Explain how a DNA double helix is formed

Explain the process of translation, leading to polypeptide formation.

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain

Ribosomes bind to mRNA in the cell's cytoplasm and move along the mRNA molecule in a 5' - 3' direction until it reaches a start codon (AUG)

Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. UAC will align with AUG)

Each tRNA molecule carries a specific amino acid (according to the genetic code)

Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via a condensation reaction)

The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is released

Explain DNA replication

Helicase

Unwinds the DNA and separates the two polynucleotide strands by breaking the hydrogen bonds between complementary base pairs

The two separated polynucleotide strands act as templates for the synthesis of new polynucleotide strands

DNA Polymerase

Synthesises new strands from the two parental template strands

Free deoxynucleoside triphosphates (nucleotides with three phosphate groups) are aligned opposite their complementary base partner and are covalently bonded together by DNA polymerase to form a complementary nucleotide chain

The energy for this reaction comes from the cleavage of the two extra phosphate groups.

Explain enzyme–substrate specificity

The active site of an enzyme is very specific to its substrates as it has a very precise shape. This results in enzymes being able to catalyze only certain reactions as only a small number of substrates fit in the active site. This is called enzyme-substrate specificity. For a substrate to bind to the active site of an enzyme it must fit in the active site and be chemically attracted to it. This makes the enzyme very specific to it’s substrate. The enzyme-substrate complex can be compared to a lock and key, where the enzyme is the lock and the substrate is the key.

Explain the effects of temperature, pH and substrate concentration on enzyme activity

Temperature

Low temperatures result in insufficient thermal energy for the activation of a given enzyme-catalysed reaction to be achieved

Increasing the temperature will increase the speed and motion of both enzyme and substrate, resulting in higher enzyme activity

This is because a higher kinetic energy will result in more frequent collisions between enzyme and substrate

At an optimal temperature (may differ for different enzymes), the rate of enzyme activity will be at its peak

Higher temperatures will cause enzyme stability to decrease, as the thermal energy disrupts the hydrogen bonds holding the enzyme together

This causes the enzyme (particularly the active site) to lose its shape, resulting in a loss of enzyme activity (denaturation)

pH

Changing the pH will alter the charge of the enzyme, which in turn will protein solubility and may change the shape of the molecule

Changing the shape or charge of the active site will diminish its ability to bind to the substrate, abrogating enzyme function

Enzymes have an optimum pH (may differ between enzymes) and moving outside of this range will always result in a diminished rate of reaction.

Substrate Concentration

Increasing substrate concentration will increase the activity of a particular enzyme

More substrate means there is an increased likelihood of enzyme and substrate colliding and reacting, such that more reactions will occur and more products will be formed in a given time period

◾After a certain point, the rate of reaction will cease to rise regardless of further increases to substrate concentration, as the environment has become saturated with substrate and all enzymes are bound and reacting (Vmax).

Explain what happens in aerobic respiration.

If oxygen is available, the pyruvate is taken up into the mitochondria and is broken down into carbon dioxide and water. A large amount of ATP is released during this process.

Explain the use of lactase in the production of lactose-free milk

Lactose is a disaccharide of glucose and galactose which can be broken down by the enzyme lactase

Historically, mammals exhibit a marked decrease in lactase production after weaning - leading to lactose intolerance (incidence is particularly high in Asian / African / Native American / Aboriginal populations)

Lactose-free milk can be produced by purifying lactase (e.g. from yeast or bacteria) and binding it to an inert substance (such as alginate beads)

Milk passed over this immobilised enzyme will become lactose-free

The generation of lactose-free milk can be used in a number of ways:

As a source of milk for lactose-intolerant individuals

As a means to increase the sweetness of milk (glucose and galactose are sweeter in flavour), thus negating the need for artificial sweeteners

As a way of reducing the crystallisation of ice-creams (glucose and galactose are more soluble than lactose)

As a means of shortening the production time for yogurts or cheese (bacteria ferment glucose and galactose more readily than lactose)

Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass

Photosynthesis can be measured in many ways as it involves the production of oxygen, the uptake of carbon dioxide and an increase in biomass. For example, aquatic plants release oxygen bubbles during photosynthesis and so these can be collected and measured. The uptake of carbon dioxide is more difficult to measure so it is usually done indirectly. When carbon dioxide is absorbed from water the pH of the water rises and so this can be measured with pH indicators or pH meters. Finally, photosynthesis can be measured through an increase in biomass. If batches of plants are harvested at a series of times and the biomass of these batches is calculated, the rate increase in biomass gives an indirect measure of the rate of photosynthesis in the plants.

Explain what happens in anaerobic respiration

In anaerobic cell respiration the pyruvate stays in the cytoplasm and in humans is converted into lactate which is the removed from the cell. In yeast the pyruvate is converted into carbon dioxide and ethanol. In either case, no ATP is produced.

Coolant

Both plants and animals use the evaporation of water from the surfaces of their bodies to facilitate cooling (sweating and panting in animals, transpiration from leaves in plants)

Water can be used to carry heat to cooler places in our bodies (countercurrent exchange of thermal energy)

Medium for Metabolic Reactions

Water can dissolve many organic and inorganic substances to facilitate chemical reactions

Water can also absorb thermal energy released as a by-product of many chemical reactions

Transport Medium

The forces of attraction between water molecules help facilitate the transport of water up the xylem of plants

Water is an effective transport medium for dissolved substances (in plants, minerals from the soil and sugars from the leaves can be transported in water through the xylem andphloem respectively; while in animals, water in the blood is used to transport oxygen, glucose and urea)

Surface Tension

The force of attraction between water molecules makes water sufficiently dense for some smaller organisms to move along its surface

Each of the nitrogenous bases can only pair with its complementary partner (A=T ; G=C)

Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase:

The new strands formed will be identical to the original strands separated from the template

The two DNA molecules formed will be identical to the original molecule.