Chapter 2. Molecular and Cellular Biology
2.1.3 Types of Bio-molecules,
§ 2.1.3a Lipids & Carbohydrates

We have now completed our introduction basic Physics and basic Chemistry as a foundation for understanding how Biochemical reactions support Life. You should be able to visualize the structure of an atom, and the four most important chemical bonds for Biology. I expect that you have a sense of how Biochemical reactions take place: from hydrogen bonding of the reactants and the rearrangement of the atoms and/or molecules to form transient intermediate structures which can transform into the final products of the reaction, but without detailed chemical knowledge of how this happens, nor its implications for Chemistry Theory. We also have seen a rather superficial review of pH and how buffers serve to stabilize the pH of the buffered solution.

As an overview of what we shall cover in this chapter, the biologically important molecules are the Lipids (fats, oils and waxes), Carbohydrates (sugars, starches and celluloses), Proteins, and Nucleic acids. As is my preference, these will be covered in a non-traditional order and are listed in the order in which I intend to discuss them. Again, the rationale for the order in which I present them is to put them in order of the complexity of how they are assembled from their basic units, which seems to assist students in following the increasing complexity.

    The biologically important molecules include the most complex and the largest chemical structures known. In order to understand the complexity of these molecules, we need to recognize that these molecules are polymers which means that they are “large, often linear molecules made of repeating units.” “Poly” means many, and “mer” means unit; but “Polly” is usually portrayed as a parrot standing on a Pirate’s shoulder, saying “Awk!, Polly want a cracker!” Polymers are also considered to be plastics, in the Chemistry sense of the term; which is also the familiar definition in a non-technical sense.

Lipids

structure

The basic unit of a lipid is the fatty acid. Fatty acids are hydrocarbons with a terminal acid group (COOH), which may be linear, branched linear, or even ring structures. A hydrocarbon is a compound that is made entirely of Carbon and Hydrogen. The simple (linear) hydrocarbons have a generic structure
CH₄ [Methane], CH₃CH₃ [Ethane], or
CH₃(CH₂)_nCH₃ [Butane, Propane, Pentane, …],where n is a number from 1 to over 70 [Asphalt],
and a simple fatty acid is
CH₃(CH₂)_nCOOH, where n is an odd number (usually) from 11 to 17.
Chemists number the Carbon atoms in organic molecules starting from the ‘end’ which can be easily found, and the starting end is called the 1'-C end. In the case of fatty acids, the 1'-C end is the Carbon to which the COOH is attached (the 1'-C end is sometimes also called the α-1 end). Nutritionists number the Carbon atoms of a fatty acid from the opposite end (called the Omega end, or Ω-1 end; because Ω is the last letter [upper case] of the Greek alphabet and α [alpha] is the first letter [lower case] of the Greek alphabet).

    The simplest polymer of fatty acids is a Triglyceride (fat), which is three fatty acids dehydrated onto a molecule of Gylcerol (an alcohol or “ any organic molecule with at least one Hydroxyl on it”). Glycerol is (H₂C-OH) - (HC-OH) - (H₂C-OH).
A “dehydration reaction” occurs when two small organic molecules are combined to a larger molecule plus a Water. In this case the Water is formed from the -OH of the Glycerol and the H- of the COOH on the fatty acid.
H₂C-OH + HO-(C=O)-(CH₂)_n-CH₃
     |
HC-OH
     |
H₂C-OH
yields
H₂C-O-(C=O)-(CH₂)_n-CH₃ + H-OH
     |
HC-OH
     |
H₂C-OH
This is a Monoglyceride. Repeating this for the next -OH on the Monogylceride yields a Diglyceride, and repeating this for the last -OH on the Diglyceride yields a Triglyceride.
More information about Lipids (and their importance in Nutrition) can be found at my Nutrition for Liberal Arts students, Lipids chapter.

