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6.2 Applications and skills

6.2.1 Membrane transport in the small intestine
6.2.2 Circulation of the blood
6.2.3 Thrombosis, penicillin and HIV
6.2.4 Practical 6: Lung cancer, emphysema and the effect of exercise on ventilation
6.2.5 The synapse
6.2.6a Therapeutic and clinical uses of hormones
6.2.6b The mystery of reproduction: IVF and William Harvey

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Course: Biology Support Site
Book: 6.2 Applications and skills
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Date: Wednesday, 23 September 2020, 12:10 AM

6.2.1 Membrane transport in the small intestine

Absorption in the small intestine involves nutrients moving from the lumen of the small intestine, through the epithelium cell, into the interstitial fluid, and finally into the capillary.

  • All types of membrane transport are involved when the small intestine is at work. However, it is important to note that all absorption into the bloodstream is passive.
  • Active transport, in the form of sodium and potassium pumps, maintains concentration gradients that allow passive transport, including osmosis, to occur in the epithelium of the small intestine (see Figure 6.2.1a). 

Figure 6.2.1a – Glucose loadingFigure 6.2.1a – Glucose loading
Glucose loading is an example illustrating how energy-requiring processes maintain concentration gradients for passive transport.

Nutrient General method Detail
Water (and electrolytes, like Na+, K+) Osmosis and simple diffusion
  • Water moves in and out of the epithelium in response to osmolarity differences maintained by sodium pumps.
  • Water and electrolytes diffuse into the capillary from the interstitial fluid.
Glucose (and other monosaccharides)

Facilitated diffusion using:

  1. sodium co-transporter proteins
  2. glucose channel proteins
  • Sodium diffuses into the cell from the lumen through the co-transporter protein, bringing glucose with it.
  • Protein channels allow glucose to diffuse from cell to interstitial space and into the capillary.
Amino acids

Facilitated diffusion using:

  1. sodium co-transporter proteins
  2. amino acid channel proteins

Uses four different sodium co-transport proteins.

  • The proteins change shape when in contact with sodium, allowing for specific amino acids to bind and be transported into the epithelium from the lumen (mechanism similar to glucose absorption).
Fatty acids

Simple diffusion  

Facilitated diffusion  

Exocytosis

Small monoglycerides and fatty acids diffuse directly through the epithelial plasma membrane as micelles.

  • Longer fatty acids use specific transporter proteins to get through the plasma membrane
  • Once inside the cell, smaller components are assembled into chylomicrons (Figure 6.2.1b).
  • Chylomicrons exit the cell by exocytosis, then enter the lacteal and travel through the lymph.
  • Fats are deposited into the bloodstream by the lymphatic vessels (Figure 6.2.1c).


Figure 6.2.1b - Chylomicrons
Figure 6.2.1b – Chylomicrons
Chylomicrons are assembled by the Golgi apparatus of epithelial cells and are made of many components in addition to triglycerides.

Application: Modelling the small intestine with dialysis tubing

Dialysis tubing is a semi-permeable membrane that is used in laboratories to separate molecules from a solution based on differential diffusion. It is often used as a teaching tool to model absorption in the small intestine.

Figure 6.2.1d – A model gut using dialysis tubingFigure 6.2.1c – A model gut using dialysis tubing

Figure 6.2.1c shows the initial set-up of a demonstration of a model gut. At the beginning of the experiment, the dialysis tubes contain different substances, and the height of the column in each capillary tube is identical.

  • Predict what will happen to the contents of the tubing and the level in the capillary tubes after one hour. Explain your predictions.
  • Outline the tests you would perform to determine the contents of the dialysis tubing after one hour.
  • Describe how this set-up models the working of the small intestine.
  • Discuss limitations of this model.

Key questions

  • Outline digestion and absorption of starch, proteins and lipids.
  • Describe three differences between the mechanisms for absorption of lipids, and monosaccharides/amino acids.

Course link

  • Review the different methods of membrane transport in Page 1.1.4.
  • Review the structure of micelles in Page 1.1.5.

Language help

  • A co-transporter protein is called a symport protein in some texts.
  • A membrane protein that facilitates diffusion in opposite directions is called a counter-transporter or an antiport protein.

support proteinFigure 6.2.1d – Symport protein 

Did you know?

  • Iodine solution turns blue/black when in contact with starch.

Lab extension

Change the concentration of the solution, the type of solution or the temperature of the set-up.

Figure 6.2.1c – Fatty acid absorptionFigure 6.2.1e – Fatty acid absorption
The pathway of absorption for fatty acids is different from that for glucose and amino acids. Click image to reveal steps in the process

6.2.2 Circulation of the blood

The heart is the only organ driving the circulatory system. Let’s have a closer look.

