Download topic 3 - Essentials Education

TOPIC 3
O3
Exchange of materials may take place at special structures or organs
O3.1
Explain why exchange surfaces in the body must be thin, moist, and have a large
surface area
Animals require a supply of oxygen and digested food. These materials are necessary for the
process of respiration that occurs in the cells. In addition cells need a supply of water and need
to dispose of carbon dioxide and a chemical which contains waste nitrogen called urea. These
materials are exchanged with the external environment primarily by the process of diffusion
that has been discussed in an earlier section. See C5.1.
organisms
Diffusion is the process of movement of particles from an area of high concentration to an area
of low concentration so as to become equally distributed through all of the space available. Very
often materials diffuse through cell membranes and pass easily into and out of cells. Such is
the case with the substances mentioned above. The ease with which diffusion occurs depends
largely on the concentration of the substance on either side of the membrane and also the
thickness of the membrane. The thinner the membrane, the more quickly will diffusion occur.
Most substances are soluble in water and can only diffuse through membranes as solutes or
dissolved substances. The membrane has to be moist and once it begins to dry out will not
allow efficient diffusion. Another factor that will influence the rate of diffusion is the total area
of membrane available. This is an important concept in biology and has been discussed in C1.2.
The idea can be illustrated very simply by taking a sheet of paper towelling and tipping water
on it, notice how quickly it absorbs the water. Now take another piece of towelling of a similar
size and screw at up tightly into a ball, again tip the water on it and notice how long it takes to
absorb the water. Clearly, even though the mass of the paper is the same, the flat piece of paper
will absorb the water much more quickly because it has a much greater surface area through
which absorption can occur. It is important that organisms have thin moist membranes that
provide a large surface area to allow all of the rapid and efficient exchange of materials with
the external environment. In animal bodies much of this diffusion occurs into and out of the
blood plasma as it circulates around the body. The following table summarises the main organs
involved in the diffusion of materials between the internal and external environment.
Organ
Structural units
Substances exchanged
Kidney
nephrons
Urea, water, salts
Lungs
alveoli
Carbon dioxide, oxygen, water
Intestine
villi
Water, amino acid, sugars, fatty
acids, glycerol
Key Points
• It is vital that materials are exchanged between the internal and external environment.
• The following organs contain the special structures listed to maximise the exchange
kidney: nephron; lungs: alveoli; intestine: villi
• Exchange surfaces are:
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thin: to decrease the distance that materials need to diffuse across
moist: to keep cells alive and provide a layer for substances like oxygen to dissolve in prior to diffusion
large surface area: to provide a larger area over which diffusion can occur.
BIOLOGY: Key Ideas – THIRD EDITION
ORGANISMS
O3.2
Explain the role of blood capillaries and lymph capillaries in the exchange of materials
Most multi-cellular organisms, including mammals, have quite a complex transport system
within their bodies. The transport medium is blood, the pump is the heart, and the vessels are
arteries, veins and capillaries. Humans have a four-chambered, double–action pump called the
heart with two separate circulations. The right side of the heart receives blood from the body
through the vena cava and pumps it through the pulmonary artery to the lungs. As blood passes
through the lungs it receives fresh oxygen and disposes of carbon dioxide, as explained earlier.
The blood then returns to the heart through the pulmonary vein and is pumped to various parts
of the body through the main artery, which is called the aorta.
Exchange
surfaces
In humans the aorta is about the same diameter as your thumb. Soon after leaving the heart,
smaller arteries branch off to the head, arms, other internal organs and eventually to the
legs. Arteries have thick, elastic walls that are able to accommodate the high and fluctuating
pressure produced by the contraction of the heart muscle. The walls of the arteries consist of
connective tissue outside of muscle tissue with a layer of epithelial tissue innermost. When this
muscle tissue contracts, it is able to restrict the flow of blood in certain arteries. The various
arteries continue to branch into smaller arterioles that have basically the same structure as the
larger arteries. It is important to realise that these arteries and arterioles do not leak, because
their function is to carry blood quickly and efficiently to the tissues without significant loss of
pressure. As the arterioles continue to branch and become smaller in diameter they eventually
lead into capillaries.
