6.1 Structure of the digestive system Flashcards
1
Q
Diagram of the digestive process
A
2
Q
Give an overview of the digestive system
A
-The digestive system can be compared to a complex tube from your mouth to your anus.
-It has several organs attached to it by small tubes secreting enzymes and hormones into the food mixture.
-The organs forming the tube are often called the organs of the digestive tract, or alimentary canal.
-The organs that secrete chemicals into the tube are often called accessory organs.
3
Q
Labeled diagram of the human digestive system
A
4
Q
What happens when food enters the mouth and what is the function of the saliva?
A
-In the mouth, you chew your food, and it is mechanically mixed with saliva.
-The saliva contains enzymes (amylase) that start the digestion of starches.
-The saliva also moistens the mixture to help you to swallow.
5
Q
What happens after you chew the food?
A
Now the food mixture (called bolus) enters the oesophagus (also called the gullet), and moves by peristalsis to the stomach.
6
Q
Explain peristalsis from the esophagus to the stomach
A
-The longitudinal and circular muscles work antagonistically, that is they work opposite to each other: when one contracts, the other relaxes and vice versa.
-The longitudinal muscle contracts to widen the lumen of the alimentary canal while the circular muscle contracts to constrict the lumen.
7
Q
Define peristalsis
A
A wave of contraction and relaxation of the longitudinal and circular muscles of the alimentary canal, by which the contents are forced along the tube.
8
Q
Diagram of perstalsis movement in the esophagus and small intestine
A
9
Q
What happens to the bolus in the stomach?
A
-In the stomach, the food is churned (mixed and kneaded) with acid.
-At this point, the food and acid mixture is called chyme.
-The stomach acid kills bacteria and starts the digestion of proteins.
10
Q
Where does the food enter after it has passed through the stomach?
A
The small intestine
11
Q
What is the first section of the small intestine called?
A
The duodenum
12
Q
What happens to the food in the small intestine?
A
-The acid passing from the stomach needs to be neutralised so that enzymes in the small intestine can function properly.
-These enzymes have an optimum pH of neutral to alkaline, and some of them digest fats and lipids: others further digest carbohydrates; and others further digest proteins.
13
Q
What has happened by the time the food reaches the end of the small intestine?
A
-Digestion has been completed and the digested products have been absorbed from the last section, the ileum.
-The small intestine is where most of the absorption takes place.
14
Q
The part of the digestive system where most of the absorption takes place is the ___
A
Small intestine
15
Q
What happens to the food once it has passed through the small intestine?
A
Next, the food – now a liquid mixture of undigested material and digestive juices – enters the large intestine.
16
Q
What happens to the food in the large intestine?
A
-Here water, along with vitamins (mainly vitamin K and B) made by bacteria that normally live in the large intestine, are reabsorbed.
-The anaerobic bacteria in the large intestine can also ferment undigested polysaccharides to produce energy.
-In the last parts of the large intestine, faeces is formed and stored in the rectum prior to egestion.
17
Q
What are the other organs that aid the digestive process called?
A
Accessory organs
18
Q
What are the accessory organs in the digestive system?
A
-The pancreas
-The liver
-The gall bladder
19
Q
What is the function of the pancreas?
A
It secretes amylase, lipase and protease (that aid in the digestion of starch, lipids and proteins, respectively) into the small intestine.
20
Q
What is the function of the liver?
A
It secretes bile to emulsify lipids.
21
Q
What is the function of the gall bladder?
A
It stores bile.
22
Q
What is bile and what is its function?
A
-Bile is not an enzyme.
-Bile breaks large lipid globules into smaller lipid globules (this is what emulsify means).
-Bile does not facilitate hydrolysis reactions as enzymes do.
23
Q
What is ingestion?
A
Food is taken in through the mouth as large particles
24
Q
What is digestion?
A
Food is broken down both physically (e.g. mastication) and chemically (e.g. enzymatic hydrolysis)
25
What is absorption?
Digested food products are absorbed into the bloodstream and transported to cells
26
What is assimilation?
Digested food products are converted into the fluid and solid parts of a cell/tissue
27
What is egestion/elimination?
Undigested food residues are egested from the body as semi-solid feces
28
Describe the structure of the small intestine
-It is a muscular tube of about 6-7 m in humans that lies betewen the stomach and the large intestine.
-It is made up of three parts: the duodenum, jejunum, an ileum.
-The inner surface of the small intestine is covered with specialized structures called villi, whihc increase its surface area. This increase in SA is essential for aborption.
29
What are the tissue layers in the small intestine? (outside to inside)
-Serosa
-Longitudinal muscles
-Circular muscles
-Submucosa
-Mucosa
30
What is the serosa?
