Aqa A Level Biology Paper 1 hard exam questions

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs. Details of breathing are not required (4 marks)

1. Oxygen enters the mouth then moves down the trachea into the bronchi then into bronchioles

I2. t moves down a pressure gradient

3. The oxygen then reaches the alveoli where it diffuses down a diffusion gradient

4. across the alveolar epithelium and across the capillary endothelium

Contrast the processes of facilitated diffusion and active transport. (3 marks)

1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins

2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP

3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.

Explain why, during DNA replication, new nucleotides can only be added in a 5' to 3' direction? (4marks)

1. DNA polymerase enzyme is used to join nucleotides together during DNA replication

2. Enzyme is specific/ has specific active site

3. Complementary to the 5' end of the strand

4. Shapes of 5' and 3' end are different ( so enzyme cannot go in other direction)

Describe how proteins are digested in the human gut ( 4 marks

1. Hydrolysis of peptide bonds

2. Endopeptidases break polypeptides into smaller peptide chains

3. Exopeptidases remove terminal amino acids ( amino acids at the end of the peptide chain)

4. Dipeptidases hydrolyse / break down dipeptides into amino acids.

Give two types of molecule from which a ribosome is made (1 mark)

1. RNA

2. Proteins/ polypeptides

Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer (3 marks)

1. mRNA binds to ribosome;

2. Idea of two codons/binding sites;

3. (Allows) tRNA with anticodons to bind/associate;

4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);

5. Moves along (mRNA to the next codon)/translocation described;

Underlined - key points i would make sure to mention

Sugggest how glycogen acts as a source of energy (2 marks)

1. Glycogen hydrolysed to glucose

2. Glucose is used in respiration

High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries. This can lead to a build up of tissue fluid. Explain how. (2 marks)

1. Higher salt results in lower water potential of tissue fluid;

2. So less water returns to capillary by osmosis (at venule end);

OR

3. Higher salt results in higher blood pressure/volume;

4. So more fluid pushed/forced out (at arteriole end) of capillary;

Bacteria are often used in industry as a source of enzymes. One reason is because bacteria divide rapidly, producing a large number of them in a short time.

Describe how bacteria divide. (

1. binary fission

2. replication of circular DNA

3. division of cytoplasm to produce 2 daughter cells

4. each with single copy of circular DNA

Underlined - key points I would mention

Some proteases are secreted as extracellular enzymes by bacteria. Suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment. Explain your answer. (2 marks)

1. To digest protein;

2. So they can absorb amino acids for growth/reproduction/protein synthesis/synthesis of named cell component;

OR

Sothey can destroy a toxic substance/protein;

The scientist suggested that people newly diagnosed with this bladder infection should be treated with both the current antibiotic and the new antibiotic.

Explain why the scientist made this suggestion

Use information in figure( graph showing the effect of the antibiotic on the number of bacteria in the bladder) and your knowledge of evolution of antibiotic resistance in bacteria in your answer. (3 marks)

1. New/old antibiotic does not kill all bacteria /. Some bacteria become resistant to the antibiotic

2. Resistant bacteria will survive and reproduce to produce more resistant bacteria

3. Use of both antibiotic will kill bacteria that are resistant to one of the antibiotics/ very unlikely that bacteria will be resistant to both antibiotics

Describe how phagocytosis of a virus leads to presentation of its antigens. (3 marks)

1. Phagosome/vesicle fuses with lysosome;

2. Virus destroyed by lysozymes/hydrolytic

enzymes;

3. Antigen from virus are displayed on

the cell membrane

Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen. (3 marks)

1. Helper T cell binds to antigens on the antigen presenting cell

2. T cell stimulates B cell to undergo clonal selection ( specific B cell clones/ divides by mitosis)

3. Forms plasma cells that release antibodies

Collagen is a protein produced by cells in joints, such as the knee.

Rheumatoid arthritis (RA) is an auto-immune disease. In an auto-immune disease, a person's immune system attacks their own cells. RA causes pain, swelling and stiffness in the joints.

Scientists have found a virus that produces a protein very similar to human collagen.

Suggest how the immune response to this viral protein can result in the development of RA. (2 marks)

1. The antibody against virus (antigen) will bind to collagen;

2. This results in the destruction of the (human) cells/collagen;

Explain three ways in which an insect's tracheal system is adapted for efficient gas exchange. (3 marks)

1. Tracheoles have thin walls so short diffusion distance to cells;

2. Highly branched/large number of tracheoles so short diffusion distance to cells;

3. Highly branched/large number of tracheoles so large surface area (for gas exchange);

4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);

5. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface;

OR

Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);

6. Body can be moved (by muscles) to move air so maintains diffusion/concentration gradient for oxygen/carbon dioxide;

Points 1,3,4 are points i would mention

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6 marks)

1. TEM use electrons and optical use light;

2. TEM allows a greater resolution;

3. (So with TEM) smaller organelles/named cell structure can be observed

OR

greater detail in organelles/named cell structure can be observed;

4. TEM view only dead/dehydrated specimens and optical (can) view live specimens;

5. TEM does not show colour and optical (can);

6. TEM requires thinner specimens;

7. TEM requires a more complex/time consuming preparation;

8. TEM focuses using magnets and optical uses (glass) lenses;

Points 1,2,3,4,5,6 are key points i would mention

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.

He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between the two pop. of tree

1. Use a random sample of seeds from each population

2. Use a large enough sample to be representative of whole population.

3. Measure size/mass of seeds

4. Calculate a mean and standard deviation for each population.

5. Use a students t-test and analyse whether there is a significant difference between the mean and standard deviation of two populations

What is meant by species richness? (1mark)

A measure of the number of different species in a community

Formation of an enzyme-substrate complex increases the rate of reaction.

Explain how (2 marks)

1. When substrate binds to complementary active site of enzyme

2. Stress/tension of bonds / weakening of bonds lowers the activation energy needed for the reaction. Hence increasing the rate of reaction

A mixture of venoms from several snakes of the same species is used. Suggest why.

1. May be different form of antigen/toxin (within one species)

OR

Snakes (within one species) may have different mutations/alleles

2. Different antibodies (needed in the antivenom)

OR

(Several) antibodies complementary (to several antigens)

Mammals such as a mouse and a horse are able to maintain a constant body temperature.

Use your knowledge of surface area to volume ratio to explain the higher metabolic rate of a mouse compared to a horse. (3 marks)

Mouse

1. (Smaller so) larger surface area to volume ratio;

2. More/faster heat loss (per gram/in relation to body size);

3. (Faster rate of) respiration/metabolism releases heat;

Explain five properties that make water important for organisms.(5 marks)

1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;

2. A solvent so (metabolic) reactions can occur

OR

A solvent so allowing transport of substances;

3. High heat capacity so buffers changes in temperature;

4. Large latent heat of vaporisation so provides a cooling effect (through evaporation);

5. Cohesion (between water molecules) so supports columns of water (in plants);

6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms;