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CELL BIO > WEEK 3 > Flashcards

WEEK 3 Flashcards

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Biology 101 flashcards
1
Q

Functions of the golgi apparatus:

A

Proteins destined for secretion and for a variety of organelles/vesicles within the cell are sorted, modified and dispatched from the Golgi apparatus

It is also a major site of carbohydrate synthesis in the form of glycoproteins and proteoglycans

Therefore the Golgi tends to be prominent in secretory cells like this intestinal goblet cell

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2
Q

Describe the structure of the golgi apparatus:

A

Endoplasmic reticulum
Golgi vesicles
Vesicular tubular cluster

Cis Golgi network (CNG)
Cis cisterna
Medial cisterna
Trans cisterna
Trans Golgi Network (TNG)

Secretory vesicles

Plasma membrane or other organelles

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3
Q

The cis face…

A

lies nearest the ER and is the site at which vesicles from the ER dock

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4
Q

The golgi

A

stack of flattened membrane bound compartments (cisternae)

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5
Q

Dispatch from the endoplasmic reticulum to the golgi:

A

Protein folding must be complete before a protein leaves the ER

  • almost all proteins in the ER lumen are glycosylated and this acts as a folding tag
  • during the intial phases of folding 2 of the 3 terminal glucose molecules are removed
  • Calnexin is a chaperone which recognises the single glucose of incompletely folded proteins and prevents their export to Golgi

The final glucose is then removed:
1. If partially folded gucosyl transferase adds another glucose and it tries again
2. Misfolded proteins are chaperoned back to the ER protein translocator and are sent to the cytoplasm for degradation
3. Correctly folded proteins go on to be exported to the Golgi

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6
Q

Golgi Maturation and Processing

A
  • For soluble proteins this involves interactions with transmembrane receptors

Proteins need “exit” signals for efficient export: non-cargos are packaged at a much lower rate

  • Most exit signals are not known
  • The coat protein COPII interacts with the cytosolic tail of the receptor causing a vesicle to bud off
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7
Q

Movement of the transport vesicles:

A
  • Once COPII coated vesicles bud from the ER they rapidly shed their coat
  • They undergo homotypic fusion – like joining with like (Details of how vesicles fuse next set of lecture bites – vesicular transport)
  • The resulting vesicular tubular cluster (VTC) moves along microtubules (see cytoskeleton lectures – week 4)… …to deliver its contents to the Golgi
  • Once the cargo reaches the Golgi it is released from its receptor…. …. Mediated by a decrease in pH – lots more hydrogens around so amino acids will change charge and therefore proteins change structure – then cargo and receptor no longer complementary and is released into cis golgi network
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8
Q

Why might the change in pH cause the cargo to be released from its receptor?

A

Decreasing pH (contintue from lecture…)

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9
Q

Transport back to the endoplasmic reticulum

A

ER proteins are retrieved from the golgi

  • Proteins that participate in ER budding (receptors) retrieved from the vesicular tubule cluster (recycled)
  • Also proteins which have escaped the ER by mistake need retrievingCOPI coated vesicles bud from VTC / Golgi, are uncoated & transported back to ER
  • Resident ER membrane proteins contain the cytosolic sequence -KKXX which interacts directly with COPI
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10
Q

Why are modifications needed:

A
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11
Q

Point of glycosylation as a protein marker…

A
  • for complete folding - for targeting transport between the ER and Golgi - for sorting within the Golgi
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12
Q

Point of glycosylation as a protector…

A
  • relatively inflexible oligosaccharides prevent proteases approaching and digesting extracellular proteins
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13
Q

Point of glycosylation as a cellular marker:

A
  • Oligosaccharides on cell surface proteins form part of the cell-cell recognition and adhesion mechanism. Different cell types may express a slightly different complement of glycosyltransferases, allowing them to modify their surface proteins in a cell specific way
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14
Q

Point of glycosylation : regulatory roles…

A
  • Cell surface signalling receptors may be glycosylated and alterations to their glycosylation pattern may affect their activity in different cell type
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15
Q

Transport through and out of the golgi:

A

Vesicles from the ER fuse to form VTCs and Cis Golgi Network Once completed CGN displaces upwards and becomes cis cisterna…

… matures into the medial…

… and then trans cisternae

Enzymes etc retrieved by retrograde transport (recyclying enzymes)

Vesicles bud from TGN for dispatch or retrieval, until it is all gone and another cisterna replaces it

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16
Q

Exocytosis =

A
17
Q

Endocytosis =

A
18
Q

Types of vesicular transport

A

exocytosis

endocytsosis

19
Q

Types of Exocytosis pathways..