The outermost layer consisting of connective tissue that is in contact with body cavities.
31
What are the longitudinal muscles?
Muscles responsible for peristalsis.
32
What are the circular muscles?
Muscles responsible for peristalsis.
33
What is the submucosa?
Connective tissue that supports the mucosa and that contains large veins and arteries which give rise to the capillary bed of the mucosa.
34
What is the mucosa?
The innermost layer, forming the soft lining of the tube comprised of the epithelium (which lines the lumen of the digestive tract), connective tissue, and smooth muscle (villi form part of this layer).
35
Diagram of the transverse section of the small intestine with villi lining it
36
The wall of the small intestine under a light microscope
37
What sequence of organs do substances pass through as they move through the human digestive system?
Mouth → oesophagus → stomach → small intestine → large intestine → anus
38
The following is a diagram of the human digestive system.
What is the name of structure II?
Large intestine (colon)
39
What is dialysis tubing also known as?
Visking tubing
40
What is dialysis tubing?
-Partially permeable cellulose tubing that contains microscopic pores.
-It allows water, small molecules and ions to pass through freely, but does not allow the movement of large molecules.
41
What is dialysis tubing/visking tubing used in?
In separation techniques – dialysis – that enable the removal of small molecules from macromolecules in solution based on differential diffusion.
42
Define dialysis
The separation of smaller molecules from larger molecules in solution by selective diffusion through a partially permeable (also known as selectively permeable or semipermeable) membrane.
43
Explain how dialysis tubing can be used as a model of the small intestine.
-The medium outside (water, in this case) represents the blood into which digested products are absorbed.
-The tubing represents the epithelium of the small intestine.
-It is worth noting that the high concentration of the glucose solution inside the tubing is what is normally observed after a starchy meal (such as spaghetti) has been fully digested.
-Because the size of glucose molecule is small enough to pass through the pores of the tubing, it will diffuse from a region of higher concentration (in the tubing) to a region of lower concentration (in the beaker).
44
Diagram of the use of dialysis tubing to model absorption
45
Describe the possible results of a visking tubing experiment
-The movement of glucose mimics the absorption of glucose via the epithelial cells, in this case, represented by the dialysis tubing, into the blood supply.
-Thus, if the water in the beaker was tested for glucose, the result would be positive.
-If a starch solution was added inside the tubing through the capillary tube, and samples of water in the beaker were tested for the presence of starch at intervals of 10 minutes, a negative result would always be found.
-The reason for this observation is that starch molecules are too big to pass through the pores of the dialysis tubing.
-The same thing would occur inside the small intestine: starch and other complex undigested molecules are not absorbed.
46
What is used to test for the presence of starch and what happens if starch is present?
-Iodine solutions are used to test for the presence of starch: when brownish-orange, no starch is present.
-When the solution turns blue-black, starch is present.
47
What is the main shortcoming of the Visking tubing model?
It can only account for absorption by diffusion or osmosis, and cannot be used to model absorption by active transport.
48
An experiment was set up as shown below.
After 1 hour the solution inside the dialysis bag was dark blue. A possible explanation for this color change is ___
The potassium triiodide molecule present in the iodine solution is small enough to pass through the pores of the dialysis tubing.
49
An experiment was set up as shown below.
After 1 hour the solution in the beaker was tested for the presence of glucose. Which of the following would clearly explain the result obtained?
Glucose would be present because it would have diffused from the dialysis bag into the iodine solution due to its small size.
50
An experiment was set up as shown below.
If you test for glucose, where would you expect those tests to be positive?
In the dialysis bag and beaker in diagram B
51
What are the two stages of digestion?
-First, the large food molecules are broken down into smaller units.
-Second, these smaller units are broken down into monomers that can be easily absorbed.
52
Where are the enzymes needed for digestion produced and where are they secreted to?
The pancreas and the wall of the intestine produce the necessary enzymes for this process and secrete them into the lumen (space in the centre) of the small intestine.
53
Diagram of the pancreas (production and secretion of digestive enzymes)
54
What are the digestive enzymes produced by the acinar cells of the pancreas?
-Amylase
-Endopeptidase (called trypsin)
-Lipases and phospholipases
55
What is the function of amylase?
Breaks down starch into maltose
56
What is the function of endopeptidase (called trypsin)
Breaks down proteins into smaller polypeptides.
57
What is the function of lipases and phospholipase?
-Break down lipids and phospholipids, respectively, to glycerol and fatty acids.
-In the case of the phospholipid, phosphate is also produced.
58
What happens to the pancreatic juice (containing the enzymes) once it is produced?
It is carried via the pancreatic duct to the duodenum, where it is released into the lumen of the small intestine.
