Topic 3.2- Digestion and Absorption Flashcards
Describe the structure of the human digestive system.
The oesophagus leads into the stomach, which connects to the small intestine (made up of the duodenum, the jejunum and the ileum). These lead into the large intestine (the ascending, transverse and descending colon) which lead to the rectum and anus. Other organs such as the liver, gall bladder and pancreas secrete chemicals such as enzymes and hormones into the digestive tract.
How is food physically broken down?
By teeth in the mouth and by muscular stomach
How is food chemically broken down?
Hydrolysis of large insoluble molecules into small soluble molecules carried out by specific enzymes
What are carbohydrates broken down into?
Disaccharides and then monosaccharides
What are lipids broken down into?
Fatty acids and monoglycerides
What are proteins broken down into?
Amino acids
Name 2 enzymes that break down carbohydrates
- Amylase
- Membrane-bound disaccharidases
What reaction does amylase catalyses?
Conversion of starch into maltose
Where is amylase produced & where is it released to?
- Salivary glands - release amylase into mouth
- Pancreas - releases amylase into small intestine
What are membrane-bound disaccharidases?
Enzymes that are attached to cell membranes of epithelial cells lining the ileum
What reaction do membrane-bound disaccharidases catalyses?
Hydrolysis of disaccharides (e.g. maltose, sucrose, lactose) into monosaccharides (e.g. glucose, fructose and galactose)
Fill in the blanks
What reaction does lipase catalyse?
Hydrolysis of lipids into monoglycerides and fatty acids
Where is lipase made and where does it work?
Pancreas - works in small intestine
Where are bile salts produced?
Liver
What do bile salts do?
Emulsify lipids which cause lipids to form small droplets
What is the benefit of bile salts causing lipids to form small droplets?
Several small lipid droplets have bigger SA than single large droplet so lipase can digest lipids faster
What happens after the lipid has broken down?
Monoglycerides and fatty acids stick with bile salts to form tiny structures called micelles
Name the different proteases (or peptidases) that break down proteins
- Endopeptidases
- Dipeptidases
- Exopeptidases
What do endopeptidases do?
Break polypeptides into smaller peptide chains
Where are trypsin and chymotrypsin (endopeptidases) synthesised and secreted into?
Synthesised in pancreas and secreted into small intestine
Where is pepsin (endopeptidases) synthesised and secreted into?
Released into stomach by cells in stomach lining
What do exopeptidases do?
- Act to hydrolyse peptide bonds at ends of protein molecules
- Remove terminal amino acids
What are dipeptidases?
Exopeptidases that work specifically on dipeptides
What do dipeptidases do?
- Hydrolses dipeptides into amino acids
- Breaks peptide bond between them
Where are dipeptidases located?
In cell-surface membrane of epithelial cells in small intestine
How is glucose and galactose absorbed?
By active transport with sodium ions via co-transporter protein
How is fructose is absorbed?
Via facilitated diffusion through different transporter protein
Describe how glucose is absorbed from lumen (middle) of ileum by co-transport
- Sodium ions actively transported out of ileum epithelial cells into blood by sodium-potassium pump. This creates concentration gradient.This creates a higher conc. of sodium ions in lumen than inside cell which causes sodium to diffuse into epithelial cell by facilitated diffusion (down their concentration gradient) via sodium-glucose co-transporter proteins. Glucose is absorbed with sodium ions against their concentration gradient which causes the concentration of glucose inside cell to increase. So glucose diffuses out of cell into blood, down its concentration gradient through protein channel by facilitated diffusion
How are amino acids absorbed?
By co-transport (similar to glucose)
Name two ways the concentration gradient between the inside of the ileum and blood is maintained
- Constant circulation of blood
- Ileum has muscles which uses peristalsis to keep food moving along the gut
Describe how in detail how monoglycerides and fatty acids are absorbed and how they’re transported around the body
- Micelles help to move monoglycerides and fatty acids towards epithelium. Micelles constantly break up and reform so they can ‘release’ monoglycerides and fatty acids which allows them to be absorbed. (This makes lipid-soluble so they can diffuse directly across epithelial cell membrane)
- Monoglycerides and fatty acids transported to ER & combine to form triglycerides
- Triglycerides associated with cholesterol and lipoproteins to form chylomicrons (lipid transport structures)
- Chylomicrons enter lacteal (part of lymphatic system) by exocytosis
- Only drain into bloodstream near heart
The walls of the small intestine have lots of finger-like projections called ____
villi
The epithelial cells on the surface of the villi have folds in their cell-surface membranes called ______
microvilli
Explain the presence of bile in the colon (large intestine)
- Secreted by the liver
- Bile passes unchanged from small intestine to colon
Suggest why the peptides released when gluten is digested cannot by absorbed by facilitated diffusion
- Too big/wrong shape
- Cannot pass through channel protein
Describe the function of part X
Transports chylomicrons
Suggest an advantage of having muscle cells in the villi (2)
- Enables villi to move
- Increased contact with food
Suggest how chylomicrons leave the epithelial cells. Give an reason for your answer. (2)
- Exocytosis
- Too large to leave by other methods
