A2.2.10

• How to recognise prokaryotes in micrographs:

  • single cells, small
  • cell wall
  • no nucleus, but nucleoid
  • simple internal structure
  • no vacuoles or other internal membranes
• How to recognise plant cells in micrographs:

  • always multicellular, larger
  • cell wall
  • nucleus (may not be visible)
  • plastids, f.e. chloroplasts or amyloplasts
  • often large vacuole
• How to recognise animal cells in micrographs:

  • always multicellular, larger
  • no cell wall
  • nucleus (not always visible)
  • no chloroplasts, but cytoplasm contains many other organelles
  • only small vacuoles
• Nucleus

  • double membrane with pores
  • contains chromosomes consisting of DNA with histone proteins
  • DNA is replicated and transcribed to form mRNA gere
  • export through pores to the cytoplasm
• Rough endoplasmic reticulum:

  • consists of flattened membrane sacs called cisternae
  • ribosomes are attached to the outside
  • function: to synthesise protein for secretion from cell
  • proteins synthesised by the ribosomes move to the cisternae and are then carried to the Golgi apparatus by vesicles
• Smooth endoplasmic reticulum:

  • consists of a branched network of tubular membranes
  • no ribosomes attached
  • function: to synthesise lipids, phospholipids and steroids
• Golgi apparatus:

  • consists of flattened membrane sacs called cisternae (as in rER)
  • there are vesicles nearby
  • function: processes proteins brought in vesicles from the rER
  • these proteins are then carried in vesicles to the plasma membrane for excretion
• Lysosome:

  • spherical with a single membrane
  • formed from Golgi vesicles
  • contain high concentrations of protein -> densely stained in micrographs
  • contain digestive enzymes which can be used to break down ingested food in vesicles
  • digestive enzymes can also break down organelles or cells
• Mitochondrion:

  • surrounded by double membrane
  • inner membrane forms cristae
  • the fluid inside is called matrix
  • produce ATP for the cell by aerobic respiration
  • fat is digested, if it is used as an energy source
• Free ribosomes:

  • no membrane
  • 80S
  • they synthesise protein, releasing it to work in the cytoplasm
  • they are made in the nucleolus
• Chloroplast:

  • double membrane
  • stacks of thylakoids (which are flattened sacs of membrane)
  • produce glucose (and other organic compounds) by photosynthesis
• Vacuoles and Vesicles:

  • single membrane with fluid inside
  • some plants have big vacuoles that occupy more than half of the cell
  • some animals absorb foods from outside and digest them in vacuoles
  • some unicellular organisms use vacuoles to expel excess water
  • vesicles are very small vacuoles used to transport materials inside the cell
• Microtubules and centrioles:

  • microtubules: small cylindrical fibres
  • roles: f.e. moving chromosomes during cell division
  • animal cells have centrioles which consist of two groups of non triple microtubules
  • centrioles from an anchor points for micro tubules during cell division and also from microtubules inside cilia and flagella
• Cytoskeleton:

  • constructed from several types of protein fibre
  • tubulin is used to make microtubules
  • actin is used to make microfilaments
  • can easily be constructed or deconstructed -> cytoskeleton is dynamic
  • microtubules guide the movement of components within the cell
  • they help plant cells to construct the cell wall
  • helps animal cells to maintain their shape
• Cilia and flagella:

  • project from the cell surface
  • contain a ring of 9 double microtubules + 2 central ones
  • flagella are large and usually only 1 is present
  • cilia are smaller and many are present
  • can be used for locomotion
  • cilia can be used to create a current in the fluid