digestion and absorption

digestion

Digestion is the process in which large molecules (in food are too big to cross cell membranes so can’t be absorbed from the gut into the blood) are hydrolysed by enzymes into small molecules which can be absorbed and move across cell membranes into the blood to to be transported around the body for use by the body cells. 

PHYSICAL BREAKDOWN

• If the food is large, it is broken down into smaller pieces by means of structures such as the teeth. 

• This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion.

• Food is churned by the muscles in the stomach wall and this also physically breaks it up.

CHEMICAL DIGESTION

• Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones. 

• lt is carried out by enzymes. 

• All digestive enzymes function by hydrolysis. Hydrolysis is the splitting up of molecules by adding water to the chemical bonds that hold them together. 

broken down into

• Carbohydrates are hydrolysed into disaccharides and then monosaccharides. 

• Fats are broken down into monoglycerides and fatty acids. 

• Proteins are broken down into amino acids.

DIGESTIVE ENZYMES

 

TYPE OF ENZYME		HYDROLYSES?	PRODUCTS	PRODUCED WHERE?	RELEASED WHERE?
CARBOHYDRASE	Amylase	Carbohydrates/ starch	Maltose	Salivary glands + pancreas	Mouth + small intestine
MEMBRANE- BOUND DISACCHARIDASES	Maltase	Maltose (starch)	Alpha glucose	Small intestine, epithelial lining	Attached to the cell membranes of epithelial cells lining the ileum
	Sucrase	Sucrose (nat. foods)	Glucose + fructose	Small intestine, epithelial lining
	Lactase	Lactose (milk)	Glucose + galactose	Small intestine, epithelial lining
LIPASE		Lipids	Glycerol + fatty acids	Pancreas + salivary glands	Small intestine
PROTEASE		Proteins	Amino acids	Stomach (pepsin) + pancreas	Stomach + small intestine.

carbohydrates by amylases and membrane-bound disaccharidases

Amylase hydrolyses starch to maltose (polysaccharide to disaccharide). 

1. Amylase produced by salivary glands, released into mouth. 

2. The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of the starch.

3. After a time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called pancreatic juice. The pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch to maltose.

4. Maltase is a membrane-bound disaccharidase (attached to epithelial cells lining the ileum of the small intestine) which hydrolyses maltose to alpha glucose by hydrolysis of glycosidic bond.

Membrane-bound disaccharidases

they help to break down disaccharides into monosaccharides which involves the hydrolysis of glycosidic bonds. (attached to epithelial cells lining the ileum of the small intestine)

Maltose comes from hydrolysis of starch, sucrose and lactose are naturally found in foods. 

co-transport mechanisms for the absorption of amino acids and of monosaccharides

Sodium potassium pump  (carrier protein between epithelial cell and bloodstream) 

1. Na+ from epithelial cell→ bloodstream by active transport in the sodium potassium pump. 

   1. K+ from blood → epithelial cells  

2. As ileum has a high concentration of Na+ ions following digestion, concentration of Na+ ions now lower in epithelial cells than ileum, creating a concentration gradient between lumen of ileum and epithelial cells. 
   

Sodium glucose co-transporter (protein in membrane of epithelial cell)

3. Na+ ions into the epithelial cells from ileum via facilitated diffusion down concentration gradient. Attaches to complementary shape receptor.

4. In the same co-transporter (symport), glucose then attaches to it and is absorbed into epithelial cells from the ileum, against its concentration gradient. 

5. The Na+ is then released on the other side which then enables glucose to also be released. Transported together. 

6. Glucose is absorbed by active transport which ensures glucose is absorbed at a fast rate.  

7. The energy for active transport of glucose comes indirectly from the concentration gradient of the Na+ ion.

8. Glucose then enters the bloodstream by facilitated diffusion. Glucose conc. is high in the epithelial cells and is carried straight away in blood as it always flows, so there is always a low concentration in the bloodstream.   

proteins digestion

Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases) which hydrolyse the peptide bonds between amino acids. There are a number of different peptidases:

proteins by endopeptidases, exopeptidases and membranebound dipeptidases.

• ENDOPEPTIDASES hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules. (trypsin, pepsin)

• EXOPEPTIDASES hydrolyse the peptide bonds on the terminal (end) amino acids of the peptide molecules formed by endopeptidases. In this way they progressively remove single amino acids.

• DIPEPTIDASES hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.

lipid digestion

Lipids are hydrolysed into monoglycerides and fatty acids by enzymes called lipases. Lipases are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides. A monoglyceride is a glycerol molecule with a single fatty acid molecule attached. 

lipids by lipase, including the action of bile salts

1. Emulsification of lipids by bile salts. 

   • The fats and oils are split up into tiny droplets. 

   • This increases the surface area of the lipids so there is  faster hydrolysis of lipids (hydrolyse the ester bond) by lipase enzymes (made in pancreas, secreted into small intestine). 

2. Once the lipid has been broken down by lipase, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles. 

   • Micelles help the products of lipid digestion to be absorbed. They carry fatty acids and glycerol / monoglycerides through membrane to intestinal epithelial cell.