Molecular weight of a Lipid (the smallest Triglyceride, H83C42O6)
element	quantity	atomic mass	contribution to molecular wt
H	80	1	80
C	42	12	504
O	6	16	96
total molecular weight			680

The smallest Triglyceride (with three 12 Carbon fatty acids, the COOH is not counted to describe the number of C’s in the fatty acid, but is counted in determining the molecular weight) attached to the glycerol.
H₂C-O-(C=O)-(CH₂)₁₁-CH₃
     |
HC-O-(C=O)-(CH₂)₁₁-CH₃
     |
H₂C-O-(C=O)-(CH₂)₁₁-CH₃
Each fatty acid has
      11 H₂ plus the H₃ on the Ω-1 C = 25 H
      11 C plus the Ω-1 C and the C in the (C=O) = 13 C
      2 O in the (O-C=O) = 2 O
There are 3 fatty acids, so there are 75H, 39C, and 6O.
the glycerol has 5H, 3C, no O
so this Triglyceride has 80H, 42C, 6O, and as an example of a small Lipid, has a molecular weight of 680 a.m.u.

functions

(1) The Triglycerides serve as an energy dense source of slowly releasing nutritional energy (density is the concentration of a nutrient [in this case energy] per gram, or per liter, of the food stuff) . The digestion of the triglyceride requires 3 hydration reactions [the reverse of the dehydration reaction, and it consumes a Water molecule] to release the fatty acids from the Triglyceride, then processing the products (in a reaction we will examine in a later chapter (2.1.5 Bioenergetics) to release Water, Carbon dioxide, and energy which must be captured and stored in chemicals to be named later. You also “need to know” that some fatty acids have double bonds in them:
…-CH₂-CH=CH-CH₂-…
When there is a single double bond (a “mono-unsaturated” fatty acid), it will be at Ω-9, and the fatty acid is an Ω-9 fatty acid. When there is a second double bond, it will be at Ω-6; and the third double bond will be at Ω-3 (poly-unsaturated fatty acids), and they are called Ω-6 and Ω-3 fatty acids, respectively. When there are no double bonds, the fatty acid is saturated (no more H’s can be addedto the molecule) and if there is one or more double bonds, the fatty acid is unsaturated (more H’s can be added to the molecule). [The term “poly-unsaturated” is an advertising term, not a chemical term]. These double bonds are important because the saturated and Ω-9 fatty acids contribute to plaque in arteries [hardening of the arteries], and increased risk of heart attack and stroke; the Ω-6 and Ω-3 fatty acids do not plaque out well (with no effect on risk of heart attack and stroke); while the Ω-3 fatty acids are believed to dissolve existing plaque deposits (reducing the risk of heart attack and stroke).

    (2) The fatty acids with ring structures (such as Cholesterol) serve as the foundation upon which many important hormones (the steroids, such as Estrogen, Testosterone, human growth hormone, …) are built. Thus Cholesterol is a dietary requirement for Humans [this statement has been verified in clinical testing]. Diets which are very low in Cholesterol are potentially dangerous to long-term health and Life Expectancy [the age at which 50% of the population can be expected to die].

    (3) The lipids can serve as a protective layer on Plant and Animal parts. Many leaves have a wax coating; ear wax is another protective Lipid. Skin oils serve to reduce friction damage to skin. Some microscopic creatures are encased in a protective layer of fats, oils, or waxes. Mucus is another protective layer, but is mostly Protein.

    (4) Lipids, especially Triglycerides (also known as fat) are important molecules used for very long-term storage of energy in Animals. The major problem for Humans is that energy reserves stored as fats are used only as the last resort to meet current energy needs; basically a couple of months into Winter (for cavemen who had to live on fat reserves for most of the late Winter when fresh foods were almost impossible to find [especially during the Pleistocene]). Weight loss programs, regardless of their claims, rarely mobilize fat as the energy source. Some of the more effective ‘programs’ for weight loss which actually do mobilize fat as the preferred energy source are not popular among American dieters, because these programs involve substantial increases in exercise (at least 30 minutes of anaerobic exercise per day for only 5 to 7 days a week; anaerobic exercise is sufficiently strenuous that the person can not speak a full sentence without catching their breath).

