Option D: Human Physiology

Question 1

What are essential nutrients?

Answer 1

Essential nutrients cannot be synthesized by the body, therefore they have to be included in the diet.

Question 2

What are dietary minerals?

Answer 2

Dietary minerals are essential chemical elements. They are major constituents of structures such as teeth and bones (e.g. Ca - plant root and shoot, P, Mg - chlorophyll), important components of body fluids (e.g. Na, K, Cl) and cofactors for specific enzymes or components of proteins and hormones (e.g. Fe, P, I)

Question 3

What are vitamins?

Answer 3

Vitamins are chemically diverse carbon compounds that cannot be synthesized by the body. They can be categorized into two groups: water soluble vitamins (excess lost in urine - B, C) and fat soluble vitamins (can be stored - A, D, E, K)

Question 4

What are fatty acids?

Answer 4

Some fatty acids and some amino acids are essential. Alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid) cannot be synthesised by the body. They are required to introduce double bonds. Valine and lysine are also essential

Question 5

What causes malnutrition? (2)

Answer 5

1. It can be caused by an improper dietary intake of nutrients – e.g. overnutrition (too much) or undernutrition (not enough).
2. This can lead to obesity (>30 bmi)associated with type 2 diabetes and hypertension) or anorexia (heart muscles break down)

Question 6

What is the result of vitamin D deficiency?

Answer 6

A lack of vitamin D or calcium may cause rickets (where bones deform, dental problems) or osteomalacia (where bone soften). Vitamin D can be stored in the liver when levels are low but the body ultimately requires sun exposure.

Question 7

What is the result of vitamin C deficiency?

Answer 7

A deficiency in vitamin C levels can lead to scurvy (anemia and swelling of joints). Ascorbic acid, vitamin C, cannot be made by humans but can be made by birds. In mammals it functions as a potent antioxidant and also plays an important role in immune function. It is also involved in the synthesis of collagen (a structural protein) and in the synthesis of lipoproteins

Question 8

How is appetite controlled? (4)

Answer 8

1. Appetite is controlled by a centre in the hypothalamus.
2. Hormones produced in the pancreas, stomach, intestines and adipose tissue, send messages to the hypothalamus,
3. Hormones that trigger a hunger response include ghrelin (from stomach) and glucagon (from pancreas)
4. Hormones that trigger a satiety response include leptin (from adipose tissue) and CCK (from intestine)

Question 9

What are some of the effects of obesity and anorexia?

Answer 9

1. High cholesterol levels in the bloodstream lead to the hardening and narrowing of arteries (atherosclerosis) as LDL particles will form deposits in the walls of the arteries. This restricts blood flow and if coronary arteries become blocked, coronary heart disease (CHD) will result
2. Breakdown of heart muscle due to anorexia - blood pressure reduces and heart may become smaller

Question 10

What are the causes of PKU?

Answer 10

1. Phenylketonuria (PKU) is a genetic condition that results in the impaired metabolism of the amino acid phenylalanine
2. It is an autosomal recessive disease which results in a toxic build up of phenylketone in the blood and urine.
3. Untreated PKU can lead to brain damage and mental retardation
4. Infants with PKU are normal at birth because the mother is able to break down phenylalanine during pregnancy
5. Diagnosis through a simple blood test

Question 11

How can PKU be treated?

Answer 11

1. Treated by enforcing a low-protein diet that restricts the intake of phenylalanine to prevent its build up within the body.
2. This diet should be supplemented with a medical formula that contains precise quantities of essential amino acids
3. Patients who are diagnosed early and maintain this strict diet can have a normal life span without damaging symptoms

Question 12

What is calorimetry? (4)

Answer 12

1. The energy content of food can be estimated by burning a sample of known mass and measuring the energy released via calorimetry:
2. Combustion of the food source causes the stored energy to be released as heat, which raises the temperature of water
3. The amount of energy required to raise 1 g of water by 1ºC is 4.18 J – this is the specific heat capacity of water
4. Energy (joules) = Mass of water (g) × 4.2 (J/gºC) × Temperature increase (ºC)

Question 13

What is the biggest source of error in calorimetry?

Answer 13

It is usually caused by the unwanted loss of heat to the surrounding environment. Therefore food sources should be burnt at a constant distance from the water to ensure reliability of results and the initial temperature and volume of water should also be kept constant.

Question 14

What are the nervous mechanisms associated with the secretion of digestive juices? (3)

Answer 14

1. The sight and smell of food triggers an immediate response by which gastric juice is secreted by the stomach pre-ingestion.
2. When food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining.
3. Signals are sent to the brain, which triggers the release of digestive hormones to achieve sustained gastric stimulation.

