Digestion Flashcards
What are the stages of food processing? (4)
- Ingestion – voluntarily taking food through the mouth
- Digestion – food is broken down into small molecules
- Absorption – small molecules move into the blood
- Egestion – undigested material is excreted
Components of the Digestive System =
Gastrointestinal Tract (GIT) / Alimentary canal + Accessory glands
Gastrointestinal tract = (8)
mouth → pharynx → oesophagus → stomach → small
intestine → large intestine → rectum → anus
Accessory glands = (4)
salivary glands + liver + gallbladder + pancreas
What are sphincters?
Regulate the passage of material between components and slow processes down to aid digestion and absorption
How is food digested? (6)
- Upper oesophageal sphincter (UOS) → after swallowing, food goes through UOS and then goes through Lower oesophageal sphincter (LOS) 5 – 8 s later.
Malfunction of LOS → heartburn/gastric reflux → acidic content of stomach pushes up into oesophagus if LOS does not close properly → acidic content reacts with sensitive lining of oesophagus causing pain
- Stomach → food stays in stomach for 2 -6 h after ingestion → small amounts of food exist from Pylorus slowly to allow small intestine to digest content
- Sphincter of Oddi → regulates digestive enzymes + bile from gallbladder
- Ileocecal valve → food exits small intestine from this sphincter into the large intestine → 5 – 12 h after ingestion before food moves into large intestine/colon
- Food moves through colon → takes 7 – 72 h until it reaches rectum
- Expelled after 24 – 72 h after ingestion from internal + external sphincters
What is mechanical digestion?
Mastication: grinding of bolus by teeth to breakdown food into smaller pieces → helps increase SA for enzymatic activity
What is chemical digestion?
→ enzymatic hydrolysis → breakdown of larger food into simpler molecules by enzymes in presence of water
Starch →
Glucose [Amylase]
Protein →
Amino acids [Protease]
Fat →
Fatty acids + glycerol [Lipase]
Chemical digestion of starch →
Salivary amylase in the mouth helps breakdown/hydrolyse starch into maltose + dextrins
What are salivary glands?
→ exocrine glands (glands with ducts) that produce
saliva
What are the types of salivary glands? (3)
- Parotid gland
- Location: back of the cheek
- Produces: amylase for chemical digestion - Submandibular gland
- Location: jaw - Sublingual gland
- Location: under the tongue
What do the Submandibular + Sublingual glands produce?
Submandibular + Sublingual gland produce mucus to lubricate food to facilitate swallowing → forms bolus
What are the stages of Swallowing/Deglutition? (3)
During mechanical and chemical digestion, nasal cavity is open, allowing for air to go down the trachea into the lungs
1. Oral phase
2. Pharyngeal phase
3. Oesophageal phase
What is the oral phase? (4)
- Voluntary
- Tongue presses food against roof of the mouth, triggering pressure receptors which causes swallowing centre, that initiates reflexes preventing food entry into nasal passages
- As you prepare for swallowing, soft palate/uvula at the back of the throat, blocks nasal cavity, preventing bolus going into nasal cavity
- Tongue blocks the oral cavity
What is the Pharyngeal phase? (2)
- Involuntary
- Larynx elevates and glottis closes by the epiglottis to ensure bolus of food does not go down trachea and cause choking
What is the Oesophageal phase? (2)
- Involuntary
- Food passes through UOS, which then closes, allowing for the nasal cavity to open again
What is peristalsis? (2)
→ part of the oesophageal phase
- Wave-like motions of the alimentary canal, moving food bolus forwards
- Contraction of smooth muscle in the walls of the oesophagus, forcing bolus down the oesophagus
What is the stomach? (3)
Stores: Rugae → gastric folds → allow stretching of the stomach (can stretch to store 2 L)
Fundus actively expands with the amount of food taken in
Disinfects food: Has acidic content not allowing microorganisms to survive
What are the 3 muscle layers of the stomach?
= oblique + circular + longitudinal → contraction aids mechanical digestion and mixing of food with secreted gastric juices
Chyme =
Ingested food + gastric juices
Gastric juices =
Hydrochloric acid (HCL) denatures proteins + Pepsin (protease enzyme) breaks down proteins into smaller peptides
Rugae have ___ ____ which are the openings of the gastric glands which are lined with cells.
gastric pits
What are the Cells in Stomach lining that protect stomach underlying tissues from getting digested? (4)
- Parietal cells
- Chief cells
- Mucus cells
- Epithelial cells
What are parietal cells?
