Cell Biology
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Cell Structure: Plant vs Animal Cells 20 questions
The diagram shows a generalised animal cell, with four structures labelled A-D.
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The diagram shows a generalised plant cell, with five structures labelled A-E.
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State two structures that are found in a plant cell but are NOT found in an animal cell.
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Any two of: the cell wall, a large permanent vacuole, and chloroplasts. Both cell types share a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes, but only plant cells have these three additional structures.
Answer the following about plant and animal cell structure.
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State the function of each of the following organelles.
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Root cells (found underground) and leaf palisade cells (found inside a leaf) are both plant cells, and both contain a nucleus, cytoplasm, a cell membrane and a cell wall.
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Explain why the cell is described as “the basic unit of life”, according to cell theory.
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Cell theory states that all living organisms are made up of one or more cells, and that the cell is the smallest unit that can carry out all the processes needed for life (such as respiration, growth and reproduction) on its own. Because nothing smaller than a complete cell can independently carry out all of these life processes, the cell is considered the basic (smallest) structural and functional unit of every living thing.
A student wants to compare the structures visible in plant cells and animal cells by preparing and observing an onion epidermis cell sample (stained with iodine solution) and a human cheek cell sample (stained with methylene blue) under a light microscope.
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When preparing a wet mount slide of cheek cells, a student notices dark, round bubbles trapped under the coverslip that could easily be mistaken for cell structures.
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A student claims: “All plant cells contain chloroplasts, and all animal cells lack a cell wall.” They plan to test this by examining prepared slides of 6 different plant tissues and 6 different animal tissues under a microscope.
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The table shows which structures were observed in two unidentified cell samples viewed under a microscope.
| Structure | Sample 1 | Sample 2 |
|---|---|---|
| Nucleus | ? | ? |
| Cell wall | ? | ? |
| Chloroplast | ? | ? |
| Large (permanent) vacuole | ? | ? |
| Mitochondria | ? | ? |
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The table shows the number of chloroplasts counted in five different plant cells viewed under a microscope.
| Cell | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Chloroplasts counted | 30 | 28 | 32 | 26 | 29 |
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$$ \text{mean} = \frac{30+28+32+26+29}{5} = \frac{145}{5} $$
Answer: the mean number of chloroplasts per cell is $29$.
The table shows the number of nuclei counted in each of five plant cells viewed under a microscope.
| Cell | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Nuclei counted | 1 | 1 | 1 | 2 | 1 |
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The table shows the diameter of the nucleus, measured in five different cells from the same tissue sample.
| Cell | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Nucleus diameter (?m) | 8.2 | 8.5 | 8.1 | 8.4 | 8.3 |
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The table shows whether certain structures were seen in three different cell samples.
| Structure | Onion epidermis cell | Elodea (pondweed) leaf cell | Human cheek cell |
|---|---|---|---|
| Nucleus | ? | ? | ? |
| Cell wall | ? | ? | ? |
| Chloroplast | ? | ? | ? |
| Vacuole | ? (large) | ? (large) | ? |
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A textbook states: “plant cells are always bigger than animal cells.” A student collects data: the mean diameter of 10 onion epidermis cells measured $120\,\mu\text{m}$; the mean diameter of 10 human cheek cells measured $40\,\mu\text{m}$.
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Four students each examined the same prepared slide of leaf tissue and counted how many of 50 cells in their field of view contained visible chloroplasts.
| Student | A | B | C | D |
|---|---|---|---|---|
| Cells with chloroplasts (out of 50) | 38 | 41 | 12 | 39 |
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A: $\dfrac{38}{50}\times100=76\%$ B: $\dfrac{41}{50}\times100=82\%$ C: $\dfrac{12}{50}\times100=24\%$ D: $\dfrac{39}{50}\times100=78\%$
Averaging several independent counts reduces the effect of any one student's individual error or an unrepresentative field of view, giving a more reliable overall estimate than relying on a single count. Excluding Student C's anomalous result:
$$ \text{mean} = \frac{76+82+78}{3} = \frac{236}{3} = 78.7\% \ (\text{3 s.f.}) $$
Scientists can now grow “cultured” (lab-grown) meat by taking a small sample of muscle cells from a living animal and growing them in a nutrient-rich liquid in a lab, without raising and slaughtering a whole animal.
