Cell-The Fundamental Unit of Life
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Chapter-5, The Fundamental Unit of LifeCell is the basic structural and functional unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. A cell performs functions like respiration, obtaining nutrition, clearing of waste material, forming new proteins etc. All living things are composed of cells. The term CELL was coined by Robert Hooke in 1665. He examined a thin slice of cork ( a substance from the bark of a tree) under the microscope. Since cork is made up of dead cells, so what Robert Hooke saw was empty compartments lined by cell wall. He described it as a structure similar to honey-comb consisting of many inner compartments. He called these empty compartments as cells. ( in latin language: cellula means a little room. Living Organisms are made up of cells. Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 m; a typical cell mass is 1 nanogram. The largest known cell is an unfertilized ostrich egg cell. Longest cells are nerve cells that may be several meters long. Observing cells in onion bulb: An onion bulb is a modified part of onion plant. It has dry and fleshy scale leaves, a reduced disc like stem and adventitious roots. On observing a small piece of skin (peel also called epidermis) from the concave side of fleshy scale leaf by preparing a slide and observing under the microscope, a number of elongated thin walled cells are seen. Each cell has a distinct nucleus and may also have a vacuole. These cells are the building blocks of onion bulb.
Organisms are divided into two categories based on the number of cells present in their body. When a single cell constitutes the whole organism, such organisms are known as unicellular organisms. ( Uni=Single) Eg. Amoeba, Chlamydomonas, Paramoecium and bacteria. When many cells group together to form one organism, such an organism is known as a multicellular organism. (Multi=Many) Eg. animals , plants and most fungi. Where do cells come from: All cells come from pre-existing cells by the process of cell division. Relation between structure and function of a cell: Shape and size of cells are related to the specific function performed by them. For eg. A nerve cell has a long axon because it helps in transmission of nerve impulses. Division of Labour: Division of labour is seen in organisms at different levels. In man, heart pumps blood, stomach digests food and so on. Similarly, division of labour is seen in cells also. Similarly, in cells, there are structures called cell organelles that perform specific functions so that the cell can exist as a living unit. For eg, lysosomes help in digestion of food within the cell and mitochondria helps in production of energy. Structural organization of a cell The three main structural and functional regions inside a cell are: 1. Plasma membrane (Cell Membrane) 2. Nucleus and 3. Cytoplasm.
Functional Regions of a Cell PLASMA MEMBRANE Outer most covering of cells. The plasma membrane is selectively permeable i.e. it permits the entry and exit of some substances in and out of the cell. Due to its flexible pore size substances can move in and out of the cell depending on the requirement of the cell. Gases (CO2 and O2 ) move across the cell membrane by the process of diffusion. Diffusion means spontaneous movement of a substance from a region of high concentration to a region of its lower concentration. Water also moves in and out of the cell membrane. Movement of water molecules through a selectively permeable membrane is known as Osmosis. Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a semi permeable membrane. Hypertonic, isotonic and Hypotonic solutions When a cell is placed in a solution, three things can happen: a) If the outer solution has a higher water concentration than the inside of the cell then water will move inside the cell, i.e. the cell will gain water by osmosis. Such a solution is HYPOTONIC. b) If the outer solution has a lower water concentration than the inside of the cell then water will move out of the cell, i.e. the cell will lose water by osmosis. Such a solution is HYPERTONIC. c) If the outer solution has exactly the same water concentration than the inside of the cell then there is no net movement of water across the cell membrane. Such a solution is ISOTONIC.
