Exchange between Organisms - Digestion and Absorption Flashcards
1
Q
What is the human digestive system made up of?
A
The human digestive system is made up of a long muscular tube and its associated glands. The glands produce enzymes that hydrolyse large molecules into small ones ready for absorption across cell membranes. The digestive system is therefore an exchange surface through which food substances are absorbed.
2
Q
What is the oesophagus?
A
The oesophagus is a muscular tube that carries food from the mouth to the stomach using peristalsis. No digestion takes place here.
3
Q
What is the stomach?
A
The stomach is a large, muscular sac that links the oesophagus and small intestine. It has an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands that secrete hydrochloric acid and produce enzymes which digest protein. Other glands in its wall produce mucus. The mucus prevents this organ being digested by its own enzymes.
4
Q
What is the ileum (small intestine)?
A
- The ileum is a long muscular tube.
- Food is further digested in the ileum by enzymes (maltase, sucrase and lactase enzymes) that are produced by its walls and by glands that pour their secretions into it.
- The inner walls of the ileum are folded into villi, which gives them a large surface area.
- The surface area of these villi is further increased by millions of tiny projections, called microvilli, on the epithelial cells of each villus.
- This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream.
5
Q
What is the colon (large intestine)?
A
The large intestine is a muscular tube that links the small intestine with the rectum and anus. It absorbs water and mineral ions. Most of the water that is absorbed is water from the secretions of the many digestive glands. The food within this part therefore becomes drier and thicker in consistency and forms faeces.
6
Q
What is the rectum?
A
The rectum is the final secretion of the intestines of undigested material. The faeces are stored here before periodically being removed via the anus in a process called egestion.
7
Q
Is digestion extracellular or intracellular?
A
Extracellular as it happens outside of cells. As it happens outside of your body, waste being removed is called egestion and no excretion.
This means that the contents of the intestines are not inside the body either. Molecules and ions only truly enter the body when they cross the cells and cell-surface membranes of the epithelial lining of the intestines.
8
Q
What are the salivary glands?
A
The salivary glands are situated near the mouth. They pass their secretions (saliva, composed of water, sodium ions, mucus and amylase) via a duct into the mouth. These secretions contain the enzyme amylase (which is activated by the sodium ions), which hydrolyses starch into maltose. Water helps to dissolve soluble substances, and mucus is for lubrication.
9
Q
What is the pancreas?
A
The pancreas is a large, glandular organ situated below the stomach. It produces a secretion called pancreatic juice, which is delivered to the small intestine (duodenum) by the pancreatic duct.
This secretion contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch. It also produces the hormones insulin and glucagon that are secreted into the blood.
10
Q
What is the difference between endopeptidase and exopeptidase?
A
Endopeptidases break polypeptides into shorter polypeptide chains. Exopeptidases break polypeptides into dipeptides at the ends of the chain.
11
Q
What is the importance of endopeptidases?
A
You need endopeptidases to shorten the polypeptides and get more ends for the exopeptidases to digest.
12
Q
What two stages does digestion take place in in humans?
A
- physical breakdown
2. chemical digestion
13
Q
What is physical breakdown in humans?
A
- If the food is large, it is broken down into smaller pieces by means of structures such as the teeth.
- This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion.
- Food is churned by the muscles in the stomach wall and this also physically breaks it up.
14
Q
What is chemical digestion in humans?
A
- Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones.
- It is carried out by enzymes.
- All digestive enzymes function by hydrolysis.
- Hydrolysis is the splitting up of molecules by adding water to the chemical bonds that hold them together.
- Enzymes are specific and so it follows that more than one enzyme is needed to hydrolyse a large molecule.
- Usually one enzyme hydrolyses a large molecule into sections and these sections are then hydrolyses into smaller molecules by one or more additional enzymes.
15
Q
What does amylase do?
A
enzyme that breaks the glycosidic (chemical) bonds in starch by hydrolysis to release maltose units
16
Q
What does carbohydrase do?
