1.2 - Ultrastructure

• Ultrastructure is all the structures of a biological specimen that are at least 0.1nm in their smallest dimension
• • Light microscopes allow us to see the structure of cells
  • Electron microscopes allow us to see the ultrastructure of cells, such as these pancreatic exocrine cells
• Prokaryotes reproduce asexually using the process of binary fission:

  • The DNA is replicated semi conservatively
  • The two DNA loops attach to the membrane
  • The membrane elongates and pinches off (cytokinesis) forming two separate cells
  • The two daughter cells are genetically identically (clones)
• There are several advantages in being compartmentalized:

  • Efficiency of metabolism - enzymes and substrates can localized and much more concentrated
  • Localised conditions - pH and other such factors can be kept at optimal levels. The optimal pH level for one process in one part of the cell 
  • Toxic / damaging substances can be isolated, e.g. digestive enzymes (that could digest the cell itself) are stored in lysosomes
  • Numbers and locations of organelles can be changed dependent on the cell’s requirements
• Nucleus

  • Generally spherical with a double membrane(DNA is too large to move through the pores)
  • Pores (holes) are present in the membrane
  • Contains genetic information in the form of chromosomes (DNA and associated histone proteins)
  • Uncoiled chromosomes are referred to as chromatin – they stain a dark colour and are concentrated at the edges of the nucleus
  • mRNA is transcribed in the nucleus (prior to use in protein synthesis in the cytoplasm)
  • mRNA leaves the nucleus via the pores
• The Mitochondrion

  • Has a double membrane
  • A smooth outer membrane and a folded inner membrane
  • The folds are referred to as cristae
  • Variable in shape
  • Site of ATP production by aerobic respiration (if fat is used as a source of energy it is digested here)
• Free ribosomes

  • 80S Ribosomes (approx. 20nm diameter) - larger than the ribosomes found in prokaryotes
  • No membrane
  • These appear as dark granules in the cytoplasm
  • Synthesizes proteins to function in the cytoplasm, for use within the cell, e.g. enzymes
• The Rough Endoplasmic Reticulum (rER)

  • The consists of flattened membrane sacs, called cisternae
  • Often located near to the nucleus
  • 80S Ribosomes are attached to the outside of the cisternae are ribosomes
  • rER synthesizes proteins which are transported, by vesicles, to the golgi apparatus for modification before secretion outside the cell
• The Golgi apparatus

  • This organelle also consists of flattened membrane sacs called cisternae, like rER.
  • Different to rER:
  • No attached ribosomes
  • Often sited close to the plasma membrane
  • The cisternae are shorter and more curved that those of the rER
  • The Golgi apparatus processes (modifies) proteins from from the rER. The proteins are then repackaged in vesicles for secretion outside the cell.
• Vesicles

  • A single membrane with fluid inside
  • Very small in size
  • Used to transport materials inside of a cell
• Lysosomes

  • Generally spherical with a single membrane
  • Formed from Golgi vesicles.
  • They contain digestive enzymes for breakdown of:
  • ingested food in vesicles
  • unwanted/damaged organelles
  • The cell itself
  • High concentration of enzymes (a type of protein) cause this organelle to stain heavily and hence appear dark on micrographs
• Vacuoles

  • Single membrane with fluid inside
  • In Plant cells vacuoles are large and permanent, often occupying the majority of the cell volume
  • In animals vacuoles are smaller and temporary and used for various reasons, e.g. to absorb food and digest it
• Flagellum

  • Thin projection (usually singular) from the cell surface.
  • Contain microtubules
  • Used to move the cell
• Cilia

  • Thin projections from the cell surface.
  • Contain microtubules
  • Used to either move the cell or to move the fluids next to the cell
• Microtubules

  • Small cylindrical fibres called microtubules
  • Have a variety of functions, e.g. part of the structure of flagella and they play a role in cell division
• Centrioles

  • Consist of two groups of nine triple microtubules
  • Are mainly found in animal cells, not present in vascular plants or fungi.
• Chloroplast

  • Many plant cells contain chloroplasts
  • A double membrane surrounds the chloroplast
  • Inside are stacks of thylakoids
  • Each thylakoid is a disc composed of a flattened membrane
  • The shape of chloroplasts is variable but is usually ovoid
  • The site of photosynthesis and hence where glucose is produced.
  • Starch grains maybe present if photosynthesis is happening quickly
• Cell wall

  • an extracellular component not an organelle
  • secreted by all plant cells (fungi and some protists also secrete cell walls).
  • Plant cell walls consist mainly of cellulose which is:

  1. Permeable - does not affect transport in and out of the cell
  2. Strong – gives support to the cell and prevent the plasma membrane bursting when under pressure
  3. Hard to digest –resistant to being broken down, therefore lasts along time without the need for replacement/maintenance