3.3 Digestion and Absorption Flashcards
(22 cards)
Explain what happens in digestion
● Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules
● That are small enough be absorbed across cell membranes into blood
Describe the digestion of starch in mammals
● Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose
● Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose
● Hydrolysis of glycosidic bond
Describe the digestion of disaccharides in mammals
● Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:
○ Maltase - maltose → glucose + glucose
○ Sucrase - sucrose → fructose + glucose
○ Lactase - lactose → galactose + glucose
● Hydrolysis of glycosidic bond
Describe the digestion of lipids in mammals,including action of bile salts
● Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
● This increases surface area of lipids for increased / faster lipase activity
● Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids
● Hydrolysis of ester bond
Describe the digestion of proteins by a mammal
● Endopeptidases - hydrolyse internal (peptide) bonds
within a polypeptide → smaller peptides
○ So more ends / surface area for exopeptidases
● Exopeptidases - hydrolyse terminal (peptide) bonds at ends of polypeptide → single amino acids
● Membrane-bound dipeptidases - hydrolyse (peptide) bond between a dipeptide → 2 amino acids
● Hydrolysis of peptide bond
* protein digestion starts in the stomach and continues in the duodenum and is fully digested in the ileum
Suggest why membrane bound enzymes are important in digestion
● Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum
● (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption
Describe the pathway for absorption of products of digestion in mammals
Lumen of ileum —>cells lining ileum —> blood
Describe the absorption or amino acids and monosaccharides in mammals
Co transport:
1 ● Na+ actively transported from
epithelial cells lining ileum to
blood (by Na+/K+ pump)
● Establishing a concentration
gradient of Na+(higher in
lumen than epithelial cell)
2 ● Na+ enters epithelial cell down
its concentration gradient with
monosaccharide or amino
acid against its concentration
gradient
● Via a co-transporter protein
3 ● Monosaccharide or amino
acid moves down a
concentration gradient into
blood via facilitated diffusion
Describe the absorption of lipids by a mammal including the roles of micelles
● Bile salts combine with monoglycerides and fatty acids to form micelles
○ Micelles make monoglycerides and fatty acids (more) soluble in water
○ Micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to
release them
○ This maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum
● Monoglycerides / fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble)
● Triglycerides reformed in (epithelial) cells and aggregate into globules
● Globules coated with proteins forming chylomicrons which are then packaged into vesicles
● Vesicles move to cell membrane and fuse with it, releasing chylomicrons via exocytosis
○ Chylomicrons enter lymphatic vessels and eventually return to blood circulation1az\
Define digestion
Biological process of biological molecules being hydrolysed into smaller molecules that can be absorbed across cell membranes
Where is the Amylase produced and desribe the digestion pathway of amylase and what is its substrate it binds to
Salivary gland and pancreas
Starts from the mouth then continues in the duodenum and completed in the ileum
Binds to polysaccharides and hydrolyses them to maltose by hydrolysing the glycosidic bond
What are the membrane bound disaccharidases
Sucrase and Lactase hydrolyse sucrose and lactose into monosaccharides
located in the duodenum
Where is lipase produced and what does it do
Produced in the pancrease and it can hydrolyse the ester bonds in tryglycerides to form monoglycerides and fatty acids
Features of bile salts
Produced in the liver
Can emulsify lipds to fro micelles
Thsi increases the SA for lipase to act on
Features of the phsycial digestion of lipids (Micelle formation and emulsification)
Lipids are coated in bile salts to create an emulsion
This is an advanateg as you get many small droplets creating a large SA which enables faster hydrolysis action by lipase
Features of the chemical digestion of lipids(Lipase)
Lipase hydrolyses lipidsinto glycerol and fatty acids
What are micelles
Water soluble vescicles formed of the fatty acifd, glycerol monoglycerides and bile salts
What do Micelles do
Deliver the fatty acids,glycerol and monoglycerides to the epiethielial cells of the ileum for absoprtion
How does the properties of the ileum increase the efficiency of absorption
- The wall of the ileum is folded and possess finger like projections which increase surface area called villi
- They are very thin walled, thus reducing the distance over which diffusion takes place.
- They contain muscle and so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum. This ensures that, as the products of digestion are absorbed from the food adjacent to the villi, new material rich in the products of digestion replaces it.
- They are well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.
- The epithelial cells lining the villi possess microvilli
-These are finger-like projections of the cell-surface membrane that further increase the surface area for absorption.
Features of lipid absorption
When the micelles encounter the ileum epithelial cells, due to the non-polar nature of the fatty acids and monoglycerides, they can simply diffuse across the cell surface
membrane to enter the cells of the epithelial cells.
Once in the cells, these will be moedified back into tryglcyerides inside of the endoplasmic reticulum and golgi bodi
Absorption of triglycerides
Once formed during digestion, monoglycerides and fatty acids remain
in association with the bile salts that initially emulsified the lipid droplets
-The structures formed are called micelles. Through the
movement of material within the lumen of the ileum, the micelles come into contact with the epithelial cells lining the villi of the ileum.
-Here the micelles break down, releasing the monoglycerides and fatty acids. As these are non-polar molecules, they easily diffuse across the cell-surface membrane into the epithelial cells.
-Once inside the epithelial cells, monoglycerides and fatty acids are
transported to the endoplasmic reticulum where they are recombined to form triglycerides. Starting in the endoplasmic reticulum and continuing in the Golgi apparatus, the triglycerides associate with
cholesterol and lipoproteins to form structures called chylomicrons.
-Chylomicrons are special particles adapted for the transport of lipids.
-Chylomicrons move out of the epithelial cells by exocytosis.
They enter lymphatic capillaries called lacteals that are found at
the centre of each villus
-From here the chylomicrons pass via the lymphatic vessels inbto the blood system
-The triglycerdies in the chylomicrons are hydrolysed by enzymes in the endothelium cells of blood capilliaries fropm where rthey diffuse into cells
Milk is the only food of human babies and so they produce a relatively large amount of lactase, the enzyme
that hydrolyses lactose, the sugar in milk. As milk forms a less significant part of the diet in adults, the production of lactase diminishes as children get older. This reduction
can be so great in some adults that they produce little, or no, lactase at all.
This was not a problem to our ancestors but can be to humans of today. Humans that produce no lactase cannot hydrolyse the lactose they consume. When the undigested lactose reaches the large intestines,
microorganisms hydrolyse it. This gives rise to small soluble molecules and a large volume of gas. This can
result in diarrhoea because the soluble molecules lower the water potential of the material in the colon.
The condition is known as lactose intolerance.
1 a Suggest the process by which micoorganisms produce ‘a large volume of gas’ in lactose
intolerant individuals.
b Suggest a reason why this gas is unlikely to be carbon dioxide.
2 Suggest an explanation why lactose intolerance is a problem for modern day humans but wasn’t for our ancestors.
3 Explain how the lowering of water potential in the colon can cause diarrhoea.
Lactose intolerance
1 a respiration
b Carbon dioxide is formed in aerobic respiration, whereas conditions in the colon are anaerobic.
2 Modern storage and distribution methods mean that milk and milk products are readily available.
Without these our ancestors rarely consumed milk as adults.
3 Low water potential in the colon causes water to move from epithelial cells into the lumen of the colon creating watery stools.
