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Milk Composition and Biosynthesis- Anatomy & Physiology (view source)
Revision as of 17:49, 7 December 2010
, 17:49, 7 December 2010moved Lactation - Milk Composition and Biosynthesis- Anatomy & Physiology to Milk Composition and Biosynthesis- Anatomy & Physiology
== Milk Composition ==
== Milk Composition ==
* Milk separates into two phases:
* Milk separates into two phases:
=== Inorganic Ash ===
* Comprises 0.5-1.0% of milk. This is the mineral content.
** Major components: calcium, phosphate, magnesium, sodium, potassium and chloride. The concentration of calcium, phosphate and magnesium in milk is much greater than the plasma concentration (x14,x7 and x4 respectively).
** Major components: calcium, phosphate, magnesium, sodium, potassium and chloride. The concentration of calcium, phosphate and magnesium in milk is much greater than the plasma concentration (x14,x7 and x4 respectively).
** 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.
* '''Vitamins''' are present in trace amounts,but are important for nutritional value.
=== Vitamins ===
* Present in trace amounts,but are important for nutritional value.
** Major components: A and Riboflavin (B)
** Major components: A and Riboflavin (B)
** Minor components: E, K and the other B vitamins
** Minor components: E, K and the other B vitamins
* '''Fat''' comprises 1-30% of milk.
=== Fat ===
* Comprises 1-30% of milk.
** 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)
=== Protein ===
* Can be divided into two groups, caseins and whey.
Proteins are synthesised on the rough endoplasmic reticulem and migrate to the golgi, where they are processed and complexed with minerals. Golgi vesicles containing the proteins pinch off and migrate towards the plasma membrane where their contents is released by exocytosis.
==== Caseins ====
* Precipitate at pH 4.5 or lower.
* They comprise 40-80% of total milk protein and exist in colloidal suspension complexed to calcium, phosphate and magnesium. This allows the mineral concentration in milk to exceed that of plasma.
* They have little secondary structure, so are easily digestable by Rennin. They have a high content of nutritionally essential amino acids.
* '''Carbohydrate''': '''Lactose''' is the only carbohydrate in milk.
==== Whey ====
* Remain in solution at pH 4.5 or lower.
* They comprise 20-60% of the total milk protein.
* They are rich in α lactalbumin which is required for the synthesis of the milk carbohydrate lactose.
** In ruminants, β lactoglobulin is also a majoy whey protein component, supplying the neonate with amino acids.
==== Minor Milk Proteins ====
* Albumin
* Immunoglobulin
* Enzymes
=== Carbohydrate:Lactose ===
* The only carbohydrate in milk.
** Lactose is a '''dimer''' of '''glucose and galactose''', which are epimers. Lacotse acts as a source of glucose to the neonate, as lactase in the gastrointestinal tract is capable of splitting the dimer.
** Lactose is a '''dimer''' of '''glucose and galactose''', which are epimers. Lacotse acts as a source of glucose to the neonate, as lactase in the gastrointestinal tract is capable of splitting the dimer.
*** Synthesised on the golgi, then packaged into golgi vesicles which pinch off and migrate towards the plasma membrane. At the plasma membrane, the contents of vesicles are released by exocytosis.
*** Synthesised on the golgi, then packaged into golgi vesicles which pinch off and migrate towards the plasma membrane. At the plasma membrane, the contents of vesicles are released by exocytosis.
== Colostrum ==
* The milk produced in the first few days after birth. It is especially rich in protein, vitamin A and fat to enhance chances of survival of the neonate. It contains less potassium and lactose than milk.
** The extra protein is mainly immunoglobulins. These are especially important in species where antibodies cannot cross the placenta. They convey passive immunity, as they can be absorbed from the duodenum in the early neonatal period without being digested.
** Vitamin A imparts the yellow colour to colostrum, it is elevated in colostrum as it is required for growth and differentiation.
** Fat is important for energy generation and thermal insulation.
== Biosynthesis ==
== Synthesis of Lactose ==
=== Synthesis of Lactose ===
[[Image:Synthesis of Lactose.jpg|thumb|right|150px|Synthesis of Lactose,Copyright RVC 2008]]
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 ===
[[Image:Milk Fat and Protein Synthesis.jpg|thumb|right|150px|Synthesis of the Fat and Protein Component,Copyright RVC 2008]]
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 synthesised 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%).
1. Triacylglycerol from lipoproteins is broken down by '''lipoprotein lipase''' to three free fatty acids and glycerol. These are then reassembled in cells to form new triacylglycerols.
1. Triacylglycerol from lipoproteins is broken down by '''lipoprotein lipase''' to three free fatty acids and glycerol. These are then reassembled in cells to form new triacylglycerols.
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 ===
[[Image:The Lactating Mammary Cell.jpg|thumb|right|150px|The Lactating Mammary Cell,Copyright RVC 2008]]
1. Polypeptide chains are synthesised on the rough endoplasmic reticulem from amino acids in the blood.
2. Polypeptide chains are transferred to the golgi, where they are processed with the addition of sugars to form gylocoproteins. Caseins are complexed with minerals.
==== FIL protein ====
* Produced in milk synthesis
* Inhibitory
* It accumulates in the alveolar lumen when milk is not being removed.
** Thus, when milk is removed during suckling, the inhibition is removed to allow further milk synthesis.
[[Category:Female Reproduction]]