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[[Image:Dissacharidase.jpg|thumb|right|150px|Dissacharidase - © RVC 2008]]

[[Image:Dissacharidase.jpg|thumb|right|150px|Dissacharidase - © RVC 2008]]

*In digestion, large, complex molecules are broken down into their constituents. They are then absorbed and used by the body for energy or used as building blocks for new complex molecules. The three main molecules that undergo digestion are carbohydrates, triacyglycerols and proteins.

*In digestion, large, complex molecules are broken down into their constituents. They are then absorbed and used by the body for energy or used as building blocks for new complex molecules. The three main molecules that undergo digestion are carbohydrates, triacyglycerols and proteins.

'''Carbohydrate Digestion and Absorption'''  

====Carbohydrate Digestion and Absorption====  

*The main soluble carbohdrates found in food are starch, found mainly in plants, and glycogen, found mainly in animal meat.

*The main soluble carbohdrates found in food are starch, found mainly in plants, and glycogen, found mainly in animal meat.

**There are two types of starch, ''amylose'' which has α1-4 glycosidic links and, ''amylopectin'' which has α1-4 glycosidic links and α1-6 glycosidic links making it branched (branches every glucose 25 residues).

**There are two types of starch, ''amylose'' which has α1-4 glycosidic links and, ''amylopectin'' which has α1-4 glycosidic links and α1-6 glycosidic links making it branched (branches every glucose 25 residues).

**The energy release is used to transport glucose and galactose up their concentration gradients into the enterocyte.

**The energy release is used to transport glucose and galactose up their concentration gradients into the enterocyte.

**Glucose and galactose can then diffuse into the blood (portal vein) by carrier mediated diffusion via a GLUT-5 transporter.

**Glucose and galactose can then diffuse into the blood (portal vein) by carrier mediated diffusion via a GLUT-5 transporter.

 

====Triacylglycerol Digestion and Absorption====

'''Triacylglycerol Digestion and Absorption'''

*Triacylglycerols (TAGs) are digested by lipases.

*Triacylglycerols (TAGs) are digested by lipases.

*TAG digestion begins in the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]], where lingual lipase is secreted in the saliva. It removes a fatty acid from the 3 position on the glycerol molecule producing 1,2-diacylglycerol(1,2 DAG) and a free fatty acid.

*TAG digestion begins in the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]], where lingual lipase is secreted in the saliva. It removes a fatty acid from the 3 position on the glycerol molecule producing 1,2-diacylglycerol(1,2 DAG) and a free fatty acid.

***Chylomicrons are too large to enter the capillaries but instead enter the lymph to eventually join the blood via the thoracic duct.

***Chylomicrons are too large to enter the capillaries but instead enter the lymph to eventually join the blood via the thoracic duct.

***This enables the lipid soluble TAG to be transported in the blood.

***This enables the lipid soluble TAG to be transported in the blood.

 

====Protein Digestion and Absorption====

'''Protein Digestion and Absorption'''

*Protein digestion begins in the [[Forestomach - Anatomy & Physiology|stomach]] where pepsin is secreted as a zymogen, pepsinogen. Pepsin is an endopeptidase and produces smaller polypeptides.

*Protein digestion begins in the [[Forestomach - Anatomy & Physiology|stomach]] where pepsin is secreted as a zymogen, pepsinogen. Pepsin is an endopeptidase and produces smaller polypeptides.

**Pepsin prefers to break peptide bonds of larger polypeptides, where there is a large hydrophobic amino acid on the N-terminal side.

**Pepsin prefers to break peptide bonds of larger polypeptides, where there is a large hydrophobic amino acid on the N-terminal side.