Define the term digestion

Digestion is the process by which large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.

Which type of chemical reaction breaks down large biological molecules during digestion?

Hydrolysis reactions break down large biological molecules during digestion, using water to break the chemical bonds.

What are the products when proteins , carbohydrates and lipids are hydrolysed during digestion?

Proteins are hydrolysed into amino acids . Carbohydrates are hydrolysed into simple sugars . Lipids are hydrolysed into glycerol and fatty acids .

During digestion, proteins are hydrolysed into .

During digestion, proteins are hydrolysed into amino acids .

Lipids are hydrolysed into glycerol and during digestion.

Lipids are hydrolysed into glycerol and fatty acids during digestion.

Why must large biological molecules be hydrolysed before they can be absorbed?

Large biological molecules are too big to be absorbed across cell membranes. Hydrolysis produces smaller, soluble molecules that can cross the cell membranes lining the ileum.

How do cells use the small molecules produced by digestion?

To release energy via respiration . To build new molecules for cell growth, repair and function.

True or False? During digestion, small biological molecules are hydrolysed into larger molecules.

False. During digestion, large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.

Where does the digestion of starch begin, and which enzyme is responsible?

Starch digestion begins in the mouth . The enzyme amylase , secreted in saliva by the salivary glands, begins to digest starch into maltose .

What are the roles of hydrochloric acid in the stomach?

It provides a suitable pH for protease enzymes to work. It destroys any pathogens present in food.

In which part of the small intestine are the products of digestion absorbed , and how?

Absorption occurs in the ileum . Food and water are absorbed into the blood via villi in the lining of the ileum.

What are the three main types of digestive enzyme ?

Carbohydrases Lipases Proteases

What is meant by describing digestive enzymes as extracellular enzymes ?

Extracellular enzymes are enzymes that function outside the body cells that produce them.

What reaction does amylase catalyse during carbohydrate digestion?

Amylase hydrolyses starch into the disaccharide maltose .

Where in the body is amylase produced?

The salivary glands , the pancreas and the small intestine .

What is the role of maltase in carbohydrate digestion?

Maltase hydrolyses the disaccharide maltose into the monosaccharide glucose .

What is a membrane-bound disaccharidase ?

A disaccharidase that is attached to the cell-surface membranes of the epithelial cells lining the small intestine. It hydrolyses disaccharides into monosaccharides .

Amylase hydrolyses starch into maltose, which is then hydrolysed into glucose by .

Amylase hydrolyses starch into maltose, which is then hydrolysed into glucose by maltase .

How do bile salts aid lipid digestion through emulsification ?

Bile salts bind to large lipid droplets and break them into smaller droplets . This gives the lipids a larger surface area on which lipase enzymes can act.

What are the products when lipase hydrolyses lipids?

Glycerol , monoglycerides and fatty acids .

True or False? Bile chemically digests lipids by hydrolysing them into fatty acids.

False. Bile is not an enzyme and does not carry out chemical digestion. Bile salts emulsify lipids into smaller droplets, increasing the surface area for lipase to act on.

What is the difference between the action of endopeptidases and exopeptidases ?

Endopeptidases hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains. Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids.

What are dipeptidases , and where are membrane-bound dipeptidases found?

Dipeptidases are a type of exopeptidase that hydrolyse dipeptides into individual amino acids . Membrane-bound dipeptidases are attached to the cell-surface membrane of the epithelial cells lining the small intestine.

Why can the iodine test be used to follow the progress of the reaction catalysed by amylase ?

Amylase digests starch into maltose . Iodine turns blue-black in the presence of starch, so the loss of this colour shows that starch is being broken down.

What do a strong positive , a weak positive and a negative iodine result tell you about amylase activity?

Strong positive (blue-black): starch is present at high concentrations, so amylase activity is absent or very low. Weak positive : starch is present at a low concentration, so amylase activity is high. Negative (orange-brown): no starch remains, so amylase has broken all the starch down into maltose.

