3.3 Digestion and absorption Flashcards
What is digestion?
The chemical or mechanical process where large molecules are hydrolysed into small, soluble molecules that can be absorbed across the cell membrane, and into the bloodstream.
What does amylase do?
Hydrolyses starch into maltose.
How does amylase hydrolyse starch into maltose?
By hydrolysing the glycosidic bonds.
What does maltase do?
Hydrolyses maltose into glucose.
What is maltase?
Membrane-bound disaccharidase.
How does maltase hydrolyse maltose into glucose?
By hydrolysing the glycosidic bonds.
What does sucrase do?
Hydrolyses sucrose into glucose and fructose.
What does lactase do?
Hydrolyses lactose into glucose and galactose.
Where can starch be broken down?
In the mouth and small intestine.
Where is amylase released from?
From the mouth and pancreas.
What enzyme digests proteins in the stomach?
Endopeptidases.
What do endopeptidases do?
Hydrolyse peptide bonds within an amino acid chain.
What is the purpose of endopeptidases?
Increase surface area.
What enzymes break down proteins in the duodenum?
Endopeptidases, exopeptidases, and dipeptidases.
What do exopeptidases do in the duodenum?
Hydrolyse peptide bonds at the end of the amino acid chain.
What do dipeptidases do in the duodenum?
Hydrolyse peptide bonds between 2 amino acids (dipeptides).
What is a triglyceride?
Glycerol and 3 fatty acids.
What does lipase do?
Hydrolyses the ester bond in triglycerides to form monoglycerides and fatty acids.
Where are bile salts formed?
In the liver.
Where are bile salts stored?
In the gall bladder.
Where are bile salts released?
Into the bile duct.
What do bile salts do?
Emulsify lipids.
What do bile salts emulsify lipids into?
Large droplets into small droplets.
Why is it useful that bile salts emulsify lipids into smaller droplets?
Provides a larger surface area for faster hydrolysis and more enzyme-substrate complexes formed.
Where is lipase released from?
From the pancreas.
Where does the digestion of lipids take place?
In the duodenum.
What is produced when lipase hydrolyses the ester bonds in lipids?
3 fatty acids + 1 glycerol.
What reaction does amylase catalyse?
Starch to maltose.
What glands secrete amylase?
Salivary and pancreas.
What is the site of amylase action?
In the mouth and duodenum.
What are the optimum conditions for amylase?
pH 7 and 37°C.
What reactions do membrane-bound disaccharidases catalyse?
Disaccharides to monosaccharides.
Give examples of membrane-bound disaccharidases.
Maltase, lactase, and sucrase.
What secretes membrane-bound disaccharidases?
Epithelial cells in the ileum and duodenum wall.
What is the site of membrane-bound disaccharidases action?
In the ileum.
What are the optimum conditions for membrane-bound disaccharidases?
pH 7 and 37°C.
What reaction does lipase catalyse?
Lipids to fatty acids + glycerol.
What secretes lipase?
The pancreas.
What is the site of lipase action?
In the duodenum.
What are the optimum conditions for lipase?
pH 7 and 37°C.
What reaction do exopeptidases catalyse?
Hydrolyse the peptide bonds at the end of the amino acid chain.
What secretes exopeptidases?
The pancreas.
What is the site of exopeptidases action?
In the duodenum.
What are the optimum conditions for exopeptidases?
pH 7 and 37°C.
What reactions do endopeptidases catalyse?
Hydrolyse peptide bonds within the amino acid chain.
What secretes endopeptidases?
The stomach and pancreas.
What is the site of endopeptidases action?
In the stomach and duodenum.
What are the optimum conditions for endopeptidases?
pH 2/7 and 37°C.
What reactions do membrane-bound dipeptidases catalyse?
Hydrolyse peptide bonds between 2 amino acids (dipeptides).
What secretes membrane-bound dipeptidases?
Cells in the ileum wall.
What is the site of membrane-bound dipeptidases action?
In the ileum wall.
What are the optimum conditions for membrane-bound dipeptidases?
pH 7 and 37°C.
What is the role of hydrochloric acid in the stomach?
Maintains a low pH, which is optimum for enzymes within the stomach, and kills pathogens.
How is the ileum lining specialised for absorption?
Villi and microvilli on epithelial cells increase surface area, capillary beds maintain a concentration gradient, and muscular walls contract for constant movement.
