BIOLOGY ESSAY EXAMPLES:
(with answers that received 100%)
GRADE 11: NOVEMBER EXAM
All is calm in Arderne house and the Grade 11 boarders have pulled rank and are
dominating the television to watch the final of ‘The Bachelor’. Joëlle decides that it
is time for coffee and puts on the kettle. She lines up the mugs for herself, Imogan,
Jemma, Gabriella and Reece. While pouring, she accidently spills boiling water on
her hand and she shouts out in pain. Explain how she is able to pull her hand away
immediately and explain how the other girls will hear her shout of pain.
Reflex action
The action of her pulling her hand away immediately is known as a reflex action which is
an automatic or rapid response to a stimulus. In this case, she uses her spinal cord as
she moves in a rapid way before she realises what has happened (without the brain). A
simple reflex arc occurs. Pain receptors in the skin of her hand pick up the stimulus which
is converted into a nerve impulse. The impulse is then transmitted by the dendrite of the
sensory neuron. The nerve impulse travels to the cell body of the neuron in the dorsal root
ganglion and is then transmitted to the axon of the sensory neuron in the dorsal root of
the spinal nerve. It is then transmitted across a synapse into the grey matter of the spinal
cord where it will be interpretated by an interneuron which is a part of the central nervous
system. The interneuron will then send an impulse acrosss a synapse to a motor neuron
which will leave the spinal cord via to the ventral root of the spinal nerve. The impulse will
then travel along the axon of the motor neuron until it reaches the muscles in Joëlle’s
hand. This allows the effectors or muscles to carry out the correct response to the
stimulus which will be to remove her hand from the hot water.
Hearing
The sound waves from her shouts will be collected by the other girls’ pinnae. The pinna
will direct the sound waves towards the auditory canal and tympanic membrane or ear
drum. The ear drum will convert the sound waves into vibrations of the same frequency
which is made possible by the eustachian tube maintaining equal air pressure on both
sides of the ear drum. The vibrating ear drum will cause the three bony ossicles in the
middle ear to vibrate. They will intensify, amplify and transmit the sound vibrations from
the outer ear to the inner ear. The vibrating foot plate of the stirrup causes the oval
window to vibrate which results in waves of the same frequency in the perilymph of the
vestibular canal. These vibrations are then transmitted through the Reissners membrane
into the endolymph of the cochlea canal. This causes the basilar membrane with the
organs of corti to vibrate. The sensory hair cells with their endings embedded in the
tectorial shelf are bounced up and down with their endings being bent or stretched. This
mechanical stimuli stimulates the hair cells to initiate a nerve impulse which will travel via
the auditory nerve to the cerebrum of the brain. Therefore, the girls can interpretate the
shouts of Joëlle. The remaining sound vibrations in the perilymph of the tympanic canal
are absorbed by the round window to prevent the sound from going backwards.
[20 marks]
GRADE 12: FEBRUARY BLOCK TEST
Question 7: (MINI ESSAY)
Write a mini essay in which you explain the homeostatic control of the Carbon
Dioxide level in the blood. Your explanation must include the concept of negative
feedback.
CO2 levels too high
When cellular respiration increases such as after doing exercise, more carbon dioxide is
produced. CO2 dissolves in the blood plasma to produce bicarbonate ions and hydrogen
ions. This causes the blood to become more acidic i.e. blood pH lowers. This stimulus is
picked up by chemoreceptors in the aortic arch, carotid artery and medulla oblongata. An
impulse is sent to the breathing centre of the medulla oblongata which sends a message
to the respiratory muscles and heart. This causes the breathing rate to increase and
deepen therefore causing more carbon dioxide to be exhaled or CO2 concentration in the
blood decreases.
CO2 levels too low
If there is a decrease in cellular respiration, less carbon dioxide will be produced and so
there will be less carbon dioxide in the blood. This will raise the pH of the blood which is
picked up b chemoreceptors in the aortic arch, carotid artery and medulla oblongata. This
causes an impulse to be sent to the breathing centre of the medulla oblongata which
sends an impulse to respiratory muscles and the heart. This causes the breathing rate t
decrease and become shallower therefore less CO2 is exhaledand carbon dioxide
concentration in the blood increases.
