Everything you need to know for the AQA AS Biology Digestion Topic

Includes Digestion Basics, Alimentary Canal, Absorption in the Ileum, Digestion Of Proteins, Carbohydrates and Lipids.

By Natalie Beard :)

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Digestion Basics

Digestion is essentially five stages:

  • Ingestion: Taking large pieces of food into the body
  • Digestion: Breaking down of food by mechanical and chemical means
  • Absorption: Taking up the soluble products of digestion into the bodys cells
  • Assimilation: The use of these absorbed materials
  • Egestion: Eliminating undigested material from the body caviry
  • Excretion: Elimination of substances from within body cells

The human digestive system is designed to cope with the above , it is essentially a long tube called the alimentary canal, (the gut) which runs from the mouth to the anus. The lining which aids with the properties is explained on the next side.

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The Gut Lining

The layers of the gut lining are explained below:

The Mucosa: This secretes digestive juices and absrobs digested food, folded to increase surface area. On the inside near the lumen is are the epitheilial cells which line the digestive tract. Constantly being worn away and replaced.

The Submucosa: Contains blood and lymph vessels as well as nerves to control the muscles, may also contain secretory glands.These nerves regulate muscle movement and the digestive secretions into the lumen of the gut.

The Muscle Layer: Basically smooth muscle under involuntary control. Made of circular muscle which squeezes the gut when it contracts and the longitudinal muscle which shortens the gut when it contracts. These two layers create the contractions known as peristalsis, the circular muscle contracts behind food whereas the longitudinal contracts in front, this combo helps food to move along the gut.

The Serosa: Tough layer of connective tissue which holds gut together and attaches it to the abdomen. Protects and anchors digestive tract.

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The Alimentary Canal (Villi and Microvilli and the

Villi and Microvilli: Tiny finger-like projections of the small intestine, microvilli are extensions of these. Both increase surface area and are thin to aid diffusion. Enzymes on surface to aid digestion.

The Mouth (Buccal Cavity): Salvia relased via conditional reflex before food and unconditional during.

  • Salvia consists of mucus to lubricate the food.
  • Mineral salts to activate enzymes.
  • Lysozyme which kills bacteria.
  • Amylase to break starch down into maltose.

Mechanical chewing breaks food to produce a larger surface area for the amylase to work. The mixture of food and salvia produced is called a bolus, this can be swallowed.

The Oesophagus: Soft tube that can be closed. Connects mouth to gut and no digestion takes place here as there is no villi, but there a few glands that secrete mucus. It has a thick muscle layer to propel food via peristalsis.

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The Alimentary Canal (The Stomach)

The Stomach: Expandable bag where can food can be stored for a few hours. Three functions: Food Storage, Digestion and Delivery Regulation.

The contractions within blend mix and churn food, using the an extra layer of oblique muscle on the lumen side of the circular muscle. The muscosa contains golbelt cells which secrete mucus which creates a barrier and prevents autodigestion, (digestion of lining by enzymes.) In the stomach wall are the gastric glands, created by infolding of the epithelium, some cells lining these pits are known as chief cells they relase an inactive enzyme, Pepsinogen.

Oxyntic cells release HCL, three effects: Right PH for efficent enzyme work. Kills bacteria. Converts Pepsinogen to Pepsin, this can then digest proteins. The combination of pepsinogen and HCL is called the gastric juice, this is released by the gastric glands when an impulse because of the presence of food in the mouth is sent to the brain. It can also be triggered by the hormone gastrin, which is triggered via the movement of the stomach during churning.

Gastrin also stimulates the pyloric sphincter to relax, chyne then moves to duodenum.

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Small Intestine: Duodenum

Duodenum: Most chemical digestion takes place here. The mucosa is folded extensively, resulting in loads of villi and the folding of the submucosa results in folding of cell membranes aka microvilli. Both of these massively increase surface area.

Some enzymes aren't released into the lumen and are instead bound to the microvilli of the epithelial cell surface. Other enzymes found free in the lumen have been released by the pancreas. As well as these, sodium bicarbonate solution is secreted to provide the optimum PH for enzymes at work here.

