Digestion and Absorption

Digestive System

Alimentary Canal

Accessory Organs

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

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

Movement of Food

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

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

Small Intestine

Absorption

Starch Digestion

Stages of Digestion

Lipid Digestion

Lipid Absorption

Sections of the Gut

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

Examples of Digestive Enzymes

Carbohydrates


  • Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
  • Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
  • Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
  • Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested

Proteins


  • Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
  • Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
  • These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine

Lipids


Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
The smaller fat droplets are then digested by lipases released from the pancreas

Nucleic Acids


The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides

Locations of Enzymatic Digestions

Esophagus

Stomach

Liver/ Gall Bladder

Pancrease

Small Intestine

  • Amylase (starch to maltose)
  • Lipase (Triglycerides to fatty acids)
  • Endopeptidase (peptides to amino acids)
  • Nuclease (DNA/RNA to nucleosides)
  • Bicarbonate ions (neutralise stomach acids)
  • proteases (protein to polypeptides)
  • Salivary amylase (starch to maltose)
  • Bile salts (emulsification of lipids)
  • Membrane-bound enzymes (e.g. maltase)

Structure of Small Intestine

Features of Villi

Structure of Vilius Epithelium

  • Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
  • Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
  • Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
  • Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen
  • 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

Tight Junctions


Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement

Mitochondria


Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis

Microvilli


Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake

Pinocytotic Vesicles


Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle

Membrane Transport Mechanisms

Bulk Transport

Secondary Active Transport


A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)
Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na+)

Facilitated Diffusion


Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)
Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion

Osmosis


Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)
The absorption of water and dissolved ions occurs in both the small and large intestine

Simple Diffusion


Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane
Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood

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

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)

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

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

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

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