Extended response questions

Question 1

Hydrophilic signaling molecules and signal transduction

Answer 1

• Receptors are transmembrane proteins which are specific to to signalling molecules (2)
• Binding to the receptor occurs at the cell surface (1)
• Binding to the receptor causes conformational change which initiates a change inside the cell (2)
• Transduced signals activate:
  enzymes/phosphorylation (1)
  G-proteins (1)

Question 2

Insulin signalling and diabetes (4)

Answer 2

• Binding of insulin to its receptor triggers GLUT-4 recruitments to cell membrane (1)
• Type two diabetes is caused by a loss of insulin receptor sensitivity (1)
• Type two diabetes is associated with obesity (1)
• Exercise increases GLUT-4 recruitment (1)

Question 3

* Types of R groups, Structure at each level

Discuss the structure of amino acids and the four levels of protein structure (9)

Answer 3

• Amino acids only differ in structure of R group (1)
• Types of R groups are basic, polar, hydrophobic and acidic (1)
• Primary structure is the order of amino acids in a polypeptide (1)
• Amino acids are linked by peptide bonds (1)
• Secondary structure is from the hydrogen bonding along the backbone of the protein (1/2)
• Alpha helices, beta plated sheets and turns are all types of secondary structure (1)
• Tertiary structure is the folding of a polypeptide (1)
• Tertiary structure is established by the interactions between r groups (1)
• Quaternary structure is the spatial arrangement of subunits (1)

Question 4

Describe the generation and transmission of a nerve impulse in a neuron (9)

Answer 4

• Resting membrane potential is when there is no net flow of ions across the membrane (1)
• Neurotransmitters bind to specific receptors (1)
• Binding of neurotransmitter opens ligand gated channels (1)
• Sodium ions enter the cell by moving down their concentration gradient (1)
• When the membrane is polarised beyond a threshold value (1) the opening of voltage gated sodium channels is triggered (1)
• Further depolarisation occurs and the sodium channel is closed (1)
• Voltage gated potassium channels open (1)
• Potassium ion move out of the cell (1)

Question 5

Discuss animal reproduction strategies under the following headings:
(i) mating systems in animals
(ii) courtship and female choice.

Answer 5

(i) Mating systems are based on how many mates an individual has during one breeding system (1)
Monogamy: the mating of a pair of animals to the exclusion of all others (1)
Polygamy: individuals (of one sex) have more than one mate. (1)
Polygyny: one male mates
exclusively with a group of
females (1)
Polyandry: one female mates with a number of males (1)

(ii) Animals use courtship rituals to attract a mate (1)
Sexual selection increases chance of mating (1)
In sexually dimporhic species males usually have more conspicuous markings (1)
Female choice involves females assessing honest signals displayed by males (1)
Honest signals indicate alleles that increase male fitness (1)
In lekking males gather to display at a lek, where female choice occurs (1)

Question 6

Discuss changes in allele frequency under the following headings:
(i) natural selection
(ii) genetic drift.

Answer 6

(i) natural selection
Natural selection is non-random (1)
Variation in new alleles arise as a result of mutation (1)
Mutations can be beneficial/harmful to the fitness of an individual (1)
Individuals with favourable
alleles/that are better adapted
are more likely to survive and
reproduce/produce offspring (1)

(ii) Genetic Drift
(Genetic drift occurs when)
chance/random events cause
unpredictable/random
fluctuations in allele frequencies (1)
Genetic drift is more likely to affect small populations (1)
Alleles are more likely to be lost from the gene pool in small populations (1)
Population bottlenecks occur
when a population is reduced for at least one generation (1)
Founder effects occur through the
isolation of a few members of a population from a larger population (1)

Question 7

Describe the roles of cells of the immune system in defence against
parasitic attack under the following headings.
(i) Non-specific cellular responses
(ii) Specific cellular responses

Answer 7

(i) Non-specific cellular responses
Phagocytes engulf pathogens (1)
Pathogen is enclosed within vesicle (1)
Lysosomes fuse with the vacuole and hydrolases destroy the pathogen (1)

(ii) Specific cellular responses
White blood cells release cytokines if tissue damaged (1)
Specific white blood cells accumulate at the site of damage (1)
Lymphocytes have receptors on their surface, which recognise antigens (1)
Binding of antigen to lymphocyte receptor causes the lymphocyte to divide (1)
This produces a clonal population of lymphocytes, of which some induce apoptosis and others produce antibodies (2)

Question 8

Discuss the role of sex chromosomes in the development of male and
female mammals under the following headings.
(i) Sex determination and sex-linked patterns of inheritance
(ii) X-inactivation

Answer 8

(i) Sex determination and sex-linked patterns of inheritance
Males are heterogametic and females are homogametic (1)
The SRY gene determines the development of male characteristics (1)
Males have only one copy of sex linked genes (1)
Females can be carriers of recessive sex-linked alleles (1)

(ii) X-inactivation
In females one x chromosome is inactivated (1)
X chromosome inactivation happens early in development (1)
Half of the cells in a tissue will have a working copy of the gene (1)
X chromosome inactivation prevents a double dose of gene products (1)

Question 9

Discuss control of the cell cycle under the following headings.
(i) Phases of the cell cycle and the importance of cell cycle checkpoints
(ii) The role of cyclins and cyclin-dependent kinases

Answer 9

(i)
Lakc of control of the cell cycle can result in degenerative diseases (1)
Phases are G1, S, G2 and M (1)
G1 and G2 are growth stages (1)
DNA replication in S phase (1)
Mitosis and cytokinesis occur in the M phase (1)
Checkpoints at G1, G2, M which regulate progression to the next stage (1)
e.g. the size and mass are assessed at the G1 checkpoint (1)

(ii)
Cyclins accumulate during G1 (1)
Cyclins combine with cycline-dependant kinases (Cdks) (1)
This activates the Cdks (1)
Active Cdks phosphorylate proteins that stimulate the cell cycle (1)
If there is a sufficient threshold of Phosphorylation, cell cycle progresses (1)

Question 10

Discuss the formation of variable gametes during meiosis under the
following headings:
(i) the activity of homologous chromosomes;
(ii) meiosis II.

Answer 10

(i)
Homologous chromosomes have the same centromere position (1)
Chiasmata form where chromosomes touch (1)