Membrane Structure, Composition of Cells, Part of Cell Volume Flashcards

1
Q

What kind of phospholipid motility is seen in plasma membrane?

A

Lateral diffusion; flexion; rotation. Flip-flop rarely occurs and usually requires ATP driven Flippase

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2
Q

Name the 3 classes of lipids in a membrane? What do they all have in common?

A

Phospholipids; sphingolipids; cholesterol. All are amphipathic (have hydrophilic and hydrophic domains) and all are synthesized in ER

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3
Q

What are the most common phospholipids?

A

phosphatidylethanolamine (PE); phosphatidylcholine (PC); phosphatidylserine (PS); and phosphatidylinositol (PI).

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4
Q

Describe the structure of cholesterol

A

Has a polar hydroxyl group; a rigid steroid ring group; and a nonpolar hydrocarbon tail

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5
Q

What effects does cholesterol have on a membrane?

A

Interaction of the steroid ring with the hydrophobic tail of other phospholipids immobilizes the lipid and decreases fluidity. Cholesterol straightens lipids and determines membrane thickness. More cholesterol = thicker membranes. Thus; intracellular membranes have less cholesterol (are thinner) than plasma membranes

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6
Q

Describe the distribution of certain lipids among the two layers of the plasma membrane

A

Negatively charged phosphatidylserine (PS); phosphatidylethanolamine (PE) and phosphatidylinositol (PI) are more abundant on the internal surface. PC; sphingomyelin and glycolipids are more abundant on the external surface. Cholesterol is distributed equally btwn the two layers

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7
Q

What are 2 ways we get cholesterol?

A

Ingestion/uptake and synthesis by the liver

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8
Q

How is uptake/cholesterol synthesis regulated?

A

Negative feedback for cholesterol production. If you get enough in the diet; you decrease synthesis and vice versa. Uptake depends on low-density lipoprotein receptor (LDLR)

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9
Q

What is HMGCoA reductase?

A

The first and rate-limiting enzyme in cholesterol synthesis pathway.

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10
Q

What do statins do?

A

Lower cholesterol by blocking the HMGCoA reductase step.

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11
Q

What does the sterol regulatory element binding protein (SREBP) do?

A

Contains a transcription factor that regulates both uptake (via LDLR) and synthesis (via regulation of all 30 synthesis proteins) of cholesterol.

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12
Q

Where are the sensor and SREBP located? Why is the sensor there?

A

Sensor is in the ER membrane since that is where cholesterol is lowest and changes are easiest to detect.

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13
Q

Describe the transcription factor of SREBP

A

It is a basic helix-loop-helix (bHLH) DNA-binding protein. It is inactive when it is bound to SREBP and only becomes active when it is cleaved from SREBP (when cholesterol is low) and translocates to the nucleus to active all 30 cholesterol synthesis enzymes. Also activates genes to produce more LDLR to bring cholesterol into the cell

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14
Q

Where are the proteases that cleave SREBP to release the bHLH? What does this mean for SREBP?

A

Both are located in the Golgi. SREBP must be held in the ER until cholesterol is low and then SREBP must move to the Golgi where it gets cleaved and the bHLH released.

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15
Q

What does SCAP do?

A

It is a SREBP cleavage activating protein. It binds to both SREBP and sterols like cholesterol. When SCAP is bound to SREBP there is a SCAP signal domain that is recognized by a coat protein (COPII) for vesicles that move from the ER to the Golgi. This is how the SCAP/SREBP complex gets packaged into vesicles to go to the Golgi for cleavage to release the transcription factor

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16
Q

What does Insig do?

A

It is a protein that binds to SCAP when cholesterol is high. Prevents SCAP from binding to SREBP and blocks the signaling part of SCAP

17
Q

What is RIP - regulated intramembrane proteolysis? Why is it important?

A

Two step proteolysis that occurs within the membrane. This is how bHLH is cleaved from SREBP. Also important for Notch signaling in development and for cleavage of the amyloid precursor protein (APP) to produce the beta amyloid peptide in Alzheimer’s disease

18
Q

What do S1P and S2P (site-1 protease and site-2 protease) do?

A

Once in the Golgi the SREBP-SCAP complex encounters active S1P. S1P cleaves SREBP at site 1 (this is luminal)- cutting it into two halves. Because each half still has a membrane-spanning helix; each remains bound in the membrane. The amino-terminal half of SREBP (the “business end” of the molecule) then goes on to be cleaved at site 2 (lies within its membrane) by S2P. This releases the bHLH signal

19
Q

What is a GPI linkage and why is it important with regard to some bacteria?

A

Some proteins are anchored to the membrane on the extracellular face of the cell by a GPI linkage. This GPI linkage is a target for some bacterial toxins which binds to the GPI linkage and then inserts into the membrane; forms a pore and kills the cell.

20
Q

On to composition of cells

A

OK

21
Q

What is typical volume of plasma?

A

3 liters (Note: this is part of the 13 liters of ECF)

22
Q

What is typical volume of extracellular fluid (ECF)?

A

13 liters + 5 L for the “3rd space”

23
Q

What is typical volume of intracellular fluid (ICF)?

A

27 liters

24
Q

Describe the [Na+] in the ICF and ECF and describe its membrane permeability

A

About 10x more Na+ outside the cell than inside. Concentration is 14 mM in ICF and 140 mM in ECF. Is EFFECTIVELY not permeable (is permeable but gets pumped out)

25
Q

Describe the [K+] in the ICF and ECF and describe its membrane permeability

A

About 29x more K+ inside the cell than outside. Concentraion is 145 mM in ICF and 5 mM in ECF. Is permeable

26
Q

Describe the [Cl-] in the ICF and ECF. Describe its membrane permeability. (Note: this also includes HCO3-)

A

About 29x more Cl- outside the cell than inside. Concentraion is 5 mM in ICF and 145 mM in ICF. Is permeable

27
Q

Describe the [A^-n] in the ICF and ECF and describe its membrane permeability. (Note: A^-n describes the big anions)

A

Effectively all of these are inside the cell. Concentration is 126 mM in ICF and ~0 in ECF. Not permeable

28
Q

Describe the concentration of water in ICF and ECF. Describe its permeability

A

About the same inside and outside the cell. Concentration is ~55000 mM in ICF and ECF. Is permeable

29
Q

On to cell volume regulation

A

OK

30
Q

What are 3 ways to counter osmotic force?

A

Water impermeable membrane; cell wall (hydrostatic force); osmotic balance in and out