Comprehension (AS & A Level) (AQA A Level Biology): Exam Questions

The BRCA1 gene helps to regulate the cell cycle and is active in many healthy mammalian tissues. It codes for a protein that is involved in detecting damaged DNA. BRCA1 is often described as a tumour suppressor gene.

In non-dividing cells the BRCA1 gene is usually inactive. It is activated during cell division by a transcription factor called E2F1, which enters the nucleus during specific phases of the cell cycle.

In some forms of breast and ovarian cancer, the BRCA1 gene is no longer transcribed. This can occur even if the DNA sequence of the gene is unchanged. Researchers have found that in some tumour cells, the DNA near the BRCA1 promoter region is highly methylated, and the associated histone proteins lack acetyl groups.

A drug called Vorinostat, which belongs to a class of compounds known as histone deacetylase (HDAC) inhibitors, is being tested as a treatment for cancers in which BRCA1 is inactive. In one clinical trial involving 120 patients with breast cancer, Vorinostat restored BRCA1 gene expression in 68% of tumour samples. In a smaller group of patients with ovarian cancer, 54% showed a partial reduction in tumour size after receiving the drug. However, delivering Vorinostat effectively to all tumour cells remains a challenge.

Nitrogen is an essential element in all living organisms. In aquatic ecosystems, nitrogen availability often limits plant growth, particularly in freshwater environments where nitrate concentrations are naturally low.

Aquatic plants have evolved various strategies to obtain nitrogen from their environment. Some species, such as water hyacinth, possess specialised root structures that can absorb both nitrate and ammonium ions directly from the water.

Recent research has investigated the growth of water hyacinth in nutrient-poor freshwater systems. Scientists compared the growth of plants in water with different nitrogen concentrations over a 28-day period. Plants grown in high-nitrogen conditions showed significantly greater biomass accumulation and darker green colouration in their leaves. Plants in low-nitrogen environments developed longer, more branched root systems compared to those in nitrogen-rich water. Analysis of gene expression revealed that genes coding for nitrogen transport proteins were expressed at higher levels in nitrogen-starved plants.

While water hyacinth can grow in nutrient poor environments, there are concerns about introducing the species into new freshwater systems, as this species can rapidly colonise water bodies and outcompete native plant species, potentially disrupting local ecosystems.

Iron is an essential mineral required for many processes in both animals and plants. In plants, iron is needed for the synthesis of chlorophyll and acts as a cofactor for enzymes involved in photosynthesis and respiration. However, in many soils, particularly those with alkaline conditions, iron is not readily available because it forms insoluble compounds.

To overcome this, some plants release organic compounds called phytosiderophores into the surrounding soil. These molecules bind with iron ions to form soluble complexes that can be absorbed by specific transport proteins located in the root cell membranes.

Scientists have genetically modified rice plants to overproduce phytosiderophores to improve their ability to grow in iron-poor soils. In a recent experiment, the growth of these genetically modified (GM) rice plants was compared with that of unmodified rice plants when grown in alkaline soil with low iron availability.

After 21 days, the GM plants had significantly greater shoot length and chlorophyll concentration. However, no significant differences in root length were observed between the two types of plants. Gene expression analysis also showed that genes involved in transporting iron from roots to shoots were more highly expressed in the GM plants than in the unmodified plants.

One concern about these GM plants is the potential that increased phytosiderophore production may enhance the availability and uptake of harmful heavy metals, such as cadmium, in soils that are contaminated. If absorbed, these metals could accumulate in the grain, which may pose a risk to human health.

Maintaining blood glucose levels within a narrow range is essential for proper cellular function in mammals. When blood glucose levels rise after a meal, the pancreas releases insulin, which promotes glucose uptake by cells and stimulates the conversion of glucose to glycogen in the liver and muscles.

Insulin binds to specific insulin receptors on target cell membranes, triggering a cascade of intracellular events. This binding causes conformational changes in the receptor, which then activates glucose transporter proteins (GLUT4) that are stored in vesicles within the cytoplasm. These transporters are rapidly moved to the cell membrane through exocytosis, allowing glucose to enter the cells more efficiently.

In type 2 diabetes, cells become resistant to insulin, meaning that they respond poorly to the hormone even when it is present in normal or elevated concentrations. Research has shown that this insulin resistance is often associated with reduced numbers of functional insulin receptors on cell membranes and impaired translocation of GLUT4 transporters to the membrane.