A
hydrolyses carbohydrates, ultimately into monosaccharides
17
Q
What does lipase do?
A
hydrolyses lipids (fats and oils) by breaking the ester bonds in triglycerides into glycerol and fatty acids
18
Q
What does protease do?
A
hydrolyses proteins, ultimately to amino acids, by breaking peptide bonds
19
Q
What do glandular tissues in the stomach secrete?
A
hydrochloric acid, Pepsin (a protease enzyme), and a thick layer of mucus for protection
20
Q
How many enzymes are needed to hydrolyse a large molecule?
A
It usually takes more than one enzyme to completely hydrolyse a large molecule. Typically one enzyme hydrolyses the molecule into smaller sections and then other enzymes hydrolyse these sections further into their monomers. These enzymes are usually produced in different parts of the digestive system. It is important that enzymes are added to the food in the correct sequence.
21
Q
Describe starch digestion.
A
Firstly, the enzyme amylase is produced in the mouth and the pancreas. Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose. The maltose is in turn hydrolysed into the monosaccharide alpha-glucose by a second enzyme called maltase.
22
Q
Where is maltase produced?
A
the lining of ileum
23
Q
Describe the process of starch digestion in humans.
A
- Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing.
- Saliva contains salivary amylase. This starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the pH at around neutral. This is the optimum pH for salivary amylase to work.
- The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of the starch.
- After a time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called pancreatic juice.
- The pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch to maltose. Alkaline salts are produced by both the pancreas and the intestinal wall to maintain the pH at around neutral so that the amylase can function.
- Muscles in the intestine wall push the food along the ileum. Its epithelial lining produces the disaccharidase maltase. Maltase is not released into the lumen of the ileum but is to part of the cell-surface membranes of the epithelial cells that line the ileum. It is therefore referred to as a membrane-bound disaccharidase. The maltase hydrolyses the maltose from starch breakdown into alpha-glucose.
24
Q
What other common disaccharides in the diet are hydrolysed?
A
Sucrose - found in many natural foods, especially fruits.
Lactose - found in milk, and hence in milk products, such as yoghurt and cheese.
25
What does sucrase do?
Sucrase hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolysis produces the two monosaccharides glucose and fructose.
26
What does lactase do?
Lactase hydrolyses the single glycosidic bond in the lactose molecule. This hydrolysis produces the two monosaccharides glucose and galactose.
27
Describe lipid digestion.
- Lipids are hydrolysed by enzymes called lipases.
- Lipases are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides.
- A monoglyceride is a glycerol molecule with a single fatty acid molecule attached.
- Lipids (fats and oils) are firstly split up into tiny droplets called micelles by bile salts (which stay attached), that are produced by the liver.
- This process is called emulsification and increases the surface area of the lipids so that the action of lipases is speeded up.
- The bile salts are released.
- The monoglycerides and fatty acids at the epithelial membrane are to be absorbed by simple diffusion.
- Triglycerides are reformed in the SER.
- The Golgi produces chylomicrons from triglycerides and lipoproteins.
- The chylomicrons are exported by exocytosis.
- Chylomicrons are absorbed into the lacteals in the villi or the hepatic portal vein (into the lymphatic system).
Short and medium-chain fatty acids and glycerol (small products) are absorbed into the blood via capillaries.
Long-chain fatty acids and monoglycerides (large products) form into triglycerides and are transported in chylomicrons (inside Golgi apparatus). Chylomicrons are extruded from the epithelial cell and enter a lacteal (lymph capillary). Lymph in the lacteal transports chylomicrons away from intestine.
28
What hydrolyses proteins?
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases).
29
What are endopeptidases?
Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of polypeptide molecules.
30
What are exopeptidases?
Exopeptidases 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.
31
What are dipeptidases?
Dipeptidases 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.
32
What is lactose intolerance?
Milk is the only food of human babies and so they produce a relatively large amount of lactase, the enzyme that hydrolyses lactose, the sugar in milk. As milk forms a less significant part of the diet in adults, the production of lactase diminishes and children get older. This reduction can be so great in some adults that they produce little, or no, lactase at all.
