Biology-Cell structure

• Light microscope


  • Magnification: x1500
  • Resolution: 200 nm

  
• Electron microscope


  • Magnification: x250,000 - x500,000
  • Resolution: SEM - 3 nm, TEM - 0.5 nm

  
• Magnification calculations


  1. 1 mm = 1000 or 103 µm

  2. 1 µm = 1000 or 103 nm
• Magnification


  How much bigger a sample appears to be under a microscope than it is in real life

  
• Resolution


  The ability to distinguish between 2 separate points
• As resolution increases


  Image clarity and detail also increase
• Light microscopes


  • Limit of resolution: half the wavelength
  • Ribosomes (25nm) can't be seen as they don't interfere with the light waves
  • Different stains are absorbed by different cell organelles so they can be observed more clearly

  
• Electron microscopes


  • Require a vacuum as electrons cannot be focused without a vacuum
  • Sample must be dehydrated as water boils at room temperature in a vacuum
• Advantages of a light microscope over an electron microscope


  • Can observe living tissue
  • More portable
  • Easier to use - no technical training required
  • Possible to see real/natural colours and a live specimen
  • Can stain particular types of tissue for better visibility
• The cell is the basic unit of all living organisms
• The interrelationships between cell structures show how cells function to transfer energy, produce biological molecules including proteins and exchange substances with their surroundings
• Plant cells


  • Image 1
  • Image 2

  
• Animal cells


  • Image 1
  • Image 2
• Cell surface membrane


  • Selectively permeable membrane that allows for the exchange of certain substances
  • Barrier between cytoplasm and external environment
  • Cell recognition (surface antigens)
  • Selection of substances that enter/leave cells
• Nucleus


  • Controls cell's activities
  • Very dense, takes up colour the most when stained
  • Divides first during cell division
  • Surrounded by 2 membranes, known as the nuclear envelope which is continuous with the RER
  • Contains nuclear pores and nucleolus
• Ribosomes


  • Composed of 2 subunits
  • Carry out protein synthesis
  • 80S - cytoplasm, 70S - chloroplasts & mitochondria
• Rough endoplasmic reticulum (RER)


  • Membranes that form an extended system of fluid-filled sacs (cisternae)
  • Ribosomes are attached and are the site of protein synthesis
  • Proteins made by the ribosomes enter the sacs and are often modified
  • Small sacs (vesicles) break off and join to form the golgi body
• Golgi body / apparatus / complex


  • Stack of flattened sacs (cisternae) formed by the vesicles which bud off from the RER
  • Packages substances into vesicles for transport
  • Glycosylation
  • Phosphorylating proteins
  • Assembly of polypeptides into proteins (4º structure)
  • Folding proteins
  • Removing the 1st amino acid methionine to activate proteins
• Smooth endoplasmic reticulum (SER)


  Synthesizes lipids and steroids such as cholesterol and the reproductive hormones estrogen and testosterone
• Lysosomes

  • Spherical sacs surrounded by a single membrane
  • Not permanent structures
  • No internal structure
  • Contain hydrolytic enzymes
  • Responsible for digestion/breakdown of unwanted structures
• Mitochondria


  • Carries out aerobic respiration
  • Synthesizes ATP (adenosine triphosphate)
  • More in cells that have a higher demand for energy
  • Outer membrane contains a transport protein called porin
  • Energy released from energy-rich molecules is transferred to molecules of ATP
  • ATP leaves the mitochondrion and can spread rapidly to all parts of the cell where energy is needed
• Microtubules

  • Long, rigid, hollow tubes found in the cytoplasm
  • Made of a protein called tubulin
  • Supports and gives shapes to the cell
  • Assembly is controlled by microtubule organizing centers (MTOCs)
• Centrioles (and centrosomes)


  • Outside the nucleus of animal cells, 2 centrioles are present close together at right angles in a region called the centrosome
  • Centrioles are hollow cylinders about 500 nm long
  • Produces spindle fibers
  • Organizes microtubules
• Chloroplasts


  • Diameter 3-10 um
  • Carries out photosynthesis
  • Contains starch grains, circular DNA, and 70S ribosomes
  • ATP is also produced here
• Cell wall


  • Gives cell definite shape
  • Rigid as made of cellulose
  • Freely permeable
  • Prevents cell from bursting
• Plasmodesmata


  • Plant cells are linked to neighboring cells by fine strands of cytoplasm
  • Allows the transport of water, sucrose, amino acids, ions, etc., between cells without crossing membranes
  • Allows communication/signaling between cells
• Vacuoles


  • Surrounded by a partially permeable tonoplast which controls exchange between the vacuole and cytoplasm
  • Helps regulate osmotic properties of cells
  • Fluid present consists of water, salts, waste products, etc.
• Prokaryotic cells


  • Organisms that lack nuclei or proper nuclear membranes
  • Unicellular
  • 1-5um
  • Cell wall made of murein (peptidoglycan = protein + polysaccharides)
  • No membranes around organelles
  • 70S (smaller) ribosomes
  • Genetic material in the form of circular DNA
  • Have no ER
• Differences between eukaryotic and prokaryotic cells

  • Image

  
• Viruses


  • Noncellular/acellular
  • Protein coat
  • Nucleic acid core; DNA/RNA strand
  • Replicate inside host cells only
  • Show no characteristics of living organism
  • Symmetrical shape
  • The virus DNA/RNA takes over the protein synthesizing machinery of the host cell which helps to make new virus particles