Discuss one benefit and one drawback of this technology.
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Benefit: because a whole animal does not need to be raised and slaughtered, cultured meat could greatly reduce the farmland, water and animal feed currently used for livestock farming, while also reducing animal suffering, all while still producing real animal muscle cells for meat.
Drawback: the technology to grow cells at a large enough scale is currently very expensive and energy-intensive, meaning cultured meat costs far more than farmed meat and is not yet widely available or affordable; some consumers may also be reluctant to eat lab-grown food, limiting how much difference it can make in the short term.
Because a single plant cell can, under the right conditions, divide and develop into a whole new plant (a property most animal cells do not have), scientists use “tissue culture” to grow thousands of genetically identical clones of a high-yielding or disease-resistant crop plant from just a few starting cells.
Discuss one benefit and one drawback of using this technology to produce crops.
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Benefit: tissue culture allows farmers to rapidly produce huge numbers of identical, high-quality plants with desirable traits (such as disease resistance or high yield), much faster and more consistently than traditional breeding, which can improve food security and crop reliability.
Drawback: because all of the cloned plants are genetically identical, a field of clones has no genetic diversity. If a new disease or pest evolves that can attack that particular genetic variety, it could wipe out the entire crop, since no plants in the field would carry natural resistance — unlike a genetically varied population, where some plants might survive and the crop as a whole would not be lost.
Plant cell walls are made mostly of cellulose, a tough carbohydrate. Scientists can break down cellulose from waste plant material (such as stalks and husks left over after harvest) and ferment it to produce bioethanol, a fuel that can replace some petrol/diesel in vehicles.
Discuss one benefit and one drawback of producing fuel this way.
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Benefit: bioethanol is made from renewable plant material, and can use waste crop parts that would otherwise be thrown away. Because the plants absorbed carbon dioxide as they grew, burning this fuel can release less net carbon dioxide than burning a fossil fuel, helping reduce its overall contribution to climate change.
Drawback: breaking down tough cellulose requires significant processing, energy, and often specific enzymes or technology, which is currently expensive. If crops are grown deliberately for fuel rather than only using waste material, this can compete with food production for farmland and water, potentially raising food prices or reducing food security in some regions.
Using Microscopes 20 questions
The diagram shows a light microscope, with five parts labelled A-E.
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Explain why you should always start viewing a specimen under the LOWEST-power objective lens first, before switching to a higher-power lens.
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The lowest-power lens gives the widest field of view, making it much easier to locate the specimen on the slide. It also keeps the objective lens further from the slide, reducing the risk of the lens touching and damaging the slide (and specimen) when first focusing.
A microscope has an eyepiece lens of magnification $\times10$ and is set to use an objective lens of magnification $\times100$. Calculate the total magnification being used.
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Step 1 — State the formula:
$$ \text{total magnification} = \text{eyepiece magnification} \times \text{objective magnification} $$
Step 2 — Substitute and calculate:
$$ 10 \times 100 = 1000 $$
Answer: total magnification $= \times1000$
State the function of each of the following microscope parts.
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Explain why specimens viewed under a light microscope are usually cut into very thin slices before being placed on a slide.
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A light microscope works by shining light up through the specimen and lenses to the eye, so light needs to be able to pass through the specimen. A thin slice lets light through clearly, and avoids overlapping layers of cells sitting on top of one another, which would make the image blurry and much harder to interpret.
A student sets up a microscope with a $\times15$ eyepiece lens and can choose between $\times4$, $\times10$ and $\times40$ objective lenses.
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The highest total magnification uses the eyepiece with the highest-power objective lens available:
$$ 15 \times 40 = 600 $$
Answer: the highest total magnification available is $\times600$.
Explain the difference between magnification and resolution.
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A student wants to investigate whether the length of time a specimen is left in iodine stain affects how clearly its structures can be seen under a microscope.
