A2.2 Cell structure

• Cells as the basic structural unit of all living organisms
• cell theory

  • The cell is the smallest unit of life
  • All living things are composed of cells
  • Cells only arise from pre-existing cells
• All cells share four basic features

  • They are enclosed by a membrane, which separates internal contents from the external environment.
  • They contain an internal fluid called the cytosol, in which various biological processes are able to occur
  • There is genetic material, which functions as a set of instructions for cellular activity
  • They possess ribosomes, which function to translate specific gene instructions within the cell
• ATYPICAL CELLS - eukaryotic

  • Striated muscle fibres
  • Aseptate fungal hyphae
  • Sieve tube elements
  • Red blood cells
• Striated muscle fibres

  • formed from the fusion of individual muscle cells and so are multinucleated
  • consist of fused muscle cells, resulting in widths of 100 µm and lengths of >12 cm
• red blood cells

  • have no nucleus and lack the capacity to replicate (new cells produced by bone marrow)
  • need to squeeze through narrow capillaries and so have a diameter of only 7 – 8 µm
• MRS HENG - functions of life

  • Metabolism
  • Reproduction
  • Sensitivity
  • Homeostasis
  • Excretion
  • Nutrition
  • Growth / Movement
• cell wall 


  a rigid structure that surrounds the bacterial cell maintaining its shape and preventing bursting. Made of peptidoglycan
• cytoplasm

  gel like fluid inside the cell membrane where metabolic reactions occur.
• flagellum

  long, thin, whip-like structures, with a core of microtubules that enables movement
• pilus 


  hair-like protein structures that allow bacteria to attach to things. Also involved in conjugation
• Nucleoid region 


  a dense region of DNA in a prokaryotic cell
• plasma membrane


  phospholipid bilayer that surrounds the cell
• ribosomes 


  organelles made of RNA that are the site of protein synthesis
• plasmid

  a small circular double-stranded DNA molecule,

  often the genes carried in plasmids provide bacteria with genetic advantages such as antibiotic resistance
• capsule

  functions include adhesion, transmission, resistance to innate host defences and resistance to the hosts immune responce
• mitochondria

  an organelle of eukaryotic cells that is the site of respiration, energy production and extra-nuclear genes
• cytoplasm 


  everything found inside the eukaryotic cell membrane except the nucleus

  • site of chemical reactions
• chloroplast 


  the chlorophyll-containing organelles in green plant cells where photosynthesis occurs
• vesicle 


  small cellulous container that can be used to move molecules , secrete substances , digest materials or regulate the pressure in the cell
• vacuole 


  a membrane bound fluid filled organelle of eukaryote cells that may contain nutrients of waste materials
• nucleus 


  region in the eukaryotic cell in which the major portion of the genetic code resides.

  its bounded by a double membrane
• nuclear membrane/envelope 


  the double lipid layer enclosing the nucleus of the eukaryotic cell
• golgi apparatus 


  an organelle distinguished by a series of stacked membranes sacs that is important in the packaging and transport of macromolecule cell products
• smooth endoplasmic reticulum


  synthesizes lipids, phospholipids, as in plasma membranes and steriods
• rough endoplasmic reticulum

  has ribosomes on it to produce proteins for the rest of the cell to function
• Aseptate fungal hyphae


  e lack internal partitions between cells and so have a continuous cytoplasm
• Sieve tube elements


  connected by plasmodesmata to form supracellular assemblies along the stem
• centimetre


  10-2
• millimetre 


  10-3
• micrometre


  10-6
• nanometre


  10-9
• metres to millimetres to micrometres to nanometres 


  x1000 each time
• nanometres to micrometres to millimetres to metres

  /1000
• As cells are typically too small to view with the naked eye, they may be visualised instead via the use of microscopes
• 2 types of microscopes

  light and electron
• light microscopy

  • Views living specimens in natural colour (uses lenses to bend light)
  • Has a much lower resoluCon and magnificaCon (roughly 100-fold)
• electron microscopy

  • Views dead specimens in monochrome (uses electromagnets)
  • Has a much higher resolution and magnification (can view in nm)
  • Transmission electron microscopes generate a cross-section
  • Scanning electron microscopes will render a 3D surface map
• improvements in microscopy

  • Immunofluorescence
  • Cryogenic electron microscopy
• Cryogenic electron microscopy

  • involves freezing biological specimens prior to visualisation with an electron microscope.
  • This allows for the determination of molecular structures at near-atomic resolution without requiring the crystallization of the specimen.
  • If the frozen specimen is physically broken along a specific plane via freeze fracturing, then internal cellular structures can also be studied at high resolution.