    (5) The fatty acids are a class of chemicals which have a water-soluable end (the 1'-C end) and a water-insoluable [fat-soluable] end (the Ω-1 end). These molecules are “detergents” in the Chemistry sense of the term. Detergents in the advertising sense of the term are also detergents in the Chemistry sense of the term. One group of triglycerides is extremely important because the cell membrane is a plasma membrane made of phosphorylated triglycerides (a triglyceride with a Phosphate ion between the fatty acid on the 1'-C end of the Glycerol and the fatty acid). A Human with a dietary deficiency of fats, will not be able to form cell membranes well, so will exhibit slowed wound healing, as well as reduced growth during “growth spurts”. While this is not a major problem with the typical American diet, it is a potential risk in “fat-free” diets promoted for weight loss. Children, especially, must have fats and Cholesterol in their diets, at least until they have completed their growth (18 to 23 years).

the plasma membrane

As noted above, the plasma (cell) membrane is made of detergent molecules. Another detergent solution is soap bubble solution. The concept of the cell membrane can be illustrated by having students blow soap bubbles (I had a Principal, when I taught High School Biology, tell me that I had no classroom management skills as evidenced by my students running around the room blowing soap bubbles! He also didn’t like that my Earth Science students ran around the room throwing paper airplanes while we were studying aeronautics). The students were to write lab reports describing how to cause the soap bubbles to remain “alive” longer (the bubble “dies” when it pops; the definition of when they were “born” was described by the student lab teams). There was extra credit for which lab team had the oldest bubbles, provided the lab report described the procedures to keep the bubbles alive. Approximately 100% of the students reported accurate descriptions of the cause of ‘death’ for most bubbles. [The expression “extra credit” causes High School students to do things that are known to be impossible. The all-time record for soap bubbles was 8.5 minutes (using a bubble solution to which glycerin [available at auto parts stores to keep window weather stripping flexible] had been added.] This trick to extend the life expectancy of the bubble is important to Biology, because it illustrates why Animal cell membranes have Cholesterol in them to make them strong enough to support the weight of an Elephant [Elephant feet have skin made of Elephant cells on the bottom, so the entire weight of the Elephant is supported on a layer of soap bubble-like cells]. Plant cells are inside cell walls (made of cellulose, or &$8216wood’), so do not require Cholesterol for strength to support the weight of giant Sequoia trees. Vegetables typically contain no Cholesterol.
    After the bubble experiment, we discussed the theoretical structure of soap bubbles [the fat soluable ends of the detergent molecules dissolve in each other, and the water soluable ends dissolve in the water, so the bubble is made of patches of soap surrounded by patches of water. This resembles a soccer ball with the black pentagons representing the soap patches, and the white hexagons representing the water patches.] Then, with a few drops of dish detergent in about 5-10oz water in a 12-20oz plastic water bottle, and the bottle tightly closed, shake the bottle vigorously. You and your students will be able to observe a milkly white fluid, which slowly separates into a clear lower layer and a milky upper layer between which you can see very tiny white bubbles swirling around. These bubbles are called coacervates (the size of a Paramecium, a large enough Protistan to be seen with the naked eye) and have a double layer of detergent around them and water inside them. In the double layer of detergent, the fat soluable ends of the detergent point away from the water outside the bubble and away from the water inside the bubble. This makes the surface of the coacervate bubble a plasma membrane; and the students begin to understand the cell membrane.
    The coacervate plasma membrane has the outer layer of detergent molecules with their water soluable ends dissolved in the water outside the bubble, and the inner layer with their water soluable ends dissolved in the water inside the bubble. The fat soluable ends of the outer and inner layers are dissolved in each other. The illustration of the plasma membrane is from http://micro.magnet.fsu.edu/cells/plasmamembrane/plasmamembrane.html (downloaded on 22 Apr 2011).