→ produce H+ and CL- ions into lumen of stomach where they combine to form HCL
What are Chief cells? (3)
→ release pepsinogen (inactive form of pepsin) so that it doesn’t attack cell itself Pepsinogen is converted into Pepsin (process activated via HCL)
Positive feedback loop → pepsin production causes more activation of pepsinogen leading to more pepsin being produced so that there is enough to digest food in stomach
pH: 1.5 – 3.0
What are mucus cells? (3)
→ produces thick coat of mucus
→ physical barrier from gastric juices Mucus contains Bicarbonate ions (alkaline)
→ chemical barrier to neutralize the acid
What are epithelial cells?
→ have tight junctions which are frequently replaced
What is Gastric Ulcers formation?
Normally gastric juices are only released when food is present, but smoking + stress can lead to gastric juices being released in the absence of food. These gastric juices start eroding the stomach lining, leaving a open sore (ulcer) which can bleed. Bacteria (Helicobacter pylori) also cause ulcers as they are acid-tolerant, produce alkaline to neutralize acid in stomach to make environment suitable for habitat.
What is the small intestine? (2)
- Acidic chyme moves into the small intestine via the pyloric sphincter.
- Where most digestion + absorption occurs.
What are the 3 parts of the small intestine?
- Duodenum: shortest, 25 cm, pancreatic enzymes + bile mixed with chyme to aid digestion
- Jejunum: 2.5 m, majority for nutrients are absorbed
- Ileum: 3.5 m, remaining nutrients are absorbed until they move into larger intestine
What is the duodenum?
Acid food (chyme) from stomach mixes with digestive juices from accessory glands.
What are the accessory glands? (3)
- Pancreas → pancreatic juice rich in enzymes
- Liver → produces bile
- Gall bladder → stores bile
What is bile? (3)
Neutralizes (pH=8) and dilutes the content of the stomach, creating an optimal pH for pancreatic enzymes
Bile = water + bile salts + bile pigments + fats
Bile does not have any enzymes, its bile salts emulsify
fats, increasing the surface area for the enzyme lipase to work
Micelles =
bile salts + cholesterol + lecithin
What are the two glands of the pancreas?
Endocrine gland and Exocrine gland.
What is the endocrine gland of the pancreas?
- Islets of Langerhans cells secrete hormones (beta cells = insulin [when glucose gets too high], alpha cells = glucagon [when glucose gets too low]) into blood vessels
What is the exocrine gland of the pancreas? (3)
- Acinar cells secrete digestive enzymes into pancreatic duct
- Pancreatic juices = H2O, HCO3 -, digestive enzymes
- Bicarbonate ions (HCO3-) neutralize acidic chyme thereby providing optimal environment for pancreatic enzymes
The pancreas secretes digestive pancreatic enzymes: (4)
- Lipase → digests fats
- Amylase → digests carbohydrates
- Proteases → digests proteins → trypsin, chymotrypsin, carboxypeptidase
- Nucleases → digests nucleic acids
How are proteins digested? (5)
- Proteases (digest proteins to form smaller polypeptides) are released in inactive form called ‘zymogens’ so that pancreas is not digested itself
- Trypsinogen (inactive form) is activated by enzyme, Enterokinase found at brush border of enterocyte cells lining the small intestine, forming Trypsin (active form)
- Trypsin then further activates zymogens + inactive proteases to create active proteins to digest proteins in small intestine
- Gall stone or if trypsin is activated in pancreatic duct → pancreatitis, digestion of pancreas
- Trypsin has positive feedback on Trypsinogen
How are amino acids digested? (5)
- Peptidases (further breakdown polypeptides into amino acids)
- Peptidases are known as membrane-bound enzymes → bound to brush border of enterocytes (columnar epithelial cells) lining the small intestine → ensures amino acids are broken away close to site where they can be absorbed + ensures peptidases are not excreted with chyme
- Dipeptidases → breaks peptide bonds into 2 amino acids
- Carboxypeptidases → cutting off amino acid off carboxy end of the peptide
- Aminopeptidases → cutting off amino acid off amino end of the peptide
How are disaccharides broken down? (3)
- Disaccharidases (break down disaccharides into monosaccharides)