Functions of cell membrane: Diffusion and Osmosis are special processes that help in : Absorption of water by plant roots, movement of molecules like ATP, movement of food molecules etc. In some organisms like Amoeba, cell membrane also helps the organism to engulf food from the outer environment. Movement of food particles inside the cell by infolding of the cell membrane and subsequent formation of the food vacuole is known as ENDOCYTOSIS. Therefore, transport of substances takes place across a cell membrane in the following ways: By diffusion (eg. Gases) By movement of water (osmosis). Substances that are water soluble also move when there is a difference in concentration. Some substances have to be moved against the concentration gradient ( low to high) and this requires energy. Movement of food particles in a cell eg. in amoeba by endocytosis. Composition of Plasma Membrane: it is chiefly composed of lipids + Proteins How can the structure of Plasma Membrane be observed: Through Electron microscope. firstname.lastname@example.org 2
CELL WALL Plant cells have an additional covering on the outside in addition to the cell membrane. This is the cell wall. It is rigid, non living and gives additional strength to the cell. It is mainly composed of cellulose.Cellulose provides structural strength to the cell. (Cellulose is a complex carbohydrate.) Other substances that constitute the cell wall are pectin and lignin. Lignin is one of the strongest substances known to man. When a living plant cell loses water through osmosis, there is shrinkage of the cell cytoplasm and cell contents away from the cell wall. This is known as Plasmolysis. Advantage of cell wall: cell wall permits the plant cells, fungi and bacteria to withstand pressures created when these cells are placed in very dilute (hypotonic) media without bursting. Plant cells, cells of fungi and bacteria are able to withstand greater changes in the surrounding medium than the animal cells. NUCLEUS
Nuclear Membrane: is a double layered covering that protects the nuclear contents. Nuclear pore: allows the transfer of material from inside the nucleus to outside Chromatin Material: is an entangled mass of thread like structures.It gets organized into chromosomes during cell division. Nucleolus Nucleoplasm (Fluid in the nucleus)
Functions of Nucleus: 1. Plays a central role in cell reproduction 2. Has chromosomes that contain information for inheritance of features from parents to offsprings. 3. Controls activities of the cell.Chromosomes: Are visible as rod shaped structures during cell division. Contain information for inheritance Are made up of DNA and Proteins. DNA
= Deoxy ribo nucleic acid Contain information necessary for constructing and organizing cells DNA has functional segments known as genes.
PROKARYOTIC AND EUKARYOTIC CELLS The term prokaryote comes from Greek words that mean before nucleus or prenucleus, while eukaryote means true nucleus. The cell structure of prokaryotes differs greatly from that of eukaryotes. The defining characteristic is the absence of a nucleus. The genetic material of prokaryotes is present as irregular DNA/protein complex in the cytoplasm called the nucleoid, which lacks a nuclear envelope. Prokaryotes generally lack membrane-bound cell compartments: such as mitochondria and chloroplasts. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells. Prokaryotes also differ from eukaryotes in that they contain only a single loop of stable email@example.com 3
chromosomal DNA stored in an area named the nucleoid, while eukaryote DNA is found on tightly bound and organized chromosomes. In photosynthetic prokaryotic bacteria, the chlorophyll pigment is present in bag-like membrane bound structures. PROKARYOTIC CELL 1. Generally smaller in size than eukaryotic cells 2. Have a poorly defined nuclear region due to absence of nuclear membrane. 3. Single chromosome (Nucleoid) is present. 4.Membrane bound cell organelles are absent EUKARYOTIC CELL 1. Generally larger in size than prokaryotic cells 2. have a well defined nuclear region surrounded by a nuclear membrane 3. More than one chromosomes are present in the nucleus. 4. Membrane bound cell organelles like mitochondria, ER, Golgi bodies etc are present.
A Prokaryotic Cell
A Eukaryotic Cell CYTOPLASM: The cytoplasm is the part of a cell that is enclosed within the plasma membrane. In eukaryotic cells the cytoplasm contains organelles, such as mitochondria, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes. Such membrane bound structures are absent in prokaryotic cells. The cytoplasm is the site where most cellular activities occur. firstname.lastname@example.org 4
Organelles are membrane-bound compartments within the cell that have specific functions. Some major organelles that are suspended in the cytoplasm are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles and in plant cells Plastids like chloroplasts. The Endoplasmic Reticulum (ER) is a eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. It has a similar structure as the plasma membrane. On the basis of presence or absence of ribosomes, ER may be classified into two groups: Rough ER: Th