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Digestion and Absorption (Locations of Enzymatic Digestions (Esophagus…
Digestion and Absorption
Digestive System
Alimentary Canal
Oesophagus
• A hollow tube connecting the oral cavity to the stomach (separated from the trachea by the epiglottis)
Stomach
• A temporary storage tank where food is mixed by churning and protein digestion begins
• It is lined by gastric pits that release digestive juices, which create an acidic environment (pH ~2)
Small Intestine
• A long, highly folded tube where usable food substances (nutrients) are absorbed
• Consists of three sections – the duodenum, jejunum and ileum
Large Intestine
• The final section of the alimentary canal, where water and dissolved minerals (i.e. ions) are absorbed
• Consists of the ascending / transverse / descending / sigmoidal colon, as well as the rectum
Accessory Organs
Salivary Glands
• Release saliva to moisten food and contains enzymes (e.g. amylase) to initiate starch breakdown
• Salivary glands include the parotid gland, submandibular gland and sublingual gland
Pancreas
• Produces a broad spectrum of enzymes that are released into the small intestine via the duodenum
• Also secretes certain hormones (insulin, glucagon), which regulate blood sugar
Liver
• Takes the raw materials absorbed by the small intestine and uses them to make key chemicals
• Its role includes detoxification, storage, metabolism, bile production and haemoglobin
Gall Bladder
• The gall bladder stores the bile produced by the liver (bile salts are used to emulsify fats)
• Bile stored in the gall bladder is released into the small intestine via the common bile duct
Mechnical Digestion
Mechanical Digestion
Chewing (Mouth)
- Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
- The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
- The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity
Churning (Stomach)
- The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices ('churning’)
- Food is digested within the stomach for several hours and is turned into a creamy paste called chyme
- Eventually the chyme enters the small intestine (duodenum) where absorption will occur
Movement of Food
Peristalsis
- Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
- Continuous segments of longitudinal smooth muscle rhythmically contract and relax
- Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
Segmentation
- Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
- Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices
- While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement
Chemical Digestion
Stomach Acids
The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine
Bile
The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
The emulsification of fats increases the total surface area available for enzyme activity (lipase)
Enzymes
Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations
Small Intestine
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Features of Villi
- Microvilli – Ruffling of epithelial membrane further increases surface area
- Rich blood supply – Dense capillary network rapidly transports absorbed products
- Single layer epithelium – Minimises diffusion distance between lumen and blood
- Lacteals – Absorbs lipids from the intestine into the lymphatic system
- Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
- Membrane proteins – Facilitates transport of digested materials into epithelial cells
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Absorption
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Bulk Transport
Endocytosis
Endocytosis involves the invagination of the plasma membrane to create an internal vesicle containing extracellular material
Vesicle formation requires the breaking and reforming of the phospholipid bilayer and hence is an energy-dependent process
In the intestines, vesicles commonly form around fluid containing dissolved materials (pinocytosis – cell ‘drinking’)
Pinocytosis allows materials to be ingested en masse and hence takes less time than shuttling via membrane proteins
Starch Digestion
Role of Pancreas
The pancreas serves two functions in the breakdown of starch:
It produces the enzyme amylase which is released from exocrine glands (acinar cells) into the intestinal tract
It produces the hormones insulin and glucagon which are released from endocrine glands (islets of Langerhans) into the blood
The hormones insulin and glucagon regulate the concentration of glucose in the bloodstream (controls availability to cells)
Insulin lowers blood glucose levels by increasing glycogen synthesis and storage in the liver and adipose tissues
Glucagon increases blood glucose levels by limiting the synthesis and storage of glycogen by the liver and adipose tissues
Starch is a polysaccharide composed of glucose monomers and accounts for ~ 60% of the carbohydrates consumed by humans
Starch can exist in one of two forms – linear chains (amylose) or branched chains (amylopectin)
The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
Starch digestion by amylase does not occur in the stomach as the pH is unsuitable for amylase activity (optimal pH ~ 7)
Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins
Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
The hydrolysis of maltose / dextrin results in the formation of glucose monomers
Glucose can be hydrolysed to produce ATP (cell respiration) or stored in animals as the polysaccharide glycogen
Glucose monomers can also be generated from the breakdown of other disaccharides (such as lactose and sucrose)
Stages of Digestion
Ingestion – food is taken into the body via the act of eating
Digestion – food is broken down both physically (e.g. mastication) and chemically (e.g. enzymatic hydrolysis)
Absorption – digested food products are absorbed into the bloodstream and transported to cells
Assimilation – digested food products are converted into the fluid and solid parts of a cell / tissue
Elimination – undigested food residues are egested from the body as semi-solid faeces
Lipid Digestion
Lipids are hydrophobic (water ‘hating’) and hence tend to be insoluble within the aqueous environments of the body
Being hydrophobic, lipids will group together (coalesce) to form large globules of fats
The enzyme responsible for lipid digestion (lipase) is generally water soluble and is only hydrophobic at the active site
This means lipase can only bind to lipid globules at the lipid-water interface (i.e. the outer extremity of the globule)
As the interior of the fat globule is inaccessible to lipase, digestion of lipids in this form is normally very slow
Bile is a watery fluid that contains bile salts and pigments (bilirubin) – it is made by the liver and released from the gall bladder
Bile salt molecules have both a hydrophobic surface and a hydrophilic surface
The hydrophobic end interacts with the lipid while the hydrophilic end faces out and prevents lipids from coalescing
This divides the fat globule into smaller droplets (emulsification), increasing the total surface area available for enzyme activity
Lipid Absorption
Lipids within the digestive system will tend to hydrophobically aggregate to form large fat globules
Bile salts, secreted from the gall bladder, emulsify these fat globules and break them up into smaller droplets
Hydrolytic enzymes called lipases then digest the fats into their component parts
When the fatty acids are absorbed into the epithelial cells of the intestinal lining, they are combined to form triglycerides
The triglycerides are combined with proteins inside the Golgi apparatus to form chylomicrons
Chylomicrons are released from the epithelial cells and are transported via the lacteals to the liver
While in the liver, chylomicrons may be modified to form a variety of lipoproteins
Low density lipoproteins will transport lipids via the bloodstream to cells
High density lipoproteins will scavenge excess lipids from the bloodstream and tissues and return them to the liver
Sections of the Gut
Duodenum
First segment of the small intestine which is fed by digestive fluids from the pancreas and gall bladder
Bile emulsifies fat globules into smaller droplets and pancreatic juice contains digestive enzymes
Sodium bicarbonate is released from the pancreas to neutralise stomach acids such that intestinal pH is ~ 7
Jejunum
Second segment of the small intestine where the digestive process is largely completed
Pancreatic enzymes and enzymes released from intestinal glands complete the break down of sugars, proteins and lipids
Ileum
Final segment of the small intestine with the principal function of nutrient absorption
The intestinal tract is highly folded (villi and microvilli) to increase surface area and optimise material absorption
Bile is also absorbed here and returned to the liver via blood vessels
Large Intestine
The principle function of the large intestine is to absorb any remaining water and mineral ions
The large intestine is divided into the ascending colon, transverse colon, descending colon, sigmoidal colon and rectum
The appendix is also considered a part of the large intestine although it is a vestigial remnant without an important function
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