3. Digestion & Absorption Flashcards
Ingestion
Taking in food
Digestion
The breakdown of large insoluble molecules into small soluble molecules
Absorption
Movement of nutrients into cells
Assimilation
Using products of digestion to build new parts of the cell
Elimination/Egestion
Removing undigested food/material
Physical breakdown
Broken into smaller pieces by the teeth and tongue
Why is physical breakdown useful?
Increases surface area for action of enzymes
Where/how does physical breakdown continue after the mouth?
Continues by churning in the stomach
Chemical breakdown
Hydrolysis of large insoluble molecules into smaller soluble ones by enzymes, often in stages.
What 3 things does saliva contain?
- Water - dissolves soluble substances
- Mucus - lubrication
- Amylase - hydrolyses starch (which consists of amylose and amylopectin)
Does digestion take place in the oesophagus?
NONONO
What process moves food through the alimentary canal?
Peristalsis
What type of muscles are in the oesophagus?
Circular and longitudinal - they are an antagonistic pair (one relaxes while the other contracts)
What do the glands of the stomach secrete?
Gastric juice
What does gastric juice contain?
Hydrochloric acid (pH 2) to kill bacteria Some protease enzymes
What does the bolus become in the stomach?
Chyme
What is emptied into the duodenum?
Bile from the gall bladder and liver
Pancreatic enzymes
What does bile do?
Neutralise acidic contents from stomach
Emulsifies lipids
What happens in the large intestine?
Absorption of water
What does bacteria in the large intestine do?
Aid digestions & helps to synthesise vitamin K
Where is faeces stored?
Rectum
The process of removing faeces via the anus is called…
Egestion
Pancreatic juice containing pancreatic enzymes is released via…
the pancreatic duct
The pancreas synthesises three main types of digestive enzyme:
- Amylase
- Lipases
- Proteases
Polysaccharides and disaccharides are digested into monosaccharides by…
hydrolysis of the glycosidic bonds
Where are membrane-bound disaccharidases found, and what do they do?
The epithelial lining of the ileum
They hydrolyse the single glycosidic bond
How is maltose hydrolysed?
Using maltase: glucose + glucose
How is sucrose hydrolysed?
Using sucrase:
glucose + fructose
How is lactose hydrolysed?
Using lactase:
glucose + galactose
Why is it beneficial for enzymes to be attached to the membranes of the epithelium?
- Always be in contact: more efficient
- Don’t want to egest enzymes!
- Could be auto-digested if they come into contact with proteases
- Enzymes can be reused over and over again
What does bile contain?
Bile salts and pigments (bilirubin)
Bile salt molecules have…
A hydrophobic surface and a hydrophilic surface.
They are amphipathic!
The hydrophobic end of the bile salt molecule…
interacts with the lipid
The hydrophilic end of the bile salt molecule…
faces out and prevents lipids from coalescing
What do bile salts do?
Divide the fat globule into smaller droplets (emulsification)
Why is emulsification useful?
Increases the total surface area available for lipase activity
Bile salts enter the ileum via the…
bile duct
Bile salts emulsify lipids into…
micelles
How do lipases digest lipids?
Hydrolyse the ester bond to form fatty acids and monoglycerides
Endopeptidases
Hydrolyse the peptide bond between amino acids in the central region of the protein, forming peptide molecules
Exopeptidases
Hydrolyse peptide bonds on the terminal amino acid of the peptides formed by endopeptidases
Dipeptidases
Hydrolyse bond between two amino acids of dipeptide
What is special about dipeptidases?
They are membrane-bound in the ileum
What type of protease is found in the gastric juice in the stomach?
Endopeptidase
What type of protease is found in the pancreas?
Endopeptidase AND exopeptidase
What type of protease is found in the epithelium of the small intestine?
Dipeptidase
Serosa
Outermost layer of the ileum. It is strong and protective.
Muscularis
Right after the serosa: consists of inner (circular) and outer (longitudinal) muscles which perform peristalsis
Sub-mucosa
Right after the muscularis: connective tissue between the mucosa and muscle
Mucosa
Right after the sub-mucosa
Inner lining which includes villi
Location of villi and microvilli
Circular folds in the mucosa contain villi.
Epithelial cells on each villus contain microvilli.
Co-transport
Glucose & amino acids
Facilitated diffusion
Monosaccharides
Simple diffusion
Triglycerides
Initially, absorption of monosaccharides and amino acids will be by…
facilitated diffusion
What is the problem with only facilitated diffusion in absorption of monosaccharides and amino acids? What is the solution?
Diffusion can only result in equal concentrations either side of the intestinal epithelium.
Therefore active transport is involved in the uptake of these monomers.
How is active transport of monosaccharides and amino acids carried out?
Via co-transport using sodium ions (review from previous topic)
What happens following emulsification?
The bile salts remain associated with monoglycerides and fatty acids, forming micelles
What happens when micelles come into contact with the epithelium of the ileum? What happens after that?
They break down, releasing monoglycerides and fatty acids.
Because they are non-polar molecules, they easily diffuse across the cell surface membrane into the epithelial cells.
What happens to the monoglycerides and fatty acids once inside the epithelial cells?
They are transported to the smooth endoplasmic reticulum and reformed into triglycerides.
What happens after triglycerides are reformed in the epithelium?
They associate with cholesterol and lipoproteins in the ER and Golgi to form chylomicrons
What are chylomicrons?
Lipoprotein particles that consist of triglycerides, phospholipids, cholesterol and proteins that transport dietary lipids from the intestines to other locations in the body
What happens right after chylomicron formation?
They enter the lacteal via exocytosis, and enter the blood circulation via the lymphatic system.
Enzymes in the endothelium of the blood vessels hydrolyse the triglycerides and they diffuse into cells.