Pressure changes during the cardiac cycle

  • One cardiac cycle is one complete heartbeat.
  • The heartbeat occurs in a continuous wave pattern originating at the sinoatrial node. See Page 6.1.2.
  • Although the process is continuous, it can be divided into two main stages as follows:
Stage Atrioventricular (mitral and tricuspid) valves Semilunar (aortic and pulmonic)valves Description of atria and ventricles Blood pressure (aorta)
Atrial systole/ventricular diastole Open Closed
  • Atria contracting
  • Ventricles relaxing
Lower (diastolic)
Atrial diastole/ventricular systole Closed Open
  • Atria relaxing
  • Ventricles contracting
Higher (systolic)

Figure 6.2.2a – A normal cardiac cycle takes about 0.8 secondsFigure 6.2.2a – A normal cardiac cycle takes about 0.8 seconds

Data analysis

Figure 6.6.2b shows pressure and volume changes in the ventricles, atria and aorta during the cardiac cycle.

Figure 6.2.2b – Pressure changes during the cardiac cycleFigure 6.2.2b – Pressure changes during the cardiac cycle

Answer the following questions:

  • How many complete cycles are shown?
  • Locate the point on the graph where ventricular blood volume is highest. Explain the relationship between blood volume and aortic pressure during the systole.
  • Which valves open and close at the points labelled 1, 2, 3 and 4? (Click on the image for answers.)
  • Explain why the pressure stays low in the atria compared with pressure in the ventricles.
  • Explain the small spike in atrial pressure at point a.

Heart disease is caused by occlusion in the coronary arteries

  • Blood flow through an artery can be blocked by hard fatty acid deposits, called atheroma, that develop in the arterial wall.
  • Atheroma develops when fatty acids, such as low-density lipids and cholesterol, accumulate in the artery, and are incorporated by phagocytosis, underneath the smooth muscle layer.
  • The disease resulting from vascular occlusion is called atherosclerosis. Some of the risk factors are advanced age, family history, smoking, obesity, diabetes or a high sugar diet.
  • Atherosclerosis causes the arteries to become less flexible. The heart has to pump harder to supply oxygen to the tissues beyond the occlusion. This means that most sufferers also have hypertension.
  • Like all tissues, there is a steady blood supply to the walls of the heart by way of coronary arteries.
  • When coronary arteries are partially blocked, the chest pain that results is called angina. Some forms of angina are more severe than others.
  • When a small part of the heart muscle isn’t able to get enough oxygen because of the occlusion, a myocardial infarction (heart attack) occurs.

Figure 6.2.2c - Occlusion in coronary arteriesFigure 6.2.2c – Occlusion in coronary arteries
When coronary arteries cannot supply blood to the heart, a heart attack occurs.

Nature of Science: William Harvey and circulation

William Harvey was an English physician who first demonstrated that blood circulation relies on the heart as a pump. His methods were diverse:

  • Using deductive reasoning, he refuted the long-standing belief that the liver produces new blood. He calculated the volume of blood that the liver would need to produce in order for that to be true, and found the value to be unrealistic.
  • Using empirical evidence, he demonstrated the unidirectional nature of blood flow.
  • Using theoretical models, he predicted the existence of capillaries that join arteries to veins.
  • Harvey’s ideas overturned long-held beliefs that were attributed to the Greek philosopher/physician known as Galen of Pergamon. You will find a quick comparison in the following table.
Galen Harvey
Blood is formed in the liver from ingested food, is pumped through the right side of the heart, and is consumed daily. Blood is pumped through two sides of the heart and recycled over and over again.
The left and right sides of the heart are separated by a porous septum. The left and right sides of the heart are completely separate.
Blood does not circulate in one direction, but rather moves by ‘ebb and flow’. Blood flow is unidirectional.
Air diffuses in and out of the blood at the ends of veins and arteries. Predicted the existence of capillaries (though he couldn’t see them – he had no microscopes).


Figure 6.2.2d – Harvey blood flowFigure 6.2.2d – Harvey blood flow
William Harvey used this simple experiment to show that backflow is prevented in the circulatory system.

Concept help

  • Normally, the cardiac cycle is described beginning with the filling of the atria (ventricular diastole). However, as a cycle, it has no beginning to speak of!
  • Blood pressure is a measure of the pressure exerted by the blood on the walls of arteries. For example, a blood pressure of 120/80 (normal blood pressure) means that during the ventricular systole, 120mmHg of pressure is put against the arterial wall. During the diastole, it is 80mmHg.
  • The heartbeat makes a characteristic ‘lub-dub’ sound. The first sound (lub) is made when the atrioventricular valves close. The second sound (dub) happens when the semi-lunar valves close. Locate ‘lub’ and ‘dub’ on the phonocardiogram line in Figure 6.2.2b.

