GCSE BIOLOGY

• When someone first peered down a microscope at a slice of cork and drew the boxes they saw, little did they know that they'd seen the building blocks of every organism on the planet...
• Organisms


  Can be Prokaryotes or Eukaryotes
• All living things are made of cells
• Cells


  • Can be either prokaryotic or eukaryotic
  • Eukaryotic cells are complex and include all animal and plant cells
  • Prokaryotic cells are smaller and simpler, e.g. bacteria
• Eukaryotes


  Organisms that are made up of eukaryotic cells
• Prokaryote


  A prokaryotic cell (it's a single-celled organism)
• The different parts of a cell are called subcellular structures
• Subcellular structures found in most animal cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
• Nucleus


  • Contains genetic material that controls the activities of the cell
• Cytoplasm


  • Gel-like substance where most of the chemical reactions happen
  • Contains enzymes that control these chemical reactions
• Cell membrane


  • Holds the cell together and controls what goes in and out
• Mitochondria


  • Where most of the reactions for aerobic respiration take place
  • Respiration transfers energy that the cell needs to work
• Ribosomes


  • Where proteins are made in the cell
• Additional subcellular structures found in plant cells

  • Rigid cell wall
  • Permanent vacuole
  • Chloroplasts
• Rigid cell wall


  • Made of cellulose, supports and strengthens the cell
• Permanent vacuole


  • Contains cell sap, a weak solution of sugar and salts
• Chloroplasts


  • Where photosynthesis occurs, which makes food for the plant
  • Contain chlorophyll, which absorbs the light needed for photosynthesis
• Bacterial cells are much smaller
• Bacterial cells


  • Don't have chloroplasts or mitochondria
  • Have a single circular strand of DNA that floats freely in the cytoplasm
  • May also contain one or more small rings of DNA called plasmids
• You could get asked to estimate the area of a subcellular structure in your exam. If you do, treat it as a regular shape. For example, if it's close to a rectangle, use the area formula 'area = length x width'.
• Microscopy


  The study of small objects using microscopes
• Microscopes


  • Important for biology
  • Let us see things we can't see with the naked eye
  • Microscopy techniques have developed over the years as technology and knowledge have improved
• Light microscopes


  • Use light and lenses to form an image of a specimen and magnify it (make it look bigger)
  • Let us see individual cells and large subcellular structures, like nuclei
• Electron microscopes


  • Use electrons instead of light to form an image
  • Have a much higher magnification than light microscopes
  • Have a higher resolution (ability to distinguish between two points, so a higher resolution gives a sharper image)
  • Let us see much smaller things in more detail, like the internal structure of mitochondria and chloroplasts, and even tinier things like ribosomes and plasmids
• Using a light microscope

  1. Clip the slide onto the stage

  2. Select the lowest-powered objective lens

  3. Use the coarse adjustment knob to move the stage up to just below the objective lens

  4. Look down the eyepiece and use the coarse adjustment knob to move the stage downwards until the image is roughly in focus

  5. Adjust the focus with the fine adjustment knob, until you get a clear image

  6. If you need to see the slide with greater magnification, swap to a higher-powered objective lens and refocus
• Cell Differentiation


  The process by which a cell changes to become specialised for its job
• Preparing a slide to view onion cells

  1. Add a drop of water to the middle of a clean slide

  2. Cut up an onion and separate it out into layers, use tweezers to peel off some epidermal tissue from the bottom of one of the layers

  3. Place the epidermal tissue into the water on the slide

  4. Add a drop of iodine solution (a stain)

  5. Place a cover slip on top, trying not to get any air bubbles
• Cell Differentiation


  1. Cells develop different subcellular structures

  2. Cells turn into different types of cells
• Stain


  Used to highlight objects in a cell by adding colour to them
• Most differentiation occurs as an organism develops
• In most animal cells, the ability to differentiate is then lost at an early stage, after they become specialised
• Lots of plant cells don't ever lose the ability to differentiate
• Standard form


  A way to write very big or small numbers with lots of zeros in a more manageable way, e.g. 0.017 can be written 1.7 x 10^-2
• Undifferentiated cell


  A cell that has not yet become specialised
• A mitochondrion is approximately 0.0025 mm long, written in standard form this is 2.5 x 10^-3 mm
• Differentiated cell


  A cell that has become specialised for a particular function
• The cells that differentiate in mature animals are mainly used for repairing and replacing cells, such as skin or blood cells
• Drawing observations from a microscope

  1. Draw what you see using a sharp pencil

  2. Make sure the drawing takes up at least half the space and has clear, unbroken lines

  3. Do not include any colouring or shading

  4. Draw subcellular structures in proportion

  5. Include a title and the magnification used
• Stem cells


  Undifferentiated cells
• You can work out the real size of a cell by counting the number of cells you can see along 1 mm