Example essays (AQA A Level Biology): Revision Note
Exam code: 7402
Essay 1: 21-25 marks
Worked Example
Write an essay on the topic below.
The importance of shapes fitting together in cells and organisms.
[25 marks]
Answer:
Enzymes are biological catalysts that help speed up chemical reactions. They are made from proteins, which means they have a specific tertiary structure formed by the way the amino acids are arranged in the polypeptide chain. This arrangement allows bonds like hydrogen bonds to form in certain places, giving the enzyme a unique 3D shape. The active site on the enzyme has a shape that matches the shape of a specific substrate, so they fit together like puzzle pieces. There are two main models that explain this: the lock and key model, where the substrate fits exactly into the enzyme’s active site, and the induced fit model, where the active site slightly changes shape to fit the substrate more closely once it binds. Both models show how the enzyme can only act on one specific substrate. This shape matching is really important because it allows an enzyme-substrate complex to form, which speeds up the reaction by lowering the activation energy. Without this, lots of vital reactions would happen too slowly. For instance, ATPase helps convert ADP and inorganic phosphate into ATP. If ATPase didn’t have the right shape, it couldn’t bind to its substrates and ATP would be made much more slowly. Since ATP is needed for things like muscle contraction, this could have serious effects—an animal might not be able to move fast enough to escape from a predator and could be caught.
DNA’s structure also shows why shapes fitting together is essential. DNA is a polymer made of nucleotides, and each nucleotide is made up of a deoxyribose sugar, a phosphate group, and a nitrogenous base. There are four different bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair in a specific way due to their shapes—A always pairs with T using two hydrogen bonds, and C always pairs with G using three hydrogen bonds. These specific pairings allow the two strands of DNA to form a stable double helix. The base pairing rules keep the structure of DNA consistent and accurate during replication. The hydrogen bonds are weak enough to break easily when DNA needs to unzip, such as during DNA replication or protein synthesis. Because the base pairing is complementary, each new DNA strand formed will be identical to the original, helping to prevent mutations and keep the genetic code accurate. The sugar-phosphate backbone also helps maintain the DNA’s shape, offering protection to the genetic material stored inside.
Carrier proteins in the cell membrane are another good example of the importance of shape. These proteins help move ions across the membrane, either through active transport or facilitated diffusion. Because ions are charged particles, they can’t pass directly through the phospholipid bilayer on their own. Carrier proteins have a specific tertiary structure that creates a binding site for a particular ion. Only ions with the right shape can fit into that binding site. When the ion binds, it causes the protein to change shape and move the ion to the other side of the membrane. If the protein’s shape wasn’t right, this process wouldn’t happen. One key process that relies on carrier proteins is the electron transport chain in aerobic respiration. During this, protons (H⁺ ions) are moved across the inner mitochondrial membrane. If carrier proteins didn’t have the correct shape to bind and move these ions, no electrochemical gradient would form. That would stop protons from flowing back through ATP synthase to make ATP, and energy production in the cell would be affected. This would have knock-on effects on lots of other processes in the body that rely on ATP.
Muscle contraction is another process that relies on the precise fit of shapes. Muscle fibres are made up of myofibrils, which contain repeating units called sarcomeres. These are built from two proteins: actin and myosin. Myosin has a tail and a globular head, and actin is made of globular units twisted into a helical shape. When a nerve impulse triggers muscle contraction, calcium ions are released from the sarcoplasmic reticulum. These ions move a molecule called tropomyosin, exposing the binding site on the actin filament. The myosin heads then attach to this site, forming actin-myosin cross bridges. Myosin heads rotate and pull the actin filament, causing the muscle to contract. ATP then binds to the myosin head, making it release the actin and return to its starting position. If actin and myosin didn’t have complementary shapes, they wouldn’t be able to bind and muscle contraction wouldn’t happen. This would prevent organisms from moving, which could be fatal if they couldn’t escape danger or catch food.
Finally, antibodies also show why shape is so important in biology. Antibodies are proteins made from four polypeptide chains—two heavy and two light chains. The variable region at the top of each chain has a different sequence of amino acids, giving each antibody a unique shape. This is the part that binds to antigens, which are protein markers found on the surface of pathogens. Each antibody can only bind to one specific antigen, forming an antigen-antibody complex. This triggers a response such as agglutination (clumping of pathogens), neutralisation of toxins, or signalling phagocytes to engulf the pathogen. Because antibodies have to fit their antigens exactly, if they were the wrong shape, they wouldn’t be able to bind. That would slow down the immune response and could allow harmful pathogens to multiply and cause illness. In some cases, if antibodies bind to the body’s own cells, it can lead to autoimmune diseases like type 1 diabetes. So, the specific shapes of antibodies and antigens are key to a healthy and effective immune system.
