Lecture 5: Endomembrane System and Bulk Transport Processes

Question 1

What is the endomembrane system made up of?

Answer 1

nuclear envelope
endoplasmic reticulum
golgi apparatus
vesicles
lysosomes
vacuoles
plasma membrane

Question 2

Endoplasmic Reticulum (ER) is made up of?

Answer 2

Smooth ER (sER)
Rough ER (rER)

Question 3

Functions of the endoplasmic Reticulum (sER)

Answer 3

Metabolism of carbohydrates
Lipid synthesis for membranes
Detoxification of drugs and poisons
Storage of calcium ions (used as a signal in the cell)
Extensive sER in cells active in these processes
The amount of sER can be increased or decreased
to meet demand

Question 4

Functions of the Rough Endoplasmic Reticulum (rER)

Answer 4

Rough appearance due to ribosomes
Involved in protein synthesis
Secreted and membrane-bound proteins enter the
lumen (interior) of the rER
and are processed by the rER and the rest of the
endomembrane system for release from the cell or
retention on the cell membrane

NOTE: Synthesis of cytoplasmic proteins occurs
on free ribosomes

Question 5

Function of the Golgi Complex:

Answer 5

Series of membrane sacs and
associated vesicles

Receives, modifies, sorts and ships
proteins arriving from the rER

Has polarity: cis and trans face

Vesicles from endoplasmic
reticulum arrive at the cis face

Processed vesicles leave at the
trans face

Question 6

where do vesicles from the endoplasmic reticulum arrive on the golgi complex?

Answer 6

cis face

Question 7

Where do the processed vesicles leave on the golgi complex?

Answer 7

trans face

Question 8

where does the golgi complex receive the proteins from?

Answer 8

the rough endoplasmic reticulum

Question 9

What is the function of the golgi complex?

Answer 9

glycosylation
sorting proteins
directing vesicle trafficking

Question 10

what is glycosylation?

Answer 10

addition or modification of carbohydrates to proteins

important for secreted or cell surface proteins

golgi also produces many polysaccharides which may also be secreted from the cell

Question 11

what is sorting proteins in the golgi complex?

Answer 11

adds molecular markers to direct proteins to the correct vesicles before “budding” from the trans face

Question 12

what is directing vesicle trafficking in the golgi complex?

Answer 12

adds molecular tags to vesicles leaving the trans face to direct them to the correct targets

such tags are often short proteins exposed on the vesicle side

act as docking sites when they reach their target

Question 13

what are the two directions the tags from the golgi complex that the vesicles could take?

Answer 13

some tags direct vesicles to the lysosome

others direct to secretory pathways

Question 14

what is exocytosis?

Answer 14

exocytosis is the transport of glycoproteins out of the cell or delivers it to the cell surface

Question 15

what is constitutive exocytosis

Answer 15

releases extracellular matrix proteins

Question 16

what is regulated exocytosis

Answer 16

releases hormones and neurotransmitters

Question 17

what is endocytosis?

Answer 17

the cell takes in molecules and particulate matter at the plasma membrane

Question 18

what is phagocytosis?

Answer 18

cell ‘eating’
uptake of ‘food’ particles
forms a phagocytic vacuole which is digested by the lysosomes
in humans this occurs in macrophages

Question 19

what is pinocytosis?

Answer 19

cell ‘drinking’
up-take of extracellular fluid containing various solutes such as protein and sugars
up-take vesicle is formed with the aid of a coat protein
up-take is non-selective

Question 20

what is receptor-mediated endocytosis?

Answer 20

specialised form of pinocytosis
allows the cell to take up bulk quantities of specific substances which may be present at only low concentration in the extracellular fluid
receptor proteins are used to selectively capture the required solute

Question 21

Lysosomes

Answer 21

Lysosomes are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids.

Question 22

How are lysosomes made?

Answer 22

Lysosomes are membrane bound organelles made by the rER

and Golgi body containing hydrolytic enzymes

Question 23

Is the inside of a lysosome acidic or basic?

Answer 23

The interior of a lysosome is acidic which is required for the
enzymes to be active

They degrade proteins, lipids, carbohydrates and nucleic
acids and release breakdown products into the cell

Question 24

What do lysosomes digest? What is this process called?

Answer 24

Lysosomes digest and recycle unwanted cellular materials

This process is called autophagy and is important for cell
health

Lysosomal digestion is also important in programmed cell
death in which whole cells “intentionally” die

Defects in lysosomal enzymes can result in lysosomal
storage diseases

Question 25

are vacuoles important in plants?

Answer 25

can perform lysosome-like functions

large central vacuole absorbs water allowing plant cells to grow
without a large increase in cytoplasm

Question 26

What are the components of the endomembrane system?

Answer 26

nuclear envelope
endoplasmic reticulum
golgi apparatus
vesicles
lysosomes
vacuoles
plasma membrane

Question 27

What is the overall function of the endomembrane system?

Answer 27

The endomembrane system (endo- = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins.

Question 28

What are the structural and functional differences of

sER and rER?

Answer 28

The most basic difference between RER and SER is the presence of ribosomes. When ribosomes attach to the surface of an ER, it gives a characteristic rough appearance; hence it is called Rough ER. On the other hand, a smooth ER does not have ribosomes on its surface. It possesses ribosomes attached to its membrane.

Question 29

What is the function of the golgi complex?

Answer 29

The Golgi apparatus, or Golgi complex, functions as a factory in which proteins received from the ER are further processed and sorted for transport to their eventual destinations: lysosomes, the plasma membrane, or secretion. In addition, as noted earlier, glycolipids and sphingomyelin are synthesised within the Golgi.

Question 30

What is the difference between constitutive and

regulated exocytosis?

Answer 30

In constitutive exocytosis, secretory materials are released continuously. But, in regulated exocytosis, secretory materials are released into secretory vesicles on-demand via secretagogues and signal transduction. So, this is the key difference between constitutive and regulated exocytosis

Question 31

What are the three types of endocytosis?

Answer 31

pinocytosis, phagocytosis and receptor-mediated endocytosis

Question 32

Why is the inside of a lysosome acidic?

Answer 32

Lysosomes have many enzymes, which need an acidic environment for proper functioning, they are referred to as acid hydrolyses. These enzymes assist the disintegration of polysaccharides, proteins, lipids, nucleic acids etc. Such enzymes require low pH compared to the cytoplasm to stay active.