Intracellular Membrane Traffic

Question 1

What are some examples of cell polarity through regulated endocytosis and exocytosis in plants?

Answer 1

The pollen tube in plants grows highly polarized, where vesicles delivering cell wall materials and membrane proteins are specifically targeted to the growing tip.

Root hairs and other specialized cells rely on targeted vesicle delivery to expand at specific points, maintaining structural and functional polarity.

Question 2

What are some examples of cell polarity through regulated endocytosis and exocytosis in animals?

Answer 2

In epithelial cells, specific molecules are transported selectively to the apical or basolateral side, maintaining cell polarity.

In nerve cells, polarity ensures that signals are sent and received at the correct locations.

Question 3

Transcytosis in plants?

Answer 3

Its seen in polarized cells, PIN auxin efflux carriers are involved in hormone transport.

Question 4

What is Transcytosis?

Answer 4

Transports materials across cells by endocytosis at one surface and exocytosis at another.

Question 5

What is Receptor-mediated endocytosis?

Answer 5

It occurs when ligands bind to receptors, which triggers clathrin-coated vesicle formation. Low pH in endosomes causes receptors to release their ligands, allowing the receptor to recycle back to the plasma membrane.

Question 6

What is the Autophagy process?

Answer 6

It is one of the processes for sending materials to lysosomes for degradation.

It is when damaged/unnecessary cellar components are enclosed in a double membrane autophagosome that fuses with the lysosome/vacuoles

essential in both plants and animals, with variations seen in plant seed development and stress responses

Question 7

What is the Phagocytosis process?

Answer 7

It is one of the processes for sending materials to lysosomes for degradation.

It is when large particles are engulfed by phagosomes, which fuse with lysosomes.

Question 8

What is the Endocytosis process?

Answer 8

It is one of the processes for sending materials to lysosomes for degradation.

It is when cellular material from outside the cell is engulfed in clathrin-coated vesicles, which fuse with the lysosomes after maturation through early and late endosomal stages

Question 9

What is the direct pathway from the ER to protein storage vacuoles?

Answer 9

Proteins can bypass the Golgi and go directly to the ER to the protein storage vacuoles via

Carried by Dense vesicles: Seed storage globulins

Protein bodies are delivered to vacuoles by autophagy Seed storage prolamins

Question 10

Protein transport to vacuoles from the Golgi?

Answer 10

In plants, vacuolar proteins contain specific sorting proteins, such as the NPIR sequence at the N-terminal/Hydrophobic C-terminal sequences. They are then recognized in the Golgi for proper sorting to the vacuoles:

-Dense vesicles carry storage proteins to storage vacuoles.

-Clathrin-coated vesicles transport lytic enzymes to lytic vacuoles.

Question 11

Protein transport to lysosomes from the Golgi?

Answer 11

In animals, proteins have an Mannose-6-Phosphate marker, which is added to their oligosaccharides. This marker is recognized and binds to the mannose-6-phosphate receptors which directs proteins into vesicles bound for lysosomes.

Question 12

Vacuoles in plants?

Answer 12

Vacuoles regulate cell size and turgor pressure, supporting the structure integrity of plant cells and facilitating cell growth without increasing the cytoplasmic volume.

Lytic: similar to animal lysosomes and plays a crucial role in intracellular digestion.

Storage: stores nutrients, metabolites, and waste products.

Question 13

Lysosomes in animals?

Answer 13

It serves as the cell’s primary degradative compartment. It has a low pH and maintains proton pumps, which optimize the environment for waste breakdown, recycle cellular components through enzymes, and protect the cytoplasm from lytic enzyme activity.

Question 14

What is the difference between the Cis, medial, and Trans Golgi?

Answer 14

The cis Golgi is the first compartment and is closest to the nucleus. Proteins and lipids enter the CGN from the ER in vesicles. Proteins can either move on in the Golgi or be returned to the ER.

The medial Golgi is the middle compartment where protein modification occurs.

The trans-Golgi is the final compartment and is farthest from the ER. Proteins exit the TGN and are sorted for their destination: the cell surface, secretory vesicles, or lysosomes. Proteins can also be returned to an earlier compartment.

Question 15

What is the fate of the Golgi during the cell cycle in plants?

