AQA GCSE Biology (8461) — Biological Organisation
A single cigarette smoked by a pregnant woman can deprive her developing fetus of optimal oxygen for up to minutes. Cigarette smoke contains thousands of chemicals, but two specific substances—carbon monoxide and nicotine—cause significant and distinct harm to an unborn baby.
Carbon monoxide () is a toxic gas that crosses the placenta, the organ that provides the fetus with nutrients and removes waste. Inside the bloodstream, carbon monoxide binds preferentially to haemoglobin in red blood cells, forming a stable complex called carboxyhaemoglobin.
Because carbon monoxide has an affinity for haemoglobin to times greater than oxygen, it drastically reduces the oxygen-carrying capacity of the mother's blood. Consequently, less oxygen diffuses into the fetal bloodstream. In regular smokers, up to of haemoglobin may be bound to carbon monoxide rather than oxygen.
Oxygen is essential for aerobic respiration. With less oxygen available, the fetal cells release less energy, which severely restricts fetal growth and development.
Nicotine is an addictive stimulant that acts as a vasoconstrictor, meaning it narrows the lumen of blood vessels in the mother and the umbilical cord. This reduces the overall volume of blood reaching the placenta.
A restricted blood supply limits the delivery of essential nutrients, such as glucose and amino acids, to the fetus. This combined lack of oxygen and nutrients is the primary cause of restricted growth and low birth mass, with babies of smokers weighing to less on average.
Smoking also places cardiovascular strain on the baby by increasing the fetal heart rate. The risk of stillbirth rises significantly; non-smoking mothers have a risk, whereas this increases to roughly for heavy smokers (accounting for an estimated of all UK stillbirths).
When a pregnant woman drinks alcohol, the active chemical, ethanol, crosses the placenta via passive diffusion directly into the fetal bloodstream. The developing fetus is highly vulnerable because its liver is immature and lacks the specific enzymes necessary to metabolise or break down the ethanol.
As a result, alcohol remains in the fetal system longer and acts as a teratogen—a substance that interferes with embryonic development and causes irreversible damage. Damage to organs is most severe during the first trimester, but brain development is affected throughout the entire pregnancy. Medical advice strongly recommends total abstinence, as there is no established safe limit.
Heavy or frequent drinking can lead to Fetal Alcohol Syndrome (FAS), a cluster of permanent physical and mental birth defects. Physical symptoms include characteristic facial features (such as a smooth philtrum and narrow eyes), stunted height, and microcephaly, a condition where the baby's head and brain are abnormally small.
FAS also causes severe, lifelong mental and developmental issues. These include lower IQ, learning disabilities, and behavioural problems such as hyperactivity, impulsiveness, and poor social judgement.
Students often confuse the effects of carbon monoxide and nicotine. Remember: Carbon monoxide causes oxygen deprivation, while nicotine causes vasoconstriction (narrowing of blood vessels).
For "Explain" questions on smoking, always build a step-by-step causal chain: CO binds to haemoglobin → reduces oxygen-carrying capacity → less oxygen reaches fetus → less aerobic respiration → less energy released for growth.
If an exam question asks WHY alcohol affects a fetus more severely than the mother, you must explicitly state that the fetal liver is immature and lacks the enzymes to metabolise (break down) the alcohol.
Use accurate scientific terminology for outcomes: write "restricted growth" or "low birth mass" rather than simply saying the mother will have a "small baby".
Carbon monoxide
A toxic, colourless, and odourless gas found in cigarette smoke that binds to haemoglobin, reducing oxygen transport.
Placenta
The organ that provides the fetus with oxygen and nutrients from the mother's blood and removes waste products.
Haemoglobin
The red pigment in red blood cells that normally binds to oxygen to transport it around the body.
Carboxyhaemoglobin
A stable complex formed in red blood cells when carbon monoxide binds to haemoglobin, preventing it from carrying oxygen.
Aerobic respiration
The process of releasing energy from glucose using oxygen, which is essential for fetal growth.
Nicotine
An addictive stimulant found in tobacco that causes the narrowing of blood vessels.
Vasoconstriction
The narrowing of the lumen of blood vessels, which reduces blood flow to the placenta.
Low birth mass
A clinical outcome where a baby is born weighing significantly less than average, often due to restricted nutrient and oxygen delivery.
Ethanol
The specific chemical agent in alcoholic drinks that acts as a toxin to the developing fetus.
Passive diffusion
The movement of molecules like ethanol across the placenta from an area of high concentration to an area of low concentration without requiring energy.
Teratogen
A substance that interferes with normal embryonic development, causing physical or mental abnormalities.
Fetal Alcohol Syndrome (FAS)
A cluster of permanent, irreversible physical and mental birth defects caused by maternal alcohol consumption during pregnancy.
Microcephaly
A physical condition where a baby's head is much smaller than expected due to abnormal brain development.
Put your knowledge into practice — try past paper questions for Biology
Carbon monoxide
A toxic, colourless, and odourless gas found in cigarette smoke that binds to haemoglobin, reducing oxygen transport.
Placenta
The organ that provides the fetus with oxygen and nutrients from the mother's blood and removes waste products.
Haemoglobin
The red pigment in red blood cells that normally binds to oxygen to transport it around the body.
Carboxyhaemoglobin
A stable complex formed in red blood cells when carbon monoxide binds to haemoglobin, preventing it from carrying oxygen.
Aerobic respiration
The process of releasing energy from glucose using oxygen, which is essential for fetal growth.
Nicotine
An addictive stimulant found in tobacco that causes the narrowing of blood vessels.
Vasoconstriction
The narrowing of the lumen of blood vessels, which reduces blood flow to the placenta.
Low birth mass
A clinical outcome where a baby is born weighing significantly less than average, often due to restricted nutrient and oxygen delivery.
Ethanol
The specific chemical agent in alcoholic drinks that acts as a toxin to the developing fetus.
Passive diffusion
The movement of molecules like ethanol across the placenta from an area of high concentration to an area of low concentration without requiring energy.
Teratogen
A substance that interferes with normal embryonic development, causing physical or mental abnormalities.
Fetal Alcohol Syndrome (FAS)
A cluster of permanent, irreversible physical and mental birth defects caused by maternal alcohol consumption during pregnancy.
Microcephaly
A physical condition where a baby's head is much smaller than expected due to abnormal brain development.