What polar and charged molecules will be absorbed in the small intestine?
Sodium ions, potassium ions, chlorine ions, and iron ions.
Why is facilitated diffusion alone insufficient for the absorption of key nutrients?
Concentrations would reach equilibrium, resulting in less glucose and amino acids absorbed.
What is formed after lipase hydrolyses small droplets?
Micelles.
What is a micelle?
A spherical vesicle that is water soluble.
What do the micelles contain after the hydrolysis of lipids?
1 fatty acid, 1 monoglyceride, and bile salts.
What happens after a micelle has been formed?
The micelles transport the fatty acid, monoglyceride, and bile salts to the epithelial cell membrane.
What happens once the micelles reach the epithelial cell membrane?
Micelles release fatty acid and monoglyceride, which diffuse across the membrane.
What happens inside the epithelial cells after fatty acid and monoglyceride diffuse across the membrane?
The triglycerides reform.
Why do the triglycerides reform inside the epithelial cells?
So they become useful again.
What happens once the triglyceride has reformed inside the epithelial cell?
The triglyceride is packaged and modified by the Golgi apparatus and combines with a protein to form a chylomicron.
What is a chylomicron?
A triglyceride bound to a protein.
Where is the triglyceride packaged and modified?
In the Golgi apparatus.
Where are triglycerides reformed within the epithelial cells?
In the endoplasmic reticulum.
Once the chylomicron has been formed, where is it released into?
A Golgi vesicle.
What happens once the chylomicron has been released into a Golgi vesicle?
It is transported to the other end of the epithelial cell, the vesicle fuses with the membrane, and the chylomicron is released via exocytosis.
What happens after the Golgi vesicle containing the chylomicron has fused with the membrane?
It is released via exocytosis.
What is the first step of glucose and amino acid absorption?
An Na+ ion binds to the carrier protein.
What type of protein forms the sodium/potassium pump?
Carrier protein.
Where does ATP attach to during glucose and amino acid absorption?
To the carrier protein.
What happens once ATP has attached to the carrier protein?
It is hydrolysed to form ADP and an inorganic phosphate.
What happens once ATP has been hydrolysed?
The phosphate remains attached to the carrier protein.
What does the phosphate do to the carrier protein?
Alters its shape.
What happens after the phosphate has altered the shape of the carrier protein?
The Na+ ion is released out of the epithelial cell.
What is the significance of the Na+ ion being released out of the epithelial cell?
The concentration of Na+ ions in the cell decreases, making it lower than in the ileum.
What happens once the Na+ ion is released out of the epithelial cell?
The phosphate is released from the carrier protein, restoring the shape of the protein.
What happens once the phosphate has been released from the carrier protein?
There is a lower concentration of Na+ ions in the epithelial cell.
What is the significance of there being a lower concentration of Na+ ions in the epithelial cell?
It means that Na+ ions can move into the cell.
By what process can Na+ ions move into the epithelial cell?
Facilitated diffusion.
By what protein are Na+ ions transported into the cell?
Via a co-transport protein.
What does a co-transport protein require to move an Na+ ion?
2 molecules: Na+ ion AND glucose or an amino acid.
Where does the Na+ ion attach to allow glucose or an amino acid to attach?
To the complementary receptor.
What does Na+ ion binding to the complementary receptor do?
Alters the shape of the co-transport protein.
What is the significance of altering the shape of the co-transport protein?
Na+ ion can be released into the cell, along with glucose or an amino acid.
What happens once glucose or an amino acid has entered the epithelial cell via the co-transport protein?
Glucose or amino acid moves into the bloodstream.
By what process does glucose or amino acid move into the bloodstream from the epithelial cell?
By facilitated diffusion.
Why does glucose or amino acid move into the bloodstream via facilitated diffusion?
Because it is moving from a higher concentration in the epithelial cell to a lower concentration in the bloodstream.
How do endopeptidases and exopeptidases increase the rate of protein digestion?
Exopeptidases hydrolyse peptide bonds at the end of a polypeptide, and endopeptidases hydrolyse internal peptide bonds within a polypeptide, resulting in a higher surface area.
Name one body protein that could result in reduced muscle power.
Myosin.
Name one body protein that could result in reduced immunity.
Antibodies.
Describe the role of enzymes in the digestion of proteins in a mammal.