This was not a problem to our ancestors but can be to humans of today. Humans that produce no lactase cannot hydrolyse the lactose they consume. When the undigested lactose reaches the large intestines. microorganisms hydrolyse it. This gives rise to small soluble molecules and a large volume of gas. This can result in diarrhoea because the soluble molecules lower the water potential of the material in the colon. The condition is known as lactose intolerance. Some people with the condition cannot consume milk or milk products at all while other can consume them only in small amounts.
33
What is emulsification?
The process by which lipids are split up into tiny droplets called micelles by bile salts, which are produced by the liver, stored in the gall bladder and emptied into the small intestine (duodenum) by bile ducts. It increases the surface area of the lipids so that the action of lipases is sped up. Bile also neutralises gastric juices producing the optimum pH for pancreatic enzymes.
34
What are micelles?
Tiny structures (4-7nm in diameter) formed when monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets. They break down as they come into contact with the epithelial cells (when the bile salts are released) lining the villi of the ileum and release the monoglycerides and fatty acids which diffuse across the cell membrane into the epithelial cells.
35
What are chylomicrons?
A structure formed when triglycerides associate with cholesterol and lipoproteins, which are adapted for the transport of lipids. They transport dietary lipids from the intestines to other locations in the body. Start forming in the endoplasmic reticulum and continuing in the Golgi apparatus.
36
What are lacteals?
Lymphatic capillaries that are found in the centre of each villus, where chylomicrons pass into and then enter the bloodstream.
37
What is exocytosis?
The outward bulk transport of materials through the cell-surface membrane. Chylomicrons move out of the epithelial cells by this process.
38
What is the lumen?
The hollow cavity inside a tubular structure such as the gut or a xylem vessel.
39
What are villi?
Folded finger-like projections of the ileum wall, about 1mm long, which are increase the surface area of the ileum and therefore accelerate the rate of absorption.
40
What are microvilli?
Tiny finger-like projections from the cell-surface membrane of some animal cells.
41
Why can lipids diffuse easily across the cell membrane?
Monoglycerides and fatty acids are nonpolar so they can easily diffuse across the cell membrane into the epithelial cells lining the epithelium. Once inside they are transported to the endoplasmic reticulum where they are reformed into triglycerides again. Again this they move out of the cells by vesicles into the lymph system.
42
What is the structure of the ileum? What are the properties of the villi?
The ileum is adapted to the function of absorbing the products of digestion. The wall of the ileum is folded and possesses finger-like projections, about 1mm long, called villi. They have thin walls, lined with epithelial cells on the other side of which is a rich network of blood capillaries. The villi considerably increase the surface area of the ileum and therefore accelerate the rate of absorption.
43
Where are the villi situated?
Villi are situated at the interface between the lumen (cavity) of the intestines (in effect outside the body) and the blood and other tissues inside the body. They are part of a specialised exchange surface adapted for the absorption of the products of digestion.
44
How do the properties of villi increase the efficiency of absorption?
- They increase the surface area for diffusion.
- They are very thin walled, thus reducing the distance over which diffusion takes place.
- They contain muscle and so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum. This ensures that, as the products of digestion are absorbed from the food adjacent to the villi, new material rich in the products of digestion replaces it.
- They are well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.
- The epithelial cells lining the villi possess microvilli. These are finger-like projections of the cell-surface membrane that further increase the surface area for absorption.
45
How are amino acids and monosaccharides absorbed?
The digestion of proteins produces amino acids, while that of carbohydrates produces monosaccharides such as glucose, fructose and galactose. The methods of absorbing these products are the same, namely diffusion and co-transport. Monosaccharides are absorbed by the epithelial cells lining each villus.
46
How are triglycerides absorbed?