Arteriole
Lymph vessel
Lymphatic
capillaries
Spaces between
cells filled with
tissue fluid
Tissue
cells
Venule
Blood
capillary
Figure 314 Blood and lymph vessels
The capillaries have a diameter of approximately 10 micrometres and consist of an epithelium
surrounded by a membrane. These capillaries come in very close contact with all of the cells
in the body and enable exchange of materials between the blood and the cells. It has been
estimated that the total length of capillaries in the human body is several hundred thousand
kilometres. It has also been estimated that there are thousands of square metres of capillary
membranes across which diffusion can occur. These capillaries not only enable diffusion of
materials between the blood cells and plasma and the fluids surrounding cells, but they also
‘leak’.
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TOPIC 3
Thoracic duct emptying
into vena cava
Thoracic duct
Lymph nodes
in armpit
Lacteals
draining
intestine
organisms
Lymph nodes in groin
Lymph nodes behind knee
Figure 315 The human lymph system
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ORGANISMS
The pressure of the blood, particularly near the arteriole end, is sufficient to force some of
the plasma out through the epithelium and membrane and in between cells where it is called
extracellular fluid. This allows even greater exchange of materials with cells. This plasma will
eventually drain back into either blood or lymph capillaries. See Figure 314. From the blood
capillaries the blood drains into small veins or venules, these veins join together to form larger
veins and eventually drain back to the heart.
After the blood has passed through the capillaries it has lost most of its pressure and so the
blood in the veins can only move with the assistance of the contractions of skeletal muscle and
valves. Some of the extracellular fluid drains into small lymph capillaries which, in turn, drain
into larger lymph vessels. These are very much like veins in that they rely on muscle movement
and valves for movement of the fluid that is eventually emptied into the vena cava near the
heart. See Figure 315.
Key Points
• Arteries carry blood away from the heart and veins carry blood back to the heart.
• Blood capillaries link the arteries to the veins.
• Blood capillaries are very thin and allow the exchange of materials between the internal
and external environment.
• Lymph capillaries belong to the lymphatic system and drain excess fluid from the tissues
and return this fluid back to the circulatory system near the heart.
Focus Questions
1. Explain how the extensive network of capillaries in the human body assists in the
exchange of materials by diffusion.
2. State two differences between blood in capillaries and lymph in lymphatic vessels.
3. Explain how water moves by osmosis from the tissue spaces back into blood
capillaries and lymphatic vessels.
O3.3
Know the structural features of the nephrons in the kidney and understand the
importance of filtration and reabsorption
The kidneys are the major organs of excretion in the human body and also in the bodies of most
of the vertebrates. The major functions of the kidney are to remove waste materials containing
nitrogen such as urea and also to balance the amount of water and other dissolved substances in
the body fluids and blood plasma. The structural and functional unit of the kidney is the nephron,
as illustrated in Figure 316. Each nephron consists of a Bowman’s capsule, containing a ball of
capillaries, called the glomerulus and a long tubule that drains into a collecting duct.
The kidney acts very much like a swimming pool filter by diverting a certain proportion (about 20%)
of the blood from the aorta as it circulates around the body. This blood passes into the renal artery
and then into a system of arterioles that provide blood to the glomeruli and then branch further to
form a capillary network around the nephrons. The purified blood leaves the kidney through the
renal veins.
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TOPIC 3
Key Points
• The nephron is the unit of structure and function in the kidney that maximises the surface
area for exchange.
• One nephron consists of:
- a Bowmans capsule
- a ball of capillaries called the glomerulus
- a long thin tubule.
• Nephrons have a rich blood supply; thin and moist membranes
Filtration
organisms
As the blood passes through the kidneys about 20% of the plasma is forced from the glomerulus,
which consists of the specialised capillaries, through the Bowman’s capsule and into the tubules.
A person with a mass of 70 kilograms will have about 5.6 litres of blood per minute passing through
the kidneys, of which about 1.2 litres per minute will be filtered. Generally the capsules are located
in the outer part of the kidney called the cortex and the tubules, which may be several centimetres
long and very small in diameter, form a loop towards the inside or medulla of the kidney, as shown
in Figure 316. The tubules drain into larger collecting ducts that in turn drain into the pelvis of the
kidney, which empties into the bladder through the ureter. Check Figure 316 again.