Possible mark: 24 / 25
This essay meets the criteria for the 21–25 mark band and demonstrates many features of an excellent synoptic response.
Mark scheme criterion | Description of 21–25 mark level in the mark scheme | Evidence for the mark awarded |
|---|---|---|
Breadth | Several different topics are covered | The essay covers six distinct areas:
|
Links with the question theme | The answer makes clear links between several different topics and the theme of the question | All examples are linked to the idea that specific shapes are essential for function, e.g.:
Links to survival, replication and energy production are clearly stated throughout |
Accuracy | No significant errors | Explanations of enzyme function, DNA base pairing, and membrane transport are accurate. Some details, e.g. ATPase instead of ATP synthase, are not required by the spec but are not incorrect and support top-band performance |
Level of detail | Biology is detailed and comprehensive A-level content Biology is always clearly explained | Paragraphs show A-level understanding of mechanisms, e.g.:
Explanations are well above GCSE level |
Use of terminology | Uses appropriate terminology consistently | Key terms are used accurately throughout, e.g.:
|
Relevance | No irrelevant material | All chosen examples support the essay theme Points that go beyond the spec, e.g. autoimmune disease, are relevant and enhance the argument |
Strengths:
An excellent range of topics has been addressed
Each paragraph is clearly focused on one process, with an explanation and a link back to the theme
Key A level processes, such as the induced fit model, antigen recognition, and muscle contraction are explained in detail
Terminology is precise and used consistently in appropriate places
Areas for improvement:
The reference to “ATPase” for ATP synthesis is not correct; ATPase is involved with ATP hydrolysis, whereas ATP synthase is the enzyme responsible for ATP synthesis
One more example that extends beyond the specification, e.g. receptor-hormone interaction or transcription factors, could increase the security of the mark in the highest band
A few points do not add anything, or are revisited without adding new insight, e.g. importance of shape in enzyme-substrate binding and references to the lock and key mechanism
Essay 2: 16-20 marks
Worked Example
Write an essay on the topic below.
The causes & importance of variation & diversity in organisms.
[25 marks]
Answer:
Variation refers to the differences between individuals. These differences may arise through genetic or environmental factors. Genetic variation occurs due to mutations, meiosis and random fertilisation, and is essential for survival and evolution.
Mutations are changes in the base sequence of DNA. Some mutations are silent, while others may result in a change in phenotype. They introduce new alleles into a population, contributing to genetic variation. For example, a mutation that gives resistance to a disease can be passed on to offspring, increasing their chances of survival.
Meiosis increases variation through independent segregation and crossing over. During meiosis I, homologous chromosomes line up, and the random arrangement of maternal and paternal chromosomes is random. Crossing over between chromatids then leads to new combinations of alleles. This means that each gamete, and hence each of the offspring, is genetically unique.
Fertilisation is also random, adding to genetic variation. Any sperm can fertilise any egg, further increasing the genetic differences between offspring.
Variation is important for natural selection. In a population with genetic variation, individuals with advantageous alleles are more likely to survive and reproduce. These alleles are passed on, and over time the frequency of beneficial alleles increases in the population. This leads to adaptation.
The evolution of resistance to pathogens is important for the survival of populations, as it means that a population will not be wiped out by the arrival of a new disease. The presence of bacteria or viruses in an environment may select for individuals with immunity or resistance, such as through the production of antibodies.
Ecological succession is another process where variation is important. As the environment changes, different species are better adapted to survive. This leads to a change in the community over time and eventually results in a stable climax community. Variation ensures that some individuals will always be suited to the new conditions.
Speciation is the formation of new species from existing ones. It often occurs when populations become geographically isolated. Each population experiences different selection pressures and mutations. Over time, genetic differences accumulate until the populations can no longer interbreed. This is an example of how variation leads to biodiversity.
Biodiversity is essential for ecosystems. Greater genetic diversity within a species allows for better adaptation to changing environments and increases the chance of survival. Species diversity contributes to ecosystem stability.
In conclusion, variation is the foundation of biological diversity and is vital for the survival of species, adaptation to changing environments, and the process of evolution.
Possible mark: 17 / 25
This essay meets the criteria for the 16–20 mark band; it shows sound biological understanding with several clear topic links, though several are not fully developed or explained at A-level depth.
