Cell: The Unit of Life - GEAR Innovative International School PDF

GEAR Innovative International School “A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.” Cells are the structural, functional, and biological units of all living beings. A cell can replicate itself independently. Hence, they are known as the buil...

GEAR Innovative International School “A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.” Cells are the structural, functional, and biological units of all living beings. A cell can replicate itself independently. Hence, they are known as the building blocks of life. Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm are several biomolecules like proteins, nucleic acids and lipids. Moreover, cellular structures called cell organelles are suspended in the cytoplasm. GEAR Innovative International School Discovery of cell The smallest functional unit of life is cell, discovered by Robert Hooke in 1665. He first observed the cells under a microscope while studying cork cells. He saw the cork , which comes from the tree bark, resembled the structure of honeycomb cell made up of a number of small apartments. Hooke named the boxes, Cells. Cell is a Latin word meaning “a little room.” The first living cell was discovered by Anton Van Leeuwenhoek. GEAR Innovative International School  The first living cell was discovered by Anton Van Leeuwenhoek. He discovered the cell of a free-living algae, Spirogyra cells in water in the pond in 1674 with the improved microscope. GEAR Innovative International School Discovery of Nucleus The nucleus was the first organelle to be discovered in 1831. Robert Brown was studying orchids under the microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer. GEAR Innovative International School Cell theory: The cell theory, that all the plants and animals are composed of cells and that the cell is basic unit of life, was presented by two biologists, M. Schleiden (1838) and T. Schwann (1839). The cell theory was further expanded by a german physiologist, Rudolf Virchow (1855). He gave the phrase Omnis cellula-e-cellula, i.e., all cells arises from pre-existing cells. Modified cell theory is termed as cell principle or modern cell theory which postulates that: The cell is the fundamental unit of structure and function in living things. All organisms are made up of one or more cells. Cells arise from other cells through cellular division. GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School Types of cells GEAR Innovative International School GEAR Innovative International School  Prokaryotic Cell Eukaryotic cell Size is 0.1- 5.0 um Size is 5-100 um Nucleus is absent Nucleus is present Ribosomes smaller Ribosomes larger Cell organelles are absent Cell organelles are present Sexual reproduction is absent Sexual reproduction is present Examples: Bacteria and Archaea Examples: Protists, Fungi, Plants, and Animals GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School Unicellular Organisms Multicellular Organisms Unicellular organisms are composed of a single cell Multicellular organisms are composed of more than one cell Simple body organization Complex body organization A single cell carries out all necessary life processes Multiple cells perform different functions The total cell body is exposed to the environment Only the outer cells are exposed to the environment Division of labour is at the organelle level Division of labour is at cellular, tissue, organs and organ system level Includes both eukaryotes and prokaryotes Includes only eukaryotes A lifespan of a unicellular organism is usually short Multicellular organisms have a comparatively longer lifespan Injury to the cell leads to the death of the organism Injury to a cell does not cause the death of the multicellular organism They are microscopic in nature Most are macroscopic in nature Bacteria, amoeba, paramecium and yeast are examples of unicellular Humans, animals, plants, birds and insects, are examples of organisms multicellular organisms GEAR Innovative International School GEAR Innovative International School Cell Diversity The diversity in cells can be seen according to their Shapes, sizes and numbers. GEAR Innovative International School A. Cell shape: The shape of cells is related to the specific function they perform. Some cells like Amoeba and WBCs have changing shapes. In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape. The shape of cells varies in different organisms and different parts of same organism- flat, columnar, round, spindle, long thread like. GEAR Innovative International School GEAR Innovative International School B. Cell size: The size of the cell also varies considerably in different animals and plants. The average cell size varies from 0.5 to 20µ (µ = micrometer). In human body, the smallest cell is sperm cell ,ovum is one of the largest cells, and the longest one is the nerve cells. GEAR Innovative International School C. Cell number: The number of cells in living beings differs from the one in unicellular organisms to many in multicellular forms. The number of cells is not definite in multicellular organisms, and may increase along with the growth and volume of organism. GEAR Innovative International School Animal cell and Plant cell Plants and animals are made up of millions of cells and these cells have several similarities and differences. Considering that they are both eukaryotic cells, which means they have a true nucleus, that is enclosed and separated from other organelles by a nuclear membrane, is a crucial factor that defines their modes of multiplication. They have similar reproduction processes of mitosis and meiosis, using their DNA that is housed by the cell nucleus. GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School  Cell Organelles An organelle is a subcellular structure that has one or more specific jobs to perform in the cell. GEAR Innovative International School Cell Wall In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane. Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a complex carbohydrate (polysaccharide) and it provides structural strength to the plant. GEAR Innovative International School GEAR Innovative International School Functions: 1. Cell wall protects cell membrane and the internal structures of the cell. 2. It provides rigidity and determines the shape of the plant cell by supporting and protecting it. 3. It prevents drying of the cell and helps it to bear unfavourable conditions. 4. It provides mechanical strength to the plant cells. 5. It is freely permeable to many substances 6. It gives inward wall pressure to the cell. GEAR Innovative International School Plasma membrane (cell membrane): Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment. Plasma membrane is living, thin, delicate, elastic, selectively permeable membrane. The plasma membrane is flexible and made up of organic molecules called lipids and proteins. GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School Functions: Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other material. Therefore, the plasma membrane is called a selectively permeable membrane. Substances can pass across a membrane by two type of transport- Active and Passive GEAR Innovative International School Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. “Diffusion is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.” Diffusion is one form of passive transport that doesn’t require the expenditure of cellular energy. Example: movement of carbon dioxide and oxygen during respiration in organisms. GEAR Innovative International School Diffusion is one form of passive transport that doesn’t require the expenditure of cellular energy. A molecule can diffuse passively through the cell membrane if it’s lipid-soluble, uncharged, and very small, or if a carrier molecule can assist it. Example: movement of carbon dioxide and oxygen during respiration in organisms. GEAR Innovative International School GEAR Innovative International School Importance of Diffusion 1. Helps in uniform distribution of materials in the cytoplasm. 2. Helps in exchange of gases during respiration. 3. Odoriferous chemicals of flowers attract the pollinating agents. 4. Transpiration (loss of water vapours) in the plants. 5. Inward diffusion of sodium ions in the cytoplasm helps in muscle contraction and conduction of nerve impulses. GEAR Innovative International School Osmosis: The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called osmosis. OR, Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration. GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School GEAR Innovative International School a) The cell gains water and swells, if the medium surrounding the cell has a higher water concentration (Hypotonic solution) than the cell. b) The cell maintains the same water concentration as the medium (Isotonic solution), water crosses the cell membrane in both directions. c)The cell loses water and shrinks, if the medium has lower water concentration (Hypertonic solution) than the cell. (The cell drinking is endosmosis; omission of water is called ex-osmosis.) GEAR Innovative International School Before Plasmolysis After Plasmolysis GEAR Innovative International School Plant cell undergoing Plasmolysis in a Hypertonic solution (x400 magnification) GEAR Innovative International School GEAR Innovative International School Importance of Osmosis It helps in absorption of water from the soil by root hair. It helps in opening and closing of stomata on the leaves. It also helps in growth of embryo during seed germination. It maintains turgidity of cells which helps in deeper penetration of roots, stretching of stem, etc. Intake of water by unicellular freshwater organisms (e.g. Amoeba, Paramecium, etc.) by endosmosis. GEAR Innovative International School Facilitated transport Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins. GEAR Innovative International School GEAR Innovative International School Active Transport The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy. GEAR Innovative International School 1. Endocytosis Movement of particles, such as large molecules, parts of cells, and even whole cells, into a cell. 1. Pinocytosis - Intake of large sized liquid molecules (cell drinking) e.g intake of fat molecules by intestinal cells 2. Phagocytosis - Intake of large sized solid particles (cell eating). E.g. eating up of pathogenic microbes by phagocytes. GEAR Innovative International School  2. Exocytosis Exocytosis is a form of active transport and bulk transport in which a cell transports molecules out of the cell by secreting them through an energy-dependent process. (Cell vomiting) e.g. secretion of proteins like enzymes, hormones and antibodies from different cells, and expelling undigested food. 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