Common Pitfalls (AQA A Level Biology): Revision Note

"Energy is produced"

Energy is not created; in respiration, energy stored in substrates is released and transferred

Energy is released / transferred OR ATP is produced / synthesised

“The frequency of the gene increases in the population”

A gene is a section of DNA coding for a polypeptide; different versions of that gene are alleles

In natural selection, it is the frequency of an advantageous allele that changes

The frequency of the advantageous allele increases in the population

“The antibiotic causes a mutation for resistance in the bacteria”

Mutations are random changes in DNA; they do not occur in response to a selection pressure, e.g. an antibiotic

Selection pressures act on existing variation

A random mutation gives rise to an allele that provides antibiotic resistance; when antibiotics are used, bacteria with this allele are more likely to survive and reproduce

“Microfibrils in muscle contract”

Myofibrils are the contractile structures in skeletal muscle

Microfibrils are associated with cellulose in plant cell walls

Myofibrils in skeletal muscle contain the actin and myosin needed for contraction

"The cells lining the ileum have villi on their surface"

Villi are large folds of the ileum wall

Microvilli are small folds of the cell-surface membrane; they are present on the epithelial cells, which are on the villi

The ileum has villi, and the epithelial cells on these villi have microvilli

“The alveolus has a thin cell wall”

Only plants, algae, fungi and prokaryotes have cell walls

Animal cells may group together to form tissue walls, e.g. the walls of the alveoli or the blood vessels, but do not have cell walls

The alveolar epithelium is a single layer of thin cells

"Hydrogen bonds between bases are hydrolysed"

Hydrolysis is breaking a covalent bond using water, e.g. in ATP or polypeptides

Hydrogen bonds are weak intermolecular forces; they are broken, not hydrolysed

Hydrogen bonds between bases are broken

"DNA polymerase forms hydrogen bonds between complementary bases"

DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds in the sugar–phosphate backbone

Hydrogen bonds between bases form by complementary base pairing; this process is not catalysed by DNA polymerase

DNA polymerase catalyses the formation of phosphodiester bonds between nucleotides

"Viruses are unicellular"

Unicellular organisms consist of a single cell

Viruses are not made of cells, have no cytoplasm or organelles and consist of genetic material inside a protein coat

Viruses are acellular