Organelles in Protein Synthesis (HL) (HL IB Biology)

• Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane called the nuclear envelope 
• This provides an area where reactions of DNA and its functions can be carried out separately from the rest of the cell
• The envelop is studded with nuclear pores 
  • Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (e.g. DNA polymerases) and signalling molecules to travel in
• The outer membrane of the nucleus is a continuous structure that links to the endoplasmic reticulum, ribosomes can be attached to sections of this forming the rough endoplasmic reticulum
  • The sections of the outer membrane that do not contain ribosomes form the smooth endoplasmic reticulum
• During cellular division (mitosis and meiosis) the nuclear membrane breaks into vesicles 
  • This occurs during prophase to allow the chromosomes to separate
  • Once at telophase the nuclear envelope begins to reform 

Structure of the Nucleus Diagram

The nucleus of a cell with its double membrane containing pores and continuation into the endoplasmic reticulum

• Ribosomes are found in cells
  • Either freely in the cytoplasm (of all cells)
  • Or bound to the endoplasmic reticulum (ER) to form rough ER (only in eukaryotic cells)
• Ribosomes are the site of protein synthesis
• They consist of a large and a small subunit composed of protein and ribosomal RNA (rRNA)
  • Protein provides structure to the ribosome
  • rRNA facilitates the binding of mRNA and tRNA and catalyses the formation of peptide bonds between amino acids
• Ribosomes have three tRNA binding sites and one mRNA binding site
• mRNA sits in a groove between the two subunits and the ribosome moves along, forming a polypeptide as it travels

The Structure of a Ribosome Diagram

A diagram of a ribosome, showing the small and large subunits

Free Ribosomes

• In eukaryotic cells, protein synthesis commonly occurs at free ribosomes in the cytoplasm
• Free ribosomes can move within the cytoplasm and synthesise proteins for use primarily within the cell
  • As opposed to proteins destined to be secreted extracellularly
  • Proteins synthesised on free ribosomes are destined for use within the cytosol (the fluid part of the cytoplasm)
    • And within large organelles such as mitochondria and chloroplasts

Membrane Bound Ribosomes

• Eukaryotic cells make thousands of proteins that need to be delivered to the correct location, sometimes in different tissues/organs altogether
• When free ribosomes make proteins destined for lysosomes, or secretion from the cell, the ribosome becomes bound to the endoplasmic reticulum (ER)
• Signal sequences in the growing polypeptide chain dictate whether the free ribosome needs to move to the ER
  • The signal sequence occurs at the beginning polypeptide
  • Signal recognition proteins bind to the polypeptide, pausing translation
  • The free ribosome binds to a receptor on the ER, forming rough ER
  • Translation is re-initiated and the polypeptide chain moves inside the ER
• The synthesised protein can be carried via a vesicle to the Golgi apparatus before being secreted out of the cell

Membrane Bound Ribosome Protein Synthesis Diagram

Proteins destined for lysosomes or secretion out of the cell are synthesised by ribosomes bound to the endoplasmic reticulum

• The Golgi apparatus consists of flattened sacs of membrane called cisternae (like the rough endoplasmic reticulum)
• Its role is to modify proteins and lipids before packaging them into Golgi vesicles
  • The vesicles then transport the proteins and lipids to their required destination
  • Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles
• The position of the Golgi apparatus indicates its functions
  • One side of the sacs face the endoplasmic reticulum, this is called the cis side
    • It's role is to receive protein or lipid filled vesicles from the endoplasmic reticulum
  • The other faces the plasma membrane and is called the trans side
    • Once the received proteins or lipids have been modified the final products are sent out via the trans side

Structure of the Golgi Apparatus Diagram

The Golgi apparatus showing the cis and trans sides for receiving and transporting protein or lipid filled vesicles