In the amylase investigation, the iodine solution remains once all the starch has been digested.

In the amylase investigation, the iodine solution remains orange-brown once all the starch has been digested.

How is a specific pH maintained in each test tube when investigating amylase activity?

A buffer solution at the required pH is added to the amylase before the starch is introduced.

What is a limitation of using the iodine test to measure amylase activity, and how can it be improved?

Judging the exact point at which the iodine no longer changes colour is subjective . A colorimeter can be used to measure the progress of the reaction in a more objective way.

Which indicator is used to follow lipase activity, and what colour changes does it show?

Phenolphthalein . It is pink in alkaline conditions and colourless in acidic conditions.

Why does the solution change from pink to white during the lipase and milk investigation?

Lipase hydrolyses the lipids in milk into fatty acids and glycerol. The fatty acids lower the pH until the solution becomes slightly acidic . The phenolphthalein turns colourless, so the white colour of the milk becomes visible.

Bile salts increase the rate of lipase activity because they cause of lipids.

Bile salts increase the rate of lipase activity because they cause emulsification of lipids.

Give three control variables that should be kept constant when investigating the effect of pH on amylase activity

Equal volume and concentration of enzyme (amylase) solution. Equal volume and concentration of substrate (starch) solution. Equal volume of buffer solution.

True or False? When describing a control variable, it is acceptable to state that the "amount" of enzyme solution is kept the same.

False. You should refer to both the volume and concentration of a solution, and avoid using the word 'amount'.

By which mechanisms are amino acids , monosaccharides and lipids absorbed in the ileum?

Amino acids and monosaccharides are absorbed by co-transport . Lipids are absorbed with the help of micelles .

What is the role of the sodium-potassium pump in the co-transport of glucose and amino acids?

It actively transports sodium ions out of the epithelial cell into the blood. This lowers the sodium ion concentration inside the cell, maintaining a sodium ion concentration gradient between the intestine and the epithelial cell.

Describe how amino acids are absorbed into the blood by co-transport

Sodium ions are actively transported from the epithelial cell into the blood by the sodium-potassium pump , maintaining a sodium ion gradient. Sodium ions then move down their concentration gradient from the intestine into the epithelial cell via a co-transporter protein , carrying an amino acid with them. The amino acid concentration in the epithelial cell rises, so amino acids diffuse down their concentration gradient into the blood .

Why is the co-transport of glucose and amino acids considered to be active transport overall?

Although the co-transporter protein itself works passively , energy (from ATP) is needed to power the sodium-potassium pump that creates the sodium ion gradient .

Glucose is absorbed into epithelial cells by co-transport alongside ions.

Glucose is absorbed into epithelial cells by co-transport alongside sodium ions.

A tiny, spherical structure formed when monoglycerides and fatty acids associate with bile salts . Micelles transport these insoluble products of lipid digestion to the cell-surface membranes of the epithelial cells.

How do the products of lipid digestion move from micelles into the epithelial cells?

Micelles constantly break up and reform , releasing fatty acids and monoglycerides at the epithelial cell surface. Because these molecules are non-polar (lipid-soluble), they diffuse through the phospholipid bilayer of the cell membrane.

What happens to fatty acids once they are inside the epithelial cells?

Short fatty acid chains move directly into the blood by diffusion. Longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum . The triglycerides are packaged into chylomicrons , which eventually enter the bloodstream.

Monoglycerides and fatty acids associate with bile salts to form .

Monoglycerides and fatty acids associate with bile salts to form micelles .

True or False? The products of lipid digestion are absorbed into epithelial cells by co-transport with sodium ions.

False. Fatty acids and monoglycerides are non-polar , so they diffuse through the phospholipid bilayer after being carried to the membrane by micelles . Co-transport with sodium ions is used to absorb amino acids and monosaccharides .

What feature of the ileum epithelial cells increases the surface area for absorption?

Microvilli , which are tiny folds of the cell-surface membrane of the epithelial cells. They increase the surface area for the absorption of the products of digestion.

