3.3.3 Digestion and Absorption Flashcards
What happens during digestion?
Large biological molecules are hydrolysed to smaller
molecules that can be absorbed across cell membranes
What are the enzymes involved in the digestive system?
Carbohydrases: hydrolyses carbohydrates, ultimately into monosaccharides
Lipases: hydrolyse lipids into glycerol and fatty acids
Proteases: hydrolyse proteins, ultimately into amino acids
What are the steps of starch digestion in humans?
Mouth: saliva contains mineral salts which maintain a neutral optimum pH and salivary amylase which starts to hydrolyse starch
Stomach: acidic which denatures amylase so hydrolysis stops
Ileum: pancreatic juice contains pancreatic amylase which converts starch to maltose and alkaline salts which maintain the optimum neutral pH, muscles push the food along
Maltase is a part of the cell surface membranes of the epithelial cells which line the ileum (membrane-bound disaccharide)
Maltase hydrolyses maltose into alpha glucose
What is the process of lipid digestion?
Hydrolysed by enzymes called lipases
Lipases hydrolyse the ester bond in triglycerides to form fatty acids and monoglycerides
Lipids are firstly split up into tiny droplets called micelles by bile salts which are produced by the liver
This is emulsification which increases surface area of the lipids so the action of lipases is sped up
How are proteins digested?
They are hydrolysed by enzymes called proteases
What are the types of peptidases? (no explanation)
Endopeptidases, exopeptides, dipeptides
What is an endopeptidase?
They hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
What is an exopeptidase?
Hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids
What is a dipeptidase?
They hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum
How do villi increase the efficiency of absorption?
Increase the surface area for diffusion
Very thin walled which reduces diffusion distance
Contain muscle so can mix the contents of he ileum to maintain the concentration gradient
Well supplied with blood vessels so the blood carries away molecules to maintain a concentration gradient
Epithelial cells lining the villi possess microvilli which are finger-like projections that increase the surface area for absorption
How are triglycerides absorbed?
Monoglycerides and fatty acids are in association with bile salts
The micelles, through the movement of material, comes into contact with the epithelial cells lining the ileum
Then the micelles break down, releasing monoglycerides and fatty acids
These are non polar molecules so easily diffuse across the cell surface membrane
Once inside, they are transported to the ER where they are recombined to form triglycerides
Starting in the ER and continuing in the Golgi, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons
Chylomicrons move of of the epithelial cells by exocytosis
They then enter lymphatic capillaries called lacteals which are found in the centre of each villi
From here they pass, via lymphatic vessels, into the blood system
The triglycerides are hydrolysed in the endothelial cells of blood capillaries where they diffuse into cells
How are micelles formed?
One end of the bile salt is soluble in fat and hydrophobic, other is reversed
This means that they can arrange themselves with their ends in fatty acid droplets and their lipophobic ends sticking out
This prevents fat droplets from sticking to each other to form large droplets, leaving tiny droplets (micelles)
What is the structure of the ileum?
The wall of the ileum is folded and possesses finger-like projections called villi
They have thin walls lined with epithelial cells on the other side of a network of blood capillaries
How are amino acids and monosaccharides absorbed?
Amino acids, fructose, glucose and galactose are absorbed via diffusion and co-transport
Which organelles are numerous/well developed in an epithelial cells of the ileum?
ER to re-synthesise triglycerides
Golgi apparatus to form chylomicrons
Mitochondria to provide ATP for co-transport
