AQA GCSE Biology Paper 1, Higher TierCell Biology, Organisation, Infection and Response, Bioenergetics — every question, answered

A partly blocked coronary artery reduces the amount of blood that can flow to the heart muscle cells, because the narrowed space inside the artery restricts how much blood can pass through per minute. This means less oxygen and less glucose reach the heart muscle cells than normal.

Because there is less oxygen available, the heart muscle cells cannot respire aerobically at the normal rate, so more of their energy comes from anaerobic respiration instead. Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration and produces lactic acid as a waste product, so as the blockage gets worse, lactic acid starts to build up in the heart muscle tissue.

The build up of lactic acid causes muscle fatigue and pain in the chest, because the heart muscle cannot keep contracting normally when energy release is reduced and a waste product is accumulating around the muscle fibres. At the same time, because less blood is reaching the rest of the body from a heart that cannot pump normally, less oxygen and glucose reach other tissues too, which is why a person with a partly blocked coronary artery often also feels tired and short of breath, since their body is forced to try to compensate by breathing faster to bring in more oxygen.

Damage to the roots reduces the surface area of root hair cells available to absorb water from the soil, so less water is taken up into the plant. Since water is needed as a raw material for photosynthesis, a lower rate of water uptake leads to a lower rate of photosynthesis, meaning less glucose is produced. Because glucose is used to make cellulose for new cell walls and provides the substrate for respiration to release energy, reduced photosynthesis limits both new growth and the energy available for normal cell processes, and with less water reaching the cells generally, cells also start to lose their turgidity, weakening the support the plant relies on to hold its stems and leaves up.

Damaged roots also absorb fewer mineral ions from the soil, particularly nitrate ions and magnesium ions, since active transport of these ions depends on healthy root hair cells with working transport proteins. Nitrate ions are needed to make amino acids, which are used to build proteins for growth, so less nitrate uptake means less protein is made and the plant cannot grow or repair damaged tissue properly. Magnesium ions are needed to make chlorophyll, so reduced magnesium uptake means less chlorophyll is produced, which further reduces the rate of photosynthesis on top of the water shortage already described, compounding the energy problem in a second, independent way.

Root damage does not only reduce how much water and minerals the roots absorb in the first place, it can also destroy the xylem and phloem tissue that carries those substances once they are inside the root. This is a completely separate failure from the absorption problem above, because even if the roots below the damaged section are still absorbing water and minerals normally, none of it can get past a break in the xylem, so every cell above that point in the stem and leaves is cut off regardless of how well the roots underneath are working. In the same way, damaged phloem stops sugars made by photosynthesis in the leaves from being carried down to root cells, so root cells below the break lose their supply of glucose for respiration even though they may be absorbing water and minerals perfectly well. Because roots also physically anchor the tree in the soil, extensive root damage further reduces anchorage, making the tree more likely to be blown over in strong wind, which is a mechanical rather than a biochemical reason the tree could be lost.

Tiredness is caused by a fall in the number of red blood cells, since red blood cells contain haemoglobin, which binds to oxygen in the lungs and carries it around the body. Fewer red blood cells means less haemoglobin overall, so less oxygen is transported to body cells. With less oxygen available, cells cannot respire aerobically at their normal rate, so more anaerobic respiration takes place instead, which releases far less energy per glucose molecule than aerobic respiration and also produces lactic acid, causing the muscle fatigue that a person experiences as tiredness.

Frequent infections are caused by a fall in the number of white blood cells, since white blood cells, including phagocytes and lymphocytes, are responsible for defending the body against pathogens. Phagocytes normally engulf and digest bacteria and other pathogens, while lymphocytes produce antibodies specific to particular pathogens. With fewer white blood cells available, fewer pathogens are engulfed by phagocytosis and fewer antibodies are produced, so bacteria and viruses that enter the body are not destroyed as quickly or as effectively as normal, which is why infections become more frequent and can become more serious.

Bleeding that will not stop is caused by a fall in the number of platelets, since platelets are cell fragments responsible for helping blood clot at the site of a cut or wound. Normally, platelets clump together at a wound and trigger a series of reactions that form a clot, sealing the break in the skin and blood vessels. With fewer platelets in the blood, clots form more slowly or do not form properly at all, so a cut that would normally stop bleeding within a short time continues to bleed for much longer, which explains this third symptom as a direct consequence of platelet shortage rather than any problem with the blood vessels themselves.

