Biology

• Cell

  The basic structural and functional unit of life
• Robert Hooke observed cells in a cork slice with a primitive microscope in 1665
• Leeuwenhoek discovered free living cells in pond water for the first time in 1674
• Robert Brown discovered the nucleus in the cell in 1831
• All plants and animals are composed of cells and the cell is the basic unit of life
• Cells

  • Unicellular organisms like Amoeba, Chlamydomonas, Paramoecium and bacteria
  • Multicellular organisms like some fungi, plants and animals
• Every multicellular organism has come from a single cell, as cells divide to produce cells of their own kind
• Cells

  • Vary in shape and size
  • Vary in structure
  • Differ between different parts of a plant body
  • Share some similarities
• Plasma membrane or cell membrane

  Outermost covering of the cell that separates the contents of the cell from its external environment
• Selectively permeable membrane

  Allows or permits the entry and exit of some materials in and out of the cell, while preventing movement of some other materials
• Movement of substances across the cell membrane


  1. Diffusion - spontaneous movement from high to low concentration

  2. Osmosis - movement of water across the membrane towards higher solute concentration
• If the medium surrounding the cell has a higher water concentration than the cell (hypotonic solution)


  The cell will gain water by osmosis and swell up
• If the medium has the same water concentration as the cell (isotonic solution)


  There will be no net movement of water across the cell membrane, the cell will stay the same size
• If the medium has a lower concentration of water than the cell (hypertonic solution)


  The cell will lose water by osmosis and shrink
• Osmosis with dried raisins or apricots

  • Gain water and swell in plain water
  • Lose water and shrink in concentrated sugar or salt solution
• Unicellular freshwater organisms and most plant cells tend to gain water through osmosis
• Absorption of water by plant roots is an example of osmosis
• Diffusion is important in exchange of gases and water in the life of a cell
• Cell also obtains nutrition from its environment, different molecules move in and out of the cell through a type of transport requiring use of energy
• Plasma membrane

  Flexible, made up of organic molecules called lipids and proteins
• Endocytosis

  Process where cell engulfs food and other material from its external environment
• Cell wall

  Rigid outer covering of plant cells, lies outside the plasma membrane, mainly composed of cellulose
• Only living cells are able to absorb water by osmosis
• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting
• Nucleus

  Darkly coloured, spherical or oval, dot-like structure near the centre of each cell, has a double layered nuclear membrane
• Chromosomes

  Rod-shaped structures containing DNA, visible when cell is about to divide, composed of DNA and protein
• Chromatin material

  Entangled mass of thread-like structures containing DNA, visible when cell is not dividing
• Prokaryotes

  Organisms whose cells lack a nuclear membrane, have an undefined nuclear region called a nucleoid
• Eukaryotes

  Organisms with cells having a nuclear membrane
• Differences between prokaryotic and eukaryotic cells

  • Prokaryotic cell: Size generally small (1-10 μm), Nuclear region poorly defined and known as nucleoid, Single chromosome, Membrane-bound cell organelles absent
  • Eukaryotic cell: Size generally large (5-100 μm), Nuclear region well defined and surrounded by nuclear membrane, More than one chromosome, Membrane-bound cell organelles present
• Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply
• Cytoplasm

  Fluid content inside the plasma membrane, contains many specialised cell organelles
• Cell organelles

  Membrane-bound structures within eukaryotic cells that carry out specific functions
• Endoplasmic reticulum (ER)

  Large network of membrane-bound tubes and sheets, has two types: rough ER (with ribosomes) and smooth ER (helps manufacture lipids)
• ER serves as channels for transport of materials, especially proteins, and provides a surface for protein and lipid synthesis
• Lipids


  Help in building the cell membrane
• Membrane biogenesis

  The process of lipids helping to build the cell membrane
• Proteins and lipids

  Function as enzymes and hormones
• Endoplasmic reticulum (ER)

  • Varies greatly in appearance in different cells
  • Always forms a network system
• Function of ER

  • Serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus
  • Provide a surface for some of the biochemical activities of the cell