Negative feedback
Negative feedback is when an increase in the end product causes a decrease in
production or a decrease in the end product causes an increase in production. The
homeostatic control of CO2 is an example of negative feedback. When the CO2 level in
the blood increases, a series of processes causes the breathing rate to increase and
deepen therefore increasing the amount of CO2 that is exhaled from the body.
[12 marks]
GRADE 12: APRIL BLOCK TEST
Question 4:
Meiosis is an important process as it introduces variation in the offspring.
Write an essay where you discuss the importance of meiosis and describe the
mechanisms in meiosis that contribute to introducing genetic variation in the
offspring.
Importance of meiosis
Meiosis is known as a reduction division as it halves the chromosome number or
produces cells which are haploid compared to the diploid mother cells. This counteracts
the doubling effect of fertilization. Meiosis results in gametes which are haploid. This will
ensure that after fertilization, the zygote will be diploid which ensures that the organism
will have the correct chromosome number.
Meiosis also results in 4 new haploid gametes which can help increase the success of
fertilization. Also these 4 new cells are all genetically different/unique which will help bring
veriation into the species. This can help with the survival of the species due to natural
selection within a environment that may change.
Mechanisms that result in variation
The first mechanism happens during prophase I in which homologous chromosomes pair
up to form bivalents. These homologous chromosomes touch at particular points called
chiasma. At these chiasma, chromatids will break so that a swapping of genes can occur
between homologous chromosomes. This ensures the creation of genetic variation or new
genetic combinations. This process is known as crossing over.
The second mechanism occurs during metaphase I in which homologous chromosomes
arrange themselves in pairs along the equator of the spindle. However, this is done
randomly. In other words, random maternal and paternal chromosomes will reach each
pole resulting in the new cells having unique chromosome combinations. This process is
known as random arrangement of maternal and paternal chromosomes at the equator
during metaphase I.
The last mechanism occurs during metaphase II in which chromosomes arrange
themselves in a single row on the equator of the spindle. Each chromosome consists of
two sister chromatids. This arrangement of sister chromatids is random and so the
process of random arrangement of sister chromatids during metaphase II can also result
in variation.
[20 marks]
GRADE 12: JUNE EXAM PAPER 1
Question 4:
The pituitary gland is known as the master gland of the body. The secretions of this
gland regulate the secretions of other glands. Explain the structure of this gland and
where it is situated in the brain. Describe how the hormone secretions from the
pituitary gland controls the a) functioning of the thyroid gland and b) the functioning
of the testes.
The pituitary gland is located in the brain near the hypothalamus and the thalamus. It is
made up of two lobes, the anterior and the posterior lobe. Each lobe secretes different
hormones and these hormones control the functioning of other structures in the body
such as the thyroid gland and the testes.
Functioning of the thyroid gland
When cellular respiration drops, the anterior lobe of the pituitary gland secretes a
hormone called Thyroid Stimulating Hormone (TSH). It secretes more of this hormone
therefore there will be more TSH in the blood. TSH targets the thyroid gland to secrete
thyroxin. The increase in thyroxin in the blood causes the basal metabolic rate to increase
and therefore cellular respiration will increase. This process is controlled by a negative
feedback mechanism. So an increase in thyroxin will decrease the secretion of TSH.
Functioning of the testes
The anterior lobe of the pituitary gland secretes hormones called the Interstitial Cell
Stimulating Hormone (ICSH) and the Follicle Stimulating Hormone (FSH). ICSH targets the
interstitial cells in the testes which causes these cells to secrete testosterone.
Testosterone plays a key role in the proper production of sperm and secondary sexual
characteristics in the male. FSH targets the seminiferous tubules in the testes which
causes these tubules to produce more sperm. These mechanisms are controlled by a
negative feedback mechanism. An increase in testosterone inhibits the secretion of FSH
and ICSH and so there will be less testosterone secreted and less sperm produced. A
decrease in testosterone will cause or stimulate the pituitary gland to secrete more FSH
and ICSH thus causing more testosterone to be secreted and more sperm to be
produced.
[20 marks]
GRADE 12: JUNE EXAM PAPER 2
Question 4:
Describe the structure of DNA as well as the process of DNA replication. Also
explain the significance of DNA replication for meiosis.