What is Visking (dialysis) tubing ?

A non-living, partially permeable membrane made from cellulose . It can be used to model the process of absorption that occurs in the small intestine.

Why can glucose pass through Visking tubing while starch cannot?

The pores in the tubing are small enough to prevent large molecules such as starch from passing through. Smaller molecules such as glucose are able to pass through the pores by diffusion .

Which process in the body does Visking tubing model?

The absorption of the products of digestion across the lining of the small intestine (ileum) .

Outline the method used to model digestion and absorption with Visking tubing

Cut a section of Visking tubing and tie one end . Fill the tubing with a starch and amylase mixture. Suspend the tubing in a beaker of water for a set period of time. Take samples of the liquid outside the tubing at regular intervals and test for starch and glucose .

What results are expected from the Visking tubing absorption model, and why?

Glucose is present in the liquid outside the tubing, while starch is absent. Amylase hydrolyses the starch into glucose , which is small enough to diffuse out, whereas the larger starch molecules cannot pass through the pores.

Visking tubing is a partially permeable membrane made from .

Visking tubing is a partially permeable membrane made from cellulose .

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Digestion & Absorption (AQA A Level Biology): Flashcards

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Define the term digestion

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Cards in this collection (57)

Define the term digestion
Digestion is the process by which large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.
Which type of chemical reaction breaks down large biological molecules during digestion?
Hydrolysis reactions break down large biological molecules during digestion, using water to break the chemical bonds.
What are the products when proteins, carbohydrates and lipids are hydrolysed during digestion?
Proteins are hydrolysed into amino acids.
Carbohydrates are hydrolysed into simple sugars.
Lipids are hydrolysed into glycerol and fatty acids.
During digestion, proteins are hydrolysed into .
During digestion, proteins are hydrolysed into amino acids.
Lipids are hydrolysed into glycerol and during digestion.
Lipids are hydrolysed into glycerol and fatty acids during digestion.
Why must large biological molecules be hydrolysed before they can be absorbed?
Large biological molecules are too big to be absorbed across cell membranes.
Hydrolysis produces smaller, soluble molecules that can cross the cell membranes lining the ileum.
How do cells use the small molecules produced by digestion?
To release energy via respiration.
To build new molecules for cell growth, repair and function.
True or False?
During digestion, small biological molecules are hydrolysed into larger molecules.
False.
During digestion, large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.
Where does the digestion of starch begin, and which enzyme is responsible?
Starch digestion begins in the mouth.
The enzyme amylase, secreted in saliva by the salivary glands, begins to digest starch into maltose.
What are the roles of hydrochloric acid in the stomach?
It provides a suitable pH for protease enzymes to work.
It destroys any pathogens present in food.
In which part of the small intestine are the products of digestion absorbed, and how?
Absorption occurs in the ileum.
Food and water are absorbed into the blood via villi in the lining of the ileum.
What are the three main types of digestive enzyme?
Carbohydrases
Lipases
Proteases
What is meant by describing digestive enzymes as extracellular enzymes?
Extracellular enzymes are enzymes that function outside the body cells that produce them.
What reaction does amylase catalyse during carbohydrate digestion?
Amylase hydrolyses starch into the disaccharide maltose.
Where in the body is amylase produced?
The salivary glands, the pancreas and the small intestine.
What is the role of maltase in carbohydrate digestion?
Maltase hydrolyses the disaccharide maltose into the monosaccharide glucose.
What is a membrane-bound disaccharidase?
A disaccharidase that is attached to the cell-surface membranes of the epithelial cells lining the small intestine.
It hydrolyses disaccharides into monosaccharides.
Amylase hydrolyses starch into maltose, which is then hydrolysed into glucose by .
Amylase hydrolyses starch into maltose, which is then hydrolysed into glucose by maltase.
How do bile salts aid lipid digestion through emulsification?
Bile salts bind to large lipid droplets and break them into smaller droplets.
This gives the lipids a larger surface area on which lipase enzymes can act.
