Digestion and Absorption: Topic 6.1, part of 6.6, and D.2

Question 1

Outline why food needs to be digested.

Answer 1

Food molecules are both insoluble and too large to be absorbed as is, so they must be digested first.

• Food molecules that are ingested (taken in/ eaten) are very large (macromolecules), and they are usually insoluble
• Macromolecules must be digested (hydrolyzed/ broken down into smaller and smaller molecules) so that they are small enough (in simpler form) to be absorbed – to pass through cell membranes in your digestive tract and into your bloodstream (so you receive “nutrients” – building blocks for your body/ tissues/ cells)
• Additionally, certain foods contain materials not suitable for human tissues (these must be separated and removed)

Question 2

Explain how starch, proteins, and fats (lipids) are digested (including the names, substrates, products, sites of production, and sites of action of the specific enzymes involved).

Answer 2

PROTEINS: broken down into amino acids or dipeptides first by pepsin/pepsinogen (secreted by chief cells in the stomach) in the stomach (acidic pH) and then by trypsin/endopeptidase (secreted by the pancreas) in the small intestine (slightly alkaline pH)
CARBOHYDRATES (starch): broken down into maltose by amylase (secreted by the salivary glands and the pancreas) in the mouth and small intestine respectively (slightly alkaline pH)
LIPIDS (triglycerides): broken down into glycerol and fatty acids by lipase (secreted by the pancreas) in the small intestine (slightly alkaline pH)
NUCLEIC ACIDS: broken down into nucleotides by nuclease (secreted by the pancreas) in the small intestine (slightly alkaline pH)

Question 3

Explain the nervous and hormonal mechanisms that control the secretion, volume, and content of gastric sections.

Answer 3

1. The sight/smell of food triggers the medulla to send impulses via the vagus nerve to gastric glands in the stomach to secrete gastric juice. This ensures that gastric juice (pepsin(ogen), HCl, and mucous) will be present in the stomach by the time food arrives.
2. Food enters the stomach, causing distention. This is detected by stretch receptors in the stomach lining.
3. Impulses from the stretch receptors are sent to the medulla oblongata, which sends a signal to the stomach through the vagus nerve that triggers gastrin (hormone) secretion into the bloodstream (from endocrine cells in the stomach wall) and causes the stomach to begin producing and secreting HCl (from parietal cells) and pepsinogen (from chief cells).
4. Gastrin causes the sustained release of gastric juices (particularly the acid component, HCl).
5. If pH becomes too low (< 1.5-2.0), gastrin is inhibited by hormones: secretin from the small intestine and somatostatin from the hypothalamus.
6. Once digested food (chyme) passes into the small intestine, the duodenum releases the hormones secretin and CCK (cholecystokinin) to stimulate the pancreas to release pancreatic juices (bicarbonate ions to neutralize stomach acids and pancreatic digestive enzymes) and liver to release bile (emulsify fats).

Question 4

Outline the roles of the stomach (in general and in digestion).

Answer 4

STOMACH - stores and churns food; begins protein digestion

• Protein digestion begins in the stomach!

Role of HCl:

• Activates pepsinogen into pepsin (to digest proteins)
• Lowers pH, which helps to initially denature proteins
• Kills pathogenic bacteria and fungi in ingested foods

Role of Proton Pumps:

• (Found in parietal cells in the gastric pits)
• Maintain acidic conditions of the stomach
• Actively pump protons into the stomach, where they combine with Cl- ions to form HCl

Question 5

Outline the roles of HCl in the stomach.

Answer 5

• Activates pepsinogen into pepsin (to digest proteins)
• Lowers pH, which helps to initially denature proteins
• Kills pathogenic bacteria and fungi in ingested foods

Question 6

Outline the causes, consequences, and treatment of stomach ulcers.

Answer 6

Stomach ulcers are inflamed/damaged open sores in the stomach wall.

CAUSE: Exposure of stomach cells to stomach acids; Helicobacter pylori infection can lead to this (It damages the goblet cells in the stomach wall that secrete mucus and it secretes mucinase, which degrades the mucous lining of the stomach wall. As the mucus breaks down, the stomach wall/epithelial lining is damaged/digested by stomach acid.)

CONSEQUENCES: Prolonged damage to the stomach wall (and chronic H. pylori infection: 20+ years) may lead to stomach cancers.

TREATMENTS: Antibiotics and PPIs (proton pump inhibitors bind irreversibly to proton pumps and prevent H+ secretion, therefore stopping these protons from combining with Cl- ions to form HCl, so the pH of the stomach is raised and the acidity won’t continue to damage ulcers)

Question 7

Outline the use of PPI’s to reduce stomach acid secretion.

Answer 7

Proton pump inhibitors bind irreversibly to proton pumps and prevent H+ secretion. These protons are therefore stopped from combining with Cl- ions to form HCl, so the pH of the stomach is raised and the acidity won’t continue to damage ulcers.

Question 8

Describe how the structure of a villus is adapted to its function.

Answer 8

Villi increase the surface area of epithelium over which absorption is carried out (within the small intestine). They absorb monomers formed by digestion as well as mineral ions and vitamins.

