HomeWork Chapter 4 | Endomembrane System

Question 1

Differences between rER and sER (both in structure and function)

Answer 1

Structural differences
- Ribosomes: rER has ribosomes attached to its membrane, while sER does not.
- Appearance: Due to the ribosomes, rER appears rough under a microscope, whereas sER appears smooth.
- Membrane shape: rER often forms flattened sacs or cisternae, while sER tends to have a more tubular network.

Functional differences
The functional differences between rER and sER exist as it relates to the individual structures of the organelles
- rER function: Primarily involved in protein synthesis and folding due to the presence of ribosomes.
- sER function: Primarily involved in lipid synthesis, detoxification, and calcium storage, as it lacks ribosomes and has a more tubular structure.

Question 2

Contents of signal hypothesis

Answer 2

The signal hypothesis states that proteins secreted from a cell have a signal sequence that directs them to and through membranes and is widely used to explain the process of protein synthesis and secretion.

Involved in the signal hypothesis is a signal peptide (sequence f 15-35 amino acids including 4-12 hydrophobic amino acids located at the N-terminal or internally) and a signal recognition particle (consisting of 6 protein subunits and a small 7S L RNA with a translation stop peptide, signal peptide recognition peptide and SRP receptor binding peptide).

The process occurs as follows:
- the signal peptide is synthesized on free ribosomes
- the signal recognition particle (SRP) recognizes and binds to the signal peptide, which forms the SRP-ribosome complex causing the interruption of mRNA translation
- ribosomes bind to the translocator on ER membrane, then forms the SRP-SRP receptor-ribosome complex
- SRP dissociates with the ER and join the recycle, and the translation process continues
- the signal peptide is cleaved by the signal peptidase on the inner membrane of ER
- finally the ribosomes dissociate with the ER and join the recycle.

Question 3

Function of Golgi Complex

Answer 3

1. Cell secretion
   - constitutive secretion: continual, unregulated discharge of material from the cells
   - regulated secretion: the discharge of products stored in cytoplasmic granules, in response to appropriate stimuli
   - examples: endocytosis & exocytosis
2. Glycosylation and modification
   - the Golgi complex plays a key role in the assembly of the carbohydrate component of glycoproteins and glycolipids:
   (a) modifications of N-linked oligosaccharides
   (b) modifications of O-linked oligosaccharides
   - the Golgi networks are processing and sorting stations where proteins are modified, segregated and then shipped in different directions.
3. Protein transport and protein hydrolysis (proteolysis)
   - examples:
   (a) the transport of ER retention proteins
   (b) the process of preproinsulin
4. Vesicular transport
   - vesicular transport model: proposes that proteins move through the Golgi apparatus by being carried in small membrane vesicles that bud off from one cisterna (compartment) and fuse with the next, essentially acting as “shuttles” to transfer cargo between static Golgi cisternae, each with its own set of modifying enzymes
   - cisternal maturation model: proposes that cargo proteins move through the Golgi stack by remaining within a continuously maturing cisterna, meaning the cisternae themselves migrate from the cis face to the trans face of the Golgi, gradually changing their enzyme composition as they progress, rather than the cargo being actively transported between static cisternae via vesicles
5. Protein sorting
   - the process where proteins received from the endoplasmic reticulum (ER) are further modified and then segregated into different vesicles based on their final destination within the cell, such as the plasma membrane, lysosomes, or secretion pathways, with the sorting primarily occurring at the “trans-Golgi network” (TGN) region of the Golgi apparatus

Question 4

The types, features and functions of lysosome

Answer 4

1. Types of lysosome
   - primary lysosome: organelles that store inactive hydrolytic enzymes until they are needed to break down substances like proteins, carbohydrates, lipids, and nucleic acids; produced in the Golgi apparatus and are then transported to other parts of the cell; small, membrane-bound vesicles.
   - secondary lysosome: a membrane-bound organelle formed when a primary lysosome fuses with a phagosome or endosome
   (a) autolysosome (b) heterolysosome (c) phagolysosome
   - post lysosome: the processes that occur after substances are transported to lysosomes.
2. Features of lysosome
   - heterogenous organelle: a typical lysosome contains approximately 60 different hydrolysis enzymes; all the enzymes of the lysosome have optimal activity at acid pH and are as such acid hydrolases with a pH of approximately 5.0
   - marker enzyme: acts as an acid phosphatase
   - the lysosomal membrane:
   (a) acid pumps: keeps the internal proton concentration high
   (b) glycosylated proteins: can protect the lysosome from self-digestion
   (c) transport proteins: transport digested material
3. Functions of lysosome
   (a) phagocytosis: the process where specialized cells, like macrophages, engulf large particles like bacteria or cell debris through phagocytosis, and then these engulfed particles are transported to and fused with lysosomes, allowing the lysosomal enzymes to break down and digest the ingested material within the newly formed “phagolysosome.
   (b) autophagy: a process that involves the delivery of cytoplasmic components to lysosomes for degradation; it is a catabolic process that means “self-eating”. It’s a quality-control mechanism that removes damaged organelles and misfolded proteins to promote cell survival
   (c) endocytosis: Endocytosis is the process by which a cell takes in substances from its surrounding environment by forming vesicles from the plasma membrane, and these vesicles then typically fuse with lysosomes, where the ingested material is broken down and digested by the lysosome’s enzymes; essentially, endocytosis is the initial step of bringing material into the cell that will ultimately be processed by the lysosome for degradation.

• primary lysosomes fuse with either phagocytic or autophagic vesicles forming residual bodies that either undergo exocytosis or are retained in the cells as lipofuscin granules