digestive system order
mouth -> oesophagus -> stomach -> small intestine -> large intestine -> rectum -> anus
peristalsis
- circular smooth muscle causes the lumen to constrict
- longitudinal smooth muscle causes localised shortening of the gut
- contractions progress in wave like motions to push food along
- occurs along parts of the alimentary canal (oesophagus, small intestine and large intestine
types of digestion
- mechanical – teeth grinding, stomach churning
2. chemical – using bile and enzymes which are produced by specialised cells in glands and tissues in the gut lining
enzymes and pH in digestion
- HCl in the stomach kills bacteria and provides the correct pH for the protease enzyme, pepsin, to digest protein
- Bile and pancreatic juice contain bicarbonate ions to neutralise the stomach acid when it enters the small intestine and provide the alkaline conditions needed by the enzymes present in the small intestine
stomach acid
forms part of the innate immune system (killing pathogens)
bile
- made in the liver
- stored in the gall bladder
- gall bladder secretes it into the small intestine when stomach contents arrive in the small intestine
sections of the small intestine
- duodenum (1st section)
- jejunum (2nd section)
- ileum (3rd section)
digestive enzymes
- amylase: starch -> maltose (produced by salivary glands, pancreas and SI)
- protease: protein -> amino acids (produced by stomach, pancreas and SI)
- lipase: fat -> fatty acids+glycerol (produced by pancreas and SI)
- maltase: maltose -> glucose (SI)
bile functions
- emulsifies lipids to increase the surface area for lipases to work on -> rate of digestion is increased
- neutralises stomach acid because bile is alkaline
chyme
digested food and HCl
villi
- increases surface area of small intestine
- the walls of the villi are 1 cell thick so there is a smaller diffusion distance -> faster ROR
- glucose and amino acids pass through the gut wall and are absorbed into the blood through the capillaries
- glycerol and fatty acids pass through the gut wall and into the lymph system through the lacteal (before entering the blood too)
why does starch, protein and lipids need to be digested?
they are large, insoluble molecules and cannot pass through the gut wall
why can fibre not be digested and absorbed by humans?
the enzymes required are not present
extracellular enzymes
they are secreted into the gut lumen to break down the large insoluble molecules in food by hydrolysis
how long does digestion take?
12-24 hours
how does the body use digested food
- enzymes make new macromolecules (e.g. carbohydrates, proteins, lipids) using condensation reactions
- glucose is used in aerobic respiration to produce ATP
undigested food
- indigestible components of food, eg cellulose (fibre), from plant-based foods are passed on to the large intestine
- undigested waste matter is called faeces
- brown colour of the faeces is due to bile pigments
- faeces passes through the large intestine, into the rectum and is expelled through the anus in a process called egestion
Why is it necessary to have an amylase in the mouth and another one secreted by the pancreas?
- the amylase secreted by the salivary glands works when the pH is 7
- this amylase enzyme denatures when the food enters the stomach due to the acidic pH
- another amylase is therefore secreted by the pancreas which works in the alkaline conditions of the small intestine (pH8)
digestive sequence
Mouth: Begins digestion of carbohydrates (starch by amylase produced by the salivary glands). Chewing breaks up the food into smaller pieces for easier digestion
Oesophagus: Food pushed down by peristalsis
Stomach: Stores food to slow down its progression along the digestive system. Breaks food up by churning. Begins digestion of proteins by pepsin (a protease). Produces HCl to kill bacteria and allow pepsin to work.
Liver: Produces bile
Gall Bladder: Stores bile and releases it when needed
Pancreas: Produces digestive enzymes. Its secretions also help neutralise stomach acid
Duodenum: continues digestion of carbs and proteins, begins digestion of lipids
Ileum: completes digestion of carbs, proteins and lipids + absorption of digested food
Large intestine: Absorption of water from undigested food
Rectum: Storage of faeces
Absorption of glucose in the villi
-active transport is used to maximise the amount of glucose we get out of our food
glycogen storage
glucose is stored as glycogen in skeletal muscles and the liver
liver main functions
- removal of glucose from the blood and converting it to glycogen
- production of bile
- synthesis of cholesterol
- breakdown of hormones
stomach/gastric juice contents
- HCl
- protease (pepsin)
- lipase
pancreatic juice contents
- protease
- amylase
- lipase
