Topic 2 - Biology Paper 1

• Light microscope


  Uses a pair of convex glass lenses that can resolve images 0.2um apart
• Electron microscope


  Can distinguish between items 0.1nm apart
• Magnification


  Size of image/size of real object
• Resolution


  Minimum distance apart that two objects can be distinguished as separate objects in an image
• Transmission Electron Microscope (TEM)


  • Beam of electrons passes through a thin section of a specimen, areas that absorb the electrons appear darker
  • Produces an electron micrograph
• Scanning Electron Microscope (SEM)


  • Beam of electrons passes across the surface and scatters, the pattern of scattering builds up a 3D image depending on the contours of the specimen
• Limitations of electron microscopes

  • Whole system must be in a vacuum so living specimens cannot be observed
  • Complex staining process required which may introduce artefacts
  • Specimens have to be very thin, particularly for TEM
  • SEM has lower resolving power than TEM
• Cell fractionation


  1. Cells are blended in a homogeniser

  2. Homogenate is placed in a centrifuge and spun at increasing speeds to separate organelles

  3. Heaviest organelles like nuclei sediment first, followed by mitochondria, etc.

  
• Homogenate


  Fluid resulting from blending cells in a homogeniser
• Supernatant


  Fluid at the top after centrifugation, leaving just the sediment of the organelles
• Homogenate is placed in a cold, buffered solution to prevent organelle bursting, inactivate enzymes, and maintain pH
• Eukaryotic cells


  • Contain a nucleus and membrane-bound organelles

  
• Ultrastructure of eukaryotic cells

  • Nucleus
  • Rough endoplasmic reticulum
  • Smooth endoplasmic reticulum
  • Golgi apparatus
  • Mitochondria
  • Centrioles
  • Ribosomes
  • Lysosomes

  
• Structures in prokaryotic cells

  • Cell wall
  • Capsule
  • Plasmid
  • Flagellum
  • Pili
  • Ribosomes
  • Mesosomes

  
• Viruses


  Non-living structures consisting of nucleic acid (DNA or RNA) enclosed in a protective protein coat (capsid), sometimes with a lipid envelope

  
• Cells of multicellular organisms are organised into tissues, tissues into organs, and organs into systems
• Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  
• Interphase


  Cell grows and prepares to divide, chromosomes and some organelles are replicated, chromosomes begin to condense

  
• Cytokinesis


  Parent and replicated organelles move to opposite sides of the cell, cytoplasm divides to produce two daughter cells
• Importance of mitosis


  • Growth: produces identical cells for organism growth
  • Repair: produces identical cells to replace dead tissues
  • Reproduction: some single-celled organisms reproduce by mitosis
• Binary fission


  1. Circular DNA replicates and attaches to cell membrane

  2. Cell membrane grows inward, dividing cytoplasm

  3. New cell wall forms, producing two identical daughter cells
• Viruses do not undergo cell division, instead they inject their nucleic acids into a host cell which then replicates the virus particles

  
• Fluid mosaic model


  Structure of biological membranes, composed of a sea of phospholipids with protein molecules between them, giving the membrane fluidity and a mosaic appearance
• Components of cell membrane

  • Phospholipids
  • Proteins (intrinsic and extrinsic)
  • Cholesterol
  • Glycolipids
  • Glycoproteins
• Diffusion


  Passive movement of small, non-polar, lipid-soluble molecules from high to low concentration
• Facilitated diffusion


  Requires channel proteins to transport polar, charged, and water-soluble molecules across the membrane
• Osmosis


  Diffusion of water molecules from high to low water potential through a partially permeable membrane
• Active transport


  Requires energy (ATP) to transport molecules from low to high concentration
• Exocytosis and endocytosis


  Transport of large particles enclosed in vesicles, fusing with or budding from the cell membrane

  
• Co-transport


  Uses ions to move substances into and out of cells, particularly in epithelial cells of the ileum
• Factors that increase rate of gas exchange by diffusion

  • Increased surface area
  • Decreased diffusion distance
• Transport across cell membrane

  1. Active transport (requires ATP)

  2. Exocytosis (transport large particles out of cell)

  3. Endocytosis (transport large particles into cell)

  4. Co-transport (uses ions to move substances in and out of cell)
• Diffusion


  • Surface area increases
  • Diffusion distance decreases
  • Diffusion gradient becomes more steep
  • Temperature increases
• Bacteria


  Prokaryotic cells, genetic information in circular DNA strand
• Viruses


  Consist of nucleic acid enclosed in protein coat, genetic material can be DNA or RNA
• Differences between bacteria and viruses

  • Bacteria are prokaryotic, viruses consist of nucleic acid and protein coat
  • Bacteria can survive without host, viruses are dependent on host
  • Viruses are smaller than bacteria
  • Bacteria have cell membrane, wall, cytoplasm, organelles, viruses have no such structures
• Bacterial disease


  • Tuberculosis (TB)
• Viral disease


  • HIV/AIDS
• Physical barriers to infection

  • Skin (keratin)
  • Stomach acid
  • Gut and skin flora
• Antigen


  Proteins on cell surface that identify self vs non-self