Cell Organelles (HL IB Biology)

Compartmentalised cell structure

• Eukaryotic cells have a more complex ultrastructure than prokaryotic cells
• The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles. These compartments are either bound by a single or double membrane
• Due to the absence of a membrane the following structures are not considered organelles
  • Cell wall
  • Cytoskeleton
  • Cytoplasm
• Eukaryotic cells have a number of compartmentalised organelles including:
  • The nucleus
  • Vesicles
  • Ribosomes
  • The plasma membrane
• The compartmentalisation of the cell is advantageous as it allows:
  • Enzymes and substrates to be localised and therefore available at higher concentrations
  • Damaging substances to be kept separated, e.g. digestive enzymes are stored in lysosomes so they do not digest the cell
  • Optimal conditions to be maintained for certain processes e.g. optimal pH for digestive enzymes
  • The numbers and location of organelles to be altered depending on requirements of the cell

Eukaryotic Animal Cell Structure Diagram

The ultrastructure of an animal cell shows a densely packed cell of compartmentalised organelles

Eukaryotic Plant Cell Structure Diagram

Plant cells have a larger, more regular structure in comparison to animal cells which also contains compartmentalised organelles

Organelle Adaptations

• In complex cells organelles can become specialised for specific functions
• These specialised organelles have specific adaptations to help them carry out their functions
• For example, the structure of a organelle is adapted to help it carry out its function (this is why each organelle looks very different from each other)
• The separation of organelles from the rest of the cell, via a membrane (sometimes double), is important as it allows the organelle to carry out its own chemical reactions without interference from the rest of the cell

NOS: Students should recognise that progress in science often follows development of new techniques

• Study of the function of individual organelles has become possible following the invention of ultracentrifuges and methods of using them for cell fractionation had been developed
  • In order to study cells at a molecular level we need to be able to separate out each compartment and study them individually in a process called cell fractionation
  • To do this a pure sample is needed (containing only the specific organelle being studied)
  • This process involves breaking up a suitable sample of tissue and then centrifuging the mixture at different speeds
  • Cell fractionation can be split into three stages:
    • Homogenisation - the cell sample is broken up using a homogeniser which is a blender-like machine 
    • Filtration - the homogenate (containing the homogenised cells) is then filtered through a gauze
    • Ultracentrifugation - the filtrate is placed into a tube and the tube is placed in a centrifuge
      • A centrifuge is a machine that separates materials by spinning
      • This speed can be altered to separate different components of the cell based on their molecular weight
      • Until this was invented, research into separate organelles was limited