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what are the differences between the two types of adipocytes?

brown adipose tissue (BAT) specializes in oxidizing FA for heat production - does so with mitochondrial uncoupling protein
white adipose tissue (WAT) stores energy as TG


from where does WAT receive its FA? what must it do to acquire the FA?

from lipoprotein particles
must hydrolyze the FA to TG in the lipoprotein to absorb it
uses LPL to do so


how does WAT use glucose? How does it attain glucose?

glucose enters WAT through GLUT4 channels
WAT makes glycerol-3-phosphate, which is used to make TG


in what form does WAT store TG?

as a lipid droplet in the cytoplasm


how does WAT release FA from TG?

using lipases, including ATGL and HSL


how does insulin affect storage and release of FA in WAT?

1: increases LPL production in WAT and increases LPL transport to blood vessel surface
2: increases amount of GLUT4 on the plasma membrane
3: decreases ATGL and HSL activity
4: decreases perilipin phosphorylation => decreased HSL access to lipid droplet


brown adipocyte (summary card)

1: many small lipid droplets
2: high oxidative capacity (lots of mitochondria)
3: fatty acid oxidation uncoupled from ATP production due ot high expression of an uncoupling protein (UCP) in mitochondrial membrane => leakage of proton gradient
4: uncoupling is used for heat generation
5: for humans, mainly in infant
6: amount of BAT inversely correlated with obesity, induced by exercise and cold temperature => growing evidence that BAT is important in adults too


how do the lipid droplets in BAT compare with those in WAT?

BAT has many small lipid droplets, whereas WAT has on large droplet


how does the oxidative capacity of BAT compare with that of WAT?

BAT has high oxidative capacity, whereas WAT has low oxidative capacity


what does uncoupling protein do? what type of cells is it found in? why is it important to that cell type's function?

uncoupling protein (UPC) is in the mitochondrial inner membrane of BAT cells
it allows FA oxidation in these cells to be separated from ATP production
this allows for the BAT cells to generate heat


what is the job of BAT?

generate heat


what is the job of WAT?

store and release FA for energy


what evidence has been found for an important role of BAT in adults?

induced by exercise and cold temperatures
levels are inversely correlated with obesity


in what age group is BAT usually found?

in infants


what type of WAT causes more risk of obesity-associated problems?

visceral WAT fat (belly fat) causes more risk than peripheral WAT fat (legs, buttocks, arms)


what are the sources of TG for human adipocytes?

chylomicrons - from intestines - carry dietary fat
VLDL - from liver - synthesized during times of excess dietary glucose


how if FA taken into adipose cells?

like in muscle, FA must by hydrolyzed from TG by LPL on the surface of blood vessel endothelial cells before uptake


what governs FA uptake in adipose cells?

concentration gradients
in anabolic state, ratio of FA outside:inside is high due to:
1: increased LPL activity (creates source outside adipocyte)
2: increased TG synthesis in the cells (creating a FA sink)


what happens to FA once it's been taken up into a cell? what are the steps of this process?

FA is esterified onto glycerol backbone (converted back into TG)
1: FA converted to acyl CoA by fatty acyl CoA ligase
2: glucose enters into adipocyte through GLUT4
3: glucose converted to glycerol-3-phosphate
4: fatty acyl-CoA coupled onto glycerol-3-phosphate => phosphatidic acid
5: phosphatidic acid is dephosphorylated => diacylglycerol
6: final esterification of a third acyl-CoA onto diacylglycerol


what are the steps in making glycerol-3-phosphate from glucose in adipose tissue?

1: glucose taken into cell by GLUT4
2: glycolyzed to dihydroxyacetone phosphate (DHAP)
3: DHAP converted to glycerol-3-phosphate


what are the enzymes involved in lipolysis? what factors regulate each enzyme? (summary card)

1: ATGL (adipocyte triglyceride lipase) positioning on lipid droplet is induced by insulin
2: hormone-sensitive lipase (HSL) - activated by PKA-dependent phosphorylation - adrenaline activates, insulin inhibits
positioning of HSL on the lipid droplet is also regulated
3: monoglyceride lipase - doesn't appear to be any regulation we need to know
4: perilipin - surronds lipid droplet - PKA-dependent phosphorylation creates conformational change => movement
adrenaline allows phosphorylation, insulin inhibits


what factors regulate ATGL? what do they do?

insulin and its counter-stimulatory hormone regulate by unknown mechanism, but they both control the positioning of the enzyme on the lipid droplet


what factors regulate HSL?

activated by PKA dependent phosphorylation
adrenaline activates
insulin inhibits
it's positioning on the lipid droplet is also regulated


where is perilipin found? how is it involved in lipolysis?

surrounds lipid droplet, blocking access to the lipid
must be phosphorylated => conformational change => moves out of the way so ATGL and HSL can access lipid
adrenaline stimulates phosphorylation
insulin inhibits


what happens to FA produced by lipolysis in adipose cells?

diffuses out of the adipocyte and into the blood stream, bound to albumin


what happens to the glycerol generated by lipolysis in adipose cells?

leaves cell through aquaporin-7 channels
goes to liver to undergo gluconeogenesis => glycerol


what are the general rules of short-term energy balance? (summary card)

1: insulin activates energy storage mechanisms
2: a major metabolic focus of the body is to keep blood glucose levels uniform - does so largely through insulin
3: plasma glucose levels must be maintained to supply fuel for the brain
4: the effects of blood glucose levels drive many short-term aspects of energy balance through insulin
5: plasma FA level fluctuates more widely than glucose levels and is inversely related to insulin levels - due to effects of insulin on adipocytes, liver, muscle
6: excess dietary AA are not stored, but converted to glucose via gluconeogenesis or FA via lipogenesis - muscle can burn AA for fuel


what is the relationship between plasma insulin levels and plasma FA levels?

plasma FA levels are inversely related to plasma insulin levels


what are normal levels and ranges of glucose and FA in the blood?

glucose is maintained at a uniform level of about 90 mg/dL
FA levels fluctuate widely from 0.1-2 mM


what happens to excess dietary AA?

they are not stored
converted to glucose via gluconeogenesis
converted to FA via lipogenesis