IGCSE BIOLOGY TOPIC 15 Drugs

Summary notes

A drug is a substance that, when taken into the body, has an effect on the chemical reactions
that take place. There are a variety of different drugs which treat different diseases.
Antibiotics:
Antibiotic drugs are used to treat bacterial infections. Some antibiotics kill bacteria by
destroying their cell wall, leading to the cell bursting, whilst others inhibit the growth of the
bacteria. Viruses cannot be killed by antibiotics as they do not grow and reproduce in the same
way as bacteria, and do not have the same structure.
Some bacterial strains become resistant to antibiotics as a result of natural selection:

  1. A mutation occurs in a bacterial cell which makes it resistant to an antibiotic.
  2. When that antibiotic is administered, this cell is not killed, whereas cells which have not
    become resistant are killed.
  3. The resistant cell can therefore survive and reproduce, producing more resistant
    bacteria.
    Resistance to antibiotics results in antibiotic resistant bacterial infections in hospitals such as
    MRSA (methicillin-resistant Staphylococcus aureus). It is therefore important to try and slow
    the development of resistant bacterial strains. This can be done by only using antibiotics for
    serious infections, and always completing the full course of antibiotics to make sure that all
    the bacteria is killed.
    Misused drugs
    Alcohol and heroin:
    Alcohol and heroin are both depressants, meaning that they lower the rate of nervous impulses
    by blocking synapses. This means that reactions are slower. They also lower self-control,
    which can lead to increased crime rate and antisocial behaviour.
    These drugs also cause the release of neurotransmitters such as dopamine into synapses in the
    reward pathway which causes a ‘high’. This can be addictive and thus lead to withdrawal
    symptoms if the person stops taking the drug, such as anxiety, insomnia, headaches and
    nausea.
    They can also lead to other medical problems:
    ● Heroin is usually injected, thus infections such as HIV are common from sharing dirty
    needles.
    ● Excessive alcohol consumption leads to liver damage. The liver usually breaks down
    alcohol and other toxins.

    ● Performance-enhancing drugs:
    Some drugs are used to enhance sporting performance. In competitive sports, these drugs are
    seen as unfair and are usually banned, with those using them being disqualified.
    ● Anabolic steroids – anabolic steroids trigger the release of hormones which promote
    muscle mass and strength. Different types of steroids target different muscle groups. In
    2018, the Russian Winter Olympic team was disqualified from competing for taking
    anabolic steroids to enhance their performance.
    ● Testosterone – testosterone is a hormone which enhances athletic performance by
    improving muscle strength and size, as well as increasing energy levels and hand-eye
    coordination.
    Smoking
    Smoking and diseases:
    Smoking is addictive due to a chemical called nicotine which is inhaled with the cigarette smoke
    and causes the release of dopamine. This leads to long-term smoking habits, which have been
    linked to many diseases:
    ● Chronic obstructive pulmonary disease (COPD) – a group of diseases that decrease the
    efficiency of gas exchange in the lungs by damaging the alveoli, hence decreasing the
    surface area for diffusion, and causing inflammation in the airways. COPD causes
    breathlessness, a persistent cough and frequent chest infections. The condition cannot
    be cured, although the progression can be slowed, and symptoms treated.
    ● Lung cancer – one of the more serious forms of cancer, for which smoking accounts for
    over 85% of cases. This is because cigarette smoke contains a variety of toxic chemicals,
    many of them carcinogens.
    ● Coronary heart disease – smoking puts a strain on the heart due to the nicotine and
    carbon monoxide breathed in. Carbon monoxide is dangerous as it displaces the oxygen
    bound to haemoglobin and binds to it instead. This means that there is less oxygen
    present in the blood so the heart must work harder to supply it to the tissues. There is
    also an increased risk of blood clots, and other chemicals can damage arteries. Coronary
    heart disease can lead to heart attacks and heart failure.

