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• 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
• Purkinje coined the term 'protoplasm' for the fluid substance of the cell in 1839
• The cell theory that all plants and animals are composed of cells and the cell is the basic unit of life was presented by Schleiden and Schwann in 1838-1839
• Virchow further expanded the cell theory by suggesting that all cells arise from pre-existing cells in 1855
• The electron microscope was invented in 1940, allowing observation and understanding of the complex structure of the cell and its organelles
• 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
• Preparing temporary mounts of plant materials

  1. Take a small piece from an onion bulb

  2. Peel off the skin (epidermis) from the concave side

  3. Put the peel in water in a watch-glass

  4. Transfer a small piece of the peel to a glass slide with a drop of water

  5. Add a drop of safranin solution and cover with a cover slip
• Cells observed in onion peel

  • Cells have different shapes and sizes
  • Cells have different structures
  • Cells from different parts of a plant body can be different
• Plasma membrane or cell membrane


  The 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 the movement of some other materials
• Diffusion


  The spontaneous movement of a substance from a region of high concentration to a region where its concentration is low
• Osmosis


  The net diffusion of water across a selectively permeable membrane toward a 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 exactly the same water concentration as the cell (isotonic solution)


  There will be no net movement of water across the cell membrane, and 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 an egg

  1. Remove the shell of an egg by dissolving it in dilute hydrochloric acid

  2. Put the egg in pure water and observe - the egg swells as water passes into it by osmosis

  3. Place the de-shelled egg in a concentrated salt solution and observe - the egg shrinks as water passes out of the egg solution into the salt solution
• Osmosis with dried raisins or apricots

  1. Put dried raisins or apricots in plain water - they gain water and swell

  2. Place them in a concentrated solution of sugar or salt - they lose water and shrink
• Osmosis with an egg

  1. Remove shell by dissolving in dilute hydrochloric acid

  2. Put egg in pure water and observe

  3. Egg swells because water passes into it by osmosis
• Osmosis with an egg

  1. Place de-shelled egg in concentrated salt solution and observe

  2. Egg shrinks because water passes out of the egg solution into the salt solution
• Activity with dried raisins or apricots

  1. Put in plain water and observe

  2. Each gains water and swells

  3. Place in concentrated solution of sugar or salt

  4. Loses water and shrinks
• 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 the cell engulfs food and other material from its external environment
• Activity to find out about electron microscopes


  1. Find information from school library or internet

  2. Discuss with teacher
• Cell wall


  Rigid outer covering found in plant cells, outside the plasma membrane, mainly composed of cellulose
• Observing plasmolysis


  1. Mount Rhoeo leaf peel in water, examine under microscope

  2. Add strong sugar or salt solution, observe shrinkage

  3. Repeat with boiled leaf, no plasmolysis observed
• Cell walls permit plant, fungal and bacterial cells to withstand very dilute external media without bursting
• Observing cells from cheek

  1. Scrape inside of cheek, spread on slide

  2. Add methylene blue stain

  3. Observe under microscope, look for nucleus
• Nucleus


  Darkly coloured, spherical or oval, dot-like structure at the centre of the cell, has a double layered nuclear membrane with pores
• 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 whose cells have a nuclear membrane