Digestion Flashcards
Digestion
The breakdown of large molecules in the food into smaller molecules
Mouth
Ingestion of food, mechanical digestion by chewing, mixing with saliva containing amylase that starts starch digestion
Oesophagus
Movement of food by peristalsis from mouth to stomach
Stomach
Churning and mixing with water and HCl which kills pathogens in the food, initial stages of protein digestion
Small intestine
Digestion of lipids, carbohydrates, proteins and nucleic acids, absorption of nutrients, neutralisation of stomach acid. In the first part of the small intestine the secreted bile neutralises the acid and emulsifies lipids. Pancreatic enzymes are also secreted to digest the macromolecules. In the second part, digestion into monomers is completed and nutrients are absorbed through the villi.
Pancreas
Secretion of the enzymes lipase, amylase and endopeptidase
Liver
Produces bile with surfactants to emulsify lipids
Gall bladder
Storage and regulated release of bile
Large intestine
Re-absorption of water, formation and storage of faeces. Faeces are stored in the rectum
Anus
Egestion of faeces
Structure of small intestine wall
The inside space is the lumen
The wall of the gut is made of layers of specialised tissues (going outwards):
-epithelium of mucosa
-mucosa
-circular muscle
-longitudinal muscle
What increases the surface area of the epithelium of the mucosa?
Villi
Peristalsis
Waves of muscle contraction
Describe peristalsis
Contraction of the circular muscle behind the food constricts the gut to prevent food from being pushed back towards the mouth. Contraction of longitudinal muscle where the food is located moves it along the gut. Peristalsis only occurs in one direction.
Why is cellulose undigested?
Humans do not have the enzyme cellulase needed
Pancreatic juice
It is secreted from the pancreatic duct:
lipase, amylase and endopeptidase
Bile
Released by the gall bladder into the lumen. Emulsifies lipids thus increasing the SA for lipase to act. Neutralises the acid for pancreatic enzymes to work.
Enzymes in the wall of the small intestine
The wall of the small intestine produces enzymes and most of these enzymes are immobilised in the membrane of epithelial cells. This includes maltase, dextrinase, lactase and nucleases
Absorption
The process of taking substances into cells and the blood
Absorption by villi
Absorb the products of digestion, such as monosaccharides. amino acids and fatty acids
Structure of villus
Epithelium consists of epithelial cells with microvilli and dew goblet cells. In the internal structure of the villus, there is a dense network of capillaries and a lacteal
Adaptations of the small intestine
-approximately 7 metre long
- folds on its inner surface
-each fold has numerous villi
-the epithelial cells of villi have microvilli
Villi adaptations
-have a single-cell epithelium
- have capillaries close to the surface for glucose and amino acids to be readily absorbed
-have lymph vessels close to the surface for lipids to be readily absorbed
- have microvilli
Methods of absorption
-Simple diffusion eg fatty acids and monoglycerides
- facilitated diffusion eg. fructose and glucose
- active transport (specific pumps)- eg mineral ions such as calcium and iron
-Pinocytosis and exocytosis (small droplets of fluid are passed through the membrane by means of vesicles) eg. triglycerides and cholesterol
-co-transport eg. glucose into epithelial cells
Apical surface
The surface of the epithelial cell facing the lumen of the intestine
Basal surface
The surface facing the blood vessels is called the basal surface
Dialysis tubing
Used to model absorption by the epithelium of the small intestine. The tubing is made of cellulose and is semi-permeable
Triglycerides transportation
-triglycerides are digested into fatty acids and monoglycerides. both products are absorbed by villus epithelial cells by simple diffusion
- inside the villus epithelial cells, fatty acids combine with monoglycerides to form triglycerides. Triglycerides coalesce with cholesterol to form droplets coated in phospholipids and protein
-the lipoprotein droplets are released by exocytosis through the basal membrane. They then enter the lacteal and are carried away in the lymph.
Glucose transportation
-glucose is absorbed into the villus epithelial cells be sodium-glucose-co-transporter protein. This protein moves a glucose molecule together with a sodium ion from the lumen into the epithelial cells. The glucose can move against its conc gradient bc the sodium ion is moving down its conc gradient. The sodium gradient is generated by active transport of sodium ions out of the epithelial cell by a pump protein.
-glucose moves be facilitated diffusion from the cytoplasm inside the villus and then diffuses into adjacent capillaries
Describe the digestion of starch
-starch molecules cannot be absorbed so much be digested.