Structure of DNA
DNA is a polymer or macromolecule made up of monomers which are nucleotides. Each
nucleotide consists of a phosphate molecule, deoxyribose (sugar molecule) and a
nitrogenous base. Nitrogenous bases in DNA are adenine and guanine which are purines
and cytosine ad thymine which are pyrimidines. DNA has a double helical shape. So it
consists of two strands. If one was to unwind the DNA molecule, it would look like a
ladder. The backbone would be made up of phosphate and deoxyribose which are joined
together by sugar-phosphate bonds. Nitrogenous bases are joined to the sugar molecule.
Nitrogenous bases occur in pairs and so a purine will always pair with a pyrimidine. In
other words, adenine will always pair with thymine and guanine will always pair with
cytosine. Weak hydrogen bonds join nitogenous bases together. Nitrogenous bases can
occur in any sequence or number which allows DNA to be unique to an individual.
Process of DNA replication
During interphase before cell division, chromosomes need to replicate i.e. DNA replication
needs to occur. The DNA molecule unwinds to form a ladder. An enzyme called DNA
polymerase causes the hydrogen bonds between nitrogenous bases to break and so the
molecule unzips. Each DNA strand acts as a template onto which a replica will be
created. There are free-floating DNA nucleotides in the nucleoplasm which join onto the
exposed nitogenous bases. Since nitrogenous bases are complementary or always occur
in pairs, the new strand will be exactly the same as the old strand (A joins to T and C joins
to G). Enzymes join the nucleotides together which result in two DNA molecules with two
strands. These DNA molecules are exactly the same with each having one original strand
and one new strand.
Significance of DNA replication for meiosis
DNA replication ensures that each gamete produced by meiosis has equal sharing of
genes or DNA. It ensures that all 4 haploid gametes have equal sharing of DNA. So it
makes sure that no information is lost or misplaced.
[20 marks]
GRADE 12: AUGUST BLOCK TEST
Question 7:
Describe how meiosis, reproduction and genetic mutations increase variation within
a population.
Meiosis
During prophase I, chiasmata form and there is a crossing over of genes between
homologous chromosomes which results in new genetic combinations. During metaphase
I, there is random arrangement of maternal and paternal homologous chromosomes at
the equator while during metaphase II, there is a random arrangement of sister
chromatids at the equator of the spindle. This results in unique chromosome
combinations. Meiosis also results in four gametes which are each genetically unqiue
therefore introducing variation.
Reproduction
Chance fertilization causes variation. The combination of male and female gametes is
done randomly which means that there will be different genetic combinations each time
fertilization occurs. Random mating also results in variation. Males and females within a
population mate randomly so that new genetic variations are created.
Genetic mutations
Mutations are a spontaneus change in the DNA or genetic material of an organism. Gene
mutations are when a single base pair is replaced, deleted or added by accident. There
are two types of genetic mutations, namely point mutations and frameshift mutations.
Point mutations are when only one codon is affected or in other words, one of the bases
of the codon is replaced or the nitrogenous bases of the codon are inversed. This will only
change one amino acid. Frameshift mutations are when bases are added or deleted from
the gene which results in the sequence shifting which results in all codons changing.
These mutations can cause a different protein to be made or even no protein to be made.
In other words, it changes the code thereby not creating the correct proteins. In order to
increase the variation within a population, these mutations need to occur within a gamete
so that these mutations can be passed on to the next generations. If the mutation occurs
witin one individual in one of their somatic cells, then they will display an unique quality or
characteristic which will give variation to the population during a period of time but it will
not be passed on.
[20 marks]
GRADE 12: MOCK EXAM PAPER 1
Question 4:
Nontobeko had been walking in the desert, without water, for two days, when she
suddenly heard a sound behind her. She turned her head and saw a snake coming
towards her. She became scared and turned around to run away. As she was
running, she tripped and fell.
Describe how her body regulated water content during the two days and describe
how her balance would have been restored after she fell down.
Water regulation
By not having water for two days, Nontobeko’s water levels in her blood will decrease.
This stimulus will stimulate osmoreceptors in the hypothalamus. The hypothalamus is
where the anti-diuretic hormone is produced. A impulse is sent to the posterior lobe of the
hypophysis which will secrete or release more ADH into the blood. The increased levels of
ADH in the blood will target the distal convoluted tubules and collecting ducts in the
kidney. It will then cause the cells in the walls of these structures to become more
permeable to water. This means that more water will be reabsorbed from the filtrate into
the ‘salty’ medulla of the kidney. This will raise water levels in the body and will cause
more dilute and less volume urine to be produced. This process is called osmoregulation.
