Biologydidactics

03/04/2026

Lesson 138: A balanced diet

The human body is adapted for an omnivorous diet based on carbohydrates as the main energy source and in most people, this makes up for at least 50% of the total energy consumption. Since the origin in Africa some 200 000 years ago, our species has adapted to most types of environments on earth and there is a wide range of flexibility that seems to have worked out well for several millennia. Among Inuits living in a very cold climate, a seemingly carbohydrate deficient diet of fish and marine mammals can work out well as long as the fats are polyunsaturated. In some parts of the world and among vegetarians, the diet can be very low in proteins.
A recommended daily intake of protein is 1 gram per kilogram body weight. Thus, proteins make up for 15% of the energy intake but its major function is for repair and maintenance of body cells and the production of blood proteins. To keep the body hydrated, water is needed in a quantity of at least 1.5 kg per day and often quite a lot more if living in a hot climate and/or having a significant amount of physical activity in the daily life. To obtain a balanced diet there are also organic compounds called vitamins and inorganic minerals needed for different functions in the human body.
Protein
Proteins are macromolecules made of 20 different amino acids linked together into long chains (polypeptides). Most of those building blocks are exchangeable in transamination reactions. However, there are amino acids the body does not have the biochemical machinery to produce and those are the essential amino acids. As long as the diet contains a modest amount of protein of animal origin, there will be a good supply of these. In a strict vegetarian or vegan diet some of these may be scanty so just like a typograph running out of certain letters, the body’s protein synthesis machinery may run out of certain amino acids. To avoid kwashiorkor (a protein deficiency disease), the vegetarian may thus have to consume huge numbers of beans.
The picture shows the distribution of essential amino acids in protein-rich vegetarian food.

28/02/2026

Lesson 136: Energy requirements of the body
One can see that regardless one is walking or jogging a certain distance, virtually the same amount of energy is spent. 30 minutes of cycling, however, can bring you further than 5 km in 30 minutes and biking is quite an energy-efficient mode of transportation that actually can get you somewhere, while the joggers mainly run around in circles. A sprinter probably hits the limit of possible energy turnover of a human body, but the effort lasts just for 10-20 seconds and cannot be maintained for a sustained period. The amount of extra energy expenditure by normal physical exercise may not seem impressive (corresponds to the energy content of 20-30 gram of butter), but regular exercise usually has a beneficial effect on health and it may reduce the risk of a slow and steady weight gain. Trained muscles do also have more mitochondria, better blood supply and use more energy than the muscles in an untrained individual. The heart will also be increasingly efficient in pumping blood around the body.
If more energy is obtained through food than the energy turnover requires, the excess energy will be stored as body fat to be used during periods when food intake provides an amount of energy that is insufficient for the body’s energy expenditure. A moderately active adult man may utilize 12 000 kJ (2870 kcal) per day while a woman of similar activity level utilizes 9 000 kJ (2150 kcal).

The body of a lean, adult man contains three pools of chemical energy:
A blood glucose level at 5.0 mmol/L (0.90 g/L) in 6 L of circulating blood.
125 g of glycogen stored in liver and muscles
11 kg body fat
Glucose and glycogen contain 4.1 kcal/g and human body fat contains 7.7 kcal/g
For how long will each of these energy pools last if the daily energy expenditure is 2870 kcal.