-Amylose has unbranched chains of a-glucose linked 1,4 bonds. amylopectin has chains of a-glucose linked by a-glucose and also some 1,6 bonds making the molecules branched
-saliva contains salivary amylase but most starch digestion happens in the small intestine by pancreatic amylase.
-amylase breaks 1,4 bonds in chains of four or more glucose monomers so it can digest amylose into maltose
-amylase cannot break the 1,6bonds in amylopectin
-fragments of amylopectin containing a 1,6 bond that amylase cannot digest are called dextrins
- starch digestion is completed by enzymes in the microvilli on villus epithelial cells: maltase and dextrinase which digests maltose and dextrins.
-the produced glucose is absorbed into the villus epithelial cells by co-transport with sodium ions. It then moves by facilitated diffusion into the villus and then onto blood capillaries.
Exocrine glands
Secrete through a duct into the lumen of the gut or onto the surface of the body
Endocrine glands
Ductless and secrete hormones directly into the blood
Gastric juice
Water, hydrochloric acid, proteases, mucus
Pancreatic juice
Water, bicarbonates, enzymes including amylase, lipase and endopeptidase
Structure of exocrine glands
Consist of secretory cells and each group of cells is called an acinus. The secretory cells appear to have extensive rER and Golgi apparatus for the synthesis and modification of secreted enzymes. They also have many vesicles and many mitochondria for exocytosis
Function of parietal cells
Secrete hydrogen ions using a proton pump to make the contents of the stomach acidic
Function of chief cells
Secrete pepsinogen and in the acidic conditions of the stomach pepsinogen is converted to pepsin
Function of the acidic conditions
Help control pathogens
Favour some hydrolysis reactions
Control of gastric juice secretion
-Both nerves and hormones control the secretion of digestive juices
-the sight or smell of food causes the brain to send nerve impulses from the medulla vie the vagus nerve
-parietal cells in the stomach wall respond by secreting acid. This is a reflex action
-sodium and chloride ions are also secreted, causing water to move by osmosis into the stomach to form gastric juices
- when food enters the stomach, chemoreceptors detect peptides and stretch receptors detect distension of the stomach. Impulses are sent from these receptors to the brain.
-the brain responds by sending impulses via the vagus nerve to endocrine cells stimulating them to secrete the hormone gastrin
-gastrin further stimulates the secretion of acid and pepsinogen
-secretin and somatostatin inhibit gastrin secretion if the pH in the stomach falls too low.
Stomach ulcers
Helicobacter pylori causes stomach ulcers. Ulcers are open sores in the stomach wall causing bleeding. They are caused by partial digestion of the stomach lining by pepsin and HCl. Stomach cancer can result. Antibiotics are provided as a treatment. H. pylori survives the acidic conditions of the stomach and it secretes enzymes which digest the stomach lining and allow for further damage by the acid and pepsin.
Reduction of stomach acid secretion
Proton pump inhibitors are used to reduce it. There is a mucus barrier in the stomach and this barrier is broken down. Some people have a problem in the muscle at the top of the stomach that prevents fluid from escaping. If the acid escaped, it irritates the oesophagus, this is acid reflux known as heartburn.
The secretion of acid in the stomach is achieved by proton pump H+/K+ ATPase in parietal cells and this causes the pump uses one ATP to exchange two protons from the cytoplasm for two K+ in the lumen.
Proton pump inhibitors (PPIs)
PPIs binds irreversibly to a pump, preventing proton pumping and making the stomach contents less acidic. PPIs are consumed in an inactive form and are converted to the active form in the acidic environment of the stomach.
Serendipity
Research that takes place because of a fortuitous accident. A man suffered from a gunshot injury and the wound healed and allowed access to his stomach from the outside. He conducted experiments over 11 years and he tied food to a string and followed its digestion. He also digested food in extracted gastric juice and overturned the notion that digestion is a solely physical process but also a chemical.
Adaptations of villi epithelial cells
-the apical surface has a number of microvilli, protrusions/extensions of the membrane that increase the SA for absorption
-large number of mitochondria lot of ATP needed for AT
-epithelial cells adheres to tight junctions to prevent most material from moving between epithelial cells and ensure that they pass through the epithelial cell into the blood vessels
- vesicles are present in large numbers
- apical and basal surfaces have different types of proteins involved in transport
Examples of dietary fibre
Cellulose
Lignin
Chitin
Pectin
Effect of increase in dietary fibre
The rate of transit of materials through the large intestine increases because fibre increases the bulk of material and draws water into the intestine.
Effect of low dietary fibre
Constipation
Bowel cancer
haemorrhoids
appendicitis
Mean residence time
The average time that food remains in the gut
