Topic 2 - CELLS

• 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
• Scanning Electron Microscope (SEM)


  • Beam of electrons passes across the surface and scatters, the pattern of scattering builds up a 3D image
• 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 centrifuged at slow speed, nuclei sediment

  3. Supernatant is removed and centrifuged at faster speed, mitochondria sediment

  4. Process continues to separate out organelles by size

  
• Homogenate is placed in a cold, buffered solution of the same water potential as the cells
• 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 producing two daughter cells
• Importance of mitosis


  • Growth: produces identical cells for 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 inwards, dividing cytoplasm

  3. New cell wall forms, producing two identical daughter cells
• Fluid mosaic model


  Structure of biological membranes, composed of a sea of phospholipids with protein molecules between them

  
• 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 a channel protein 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


  Transports molecules from low to high concentration using energy in the form of ATP
• Exocytosis and endocytosis


  Transport large particles by enclosing them in vesicles made from the cell surface membrane

  
• Co-transport


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

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

  1. Active transport

  2. Exocytosis

  3. Endocytosis

  4. Co-transport
• Active transport


  • Requires energy in the form of ATP
• Exocytosis


  Transport of large particles out of the cell in vesicles
• Endocytosis


  Transport of large particles into the cell in vesicles

  
• Sodium and potassium ions pumped out of epithelial cell

  Causes these ions to move in from the lumen by facilitated diffusion, which at the same time brings glucose and amino acids into the cell
• Factors that increase rate of gas exchange by diffusion

  • Increased surface area
  • Decreased diffusion distance
  • Steeper diffusion gradient
  • Increased temperature
• Bacteria


  Prokaryotic cells with circular DNA, do not require a host to survive
• Viruses


  Consist of nucleic acid enclosed in a protein coat, can have DNA or RNA, entirely dependent on hosts

  
• Differences between bacteria and viruses

  • Bacteria are prokaryotic, viruses consist of nucleic acid and protein
  • Bacteria do not require a host, viruses are dependent on hosts
  • Viruses are significantly smaller than bacteria
  • Bacteria have cell membrane, cell wall, cytoplasm and organelles, viruses have no such structures

  
• Tuberculosis infection


  1. Mycobacterium tuberculosis infects phagocytes in lungs

  2. Bacteria lie dormant in tubercles

  3. Immune system weakens, bacteria become active and destroy lung tissue
• HIV infection


  1. Initial symptoms include fevers, tiredness and headaches

  2. Antibodies appear in blood, making person HIV positive

  3. Immune system weakens, leading to AIDS with symptoms like weight loss, diarrhoea, dementia, cancers, opportunistic infections
• Physical barriers to infection

  • Skin
  • Stomach acid
  • Gut and skin flora