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A student measures the diameter of an onion cell using a microscope fitted with an eyepiece graticule (a tiny built-in ruler), repeating the measurement on three different cells: $42\,\mu\text{m}$, $58\,\mu\text{m}$, $45\,\mu\text{m}$.
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A school with limited funding builds a simple “smartphone microscope” by taping a small lens over a phone camera, and wants to test whether it is reliable enough to replace their proper light microscopes for measuring cell sizes.
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A drawing of a cell is $20\,\text{mm}$ long. The actual cell is $0.1\,\text{mm}$ long. Calculate the magnification of the drawing.
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$$ \text{magnification} = \frac{\text{image size}}{\text{actual size}} = \frac{20}{0.1} $$
Answer: magnification $= \times200$ (magnification has no unit, since it is a ratio of two lengths).
An image of an onion cell is measured as $15\,\text{mm}$ wide. The drawing was made at a magnification of $\times150$.
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A student draws a red blood cell as part of a scientific diagram, measured below. The actual diameter of a red blood cell is about $0.007\,\text{mm}$.
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$$ \text{magnification} = \frac{\text{image size}}{\text{actual size}} = \frac{60}{0.007} = 8571.43\ldots $$
Answer: magnification $\approx \times8570$ (3 s.f.)
The same cell was drawn three times, at three different magnifications. The table shows the image size measured each time.
| Magnification used | ×100 | ×200 | ×400 |
|---|---|---|---|
| Image size (mm) | 5 | 10 | 20 |
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$$ \text{actual size} = \frac{\text{image size}}{\text{magnification}} $$
$\times100$: $\dfrac{5}{100}=0.05\,\text{mm}$ $\times200$: $\dfrac{10}{200}=0.05\,\text{mm}$ $\times400$: $\dfrac{20}{400}=0.05\,\text{mm}$
All three calculations give the same actual size of $0.05\,\text{mm}$ ($50\,\mu\text{m}$).
Four students each measured the same type of cell at a different magnification, then used the magnification formula to calculate its actual size.
| Student | Image size measured | Magnification used | Calculated actual size |
|---|---|---|---|
| A | 40 mm | ×200 | 0.2 mm |
| B | 4 cm | ×200 | 0.02 mm |
| C | 60 mm | ×300 | 0.2 mm |
| D | 30 mm | ×150 | 0.2 mm |
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Convert first: $4\,\text{cm} = 40\,\text{mm}$.
$$ \text{actual size} = \frac{40}{200} = 0.2\,\text{mm} $$
This now matches the other three students' results of $0.2\,\text{mm}$.
Under a certain magnification, the circular field of view of a microscope has a diameter of $1.5\,\text{mm}$. Approximately $30$ identical plant cells fit side by side across this diameter.
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A student is asked to determine the actual length of a plant cell. They measure their drawing as $8.4\,\text{cm}$ long, using a ruler that can be read to the nearest $0.1\,\text{cm}$. The drawing was made using a microscope set to an eyepiece magnification of $\times10$ and an objective magnification of $\times40$.
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$8.4\,\text{cm} = 84\,\text{mm}$
$$ \text{actual size} = \frac{84}{400} = 0.21\,\text{mm} $$
Answer: actual length $\approx 0.21\,\text{mm}$ (2 s.f.)
Electron microscopes can magnify specimens far more than light microscopes (up to about 2 million times, compared to around 1500 times for a typical light microscope) and can reveal much finer detail, allowing scientists to see structures such as viruses and individual organelles clearly.
Discuss one benefit and one drawback of using an electron microscope compared to a light microscope.
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Benefit: the far higher magnification and resolution of an electron microscope reveals fine detail — such as viruses and the internal structure of organelles — that is completely invisible under a light microscope, hugely advancing medical research and diagnosis.
Drawback: electron microscopes are extremely expensive and require the specimen to be placed in a vacuum, usually after being specially treated or coated, which kills the specimen. This means living processes cannot be observed happening in real time, unlike with a light microscope, and the cost and size of electron microscopes make them unavailable in most schools and smaller laboratories.
Scientists have developed cheap, portable microscope attachments that clip onto a smartphone camera, allowing health workers in remote areas with limited resources to examine blood samples under magnification, photograph what they see, and send the images to specialists elsewhere for diagnosis (for example, of malaria).