Question 15

What are the hormonal mechanisms associated with the secretion of digestive juices? (4)

Answer 15

1. Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids.
2. If stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin).
3. When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones such as secretin and cholecystokinin which stimulate the pancreas and liver to release digestive juices.
4. Pancreatic juices contain bicarbonate ions which neutralise stomach acids, while the liver produces bile to emulsify fats

Question 16

What are exocrine glands?

Answer 16

1. They produce and secrete substances via a duct onto an epithelial surface – either: the surface of the body (e.g. sweat glands, sebaceous glands) or the lumen of the digestive tract / gut (e.g. digestive glands)
2. Examples of digestive glands include: salivary glands (amylase), gastric glands (hydrochloric acid), pancreatic glands (lipase) and intestinal glands
3. Exocrine glands are composed of a cluster of secretory cells which collectively form acini. The acini are surrounded by a basement membrane and are held together by tight junctions between secretory cells.
4. The secretory cells possess a highly developed ER and golgi network for material secretion and are rich in mitochondria

Question 17

How is stomach acid produced?

Answer 17

1. The gastric glands that lines the stomach wall secrete an acidic solution that creates a low pH environment within the stomach of 1.5 – 2.0, which is the optimum pH for hydrolysis reactions by stomach enzymes.
2. This low pH environment is maintained by proton pumps in the parietal cells of the gastric pits.
3. These proton pumps secrete H+ ions (via active transport), which combine with Cl– ions to form hydrochloric acid.
4. Proton pump inhibitors (PPIs) irreversibly bind to the proton pumps and prevent H+ ion secretion to prevent gastric discomfort caused by high acidity (e.g. acid reflux)

Question 18

What are the different functions of stomach acid? (3)

Answer 18

1. Assists in the digestion of food (by dissolving chemical bonds within food molecules)
2. Activates stomach proteases (e.g. pepsin is activated when pepsinogen is proteolytically cleaved in acid conditions)
3. Prevents pathogenic infection (stomach acids destroy microorganisms in ingested food)

Question 19

How is villi adapted for the absorption of food? (9)

Answer 19

1. Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption.
2. Microvilli – Ruffling of epithelial membrane further increases surface area
3. Rich blood supply – Dense capillary network rapidly transports absorbed products
4. Single layer epithelium – Minimises diffusion distance between lumen and blood
5. Lacteals – Absorbs lipids from the intestine into the lymphatic system
6. Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
7. Membrane proteins – Facilitates transport of digested materials into epithelial cells
8. Mitochondria - Requires large amount of energy for active transport mechanisms
9. Tight Junctions - They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement

Question 20

Explain the role of fibre in egestion? (5)

Answer 20

1. The small intestine absorbs usable food substances (i.e. nutrients – monosaccharides, amino acids, fatty acids, vitamins, etc.) while the large intestine absorbs water and dissolved minerals (i.e. ions) from the indigestible food residues
2. Dietary fibre, or roughage, is this indigestible portion of food derived principally from plants and fungi (cellulose, chitin, etc.).
3. It provides bulk in the intestines to help keep materials moving through the gut.
4. Roughage also absorbs water, which keeps bowel movements soft and easy to pass.
5. It also: reduces the frequency of constipation and lowers the risk of colon and rectal cancer; lowers blood cholesterol and regulates blood sugar levels (by slowing the rate of absorption); and aids in weight management.

Question 21

Outline the causes and consequences of stomach ulcers (3)

Answer 21

1. Stomach ulcers are inflamed and damaged areas in the stomach wall, typically caused by exposure to gastric acids.
2. Helicobacter pylori is thought to be the main cause since it can survive the acid conditions of the stomach by penetrating the mucus lining. It damages the epithelial cells of the stomach and degrades the protective mucus lining, exposing the stomach wall to gastric acids and causing ulcers.
3. The prolonged presence of stomach ulcers may lead to the development of stomach cancer over many years (20 – 30 years) but can be treated by antibiotics.

Question 22

Outline the causes and consequences of vibrio cholerae

5

Answer 22

1. Vibrio cholerae is a bacterial pathogen that infects the intestines and causes acute diarrhoea and dehydration.
2. V. cholerae releases a toxin that binds to ganglioside receptors on the surface of intestinal epithelium cells.
3. This toxin is internalised by endocytosis and triggers the production of cyclic AMP (a second messenger) within the cell.
4. Cyclic AMP (cAMP) activates specific ion channels within the cell membrane, causing an efflux of ions from the cell.
5. The build up of ions in the intestinal lumen draws water from cells and tissues via osmosis – causing acute diarrhoea. As water is being removed from body tissues, dehydration will result if left untreated.