Mark scheme criterion | Description of 16-20 mark level in the mark scheme | Evidence for the mark awarded |
|---|---|---|
Breadth | Several different topics are covered | The essay addresses six topics from across the specification:
|
Links with the question theme | Answer links several topics to the main theme to form a series of interrelated points which are clearly explained | Most topics are linked clearly with the theme of the question, e.g.:
Not all topics are clearly linked in this way, e.g. the paragraph on meiosis doesn't explain why it is an advantage to produce genetically unique offspring |
Accuracy | Biology is fundamentally correct A-level content | All examples are biologically accurate and contain A-level content |
Level of detail | Contains some points which are detailed, though there may be some which are less well developed | Speciation and independent segregation are explained in reasonable detail The essay could benefit from more in-depth mechanisms or examples, e.g.:
|
Use of terminology | Appropriate use of terminology | Biological terms used include:
Use is consistent and appropriate, though not especially wide-ranging |
Relevance | One significant error and/or one irrelevant topic may be present | All material is relevant to the theme |
Strengths:
The essay covers a broad range of topics, going beyond the minimum of four topic areas
The answer shows clear understanding
There are no significant errors and all content is relevant
Areas for improvement:
The answer could explain biological processes, e.g. meiosis stages, in more detail
Some links to the essay title are stated but not explained, e.g. the student needed to explain the importance of ecosystem stability
A wider range of more technical vocabulary would improve the answer
There is a lack of specific examples, e.g. antibiotic resistance, sickle-cell anaemia and peppered moth
The essay needs at least one example that goes beyond the specification, or a very detailed description of an A level mechanism, to reach the 21-25 mark band
Essay 3: 11-15 marks
Worked Example
Write an essay on the topic below.
The importance of gradients in biological systems.
[25 marks]
Answer:
Gradients are found in many parts of biology and they help to move substances around. They are important because without them, some things wouldn’t work properly. A gradient means that something like concentration or water potential is higher in one place and lower in another.
Diffusion relies on a concentration gradient. In the lungs, oxygen moves into the blood from the alveoli because there is more oxygen in the alveoli than in the blood. The same thing happens with carbon dioxide, which moves out of the blood. This is essential so the body can get oxygen and remove waste.
In the kidney, water moves because of a water potential gradient. The loop of Henle causes water to move out of the collecting duct and be reabsorbed. This makes the urine more concentrated.
In the small intestine, sodium is pumped out of cells and this helps glucose move in. This is called co-transport. The sodium goes back into the cell along the gradient, and glucose goes with it. This can also happen in the absorption of amino acids.
In nerve impulses, sodium ions go into the neuron which changes the charge and makes the impulse travel. This uses a gradient of sodium and potassium, which is maintained by a sodium-potassium pump.
Gradients are useful in xylem and phloem. In xylem, water moves from roots to leaves because of evaporation in the leaves. In phloem, sugar moves from sources to sinks by mass flow.
In conclusion, gradients are used in lots of systems, like nerves and the kidney. They help to move substances in and out of cells and are important in many processes.
Possible mark: 13 / 25
The essay exceeds the criteria for the 6-10 mark level, and meets some of the criteria for the 11-15 band, as described below:
Mark scheme criterion | Description of 11-15 mark level in the mark scheme | Evidence for the mark awarded |
|---|---|---|
Breadth | Several aspects covered | The essay contains five topics:
|
Links with the question theme | Topics are not interrelated and links are not made to the theme of the question | Not all of the topics are linked clearly to the question theme, which asks about the importance of gradients The first paragraph states that "This is essential so the body can get oxygen and remove waste" No other paragraph clearly links the example with survival of the organism |
Accuracy | Biology is usually correct Some significant errors | Most paragraphs contain correct content, though some statements could be interpreted incorrectly, e.g.:
|
Level of detail | A-level content Usually clearly explained, but lacks detail | A level topics are covered, e.g. the loop of Henle and cotransport, so detail goes beyond GCSE level Explanations are correct at a superficial level, but every paragraph lacks detail |
Use of terminology | Generally uses appropriate terminology | Examples of appropriate terminology include:
There are many places where A level terminology could have been used but has not been, e.g.:
|
Relevance | Response mostly deals with suitable topics More than one irrelevant topic may have been included | All chosen topics are relevant to the context of gradients given in the question |
Strengths:
The essay covers a broad range of topics, going beyond the minimum of four topic areas
The paragraph on gas exchange links back to the theme of the essay by explaining why the gradient is important for survival
There are no significant errors and all content is relevant
Areas for improvement:
Consistent use of biological terminology is needed, e.g. "more oxygen" should be higher oxygen concentration and "changes the charge" should be depolarises the membrane
The level of detail is low throughout; every paragraph needs a more detailed explanation of the process being described
Clarity needs to be improved to reduce the risk of misinterpretation, e.g. the statement that "the same thing happens with carbon dioxide" needs to be altered to remove the incorrect implication that carbon dioxide moves into the blood from the alveoli
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