Blood glucose ……………………….
Glycogen …………………………
Body fat …………………………

17/02/2026

Lesson 135: Diet and nutrition

It has long been known that food is something that is essential to life. Unfortunately, scientists have recently found that food is also a major cause of death. Food causes cardiovascular diseases, cancer and obesity and as soon as famine is eliminated, type II diabetes is not far away. Even the best food, if you eat too much of it, will kill you. Our ancestors did not live long enough to encounter those problems. A diet most similar to what our ancestors have survived on as hunters and gatherers for hundreds of thousands of years is probably the diet to which the human physiology is best adapted. A diet of nuts, berries and brown rice sometimes reinforced with dried meat or fish, will keep you healthy. If you are hungry all the time and never eat anything that tastes good, you are probably on the right track and can look forward to a long and interesting life
Energy requirement of the body
To obtain chemical energy is the main reason why we need food. Energy can be measured as calories (cal) or in Joule (J). For practical reasons, the energy content of food is measured in kilocalories (kcal) or kilojoules (kJ). 1 kcal equals 4.184 kJ. Per definition, it takes 1 Joule to heat one gram of water by 1oC. The basal metabolic rate (BMR) is the minimal amount of energy required to maintain the body’s vital functions. The BMR is highest in infants, decreases rapidly during growth, levels out by the age of 20 and then slowly declines by advancing age. BMR should not be confused with daily energy expenditure which is the total amount of energy the body uses up during a 24 hour period and as it on top of BMR also includes energy spent on different types of physical activity, it is a value that usually exceeds the BMR. Similar to BMR, daily energy expenditure is related to age, but also gender and level of physical activity during a day.

The intensity of physical activity is sometimes measured in watt (W). Watts are convertible to calories and joules in that 1-Watt effect during one second equals one Joule (1 Ws = 1 J = 0.24 cal)
The following values apply for the metabolic energy transfer (MET) at different activities of an adult human being with a body surface of 1.8 m2.

Metabolic energy Energy used during
transfer (W) one hour of activity
(Kcal)
Sleep 95 82
Sitting in armchair 105 90
Walking 350 300
Bicycling moderate intensity 600 520
Jogging 800 690
Fast running (23km/h) 2 300 N.A
Sprinting (100-200m) 3 500 N.A

08/02/2026

31/01/2026

Lesson 134: Liver functions

The liver is the largest gland in the body and as such it secretes bile acting as an emulsifier in the digestion of fat. The bile is made of two major components: Bile pigment bilirubin is a degradation product of the organic heme group of the haemoglobin molecule and bile salt sodium deoxycholate is a metabolite of cholesterol transported to the liver by HDL protein. The recycling system for bile pigments reabsorbed from the intestinal tract and brought back to the liver through the hepatic portal vein is sometimes referred to as the enterohepatic circulation. When used red blood cells are degraded in the liver, degradation of haemoglobin yields iron to be stored in the liver bound to ferritin or transported to the bone marrow by transferrin and other transport proteins. The polypeptides of the globin units are degraded into amino acids that either can undergo deamination for use in catabolic processes with urea formed as a waste product or be used to assemble serum albumin and other plasma proteins.
The liver serves as a filter of the blood carried by the hepatic portal vein from the gastrointestinal tract metabolizing and excreting drugs and other potentially harmful compounds absorbed by the intestines.
The liver receives about 25% of the total cardiac output at rest.
(a) Calculate the amount of blood carried to the liver through the hepatic artery if a total of 1 500 ml of blood leaves the liver through the hepatic vein and 1125 ml blood is carried through the hepatic portal vein.
(b) The hepatic artery carries 30-40% of the total amount of oxygen used by the liver. Calculate from (a) how large percentage of the total quantity of blood that enters the liver through the hepatic artery.
The liver is no compact chunk of cells. The blood flows in sinusoids between plates of liver cells inwards to the central vein in sinusoids. Bile is secreted into canaliculi draining into bile ducts. In a healthy liver, bile and blood are maintained well segregated. Kupfer cells phagocytose used red blood cells and other particles to be cleaned from the circulating blood.

17/01/2026

Lesson 133: Hunger and satiety

Similar to many homeostatic regulatory functions, hunger and satiety are regulated from centres in the hypothalamus region of the brain. Leptin is a peptide produced by several different types of body cells outside the brain and it circumvents the blood-brain barrier by entering the choroid complex.
Ghrelin is a peptide hormone released from the gastric tract. Its transport across the blood-brain barrier is dependent on putting a hydrophobic tail on the peptide by octanoylation (Banks et al. 2002).