If you think about this layer long enough (and with a little encouragement from the teacher), you will realize that a fat molecule (or anything dissolved in fat molecules) outside the bubble (or cell) can not penetrate the water soluable ends of outer layer [hydrophyllic region], so can not enter the bubble (or cell). A water molecule (or anything dissolved in water) can not penetrate the fat soluable ends of the outer and inner layers [hydrophobic region], and can not enter the cells. Students (grade 9 through 12) will state this in class if you ask “What do you think would happen if a substance which will not dissolve in water tried to move from outside to inside the cell?” Then ask “So, what would happen if the substance will not dissolve in fat?”
Such a cell would be unable to get any nutrients in (nor wastes out) of the cell, and would die in less than 8.5 minutes. This arrangement is unacceptable to most Humans with a life expectancy of more than 70 years. The solution to this dilemma is to poke holes in the bubble. Again we can illustrate this to Middle School and High School students by offering extra credit to any one who can poke holes in their soap bubbles with a pin-shaped object (the pin does NOT have to have a sharp point; plastic ‘cocktail stirrers’ will work well) without popping the bubble.
WARNING! Danger, Will Robinson! This exercise requires Safety instructions. The students MUST be told how to handle the pins without producing injury to themselves nor to their classmates; then monitored to assure that injuries do not occur. It is, after all, “all fun and games until somebody's eye gets poked out.”
The students won’t realize that poking holes in soap bubbles is not possible, so they can achieve it by discovery [using high level cognitive skills], unless you tell them how to do it [which you should never do during a discovery exercise]. According to my student teams’ lab reports (which they had to write to get their extra credit and, for that matter, the credit for blowing the bubbles in the first place), the pin has to be covered with soap bubble solution to poke a hole in the bubbles without killing them.
Ah ha! you must keep the hole plugged and the plasma membrane must not touch the pin (there has to be another layer of membrane on the pin). So we can poke holes in a cell membrane as long as we plug the hole with a protein that can dissolve in the outer water soluable layer, the two fat soluable layers and the inner water soluable layer of the plasma membrane. If the protein [“transmembrane protein”] is donut-shaped, things can enter and leave the cells through the donut hole. To verify this, have the students poke a hole in a soap bubble with a plastic eye dropper soaked in bubble solution, and squeeze the bulb to make the bubble bigger. My students discovered [learned], before I suggested it, that injecting water with the eye dropper usually kills the soap bubble [“probably because the soap bubble membrane is not strong enough to support the weight of the water,” a High School student hypothesis, reported in one team's lab report]. Now we have a plasma membrane that will allow anything smaller than the donut hole to move into and out of the cell. If the donut could change shape to make the hole bigger or smaller (and even closed), the cell could control what can come in or go out.

Osmosis

Diffusion through a semi-permeable membrane.
Example 1. salt solution Initial conditions
		container	bag
time	substance	conc %	conc %
0	salt	0	10
	water	100	90
	total	100	100
	diffusion
1	salt	0+5 = 5	10-5 = 5
	water	100-5 = 95	90+5 = 95
	total	100	100
Example 2. Sugar solution Initial conditions
		container	bag
time	substance	quantity g/500g solution	quantity g/500g solution
0	sugar	0	50
	water	500	450
	total solution	500	500
	diffusion
1	sugar	0	50
	water	500-25 = 475	450+25 = 475
	total	475	525
	revised water conc	475/475 = 500/500	475/525 = 452.4/500

When a membrane allows only some things to pass through it, the membrane is a semi-permeable membrane. Commercially available semi-permeable membranes are “sized” by the maximum molecular weight (mw) of the substance which can pass through the membrane, although the molecular diameter is the actual characteristic determining what will and will not pass through the membrane. Commercially available semi-permeable membranes also require a budget for Science experiments, and a P.O. on letterhead.