    Although smoking is a major risk factor for all of these diseases, it does not mean that every
    smoker will develop these diseases. There are a variety of other factors that alter the
    probability of having one, such as age, fitness and amount/length of time smoking.
    Components of cigarettes:
    There is a toxic mix of over 7000 chemicals in every cigarette, many of which are poisons and
    carcinogens:
    ● Nicotine – addictive, causes high heart rate and blood pressure, and also triggers the
    release of adrenaline.
    ● Tar – tar, when inhaled, sticks to the cilia of cells in the lungs which usually transport
    mucus away from the lungs to protect them from infections. Tar prevents them from
    doing this, which is why smokers are more susceptible to chest infections. In addition, a
    build-up of tar can narrow airways.
    ● Carbon monoxide – Carbon monoxide decreases the amount of oxygen in the blood,
    thus putting a strain on the heart to supply more.
    Other chemicals include arsenic (used in rat poison), formaldehyde (poisonous) and hydrogen
    cyanide (chemical used to kill ants)

main notes

medicinal drugs

Medicinal drugs

Antibiotics:

Antibiotics attack bacteria in a variety of ways

  • Disrupt the production of the cell wall and so prevent the bacteria from reproducing, or even cause them to burst open.
  • Interfere with protein synthesis and thus arrest bacterial growth.

Animal cells do not have cell walls, and the cell structures involved in protein production are different. Consequently, antibiotics do no damage human cells although they may produce some side effects such as allergic reactions.

Development of resistant bacteria:

  • If a course of antibiotics is not completed, some of the bacteria it is being used to destroy will not be killed, but will have been exposed to the drug..
  • Some of the survivors may be drug-resistant mutants. When they reproduce, all their offspring will have the drug resistance, so the antibiotic will become less effective.
  • MRSA (methicillin-resistant Staphylococcus aureus). A type of bacteria that has developed resistance to a number of widely used antibiotics.
  • Development of this can be minimised by using antibiotics only when essential and ensuring treatment is completed.

Antibiotics and viral diseases:

  • Antibiotics are not effective against viral diseases.
  • This is because antibiotics work by disrupting structures in bacteria such as cell walls and membranes, or processes associated with protein synthesis and replication of DNA.
  • Viruses have totally different characteristics to bacteria, so antibiotics do not affect them.

15.3) Misused drugs

Effects of excessive alcohol consumption and abuse of heroin:

  • Powerful depressant drugs
  • Effect on reaction times and self-control
  • Addiction and withdrawal symptoms
  • Negative social implications, eg. crime

How heroin affects the nervous system:

  • Produces it effects by interacting with receptor molecules at synapse.
  • Heroin mimics the transmitter substances in synapses in the brain, causing the release of dopamine (a neurotransmitter), which gives a short lived ‘high’.
  • Injecting heroin can cause infections such as HIV.
  • Excessive alcohol consumption can cause liver damage.
  • Tobacco smoking can cause chronic obstructive pulmonary disease (COPD), lung cancer and coronary heart disease.
  • Liver is the site of breakdown of alcohol and other toxins.

Evidence for a link between smoking and lung cancer:

There are at least 17 substances in tobacco smoke known to cause cancer in experimental animals, and it is now thought that 90% of lung cancer is caused by smoking.

  • Nicotine, the addictive component of tobacco smoke, produces an increase in the rate if the heartbeat and a rise in blood pressure and can cause an erratic and irregular heart beat.
  • Tar in cigarette smoke is thought to be the main cause of lung cancer in smokers.
  • Carbon monoxide permanently binds with haemoglobin in red blood cells, reducing the smoker’s ability to provide oxygen to respiring cells. This results in a smoker getting out of breath more easily and it reduces physical fitness.

The nicotine and carbon monoxide increase the tendency for the blood to clot and so block the coronary arteries.

Performance-enhancing hormones:

Testosterone

  • Is made in the testes of males and is responsible for promoting male primary and secondary sexual characteristics.
  • Taking testosterone supplements (known as ‘doping’) leads to increased muscle and bone mass.
  • Which can enhance a sports person’s performance.

Anabolic steroids

  • Are synthetic derivatives of testosterone.
  • They affect protein metabolism, increasing muscle development and reducing body fat.
  • Athletic performance is thus enhanced.
  • There are serious long-term effects of taking anabolic steroids: sterility, masculinisation in women, and liver and kidney malfunction.

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