B2.2 - Organelles and Compartmentalization

Name Organelles with; no membrane

80s ribosomes, centrioles, microtubules, proteasomes, nucleoli 

name organelles with single membrane

rER, sER, golgi apparatus, lysosomes, vesicles, vacuoles

name organelles with double membrane

nucleus, mitochondria, chloroplast, amyloplasts, chromoplasts 

What are organelles? 

Organelles are parts of a cell which are adapted by their structure to perform one or more specific functions. To classify as an organelle cellular component must:
-have a specific function
-contain a discrete structure

Name structures which are not considered as organelles. 

• Cell wall as it is extracellular.  

• Cytoplasm, as it is not specialised to perform a specific function 

• Cytoskeleton, as it does not have a discrete structure. Instead, it contains different types of filaments and tubules.

Why do prokaryotic cells have fewer organelles than eukaryotes? 

They do not contain a nucleus; thus, they are less complex and have fewer organelles.  

What does differential centrifugation do? Name an application of it

• Differential centrifugation allows the study of individual organelles 

• An application of it is cell fractionation, which separates cellular components by size  

 
6. What is the advantage of separating nucleus and cytoplasm into separate compartments? 

• ‘Safeguards’ the DNA in the nucleus from other cellular processes that may be harmful 

• Allows for separation of the processes of transcription (nucleus) and translation (cytoplasm)  

• Separating these processes is crucial:  

• Enables post-transcriptional modification of mRNA (before being translated by ribosomes).  

• Stabilises the mRNA transcript  

• Removes non-coding sequences 

• Improves the efficiency of protein synthesis 

7. What are the advantages of compartmentalization of cells? 

• Enzymes can be concentrated in a small space – increases collisions between the active site and substrate 

• Substances that damage the cell are isolated, which protects remaining structures, as biochemical reactions are separate  

• Optimal conditions can be maintained (e.g., pH)  

• Large areas of the membrane can become dense with proteins for specific reactions 

• Organelles and their contents can be moved around in the cell  

 
8. Give an example of compartmentalization in the natural world. (Garlic cell compartmentalization.) 

• Garlic cells contain alliin in the vacuoles  

• The enzyme, alliinase is stored in different parts of the cell  

• Alliinase converts alliin to allicin when they react 

• This reaction occurs when biting in garlic cells or damaging them in other ways