The lungs contain many millions of tiny alveoli, and this huge number gives the lungs an extremely large total surface area in contact with the surrounding capillaries. A larger surface area means more gas molecules can diffuse across the exchange surface at the same time, since diffusion happens across the whole membrane simultaneously rather than at just one point, so the total rate of gas exchange for the lungs as a whole is much higher than it would be with only a few larger exchange surfaces.

The walls of the alveoli and the walls of the capillaries surrounding them are each only one cell thick, so together they provide a very short diffusion path between the air inside the alveolus and the blood inside the capillary. Because the rate of diffusion is faster over a shorter distance, this thin, one cell thick barrier allows oxygen to move into the blood and carbon dioxide to move out of the blood much more quickly than if the walls were made of several layers of cells.

Breathing constantly moves air in and out of the lungs, bringing in air with a high oxygen concentration and removing air that has a lower oxygen concentration and a higher carbon dioxide concentration after gas exchange has taken place. At the same time, the dense network of capillaries around each alveolus constantly brings in deoxygenated blood and carries away oxygenated blood. Together, ventilation of the lungs and the constant blood flow through the capillaries maintain a steep concentration gradient for both oxygen and carbon dioxide across the alveolar wall, and since diffusion depends on a concentration gradient, maintaining this steep gradient keeps the rate of gas exchange high rather than letting it slow down as concentrations equalise.

Meat burgers contain more protein per 100g than meat-free burgers, and protein is needed for growth and repair of body tissues, so meat burgers are the stronger source of this nutrient in the table. However, meat burgers also contain more fat and much more cholesterol than meat-free burgers, and a diet high in saturated fat and cholesterol increases the risk of narrowing of the arteries (atherosclerosis), which raises the risk of coronary heart disease and heart attacks over time. This means that although meat burgers are useful for protein, eating them regularly carries a genuine long term cardiovascular health risk that the table figures directly support.

Meat-free burgers contain considerably more fibre than meat burgers, and fibre aids digestion and helps prevent constipation by adding bulk to food moving through the intestines, which is a genuine benefit the table supports. The two types of burger have a similar amount of carbohydrate, so this table does not show one type being clearly better than the other as an energy source. The table also gives no information about vitamins or minerals in either burger, so a full judgement about which is more balanced overall cannot be made from this data alone, only from the specific nutrients that were actually measured.

Overall, meat burgers provide more protein but carry a greater long term cardiovascular health risk because of their higher fat and cholesterol content, while meat-free burgers provide less protein but more fibre and a lower cardiovascular risk. Neither is automatically the healthier choice on its own, since a balanced diet needs both adequate protein for growth and repair and enough fibre for healthy digestion, so the better choice depends on what else a person eats and how often they eat each type of burger, rather than one burger type being straightforwardly the healthiest option.

Select a group of at least 30 athletes to take part, since a larger sample size reduces the effect of individual differences on the results and makes any pattern found more reliable. Record the resting heart rate of each athlete before any exercise takes place, by counting their pulse for a fixed period of time (for example, counting beats for 15 seconds and multiplying by four to get beats per minute) while they are sitting still.

Choose at least two different types or intensities of exercise to compare, for example light exercise such as walking and vigorous exercise such as running, and have every athlete in the group complete each type of exercise for the same fixed length of time. Immediately after each type of exercise, record each athlete's heart rate again using the same counting method as before, and calculate the increase in heart rate for each athlete by subtracting their resting heart rate from their heart rate after exercise.

Calculate the mean increase in heart rate across all athletes for each type of exercise, then compare the mean increases between the different exercise types to see which type of exercise causes the largest rise in heart rate. To make this comparison fair, keep other variables constant across all athletes and both exercise sessions: use a mix of biological sexes so the sample is representative, make sure all athletes are a similar age and have a similar level of overall fitness or activity, ensure no athlete has taken caffeine or medication that could affect heart rate before testing, and allow each athlete's heart rate to return to their resting rate before starting the next type of exercise.

The trial should first be given to a small group of healthy volunteers at a very low dose, to test that the treatment is safe and to check for any side effects before it is given to anyone who is actually unwell. Only once this stage shows the treatment is safe at low doses should the dose be gradually increased in further healthy volunteers, still checking for side effects at each stage.

