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

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.

Figure 25.2. Diagram of a chicken egg (image licensed under Creative commons)

The familiar chicken egg has a yolk surrounded by egg white, contained within a hard shell.

· The Yolk: accounts for ~ ?rd 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 ~ ?rd 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.

Table 25.1. The Composition of a U.S. Large Egg2

Whole Egg

Egg White

Egg Yolk

Weight

55 grams (g)

38 g

17 g

Protein

6.6 g

3.9 g

2.7 g

Carbohydrate

0.6 g

0.3 g

0.3 g

Fat

6 g

0

6 g

Monounsaturated

2.5 g

0

2.5 g

Polyunsaturated

0.7 g

0

0.7 g

Saturated

2 g

0

2 g

Cholesterol

225 milligrams (mg)

0

225 mg

Sodium

71 mg

62 mg

9 mg

Calories

84

20

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 fertilized and unfertilized chicken eggs[footnoteRef:1]. 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 indefinitely[footnoteRef:2]. [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. ]

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.

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?

Inside an Egg

Model 2.

[footnoteRef:3] [3: Image licensed under creative commons. ]

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 Albumen[footnoteRef:4] [4: On Food and Cooking by Harold McGee, 2004, Scribner]

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….

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.

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. ( (a) N o t the plate…there is something there )Identify the labeled components of the photo below:

( (c) ) ( (b) The white stringy thing )

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 to an alkaline solution will lower the pH.[footnoteRef:5] [5: For a full lesson on pH, see Activity 8]

Figure 25.4. Carbon dioxide in water makes carbonic acid

Figure 25.5. pH scale, a measure of acidity and alkalinity
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.

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.

( Thick (middle) egg white albumen is jelly-like and very thick (not runny) Thick (middle) egg white albumen spreads a little further than grade AA but still jelly-like Thick (middle) egg white albumen is runny and spreads. It is not jelly-like. ) ( Thin (outer) egg white albumen runs more than AA ) ( Yolk is tall = good yolk membrane ) ( Thin (outer) egg white albumen – not too runny, doesn’t spread that far )

( Yolk membrane still looks tight )

( Yolk membrane is weak and enlarged yolk flattens out ) ( Thin (outer) egg white albumen is quite runny – spreads far )

Grade AA

Grade A

Grade B

Figure 25.6. Egg grade is a function of egg white and yolk membrane[footnoteRef:6]. [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. ]

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.

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?

Copyright © 2016 Wiley, Inc. Page 10

C

O

O

+

H

O

H

C

O

OO

HH

carbon dioxide

(CO

2

)

water

(H

2

O)

Carbonic Acid

(H

2

CO

3

)

This funny “double sided arrow”

indicates that carbonic acid can

become CO

2

and H

2

Oand vice versa….

IncreasingAcidity

Neutral

Decreasingacidity

(increasingalkalinity)

1

2

3

4

5

6

7

8

9

10

11

12

humangastricjuice(pH1.3-3.0)

lemonjuice(pH2.1)

distilledwhitevinegar(pH2.4)

orangejuice(pH3.0)

yogurt(pH4.5)

blackcoffee(pH5.0)

milk(pH6.9)

eggwhite(pH7.6-9.5)

Bakingsodainwater(pH8.4)

householdammonia(pH11.9)

eggcell

sperm cell

each cell hashalf the genetic material

necessary to make a new individual

sexual

reproduction

fertilized egg- has a unique

combination of allthe genetic

material necessary to make a new

individual

combines genetic

material

divides and

differentiates

a bacterial cell containing

all the genetic material

that defines the individual

a single cell grows, and

copies its own genetic

material

two full copies of the all

the genetic material

the single cell divides into two daughter

cells each daughter cell has the

complete set of genetic material

Each daughter is a cloneof the parent

Asexual reproduction

Sexual reproduction

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