Answer 15

The Golgi remains intact during cell division and aligns along the division site, known as the phragmoplast, to assist with cell plate formation. Golgi vesicles deliver materials needed for the new cell wall, actively participating in plant cell cytokinesis by forming the cell plate separating daughter cells.

Question 16

What is the fate of the Golgi during the cell cycle in animals?

Answer 16

The Golgi undergoes fragmentation during mitosis to allow equal distribution between daughter cells.
This fragmentation is prompted by microtubule depolymerization, which breaks down the Golgi’s structural support and disperses it throughout the cell. After mitosis, these fragments are reassembled to restore the Golgi apparatus in each daughter cell.

Question 17

What are SAR1, Arf, and SNAREs

Answer 17

SAR1: mediates budding for COP1-coated vesicles.
Arf: mediates budding for COP2-coated and clathrin-coated vesicles.
SNAREs: vesicle docking and vesicle target membrane fusion.

Question 18

In plants, what does the Golgi synthesize?

Answer 18

Cell wall polysaccharides: Hemicellulose and pectin are vital for cell wall structure and function.

Question 19

Types of Glycosylation?

Answer 19

N-linked: initiated in the ER and further processed into Golgi into complex and high mannose oligosaccharides dependent on protein accessibility and folding.

O-linked: occurs in the Golgi and affects proteins that will be secreted or embedded in the cell membrane.

Question 20

What modifications happen in the Golgi?

Answer 20

Glycosylation: Sugar moieties are attached to proteins and lipids, enhancing their stability and function. It prevents protein aggregation and improper folding, marks protein for sorting and transport, cell surface glycoproteins gain protection, and glycosylation of receptors influences signaling by modifying ligand specificity.

Question 21

How does the differential PH between ER and Golgi affect the retrieval process back to the ER?

Answer 21

The Golgi is more acidic, which enables the KDEL receptor to bind to ER proteins containing a KDEL sequence and release them once they return to the neutral pH of the ER lumen.

Question 22

What signals are used for ER proteins to be retrieved from the Golgi?

Answer 22

Via Retrograde transport: COP1 coated vesicles recognize specific signals such as:

-KKXX sequence at the C-terminus for membrane proteins

-The KDEL sequence for soluble ER proteins binds the KDEL receptors in the Golgi and engages with the COP1 coat for proper retrieval.

Question 23

How do proteins migrate through the Golgi?

Answer 23

As proteins move through the cis, medial, and trans cisternae of the Golgi, they undergo stepwise processing:

-diffusion through cisternae

• signaling
• The cisternal maturation model: as each cisterna matures, it moves from the cis to the trans face of Golgi, obtaining enzymes that progressively modify cargo proteins.
• The vesicular transport: presents the vesicles carrying cargo between stable Golgi cisternae.

Question 24

What is the Golgi high mobility dependent on?

Answer 24

Actin-dependent.

Question 25

How are proteins transported from the ER to the Golgi in plants?

Answer 25

Vacuum cleaner model: The Golgi stacks move along actin filaments and efficiently sweep up vesicles, enabling rapid and efficient trafficking.

Question 26

How are proteins transported from the ER to the Golgi in animals?

Answer 26

Vesicular tubular clusters transport proteins from the ER toward the cis-golgi, where they are further processed. VTCs’ movement along microtubules toward the Golgi ensures an organized cargo delivery to the Golgi.

Question 27

What are VTCs, and where are they used?

Answer 27

Vesicular tubular clusters are intermediates that form when COP2 vesicles from the ER fuse together before reaching the Golgi in animal transport.

Question 28

What type of vesicles does Anterograde transport use, and how?

Answer 28

COP2-coated vesicles are responsible for selecting the molecules for transport by recognizing exit signals.

Question 29

What type of vesicles does Retrograde transport use, and how?

Answer 29

It’s mediated by COP1-coated vesicles, which retrieve ER resident proteins accidentally transported to the Golgi and recycle other Golgi proteins.

Question 30

What is Retrograde transport?

Answer 30

The retrieval of proteins from the Golgi back to the ER and moves cargo back along the endocytic pathway.

Question 31

What is Anterograde transport?

Answer 31

The movement of vesicles transports newly synthesized proteins, lipids, and other molecules from the ER to the Golgi apparatus and eventually to the plasma membrane.