Language help

  • Systole and diastole are synonymous with muscular contraction and relaxation, respectively.

Figure 6.2.2e - William Harvey (1578–1657)Figure 6.2.2e – William Harvey (1578–1657)
William Harvey published his theories in De Motu Cordis (‘On the Motion of the Heart and Blood’) in 1628.

Nature of Science

Theories are generally regarded as uncertain and likely to be refined. William Harvey overturned theories developed by the ancient Greek philosopher Galen on movement of blood in the body. 

Figure 6.2.2f - GalenFigure 6.2.2f – Galen (c. AD200)
Galen held ideas on blood and circulation that were part of a greater philosophy of medicine common until the Renaissance.

Course link

In Page 6.2.1 you learned how fatty acids are deposited into the bloodstream as chylomicrons. Chylomicrons eventually become differentiated into different types of fatty acid and this may lead to atherosclerosis.

6.2.3 Thrombosis, penicillin and HIV

We have learned in Page 6.1.3 how the body defends itself against pathogens. Now let’s examine some applications involving disease.

Causes and consequences of coronary thrombosis

  • When a blood clot forms inside an artery or vein, it is called thrombosis. A blood clot may grow in one area or detach and move through the blood vessels as an embolus.
  • Coronary blood clots are more likely to form where the arterial wall has been damaged, or where there is a restriction in blood flow due to atherosclerosis (Page 6.2.2) because when blood flow is slowed it is more likely to coagulate.
  • Coronary thrombosis can lead to myocardial infarction (heart attack) if the oxygen supply to the cardiac muscle is blocked by the occlusion.
  • Risk factors for coronary thrombosis are: family history of atherosclerosis, diabetes, high blood pressure, high blood cholesterol, smoking.

Figure 6.2.3a - ThrombusFigure 6.2.3a – Thrombus
Coronary thrombosis develops where the artery is damaged or blocked by atherosclerosis.

Florey and Chain’s experiments with penicillin

  • 1928: Alexander Fleming accidentally discovers penicillin after his bacterial cultures become contaminated by mould of the genus Penicillum.
  • 1930s: Working at Oxford, Howard Florey and Ernst Chain develop techniques that isolate penicillin in sufficient quantities to permit an investigation into its antibiotic properties.
  • 1940: Florey and Chain inject eight mice with a lethal dose of Streptococcus bacteria. Four mice are treated with penicillin, four are not. By the following day, the untreated mice are dead. The treated mice survive.
  • 1941: Human trials begin. The first patient dies because there is not enough antibiotic to finish his treatment. Penicillin proves to be very difficult to isolate and stabilise.
  • By 1944, enough penicillin is available to treat wounded troops returning from the war. Penicillin becomes the first widely used antibiotic.

The effects of HIV on the immune system

  • A virus is composed of genetic material surrounded by diverse proteins, and a simple membrane envelope.
  • Antibiotics prevent bacterial metabolism by interfering with different enzymatic processes.
  • Viruses lack metabolism; they need a living host to perform any enzymatic processes, so they cannot be treated with antibiotics.
  • HIV is a retrovirus, meaning it has RNA as its genetic material and it uses reverse transcriptase to make DNA in the host cell.

Figure 6.2.3b – Structure of the human immunodeficiency virus (HIV)Figure 6.2.3b – Structure of the human immunodeficiency virus (HIV)

  • Transmission of HIV occurs when there is blood-to-blood contact, particularly in these situations:
    • sharing hypodermic needles (for intravenous drug use)
    • tainted blood transfusions
    • sexual activity in which the mucous membranes are torn
    • mother-to-child transmission (during pregnancy, childbirth or through breast milk).
  • HIV infects lymphocytes, specifically T-cells, of the immune system, lowering the body’s ability to produce antibodies and fight disease.
  • With immunity and defence lowered, common infections become very difficult to treat. In addition, a person whose immune system is lowered can be infected with very rare diseases that would not penetrate an uncompromised immune system.
  • When an HIV positive patient suffers from multiple infections at the same time, they are diagnosed with acquired immune deficiency syndrome (AIDS).

International mindedness

Without intervention, the rate of HIV transmission from mother to child can be as high as 45%. With intervention, it can be reduced to 5%. The WHO has a programme in place that aims to eliminate mother-to-child transmission completely by 2015.

