Biology Topic 1-Cells

• Cells


  Microscopic units that living organisms are made up of
• Cell membrane


  • Selectively permeable boundary that controls what enters or leaves the cell
  • Forms the boundary of the cell
• Cytoplasm


  • Main part of the cell where chemical reactions take place
• Chloroplast


  • Organelle found in plant cells that contains chlorophyll and helps the plant make food during photosynthesis
• Nuclear membrane


  • Surrounds the nucleus
• Animal cell


  Cells that make up animals
• Nucleus


  • Contains chromosomes made of DNA which hold the organism's genetic information
  • Control centre of the cell
• Mitochondria


  • Provide a lot of energy for animal cells
• Vacuole


  • Large permanent structure in plant cells that contains cell sap and provides support
• Cell wall


  • Stiff structure outside the cell membrane in plant cells that provides support
• Plant cell


  Cells that make up plants
• Bacterial cell


  • Microscopic single-celled organisms with a cell membrane surrounding cytoplasm but no nucleus
  • Genetic material in the form of a circular chromosome and plasmids
  • Cell wall not made of cellulose
• Most cells can only be seen using a microscope
• In school, a light microscope is used to observe cells on a slide
• Making slides


  1. Peel thin, transparent layer of epidermis cells from onion leaf

  2. Place epidermis cells on microscope slide with water, iodine solution or methylene blue

  3. Lower coverslip onto slide
• Temporary slides


  • Contain animal and plant cells
• Most slides containing animal cells will already be prepared for you
• Forceps


  Used to peel the thin, transparent layer of epidermis cells from the inside of a small section of onion leaf
• Coverslip


  Protects the lens of the microscope and prevents the cells drying out
• Light microscope


  • Eyepiece
  • Objective lenses
  • Stage
  • Focusing knob
  • Light source
• Using a light microscope

  1. Start with low power objective lens

  2. Move slide to centre cell/area of interest

  3. Rotate high power lens into place and focus
• Great care is needed when rotating high-power lens into position as it can damage the slide
• Drawing


  • Usual way of recording cells seen in a microscope
  • Uses biological knowledge to select detail
  • Made in pencil with firm, continuous lines
  • Large, with same proportions, faithful representation
  • Labelled with separate ruled lines
• Magnification


  The number of times the length of the image (in photograph or drawing) is larger than the actual length of the cell
• Length of cell is most appropriately measured in micrometres
• Scale bar


  Line drawn on or near photograph/drawing showing the actual length before magnification
• Calculating magnification using scale bar


  1. Measure length of scale bar image in millimetres

  2. Convert to micrometres

  3. Divide by actual length of scale bar
• Resolution


  Ability of microscope to show detail
• Best light microscopes can resolve details 0.2μm apart, requiring x1500 magnification
• Electron microscopes


  • Pass beam of electrons through specimen to investigate cell ultrastructure
  • Have much greater resolution, able to show details about 0.002μm apart
• Highest-powered electron microscopes can show detail of large molecules like proteins, requiring up to x500,000 magnification
• Cell membrane


  Selectively permeable, allowing some substances to pass in/out while preventing others
• Diffusion


  1. Random movement of a substance from high to low concentration

  2. Affected by concentration gradient, temperature, surface area
• Selectively permeable


  Cell membrane prevents the passage of some molecules while allowing others to pass through
• Diffusion


  1. Random movement of a substance from high to low concentration

  2. Factors affecting rate of diffusion: concentration gradient, temperature, surface area
• Diffusion is particularly important in gas exchange
• As cell size increases


  Surface area to volume ratio decreases
• There is a limit on the size of single-celled organisms due to the need for sufficient surface area to volume ratio
• Multi-celled organisms


  • Need large surface area to efficiently exchange substances with environment
  • Develop special gas exchange organs to increase surface area
• Levels of organisation in multi-celled organisms

  • Cells
  • Tissues
  • Organs
  • Organ systems