- Sucrase, maltase lactase → located on brush border of enterocyte cells
- enzyme, lactase breaks down lactose into glucose +
galactose which then enter bloodstream
Lactase breaks down ____ to form Galactose +
Glucose
Maltase breaks down ____ to form Glucose +
Glucose
Sucrase breaks down _____ to form Glucose +
Fructose
Lactose
Maltose
Sucrose
_____ helps break down nucleotides.
Nuclease
Nucleotidase →
Nucleosidase →
→ removes the phosphate group from the nucleotide
→ cleaves or breaks off the nitrogenous base
Left with 3 separate components: pentose sugar, phosphate group, nitrogenous base which can be used by the cell to ______ its own DNA.
synthesise
How are carbohydrates digested? (4)
Salivary amylase in the mouth breaks down starch molecules into smaller molecules which then travel
down the oesophagus into the stomach in the form of bolus where the salivary amylase becomes deactivated.
When bolus reaches duodenum, pancreas releases pancreatic juice via exocrine ducts which contain pancreatic amylase. This enzyme further breaks down the glucose chains and then enzymes on the brush
border (micro villi) finish the breakdown of the chain into monosaccharides.
The glucose, fructose and galactose are absorbed into the enterocyte cells and then out of the cells into the
blood stream where they travel into the liver.
In the liver:
- fructose + galactose is converted into glucose
Sucrose = glucose + _____
Maltose = glucose + ______
Lactose = glucose + _______
fructose
glucose
galactose
The actual glucose is converted to: (4)
- stored as liver glycogen
- becomes part of blood glucose
- stored as muscle glycogen
- converted/stored as fat.
What happens once proteins enter the stomach?
- Once proteins enter the stomach, chief cells in the stomach release pepsinogen (inactive enzyme). HCl
activates the pepsinogen into pepsin. Pepsin breaks the proteins into small polypeptides. The alkaline pH
of the small intestine inhibits pepsin’s function. Pancreas release trypsinogen, chymotrypsinogen and
procarboxypeptidase. These are inactive when released. An enzyme called Enteropeptidase activates the
enzymes trypsinogen into trypsin. This trypsin activates chymotrypsinogen into chymotrypsin and
procarboxypeptidase into carboxypeptidase. - Trypsin and chymotrypsin act on specific bonds between specific amino acids (dipeptides or small
fragments), while carboxypeptidase can only act on specific end of fragment, releasing one amino acid at a
time (free amino acids). - The brush border microvilli contain peptidases which complete the breakdown of the dipeptides and small
fragments. - The free amino acids and the dipeptides are absorbed by the enterocyte cells via membrane transporters.
The dipeptides further break down into separate amino acids when entering the enterocytes. The amino acids
are exported out of the cells into the blood where they travel to the liver.
In the liver, amino acids are: (4)
- taken up by liver cells
- used by cells throughout the body
- converted to glucose
- be used for cellular respiration
Triglyceride =
glycerol unit + 3 fatty acid chains
How are fatty acids digested? (5)
- Lingual lipase if released in the saliva in the mouth however it is not active in the mouth. The lingual
lipase + triglyceride travel down to the stomach. The acidic pH activates the lingual lipase. Chief cells
release gastric lipase. These lipases break the triglycerides into diglycerides and fatty acid chains. - When entering the small intestine, since they are hydrophobic, enter the small intestine in large lipid
masses, which must be broken down into smaller fragments → emulsification via bile salts + phospholipids. Bile salts and phospholipids are amphipathic molecules. The hydrophobic regions enclose the hydrophobic fats forming emulsion droplets. - Pancreatic lipase released by the pancreas breaks down the molecules into monoglycerides and 3 fatty
acids. Form micelles, much smaller than emulsion droplets. These transport monoglycerides + fatty acids to the enterocyte cells membranes, where they absorb the fatty acids + monoglycerides. - In the cells, the fatty acid s+ monoglycerides reform triglyceride. These are wrapped in proteins forming
lipoprotein = chylomicron. The chylomicrons are transported through the lymphatic vessels eventually
entering the bloodstream. - They are transported to liver, fat, muscle tissue where they can be stored or used by cells.