What are the products when lipase hydrolyses lipids?
Glycerol, monoglycerides and fatty acids.
True or False?
Bile chemically digests lipids by hydrolysing them into fatty acids.
False.
Bile is not an enzyme and does not carry out chemical digestion.
Bile salts emulsify lipids into smaller droplets, increasing the surface area for lipase to act on.
What is the difference between the action of endopeptidases and exopeptidases?
Endopeptidases hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains.
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids.
What are dipeptidases, and where are membrane-bound dipeptidases found?
Dipeptidases are a type of exopeptidase that hydrolyse dipeptides into individual amino acids.
Membrane-bound dipeptidases are attached to the cell-surface membrane of the epithelial cells lining the small intestine.
Which two factors can be investigated for their effect on the rate of digestive enzyme activity?
pH
The presence of bile salts
Why can the iodine test be used to follow the progress of the reaction catalysed by amylase?
Amylase digests starch into maltose.
Iodine turns blue-black in the presence of starch, so the loss of this colour shows that starch is being broken down.
What do a strong positive, a weak positive and a negative iodine result tell you about amylase activity?
Strong positive (blue-black): starch is present at high concentrations, so amylase activity is absent or very low.
Weak positive: starch is present at a low concentration, so amylase activity is high.
Negative (orange-brown): no starch remains, so amylase has broken all the starch down into maltose.
In the amylase investigation, the iodine solution remains once all the starch has been digested.
In the amylase investigation, the iodine solution remains orange-brown once all the starch has been digested.
How is a specific pH maintained in each test tube when investigating amylase activity?
A buffer solution at the required pH is added to the amylase before the starch is introduced.
What is a limitation of using the iodine test to measure amylase activity, and how can it be improved?
Judging the exact point at which the iodine no longer changes colour is subjective.
A colorimeter can be used to measure the progress of the reaction in a more objective way.
Which indicator is used to follow lipase activity, and what colour changes does it show?
Phenolphthalein.
It is pink in alkaline conditions and colourless in acidic conditions.
Why does the solution change from pink to white during the lipase and milk investigation?
Lipase hydrolyses the lipids in milk into fatty acids and glycerol.
The fatty acids lower the pH until the solution becomes slightly acidic.
The phenolphthalein turns colourless, so the white colour of the milk becomes visible.
Bile salts increase the rate of lipase activity because they cause of lipids.
Bile salts increase the rate of lipase activity because they cause emulsification of lipids.
Give three control variables that should be kept constant when investigating the effect of pH on amylase activity
Equal volume and concentration of enzyme (amylase) solution.
Equal volume and concentration of substrate (starch) solution.
Equal volume of buffer solution.
True or False?
When describing a control variable, it is acceptable to state that the "amount" of enzyme solution is kept the same.
False.
You should refer to both the volume and concentration of a solution, and avoid using the word 'amount'.
By which mechanisms are amino acids, monosaccharides and lipids absorbed in the ileum?
Amino acids and monosaccharides are absorbed by co-transport.
Lipids are absorbed with the help of micelles.
What is the role of the sodium-potassium pump in the co-transport of glucose and amino acids?
It actively transports sodium ions out of the epithelial cell into the blood.
This lowers the sodium ion concentration inside the cell, maintaining a sodium ion concentration gradient between the intestine and the epithelial cell.
Describe how amino acids are absorbed into the blood by co-transport
Sodium ions are actively transported from the epithelial cell into the blood by the sodium-potassium pump, maintaining a sodium ion gradient.
Sodium ions then move down their concentration gradient from the intestine into the epithelial cell via a co-transporter protein, carrying an amino acid with them.
The amino acid concentration in the epithelial cell rises, so amino acids diffuse down their concentration gradient into the blood.
Why is the co-transport of glucose and amino acids considered to be active transport overall?
Although the co-transporter protein itself works passively, energy (from ATP) is needed to power the sodium-potassium pump that creates the sodium ion gradient.
