Y12 MW - Enzymes, Digestion, Absorption and Proteins (Complete) Flashcards
What is digestion?
There process where large, insoluble biological molecules are hydrolysed into smaller, soluble molecules (which can be absorbed across cell membranes and into the bloodstream)
What are enzymes?
Biological catalysts
What is the function of the mouth in digestion?
Mechanical break down of food into smaller pieces, increasing volume to surface area ratio
Salivary glands secrete salivary amylase so the digestion of starch begins (hydrolysed into maltose)
What is the function of the oesophagus in digestion?
A hollow tube through which food passes from the mouth to the stomach
Has muscular walls which contract to help the movement of food
What is the function of the stomach in digestion?
Glands in the stomach produce enzymes, stomach acid and mucus (which forms a protective layer on the stomach lining due to the acidic conditions)
Protease enzymes are produced which begin to break down proteins
What is the function of the pancreas in digestion?
The pancreas is a large gland situated below the stomach which secretes pancreatic juice
Pancreatic juice contains the enzymes protease, lipase and amylase
What is the function of the liver in digestion?
The liver produces bile which is stored in the gall bladder and then released into the small intestine where it emulsifies fats
What is the function of the small intestine in digestion?
It is a long muscular tube
Enzymes are produced by the walls and glands (amylase, proteases and lipase are secreted as well as maltase and dipeptidases membrane-bound to the epithelium of the small intestine)
The inner walls are folded into villi and there are microvilli on the epithelial cells of each villus which increase the surface area for absorption
What is the function of the large intestine in digestion?
The water is absorbed, meaning that the food inside becomes drier and thicker and becomes faeces
Give an example of physical digestion
Teeth mechanically break down food into smaller pieces
Muscles in the stomach churn the food into smaller pieces
This helps create as larger surface area for chemical digestion
Give an example of chemical digestion
Digestive enzymes hydrolyse large, insoluble molecules into smaller, soluble molecules by hydrolysis reactions (breaking the chemical bonds between molecules with a molecule of water)
Enzymes are specific
Usually, more than one enzyme is required to hydrolyse a large molecule
Describe the hydrolysis of starch by digestive enzymes
Starch is a polysaccharide which is hydrolysed into the disaccharide maltose by the enzyme amylase
The disaccharide maltose is then hydrolysed into the monosaccharide alpha glucose by the enzyme maltase which is membrane-bound to the epithelium of the ileum
Disaccharidases are ———— ——
Membrane bound enzymes
Where in the digestive system is starch hydrolysed by enzymes?
In the mouth the salivary glands produce salivary amylase which begins to hydrolyse starch into maltose
In the oesophagus, as food is moved down from the mouth to the stomach salivary amylase continues to hydrolyse starch into maltose
The salivary amylase enzymes are then denatured by the hydrochloric acid in the stomach so hydrolysis does not continue
In the small intestine, maltose is then hydrolysed into alpha glucose by the enzyme maltase which is membrane-bound to the epithelium of the ileum
What does it mean that enzymes are specific?
Each enzyme catalyses a different reaction
What is the primary structure of a protein?
A specific sequence of amino acids joined by peptide bonds
What is the secondary structure of a protein?
The specific folding of the primary structure held in place by hydrogen bonds (e.g beta pleated sheets and the alpha helix)
What is the tertiary structure of a protein?
The specific folding of secondary structure held in place by hydrogen and ionic bonds and disulphide bridges
What is the idea used to describe enzyme action?
The induced fit hypothesis
What is the induced fit hypothesis?
The substrate and the active site of the enzyme are not initially complementary
When the substrate binds with the active site, a change in the tertiary structure occurs
Therefore, the active site changes shape to become complementary to the substrate
How do enzymes catalyse a hydrolysis reaction?
The enzyme puts stress on the bonds in the substrate
This reduces the amount of energy required for the reaction to occur (reducing the activation energy)
Describe the hydrolysis of protein by digestive enzymes
Proteins are hydrolysed into shorter chain polypeptides by the enzyme endopeptidase as the bonds in the middle of the protein are hydrolysed (creating more ‘ends’ for the next enzyme to work on) in the stomach
The polypeptides are then hydrolysed into dipeptides by the enzyme exopeptidase as every other peptide bond in the polypeptide is hydrolysed in the small intestine
The dipeptides are then hydrolysed into amino acids by the enzyme dipeptidase which is membrane-bound to the epithelium of the ileum
What is the optimum condition for the action of endopeptidase?
Acidic conditions with a low pH (hydrologic acid in the stomach means the pH is 1 or 2)
What is the optimum condition for the action of exopeptidase?
A neutral pH is optimum (in the small intestine pancreatic juice is alkaline so neutralises the acid from the stomach)
Where in the digestive system is protein hydrolysed by enzymes?
