membrane bound organelles

animal cell

• Cells are highly organised 

• Vast majority of proteins are produced on cytoplasmic ribosomes + encoded by nuclear genome 

  • Others are produced in mitochondria 

how do internal organelles evolve

Developed methods of invagination of the membrane 

• Thought to be some advantage of the way the membrane interacted with cell 

 Nuclear membrane formed first 

• Then endoplasmic reticulum 

• Are continuous with each other 

 Mitochondria and chloroplasts have a different origin 

mitochondria, chloroplasts and peroxisomes

• Mitochondria 

  • Oxidative phosphorylation 

  • Production of majority of cellular ATP 

• Chloroplasts 

  • photosynthesis 

• Peroxisome 

  • Fatty acid beta oxidation 

  • Source of energy 

endomembrane system

• endoplasmic reticulum

• rough

• smooth

• Golgi apparatus

• vesicles

endoplasmic reticulum

• Entry point of newly synthesised proteins  

• Translocon 

  • Allows proteins to go into channel into endoplasmic reticulum 

  • Doesn’t let ions in 

  • Facilitates the assembly of integral proteins 

    • If not they get into lumen of endoplasmic reticulum 

    • Can stay or move onto other organelles 

    • Integral proteins move laterally 

    • Out of translocon into membrane 

rough ER

• Studded with ribosomes 

• Cytoplasmic ribosomes (on cytoplasmic side) 

• Dynamic  

  • Interacts with system 

smooth ER

• No ribosomes 

• Involved in lipid synthesis 

Golgi apparatus

• Organised into Golgi sacs/cisternae 

• Cis face, trans face 

• New proteins travel from cis to trans 

vesicles

• Secretory 

• Release molecules to cell surface 

  • exocytosis 

• Can be diverted 

  • e.g. lysosomes 

• Form via budding 

• Fuse to release 

why are there different compartments

• Sequential processing / modification of secreted proteins 

  • Important for these to happen in the correct order 

• Keep reactions separate 

  • e.g. enzymes separate 

processes that take place in the endoplasmic reticulum

• N-glycosylation

• Disulphide bond formation

• folding and quality control

disulphide bond formation

• Lumen = reducing site 

  • So happens on lumen side 

• Proteins disulfide isomerase forms bond 

• Oxidation 

• Protein gets reduced 

folding and quality control

• Proteins not correctly folded do not leave ER 

  • Role is not in ER, so will not start incorrectly functioning 

• Binds to glucosyl transferase (adds glucose) 

  • Protein binds to calnexin 

  • Keeps protein in endoplasmic reticulum 

• Sugar removed by glucosidase 

  • If protein is still incorrectly folded then glucose added again 

• Once protein is correct, it will no longer be detected by glucosyl transferase and it can leave the ER lumen 

Golgi apparatus

• In stacks 

• Enzymes 

  • Modify sugars on lipids and proteins 

  • Cleave proteins (e.g proteases) 

N and O linked glycosylation

• N-linked 

  • Adding sugar on asparagine 

  • In ER 

• O-linked 

  • Adding sugar on threonine/serine 

• ABO blood groups 

  • Due to different sugar modifications 

  • At antigen terminus 

    • O= n/a 

    • A = N-acetylgalactosamine 

    • B=N-acetylglucosamine 

N glycosylation

What it is:
Attachment of a sugar chain to the nitrogen (N) atom of the amide group of asparagine (Asn).

Where it occurs:

• Begins in the rough endoplasmic reticulum (RER)

• Further modified in the Golgi apparatus

Amino acid requirement:

• Occurs at the consensus sequence:
  Asn–X–Ser/Thr (X ≠ proline)

Process:

• A pre-assembled oligosaccharide is transferred all at once to the protein.

• The glycan is then trimmed and modified.

Functions:

• Protein folding and stability

• Cell–cell recognition

• Protection from degradation

Examples:

• Antibodies (IgG)

• Many membrane and secreted proteinsO

O glycosylation

What it is:
Attachment of a sugar to the oxygen (O) atom of the hydroxyl group of serine or threonine.

Where it occurs:

• Mainly in the Golgi apparatus

Amino acid requirement:

• No strict consensus sequence

Process:

• Sugars are added one at a time directly to the protein.

Functions:

• Lubrication and protection (e.g., mucus)

• Structural support

• Cell signaling

Examples:

• Mucins

• Proteoglycans

lysosomes

• Degradative organelles 

• First assembled in ER 

• Ph = 5 inside lumen (for enzymes) 

  • Nucleases 

  • Proteases 

  • Glycosidases 

  • Lipases 

  • Phosphatases 

  • Sulfatases 

  • Phospholipases 

• Lumen reduced by a H+ pump 

  • ATP -> ADP + pi 

  • H+ pumped into lumen 

protein sorting

• A eukaryotic cell contains 10 billion (~10¹⁰) protein molecules 

• Cells are highly organised 

• Different organelles contain different sets of proteins. 

• If proteins were not sorted correctly chemical chaos would result. 

• Protein synthesis/degradation 

 

• Starts in the cytosol 

• 4 outcomes 

  • Stays put 

  • Nucleus 

  • ER trafficking 

  • mitochondrion