Evaluate the impact of this technology, discussing both a benefit and a limitation.
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Benefit: this makes microscopic diagnosis available in remote or low-resource communities that could never afford or access a full laboratory microscope, potentially enabling much faster diagnosis and treatment of diseases such as malaria, and saving lives in areas far from specialist doctors and hospitals.
Limitation: the image quality, magnification and resolution from a simple clip-on lens is far lower than a proper light or electron microscope, so subtle structures or early-stage infections could be missed or misdiagnosed. The system also still relies on a working smartphone, a signal or internet connection to send images, and a specialist being available to interpret them — none of which may be reliably available in the same remote areas that need this technology most.
Before microscopes were invented, nobody could see bacteria, and diseases were often blamed on “bad air” or evil spirits. The invention and improvement of microscopes allowed scientists to see microorganisms directly for the first time, eventually leading to germ theory and much of modern medicine (including vaccines and antibiotics).
Discuss the impact of the invention of the microscope on society, including one benefit and one possible concern or limitation.
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Benefit: directly seeing microorganisms let scientists finally identify the true causes of many diseases, leading to hygiene practices (such as hand-washing and sterilising medical equipment), vaccines and antibiotics that have saved an enormous number of lives and increased average life expectancy worldwide.
Concern/limitation: a light microscope alone still cannot show viruses, which are far too small to see even at its maximum magnification, so some diseases remained mysterious and hard to diagnose until the much later invention of the electron microscope. Access to microscopes, and to the trained scientists needed to use them, has also historically been — and in some poorer regions still is — unequal around the world, meaning the medical benefits of this technology have not reached everyone equally.
Specialised Cells 20 questions
The diagram shows a neuron (nerve cell), with five structures labelled A-E.
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State two ways in which a (mammalian) red blood cell is adapted for carrying oxygen efficiently.
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Any two of: it has no nucleus, leaving more space inside the cell for haemoglobin (and therefore for carrying oxygen); it has a biconcave disc shape, which increases its surface area for gas exchange; and it is packed with haemoglobin, a protein that binds to oxygen.
A sperm cell is specialised for reaching and fertilising an egg cell.
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The diagram shows a root hair cell, with two structures labelled A and B.
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Palisade mesophyll cells are found near the upper surface of a leaf, and are the main site of photosynthesis in a plant.
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Muscle cells are specialised for contraction (movement).
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All the specialised cells you have studied (e.g. nerve cells, sperm cells, root hair cells, red blood cells) develop from unspecialised cells that all originally contained the same genetic information (DNA), through a process called differentiation.
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A student investigates whether the number of root hairs on a plant root affects the rate at which the plant takes up water, using cress seedlings grown in two different conditions (one group with root hairs intact, one group with root hairs gently rubbed off).
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A group of students is given five unlabelled prepared microscope slides, each showing a different specialised cell type (for example, a nerve cell, a red blood cell, a root hair cell, a sperm cell and a palisade cell), and must identify each cell type from its structure alone.
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A student hypothesises that plants grown in drier soil develop MORE root hairs per unit length of root than plants grown in consistently moist soil, as an adaptation to maximise water absorption.
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The table shows the length of red blood cells measured in five blood smear samples.
| Sample | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Cell length (?m) | 7.2 | 7.5 | 7.1 | 7.4 | 7.3 |
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A student examined a prepared blood smear slide and counted 200 cells in total, identifying the type of each one.
| Cell type | Red blood cells | White blood cells | Platelets |
|---|---|---|---|
| Number counted (out of 200) | 188 | 10 | 2 |
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The table shows the number of mitochondria counted in each of five muscle cell samples.
| Sample | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Mitochondria counted | 850 | 910 | 875 | 120 | 895 |
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The table shows the speed of nerve impulse conduction along four different nerve fibres.
| Fibre | A | B | C | D |
|---|---|---|---|---|
| Myelinated? | Yes | No | Yes | No |
| Conduction speed (m/s) | 80 | 2 | 60 | 1.5 |
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The table shows the approximate size of four human cell types.