Question 23

What is the role of the liver? (4)

Answer 23

1. The liver is a lobed organ located below the diaphragm that functions to regulate the chemical composition of blood.
2. It receives oxygenated blood via the hepatic artery, which is used to sustain liver cells (hepatocytes)
3. It also receives nutrient rich blood from the gut via the portal vein
4. Deoxygenated blood is transported from the liver via the hepatic vein

Question 24

What is the structure of the liver? (4)

Answer 24

1. The liver is composed of smaller histological structures called lobules, which are roughly hexagonal in shape.
2. Each lobule is surrounded by branches of the hepatic artery (provide oxygen) and the portal vein (provide nutrients). These vessels drain into capillary-like structures called sinusoids (they have increased permeability), which exchange materials directly with the hepatocytes.
3. The sinusoids drain into a central vein, which feeds deoxygenated blood into the hepatic vein.
4. Hepatocytes also produce bile, which is transported by vessels called canaliculi to bile ducts, which surround the lobule.

Question 25

What are sinusoids? (2)

Answer 25

1. Sinusoids are a type of small blood vessel found in the liver that perform a similar function to capillaries (material exchange).
2. They have increased permeability, allowing larger molecules (e.g. plasma proteins) to enter and leave the bloodstream

Question 26

What is the difference between sinusoids and capillaries? (2)

Answer 26

1. The surrounding diaphragm (basement membrane) is incomplete or discontinuous in sinusoids (but not in capillaries)
2. Also the endothelial layer contains large intercellular gaps and fewer tight junctions (allowing for the passage of larger molecules)

Question 27

What is the role of the liver in detoxification? (6)

Answer 27

1. The liver acts on drugs and toxins that have entered the bloodstream. Many of these toxic compounds are fat soluble, making them difficult for the body to excrete.
2. These compounds are converted into less harmful and more soluble forms, which are then excreted from the body.
3. These conversions occur by oxidation, reduction and hydrolysis reactions which are are mediated by a group of enzymes known as the cytochrome P450 enzyme group.
4. While the conversions produce damaging free radicals, these are then neutralised by antioxidants within the liver, and the converted chemical is then attached to another substance (e.g. cysteine) via a conjugation reaction.
5. This renders the compound even less harmful and also functions to make it water soluble.
6. The water soluble compounds can now be excreted from the body within urine by the kidney

Question 28

How does the liver help to store and regulate nutrients?

3

Answer 28

1. Carbohydrate Metabolism - Excess glucose in the bloodstream (e.g. after meals) is taken up by the liver and stored as glycogen. When blood glucose levels drop, the liver breaks down glycogen into glucose and exports it to body tissues. When hepatic glycogen reserves become exhausted, the liver synthesises glucose from other sources (e.g. fats)
2. Protein Metabolism - The body can not store amino acids, so they must be broken down when in excess. This breakdown releases an amine group (NH2), which cannot be used by the body and is potentially toxic. The liver removes this (deamination) and converts it into urea which is excreted within urine by the kidneys. The liver can also synthesise non-essential amino acids from surplus stock (via transamination).
3. Fat Metabolism - The liver is the major site for converting excess carbohydrates and proteins into fatty acids and triglycerides. It is also responsible for the synthesis and storage of large quantities of phospholipids and cholesterol. Surplus cholesterol is converted by the liver into bile salts, which can be eliminated from the body via the bowels.

Question 29

What is jaundice?

Answer 29

1. Jaundice is a condition caused by an excess of bile pigment – bilirubin – within the body.
2. Normally, the liver conjugates bilirubin to other chemicals and then secretes it in bile yet when there is an excess of bilirubin, it may leak out into surrounding tissue fluids.

Question 30

How is jaundice caused?

Answer 30

1. Liver disease (impaired removal of billirubin)
2. Obstruction of the gallbladder (preventing secretion of bile)
3. Damage to red blood cells (increased destruction of erythrocytes)

Question 31

What are the effects and treatments of jaundice?

Answer 31

1. The main consequence of jaundice is a yellowish discoloration of the skin and whites of the eyes (sclera) as well as itchiness, paler than usual stools and darkened urine.
2. Jaundice may be resolved by treating the underlying cause for the build up of bilirubin within the body