Digestion, Absorption, Assimilation (Simple explanation)





Human Digestion

Mouth

Salivary glands produce saliva which contains salivary amylase. Starch is hydrolysed into maltose.

Stomach

Gastric glands produce gastric juice which contains enzyme pepsin and rennin. Proteins are hydrolysed into polypeptide. Caseinogens is converted into casein.

Duodenum

Liver produces bile. Bile emulsifies lipid. Pancreas produces pancreatic amylase, trypsin & lipase. Starch is hydrolysed into maltose. Polypeptide is hydrolysed into peptide. Lipid is hydrolysed into fatty acids & glycerol.

Ileum

Intestinal gland secretes intestinal juice which contains enzyme maltase, lactase, sucrase and erepsin. Maltase hydrolyses maltose into glucose & glucose. Lactase hydrolyses lactose into glucose and galactose. Sucrase hydrolyses sucrose into glucose & fructose. Erepsin hydrolyses peptide into amino acids

Absorption

Glucose, amino acids, Vitamins B & C are absorbed through facilitated diffusion into blood capillaries in the villi then into hepatic portal vein that leads to liver.

Fatty acids, glycerols, Vitamin ADEK are absorbed into lacteals (FA & G in the form of lipid droplets). Then lacteal converge into lymphatic system. Flows into right lymphatic duct & thoracic duct, finally enters subclavian veins into bloodstream.

Assimilation

In the liver

Liver synthesises plasma proteins from amino acids. Short supply of glucose – liver converts amino acids into glucose. Excess amino acids are broken down – deamination. Urea is produced & transported to kidney to be excreted.

Glucose is converted & stored in the liver. Blood sugar level falls, glycogen is converted back into glucose. Liver full of glucose – converted into lipids.

Lipids enter the heart through subclavian vein transported in the bloodstream to body cells.

In the cells

Amino acids – used for synthesising new protoplasm & repair damaged tissues. Build enzymes & hormones. Synthesise plasma membrane.

Glucose – oxidised to release energy (cellular respiration). Used for muscle contraction, synthesis of proteins. Excess glucose is stored in muscles (glycogen).

Lipids – phospholipids & cholesterol : components of plasma membrane

Fats are stored around organs act as cushions that protect organs from injuries. Excess fats are stored in adipose tissue as reserve energy. When body lacks glucose, fats are oxidised to release energy.