13/01/2026

Lesson 132: Acid production and peptic ulcer

Parietal cells are activated by three neurohormones; Acetylcholine, Cholecystokinin (CCK) and Histamine. Histamine acts by increasing the intracellular cAMP level while acetylcholine and CCK act by increasing the level of intracellularly available calcium ions. These hormones have a synergistic effect so if one of them is inhibited there is a significant decrease in acid secretion.
Common antacid drugs simply neutralize the hydrogen ions in the stomach content. However, a second generation antacid drugs may either act by blocking histamine (H2) receptors or blocking the proton pump that mediates an active proton transport into the stomach.

06/01/2026

Lesson 131: The hormones regulating digestion.

An endocrine regulation of digestion is mediated by several hormones released into the bloodstream. CCK stimulates contraction of the gallbladder to release bile into the duodenum. In addition, it also stimulates secretion of pancreatic enzymes. Secretin stimulates the pancreas to secrete alkaline HCO3- to neutralize HCl in the acidic content entering the duodenum from the stomach.
Deduce from the picture what function(s) each gastrointestinal hormone has in regulating digestion.

1. Secretion of bile
2. Secretion of hydrochloric acid in the stomach
3. Secretion of pancreatic enzymes
4. Secretion of alkaline HCO3- from the pancreas
5. The release of insulin into circulation

Hormone Function (1-5)
Gastrin
GLP-1 and GIP
Secretin
CCK

The diagram shows the pivotal role of GLP-1 in regulating the secretion of insulin and glucagon from endocrine cells in the pancreas.
Deduce what effect DPP-4 inhibitors will have on blood glucose level.
Discuss the DPP-4 inhibitors potential use for treating type 1- and type 2-diabetes.

20/12/2025

Lesson 130: Neuronal and hormonal regulation of digestion.

Secretion of gastric juice is started by the sight and smell of food, sometimes even before the food is in the mouth. Nervous stimulation through the vagus nerve (cranial nerve X) peaks after half an hour and in the meantime the presence of food in the stomach trigger an increased secretion of the hormone gastrin taking over stimulation of gastric juice secretion as long as there is any food left in the stomach.
Use the diagram for answering the following questions.

(a) How long time, after the start of ingesting food is the total secretion of gastric juice maintained at a maximum?
(b) How long time, after the start of ingesting food is hormonal influence the only factor affecting secretion of gastric juice?

18/12/2025

Low divorce rate.

17/12/2025

Lesson 129: Absorption of amino acids and water

Transport of amino acids across the brush border membrane is more complex than uptake of monosaccharides but follows the same principles. At least seven different transport systems are available for the 20 different amino acids that normally make up proteins. Some of these transport systems are cotransporters involving Na+, Cl- or H+ ions in addition to the amino acids. There is also a channel protein for di- and tripeptides and those are degraded into single amino acids by enzymes inside the epithelial cells. Mainly in infants, undigested proteins can be taken up by endocytosis, transported through the epithelial cells and released by exocytosis into the circulation. Antibodies (IgA) and other proteins in colostrum are absorbed in this way (transcytosis), but after the first year of life, this type of absorption diminishes. Still, there may be a minute uptake by pinocytosis of intact proteins from the adult intestine and this evokes antibody formation that sometimes causes allergic symptoms to certain types of food (most commonly seafood or fungi).
Absorption of water
In the lower parts of the small intestine (jejunum) and in the large intestine, most osmotically active components have been absorbed and the intestinal content has a high water potential. Water is then absorbed by osmosis. Cholera toxin (A) binds to GM1 receptors in intestinal cells and the toxin enters the cell through endocytosis. Inside the epithelial cell in is cleaved and a part of it binds to a G protein that, in turn, activates adenylate cyclase to produce cyclic AMP (cAMP). Permanently elevated cAMP causes a massive leak of Cl- ions through open cystic fibrosis transmembrane conductance regulator (CFTR).
(a) Predict what will happen to the Na+ in the lateral intercellular spaces and water absorption when cholera toxin is acting inside enterocytes.
(b) Discuss how the effect of cholera toxin may benefit transmission of the disease caused by water-borne bacteria (Vibrio cholera)