There are about a million nephrons in each kidney and each one consists of a Bowman’s
capsule that contains the glomerulus and then a long and convoluted tubule that is interlaced
with capillaries. As a result of filtration, about 20% of the blood plasma becomes filtrate, it is
very similar to blood plasma but does not contain any red blood cells and contains only about
1% of the proteins in the blood (these being too large to be filtered through the capillaries)
Medulla
Cortex
Renal
pelvis
(a) KIDNEY
Tubule
Blood
vessels
Capillaries
Loop
of Henle
Ureter
Efferent
arteriole
Afferent
arteriole
Nephron
Collecting
duct
Branch of
renal vein
(b) KIDNEY STRUCTURE
Bowman's
capsule
Glomerulus
Decending
limb
Ascending
limb
Loop
of
Henle
(c) NEPHRON
Figure 316 Nephrons in the kidney
Reabsorption
As the filtrate slowly flows through the tubules, dissolved substances such as salt, glucose and amino
acids are actively reabsorbed into the capillaries and, as a result, water diffuses back by osmosis.
The rate of reabsorption can be influenced by hormones, for example the Anti-Diuretic Hormone
(ADH) controls the reabsorption of water. Alcohol decreases the production of ADH and therefore
increases the production of urine that can lead to dehydration. The epithelial cells lining the tubules
have a ‘brush border’ or microvilli which increases the surface area for this reabsorption. These have
a very similar structure and function to microvilli in the intestine. See Figure 318.
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In addition some chemicals including ions, drugs and poisons are excreted directly from the
capillaries into the tubule by active transport. About 99 per cent of all of the water is reabsorbed,
and urine is actually a very concentrated and sterile sample of blood plasma! An average figure
would be that 150 litres of blood plasma is filtered through the capsules every day and 148.5 litres
is reabsorbed through the tubule, leaving about 1.5 litres of urine to be excreted per day. This urine
drains through the ureters to the bladder for storage and is then passed out through the urethra at
your convenience!
It is important to note that the nephrons provide a thin, moist, large surface area and are richly
supplied with capillaries so as to maximise the rate of diffusion and exchange of materials.
Key Points
• There are two important processes that enable the kidney to:
a) excrete soluble wastes from the bloodstream
b) act in a homeostatic function to regulate blood concentration of vital solutes.
• These processes are known as Filtration and Reabsorption
Filtration occurs from the Glomerulus → Bowmans capsule
Reabsorption occurs from the Tubule → Blood capillaries
Focus Questions
1. Name the three structural components of the nephron and state the main function of
each.
2. Explain how each of i) a large surface area and ii) a rich blood supply is important in
the processes of filtration and reabsorption in the kidney.
3. State one difference in solute concentration that you would expect to find between
a) Blood plasma and filtrate
b) Filtrate and urine
4. The renal artery carries blood into the kidney and the renal vein carries blood out of
the kidney. Describe two differences you would expect to find when comparing the
blood in each of these vessels.
O3.4
Know the structural features of alveoli in the lungs and describe how gases are
exchanged through this surface
Most organisms need to obtain a supply of oxygen from their environment and also to dispose of
surplus carbon dioxide to it. Frogs, for example, are able to do this through their thin, moist skin
and fish do so through their gills. Humans and other mammals have special structures called lungs
that have evolved to perform this function. Air which is breathed in, passes through the trachea
which then branches into the bronchi and these branch even further into bronchioles. These airways
have spiral bands of cartilage, much like the hose in your vacuum cleaner, to maintain shape when
the pressure inside is lowered. These bronchioles terminate in small air sacs called alveoli.
See Figure 317.
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TOPIC 3
Key Points
• Alveoli are small air sacs in the lungs that provide the gas exchange surface over which
oxygen and carbon dioxide are exchanged between the air and the blood.
organisms
• Alveoli provide:
- a large surface area (135 m2)
- a thin membrane (1 cell in thickness)
- a moist layer to assist with diffusion
There are about 300 million alveoli in each lung and they have a total volume of about 3 litres and
a total area of about 135 square metres which is the floor area of a medium size house. This is the
respiratory surface and it is very thin and moist to assist diffusion of gases into and out of the blood.
