Guided Inquiry Activity #25
Incredible, edible eggs
Model 1. Any egg is a type of cell that is specialized for sexual reproduction – the
process by which two parents contribute genetic material (genes) to make a new
individual. Germ cells (e.g. egg and sperm) contain half the genetic material necessary to
make an individual – the cells are haploid. Only eukaryotic organisms undergo sexual
reproduction, and not even all eukaryotic organisms…. only the most complex.
Of the two reproductive germ cells that combine their genetic material (genes) to make
a new individual, the egg is the larger, less mobile one. The egg cell receives the sperm
cell (the carrier of the other half of the genetic material), accommodates the joining of
the two haploid sets of genetic material, and subsequently divides and differentiates
into the new embryonic organism.
Figure 25.1. A comparison between sexual and asexual reproduction
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Guided Inquiry Activity #25
Asexual reproduction is how bacteria, yeast and other simple organisms multiply. By
definition, the product cells of asexual reproduction are exactly genetically identical to
the single, original parent – the daughter cells are clones of the parent. On the other
hand, sexual reproduction combines two sets of genetic material into a new, unique
individual.
The familiar chicken egg has a yolk surrounded by
egg white, contained within a hard shell.
• The Yolk: accounts for ~ ⅓rd of the weight of an
intact chicken egg. Comprised of mostly fats and
proteins – it carries 75% of the calories and most
of the iron, thiamin and vitamin A. Its purpose is
to provide food for a developing chick2.
• The White: accounts for ~ ⅔rd of the weight of
an intact chicken egg. It is 90% water – the rest
being protein. There are only traces of minerals,
fatty material, glucose (a sugar) and vitamins.
The white provides essential proteins and water,
and also provides protection to a developing
chick2.
• The shell: Made of calcium carbonate and
protein, the shell is riddled with pores (tiny
holes) that allow gases to pass in and out of the
egg.
Figure 25.2. Diagram of a chicken egg (image licensed
under Creative commons)
Table 25.1. The Composition of a U.S. Large Egg2
Weight
Protein
Carbohydrate
Fat
Monounsaturated
Polyunsaturated
Saturated
Cholesterol
Sodium
Calories
Whole Egg
55 grams (g)
6.6 g
0.6 g
6g
2.5 g
0.7 g
2g
225 milligrams (mg)
71 mg
84
Egg White
38 g
3.9 g
0.3 g
0
0
0
0
0
62 mg
20
Egg Yolk
17 g
2.7 g
0.3 g
6g
2.5 g
0.7 g
2g
225 mg
9 mg
64
Chicken eggs are created by the female hens. The hen will produce the eggs whether
they are fertilized or not – mass produced, grocery-store eggs are not fertilized. There is
no nutritional difference or noticeable difference in physical appearance between
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Guided Inquiry Activity #25
fertilized and unfertilized chicken eggs1. A chicken will lay eggs (fertilized or not) until
she has accumulated a certain number of eggs in her nest. If the eggs are removed –
perhaps by a predator…or a human – the hen will lay another to replace it, and may do
so indefinitely2.
Eggs are unmatched as a balanced source of amino acids, and they include a plentiful
supply of alpha linolenic acid – an essential, polyunsaturated, omega-3 fatty acid – as
well as several minerals and most vitamins. An egg contains everything you need to
make a chick2. Eggs also contain cholesterol - a hydrophobic molecule. Cholesterol is
also considered a lipid because fatty acids can be converted into cholesterol using many
enzymes. In humans, high blood cholesterol does increase the risk of heart disease – a
fact that has long made medical professional recommend limiting egg yolk consumption
to 2-3 per week. However, recent studies show egg consumption has little effect on
blood cholesterol, rather saturated fats have a far more powerful effect on raising blood
cholesterol. In addition, the phospholipids in eggs yolk interfere with our ability to
absorb the cholesterol…so we don’t have to count our eggs after all.
Questions:
1. In Figure 25.1, why are the egg, sperm cell and fertilized egg(s) represented with
their genetic material inside another circle? Why is this second “inside” circle absent
in the bacterial cell in the lower part of Figure 25.1? (Hint, the egg and sperm come
from eukaryotic organisms)
2. The chicken egg found in the grocery store contains which of the cells in Figure 25.1?
Explain.
1
Folklore indicates that a “blood spot” in an egg indicates fertilization. This is incorrect. The blood comes from the rupture of a
blood vessel during formation of the egg and has nothing to do with fertilization. Blood spots are not harmful nor do they affect
taste. They can simply be removed with a spoon.
2
On Food and Cooking, by Harold McGee, 2004, Scribner.
