Exchange of Substances Across Capillary Wall

[justadream]

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How exactly are fats taken up by adipocytes? I’ve read 2 methods



1) After forming chylomicrons and entering the veins (after being transported from the lymphatic vessels from lacteal in the small intestine), enzymes in the blood (specifically, lipoprotein lipase) break up the chylomicrons so that adipocytes can take-up the fatty acids/glycerol. The only part of the chlyomicrons that end up going to the liver are the cholesterol (and any excess fatty acids/glycerol).

2) Liver takes up chylomicrons for processing into lipoproteins. The lipoproteins carry fat to the adipocytes for storage. The difference here is that all chylomicrons are not processed on their way to the liver



Is it both?

 

[sillyjoe]

How exactly are fats taken up by adipocytes? I’ve read 2 methods



1) After forming chylomicrons and entering the veins (after being transported from the lymphatic vessels from lacteal in the small intestine), enzymes in the blood (specifically, lipoprotein lipase) break up the chylomicrons so that adipocytes can take-up the fatty acids/glycerol. The only part of the chlyomicrons that end up going to the liver are the cholesterol (and any excess fatty acids/glycerol).

2) Liver takes up chylomicrons for processing into lipoproteins. The lipoproteins carry fat to the adipocytes for storage. The difference here is that all chylomicrons are not processed on their way to the liver



Is it both?

The easiest way to look at this is that chylomicrons (a form of lipoprotein) are the main mechanism for absorbing and delivering EXOGENOUS fats (from diet) throughout your body intestines --> lacteal --> lymphatics etc.

Your body can also synthesize fats and in the liver it will package it into VLDL (also a lipoprotein) which is the main transport mechanism for synthesized endogenous fats.

This is a simplification but is more than sufficient for MCAT understanding. More to come in biochemistry in medical school!

 

[justadream]

@sillyjoe

So basically, adipoycytes get their fatty acids/glycerol AFTER the chylomicrons are processed in the liver?

That is what I thought before. Statement I is from a TPR SW passage (all that info was in the passage).

 

[sillyjoe]

@sillyjoe

So basically, adipoycytes get their fatty acids/glycerol AFTER the chylomicrons are processed in the liver?

No, most adipocytes in a fed human will get their fatty acids from chylomicrons before the chylomicron remnant is taken up by the liver. Remember, it goes small intestines --> lacteal --> lymph --> circulation --> adipocytes / muscle / etc --> chylomicron remnant to liver

Just think of VLDL as mostly packaged triglycerides that are synthesized in the body or a minimal amount from leftover chylomicron remnants. I think this is already much more in depth than needed for the MCAT. At least you know you will love med school biochem once you're there!

 

[justadream]

@sillyjoe

Ok thanks for the reply!

So physiologically speaking, what's the point of the liver? Just to take up extra chylomicrons and process cholesterol?

Like why (physiologically speaking) doesn't the Liver process/regulate these fatty acids/glycerol before allowing other cells to take it up (for example, the liver first processes other nutrients like sugars from the hepatic portal vein before these nutrients are released into the blood)?

 

[sillyjoe]

@sillyjoe

Ok thanks for the reply!

So physiologically speaking, what's the point of the liver? Just to take up extra chylomicrons and process cholesterol?

Like why (physiologically speaking) doesn't the Liver process/regulate these fatty acids/glycerol before allowing other cells to take it up (for example, the liver first processes other nutrients like sugars from the hepatic portal vein before these nutrients are released into the blood)?

That is a very in depth question and may even be over my head. Simply, one major function of the liver is acting as a metabolic homeostatic machine. If glucose is too low it will release glucose from stored glycogen or synthesized via gluconeogenesis. It also stores glucose as glycogen when glucose levels in the blood are high. It functions similarly with lipids in that it can store them, synthesize them, and release them. The liver also synthesizes cholesterol however when we have enough exogenous cholesterol from our diet it mainly goes to making bile.

In addition, the liver can synthesize, process, and release amino acids. It removes the nitrogen groups from amino acids via aminotransferases and turns the amino acid remnant to a usable form of energy via gluconeogenesis or ketone bodies. It can also release amino acids to the body when in need. All in all the liver maintains our metabolic homeostasis which is why the chylomicron remnants end up in the liver and why the liver can synthesize fats.

There is a physiological dilemma in your body that the liver solves. You generally want your blood to have constant levels of nutrients/waste yet you only eat and excrete at certain times. If it were just based on that, you would never have constant levels of anything. The liver plays a major role in ensuring constant levels of all nutrients/waste in your body. I should give the kidney some credit too.

 

[justadream]

@sillyjoe

Thanks - that was really helpful.

"the liver can synthesize fats" - the liver can synthesize fats from sugars, correct? (and this occurs once the liver has exceeded it's max glycogen storage)?

Also, this is somewhat of a thread self-hijack but with regard to muscle glycogen - do muscle cells synthesize that themselves?

 

[sillyjoe]

@sillyjoe"the liver can synthesize fats" - the liver can synthesize fats from sugars, correct? (and this occurs once the liver has exceeded it's max glycogen storage)?

Fat synthesis is via the condensation of acetyl CoA which comes mainly from sugars but also from fats/protein/acetate. The liver does not only synthesize fats when glycogen stores are in excess.

@sillyjoeAlso, this is somewhat of a thread self-hijack but with regard to muscle glycogen - do muscle cells synthesize that themselves?

The muscle only synthesizes glycogen for itself. This occurs as a result of two primary factors. One, glycogen becomes "insoluble" in the cytoplasm of muscle and the liver. It does this because if it was soluble it would create a huge and unsustainable osmotic pressure.

Second and more importantly, the muscle lacks glucose-6-phosphatase to remove the phosphate that is put on glycogen immediately when it enters the cells via glucose-6-phosphate. If you can't remove the phosphate, you cannot get it through the cellular membrane. The muscles are selfish and only have glycogen for their own usage. The liver is an altruistic organ providing the body with it's glucose needs via glycogen because it has glucose-6-phosphatase.