Biology 1

• Chloroplasts contain chlorophyll and are responsible for photosynthesis
• The nucleus is the control centre of the cell, containing DNA which controls all activities within the cell.
• The light microscope was invented about 350 years ago
• Light microscope


  Uses light to form magnified images of cells and other tiny objects
• The best light microscope can produce a magnification of about 2000 times
• Electron microscope


  Uses electrons to produce magnifications of up to 2 million times
• The higher magnification and resolving power of electron microscopes allow scientists to see and understand sub-cellular structure
• You can rearrange the magnification equation to find the size of the real object, if you know the magnification and the size of the image</b>
• Resolving power


  A microscope's ability to distinguish between two objects that are very close together
• Types of microscopes


  • Light microscope
  • Electron microscope
  • Scanning electron microscope
  • Transmission electron microscope
• Scale bar


  Allows you to easily see the real sizes of objects in a diagram or photo, and calculate the magnification if you don't know it
• Animal cells and plant cells contain smaller structures which have different functions
• Structures of a generalised animal cell


  • Cell membrane
  • Nucleus
  • Cytoplasm
  • Mitochondria
• Structures of a generalised plant cell


  • Cell membrane
  • Nucleus
  • Cytoplasm
  • Mitochondria
  • Chloroplasts
  • Vacuole
  • Cell wall
• Plant and algal cells contain three structures that are not present in animal cells: a permanent vacuole, a cell wall made of cellulose, and chloroplasts
• Animal cells and plant cells are eukaryotic cells, while bacterial cells are prokaryotic cells</b>
• Differences between eukaryotic and prokaryotic cells

  • Eukaryotic cells have a nucleus, prokaryotic cells do not
  • Eukaryotic cells have membrane-bound organelles, prokaryotic cells do not
  • Eukaryotic cells are larger, prokaryotic cells are smaller
• Bacterial cells have a single loop of chromosomal DNA, some have flagella, and some have plasmid DNA
• The animal cell is 10 times larger than the bacterial cell, a difference of one order of magnitude
• Ciliated cells, sperm cells, and nerve cells are examples of specialised animal cells
• Ciliated cells


  • Have cilia that can move in organised ways to push substances past the cell surface
  • Line the trachea and sweep mucus away from the lungs
• Sperm cells


  • Carry the male's genetic information to the female's egg cell
  • Have a tail and many mitochondria to provide energy for movement
  • Have an acrosome to help digest the egg cell's outer layers
• Nerve cells


  • Carry electrical impulses from one part of the body to another
  • Have a long axon and tiny dendrites to make connections with other nerve cells
• As an organism develops, cells differentiate to form specialised cells with different structures and functions
• Unlike most animal cells, many plant cells retain the ability to differentiate throughout their life
• Root hair cells


  • Absorb water and mineral ions from the soil
  • Have a large vacuole to increase the rate of water absorption by osmosis
  • Have many mitochondria to provide energy for active transport of mineral ions
• Xylem cells


  • Form the transport tissue that carries water and mineral ions from roots to the rest of the plant
  • Have lignin to provide strength and support
  • Have no end walls so water and ions can flow easily
• Phloem cells


  • Form the transport tissue that carries dissolved sugars through the plant
  • Have sieve plates between cells to allow solutions to move from cell to cell
  • Have companion cells with many mitochondria to provide energy for active transport of sugars
• Meristem cells in plant tips are undifferentiated and can differentiate into specialised cells
• Prepared slides of animal and plant cells can also be observed under the microscope
• Preparing a specimen


  1. Use the forceps to peel a thin layer of epidermal tissue from the onion

  2. Carefully lay the thin layer on the slide

  3. Add one or two drops of iodine solution

  4. Lower the coverslip over the onion layer. Remove excess liquid with tissue paper
• Observing a specimen


  1. Place a slide on the microscope stage

  2. Turn the turret to the lowest power objective lens

  3. Looking from the side, use the coarse wheel to lower the lens so it almost touches the slide

  4. Looking through the eyepiece, use the coarse wheel to raise the objective lens until the image is focused

  5. Turn the turret to select a higher power objective lens. Use the fine wheel to focus the image again

  6. Repeat steps 5-9 with other slides
• Onion skin is easily peeled away in single layers of cells
• The nucleus contains a large number of genes
• Wear eye protection when you handle iodine solution. Wash off spills straight away because it will stain skin and clothes
• You may find it easier to position the onion if you put a drop of water on the slide first. Don't let the tissue curl over
• Place one edge of the coverslip on the slide at 45 degrees. Gently lower the other edge so that liquid and bubbles are pushed out
• Light microscope


  • Coarse focusing wheel
  • Fine focusing wheel
  • Eyepiece
• Some modern microscopes have a light source built in rather than a mirror
• The cell cycle has three main stages: interphase, mitosis, and cell division