    Imagine a fish bowl about 2/3^rd full of water, and 500 ml [2 cups] of water with red food coloring added. When you pour the red water into the fish bowl, the water in the entire bowl will soon become red. This exercise illustrates diffusion, “the process by which molecules spread from areas of high concentration [the red water], to areas of low concentration [the water in the fish bowl].” “When the molecules are even throughout a space” [the combined red water and clear water in the fish bowl after a brief time], it is called EQUILIBRIUM (from Biology Corner downloaded 18 Apr 2011) [this site also provides animations of both diffusion and diffusion].
    Imagine a container of 500 milliliters (ml) [2 cups] of water with 0g of salt dissolved in the water. Now imagine a semipermeable membrane, which allows salt (mw = 58) but not sugar (mw =180) to pass through it, formed into a bag containing 500 ml of a 10% by weight salt (NaCl) solution [50g NaCL in 450 ml Water]. Place the semipermeable bag [right side of the table] into the initial container [left side of the table]:
Initially, there is 100% water on the left, so the salt concentration is 0%; and on the right the salt concentration is 10%. Since salt will pass through the membrane, salt will move from the high concentration (10%) to the low concentration (0%) until both sides have an equal concentration (in this case the average of the starting numbers, or 5%.
Water will also pass through the membrane, until both sides are equal, in this case the average of the initial numbers, or 95%. The system is now at equilibrium. “The diffusion of Water through a semi-permeable membrane” is called Osmosis. However, osmosis is much more confusing than that. When you begin to get confused, just remember ‘osmosis is just diffusion of Water through a membrane!’

    We now wish to repeat the explanation with Sugar (C₆H₁₂O₆, mw = 180) rather than salt. Our membrane will not allow Sugar to pass through it, so each side behaves as if they were totally separate containers, and no Sugar moves.
    Water, however, will pass through the membrane, so the system behaves as if it were a single container, so it has to seek equilibrium. To follow this, we need to return to concentration as g Water/g solution. We initially had 500g H₂O on the left for a concentration of 500g H₂O/500g (100% H₂O) solution. On the right we had 450g H₂O/500g (90% H₂O) solution. This is not at equilibrium, so Water must move from the high concentration (500g on the left) to the low concentration (450g on the right side; and 25g should make both sides equal to the average of the values. So we move the 25g H₂O producing 475g H₂O/475g solution on the left, and 475g H₂O/525g solution on the right. Converting to concentration as g H₂O/500g solution, we find 500g H₂O/500g solution (100% H₂O) on the left, but 425g H₂O/525g solution = 452.4g H₂O/500g solution = 90.5% H₂O on the right. This is still not at equilibrium, so more water will have to move. Since the bag is a closed system with 550g solution at first, and now has 525g solution, the pressure inside the bag increases. This pressure is called osmotic pressure. The pressure on the left remains the same as before because the left side is an open system, so has atmospheric pressure. If the left side were closed as well, the pressure would have dropped on the left side. In Biological systems (cells) an increase in internal pressure would cause swelling, and may become fatal if the cell membrane breaks (ruptures). Similarly, a decrease in internal pressure would cause shrinking, which may be fatal if the cell collapses.

Carbohydrates

structure

Carbohydrates form by a slightly more complex method than for the Lipids. The basic unit of a Carbohydrate polymer is a simple sugar. The sugars are made by green plants, as the product of photosynthesis
6H₂O + 6CO₂ + energy → C₆H₁₂O₆ +6O₂
A simple, 6-C sugar is a ring structure (illustration of glucose ring structure, downloaded 7 Feb 2009 from www.palaeos.com/Fungi/FPieces/CellWall.html).