SINGLE LAYER OF EPITHELIAL CELLS: Each villus is surrounded by a single layer of epithelial cells, which decreases the distance nutrients must diffuse to be absorbed into the bloodstream.
MICROVILLI: Epithelial cells of each villus contain microvilli that increase the surface area for absorption of nutrients.
MANY MITOCHONDRIA: Each villus contains many mitochondria to generate ATP for the active transport of nutrients into the blood.
TIGHT JUNCTIONS: Each epithelial cell is connected to the next epithelial cell through tight junctions. These form an impermeable barrier between the plasma membranes of adjacent epithelial cells, ensuring a one-way flow of food materials and ensuring that digestive fluids and body-tissue fluids remain separate at all times.
RICH CAPILLARY NETWORK: Capillary networks are close to the epithelial cells to minimize the diffusion distance of nutrients. Capillary networks also maintain a large concentration gradient that allows for the rapid absorption of nutrients.
LACTEAL: Within each villus is a lacteal; lacteals absorb lipids.

Practice looking at diagrams/micrographs!

Question 9

Describe the methods of transport used to absorb the products of starch, protein, and lipid digestion in the small intestine.

Answer 9

DIFFUSION: involves the movement of nutrients along a concentration gradient (ex. fatty acids)
FACILITATED DIFFUSION: involves movement of nutrients through channel proteins (ex. fructose)
ACTIVE TRANSPORT: involves movement of nutrients against a concentration gradient using ATP (ex. amino acids and glucose)
ENDOCYTOSIS: involves invagination of the cell membrane to form a vesicle around nutrients (ex. antibodies in breast milk)
OSMOSIS: involves water diffusing across membranes in response to movement of ions (the specific example here is not needed because it’s just water)

Question 10

Distinguish between exocrine and endocrine glands (with regard to their structure and function) - be able to identify an exocrine gland in micrographs based on recognition of structural features too.

Answer 10

HELP

Question 11

Outline the benefits of a diet rich in fiber.

Answer 11

FIBER IS IMPORTANT!
Helps clean out old/damaged intestinal cells and unabsorbed materials
Provides bulk to keep materials moving
Absorbs water to keep feces soft and easy to pass
“Works out” the body’s normal microflora
Reduces the frequency of constipation
Lowers the risk of colon and rectal cancers
Lowers blood cholesterol
Regulates blood sugar levels (slows absorption rate of glucose)
Decreases hunger (aids in weight management/prevention of obesity)

The rate of transit of materials through the large intestine is positively correlated with their fiber content—more fiber means a faster rate of transit, which means less exposure to undesirable food chemicals, etc. It decreases the contact time between the intestinal wall and food.

Question 12

State which substances are egested by the human body.

Answer 12

BELCH: Bile pigments, Epithelial cells, Lignin, Cellulose, and Human microflora/bacteria

• (Cellulose and lignin are not digested in humans because humans do not possess the enzymes (cellulase) or the gut bacteria to break them down.)

Question 13

Outline the effects of the cholera toxin.

Answer 13

Vibrio cholerae is a bacterial pathogen that infects the intestines. It causes diarrhea and dehydration. Cholera is the disease due to this infection and can cause death in a matter of hours if not treated (through rehydration).
HOW?
1. V. cholerae releases a toxin that binds to a receptor on the epithelial cells in the intestine.
2. This activates ion channels in these cells so that ions including chloride ions (Cl-) are pumped out of these cells and into the intestine.
3. Water from the cells follows the ion concentration gradient (by osmosis).
4. Excess water dilutes feces (diarrhea), and will cause dehydration if left untreated, as water is continuously removed from the body tissues.

Question 14

Explain how the pancreas maintains homeostatic levels of glucose in the blood.

Answer 14

• The pancreas produces hormones that control the levels of glucose
• If glucose levels in the blood are high, *beta-cells of the pancreas produce insulin
• Insulin causes the cells to take up/absorb glucose
• The liver stores excess glucose as glycogen
• If glucose levels in the blood are low, *alpha-cells of the pancreas produce glucagon
• Glucagon causes the liver to break down glycogen into glucose
• Glucagon increases levels of glucose in the blood
• Negative feedback controls the glucose levels

Glucagon stimulates hepatocytes (liver cells) to break down glycogen into glucose
Insulin acts on all cells in the body, triggering them to take up glucose
Glucose in liver cells (hepatocytes), adipose cells, and muscle cells is converted to and stored as glycogen, decreasing blood glucose

Question 15

Explain the causes, consequences, and treatment of Type I and Type II diabetes.

Answer 15

TYPE I DIABETES
Causes: Early onset… Beta cells damaged/destroyed by the body’s own immune system – autoimmune disease (not enough/no insulin produced)
Consequences/Symptoms: High blood sugar, glucose in the urine, increased thirst/urination, hunger, fatigue, weight loss, etc.
Treatments: Controlled by insulin injections to regulate blood glucose levels

TYPE II DIABETES
Causes: Adult onset… Insulin receptors on cells (liver, muscles, etc.) are fewer and/or become less sensitive to insulin (decreased body response to insulin)
Consequences/Symptoms: High blood sugar, glucose in the urine, increased thirst/urination, hunger, fatigue, weight loss, etc.
Treatments: Controlled by managing diet (more fiber/complex carbs/smaller meals/reduce sugar intake = slower/reduced glucose release into blood; less saturated fat = lose weight), and lifestyle

Question 16

Outline the dual supply of blood to the liver.

