Lipid Transport
Key Points:
How are fats transported around the body
How are they targeted to different tissues
What if things go wrong?
Fats taken up in the gut are transported through the circulation to the liver. The way
the body regulates fats is very different to carbohydrates, fat levels fluctuate much
more.
Lipids are transported from the liver to non-hepatic tissue as they circulate, being
taken up by mammary, muscle or adipose tissue.
Fats and Lipids in Circulation include:
-Triacylglycerol (triglycerides) (45%)
-Cholesterol esters and free cholesterol (15%)
-Phospholipids (35%)
-Free fatty acids (5%)
The %’s given in human plasma vary greatly with nutritional state. All are insoluble
in water, they are not transported completely freely dissolved in the blood. This is
because they are hydrophobic and also they interfere with membrane structure
(being components of membranes themselves)
Free Fatty Acids
The majority of lipids are transported in lipoproteins, but there is some transport of
free fatty acids. These are transported not exactly free but instead bound to albumin.
These are formed from triacylglycerides and stored in adipose tissue.
Fatty acids enter cells by simple diffusion and intracellular concentration of FFA is
kept low.
Lipoproteins
The majority of lipids are transported in the blood as plasma lipoproteins, these are
relatively large structures consisting of both the lipid and large proteins (called
apolipoproteins). There are five types of lipoproteins, classified by their density:
-Chylomicrons
-Very low density lipoproteins (VLDL)
-Intermediate density lipoproteins (IDL)
-Low density lipoproteins (LDL)
-High density lipoproteins (HDL)
Chylomicrons have the least density (and are largest in size), while HDLs have the
highest, each particle also has a different function.
, Diagram of a lipoprotein
As can be seen the lipoprotein consists of a large protein with phospholipids
orientated similarly to how they are in a membrane, but instead as a single layer
(more like a micelle). Then in the hydrophobic core is the hydrophobic compounds
like cholesterol esters and triglycerides.
The thing that distinguishes lipoproteins is the apolipoprotein present and the
amount of triacylglycerol/lipid that is present. The amounts in a single particle do
vary however depending on where in the circulation the lipoprotein is.
Chylomicrons have a very low density, they have high TG and low protein. HDLs have
a very high density with low TG and high protein.
As stated earlier a big determinant of lipoprotein is the apolipoprotein present, this
is summarised below:
Chylomicrons ----- B48, Apo C2, C3 and E
VLDL ---- B100, Apo C1, C2, C3 and E
IDL ----- B100, Apo E
LDL ---- B100
HDL ---- Apo A1, A2, C1, C3, D, E
Because of the unique apoproteins present, it can be used to determine which one it
is and where it was synthesised e.g. apoprotein B48 in chylomicrons.
These apolipoproteins have multiple functions, which include:
Structural = Form a substrate in which lipid can be constructed
To solubilise lipids = Allowing body to transport a difficult class of molecules
Key Points:
How are fats transported around the body
How are they targeted to different tissues
What if things go wrong?
Fats taken up in the gut are transported through the circulation to the liver. The way
the body regulates fats is very different to carbohydrates, fat levels fluctuate much
more.
Lipids are transported from the liver to non-hepatic tissue as they circulate, being
taken up by mammary, muscle or adipose tissue.
Fats and Lipids in Circulation include:
-Triacylglycerol (triglycerides) (45%)
-Cholesterol esters and free cholesterol (15%)
-Phospholipids (35%)
-Free fatty acids (5%)
The %’s given in human plasma vary greatly with nutritional state. All are insoluble
in water, they are not transported completely freely dissolved in the blood. This is
because they are hydrophobic and also they interfere with membrane structure
(being components of membranes themselves)
Free Fatty Acids
The majority of lipids are transported in lipoproteins, but there is some transport of
free fatty acids. These are transported not exactly free but instead bound to albumin.
These are formed from triacylglycerides and stored in adipose tissue.
Fatty acids enter cells by simple diffusion and intracellular concentration of FFA is
kept low.
Lipoproteins
The majority of lipids are transported in the blood as plasma lipoproteins, these are
relatively large structures consisting of both the lipid and large proteins (called
apolipoproteins). There are five types of lipoproteins, classified by their density:
-Chylomicrons
-Very low density lipoproteins (VLDL)
-Intermediate density lipoproteins (IDL)
-Low density lipoproteins (LDL)
-High density lipoproteins (HDL)
Chylomicrons have the least density (and are largest in size), while HDLs have the
highest, each particle also has a different function.
, Diagram of a lipoprotein
As can be seen the lipoprotein consists of a large protein with phospholipids
orientated similarly to how they are in a membrane, but instead as a single layer
(more like a micelle). Then in the hydrophobic core is the hydrophobic compounds
like cholesterol esters and triglycerides.
The thing that distinguishes lipoproteins is the apolipoprotein present and the
amount of triacylglycerol/lipid that is present. The amounts in a single particle do
vary however depending on where in the circulation the lipoprotein is.
Chylomicrons have a very low density, they have high TG and low protein. HDLs have
a very high density with low TG and high protein.
As stated earlier a big determinant of lipoprotein is the apolipoprotein present, this
is summarised below:
Chylomicrons ----- B48, Apo C2, C3 and E
VLDL ---- B100, Apo C1, C2, C3 and E
IDL ----- B100, Apo E
LDL ---- B100
HDL ---- Apo A1, A2, C1, C3, D, E
Because of the unique apoproteins present, it can be used to determine which one it
is and where it was synthesised e.g. apoprotein B48 in chylomicrons.
These apolipoproteins have multiple functions, which include:
Structural = Form a substrate in which lipid can be constructed
To solubilise lipids = Allowing body to transport a difficult class of molecules