| Cell type | Red blood cell | Egg cell | Neuron cell body | Neuron axon (longest, e.g. spinal cord to foot) |
|---|---|---|---|---|
| Approximate size | 7.5 ?m diameter | 120 ?m diameter | 20 ?m diameter | up to 1 m long |
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A neuron's cell body is microscopic, similar in size to other cells (only tens of ?m wide), but its axon is a long, extremely thin extension of that same single cell, which in some human neurons can reach up to about $1\,\text{m}$ ($1{,}000{,}000\,\mu\text{m}$) long.
$$ \frac{1{,}000{,}000}{7.5} \approx 133{,}000 $$
So the longest human axon is roughly $133{,}000$ times longer than a red blood cell's diameter, even though both cells are, in cross-section, similarly microscopic in width.
The table shows the number of root hairs counted in a fixed 1 cm section of root, for four different seedlings grown in the same conditions.
| Seedling | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Root hairs counted (per 1 cm) | 42 | 39 | 45 | 41 |
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The table shows the red blood cell count (millions per mm³ of blood) for a group of climbers, measured at sea level and again after 4 weeks living at high altitude, where oxygen is scarcer.
| Climber | Sea level count | After 4 weeks at altitude |
|---|---|---|
| 1 | 5.0 | 6.1 |
| 2 | 4.8 | 5.9 |
| 3 | 5.1 | 5.0 |
| 4 | 4.9 | 6.0 |
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Donated human blood relies on healthy red blood cells (and other blood components) from volunteer donors being collected, tested, stored, and given to patients who need transfusions after surgery, childbirth complications, or serious blood loss.
Discuss one benefit and one drawback/challenge of relying on human blood donation.
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Benefit: donated blood directly saves lives in emergencies, surgery, and for people with blood disorders, and — unlike many medical treatments — it is a relatively simple, low-cost way to help many patients using cells the body naturally makes.
Drawback: the blood supply depends entirely on enough healthy volunteers regularly donating, so shortages can occur, especially of rarer blood types or during crises when demand is high. Donated blood must also be carefully tested for infections before use, and it does not last indefinitely once stored, meaning it can go to waste if not used in time.
Stem cells are unspecialised cells that can still differentiate (develop) into many different types of specialised cell. Scientists are researching using stem cells to grow replacement specialised cells — for example, insulin-producing cells for people with diabetes, or nerve cells to help repair spinal cord injuries.
Evaluate the impact of this technology, discussing a benefit and a concern.
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Benefit: this could allow doctors to grow replacement cells or tissue for patients with currently incurable conditions — such as restoring damaged nerve cells after a spinal injury, or replacing insulin-producing cells destroyed by diabetes — potentially curing or greatly improving conditions that today can only be managed, and reducing reliance on donor organs/tissue, which are often in short supply.
Concern: some sources of stem cells, particularly from human embryos, raise ethical concerns for some people, since obtaining them can involve the destruction of a human embryo. The technology and treatments are also still expensive and not fully understood, meaning there is a risk of unintended effects (such as the new cells growing uncontrollably), and access to these advanced treatments may initially only be available to wealthier patients or countries, raising fairness concerns.
Some athletes have illegally used “blood doping” — for example, injecting themselves with extra red blood cells, or using a banned drug that makes the body produce more red blood cells — before an endurance event such as a marathon or cycling race, to increase the oxygen-carrying capacity of their blood.
Discuss why an athlete might be tempted to do this, and one serious drawback or risk of doing so.
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Reason/benefit (to the athlete): more red blood cells means more haemoglobin available to carry oxygen to the working muscles, which can significantly improve endurance performance, since muscles can respire aerobically for longer and more efficiently before fatiguing — potentially giving a real competitive advantage in a sport where races can be won or lost by fractions of a second.
Drawback/risk: blood doping is banned in almost all sports as a form of cheating, and athletes caught face bans, disqualification and damage to their reputation. Medically, an artificially high concentration of red blood cells makes the blood thicker (more viscous), which increases the risk of dangerous blood clots, strokes and heart problems, especially during intense exercise when the heart is already working hard — several documented athlete deaths have been linked to blood doping practices.