Glucose is absorbed into epithelial cells by co-transport alongside ions.
Glucose is absorbed into epithelial cells by co-transport alongside sodium ions.
What is a micelle?
A tiny, spherical structure formed when monoglycerides and fatty acids associate with bile salts.
Micelles transport these insoluble products of lipid digestion to the cell-surface membranes of the epithelial cells.
How do the products of lipid digestion move from micelles into the epithelial cells?
Micelles constantly break up and reform, releasing fatty acids and monoglycerides at the epithelial cell surface.
Because these molecules are non-polar (lipid-soluble), they diffuse through the phospholipid bilayer of the cell membrane.
What happens to fatty acids once they are inside the epithelial cells?
Short fatty acid chains move directly into the blood by diffusion.
Longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum.
The triglycerides are packaged into chylomicrons, which eventually enter the bloodstream.
Monoglycerides and fatty acids associate with bile salts to form .
Monoglycerides and fatty acids associate with bile salts to form micelles.
True or False?
The products of lipid digestion are absorbed into epithelial cells by co-transport with sodium ions.
False.
Fatty acids and monoglycerides are non-polar, so they diffuse through the phospholipid bilayer after being carried to the membrane by micelles.
Co-transport with sodium ions is used to absorb amino acids and monosaccharides.
What feature of the ileum epithelial cells increases the surface area for absorption?
Microvilli, which are tiny folds of the cell-surface membrane of the epithelial cells.
They increase the surface area for the absorption of the products of digestion.
What is Visking (dialysis) tubing?
A non-living, partially permeable membrane made from cellulose.
It can be used to model the process of absorption that occurs in the small intestine.
Why can glucose pass through Visking tubing while starch cannot?
The pores in the tubing are small enough to prevent large molecules such as starch from passing through.
Smaller molecules such as glucose are able to pass through the pores by diffusion.
Which process in the body does Visking tubing model?
The absorption of the products of digestion across the lining of the small intestine (ileum).
Outline the method used to model digestion and absorption with Visking tubing
Cut a section of Visking tubing and tie one end.
Fill the tubing with a starch and amylase mixture.
Suspend the tubing in a beaker of water for a set period of time.
Take samples of the liquid outside the tubing at regular intervals and test for starch and glucose.
What results are expected from the Visking tubing absorption model, and why?
Glucose is present in the liquid outside the tubing, while starch is absent.
Amylase hydrolyses the starch into glucose, which is small enough to diffuse out, whereas the larger starch molecules cannot pass through the pores.
Visking tubing is a partially permeable membrane made from .
Visking tubing is a partially permeable membrane made from cellulose.
In the Visking tubing model, glucose leaves the tubing by the process of .
In the Visking tubing model, glucose leaves the tubing by the process of diffusion.
How can the rate of absorption be investigated quantitatively using the Visking tubing model?
Take a series of samples of the surrounding liquid over a period of time.
Measure the concentration of glucose in each sample using colorimetry.
How could the Visking tubing model be adapted to investigate the effect of temperature or pH on digestion?
Temperature: set up multiple tubes in water baths kept at different temperatures.
pH: set up multiple tubes containing starch and amylase kept at different pH levels using buffer solutions.
Give three limitations of using Visking tubing as a model of the ileum
Visking tubing has no biological membranes or membrane features such as channel proteins.
Active transport cannot occur, as there are no carrier proteins or energy from respiration.
The surface area of the tubing is smaller than the intestinal epithelium, which has villi.
True or False?
Active transport of glucose can occur across Visking tubing, just as it does across the ileum epithelium.
False.
Active transport cannot occur across Visking tubing because it lacks carrier proteins and there is no energy from respiration.
Movement across the tubing occurs only by diffusion.
Why is the distilled water surrounding the Visking tubing a poor model of the blood in the ileum?
The distilled water does not flow like blood.
This means it does not continually remove absorbed molecules, so the concentration gradient is not maintained.

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