Figure 6.2.3c - Prevention mother-to-child transmission Figure 6.2.3c – Prevention of mother-to-child transmission
Stages in the prevention of mother-to-child transmission (PMTCT) as outlined by the WHO

Review concepts

  • Clotting factors cause blood to coagulate into blood clots Page 6.1.3
  • Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells > Page 5.1.2
  • Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains of bacteria have multiple resistance > Page 5.1.2
  • Viruses cannot be treated with antibiotics.

Language note

  • Do not confuse coronary thrombosis (the formation of a blood clot in an artery) and atherosclerosis (the narrowing of an artery due to the thickening of the arterial wall). It is more likely that a blood clot will form where the artery is narrowed by atherosclerosis, but these are two different phenomena.

Figure 6.2.3c - Deep vein thrombosisFigure 6.2.3d – Deep vein thrombosis
Thrombosis can develop in any vein or artery. Blood clots can grow very large in a deep vein, then detach to block vessels in another part of the body. This is called an embolism.

Consider this

  • Some people are more susceptible to thrombosis because their blood has a high proportion of clotting factors and clots more easily. The condition is called hypercoagulability.
  • There is a much higher risk of developing deep vein thrombosis for women who smoke while taking birth control pills. Deep vein thrombosis can dislodge to cause an embolism in the brain (stroke) or a heart attack.
  • The effect of penicillin on world history cannot be overstated. Florey and Chain’s first human patient died from an infection he developed after being cut by a thorn of a rose .

Figure 6.2.3d - Testing antibioticsFigure 6.2.3e – Testing antibiotics
The efficacy of antibiotics is assessed by measuring the size of the ‘halo’ in which bacteria do not grow around the drug.

Nature of Science

Risks associated with scientific research: current protocols regarding safety and ethics would have precluded Florey and Chain’s experiments with mice.

Figure 6.2.3f - Fleming, Florey and ChainFigure 6.2.3e – Fleming, Florey and Chain
Alexander Fleming discovered penicillin in 1928. Howard Florey and Ernst Chain discovered ways to isolate and produce penicillin in large quantities. The three scientists shared the Nobel Prize in 1945.

Course link

6.2.4 Practical 6: Lung cancer, emphysema and the effect of exercise on ventilation

Lung cancer and emphysema are two fatal diseases caused by cigarette smoking and other factors.

Causes and consequences of lung cancer

  • Lung cancer develops in the cells that line the bronchi, as well as in the alveoli. There is a high mortality rate compared with other cancers.
  • By the time symptoms appear, the cancer is often well developed or metastasised. Symptoms include shortness of breath, fatigue, weight loss, chest pain, and coughing up blood.

Figure 6.2.4a – Lung cancer mortality rates are very highFigure 6.2.4a – Lung cancer mortality rates are very high

  • Major causes of lung cancer are long-term tobacco use, second-hand smoke, exposure to asbestos or particulate matter from the workplace, and atmospheric exposure to air-borne pollutants such as radon gas.
  • Lung cancer develops quickly through the stages set out in the following table.
Stage 0 The carcinoma is in situ and has not affected nearby tissue. Carcinoma may be removed by surgery.
Survival rate >75%
Stage I The tumour is small and has affected nearby tissues (early stage cancer). Treatment is removal by surgery followed by radiation or chemotherapy.
Survival rate 55–75%
Stages II and III The cancer has grown, infected other tissues, and has moved into the lymphatic system, but not yet metastasised. Treatment is radiation to shrink the tumour followed by surgery, or a combination of radiation and chemotherapy without surgery, as the tumour may be too large to remove safely.
Survival rate 10–50%
Stage IV The cancer has spread to other parts of the body (metastasis). Treatment is unlikely to cure these cancers.
Survival rate <5%

Causes and consequences of emphysema

Figure 6.2.4d – Measurable factors related to spirometryFigure 6.2.4b – Presentation of emphysema at the alveoli

  • Emphysema is a chronic obstructive pulmonary disease. ‘Chronic’ means that damage caused by the disease is irreversible. ‘Obstructive’ means it makes ventilation more difficult physically.
  • The disease develops as the inner surfaces of alveoli became less elastic, weaken and eventually rupture. Large spaces develop where alveoli have ruptured, resulting in a reduced overall surface area for gas exchange.
  • In its initial stages, the disease causes shortness of breath during strenuous exercise. As emphysema progresses, the ‘holes’ in the lungs become larger. Air may become trapped, preventing new air from entering the lungs. Breathing at rest becomes more difficult. At the latest stage, blood oxygen levels can fall to less than 50% of normal levels.
  • Cigarette smoke is the leading cause of emphysema. Long-term exposure to air-borne irritants, including air pollution and second-hand smoke, are other causes. Continued exposure to pollutants worsens the disease.
  • In rare cases, emphysema is caused by an auto-immune response that results from an inherited protein deficiency (alpha 1 anti-trypsin, AAT). AAT prevents certain white blood cells, called neutrophils, from attacking the alveolar lining.

