Changes

= '''Milk Composition''' =

= '''Milk Composition''' =

* Milk separates into two phases:

* Milk separates into two phases:

* Colloidal suspension: cassein proteins complexed with the minerals calcium, magnesium and phosphate.

* Colloidal suspension: cassein proteins complexed with the minerals calcium, magnesium and phosphate.

2.''' Fat Phase''': triacylglycerols, fat soluble vitamins, cholesterol and phospholipids.

2.''' Fat Phase''': triacylglycerols, fat soluble vitamins, cholesterol and phospholipids.

** Minor trace elements: iodine, zinc, cobalt etc are present in concentrations <1 part per million but are essential to the nutritional value of milk.

** Minor trace elements: iodine, zinc, cobalt etc are present in concentrations <1 part per million but are essential to the nutritional value of milk.

** Copper and Iron are found at very low levels.  Iron is produced by the foetal liver, so is not really required. However, neonatal piglets cannot synthesise their full requirement for iron, so need daily iron injections.  Copper must be at low concentrations to avoid toxixity, especially in sheep.

** Copper and Iron are found at very low levels.  Iron is produced by the foetal liver, so is not really required. However, neonatal piglets cannot synthesise their full requirement for iron, so need daily iron injections.  Copper must be at low concentrations to avoid toxixity, especially in sheep.

** Minor components: E, K and the other B vitamins

** Minor components: E, K and the other B vitamins

** Particularly low in C and D

** Particularly low in C and D

** Synthesised in the smooth endoplasmic reticulem, then packaged into lipid droplets, which increase in size as they move towards the plasma membrane.  Eventually they pinch off and enter the milk as fat globules surrounded by membrane.

** Synthesised in the smooth endoplasmic reticulem, then packaged into lipid droplets, which increase in size as they move towards the plasma membrane.  Eventually they pinch off and enter the milk as fat globules surrounded by membrane.

** Globules consist mainly of triacylglycerols surrounded by a membrane containing cholesterol and phospholipid.  The membrane is derived from the mammary gland plasma membrane as the globules pinch off.  Globules also contain the fat soluble vitamins (D,K,E)

** Globules consist mainly of triacylglycerols surrounded by a membrane containing cholesterol and phospholipid.  The membrane is derived from the mammary gland plasma membrane as the globules pinch off.  Globules also contain the fat soluble vitamins (D,K,E)

== Synthesis of Lactose ==

== Synthesis of Lactose ==

1. In the cytosol of the mammary cell, glucose is converted to UDP- glucose. UDP glucose is then epimerised by the enzyme '''epimerase''' to form UDP galactose.

1. In the cytosol of the mammary cell, glucose is converted to UDP- glucose. UDP glucose is then epimerised by the enzyme '''epimerase''' to form UDP galactose.

4. UDP is recycled

4. UDP is recycled

== Synthesis of the Fat Component ==

== Synthesis of the Fat Component ==

The mammary gland synthesises triacylglycerol from glycerol and three fatty acids.  The fatty acids originate from triacyclglycerol in plasma lipoproteins (60%) or from synthesis in the mammary gland (40%).

The mammary gland synthesises triacylglycerol from glycerol and three fatty acids.  The fatty acids originate from triacyclglycerol in plasma lipoproteins (60%) or from synthesis in the mammary gland (40%).

3.  In '''ruminants''', the enzyme citrate lyase is lacking, so Acetyl CoA from glucose metabolism cannot be used for fatty acid synthesis.  However, they have an abundant supply of '''acetate, butyrate and β hydroxybutyrate''' from the digestion of carbohydrate in the rumen.  These are therefore used as the precursors for the synthesis of acyl CoA. '''Acyl CoA''' and '''gylcerol-3-phosphate''' then combine to form triacylglycerol.

3.  In '''ruminants''', the enzyme citrate lyase is lacking, so Acetyl CoA from glucose metabolism cannot be used for fatty acid synthesis.  However, they have an abundant supply of '''acetate, butyrate and β hydroxybutyrate''' from the digestion of carbohydrate in the rumen.  These are therefore used as the precursors for the synthesis of acyl CoA. '''Acyl CoA''' and '''gylcerol-3-phosphate''' then combine to form triacylglycerol.

== Synthesis of Protein ==

== Synthesis of Protein ==

1. Polypeptide chains are synthesised on the rough endoplasmic reticulem from amino acids in the blood.

1. Polypeptide chains are synthesised on the rough endoplasmic reticulem from amino acids in the blood.