A recent study investigated the effectiveness of a new drug treatment for type 2 diabetes. The drug works by increasing the sensitivity of cells to insulin, thereby improving glucose uptake. Patients were divided into two groups: one receiving the new drug and a control group receiving a placebo (a treatment containing no active ingredients). After 12 weeks of treatment the drug group showed significantly lower fasting blood glucose levels and improved glucose tolerance compared to the control group. Some patients in the drug group experienced side effects, including hypoglycaemia, a condition in which they developed abnormally low blood glucose levels.

Natural selection is a fundamental mechanism of evolution that shapes populations over time. When environmental pressures favour certain traits, individuals with advantageous characteristics are more likely to survive and reproduce, passing these beneficial alleles to their offspring.

In a population of peppered moths (Biston betularia), two main colour variants exist: light-coloured moths and dark-coloured (melanic) moths. The colour is controlled by a single gene, with the dark allele being dominant over the light allele. During the Industrial Revolution, pollution darkened tree bark in many areas, providing better camouflage for dark moths against predation by birds.

Research has shown that in polluted environments, dark moths have significantly higher survival rates compared to light moths. This selective advantage leads to an increase in the frequency of the dark allele in the population over successive generations. Conversely, in unpolluted areas with light-coloured tree bark, light moths have the survival advantage.

A recent study investigated changes in allele frequencies in a peppered moth population over a 50-year period following the Clean Air Act, which reduced industrial pollution. Researchers found that as pollution levels decreased and tree bark became lighter, the frequency of the dark allele declined from 90% to 30% in the study population.

However, the researchers noted that some dark moths persisted even in the cleanest environments, suggesting that factors other than camouflage alone may influence survival.

In heavily polluted environments, the frequency of the light allele in peppered moth populations can become very low due to strong selection pressure.

Protein digestion is a multi-stage process that begins in the stomach and continues in the small intestine. In the stomach, the enzyme pepsin hydrolyses proteins into smaller polypeptides under acidic conditions. The majority of protein digestion then takes place in the small intestine, where pancreatic enzymes, such as trypsin and chymotrypsin, break down these polypeptides into shorter peptide chains. Much of the final stage of protein digestion occurs on the surface of the small intestine’s epithelial cells. These cells possess membrane-bound peptidases on their brush border, which hydrolyse dipeptides and tripeptides into single amino acids. These amino acids are then absorbed into the epithelial cells and enter the bloodstream for transport around the body.

Scientists investigated protein digestion in individuals with pancreatic insufficiency — a condition in which the pancreas does not produce enough digestive enzymes. Participants ate a protein-rich meal and had blood sampled two hours later. Compared with healthy individuals, those with pancreatic insufficiency showed markedly lower blood amino-acid concentrations. When affected participants were given enzyme supplements, amino-acid absorption improved, although in many cases it did not reach the levels seen in healthy controls, and some participants continued to report digestive symptoms.

Further analysis revealed elevated levels of undigested peptides in the faeces of those with pancreatic insufficiency, indicating incomplete protein breakdown. Despite enzyme supplementation, faecal peptide levels often remained higher than in healthy individuals. Together, these findings show that pancreatic enzymes are essential for nutrient absorption, and that while supplementation helps, it may not fully restore normal digestion in all patients.

One common genetic cause of pancreatic insufficiency is cystic fibrosis (CF), a hereditary disorder caused by mutations in the CFTR gene. This mutation results in the production of thick, sticky mucus that can block the pancreatic duct. Cystic fibrosis is inherited in an autosomal recessive pattern.

Researchers studying bacteria from Antarctic sea ice have discovered several enzymes that remain active at temperatures close to 0 °C. These cold-adapted enzymes allow the bacteria to carry out essential metabolic reactions even in extremely cold conditions.

One such enzyme, called lipase-A, helps the bacteria break down lipids for energy. Compared to similar enzymes from bacteria in warmer environments, lipase-A is made up of a slightly different strcuture, giving it a more flexible structure. This flexibility appears to allow it to function efficiently at lower temperatures.

Scientists sequenced the gene for lipase-A and compared it to the version found in a closely related species that lives in temperate waters. The coding region for lipase-A in the Antarctic bacteria contained several base substitutions. However, the amino acid sequence was still very similar between the two species, suggesting that many of the mutations were neutral.

To study how the enzyme is produced, researchers isolated the mRNA for lipase-A. They then inserted the gene into bacterial cells to produce large amounts of the lipase enzyme in the laboratory.