Monitoring ventilation rate in humans (Practical 6)

Figure 6.2.4c – A spirometer measures lung capacityFigure 6.2.4c – A spirometer measures lung capacity

Figure 6.2.4d – Measurable factors related to spirometryFigure 6.2.4d – Measurable factors related to spirometry

  • Tidal volume – the volume of air displaced in a single breath.
  • Residual volume – the volume of air that remains in the lungs after expiration.

The aim of this experiment is to design a lab to investigate the effect of exercise on one aspect of ventilation.

Use this template to help you plan your experiment:

Independent variable At least five different values are necessary. This variable could be categorical (e.g. type of exercise) or continuous. For example, you could measure the ‘level of exertion’ by:
  • heart rate?
  • speed of walking on a treadmill?
  • number of push-ups or sit-ups?
Do you need to perform a pre-lab to determine an appropriate range of values for your independent variable?
Dependent variable This variable must be quantitative:
  • ventilation rate (number of breaths/minute)?
  • tidal volume?
Do you need to calibrate your equipment to an appropriate range of values? Do you need to practise the technique of using the equipment?
Control variables
  • Sex, age, height, weight and fitness level of participants
  • Trial duration, rest time, etc.
Ensure all your participants sign a consent form. Ensure you are not causing the participants harm.
Collecting data
  • How many repeated trials are sufficient? (Normally three or until results are consistent)
  • Do you need to re-calibrate your equipment?
Analysing data
  • Continuous data? Use a line graph.
  • Categorical data? Use a bar graph.
  • Which statistics are appropriate?
Concluding and discussing
  • What is the relationship between the two variables?
  • How reliable is your conclusion?
  • What are some limitations of your experimental design? How could you improve them?

Course link

Cancer is caused by mutations that result in uncontrolled mitosis. Review the cell cycle, control and how tumours form in Page 1.2.6.

Language help

  • In situ means ‘in place’. The tumour is growing at its origin. It hasn’t moved or invaded other tissues yet.
  • Carcinoma is a tumour that grows from epithelial cells.

6.2.4e - ChemotherapyFigure 6.2.4e – Chemotherapy
Chemotherapy kills cells that divide quickly. Cancer cells, but also hair follicles, cells of the intestinal lining, and white blood cells, are affected. That’s why patients often experience hair loss and a weakened immune system.

Nature of Science

Evidence: Epidemiology is the study of risk factors, and patterns of incidence and distribution of disease. It is also concerned with identifying possible control methods for disease and health-related issues. Epidemiological research uses non-experimental evidence-based methods. These methods are evidence-based because they rely on quantitative data, but do not involve controlled experiments (like a clinical trial). Therefore they are known as observational studies. For example:

  • Case studies – two groups (e.g. presence or absence of disease) are compared with respect to possible causes.
  • Cohort study – a group of people are repeatedly observed over a long period of time with respect to certain variables (e.g. lifestyle, progression of disease).
  • Cross-sectional study – data is collected from a subset of population at a specific moment in time (e.g. during an outbreak).

Epidemiological studies have contributed to our understanding of the causes of lung cancer.

Figure 6.2.4f – Lung cancer risk factorsFigure 6.2.4f – Lung cancer risk factors
Risk factors are identified and calculated using epidemiological research.

Further reading

To learn more about the history of cancer research, and especially how epidemiological research is performed and interpreted, see the Pulitzer Prize winning book:
Mukherjee, Siddartha (2010) The Emperor of all Maladies: A biography of cancer. New York: Scribner.

IA

Use this template to help you plan any lab in this course.

Download IA template >

6.2.5 The synapse

  • The junction between two neurons, as well as at the junction between a neuron and a sensor/effector, is called a synapse.
  • The functional unit of a synapse between two neurons consists of an axon terminal (of the presynaptic neuron), a synaptic cleft, and a dendrite (of the postsynaptic neuron).
  • When the axon terminal of a presynaptic neuron is depolarized, calcium ions, Ca2+, diffuse through the membrane into the cell.
  • Calcium signals vesicles of neurotransmitters to fuse with the membrane.
  • Molecules of neurotransmitters are released into the gap between two neurons. At the postsynaptic neuron, they bind to protein receptors.
  • Binding of neurotransmitters changes the permeability of the postsynaptic cell to sodium ions. Sodium ions from the extracellular fluid rush in to depolarize the postsynaptic neuron.
  • The synapse is a chemical signal as opposed to an electrical impulse. Click Figure 6.2.5a to review how the synapse works.