Breathing in, or inspiration, occurs because muscles contract. The diaphragm, which is beneath
the lungs, contracts and pulls down, and some of the intercostal muscles between the ribs contract
to pull them upwards and outwards. The contraction of these muscles causes an increase in the
volume of the chest cavity and therefore a decrease in pressure. Air is then forced into the lungs by
the greater pressure outside of the body. Breathing out or expiration is essentially a passive process
that occurs when the muscles relax. However if a person is exercising the air can be expelled more
efficiently if the abdominal muscles are used to exert pressure on the intestines which in turn push
up on the diagram, decrease the volume, increase the pressure and force air out of the lungs. This
process is also assisted by another group of intercostal muscles that pull the ribs downwards. This is
sometimes called ‘forced expiration’.
The normal composition of the atmosphere is oxygen 20%, nitrogen about 80% and other gases,
including carbon dioxide, only a tiny fraction of a %. The blood returning from the body to the
lungs usually contains between 12 and 15% dissolved oxygen, again about 80% nitrogen and 3-4%
carbon dioxide. Oxygen is normally attached to haemoglobin molecules inside the red blood cells
where it forms oxy-haemoglobin. Some of the nitrogen is simply dissolved in the plasma, although
much more dissolves under high pressure, for example when a person is using SCUBA equipment.
Carbon dioxide is transported; both dissolved in the plasma as bicarbonate ions and also attached
to haemoglobin in the red blood cells. It is actually the build up of carbon dioxide, rather than the
lack of oxygen which is detected by a special part of the brain called the ‘respiratory centre’, as the
stimulus to breathe more rapidly and deeply.
Key Points
• Gas exchange occurs across the alveolar membrane in the following manner:
Oxygen goes from Alveolus →Blood capillaries
Carbon dioxide goes from Blood capillaries →Alveolus
• Oxygen and carbon dioxide move across the alveolar membrane by diffusion, i.e. from high
concentration to low concentration.
• The rate of diffusion of the gases can be altered by such factors as:
- the concentration gradient of the gases
- the rate of blood flow
- the rate and/or depth of breathing.
As the blood flows through the thousands of kilometres of capillaries, it comes in very close
proximity to the walls of the alveoli. When it does so, diffusion will occur from a region of high
concentration to low concentration. This means that oxygen will diffuse from the alveoli across
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the membranes and into the blood plasma and red blood cells until the concentration of oxygen
is equalised. Diffusion of carbon dioxide will also occur from the blood cells and plasma in the
opposite direction into the alveoli until the concentration is equalised. Very little diffusion of
nitrogen will occur because it is not produced or used up by the body tissues and the concentration
in the blood and the alveoli remains virtually the same.
Artery
Pharynx
Vein
Bronchiole
Larynx
Left lung
Alveolus
Alveolus
Right lung
Bronchus
Bronchiole
Heart
Diaphragm
Capillaries
(a) GAS EXCHANGE SYSTEM
Capillary
(b) LUNG
Red blood
cell
(c) ALVEOLUS
Figure 317 Alveoli in the lungs
The nett effect, therefore, is that, as the blood passes through the lungs it gains fresh oxygen
(oxygenation) and loses dissolved carbon dioxide. The rate of diffusion depends on the concentration
gradient which, in turn, depends largely on the rate of blood flow and the rate of breathing. It is
important to note that the alveoli provide a thin, moist, large surface area and are richly supplied
with capillaries so as to maximise the rate of diffusion and exchange of materials.
Lung function is impaired if a person is suffering from asthma because the bronchioles may
be constricted, thereby restricting the volume and rate of breathing. A person suffering from
emphysema has a reduced area of alveolar membranes which also limits diffusion rate, such people
often need to breathe an air mixture containing more oxygen. Lung infections, including pneumonia,
may cause the accumulation of fluid on the lungs, which again reduces effective volume and so limits
the rate of diffusion. Carbon monoxide, which is a product of car engines, is more than 200 times
more strongly attracted to haemoglobin than oxygen and will rapidly make it unavailable for the
transport of oxygen and carbon dioxide. If a person breathes too much carbon monoxide, they may
lose consciousness or even die. The medical treatment is to provide air that has a high level of oxygen
to compete with the carbon monoxide and also some carbon dioxide to stimulate breathing.
Focus Questions
1. Explain how each of the following features of the alveolus contribute to the efficiency
of diffusion of oxygen and carbon dioxide.
a) Large surface area
b) Thin membrane
c) Moist membranes
d) Rich blood supply
2. Explain why the efficiency of gas exchange is increased in the following situations:
a) A faster beating of the heart leading to increased blood flow
b) An increase in the rate and/or depth of breathing.