Once safety has been established, the treatment should then be trialled on patients who actually have the Candida albicans infection, to test both the correct or optimum dose needed to treat the infection and to check for side effects in people who are unwell, since a safe dose in a healthy person is not necessarily the correct dose to actually treat a disease. This stage should also test whether the treatment is effective (its efficacy) at treating the infection, not just whether it is safe.

The trial with patients should be a double blind trial, where neither the patients nor the doctors treating them know who has been given the real mAbs treatment and who has been given a placebo or an alternative existing treatment, since this prevents the patients' or doctors' expectations from influencing the results. The trial should also involve a reasonably large number of patients and run for a sufficient duration to be confident the results are reliable, and appropriate control variables, such as similar severity of infection between groups, should be used so that the comparison between the treatment group and the control group is fair.

A red blood cell has no nucleus, since it loses its nucleus during development in order to make more room to carry oxygen, but a plant cell has a nucleus, which contains the genetic material controlling the cell's activities. A red blood cell also has no cell wall, since animal cells never have a cell wall, but a plant cell has a cell wall made of cellulose, which gives the cell a fixed, rigid shape and support.

A red blood cell is a biconcave disc shape, a shape that increases its surface area for absorbing oxygen, whereas plant cells come in many different regular shapes depending on their type and function, often described as rectangular or box shaped because of their rigid cell wall. A red blood cell contains haemoglobin, a red pigment that binds to oxygen, while a plant cell does not contain haemoglobin, though some plant cells do contain the green pigment chlorophyll for photosynthesis, which a red blood cell never contains.

A red blood cell also has no permanent vacuole, whereas many plant cells have a large, permanent vacuole filled with cell sap that helps keep the cell rigid through turgor pressure. Despite all of these differences, both cell types share some features in common: both have cytoplasm, where chemical reactions take place, and both have a cell membrane, which controls what substances move into and out of the cell.

Xylem tissue is made of dead cells, which have no cytoplasm and are essentially hollow tubes, whereas phloem tissue is made of living cells, which retain their cytoplasm. Xylem cell walls are strengthened with a substance called lignin, giving xylem vessels a rigid structure, but phloem cells do not contain lignin. Phloem cells also have small pores in their end walls, forming structures called sieve plates that allow substances to pass between adjacent cells, while xylem cells do not have pores in their end walls, since they form continuous hollow tubes instead.

In terms of function, xylem transports water and dissolved mineral ions, moving them only in one direction, upwards from the roots to the stem and leaves, as part of the transpiration stream. Phloem, on the other hand, transports dissolved sugars made during photosynthesis, and it can move substances in both directions around the plant, upwards or downwards depending on where sugars are needed, in a process called translocation.

Despite these differences, xylem and phloem do share some features in common: both are made up of columns of cells forming continuous tubes running through the plant, and both are involved in transporting liquids and dissolved substances around the stem, roots and leaves. The key difference in what they transport and in which direction is why a plant needs both tissues rather than just one, since neither tissue on its own could deliver both water and minerals upwards and sugars to wherever they are needed.

Scientists incubate bacteria at 37 degrees Celsius because this matches normal human body temperature, and bacteria such as Salmonella grow best at this temperature, so using 37 degrees Celsius gives scientists the fastest, most reliable growth for their investigation.

Students in school must use 25 degrees Celsius instead, because this lower temperature reduces or prevents the growth of bacteria that could be harmful to humans. Since many of the bacteria that cause disease in people grow best around body temperature, keeping the incubator below that range in a school lab lowers the risk to students handling the culture.

Magnification = size of image / size of real object, so rearranging this gives: size of real object = size of image / magnification.

Real length of cell Z = 4.8 / 400 = 0.012 cm. Converting this to micrometres: 0.012 cm = 0.12 mm = 120 micrometres.

At both 5 degrees Celsius and 80 degrees Celsius, the iodine solution never turned blue-black, showing that starch was still present because it had not been broken down by the amylase. At 5 degrees Celsius, the amylase and starch molecules have low kinetic energy, so there are fewer successful collisions between the enzyme and its substrate, meaning enzyme-substrate complexes form far more slowly than at a warmer temperature.

At 80 degrees Celsius, the amylase has been denatured, since the high temperature has broken the bonds holding the enzyme in its normal three dimensional shape. This changes the shape of the amylase's active site, so the starch molecule can no longer fit into it, meaning no new enzyme-substrate complexes can form at all at this temperature.