Figure 6.2.5a – The synapseFigure 6.2.5a – The synapse
Neurotransmitters are released at the synapse when presynaptic neurons are depolarized.

Application: Secretion and reabsorption of acetylcholine

  • Acetylcholine, ACh, is a neurotransmitter that operates at neuromuscular junctions and other cholinergic synapses.
  • Figure 6.2.5b shows a neuromuscular synapse. Describe what is happening at each step.

Figure 6.2.5b – A cholinergic neuromuscular synapseFigure 6.2.5b – A cholinergic neuromuscular synapse

  • Acetylcholine is bound to receptors at the postsynaptic cell for a short time. It is quickly broken down into acetic acid and choline by an enzyme called acetylcholinesterase, AChE.
  • AChE prevents excessive firing of nerves by ensuring that only one action potential is propagated through the synapse.
  • Choline molecules are reabsorbed into the presynaptic neuron and are used to form new molecules of ACh.

Figure 6.2.5c - ReabsorptionFigure 6.2.5c – Reabsorption
Acetylcholine is rapidly broken down by acetylcholinesterase.

Application: Neonicitinoid pesticides block acetylcholine receptors

  • Neonicitinoids are organic pesticides that bind permanently to some types of ACh receptors, blocking the attachment of the neurotransmitter at the postsynaptic cell.
  •  Once the pesticide is bound, sodium channels open and remain open. This results in constant firing of the synapse and eventually paralysis and death to the affected insect.
  • Neonicitinoids are less toxic to birds and mammals than older pesticides such as DDT and organophosphates. They were developed during the 1990s and are the most widely used class of pesticides today.
  • There is growing evidence that neonicitinoids are at least partially responsible for declining honeybee and bird populations. The future of these pesticides remains in question.

Skill: Analysing oscilloscope traces showing resting and action potentials

Figure 6.2.5d shows the typical shape of an oscilloscope trace during a nervous impulse.

Figure 6.2.5d – Oscilloscope trace

Figure 6.2.5d – Oscilloscope trace

Language help

  • Cholinergic (adjective) refers to any process that uses or produces acetylcholine (noun), e.g. cholinergic receptor, cholinergic synapse.
  • Acetate’ is another word for acetic acid.
  • Neonicitinoids’ are so named because they are molecules that chemically resemble nicotine. An example is shown in the following diagram:

Figure 6.2.5e - Imadocloprid is a neonicitinoid pesticide that is chemically similar to nicotineFigure 6.2.5e – Imadocloprid and nicotine
Imadocloprid (top) is a neonicitinoid pesticide that is chemically similar to nicotine (bottom).

Science and Science Responsibility (Aim 8)

  • Many psychoactive drugs affect the functioning of a neurotransmitter called dopamine. Discuss some of the social effects of drug abuse.
  • Botox is a neurotoxin that inhibits the release of acetylcholine at neuromuscular junctions, effectively paralysing muscles. Should there be limits on its use for cosmetic purposes? 

Figure 6.2.5f - Botox®Image Fig 6.2.5f – Botox®
Injections of botulinum toxin (Botox®) are very commonly used for cosmetic purposes.

Course link

  • Acetyl CoA is an important molecule for cell respiration. HL students will learn more about it in Page 8.1.2.

International mindedness

How do different governing bodies deal with ambiguous evidence? In 2013, the European Union announced a two-year ban on neonicitinoids. This was to allow for evaluation of scientific evidence related to honeybee populations and other environmental effects. A similar proposal was made in the United States in 2013, but it did not leave Congress.

6.2.6a Therapeutic and clinical uses of hormones

Many health conditions are caused by abnormal hormone function. Hormones are used in a variety of therapies, such as replacement therapies and fertility treatments.

Causes and treatment of Types I and II diabetes

  • Diabetes is actually a group of diseases that cause a person to have consistently elevated blood sugar, either because the body doesn’t respond to insulin, or because the body cannot produce insulin.
  • Type I diabetes is also called early-onset diabetes.
  • Type II diabetes is also called late-onset diabetes.
  • There is no cure for diabetes. When left untreated, diabetes will cause blindness, kidney failure, nerve damage, cardiovascular disease and death.
Type Cause Treatment
I
  • An autoimmune disease, b cells of the pancreas are destroyed by the body’s own immune system.
  • Results in the inability to produce insulin.
  • Symptoms (thirst, hunger, frequent urination, glucose in the urine) arise quickly.
  • Normally diagnosed in childhood ('early-onset').
  • Regular blood glucose level testing and injections of insulin.
  • Insulin injections need to be taken before peaks in blood sugar (i.e. before meals).
II
  • Body produces insulin, but is unable to process or respond to it normally.
  • Symptoms develop slowly and may go unnoticed for years ('late onset').
  • Normally associated with genetic factors, fatty or high-sugar diet, obesity and sedentary lifestyle.
  • Managed through diet: increasing high-fibre foods (which slow digestion) and decreasing high sugar foods (which cause short-term spikes in blood sugar).
  • Weight loss and regular exercise are also advised to improve insulin function.