59
Pancreatic enzymes have an optimum pH of ___
Neutral to alkaline
60
Why are more enzymes needed to complete digestion?
Because despite the involvement of pancreatic enzymes in the digestion process, most of the molecules are still too large to be taken up by villi.
61
The wall of the small intestine contains some glands that produce ___
Enzymes that are immobilised in the intestinal epithelial cells.
62
What enzymes complete digestion?
-Nucleases
-Maltase
-Lactase
-Exopeptidases
-Dipeptidases
63
What is the function of nucleases?
Break down DNA and RNA.
64
What is the function of maltase?
Breaks down maltose into alpha glucose.
65
What is the function of lactase?
Breaks down lactose into beta galactose and alpha glucose.
66
What is the function of exopeptidases?
They remove a single amino acid from the end of the small polypeptides.
67
What is the function of dipeptidases
Break down a dipeptide into two amino acids.
68
Where do enzymes break down molecules that are too large to be taken up by villi?
In the small intestine
69
What is the function of peristalsis other than helping food pass through the gut?
As well as helping the food to move along the gut, the contraction of circular and longitudinal muscles of the small intestine also mixes the food with enzymes.
70
What is the overall function of enzymes in digestion?
Enzymes (from pancreas and small intestine) digest most macromolecules in food into monomers in the small intestine. These monomers are then absorbed into the blood or lymph.
71
What can be digested in the human body and what remains undigested?
-The great variety of enzymes secreted by the human body allows the digestion of starch, glycogen, lipids and nucleic acids into their respective monomers.
-However, cellulose remains undigested, as the enzyme cellulase (which breaks down cellulose) is not produced by humans.
72
Where does the actual absorption of food molecules mainly take place?
In the small intestine, across the epithelium.
73
Why is a large surface area necessary in the small intestine?
As transport proteins are often necessary for this absorption, a large surface area is necessary to ensure enough transport proteins to absorb all the necessary nutrients.
74
How is the surface area of the small intestine increased?
By the presence of villi (singular: villus)
75
The villi are the location of ___
Absorption of all the monomers produced by the digestive processes in the small intestine.
76
Diagram and micrograph of a villus
77
What is the function of the epithelial cells of the villus?
This is where absorption takes place.
78
What is the function of goblet cells on the villus?
They produce mucus.
79
What is the function of the capillary network and the lacteal in the villus?
They carry absorbed nutrients away from the intestine.
80
What is the lamina propria?
Connective tissue of the villus
81
Define absorption
The taking in of digested food substances as well as minerals and vitamins from the lumen of the small intestine into the blood.
82
Describe the structure of an epithelial cells of a villus
-Each epithelial cell of the villus has projections of its plasma membrane that protrude into the lumen of the small intestine.
-These projections are called microvilli and they further increase the surface area of the small intestine.
83
Which molecules, which are the end products of digestion, are directly absorbed by the villi?
-Bases and phosphates from nucleic acids
-Fatty acids and glycerol
-Amino acids
-Monomeric carbohydrates, such as fructose, glucose, galactose and ribose.
84
What happens to the vitamins and minerals in food during digestion?
-These can be absorbed without further digestion.
-Once these end products of digestion are distributed around the body through the circulatory system they can be used by cells for anabolic processes or for respiration.
-This use of molecules resulting from digestion is called assimilation.
85
What happens to the contaminants or poisons in food during digestion?
-Some food may also contain some contaminants or poisons – including alcohol, if you drink wine, beer or spirits (containing ethanol).
-Most of these contaminants can pass directly into the blood.
-The liver is able to detoxify some of the compounds.
-However, if they cannot be broken down by the liver, they can be secreted from the body in the urine.
86
How are most medicinal drugs taken up?
Directly into the blood and are broken down by the liver
87
What are microvilli?
-Hairlike folds in the membrane of the epithelial cells of the villus.
-This is where the absorption takes place by means of (facilitated) diffusion, passive and active transport.
88
The food molecules, minerals and vitamins are absorbed into the ___
Blood or the lymph
89
What needs to happen to food molecules to be absorbed into the blood?
The molecules need to pass into the capillaries of the villus.
90
How are fats absorbed?
Fats are absorbed into the lymph, which circulates in the lacteal in the centre of the villus.
91
What are the two steps of the absorption of food molecules into the bloodstream?
1. Substances to be absorbed move from the lumen into the epithelial villi.
2. Amino acids and monosaccharides move from the villi into the capillaries and monoglycerides move into the lacteals.
92
Diagram of two-step absorption from the lumen of the small intestine into capillaries and lacteals of a villus
.
93
What modes of absorption are used to execute the two steps of absorption of food molecules?