A

Constitutive Secretory Pathway

Regulated Secretory Pathway 1

20
Q

Constitutive secretory pathway:

A
  • happening all the time

In the Golgi, any protein which is not either returned to the ER, retained in the Golgi lumen, directed to the lysosomes or regulated secretion is transported by this route – i.e. it is the default pathway

Vesicles bud from the trans Golgi network are targeted to the plasma membrane and fuse with it immediately

21
Q

Regulated secretory pathway 1

A

Allows concentration of secretory proteins

  • Some cells concentrate and store secretory products in special vesicles called secretory granules.
  • In the acidic conditions of the Golgi secretory proteins aggregate, then leave the trans Golgi network as loosely bound immature secretory vesicles.
  • The concentration of secretory proteins increases as the vesicles mature due to:- - retrieval of the membrane back to the Golgi - increased acidity in the maturing vesicle lumen, causing tighter aggregation
22
Q

Endocytosis

A
  • Following the release to secretory proteins to the extracellular space there is a transient increase in the surface area of the plasma membrane. This can be huge!
  • These extra components must be removed and recycled
  • Cells also take up macromolecules and even other cells
  • The process by which this occurs is endocytosis and there is a balance between secretion and internalisation – the endocytotic - exocytotic cycle
23
Q

Types of endocytosis

A

Pinocytosis
Receptor - mediated endocytosis
Phagocytosis

24
Q

Pinocytosis

A

Clathrin-coated pits typically occupy 2% of the cell surface. They rapidly invaginate and form vesicles which lose their coat and fuse with the early endosomes.

Takes some from outside to see what the conditions are like…

Types of endocytosis In this way extracellular constituents are delivered to the lysosomes for digestion (more on this in next topic) and membrane components are recycled.

25
Q

Receptor - mediated endocytosis

A

E.g. low density lipoproteins (LDLs) containing cholesterol interact with LDL-receptors on animal cells. The cytoplasmic domain interacts with adaptin, guiding its into pits for internalisation by endocytosis. In the endosome, LDL is released from its receptor (recycled) and delivered to the lysosome where the cholesterol is released.

26
Q

Phagocytosis

A
  • Specialised white blood cells (macrophages, neutrophils and dendritic cells) can take up large particles such as microorganisms and dead cells
  • Such particles bind to the surface of phagocytes and are recognised by cell surface receptors, e.g. micro organisms are rapidly coated in antibodies, which trigger receptors on the surface of macrophages and neutrophils
  • This stimulates the cell to extend pseudopods which fuse to engulf the particle
  • The resulting large endocytic vesicles, called phagosomes, fuse with lysosomes so that their contents can be digested (next lecture)
27
Q

(7) Lysosome functions

A
  • Hydrolytic enzymes are transported from the endoplasmic reticulum where they are synthesised.
  • Macromolecules targeted for digestion come from a variety of sources.
  • Digestion products (sugars, amino acids and nucleotides) are transported out of the lysosome by transporters in the membrane.
28
Q

What happens to endocytosed contents?

A

Fuse with early endosomes: a sorting site for endocytosed molecules
THEN:
1. Recycling

  1. Transcytosis
  2. Degradation
29
Q

Recycling

A

Membrane & many receptors sent to “recycling endosome” Vesicles return to plasma membrane

30
Q

Transcytosis

A

Vesicles return to different part of PM; transports material across cell

31
Q

Degradation

A

Cargoes (& some receptors) sent to late endosomes (mechanism not precisely known) which mature into lysosomes

Macromolecules degraded & their components used to make new molecules.

32
Q

Transport to the lysosomes

A
  • signal peptide directs synthesis of lysosomal proteins into the endoplasmic reticulum, N-linked glycosylation
  • cis Golgi network: signal patch directs phosphorylation of terminal mannose on N - linked oligosaccharide

Mannose-6-phosphate (M6P): The signal for lysosomal delivery

In the trans Golgi network:
M6P receptors in the membrane of the trans Golgi network bind M6P - labelled lysosomal hydrolases on the luminal side…

….and to adaptins on the cytosolic side…

…packaging the hydrolases into clathrin coated transport vesicles which bud from the Golgi

33
Q

How the late endosome is formed:

A
  • Transported vesicles fuse with the late endosome
  • acidic pH (-6) causes the M6P receptor to dissociate from M6P - labelled hydrolase, releasing it into the lumen
  • A signal in the cytoplasmic tail of the M6P receptor targets it into a transport vesicle which takes it back to the trans Golgi network
34
Q

Maturation to form a lysosome

A

-In the late endosome phosphate is removed from mannose sugars on the hydrolases to ensure that they are not returned to the Golgi

  • As the H+ pump lowers the pH the acid hydrolases begin to digest macromolecules delivered from the early endosomes…
  • …and the endosome mature into a lysosome
35
Q

Lysosomal membrane is semi-permeable with the usual routes out:

A
  • passive diffusion
  • membrane transporters
  • secretory lysosomes
36
Q

Passive diffusion

A

Small hydrophobic molecules can pass out of the lysosome by diffusion

37
Q

Membrane transporters

A

Digestion products (sugars, amino acids and nucleotides) are transported out of the lysosome by transporters in the membrane.

38
Q

Secretory lysosomes

A

Lysosomal secretion allows cells to eliminate indigestible debris

Usually this is a minor pathway used mainly under stress, however in some cells specially adapted lysosomes fuse with the cell membrane

39
Q

The principles of Vesicular transport

A
  • membrane bound vesicles carry cargo in their lumen and membrane from a donor to a recipient compartment
  • vesicles bud from specialised coated regions of the donor compartment
  • the coat is later discarded, allowing the membranes of the vesicles and recipient to interact and fuse.

CELL BIO (8 decks)

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