6.1 Digestion and Absorption Flashcards
What are the two major groups of organs which comprise the human digestive system?
- The alimentary canal consists of organs through which food actually passes (oesophagus, stomach, small & large intestine)
- The accessory organs aid in digestion but do not actually transfer food (salivary glands, pancreas, liver, gall bladder)
What organs are part of the alimentary canal?
Oesophagus, stomach, small intestine, large intestine
What are the accessory organs?
Salivary glands, pancreas, liver, gall bladder
What is the oesophagus and what does it do?
- A hollow tube connecting the oral cavity to the stomach (separated from the trachea by the epiglottis)
- Food is mixed with saliva and then is moved in a bolus via the action of peristalsis
- Movement of food by peristalsis from the mouth to the stomach
What is the stomach and what does it do? What pH?
- Churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of protein digestion
- 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)
What is the small intestine and what does it do?
Small Intestine
- A long, highly folded tube where usable food substances (nutrients) are absorbed
- Consists of three sections – the duodenum, jejunum and ileum
- Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralising stomach acid, plus absorption of nutrients
What doess the salivary gland do?
- Release saliva to moisten food and contains enzymes (e.g. amylase) to initiate starch breakdown
What is the pancreas and what does it do?
- Produces a broad spectrum of enzymes that are released into the small intestine via the duodenum
- Secretion of lipase, amylase, and protease
What is the liver and what does it do?
- Secretion of surfactants in bile to break up lipid droplets
- Its role includes detoxification, storage, metabolism, bile production and haemoglobin breakdown
What is the gall bladder and hat does it do?
- Store and regulate release of bile produced by the liver
- Bile stored in the gall bladder is released into the small intestine via the common bile duct
What are the two methods food can be digested in?
Mechanical and chemical digestion
What is involved in 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
How does the food move?
Peristalsis
- Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
- The contraction of circular and longitudinal smooth muscle layers of the small intestine mixes the food with enzymes and moves it along the gut
- Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
- Exerts continuous moderate force, interspersed with short periods of more vigorous contraction, rather than remaining relaxed unless stimulated to contract
What is segmentation? Where does it occur? What do it do? How does it help?
- 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
In chemical digestion, how is food broken down?
by the action of chemical agents (such as enzymes, acids and bile)
How is stomach acid released?
- The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
Where is bile produced and stored?
What does bile interact with?
- Produced by liver
- Stored in gall bladder
- Release into the intestine
- Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
Why is emulsification needed?
Bile emulsifies the fat globules and it increases the total surface area available for enzyme activity (lipase)
What are enzymes? What do they do?
- 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
What are some examples of digestive enzymes from the pancreas and liver?
Where are digestive enzymes secreted from?
Digestive enzymes are secreted predominantly by the pancreas, although other organs also contribute (salivary gland, stomach)
What does carbohydrate digestion begin?
What is the enzyme secreted by?
Can be digest cellulose?
- begins in the mouth with the release of amylase from the salivary glands (starch digestion)
- secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
- Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence passes through the body undigested
What enzymes are often immobilised and where?
Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
Where does protein digestion begin? What does it release? What pH?
- Begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH2)
- Then in the small intestine, the smaller polypeptide chains are broken down by endopeptidases released by the pancreas
- Endopeptidases work optimally in neutral environments (pH 7) as the pancreas neutralises the acids in the intestine
Where do lipids break down? With what enzyme?
- Lipid breakdown occurs in the intestine, 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
What enzyme released from where digests nucleic acids (DNA,RNA) and into what?
- The pancreas releases nucleases which digest nucleic acids into smaller nucleosides
What specialised structures do the human intestine have to absorb the products of digestion?
- The small intestine absorbs usable food substances (i.e. nutrients – monosaccharides, amino acids, fatty acids, vitamins, etc.)
- The large intestine absorbs water and dissolved minerals (i.e. ions) from the indigestible food residues
What are the 4 main tissue layers of the small intestine?
Identify them in this image
From the outside of the wall going inwards:
- 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
What are the finger-like projections in the inner epithelial lining of the intestine?
Villi (singular: villus)
What are the features in 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
Mnemonic: MR SLIM
Why is the inner epithelial lining of the intestine highly folded?
The vili increase the available surface area for material absorption
What are some structural features which optimise villi’s ability to absorb materials?
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
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
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
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
During absorption, where must the digested food monomers pass through?
Pass from the lumen into the epithelial lining of the small intestine
What do the tight junctions between epithelial cells do?
It occlude any gaps between cells - all monomers must cross the membrane
Do different monomers/nutrients undertake different membrane transport?
Yes, different methods of membrane transport are required to absorb different nutrients
What is secondary active transport and what monomers uses this method to be absorbed?
- 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+)
What is facilitated diffusion and what monomers use this method?
- 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
What is osmosis and which monomers use this method?
- 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
What is simple diffusion and which monomers use this method?
- 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
What does endocytosis involve? How do vesicles form? Does it require energy? What is it known as in the intestine?
- 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
Summary of absorption methods
Overview of starch hydrolysis
What do 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
What is the permeability of dialysis tubing?
Selectively-permeable
What are the 5 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
How does bile divide the fat globule into smaller droplets (emulsification)
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
How is lipid absorbed?
After they are emulsified by bile salts, they can enter the epithelial cells. 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