Leptin and obesity

  • In a healthy body, leptin released by adipose tissue acts on the hypothalamus to reduce appetite. When the blood plasma concentration of leptin falls, appetite is restored. The amount of adipose tissue remains in homeostasis by this negative feedback mechanism.
  • In the 1950s, scientists discovered a strain of mouse with a deficiency in a gene called ‘Ob’. These mice were obese and had insatiable appetites because their body was not producing leptin.
  • Scientists hypothesised that they had found a genetic basis for obesity.
  • Clinical trials using leptin as a therapy for obesity have shown limited success, because while a small proportion of patients have a deficiency, the majority of patients have abnormally high concentrations of leptin.
  • A more current model of the relationship between leptin and obesity is as follows: As more adipose tissue is deposited, more leptin is produced. The hypothalamus is desensitised to high levels of hormone. Appetite is not suppressed, and calorie intake is converted to adipose tissue in a positive feedback loop. Leptin insensitivity could predispose certain individuals to developing obesity.

Figure 6.2.6a.a - Positive feedback loop of an obese body
Figure 6.2.6a.a - Negative feedback loop of a healthy body

Figure 6.2.6a.a – Negative vs positive feedback
Appetite and adipose deposition are linked in positive (left) and negative (right) feedback loops. Top: Obese body. Bottom: Healthy body.

Melatonin and jet lag

  • The body’s ‘internal clock’ is located in the SCN, or suprachiasmatic nucleus, which is directly connected by neurons to the retina.
  • The SCN maintains a circadian rhythm of sleep and wakefulness approximately equal to 24 hours. External stimuli synchronise the clock to the natural day/night cycle.
  • Light activates photoreceptors on the retina, which synchronises the SCN to daytime. In the absence of light, the pineal gland produces a hormone, called melatonin, which synchronises the SCN to night.

Figure 6.2.6a.b – Normal melatonin profile in an adultFigure 6.2.6a.b – Normal melatonin profile in an adult

  • Melatonin promotes sleep. Production starts in the evening and peaks at night. The hormone is rapidly removed from the blood by the liver in the morning.
  • When you travel across a number of time zones very quickly (i.e. on a jet), circadian rhythms become temporarily desychronised. It can take up to five days for external stimuli to resynchronise the SCN to a new day/night cycle.
  • Sleep disturbance, indigestion, fatigue and irritability are all symptoms of ‘jet lag’, or desynchronosis.
  • Melatonin supplements may be taken to help normalise sleep patterns.

Figure 6.2.6a.c - Blood glucose levelFigure 6.2.6a.c – Blood glucose level
People with diabetes must monitor their blood glucose levels closely.

Did you know?

  • A third type of diabetes, called gestational diabetes, may develop in pregnant women during the last trimester of pregnancy. Normally, it subsides four to six weeks after the birth of the baby.
  • Type II diabetes is a growing problem all over the world, and the typical patient is getting younger. High fat and sugar diets, and lack of exercise are to blame.

Figure 6.2.6a.d - Fat miceFigure 6.2.6a.d – Fat mice 
Leptin deficiency causes obesity in mice.

Concept help

  • A circadian rhythm is any biological process that displays an internal, and trainable, cycle of approximately 24 hours. The best understood example is the sleep–wake cycle, but daily cycles of body temperature and bowel movement are other examples.
  • The retina is the photosensitive layer of cells of the eye.

Language help

  • Circadian, from the Latin circa = approximately and diem = day.

Figure 6.2.6a.e - Adjusting flying east/westFigure 6.2.6a.e – Adjusting flying east/west
It is easier for the SCN to resynchronise after east-to-west travel than west-to-east travel. Suggest a reason why.

Figure 6.2.6a.f - Sleepy teenFigure 6.2.6a.f – Sleepy teen
In teenagers, melatonin production starts and peaks later in the night than in adults. There is a reason why you are so sleepy at school!

6.2.6b The mystery of reproduction: IVF and William Harvey

  • IVF is a form of assisted reproduction that is available to people who are unable to conceive naturally due to infertility.
  • IVF involves using drugs to suspend the normal secretion of hormones, followed by artificial doses of hormones to induce superovulation. Eggs are harvested and fertilised outside of the body, and then implanted in the uterus to establish pregnancy.
  • William Harvey’s study of reproduction in deer is related to many themes in the nature of science.