Bile is also secreted by gall bladder, after it has been made by the liver. Bile consists of bile salts which emulsify fats, making large globules into smaller ones, giving a larger surface area for lipases to act.

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Small Intestine: Duodenum (Continued)

The release of these substances is controlled by the chyme touching the mucosa of the duodenum, two seperate hormones bring this about:

  • An electric impulse is sent to brain. Brain then contacts the gastric glands and slows down gastric juice production as food has moved on. As well as this, secretin, a hormone, is released. This stimulates sodium bicarbonate release and production of bile in liver.

The presence of food also stimulates the glands to produce their enzymes and the release of a hormone known as CCK, functions of this are:

  • Release of bile from the liver
  • Production and release of pancreatic juice which contain enzymes.
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Enzymes in the Duodenum

  • Aminopeptidase: Released by duodenum: Changes peptides to amino acids
  • Enterokinease: Released by duodenum. Changes inactive trypsinogen to trypsin
  • Maltase: Released by duodenum. Changes maltose to glucose
  • Sucrase: Released by duodenum. Changes sucrose to glucose and fructose
  • Exopeptidase: Released by duodenum. Changes peptides to amino acids
  • Lactase: Released by duodenum. Changes lactose into glucose and galactose
  • Amylase: Released by duodenum and pancreas. Changes starch/glycogen into maltose
  • Chymotrypsin: Released by pancreas. Changes proteins into amino acids
  • Trypsin: Released by pancreas. Changes proteins into amino acids and chymotrypsinogen into chymotrypsin
  • Lipase: Released by pancreas. Changes lipids into fatty acids and glycerol
  • Carboxypeptidase: Released by pancreas. Changes peptides to amino acids
  • Endopeptiase: Released by pancreas. Changes proteins into peptides.
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Small Intestine: Ileum and Large Intestine

Ileum: Here is where most absorption takes place. It has a large surface area with many folds and is essentially a long tube with many villi and microvilli.

It also has a good blood supply and thin walls, these aid diffusion as with a good blood supply diffusion is faster as it is closer and with the thin walls the same is true due to the thiness of the walls.

Absorptions happens using facilitated diffusion, diffusion, osmosis and active transport. Glucose, amino acids and water are absorbed into the blood stream and fats are absorbed into the lacteals.

Large Intestine: Made up of the colon, rectum and the anus, these are mainly for absorbing water which in turn forms solid faeces from undigested food, dead cells and bile pigments. These are then egested.

Sodium is transported via active transport out of the lumen water follows by osmosis, solidifying what is left. Faeces are then stored in the rectum until elimination via the anus when the rectum is distended.

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Absorption in the Ileum

Essentialy the pasage of molecules being absorbed through the intestine wall along with mucus and digestive juices.

  • Monosaccharides, amino acids, dipeptides and tripeptides are actively transported into the epithelial cells lining the wall. Their transport is coupled with sodium and so the protein carriers will have two slots for each, which must both be filled before transport, after this they'd diffuse into the capillaries.
  • Proteins and Carbohydrates: Active Transport
  • Lipids: Broken down and reassembled
  • Water: Osmosis. Vitamins and Minerals: Varies.

The active transport of absorption requires ATP as it is moving against the concentration gradient, the microvilli provide a large surface area which makes for quicker absorption. The actual active transport maintains the low concentration in the cell which enables facilitated diffusion to take place.

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Carbohydrate Digestion

Starch digestion happens in two stages:Hydrolysis of starch to maltose via amylase and hydrolysis of maltose to glucose via maltase.

Stages: Amylase released in buccal cavity but little time to actually work so amylase is also released in the duodenum. Amylase is denatured by PH 2 in the stomach.

The acid chyme is then neutralised to PH 8 by the sodium bicarbonate secretion from the glands in the small intestine and the hydrogen carbonate secretion from the pancreas. The amylase secreted in the duodenum then changes the starch to maltose.

Maltose is then broken down to glucose by maltase which is a membrane enzyme on the epithelial cells in the duodenum. Glucose and sodium are co-transported into the epithelial cells of the villi via active transport and then moves into the capilary via facilitated diffusion, the capillaries of the villi drain into the hepatic portal vein and the glucose is taken to the liver, where it is stored as glycogen. The excess is converted to fat for storage as adipose tissue, glucose can then be used for respiration.

Carbohydrate in plant fiber aka celluose cannot be digested.

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Protein Digestion

Stomach: Two enzymes. Rennin and Pepsin. Rennin is found in gastric juice and converts the soluble milk protein caesin into insoluble calcium salt. It can then stay in stomach for further digestion by Pepsin. Pepsin: Found in gastric juice, digests proteins to peptides. Is a endopeptidase, unusual as its optimum is PH 2.

Duodenum: Pancreatic endopeptidases continue the digestion, digesting proteins and peptide chains to dipeptides. All endopeptidases cut at different places on the chain and are all specfic to one type of amino acid only, this is efficent and makes many free ends for later enzymes to work with.

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Protein Digestion (Continued)

Ileum: Exopeptidases in the membrane of epithelial cells of the ileum digest dipeptides to single amino acids, these are then actively transported into the eptheilial cells of the ileum where they diffuse into the blood capillaries of the villi.

Protein enzymes and issues: They can digest other enzyme and themselves, to stop this they are produced as inactive and activated when required, once they are activated, the lining of mucus on stomach wall protects stomach from autodigestion.

Pepsin is made as pepsinogen and is activated by HCL in the stomach.

Rennin is made as Prorennin and activated by Pepsin in stomach.

Pancreatic Exopeptidases: Activated by specfic enzymes in the duodenum.

Membrane-bound peptidases: Fixed and don't have a problem

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Lipid Digestion


  • Emulsified by bile salts in the duodenum to form micelles, which have a large surface area.
  • Triglyceride is hydrolysed to fatty acid and glycerol by the pancreatic lipase enzymes in the duodenum.
  • These are lipid soluble and so easily diffuse through into the epitheial cells of the villi in the ileum.
  • These are then re-synthesised to join with proteins to form lipoproteins particles called chylomicrons.
  • These then diffuse in the lacteal which is the lymph vessel in each villus.
  • These then enter the bloodstream and are carried all round the body, excess stored as adipose tissue.
  • It isn't properly digested till used in the muscle or liver cells during respiration.
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Digestion Diseases

Lactose Intolerance: Inablity to digest and absorb lactose, the sugar in milk. In order for lactose to be digested, in needs to be broken down into glucose and galactase by its enzyme, lactase. With this condition, lactase is either not produced at all or produced in tiny volumnes.

It can be genetic or due to diseases that destroy the ileumn epitheial lining and the lactase enzymes with it. Can cause abdominal pain, flatulence and nausea.

Chlorea: Caused by bacterium vibrio chlorae, disease is caught by drinking water contaminated with faeces. The bacterium attaches the epithelial cells of the small intestine and releases a toxin, this causes chloride pump to be activated at permanent open position and cells lose ions at massive rate, sodium ions follow as they are attracted to negative ones. Due to this, water follows by osmosis and this results in watery diarrhorea. This can be fatal due to dehydration

ORT: Remedy to above conditions. Replaces lost water and ions and also contain glucose as a source of energy and also needed as glucose and salt are co-transported. Many ethical issues like is it worth money and chidren in trials.

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ok good. keep going



hahah im an idiot by=ut at least im better at bio than you haha

Natalie Beard


:P You know, if you weren't my friend id hate you by now :) thanks for the rate and feedback anyway! :)



You didn't mention about the difference in water potential with the cholera.

The flooding of chloride ions in the lumen of the intestine decreases the water potential of the lumen. The water then moves out of the blood to the lumen due to osmosis.

Very good otherwise though :)

Natalie Beard


Correct thanks for the heads up :P I'll have it done within the hour :P

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