Question

1. Starting with the nucleus, name 4 parts of the endomembrane system and explain how they work together to produce a mature polypeptide.
2. Name and explain the 3 different ways proteins can be sorted.
3. Name and explain 3 functions of the plasma membrane.
4. Define the 3 different types of passive transport and be sure to tell me what direction molecules move in each (down or against the concentration gradient?)

5. Explain the two main distinguishing differences between passive transport and active transport (be specific about which process does what)?

Answer 1

Answering in the order of question asked:

1. Endomembrane system to form the mature polypeptide: endomembrane system consists of nucleus, ER, golgi apparatus, lysozyme, endosomes, vesicle and plasma membrane. Starting from the nucleus the organelle carry out the process of transcription and makes RNA out of the genomic DNA and process it for the translation which is carried out by ribosomes sitting on the ER mainly. This transcribed RNA is transported to the ribosome which translates the mRNA into protein directly into the lumen of ER where it gets modified by adding the target specific sequences for the purpose of sorting the peptide. This matre and properly folded peptide s then transported to the golgi where the organelle decides the fate of peptide based on the target sequence attached with it and helps the peptide to get to the target location with the help of endosomes or vesicles. Proteins without target sequence are retained in the cytoplasm.
2. 3 types of protein sorting:
   1. Co-translational localization: protein are sorted to the target organelle while it is still in the process of translation, e.g. translation of proteins by ER bound ribosomes, they directly translocate proteins into the ER while it is still translating.
   2. Post translational translocation: proteins first completes the translation by cytoplasmic ribosomes and based on the peptide signal they have the proteins are targeted to the destination.
   3. Vesicle mediated translocation: particularly there are target vesicles such as V-snares and T-snare which functions in the protein targeting to the desired location.
3. Three basic functions of plasma membranes are:
   1. It separates the cell from surrounding and establish a independently functioning unit.
   2. Allows selective permeability for the molecules needed for the cell.
   3. They separate vital but incompatible metabolic processes conducted within organelles.
4. Passive Transport: Passive transport involves the movement of material along a concentration gradient without involvement of energy (high concentration ⇒ low concentration). There are three main types of passive transport:
   1. Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.) (high concentration ⇒ low concentration).
   2. Osmosis – movement of water molecules (low solute concentrations ⇒ high solute concentration)
   3. Facilitated diffusion – movement of specific molecules (large or charged) via specialized membrane proteins (e.g. ions, sucrose, etc) (high concentration ⇒ low concentration).
5. Difference between active and passive transport:
   1. Active transport:
      1. Transports the molecules against the concentration gradient
      2. Requires external energy in the form of ATP
   2. Passive transport:
      1. Transports molecules along thee concentration gradients. (high concentration ⇒ low concentration)
      2. Requires no external energy.