case 5 - teach me physiology lipid metabolism

Question 1

what are cholesterol and phospholipids used for

Answer 1

functions such as the synthesis of the cell membrane and the synthesis of steroid hormones

Question 2

what happens when energy is needed from the fat stored in adipose tossue

Answer 2

triglycerides are hydrolysed into fatty acids and glycerol by triglyceride lipase

Question 3

what is this stimulated by

Answer 3

adrenaline and glucagon

Question 4

where do these fatty acids then go and what do they do

Answer 4

they enter the circulation where they bind immediately to albumin. when conjugated to albumin, the fatty acids are soluble in blood and so can be transported to tissues including the liver

Question 5

what happens when glycerol enters the hepatocytes

Answer 5

glycerol is immediately converted into glycerol-3-phosphate, which then enters the glycolysis pathway.

Question 6

what happens in the mitochondria

Answer 6

fatty acids need to be oxidised and degraded, this occurs in the mitochondria by a series of reactions known as beta-oxiidation

Question 7

what happens in beta oxidation

Answer 7

two carbon segments are progressively released from the fatty acid chain until acetyl-CoA is generated.

NADH and FADH2 are generated as byproducts

Question 8

what then happens to this acetyl-CoA

Answer 8

it binds immediately with oxaloacetate to form citrate and then enters the TCA cycle to release energy in the form of ATP

Question 9

what is excess acetyl-CoA converted into

Answer 9

acetoacetic acid using HMG-CoA as an intermediate and then transported to other tissues

Question 10

what is the acetoacetic acid converted into

Answer 10

beta-hydroxybutyric acid, and small amounts of acetone.

Question 11

what is acetoacetic acid

Answer 11

a keto acid

Question 12

what are acetoacetic acid, beta-hydroxybutyric acid and acetone known as

Answer 12

ketone bodies

Question 13

what can ketone bodies do

Answer 13

can travel in the blood to other tossies where they are then used for energy. however, they can also play a significant role in disease states.

Question 14

what is lipogenesis

Answer 14

is an essential mechanism that provides an energy store which can be used at times when the body’s energy requirement cannot be met by glucose alone

Question 15

when and how are fatty acids synthesised

Answer 15

synthesised within the cytoplasm of the hepatocytes, following maximal conversion of glucose to glycogen.

Question 16

what happens to the remaining glucose

Answer 16

is converted to pyruvate via the glycolysis pathway, and transported into the mitochondria where it is converted to acetyl-CoA

Question 17

what is the citrate shuttle

Answer 17

If not entering the TCA cycle, acetyl-CoA needs to leave the mitochondria and enter the cytosol. However, the inner mitochondrial membrane is impermeable to acetyl-coA, and therefore it must traverse the membrane in an altered form; as citrate

Question 18

explain the steps of the citrate shuttle

Answer 18

Acetyl-CoA joins with oxaloacetate to form citrate

Citrate is able to cross the mitochondria membrane

Citrate is then converted back into acetyl-CoA and oxaloacetate within the cytosol

Question 19

what happens once the acetyl-CoA is in the cytosol

Answer 19

it can be converted to malonyl-CoA by acetyl-CoA carboxylase. this step is important in the regulation of lipogenesis as it is allosterically activated by citrate and inhibited by AMP

Question 20

what does fatty acid synthase then do

Answer 20

creates an elongated fatty acid chain from the malonyl-CoA molecules, adding two carbon atoms for each molecule of malonyl-CoA.

Question 21

what happens as the malonyl-CoA molecules are added

Answer 21

they lose a carbon atoms creating CO2

Question 22

what are triglycerides made from

Answer 22

three fatty acid molecules and a molecule of glycerol

Question 23

what transports triglycerides from the liver to their destination in adipose tissue

Answer 23

very low density lipoprotein (VLDL)

Question 24

what are the hormones that regulate lipogenesis

Answer 24

glucagon and adrenaline are the hormones that inhibit lipogenesis. alongside negative feedback from the presence of lipoproteins in the blood.

Question 25

what is the main hormone to stimulate lipogenesis

Answer 25

insulin