The endopeptidase enzymes require an acidic environment for optimum action so they hydrolyse proteins into polypeptides in the stomach (due to the hydrochloric acid which causes a low pH)
The exopeptidase enzymes hydrolyse polypeptides into dipeptides in the duodenum of the small intestine, hydrolysing every other peptide bond of the polypeptide
Dipeptidases are enzymes which are membrane-bound to the epithelium of the ileum and hydrolyse dipeptides into amino acids
Describe the hydrolysis of fats by digestive enzymes
Before the enzyme digestion of fats can take place, they must be emulsified. Bile is produced by the liver, stored in the gall bladder and then released into the duodenum. In the duodenum, the bile salts emulsify the large fat globules into smaller fats which have a larger surface area for enzymes to act on
Fats then are hydrolysed into glycerol and fatty acids in the small intestine by lipids (secreted from the pancreas) which hydrolyse the ester bonds between the three fatty acids and one glycerol molecule
What is the structure of an amino acid?
What are the different groups in the structure of an amino acid?
The carboxylic acid group
The amino group
(The variable side chain)
What does a condensation reaction between two amino acids produce?
A dipeptide with a peptide bond and a molecule of water
How is a polypeptide formed?
Hundreds of amino acids joined together with peptide bonds by a condensation reaction
This sequence of amino acids is the primary structure of a protein
What are the factors affecting the rate of reaction with enzymes?
Temperature
pH
Enzyme concentration
Substrate concentration
What does it mean that enzymes have an optimum?
These are the conditions where enzymes work best at a certain temperature and pH
What happens to enzymes at temperatures which are too high?
The weak hydrogen bonds within the tertiary structure of an enzyme are broken due to the atoms vibrating much quicker. Therefore, the tertiary structure of the enzyme changes irreversibly, altering the active site so the enzyme has become denatured. This means that the substrate can no longer bind the the active site and the enzyme-substrate complex cannot be formed
What happens to enzymes where the pH is too low / high?
If the pH is too low, there are many more H+ ions which change the charges of the amino acids
If the pH is too high, there are many more OH- ions which change the charges of the amino acids
Therefore, the tertiary structure of the enzyme is irreversibly changed and the enzyme is
denatured. The substrate can now no longer bind with the active site of the enzyme as it has been altered and the enzyme substrate complex cannot form
If there is a high substrate concentration what does this mean?
The rate of reaction with the enzymes would be higher as there are more substrate molecules to collide and bind with the active site of the enzymes, forming a enzyme substrate complex so the enzyme can catalyse the reaction
What are the different types of enzyme inhibitors?
Competitive inhibitors
Non-competitive inhibitors
How do competitive inhibitors slow the rate of reaction?
The competitive inhibitors has a similar shape to the substrate so they can both bind to the active site of the enzyme
The inhibitor competes with the substrate to bind with the active site and when it does it slows the rate of the reaction as the enzyme is now inactive, being unable to form an enzyme substrate complex with the substrate
How do non-competitive inhibitors slow the rate of reaction?
The non-competitive inhibitors are not complementary to the active site of the enzyme and instead can bind to the allosteric site of the enzyme
When the enzyme inhibitor complex is formed, the tertiary structure of the enzyme changes and so the shape of the active site is altered
The substrate can now no longer bind with the active site and form an enzyme substrate complex so the rate of reaction decreases
Do non-competitive inhibitors denature the enzyme?
No, once the inhibitor is removed from the allosteric site, the tertiary structure of the enzyme and shape of its active site will return back
With the use of a competitive inhibitor, will the maximum rate of reaction change?
No
With the use of a non-competitive inhibitor, will the maximum rate of reaction change?
Yes
Where does absorption take place?
The small intestine
How are the products of lipid digestion absorbed in the small intestine?
The fatty acids and monoglycerides are non-polar so they can move by diffusion across the phospholipid bilayer into the epithelium cells. They then are reformed into tricylcerides in the smooth endoplasmic reticulum. After being transferred to the Golgi apparatus, the triglycerides are then combined with a lipoprotein and cholesterol to form chylomicrons. By exocytosis the chylomicrons move out of the epithelial cells and into the lacteals in the centre of the villus
How are the products of starch digestion absorbed in the small intestine?
The glucose moves into the epithelium cells through co-transport with a sodium ion with the use of a specific carrier protein as the sodium ions move down a concentration gradient. The sodium ions then are actively transported out of the epithelium cell into the bloodstream and potassium ions move in the opposite direction, maintaining a low concentration of sodium ions in the epithelial cell. The glucose can then move by facilitated diffusion into the blood with the use of a protein
How are the products of protein digestion absorbed in the small intestine?
The amino acid move into the epithelium cells through co-transport with a sodium ion with the use of a specific carrier protein as the sodium ions move down a concentration gradient. The sodium ions then are actively transported out of the epithelium cell into the bloodstream and potassium ions move in the opposite direction, maintaining a low concentration of sodium ions in the epithelial cell. The amino acids can then move by facilitated diffusion into the blood with the use of a protein