3. Explain why excess fluid on the lung may impair gas exchange.
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O3.5
Know the structural features of villi in the digestive system and describe how
nutrients are absorbed
Food that is eaten consists of a mixture of materials from other organisms. This material
consists mainly of large insoluble macromolecules which must be physically and chemically
broken down before they are of any use to the organism. This process is called digestion.
Physical digestion results in the food being broken into smaller particles whereas chemical
digestion refers to processes which result in smaller molecules.
Physical digestion includes the action of teeth, the peristaltic contractions of intestines and the
emulsification of lipids by alkali (e.g. bile) in the duodenum. Emulsification means that the
droplets of fat are broken into smaller droplets that provide a larger surface to enable chemical
digestion. Chemical digestion includes the action of digestive enzymes, acids and alkalis that
are produced by the digestive organs. Generally, carbohydrates are digested by carbohydrases
to form simple sugars such as glucose; and proteins are digested by proteases to form amino
acids. Similarly, nucleic acids are digested by nucleases to form nucleotides; and lipids are
broken down by lipases to form glycerol and fatty acids.
organisms
This is summarised in the table below:
Group of food molecules
Digestive enzymes
Products
Carbohydrates
carbohydrases
soluble sugars (e.g. glucose)
Proteins
proteases
peptides and amino acids
Nucleic acids
nucleases
nucleotides
Lipids
lipases
fatty acids and glycerol
Smaller molecules resulting from digestion are soluble and are able to pass from the alimentary
canal through the cells lining it into the bloodstream. This process is called absorption. The
blood capillaries that absorb these digested materials combine to form venules, that in turn form
the hepatic portal vein which carries the digested materials to the liver. In addition, some of the
products of digestion, particularly glycerol and fatty acids are absorbed by lymph capillaries or
lacteals and eventually get back into the blood system. The inner surface of the intestine is not
smooth, it is made up of folds and tiny projections called villi as shown in Figure 319. These provide
the exchange surface that is about 300 square metres, which is about the size of a tennis court.
Figure 318 Microvilli in the epithelium of the small intestine (x1000)
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BIOLOGY: Key Ideas – THIRD EDITION
ORGANISMS
Key Points
• Villi are finger like projections from the wall of the small intestine that assist in the
absorption of soluble materials from the intestine into the bloodstream.
• Villi provide a thin, moist membrane with a large surface area and a rich blood supply.
• The structural features of the villi include:
- a lacteal in the core of each villus. Lacteals are part of the lymphatic system.
- epithelial cells surrounding the villus are covered in microvilli to increase the surface area.
- a rich capillary network.
These villi have an outer layer of epithelial cells which in turn have tiny projections called microvilli, which form a brush border much like those in the tubules in the kidney as shown in Figure 318.
The nett effect is to greatly increase the surface area of membranes to absorb the nutrients resulting
from digestion of food in the gut. When the concentration of these simple, soluble molecules in
the gut is higher than the bloodstream they will diffuse into the blood or lymph vessels. When the
concentration of a substance is higher in the bloodstream the cells lining the gut will need to use
active transport instead of diffusion. Lipids may also be absorbed by pinocytosis and carried in the
tiny lymph vessels called lacteals.
Villi
Lacteal
'Finger like
projections'
(villi)
Liver
Small
intestine
Large
intestine
Anus
(a) DIGESTIVE SYSTEM
Absorptive
cell
Blood
capllaries
Lumen
Muscular wall
of intestine
(b) SMALL INTESTINE
(c) VILLI
Figure 319 Villi in the intestine
As an example, imagine that you have just eaten a meal of bread and vegetables rich in starch. As
the starch passes through your digestive system it is broken down to glucose, most of which diffuses
from the gut into the blood. However, as the concentration of glucose in the blood rises and the
concentration of glucose in the gut falls, a point will be reached at which the concentrations are
equal. At this point no nett diffusion will occur and the glucose will need to be actively transported
from the gut into the bloodstream or it will otherwise remain in the gut and be lost in the faeces. A
similar situation will occur with all of the required materials resulting from digestion. As the material
passes through the large intestine, water and vitamins are absorbed into the blood capillaries.