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Guided Inquiry Activity #25
3. Does a chicken egg contain genetic material? Is it haploid? Explain.
4. Egg whites are often recommended as an excellent source of protein in the human
diet. Now, egg white contains about 60% of the protein in the egg, while egg yolk
contains the other 40%. Why are egg whites (vs. yolks) such a “good” (i.e. high
quality) source of protein?
5. It is known that excessive consumption of saturated fat is known to raise blood
cholesterol and contribute to heart disease. Eggs contain saturated fat…so why
might normal egg consumption not have a measurable effect on blood cholesterol
levels in humans?
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Guided Inquiry Activity #25
Inside an Egg
Model 2.
3
1. Eggshell – hard calcium carbonate and protein
2. Outer membrane - antimicrobial protein layer
3. Inner membrane – antimicrobial protein layer
4. Chalaza – protein cord that anchors yolk
5. Exterior albumen (outer thin albumen, less protein)
6. Middle albumen (inner thick albumen, more protein)
7. Yolk membrane – surrounds and protects yolk
8. Primordial white yolk: the first yolk to surround the germ cell
9. Germ cell (i.e. the egg cell) – not actually red in real life
10. Yellow yolk – fats and protein for germ cell
11. White yolk – less dense, high in iron
12. Internal albumen – the coating from which the chalazae
extend
25. Chalaza - protein cord that anchors yolk
14. Air cell – air for chick to breathe
15. Cuticle - protein coating that gives egg color and blocks
entry of bacteria
Figure 25.3. Detailed anatomy of a chicken egg
The Yolk: The yolk (10, 11) surrounds the germ cell (9). The germ cell contains half the
genetic material needed to make a chick. This germ cell is the haploid egg cell. The germ
cell is surrounded by white (or light) yolk (11) and yellow (or dark) yolk (10).
Table 25.2 The Proteins in Egg White Albumen4
Protein
% of total
Natural Function
Ovalbumin
54
Nourishment for chick, may block digestive enzymes
Ovotransferrin
12
Binds iron
Ovomucoid
11
Blocks digestive enzymes
Globulins
8
Plug defects in membranes and shell
Lysozyme
3.5
Enzyme that digests bacterial cell walls
Ovomucin
1.5
Thickens albumen, inhibits viruses
Avidin
0.06
Binds the vitamin biotin
Others
10
Bind vitamins; block digestive enzymes….
3
4
Image licensed under creative commons.
On Food and Cooking by Harold McGee, 2004, Scribner
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Guided Inquiry Activity #25
The white yolk is less dense and especially rich in iron, while the yellow yolk is denser
and rich in fats and proteins. The color of the yellow yolk depends on what the hen
eats….the pigments in her food. The yolk is much larger than the germ cell. The yolk
membrane (7) surrounds and protects the yolk and indicates egg freshness.
The White: The egg white is made of mostly water and a mixture of proteins called
albumen (5,6,12). The albumen not only nourishes the chick, it is a biochemical shield
against infection and predators. The chalazae (4,25) are dense elastic cords made of
albumen that anchor the yolk to the ends of the shell and allow it to rotate while
suspended in the middle of the egg. Chalazae are visible in a raw egg when it is cracked
open.
Membranes and shell: The membranes (2,3) line the inside of the shell and are made of
antimicrobial proteins. The shell (1) is made of calcium carbonate and protein, and since
the developing chick would need to breathe, the shell has thousands of tiny pores or
holes. These pores are invisible to the eye. The cuticle (15) is a thin protein coating on
the shell. This coating initially blocks the pores to slow the loss of water and prevent the
entry of bacteria. When the chicken deposits the proteinaceous cuticle, pigment
molecules are also deposited into the shell and give the egg its color. The pigments
deposited are dependent on the type of chicken. White, brown, even blue eggs and
yellow spotted eggs – all the colors have to do with the genetic makeup (the breed) of
chicken, for example Rhode Island Reds lay brown eggs. There is no nutritional
difference between white, brown….or even blue eggs. The air cell (14) provides the
developing chick with its first breaths of air and is also an indicator of egg freshness.
Questions:
6. Is a chicken egg made of single cell, many cells, or neither? Explain.
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Guided Inquiry Activity #25
7. A friend tells you that brown eggs are organic because they come from a small farms
and “free-range” hens. (i.e. they aren’t mass produced like typical grocery-store
eggs).
a. Is a brown egg necessarily “organic”?
b. Does a brown egg necessarily come from a “free-range” hen?
c. How is a white egg different from a brown egg?
8. When an egg is hardboiled, there is a persistent bit of uncoagulated yolk at the
center if the hard cooked egg.
a. What is at the very center of the yolk?
b. Is your answer to (a) consistent with the fact that very high concentration of
iron prevents yolk from “setting” or coagulating?
