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By the end of this section, you will be able to:
  • Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
  • Explain the classifications of carbohydrates
  • List common monosaccharides, disaccharides, and polysaccharides

Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch    and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.

Molecular structures

Carbohydrates can be represented by the stoichiometric formula (CH 2 O) n , where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.


Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See [link] for an illustration of the monosaccharides.

The molecular structures of glyceraldehyde, an aldose, and dihydroxyacetone, a ketose, are shown. Both sugars have a three-carbon backbone. Glyceraldehyde has a carbonyl group (c double bonded to O) at one end of the carbon chain with hydroxyl (OH) groups attached to the other carbons. Dihydroxyacetone has a carbonyl group in the middle of the chain and alcohol groups at each end. The molecular structures of linear forms of ribose, a pentose, and glucose, a hexose, are also shown. Both ribose and glucose are aldoses with a carbonyl group at the end of chain,and hydroxyl groups attached to the other carbons.
Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.

The chemical formula for glucose is C 6 H 12 O 6 . In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.

Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C 6 H 12 O 6 ), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon ( [link] ).

Questions & Answers

What's the function of epiglottis
Ugo Reply
What Is The Other Name For Intestinal Juice?
Justin Reply
what is the largestest planet of the universe
rick Reply
what are the types of cell
Bernard Reply
prokaryotic and eukaryotic
prokaryotic cell and eukaryotic cell
what is the protein found in the blood?
Tobias Reply
what is parasitic movement
Emmanuel Reply
Parasitic movement is a problem for all of us. So is its companion, parasitic tension. Parasitic movement is the excess contraction of muscles that you don't actually need to complete an action.
HW a u
am OK how a u
absorption may simply mean utilization of food in the body
what are eukaryotic cells
Thiza Reply
eukaryotic cells which posses a true nucleus that is the DNA is enclosed and covered by a nuclear membrane
what is the mean of pair of chromosomes
Kazula Reply
23 haploid and 23diploid
how are you studying in this quarantine? .. how are you keeping yourselves motivated?
sivajijadhav @815.com
good morning guyz
tell me if you know what can be used...than reading pls hint me pls 🙏🙏🙏
what is the important of sex
Aremu Reply
why did human being need sex?
because he/she have feelings
reproduction...to make more
due to active harmon
One important of sex is to reproduce
to ensure the countinuty of life
all of you are right
for sexual satisfaction and birth
what is momentum
Asiya Reply
The strength or force that allows something to continue or grow stronger or faster as time pass
What is Centripetal Force?
centrepital force is the inward force required to keep a body moving with constant speed in a circular path
what is the test for protein
Takii Reply
List four condition necessary for seed germination
Tedeka Reply
water, light, oxygen and temperature
water, oxygen, light temperature
water oxygen light and temperature
importance of biology
Alabina Reply
importance of boilogy
what is soil
Amina Reply
soil is the upper part of the earth
what is importance of studying biology
soil is the uppermost layer of the earth on which plant grows
soil is defined as the thin surface of the upper most layer of the earth crust on which plants grow
soil is the upper part of the earth which plants grow on
soil is the uppermost layer of the earth in which most of the necessary nutrient are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrients are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrient are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrients are found and serve as habitats for some organisms
soil is the accumulation of lose weathered materials which covers much of the land surface of the earth

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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