Ebola is a viral disease. A glycoprotein on the surface of the Ebola virus binds to a receptor protein found in the cell-surface membranes of human cells. The binding of the virus glycoprotein with the human cell receptor protein allows the virus to enter the cell. However, not all humans produce this receptor protein.

Those who don't have the receptor protein can still become infected with the virus, without actually developing the disease.

A blood test can be used to check whether a patient has the Ebola disease. The blood of patients with the disease contains large numbers of specific plasma cells and large numbers of a particular antibody. It has been suggested by researchers that patients with Ebola could be treated by giving them blood plasma transfusions from patients who have recently had the disease but have now recovered.

Researchers have also stated that the high mutation rate of the Ebola virus presents a challenge to the development of a vaccine.

Use the information provided above and your own knowledge to answer the following questions.

The milk produced by mammals is nutrient-rich and contains the correct proportions of the different nutrients that offspring need to grow and develop. This milk is produced by milk-producing cells located in mammary glands.

Milk-producing cells synthesise lipids, carbohydrates and proteins by absorbing substances from the blood. These cells have a height to breadth ratio of 1.3: 1. They are approximately cube-shaped.

Lactose is a disaccharide and is the main carbohydrate in milk. The condensation of glucose and galactose (both monosaccharides) forms lactose. Galactose has the same chemical formula as glucose but has a slightly different structure.

Lactose-synthesis occurs in the Golgi apparatus and is then transported through the cytoplasm in vesicles. The lactose inside these vesicles cannot escape. The diameter of the vesicles increases as they move to the plasma membrane. When they get there, the vesicle membranes fuse with the plasma membrane and release their lactose out of the cell.

The human body contains over 200 unique cell types. Further still, numerous human body cell types can be split further into different varieties of cell, within the cell type. For example, we can split white blood cells into several distinct varieties including lymphocytes and macrophages (a type of phagocytes).

Despite many common features present across a range of human body cell types, 5 each type is also specialised (sometimes very highly) to carry out its specific function. Although the majority of cell types contain the same organelles, the number of certain organelles that each cell type contains can differ significantly. A notably large number of mitochondria are present in muscle cells, for example, and hormone-secreting cells in the pancreas have distinctly large amounts of rough endoplasmic reticulum.

Interestingly, the amount of a certain organelle is not always fixed during a cells 'life' and this was observed by scientists looking at the cells in the tails of tadpoles. As a tadpole develops into a frog, its tail is gradually reabsorbed into the main body of the tadpole. The scientists found that the number of lysosomes in the tail cells actually increased as the tadpole matured.

Wheat stalks are mainly composed of three different organic materials. These are cellulose, hemicellulose and lignin. Molecules of cellulose are joined in chains, which then layer together to create cellulose fibres. Hemicellulose is mostly made up of pentose sugar monomers.

Hemicellulose is a small molecule and functions a bit like a glue in that it holds cellulose fibres together. The third organic material, 5 lignin, is also a polymer. Unlike the other two substances, however, it is not a carbohydrate. It provides extra strength by covering the cellulose in the cell wall.

After the harvesting of a wheat crop, the hollow wheat stalks are dried, then cut and formed into bales of straw. Straw has a very low moisture content.

The straw can be used to produce a variety of useful organic substances. For example, it can be used as an ingredient in biofuels, packaging materials and paper manufacturing. A key step in the process is converting the cellulose to glucose. Although enzymes can be used for this process, a complication with this is that the lignin covering the cellulose within the straw stops the cellulose from being broken down by such enzymes. The lignin must therefore be removed before this can occur.

Pathogens enter the bodies of organisms, where they cause disease. Viruses are a type of pathogen. People infected with the human immunodeficiency virus, commonly referred to as HIV, develop something known as acquired immunodeficiency syndrome, commonly referred to as AIDS. This is a very serious condition as it stops the infected individual from producing an immune response in defence against other pathogens.

Researchers have been trying to create vaccines against HIV for a long time but it is proving very difficult, mainly due to three properties of HIV. Firstly, HIV has high antigenic variability. Secondly, the virus can enter human body cells within a very short time of an individual being infected. Thirdly, HIV destroys T-cells.

There are other methods used to help delay the onset of AIDS. Although not a long-term cure, a drug called azidothymidine (AZT) can be used to temporarily stop and slow down the development of AIDS. The drug does not destroy HIV in the body but inhibits the enzyme responsible for synthesising DNA from the RNA of the human immunodeficiency virus.