Question

Describe the general pathways and end products of carbohydrate and lipid metabolism in each of the following mammalian tissues:

a) Liver

b) Adipose

c) Muscle (skeletal and heart)

d) Brain tissues

e) Blood

Answer 1

Carbohydrates Metabolism:-

The liver plays a central role in maintaining blood levels.Following consumption of food,excess glucose can be stored within the liver as glycogen.This is stimulated by insulin release.This glycogen can then be degraded to release glucose in times of exercise(sketetal muscle stores) or fasting (liver stores).The steps of glycogenolysis are as follows:-

-one residue of glycogen is removed and converted to glucose-1-P by glycogen phosphorylase or de-branching enzyme.

-Glucose-1-P is converted to Glucose-6-P by phosphoglucomutase.

-Glucose -6-- P is then converted to Glucose by glucose-6-phosphatase.

-This glucose then enters the bloodstream to be used through the body.

The liver can convert amino acids,lactate, pyruvate and glycerol into glucose too,via gluconeogenesis.

Lipid Metabolism:-

Lipogenesis:-

Fatty acids are synthesised within the cytoplasm of hepatocytes from Acetyl-CoA.The reaction requires ATP and NADPH.Firstly ,Acetyl-CoA is converted to Malonyl-CoA by acetyl carboxylase. This step is important in the regulation of lipogenesis as it is allosterically activated by citrate and inhibited by AMP.

Fatty acid synthase then adds these 2 carbon molecules (malonyl-CoA) to a growing fatty acid.This fatty acid is then linked to a carrier protein.Lipogenesis is stimulated by the presence of insulin and inhibited by glucagon and adrenaline.

Lipolysis:-

During fasting or stress,fatty acids can be activated in the liver to undergo B-oxidation .This occurs in the mitochondria and produces acetyl-CoA which can either enter the TCA cycle or be used to produce ketone bodies.

The long chains of fatty acids are broken down into a series of 2 carbon acetate units, which are then combined with co-enzyme A to form acety-CoA .This acetyl -CoA can then be combined with oxaloacetate to form citrate for the beginning of the TCA cycle.

Glucagon and adrenaline stimulate the process of lipolysis whereas it is inhibited by insulin.