IVF - in vitro fertilisation

One cycle of IVF takes between four and six weeks. The success rate for women under 35 is about 40%. For women over 35, it is about 20%.

Figure 6.2.6b.a - The IVF processFigure 6.2.6b.a – The IVF process

Hormonal control is essential during all steps:

Ovarian hyperstimulaton
  • The menstrual cycle is suspended with drugs that suppress the release of LH and FSH from the pituitary gland. (This allows follicular development to be better controlled and timed.)
  • Over the next week or two, high-dose intramuscular injections of FSH are administered to stimulate the development of many follicles at the same time.
Egg retrieval
  • When the follicles are large enough, another hormone, called hCG, is administered. hCG stimulates ovulation because it is functionally and chemically similar to LH.
  • Eggs are retrieved before they are released exactly 36 hours after hCG injection. The patient is under general anaesthetic.
  • Fluid from the ovaries is removed directly using a guided needle. Up to 12 eggs may be removed at once.
Fertilisation and culture
  • Not all the eggs are mature, or viable.
  • Viable eggs are fertilised under a dissecting microscope with a small needle.
  • The fertilised eggs are incubated until they reach a blastocyst stage (five or six days).
Transfer and implantation
  • Blastocysts are implanted directly into the uterus. No anaesthetic is needed.
  • Sometimes, the patient may choose to implant more than one embryo to increase the chance of a successful pregnancy.

William Harvey’s investigation of reproduction in deer

  • In the 17th century and 18th centuries, there was an important debate on the study of reproduction. Some scientists believed in preformationism, whereby an embryo developed from particles that were simply smaller versions of the adult animal.
  • On the other side of the debate was a more developmental philosophy called epigenesis, which proposed that an embryo takes on human characteristics as it develops through a process of differentiation. In Ancient Greece, Aristotle proposed this philosophy. He believed sperm when mixed with menstrual blood held all the material for development. The female provided the ‘fertile ground’ for the male ‘seed’.
  • William Harvey was educated in Aristotle’s philosophy, but after studying chicken embryos developing in eggs, carefully documenting differentiation and the stages of embryonic development, he wanted to prove that omne vivum ex ovum, or ‘every living thing comes from an egg’, including mammals and humans.
  • He thought it would be easier to observe eggs in the uterus of large mammals, so he slaughtered deer that had recently mated, and opening the uterus, he searched for eggs or other early signs of embryonic differentiation. He was not able to observe anything in the initial stages.
  • Harvey was not able to identify eggs because he was limited by his equipment – a simple hand lens. Microscopes allowed the observation of the first sperm cell in 1877, and the first mammalian egg nearly 200 years after Harvey, in 1826.

Nature of Science

Developments in scientific research follow improvements in apparatus: William Harvey was not able to visualise sperm or egg cells, as the microscope was invented 17 years after his death.

Figure 6.2.6b.b - Dissecting microscopeFigure 6.2.6b.b – Dissecting microscope
In IVF, dissecting microscopes allow the technician to manipulate the egg and sperm while observing them.

Figure 6.2.6b.c - In vitro fertilisationFigure 6.2.6b.c – In vitro fertilisation

Figure 6.2.6b.d - A blastocystFigure 6.2.6b.d – A blastocyst

Concept help

  • Infertility may be the result of a low sperm count or motility in the male partner, or fibroid polyps or other obstructions in the fallopian tubes of the female partner.
  • Natural fertility in women declines rapidly after the age of 35.
  • Single women or same-sex couples without fertility issues would probably use IVF only after attempting less invasive procedures (such as intrauterine insemination, also known as ‘artificial’ insemination).

Course link

  • Human chorionic gonadotropin, hCG, is naturally produced during pregnancy. HL students will learn more about it in Page 11.1.4.

Language help

  • In vitro is Latin for ‘in glass’. Fertilisation takes place in a petri dish.
  • In vivo is Latin for ‘in the living’. Fertilisation takes place in the fallopian tube.
  • Epigenetics is the (modern) branch of biology concerned with inheritable processes that do not directly involve DNA. Do not confuse this term with epigenesis, which is an (outdated) Aristotelian theory of development.

Figure 6.2.6b.e - PreformationistFigure 6.2.6b.e – Preformationist
Nicholas Hartsoeker (1656–1725) was a preformationist who imagined that sperm were made of tiny ‘men’. He made this drawing in 1695.

TOK

The ‘preformation–epigenesis’ debate has a modern face: the ‘nature–nurture’ debate. How far have we come in resolving it?