They hydrolyse peptide bonds, with endopeptidases acting in the middle of polypeptides, exopeptidases acting at the end, and dipeptidases acting on dipeptides.
Name the other type of enzyme required for the complete hydrolysis of proteins to amino acids.
Dipeptidases.
Why are the combined actions of endopeptidases and exopeptidases more efficient than exopeptidases alone?
Endopeptidases hydrolyse internal peptide bonds, leading to more ends and increased surface area.
Describe the role of the enzymes of the digestive system in the complete breakdown of starch.
Amylase converts starch to maltose, maltase converts maltose to glucose, involving the hydrolysis of glycosidic bonds.
Describe the processes involved in the absorption of the products of starch digestion.
Glucose moves into the epithelial cell with sodium via carrier/channel protein, sodium is removed from the epithelial cell via active transport, maintaining a low concentration of sodium, and glucose moves into the blood by facilitated diffusion.
Name the structure labelled X. (1)
Describe the function of layer labelled Y. (2)
X - Villus
y- contracts / peristalsis;
moves / pushes / forces food through gut;
Students investigated the digestion of lipids in milk by lipase. They set up three test tubes.
In tube A, milk was incubated with lipase only
In tube B, milk was incubated with lipase and bile salts
In tube C, milk was incubated with bile salts only
The results are shown in the table.
(b) The pH did not fall below a value of 6.5 in tube A. Suggest one reason why. (1 mark)
Substrate/lipids all used up
Equilibrium reached
(pH) denatures enzymes
(c) The rate at which the pH fell in tube A was different from the rate at which the pH fell in tube B. Explain why pH fell at a different rate. (2 marks)
Bile salts produce many small lipid droplets/emulsifies lipids
Greater surface area of lipids for lipase to act on in test tube B
so fatty acids produced faster and pH drops faster
(d) Explain why test tube C set up. (1 mark)
To show that lipase has to be present for pH to change/reaction to take place / to show that bile salts do not digest lipids
The pH changed in test tube A. Explain why. (2 marks)
Production of fatty acids
(Fatty) acids (produced) cause fall in pH
The addition of a respiratory inhibitor stops the absorption of amino acids. Use the diagram about cotransport to explain why?
less ATP
NO active transport
Sodium not moved into cell
NO diffusion gradient for sodium
NO concentration gradient for sodium
Describe how you would use a microscope to find the mean diameter of triglyceride droplets on a slide. (3)
Measure with eyepiece graticule / scale;
Calibrate with stage micrometer / scale on slide / object of known size;
Repeats and calculate the mean;
OR
Use a ruler to estimate the field diameter under microscope;
How many droplets go across the field;
Repeats and calculate mean;
Figure 1 shows the co-transport mechanism for the absorption of amino acids into the blood by a cell lining the ileum.
The addition of a respiratory inhibitor stops the absorption of amino acids. Use figure 1 to explain why. (3 marks)
No/less ATP produced OR
No active transport
Sodium (ions) not moved (into/out of cell) / sodium ions increase in cell
No diffusion gradient for sodium (to move into cell with amino acid)
OR
No concentration gradient for sodium (to move into cell with amino acid);
Give an advantage of lactase and other digestive enzymes being located in the plasma membranes of cells lining the small intestine, rather than being secreted into the lumen of the small intestine. (1)
Enzymes not lost (with gut contents) / more effective absorption of products formed by these enzymes;
Explain the advantages of lipids droplet and micelle formation
Droplots increases surface area
so faster hydrolysis
Micelle carry fatty acids and glycerol through membrane to cell
Glucose is absorbed from the lumen of the small intestine into epithelial cells.
Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells.
- Na+ ions leave epithelial cell and enter blood;
Penalise for Na without ions once.
- (Transport out is by) active transport / pump / via carrier protein using ATP;
Reject channel protein
- So, Na+ conc. in cell is lower than in lumen (of gut);
Maintains diffusion gradient for Na+ from lumen/into cells;
- Sodium/Na+ ions enter by facilitated diffusion;
Accept diffusion/from high to low concentration through a symport/cotransport protein
- Glucose absorbed with Na+ ions against their concentration/diffusion gradient / glucose absorbed down an electrochemical gradient;
How is the Golgi involved in the absorption of lipids
Modifies/ processes triglycerides
Combines triglycerides with protein
Packaged for release/ exocytosis
Forms vesicles
