapes of the cells could be3 to 5 um. The largest isolated single cell is the egg of anostrich. Among multicellular organisms, human red blood cells are about 7.0 um in diameter. Nerve cells are some of the longest cells. Cells also vary greatly in their shape. They may be disc-like. polygonal, columnar, cuboid, thread like, or even irregular. The shape of the cell may vary with the function they perform. 8.4 PROKARYOTIC CELLS The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells (Figure 8.2). They may vary greatly nin shape and size. The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral). The organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions. All  2, prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The fluid matrix flling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is Typical bacteria (1-2 um) basically naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called PPLO (about 0.1 um) plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacterla. One such character is resistance to antibiotics. Viruses In higher classes you will learn that this plasmid A typical eukaryotic cell (0.02-0.2 um) DNA is used to monitor bacterial transformation (10-20 um) with foreign DNA. Nuclear membrane is found Figure 8.2 Diagram showing comparison of in eukaryotes. No organelles, like the ones in eukaryotic cell with other ,are found in prokaryotic cells except organisms emee Prokarvotes have for somnething unique in the form of inclusions. A specialised differentiated form of cell membrane called mesosome is the characteristic of prokaryotes, They are essentially infoldings of cell membrane. 8.4.1 Cell Envelope and its Modifications Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope. The cell envelope consists of a tightly bound three layered structure i.e., the outermost glycocalyx followed by the cell wall and then the plasma membrane. Although each layer of the envelope performs distinct function, they act together as a single protective unit. Bacteria can be dassified into two groupsson on the basis of the differences in the cellenvelopes and the manner in whichh they respond to the staining procedure developed by Gram viz., those that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria. Glycocalyx differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule. The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing. The plasma membrane is selectively pernmeable in nature and interacts with the outside world. This membrane is similar structurally to that of the eukaryotes. A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall 2024-25 CELL: THE UNIT OF LIFE 91 formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content. In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments. Bacterial cells may be motile or non-motile. If motile, they have thin flamentous extensions from their cell wall called flagella. Bacteria show a range in the number and arrangement of flagella. Bacterial flagellum is composed of three parts - filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. i s h e d Besides lagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues. 8.4.2 Ribosomes and Inclusion Bodies In prokaryotes, ribosomes are associated with the plasma mnembrane of the cell. They are about 15 nm by 20 nm in size and are made of two subunits - 50S and 30S units which when present together form 70OS prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the NA into mRNA proteins. Inclusion bodies: Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie free in the cytoplasm, e.g. phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria. 8.5 EUKARYOTIc CELLS The eukaryotes include all the protists. plants, animals and fungi. 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 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 almost all plant cells (Figure 8.3). 2024-25  Rough endoplasmic reticulum Lysosome Smooth endoplasmic reticulum Plasmodesmata Nucleus Nucleolus Golgi apparatus Microtubule Nuclear envelope Plasma membrane Vacuole -Middle lamella Cell waln (a) Peroxisome Mitochondrion Cytoplasm Ribosomes Chloroplast Golgi Microvilli apparatus Plasma membrane Centriole Smooth Peroxiome endoplasmic reticulum Lysosome Nuclear envelope Ribosomes Nucleolus Mitochondrion Rough endoplasmic Nucleus reticulum (b) Cytoplasm Figure 8.3 Diagram showing : (a) Plant cell (b) Animal cell 2024-25 CELL: THE UNIT OF LIFE 93 Let us now look at individual cell organelles to understand their structure and functions. 8.5.1 Cell Membrane 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. These studies showed that the cell membrane is mainly composed of lipids and proteins. The major lipids are phospholipids that are arranged s h e d 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 hydrocarbons is protected rom the aqueous environment (Figure 8.4). In addition to phospholipids membrane also contains cholesterol. Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in diferent cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids. Depending on the ease of extraction, membrane proteins can be classified as integral inte and peripheral. Peripheral proteins lie on the surface of membrane while the integral proteins o e are partially r e or totally buried in the membrane. Sugar Peripheral Protein Phospholipid bilayer Integral protein Cholesterol Figure 8.4 Fluid mosaic model of plasma membrane 2024-25 94 BIoLOGY An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model (Figure 8.4). According to this, the quasi-fluid nature of lipid enables lateral mnovement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity. The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc. One of the most important functions of the plasma membrane is the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side ofit. 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, ie.. 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. 8.5.2 Cell Wall As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants. Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules. Algae have cell wall, made of cellulose. galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose. hemicellulose, pectins and proteins. The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell. The middle lamella is a layer mainly of calciumn pectate which holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.