Once formed during digestion, monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets. The structures formed are called micelles. They are tiny, being around 4-7nm in diameter. Through the movement of material within the lumen of the ileum, the micelles come into contact with the epithelial cells lining the villi of the ileum. Here the micelles break down, releasing the monoglycerides and fatty acids. As these are non-polar molecules, they easily diffuse across the cell-surface membrane into the epithelial cells.
Once inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides. Starting in the endoplasmic reticulum and continuing in the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons. Chylomicrons are special particle adapted for the transport of lipids.
Chylomicrons move out of the epithelial cells by exocytosis. They enter lymphatic capillaries called lacteals that are found at the centre of each villus. From here, the chylomicrons pass, via lymphatic vessels, into the blood system. The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells.
47
What is the role of bile in the absorption of fatty acids?
Bile salts play a role in the digestion and absorption of fatty acids. One end of the bile salt molecule is soluble in fat (lipophilic) but not in water (hydrophobic). The other end is soluble in water (hydrophilic) but not in fat (lipophobic). Bile salt molecules therefore arrange themselves with their lipophilic ends in fat droplets, leaving their lipophobic ends sticking out. In this way they prevent fat droplets from sticking to each other to form large droplets, leaving only tiny ones (micelles). It is in this form that fatty acids reach the epithelial cells of the ileum where they break down, releasing the fatty acids for absorption.
48
What is the definition of digestion?
The process in which large insoluble substances are hydrolysed by enzymes into small soluble substances so that they can be absorbed into the blood and assimilated.
49
What are brush border enzymes?
e.g. maltase, sucrase, lactase and dipeptidase.
They are bound to the cell membranes of the villi in the wall of the small intestine. They carry out the final digestion of disaccharides and dipeptides.
50
Where are endopeptidases found?
Gastric juices of the stomach (e.g. pepsin), pancreatic juice in the duodenum (e.g. trypsin) and small intestine cells.
51
Where are exopeptidases found?
Pancreatic juice and produced by small intestine cells.
52
What happens in the duodenum during digestion?
- Carbohydrates: breakdown of disaccharides into monosaccharides by maltase/sucrase/lactase enzymes.
- Proteins: breakdown by endopeptidases, exopeptidases and dipeptidases.
- Lipids: emulsification by bile, then breakdown by lipases.
53
What happens in the gall bladder and liver?
Bile is stored in the gall bladder. Bile is released from the liver to the duodenum via the bile duct.
54
Describe the structure of the small intestine.
- The small intestine is like a long tube.
- The outer walls of the tube are lined with smooth muscle.
- The inner wall which faces the lumen is lined with millions of epithelial cells.
55
What is the absorption route of nutrients?
1. Lumen of ileum - Nutrients are absorbed from the lumen of the ileum through the villi.
2. Epithelial cell - Nutrients travel into epithelial cells of the small intestine.
3. Bloodstream - Nutrients reach the bloodstream. From here, the nutrients go to the hepatic portal vein, or into lacteal cells - into the lymphatic system.
56
How is glucose absorbed?
Glucose is absorbed via co-transport alongside Na+ ions. Active transport is required to pump Na+ ions out of the epithelial cell and into the bloodstream.
57
How are monosaccharides absorbed?
Fructose is absorbed via facilitated diffusion. This involves a different carrier protein to glucose transport.
58
Where does glucose travel?
Glucose travels from the lumen of the ileum to the bloodstream through the sodium-glucose co-transport mechanism. This mechanism uses co-transporter proteins.
59
Describe the sodium/potassium pump.
1. Na+/K+ pump on basolateral membrane.
The Na+/K+ pump actively transports Na+ out of the epithelial cell and into the bloodstream. K+ comes in the other way.
2. Na+ concentration in epithelial cell falls.
The Na+ concentration in the epithelial cell becomes lower than the Na+ concentration in the ileum.
3. Na+ and glucose enter the cell via facilitated diffusion.
Na+ moves from the ileum into the epithelial cell, down a concentration gradient via facilitated diffusion. Glucose is co-transported alongside Na+.
