6.1 ~ Digestion and absorption

The digestive system

• A tube through which food passes from the mouth to the anus

• Purpose is to break down large, complex carbon compounds into smaller ions and compounds thar can be absorbed and utilised by the body

• Main groups broken down are; carbohydrates, lipids and proteins

• Surfactants are needed to breakdown lipid droplets and enzymes are required to catalyse the reactions that breakdown larger compounds

  • Produced by glandular cells that line the stomach and intestine, and by accessory organs

Accessory glands and organs

• E.g. Liver, gall bladder and pancreas

• Called such because food does not pass through these organs; however, they have a vital role in digestion

• These have ducts that connect to the rest of the digestive system

Mouth

• Voluntary control of eating and swallowing

• Mechanical digestion of food by chewing and mixing with saliva

  • Contains lubricants and enzymes that start starch digestion

Esophagus

Movement of food by peristalsis from the mouth to the stomach

Stomach

Churning and mixing with secreted water and acid which kills foreign bacteria & other pathogens in food + initial stages of protein digestion

Liver

Secretion of surfactants in bile to break up liquid droplets

Gallbladder

Storage and regulated release of bile

Pancreas

Secretion of lipase, amylase and protease

Small intestine

Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralising stomach acid + absorption of nutrients

Large intestine

Re-absorption of water, further digestion (especially carbohydrates by symbiotic bacteria) + formation and storage of feces

Rectum/anus

Receives stool from the colon, sends signals to the brain if there is stool to be evacuated & holds until evacuation can happen

Absorption and surface area

• The process of taking substances into cells and into the blood is called absorption

  • In humans, this primarily happens in the small intestine

• The rate of absorption depends on the SA

  • In adults, small intestine is approx. 7m long and 2.3-3cm wide

  • SA is further increased by the presence of villi

• Villi are finger-like projections on the inside of the intestinal wall that project into the lumen of the intestine

  • Each villi is approx. 1.5mm long & there can be up to 40 per mm2 of the intestine

  • Increases SA by a factor of about 10

Villi structure (efficient absorption)

• Epithelium is ‘ruffled’ into microvilli: further increases SA

• Epithelium is one cell thick: short distance for substances to move across

• Capillary network within each villus: rapid transport to blood

  • Maintains concentration gradient for efficient diffusion

• Lacteal absorbs lipids into lymphatic system: rapid removal

• Contain gland cells within epithelium that secrete intestinal juices

• Membrane proteins facilitate transport of digested products into the villi

Digestive tract

• Lined with smooth muscle

• Unidirectional movement of food through the digestive tract is called peristalsis

  • Involves circular and longitudinal muscle fibres that run through the tract

• Circular muscles surround the lumen, therefore make the lumen smaller

• Longitudinal muscles run lengths ways, therefore contraction shortens the tract

• Contraction and relaxation of these two muscles moves the bolts (undigested food_ through the tract from mouth to anus (unidirectionally)

• In the intestine, peristalsis is used to churn the food to mix it with enzymes and speed up the process of digestion

Epithelial layer

Outer layer of villi

Mucosa

Innermost layer —> exposed to lumen of digestive tract

Submucosa

Contains blood cells, lymph vessels, nerves, collagen and elastic fibres

Muscles layers

Circular and longitudinal muscles for peristalsis

Serosa

Outermost, protective layer

Pancreatic enzymes

• Contains two types of gland tissue that either

  • Synthesise and secret insulin and glucagon (islets of Langerhans) or

  • Digestive enzymes (acinar cells)

• Within acinar cells, digestive enzymes are synthesised in the ribosome in the RER

  • Then processed through the Golgi apparatus and secreted in vesicles by exocytosis into smaller ductS

  • These smaller ducts connect up to the larger pancreatic duct which connects to the duodenum

Amylase

Starch —> Maltose

Lipase

Triglycerides —> Fatty acids and glycerol

Endopeptidase

Proteins —> Amino acids

Phospholipase

Phospholipids —> Fatty acids, glycerol and phosphate

Enzymatic digestion in small intestine

• Further enzymatic digestion takes place through enzymes produced by gland cells in the intestinal wall

• Either secreted into the intestinal juice or remain immobilised in the PM of these cells

  • As the epithelium cells are abraded off the lining as the bolus moves through, these cells and their immobilised enzymes continue to act on the bolus

• Digestion through the small intestine takes hours, not allowing sufficient time for macromolecules to be digested and the monomers to be absorbed

• Humans do not produce enzymes to digest everything

  • As such, some substances e.g. cellulose pass through the small intestine undigested

Tubing

• Dialysis or busting tubing can be used to model digestion

• This tubing has pores between 1-10nm in size

• Like cell membranes, this tubing is semi-permeable: small molecules can pass through but not larger ones

• Unlike cell membranes, this membrane is not selectively permeable based on charge

  • As such, charged ions will move freely across the membrane

• This tubing can also not model movement by active transport