Cell

• Cell can be defined as the structural and functional unit of life
• Robert Hooke discovered the cell while studying a thin slice of cork under a self-designed primitive microscope in 1665
• Anton von Leeuwenhoek discovered free-living cells in pond water with his improved microscope in 1674
• Robert Brown discovered and named the Nucleus in the plant cell in 1831
• Purkinje gave the term protoplasm for the living fluid substance present inside the cell in 1839
• Postulates of cell theory:

  • All organisms (both plants and animals) are composed of cells
  • Cells are the structural and functional unit of life
  • All cells arise from pre-existing cells only
• Cell is called the structural unit as all living organisms are composed of one or more cells
• Cell is called the functional unit as all vital functions of an organism occur within the cells
• Plasma membrane is called selectively permeable because it allows the movement of only selected molecules across it and not all of them
• If the organization of a cell is destroyed due to some physical and chemical influence, the cell will not be able to perform basic functions like respiration, nutrition, and excretion. Lysosome will burst to release digestive enzymes digesting the entire cell
• Difference between plasma membrane and cell wall:

  • Plasma membrane is found in both plant and animal cells, while the cell wall is found only in plant cells
  • Plasma membrane is quite thin, while the cell wall is quite thick
  • Plasma membrane is flexible, while the cell wall is rigid
  • Plasma membrane is selectively permeable, while the cell wall is generally permeable
  • Plasma membrane is formed of lipoprotein, while the cell wall is formed of cellulose
• Difference between plant cell and animal cell:

  • Plant cell has a cell wall, while the animal cell does not
  • Plastids are present in plant cells, while they are absent in animal cells
  • Plant cells have a single large central vacuole, while animal cells have more small vacuoles
  • Plant cells are larger in size and regular in shape, while animal cells are comparatively smaller in size and irregular in shape
  • The stored food in plant cells is generally starch, while in animal cells it is generally glycogen
• Difference between prokaryotic cell and eukaryotic cell:

  • Prokaryotic cells are generally small (1-10µm), while eukaryotic cells are generally large (50-100µm)
  • Prokaryotic cells have a poorly defined nuclear region due to the absence of a nuclear membrane, known as nucleoid, while eukaryotic cells have a well-defined nuclear region surrounded by a nuclear membrane
  • Prokaryotic cells contain a single chromosome, while eukaryotic cells contain more than one chromosome
  • Membrane-bound cell organelles are absent in prokaryotic cells, while they are present in eukaryotic cells
• Difference between Nucleus and nucleoid:

  • Nucleus is larger in size, while nucleoid is comparatively smaller in size
  • Nucleus has a covering of a double membrane, while nucleoid has an absence of a membrane
  • Nucleus has a nucleolus present in it, while nucleoid has a nucleolus absent in it
• Functions of cell membrane:

  • It provides definite shape to the semifluid contents of the cell
  • It functions as a mechanical barrier that protects the internal contents of the cell
  • It regulates the entry and exit of substances in and out of the cell
  • In unicellular organisms like Amoeba, the flexibility of the membrane enables the organism to engulf food and other substances from the external environment by endocytosis
• Diffusion is the process of movement of a substance from the region of higher concentration to the region of lower concentration to spread uniformly in the given space
• Osmosis is the passage of water from a region of high-water concentration through a semi-permeable membrane to a region of low water concentration. Osmosis occurs only in living cells
• Behaviour of plant cell/animal cell in hypotonic, hypertonic, and isotonic solution:

  • Hypotonic solution: The cell swells up by gaining water if the surrounding medium has a higher water concentration than the cell
  • Isotonic solution: There will be no net movement of water across the plasma membrane if the medium surrounding the cell has the same water concentration as the cell
  • Hypertonic solution: The cell will shrink due to losing water by osmosis if the medium has a lower concentration of water than the cell
• Plasmolysis: When plant cells lose water in a hypertonic solution, the cell shrinks, and the shrinking protoplasm contracts from the cell wall
• Difference between diffusion and osmosis:

  • Diffusion can occur in any medium, while osmosis occurs only in a liquid medium
• Diffusion:

  • Can occur in any medium
  • The diffusing molecules may be solid, liquid or gases
  • Does not require a semipermeable membrane
  • Equalizes the concentration of substances throughout the available space
• Osmosis:

  • Occurs only in liquid medium
  • Involves movement of solvent molecules only
  • Requires a semipermeable membrane
  • Does not equalize the concentration of solvent on the two sides of the membrane
• Cytoplasm:

  • The protoplasmic mass of the cell inner to plasma membrane excluding nucleus is called cytoplasm
  • Contains various membrane-bound organelles specialized to perform specific functions for the cell
  • Along with nucleoplasm, makes up the protoplasm

  Function of cytoplasm:

  • Helps in exchange of materials between cell organelles
  • Acts as a store of vital chemicals such as amino acids, glucose, vitamins, ions, etc.
  • Site for many biochemical activities
• Nucleus:

  Function of nucleus:

  • Plays an important role in cellular reproduction
  • Determines cell development and maturity by directing the chemical activities of the cell
  • Helps in the transmission of hereditary traits from the parent to the offspring
  • Directs and controls all metabolic activities of the cell
• Genes:

  • Functional unit of DNA or chromosome that carries hereditary information
  • Responsible for the transfer of characters from parents to offspring
• Endoplasmic Reticulum:

  • Rough E.R.: characterized by the presence of Ribosomes on its surface
  • Smooth E.R.: devoid of ribosomes on its surface

  Function of Endoplasmic reticulum:

  • RER is the site for protein synthesis
  • SER helps in lipid synthesis
  • Proteins and lipids synthesized on E.R. are used for making cell membrane
  • Provides large surface area for synthesis of many chemicals and various physiological activities
• Difference between SER and RER:

  Smooth Endoplasmic reticulum:

  • Ribosomes are not attached to its surface
  • Situated near the plasma membrane and is formed of cisternae
  • Helps in lipid synthesis

  Rough Endoplasmic reticulum:

  • Ribosomes are attached to its surface
  • Situated near to the nucleus and is formed of tubules
  • Helps in protein synthesis
• Golgi Apparatus:

  • Discovered by Camillo Golgi
  • Organelle of membrane-bound sacs, tubules, and vesicles near the nucleus

  Function:

  • Involved in repair and synthesis of cell membranes
  • Complex sugars may be made from simple sugars
  • Forms Lysosomes
  • Takes part in storage, modification, and packaging of various biochemicals produced by different components of the cell
• Lysosomes:

  • Formed by the Golgi Apparatus
  • Membrane-bound sacs containing powerful digestive enzymes made by RER
  • Involved in intracellular digestion of foreign food or microbes as well as worn-out cell organelles
• Mitochondria:

  • Have two membrane coverings, outer membrane is porous while inner membrane is deeply folded
  • Folds create a large surface area for ATP generating chemical reactions

  Function:

  • Sites of cellular respiration
  • Provide energy (in the form of ATP) for vital activities of the living cell
• Energy currency of the cell:

  • ATP (Adenosine triphosphate)
• Plastids:

  • Present only in plant cells
  • Leucoplast: colorless plastids that store food materials such as protein, starch, and lipids
  • Chromoplast: colored plastids, including non-green and green chromoplast (chloroplast)

  Function of plastids:

  • Leucoplasts store food materials
  • Chromoplast imparts color to flowers and fruits
  • Chloroplast traps solar energy to manufacture food through photosynthesis
• Vacuoles:

  • Storage sacs for solid or liquid contents, bounded by a membrane called tonoplast

  Function of vacuole:

  • Play important roles in expelling excess water and some wastes from the cell in some unicellular organisms
  • Store toxic metabolic by-products or end products of plant cell
  • Provide turgidity and rigidity to the plant cells
• Cell division:

  • Process by which new cells are made
  • Two types: Mitosis and Meiosis

  Mitosis:

  • Equational division
  • Gives rise to two daughter cells
  • Number of chromosomes in daughter cells is the same as that of parents
  • Helps in growth and repair

  Meiosis:

  • Reductional division
  • Produces four daughter cells
  • Number of chromosomes becomes half in daughter cells
  • Involved in gamete formation