There are three different monosaccharides: glucose, galactose, and fructose (all of which have the same formula, but differ in their structural formula).
There are also three disaccharides: glucose-fructose or sucrose (table sugar), glucose-galactose or lactose (milk sugar) and glucose-glucose or maltose (digestion product of starch).
    The Carbohydrate polymers (polysaccharides) are polymerized from disaccharides, not simple sugars. The polymerization of these glucose units works by aligning the -OH on the 4'-C of a second glucose with the -OH on the 1'-C of the first glucose, and then dehydrating the H from one -OH and the entire other -OH so the two glucoses are joined by an -O- to make a disaccharide plus a Water. (illustration of sucrose, a disaccharide, downloaded 1 May 2011 from staff.jccc.net/pdecell/biochemistry/carbohyd.html#glucose)

[This explanation is sufficient up to and including first-year High School Biology and A.P. Human Anatomy & Physiology. College-level introductory Botany will refine this because green plants actually make simple sugars as an intermediate in the process of making disaccharides. I suggest that High School second year Biology students should be told that plants make disaccharides, but would not expect them to fully grasp the significance of this information.] Most Introductory texts describe the polysaccharides as a polymer of glucose (monosaccharides). However, polysaccharides are made by polymerizing disaccharides onto the 1'-C end of the forming polysaccharide, and are digested by removing one disaccharide at a time from the 1'-C end of the polymer. Many starches are branched, with disaccharide polymer side chains assembled on the 6'-C of the terminal disaccharide on the partially assembled polymer. The main chain continues to grow on its 1'-C end as well. Digestion of the branched polysaccharides removes disaccharides from the 1'-C end, but can not proceed past the branch point. This leaves an undigestible polysaccharide fragment, which is called “soluable fiber” by Nutritionists. (illustration of starch and cellulose polysaccharides, downloaded 1 May 2011 from staff.jccc.net/pdecell/biochemistry/carbohyd.html#glucose)

    Disaccharides may have both 6'-C HCOH’s pointing up, or the monosaccharide on the 4'-C end with its 6'-C up and the monosaccharide on the 1'-C end with its 6'-C down. The polymers of the disaccharides with the 6'-C side chains on the same side are starches, which can be digested by Animals (including Humans). The polymers of the disaccharides with the 6'-C side chains alternating up and down are celluloses, which cannot be digested by Animals, but can be digested by some Protista (one-celled animal-like creatures) and some bacteria. Cellulose is called “insoluable fiber” by Nutritionists.

Molecular weight of a Carbohydrate H12C6O6)
element	quantity	atomic mass	contribution to molecular wt
H	12	1	12
C	6	12	72
O	6	16	96
total molecular weight			180

    We can determine the molecular weight of large biologically important molecules by chromotography. For low budget Science in the classroom, you can make your own chromatography paper by cutting coffee filters into strips. You then drape the strips over the edge of a disposable (recyclable) plastic drinking glass. Place one drop each of more than one food coloring side by side near the top (outside the glass) of the filter which you draped overthe glass, then fill the glass with water until most of the filter (inside the glass) is in the water. The water will climb up the filter, then flow down the filter outside the glass. As the students watch, they can see a race between the colors, and report which color won. The race occurs because the molecules of color will move at different speeds; the smaller molecules move faster than the larger molecules. Another way to do this with older (middle school) students is paper chromatography lab or candy chromatography. With three chemicals (one large molecule of known molecular weight, one small molecule of known molecular weight, and an “unknown”), the molecular weight of the unknown can be calculated using fairly complicated ratios [but only for High School Algebra and above]. As a teacher of anything less than High School Chemistry or High School second year college-prep Biology you will not need to know how to do these calculations, and if you do decide to need to you can google paper chromatography.
    When we have done this (at the graduate student to post-doctoral research fellow level, and professorial level) we have shown starch to have molecular weight mw = 40,000 to 340,000 amu. Dividing this by the molecular weight of the polymer units gives 250 to 2,000 glucoses in the example starch (data downloaded 19 Apr 2011 from www.starch.dk/isi/applic/tapiocafood.htm). Cellulose has been shown to have molecular weight mw = 81,000 to 810,000 amu, or 500 to 5,000 glucoses (data downloaded 19 Apr 2011 from www2.chemistry.msu/faculty/reusch/VirtTxtJml/carbhyd.htm).

functions

(1) Carbohydrates (except for cellulose) are the primary energy source to maintain Life processes in all living creatures (Animals, Plants, Protists, Fungi, Bacteria). The sugars (monosaccharides and disaccharides) serve as a quick source of energy. For the higher Animals, this source can sometimes be too quick, causing dangerously high spikes in blood sugar.