-Simple diffusion
-Facilitated diffusion
-Active transport
-Pinocytosis
94
Explain the role of simple diffusion in absorption
-Occurs when molecules are small and are hydrophobic (so they can pass through the phospholipid bilayers).
-This occurs mostly with the products of lipid digestion.
95
Explain the role of facilitated diffusion in absorption
-Fructose, glucose and other hydrophilic monomers are moved by protein channels.
-Be aware, this still requires a concentration gradient.
96
Explain the role of active transport in absorption
-It is needed when the concentrations are lower in the lumen of the small intestine.
-Thus, the movement needs to occur against a concentration gradient.
-Glucose, amino acids, and some mineral ions are transported out of the lumen in this way, which requires ATP.
-The cells of the epithelium have many mitochondria that can synthesize ATP for this process.
97
Explain the role of pinocytosis in absorption
-This draws in small droplets of liquid surrounded by a small section of the phospholipid membrane.
-This is most likely to occur with fat droplets in the lumen of the small intestine.
98
Diagram of pinocytosis
99
Why does the absorption of fats differs from that of carbohydrates?
Most absorbed fat first enters the lymphatic system, whereas carbohydrates directly enter the blood.
100
Absorption of glucose in the small intestine occurs by ___
Facilitated diffusion and active transport.
101
Diagram of amylopectin and amylose
102
When does digestion of starch begin?
The moment you start chewing your food (salivary amylase is an enzyme present in saliva).
103
What happens to starch once the saliva and food have been mixed?
Amylase starts breaking down the α-1,4 glycosidic bonds that connect the glucose monomers in amylose and amylopectin.
104
What do the numbers 1 and 4 (in α-1,4 glycosidic bonds) refer to?
Specific carbon atoms within the two glucose molecules that are joined by the bond.
105
What are the end products of the digestion of starch in the mouth?
The end products are maltose, a dimer of glucose connected by α-1,4 bonds, and maltotriose, which is comprised of three glucose molecules also connected by α-1,4 bonds.
106
What type of bond does amylopectin posess and how does this affect its ability to be broken down by amylase?
Amylopectin also possesses a slightly different type of bond, called α-1,6 glycosidic bonds, however, these cannot be broken down by amylase.
107
What is the branching of polymers caused by?
It is caused by α-1,6 glycosidic bonds, as seen in an amylopectin molecule.
108
Diagram of the molecular structure of amylopectin
109
Diagram of the molecular structure of amylose
110
What needs to happen the di- and tri- saccharides produced from the starch molecules after the initial catalytic breakdown and why?
Even after the initial catalytic breakdown by amylase, the di- and tri-saccharides produced from the starch molecules are too large to pass through membranes, so they need to be broken down into monomers (monosaccharides) before they can be absorbed.
111
What forms of carbohydrates enter the small intestine and what happens to them?
-What enters the small intestine is a mixture of maltose, maltotriose, and dextrins.
-Dextrins are very small polymers still containing the α-1,6 glycosidic bond.
-Three enzymes that are immobilized in the epithelial cells of the small intestine, maltase, glucosidase, and dextrinase, break down these molecules into glucose, which can then be absorbed by the villi.
112
What happens to all absorbed monomers from food?
They are transported via the hepatic portal vein from the small intestine to the liver, from there it enters the general circulation.
113
Diagram showing nutrients absorbed from the small intestine travel through the hepatic portal vein to the liver.
There, the liver monitors the contents of the blood.
Nutrients are then delivered to the rest of the body through the rest of the circulatory system.
114
How is glucose that has been absorbed in the small intestine used by the body?
-Transport in the circulatory system makes the glucose available for use by all body cells.
-Brain cells in particular use a lot of glucose, and they need to be constantly supplied with glucose in order to maintain bodily functions.
-Any excess glucose is taken up by the liver and converted into glycogen, the animal equivalent of starch.
115
Describe the structure of glycogen
It resembles amylopectin but has more α-1,6 glycosidic bonds.
116
Diagram showing what happens to glucose once it is in the bloodstream
117
After ingestion by humans, the first category of macromolecules to be chemically digested by enzymes in the mouth is ___
Carbohydrates
Amylase starts the digestion of starches (carbohydrates) while the food is still being chewed.
118
What does salivary amylase digest?
Starches
Amylase starts the digestion of starches while the food is still being chewed.
119
Features/adaptations of villi (MR SLIM)
- Microvilli – Ruffling of epithelial membrane further increases surface area
- Rich blood supply – Dense capillary network rapidly transports absorbed products
- Single layer epithelium – Minimises diffusion distance between lumen and blood
- Lacteals – Absorbs lipids from the intestine into the lymphatic system
- Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
- Membrane proteins – Facilitates transport of digested materials into epithelial cells