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TOPIC 3
The last section of the large intestine is the rectum, which serves as a storage before defecation.
Faeces consist largely of undigested cellulose or fibre, bacteria and the remains of mucus and
digestive fluids. Because of this, with the exception of digestive juices, the faeces do not strictly
consist of material which has been excreted from cells. The ability of the villi to work effectively will
be impaired if there is an infection of the lining of the gut or if the cells have been killed or damaged
as can occur with chemotherapy for cancer. This in turn may lead to constipation or the opposite,
diarrhoea. Again it is important to note that the villi provide a thin, moist, large surface area and
are richly supplied with capillaries so as to maximise the rate of diffusion and exchange of materials.
Key Words
biotic, stimulus, hypothermic, hyperthermic, receptors, olfactory, involuntary, voluntary,
nervous system, dendrites, axon, receptors, stimulus, effector, response, hormones, endocrine,
receptors, reflex response, endothermic, homeostasis, stimulus–response, negative-feedback,
adrenalin, thyroxine, diffusion, capillaries, lymph capillaries, nephron, Bowman’s capsule,
filtration, reabsorption, haemoglobin, respiratory centre, digestion, pinocytosis, lacteals,
enzymes
Key Points
• Nutrients are absorbed from the small intestine through the villi by both active and passive
processes.
organisms
• When the concentration of substances such as glucose, amino acids, nucleotides and ions
are higher in the intestine they move via diffusion into the blood capillaries in the villi.
• Active transport is required to move substances against the concentration gradient when
the concentration is higher in the blood capillaries compared to the intestine.
• The products of fat digestion are absorbed by epithelial cells and then move by exocytosis
out of there cells into the lacteal in the villi.
Focus Questions
1. Draw a diagram of a villus showing the important structural features including:
microvilli on epithelial cells, the lacteal, blood capillaries.
2. Explain the role of each of the following in increasing the rate of absorption of
nutrients from the intestine into the bloodstream:
a) extensive capillary network
b) microvilli on the epithelial cells
c) lacteal
3. Explain why active transport would be required in some cases to move nutrients from
the small intestine.
4. The products of fat digestion diffuse into the epithelial cells and are then re-synthesized
back into fats. Explain why the process of exocytosis would be required for these fats
to move into the lacteal.
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ORGANISMS
SACE Capability example - Personal development
Georgia's story
Georgia is a student of the University of
Adelaide. She is currently undertaking her
second year in a Bachelor of Animal Science
Pre-Veterinary. To become a qualified
veterinarian a student must complete a three
year Animal Science degree and a subsequent
three year Masters of Veterinary Science
degree.
A compulsory component of the Veterinary
degree is the completion of 12 weeks of
work experience within the first three years.
Students are required to undertake work
experience at numerous places in the animal
industry, these include; a dairy farm, a cattle
farm and small animal organisations.
Georgia has so far completed placement at a
cattle farm where she was responsible for administering vaccinations, performing eartagging, drenching and other various treatments. She has also finished two weeks work
experience at the RSPCA where she went out on animal rescues and assisted with the
Veterinarian checks on the different animals.
From a very early age Georgia had a great passion for animals. Growing up she was
surrounded by a myriad of different animals including; dogs, cats, rabbits, chickens and
birds. Georgia has a history of rescuing and caring for injured and abandoned birds. Her
love of animals sparked her interest in biology and science; she wanted to discover more
about different animals’ form and function and their interactions with one another.
Georgia has a casual job at a grain, garden and pet store where she is responsible for
taking care of rabbits and numerous different types of birds and fish.
Georgia studied science throughout her high school career. In year 12, this included
Physics, Chemistry and Biology. It was necessary for Georgia to study Biology and
Chemistry in year 12 as they are pre-requisites for admission into the Animal Science
degree. In her first year of University study Georgia was required to study Physics,
Biology and Chemistry. The study of these subjects at high school level gave her an
advantageous base of knowledge with which to enter her university career.
Georgia credits both her parents as inspiration for her pursuit of a career in science.
Georgia’s mother surrounded her daughter from a very young age with animals for her
to love and care for. Georgia’s father, a biology teacher, instilled in her a love for science.
His continual encouragement and support of Georgia’s studies helped her to succeed in
her goal of undertaking a Veterinary Science degree.
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