    (2) Cellulose has been described as the most abundant of all organic molecules. It is the primary molecule in wood (trees and woody weeds). As such, it is also the primary ‘Carbon sink’ which removes excess CO₂ from the atmosphere. While it is not commonly known among environmentalists, Botanists and Forestry professionals know that any increase in atmospheric Carbon dioxide causes an increase in growth of trees, and a decrease in atmospheric Carbon dioxide causes a decrease in growth of trees. All Plant cells are encased in cell walls (outside the phospholipid cell membrane) which are mostly Cellulose. The Cellulose from dead plants in marsh environments, with at least 3mm (0.1 inch) of water covering the dead plant material, is the material which can become coal over Geologic Time. Deep in the muck in all contemporary marshes, and even marshy lakes, the sediments contain the precursors of coal. Bacteria in the muck are responsible for the formation of the coal precursors (and probably soft coal itself; we have found living bacteria of the same type in even hard coal found in deep coal mines including coal well back in the coal seam).

    (3) Starch is a major molecule used for long term storage of energy in Plants and Animals (Animals convert excess simple sugars (monosaccharides and disaccharides) into Animal starch, called glycogen (a highly branched starch). For Humans, glycogen is stored in the liver and in muscles. It is mobilized by at least 10 minutes of anaerobic exercise, and is replaced by conversion of body fat to glycogen stored in muscles and in the liver.

    (4) Starchs and to a lesser extent celluloses are used as thickeners and ‘binders’ in food preparation. Thickeners, as the name suggests, make fluid foods thicker (for example, the addition of wheat starch [flour] to pork sausage fats thickens the fats to sausage gravy [which in the South is then served over biscuits]). Binders are used to hold prepared foods together. If you attempt to make meatloaf without any binders, you will end up with ‘sloppy Joes.’ If you add an binder (such as crumbled saltine crackers, mostly starch) the meatloaf will hold together and look like meatloaf. There is an Urban Legend that some fast food restraurants add fiber (mostly cellulose) as the filler to make their hamburgers hold together as hamburger patties [this does work].

    (5) Fiber is a dietary requirement for Humans to assist in moving digested food through the digestive tract. It also serves to provide the feeling of being full with less food; and in the contemporary “biggee-sized” restraurant servings, which expect the customers to eat enough food per meal to feed two or three people an adequate amount of food (including many nutrients, but lacking in vegetables and fruits). There is also some clinical data to suggest that fiber intake reduces the risks of some cancers.

Types of Bio-molecules, continued

Works Cited

Biology Corner. “This site serves as a resource site for students in Biology 1& 1A at Granite City High School. The goal of Biology 1& 1A is to provide a general overview of major biological topics. The class includes several labs, including dissections. Biology is a freshman level class, aligned to a college-prep curriculum. Most students taking biology intend to enter college after graduation. As such, the class can be challenging and requires students to spend time studying for tests.8221

    Kimball, John W. 2011. Kimball's Biology Pages, an online biology textbook, ©John W. Kimball, 2011 (downloaded 20 Apr 2011).
Most of the material on this page either came from or was adapted from Kimball's Biology Pages. When his textbook was in print form only, I often used it as a required textbook for my Introductory Biology courses.

    “Dr. Paul's Biology 122 General Biology Home Page”
Johnson County [Kansas] Community College. staff.jccc.net/pdecell/bio122/bio122home.html

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