4. Na+ and glucose diffuse out of the basolateral membrane.
Na+ and glucose diffuse through the epithelial cell and out of the basolateral membrane, into the blood through facilitated diffusion. Glucose travels through the GLUT-2 transporter.
5. Water follows.
Water is absorbed via aquaporins, and relies on the Na+ movement to create an osmotic gradient.
60
What is ingestion?
taking large pieces of food into the body
61
What is egestion?
eliminating the undigested material
62
What is absorption?
taking up the soluble digestion products into the body's cells
63
What is assimilation?
using the absorbed materials in the body
64
What is bile?
green/yellow fluid produced by the liver that emulsifies lipids in the duodenum, making them easier to break down
65
Why is the stomach acidic?
- the acidity neutralises harmful pathogens that enter the body in food, which may potentially cause food poisoning or upset the delicate balance of gut bacteria
- the acidity activates the protein-digesting enzyme (pepsin) in the stomach which is required alongside hydrochloric acid to commence the splitting apart of protein molecules
- the acid has an important role in the ionisation of minerals for their effective absorption into the bloodstream
66
Why does the stomach not have villi or microvilli?
Villi and microvilli increase the surface area to speed up the absorption of soluble molecules. As the food in the stomach has not yet been broken down into soluble molecules, they cannot be absorbed and so villi and microvilli are unnecessary.
67
What are the stomach adaptations?
1. muscular wall - to churn food
| 2. secretes mucus - to prevent the enzymes it produces from digesting the surface of the stomach
68
Describe how carbohydrates, proteins and lipids are digested.
- physical breakdown increases surface area
- enzymes hydrolyse bonds
- starch digested by amylase to form maltose
- maltose digested by maltase
- other disaccharidases also needed to digest other disaccharides, e.g. lactase, sucrase
- endopeptidases break down large proteins into smaller polypeptides
- exopeptidases break down polypeptides into dipeptides
- dipeptidases break down dipeptides into amino acids
- bile salts emulsify large droplets of fats to increase their surface area
- lipases break down lipids into fatty acids and monoglycerides
69
Explain why different enzymes are required.
- substrates all have different shapes
- enzymes have a specific primary structure giving specific tertiary structures
- and therefore an active site with a specific shape
- only substrates that are complementary can fit in the active site
- to form an enzyme-substrate complex and allow the reaction to happen
70
Where is salivary amylase produced?
- substrate: starch
- end products: maltose
- produced: salivary glands
71
Where is protease produced?
- substrate: protein
- end products: amino acids
- produced: stomach, pancreas
72
Where is lipase produced?
- substrate: lipids (fats and oils)
- end products: fatty acids and glycerol
- produced: pancreas
73
Where is pancreatic amylase produced?
- substrate: starch
- end products: maltose
- produced: pancreas
74
Where is maltase produced?
- substrate: maltose
- end products: glucose
- produced: small intestine
75
What happens during peristalsis?
- circular muscle contracts and longitudinal muscle relaxes (narrow)
- circular muscles relaxes and longitudinal muscle contracts (wide)
76
What is the duodenum (small intestine)?
- first 30cm of the small intestine
- almost all the digestion takes place here, due to three sections: pancreatic juice, bile and glands secreting maltase in the lining
77
What is the liver?
The liver secretes bile containing bile salts and has a high pH to help neutralise stomach acid.
78
What is cholera?
- disease caused by the bacterium Vibrio cholerae
- this bacterium produces a toxin that binds to the cell surface membrane of epithelial cells of the small intestine
- this opens chloride ion channels causing Cl- to diffuse from the cells into the small intestine lumen
79
Why do lipids need to be emulsified?
Emulsification increases the lipid surface area. This helps to speed up the activity of lipase.
80
What is the structure of villi?
- each villus has capillaries and lacteal vessels that are under it
- this is to absorb the nutrients that are absorbed by the epithelial cells of the villus
- the blood capillaries connect to the hepatic portal vein (carries blood to liver)
- the lacteal vessels connect to the lymphatic system (part of your immune system)