9. In Table 25.2 we see that egg white proteins mostly bind stuff. Only one is called an
enzyme, what makes an enzyme different from other proteins?
10. Explain why chicken egg white protein could be considered a mixture. Include Table
25.2 in your answer.
11. Identify the labeled components of the photo below:
(a) Not the plate…there is something there
(b) The white stringy thing
(c)
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Guided Inquiry Activity #25
Model 3. Although an egg can remain edible for weeks if keep intact and cool, egg
quality does deteriorate over time. When an egg is freshly laid, it contains carbon
dioxide dissolved in the white and yolk. When carbon dioxide is dissolved in water (and
egg white is 90% water) it is in the form carbonic acid. When acids are dissolved in
water, they make the water acidic.
This can be measured by
something called a pH scale. On a
pH scale, 7 is neutral pH, below 7
is acidic and above 7 is alkaline.
The more acid in the water, the
lower the pH, while adding acid
Figure 25.4. Carbon dioxide in water makes carbonic acid to an alkaline solution will lower
the pH.5
As eggs age, the carbon dioxide dissolved in the
white and yolk gradually escapes through the
pores in the shell. As carbon dioxide leaves the
egg, the white and yolk become more alkaline.
This change in pH changes interactions between
egg white albumen proteins (the proteins interact
less) and the egg white albumen (5,6) is
consequently runnier.
Figure 25.5. pH scale, a measure of
acidity and alkalinity
In addition to carbon dioxide escaping through
the pores of the shell, water molecules also
escape – making the overall contents of the egg
shrink. This shrinking allows air to travel in
through the pores in the shell and enlarge the air
cell. In a typical refrigerator an egg will lose 4
milligrams of water a day.
A final indicator of egg freshness is the yolk membrane (7). Because the yolk contains
less water than the white, water gradually crosses the membrane from the white into
the yolk. In the refrigerator, the yolk gains about 5 milligrams of water per day. This
increase in water makes the yolk swell (enlarge) and the yolk membrane weaken.
5
For a full lesson on pH, see Activity 8
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Guided Inquiry Activity #25
Thick (middle) egg white albumen is
jelly-like and very thick (not runny)
Thin (outer) egg
white albumen – not
too runny, doesn’t
spread that far
Yolk is tall = good
yolk membrane
Thick (middle) egg white albumen
spreads a little further than grade AA
but still jelly-like
Yolk membrane
Thin (outer) egg white
still looks tight
albumen runs more than AA
Thick (middle) egg white albumen is
runny and spreads. It is not jelly-like.
Thin (outer) egg
white albumen is
quite runny –
spreads far
Grade AA
Grade A
Figure 25.6. Egg grade is a function of egg white and yolk membrane6.
Yolk membrane is
weak and
enlarged yolk
flattens out
Grade B
12. Explain why the egg white is more alkaline after carbon dioxide escapes from the
egg white and out the shell (through the pores).
13. Around 1750, English cookbook author Hannah Glasse noticed that fresh eggs
quickly sank in a bowl of water, while old eggs bobbed and really old (probably
rotten) eggs floated in the water. A floating egg must have a large quantity of air
inside the shell to make it less dense. What factors about egg anatomy and
chemistry explain this simple “home test” for egg freshness?
14. Given the qualities that affect egg freshness (and consequently egg quality), is a
grade AA egg any more or less nutritionally valuable that a grade B egg? That is,
have the amounts of protein, fat etc. changed between the two grades of egg?
Explain.
6
United States Standards, Grades, and Weight Classes for Shell Eggs AMS 56 and the Home and Garden Bulletin number 264, US
Department of Agriculture, Agriculture Marketing Service. Public Domain.
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Guided Inquiry Activity #25
Putting it all together:
15. Due to the public desire for cholesterol-free and lower fat eggs, food manufacturers
have come up with substitutes (e.g. egg beaters) that imitate whole beaten eggs –
these substitutes can be scrambled (for scrambled eggs or omelets) and can be used
in baking. These substitutes consist of genuine egg whites mixed with an imitation
yolk.
a. Why are the whites genuine, but the yolks imitation?
b. Based on your knowledge of egg yolk composition and what the consumer
wants, what molecules might food manufacturers include in an imitation egg
yolk?
16. Ovalbumin is the most abundant protein in eggs and the protein most likely to cause
egg allergy in humans. Since developing an allergy requires exposure to the allergen
(e.g. ovalbumin), medical professionals recommend that babies not eat egg white
until after they are one year old, and yet it is considered perfectly safe for babies to
eat egg yolk. Why?
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