Answer 16

Because the liver is the largest gland in the body and it performs over 500 different vital functions, it has a unique input of blood from two separate sources.
The HEPATIC ARTERY delivers oxygen-rich blood from the heart to the liver.
The HEPATIC PORTAL VEIN delivers nutrient-rich blood from the gut to the liver.
Blood enters the liver through the hepatic artery and hepatic portal vein and flows into liver capillaries called sinusoids.

Sinusoids are wide capillaries surrounded by a single layer of hepatocytes. They filter, detoxify, and balance the levels of nutrients in the blood.
After passing through the sinusoids of the liver, “clean”/balanced blood passes into the hepatic vein (which carries “clean”/balanced/deoxygenated blood back to the heart).

In each lobule of the liver, smaller branches of the hepatic artery (hepatic arterioles) and smaller branches of the hepatic portal vein (hepatic venules) drain into liver capillaries called sinusoids. Sinusoids filter the blood and it drains into the hepatic vein (back to the heart).

Sinusoids are small blood vessels (similar to capillaries) that facilitate material exchange (between the blood and the hepatocytes).
* Blood drains into each sinusoid through hepatic arterioles (oxygen-rich blood from heart) and hepatic portal venules (nutrient-rich blood from intestine)
* Each sinusoid is lined with a single layer of hepatocytes (one cell layer thick) and contains specialized macrophages (phagocytic cells = Kupffer cells)
* Sinusoids drain “filtered” blood into a central hepatic vein (which will take clean, deoxygenated blood, under lower pressure, back to the heart)

Question 17

Explain how the liver regulates nutrient levels in the blood (including nutrients that are stored by the liver).

Answer 17

CARBOHYDRATE METABOLISM: Excess glucose in the blood is stored as glycogen in hepatocytes (controlled by insulin from the pancreas). If blood glucose levels drop, the liver hydrolyzes glycogen into glucose and releases it back into the blood (controlled by glucagon from the pancreas). IF hepatic glycogen stores are depleted, the liver CAN synthesize glucose from other sources (such as fats).
PROTEIN METABOLISM: Using amino acids from digested proteins, hepatocytes make and secrete plasma proteins (produced by Rough ER and modified by the Golgi within these cells). Hepatocytes also break down the protein component of hemoglobin (globin) into amino acids. Amino acids from these sources can also be used by the liver to synthesize nonessential amino acids. Excess amino acids CANNOT be stored in the body, so the liver breaks them down. In doing so, it removes the amine (NH2) group (deamination) and converts it into urea (nitrogenous waste removed by the kidneys).
LIPID METABOLISM: Excess carbohydrates and proteins are converted into fatty acids and triglycerides in the liver. These are stored, or used to make cholesterol and phospholipids, which are either stored or transported to other parts of the body by lipoproteins. LDL’s take cholesterol TO cells and HDL’s bring cholesterol back to the liver. *Understand: Excess cholesterol in the liver is converted to bile salts, which are used to make bile or egested via the large intestine.

Hepatocytes (liver cells) store and release: cholesterol, triglycerides, glucose/glycogen, iron, vitamin A, and vitamin D

Question 18

Explain how the liver breaks down and recycles the components of erythrocytes (red blood cells).

Answer 18

Erythrocytes are broken down by phagocytosis. Kupffer cells (phagocytic cells in the liver sinusoids) engulf ruptured red blood cells and their “pieces”/ hemoglobin, and break down their hemoglobin molecules into heme (contains iron) and globin.

Peptidases break down globin into individual amino acids (used by hepatocytes and other cells to synthesize new proteins or deaminated by the liver)
Heme groups are broken down into iron (stored as ferritin in the liver OR transported to the bone marrow to be used in the production of new red blood cells) and bilirubin (bile pigment).

Question 19

Outline the production of bile and the causes and consequences of jaundice.

Answer 19

Bile (made of bile salts, water, cholesterol, and bilirubin/ bile pigment) is made in the liver (hepatocytes) and transported via bile ductules (in the sinusoids) to the gallbladder for storage.
Bilirubin is produced by the breakdown of hemoglobin/heme (from red blood cells) in the liver.
Excess cholesterol in the liver is converted to bile salts.

Jaundice is a condition in which there is an excess of/ high levels of bilirubin (bile pigment) in the body (and bilirubin leaks out of the liver into the blood).
Causes: Liver disease (hepatitis, liver cancer, cirrhosis), gall bladder obstruction/ blocked bile duct/ gall stones, increased RBC damage (anemia), infection (parasite), immature liver (in newborns)
Consequences: yellowing skin, yellowing sclera (whites of eyes), itchiness, pale/ grey feces, darkened urine, brain damage (in infants)