Answers For Extension Worksheet – Chapter 11

Cambridge Biology for the IB Diploma

Answers for extension worksheet – Chapter 11

1 Sperm formation takes place in the walls of the seminiferous tubules of the testes. Outer cells multiply by mitosis, and then undergo a period of growth before undergoing division by meiosis. The developing sperm cells complete their development while attached to Sertoli cells. Spermatogenesis is a continuous process that begins at puberty and continues until death.

In oogenesis, the multiplication phase takes place in the ovaries of the fetus. Several thousand primary oocytes are produced and remain in primary follicles. No further development occurs until puberty, when the menstrual cycle begins. Each month, one or two follicles develop into mature follicles. At ovulation, a mature follicle ruptures and releases the secondary oocyte into the oviduct. This will only undergo the second division of meiosis to become a haploid ovum if fertilisation occurs. Oogenesis in women ceases after the menopause. (15)

2 a Intravenous injection delivers the antibiotic directly to the bloodstream, so it reaches a peak in the infected tissue in less than one hour, but after 120 minutes the level of antibiotic has fallen back to zero.

Intramuscular injection means that the antibiotic must travel through muscle tissue before entering the blood so the level of antibiotic reaching the infected tissue does not rise as quickly or reach the same high concentration as for intravenous injection, but the level in the body remains quite high for a longer period of time (about 150 minutes).

Antibiotic taken by mouth must pass through the stomach and small intestine before reaching the blood and being carried to the infected tissue. This means that the antibiotic takes a longer time to build up to its highest level but the antibiotic remains at a high level for longer (almost 180 minutes). (3)

b Antibodies are proteins that are made in response to antigens. They are secreted into the blood plasma, where they circulate and can help to destroy antigens. Antibiotics are synthetic, manufactured antimicrobial drugs, which are also produced naturally by some fungi. They interfere with the metabolism of bacteria, killing them or preventing them from reproducing. (2)

c Ready-made antibodies may be injected into a person in circumstances when the body would not have time to identify and produce its own antibodies against a serious antigen, which could be fatal. For example, a person who has suffered a bite from a venomous snake or an animal with rabies might be given anti-venom containing antibodies. (2)

3 a Prior to muscle contraction, ATP binds to heads of myosin (molecules) so they are primed. When an action potential arrives, calcium ions are released from the sarcoplasmic reticulum. These ions cause the removal of blocking molecules so that myosin heads can bind to the actin filament. As a muscle contracts, actin filaments slide past myosin filaments towards the centre of the sarcomere. (7)

b (3)

4 a (4)

Process / Region(s) of the kidney
water regulation influenced by ADH / collecting duct
selective reabsorption of glucose and amino acids / proximal convoluted tubule
water reabsorption / descending loop of Henle
ultrafiltration / glomerulus

b ADH causes an increase in the permeability of the collecting duct so that water is reabsorbed. If there is insufficient ADH, copious amounts of dilute urine will be produced and excess water will be lost from the blood. This will produce symptoms of frequent urination, thirst and dehydration. (2)

5 a This is due to the fall in the level of estrogen, which inhibits FSH and LH production. (1)

b (4)

Egg cell / Sperm cell
large – as it is produced by unequal divisions of cytoplasm during meiosis / small – produced by even divisions of meiosis
immobile – waits to be fertilised in the oviduct / motile – must swim to meet the ovum in the female reproductive tract
one released per month – controlled by hormones of the menstrual cycle / continuous production – male hormones stimulate sperm production from puberty onwards
stores nutrients in cortical granules – nutrients are needed for survival after fertilisation until implantation / no stores of nutrients – energy is obtained from constituents of semen