Thus her water levels of her body would be regulated. If her water levels were too high,
less ADH will be secreted and a negative feedback mechanism will occur.
Regaining balance
Mechanoreceptors in the inner ear play a key role in the maintenance of balance. When
she fell down, her head position relative to gravity would have changed. The utriculus and
sacculus are sensitive to changes in the position of the head relative to gravity. In these
structures, there are structures called maculae which consist of sensory hair cells
embedded in a jelly-like substance with granulated calcium carbonate pieces called
otoliths. The otoliths sit on top of the sensory hair cells. When the head’s position
changes, the otoliths will change the amount of pressure they exert on the sensory hair
cells due to the force of gravity. This change or stimulus is converted to a nerve impulse
by the sensory hair cells and is sent to the cerebellum via the vestibular nerve. In this way,
she will be able to regain balance.
By turning around and falling down, she would have moved her head. The semi-circular
canals with ampullae are sensitive to movements of the head. At the bottom of the semicicular canals, there are ampullae which are made up of structures called cristae. These
cristae are made up of a jelly substance called cupula with sensory hair cells embedded
in the cupula. Movements of the head causes the endolymph in the semi-circular canals
to flow and move around. This causes the cupula to bend which stimulates the sensory
hair cells to convert the stimulus to a nerve impulse. The nerve impulse is sent via the
vestibular nerve to the cerebellum where movements of the head can be interpretated
and balance can be maintained.
[20 marks]
GRADE 12: MOCK EXAM PAPER 2
Question 4:
Explain how parts of the skeleton of primitive ape-like beings and those of modern
humans support the idea that the general trend in human evolution has been
towards bipedalism. Also explain the advantages that being bipedal would offer the
modern humans.
Skeletal changes towards bipedalism
Bipedalism involves walking upright on two legs. There are many skeletal changes that
happened in the evolution of modern humand from ape-like beings in order for bidepalism
to occur. Firstly, the foramen magnum moved from being at the back of the skull to the
middle of the base of the skull so that to allow the head to be balanced in upright
movement. Also the pelvis went from being narrow and long to being short, wide and
bowl-shaped in order to accommodate the internal organs as well as to support the
weight of the upper body. The foot lost its gripping ability by developing a big toe that
was no longer divergent like those in ape-like beings, but was parallel to the other toes.
So the foot become used for locomotion. Also the foot went from being flat in ape-like
beings to having an arch which would act as a shock absorber. The femur also became
more angled in order to support the weight of the upper body ad humans developed
stronger knew joints than ape-like beings. The spine became s-shaped instead of cshaped to act as a shock absorber and for flexibility. Lastly, the arms became shorter and
weaker than the legs as humans moved away from walking on all fours.
Advantages of Bipedalism
Firstly by standing upright, one is able to see far over the grass on grasslands in order to
spot predator and food. Walking on two legs also expends less energy than walking on all
fours which allows one to travel further distances over grasslands.
An important function of bipedalism is its role that it plays in temperature regulation. By
walking upright, less surface area is exposed to the direct sun rays and more surface area
is exposed to wind and air currents which promote cooling. Also a large surface area was
available for sweating which also promotes cooling. These temperature regulating
mechanisms allowed one to live in a variety of different niches.
Lastly, bipedalism allowed one’s arms to be free. Now the forelimbs could perform a
variety of tasks such as defence, holding young, gathering food and social
communication through hand gestures. It could also promote the making of tools and the
use of fire. This may have contributed to humans becoming intelligent beings with a
greater brain capacity.
[20 marks]
TIPS FOR ESSAYS:
• 17 marks are for facts and 3 marks are for style (20 marks total)
• Style relates to the following: (you do not need to be an expert English writer)
- No incorrect facts or unnecessary information
- Flows nicely, logical order (headings help)
- Contains paragraphs
• There is no need for an introduction or conclusion. It is purely factual.
• Writing headings helps with your style and helps you to organise your thinking. Try to
highlight the main points in the question and make those your headings.