2.5: المركبات العضوية الأساسية لأداء الإنسان
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أهداف التعلم
- حدد أربعة أنواع من الجزيئات العضوية الضرورية لعمل الإنسان
- اشرح الكيمياء الكامنة وراء تقارب الكربون للترابط التساهمي في المركبات العضوية
- قدم أمثلة لثلاثة أنواع من الكربوهيدرات، وحدد الوظائف الأساسية للكربوهيدرات في الجسم
- ناقش أربعة أنواع من الدهون المهمة في وظائف الإنسان
- وصف بنية البروتينات ومناقشة أهميتها بالنسبة للأداء البشري
- حدد اللبنات الأساسية للأحماض النووية وأدوار الحمض النووي والحمض النووي الريبي وATP في الأداء البشري
تتكون المركبات العضوية عادةً من مجموعات من ذرات الكربون المرتبطة تساهميًا بالهيدروجين، وعادة ما تكون الأكسجين، وغالبًا عناصر أخرى أيضًا. تم إنشاؤها بواسطة الكائنات الحية، وهي موجودة في جميع أنحاء العالم، في التربة والبحار، والمنتجات التجارية، وكل خلية من خلايا جسم الإنسان. الأنواع الأربعة الأكثر أهمية للبنية البشرية ووظيفتها هي الكربوهيدرات والدهون والبروتينات والنيوكليوتيدات. قبل استكشاف هذه المركبات، تحتاج أولاً إلى فهم كيمياء الكربون.
كيمياء الكربون
ما يجعل المركبات العضوية في كل مكان هو كيمياء نواة الكربون. تذكر أن ذرات الكربون تحتوي على أربعة إلكترونات في غلاف التكافؤ الخاص بها، وأن قاعدة الثمانية تملي أن الذرات تميل إلى التفاعل بطريقة تكمل غلاف التكافؤ الخاص بها بثمانية إلكترونات. لا تكمل ذرات الكربون قشور التكافؤ الخاصة بها عن طريق التبرع بأربعة إلكترونات أو قبولها. بدلاً من ذلك، يتشاركون الإلكترونات بسهولة عبر الروابط التساهمية.
عادةً ما تتشارك ذرات الكربون مع ذرات الكربون الأخرى، وغالبًا ما تشكل سلسلة كربون طويلة يشار إليها باسم الهيكل العظمي الكربوني. ومع ذلك، عندما يتشاركون، لا يتشاركون جميع إلكتروناتهم حصريًا مع بعضهم البعض. بدلاً من ذلك، تميل ذرات الكربون إلى مشاركة الإلكترونات مع مجموعة متنوعة من العناصر الأخرى، أحدها دائمًا هو الهيدروجين. تسمى مجموعات الكربون والهيدروجين الهيدروكربونات. إذا درست أشكال المركبات العضوية في الجزء المتبقي من هذا الفصل، فسترى العديد منها يحتوي على سلاسل من الهيدروكربونات في منطقة واحدة من المركب.
هناك العديد من التركيبات الممكنة لملء «الوظائف الشاغرة» الأربعة للكربون. قد يتشارك الكربون الإلكترونات مع الأكسجين أو النيتروجين أو الذرات الأخرى في منطقة معينة من المركب العضوي. علاوة على ذلك، قد تكون الذرات التي ترتبط بها ذرات الكربون أيضًا جزءًا من مجموعة وظيفية. المجموعة الوظيفية هي مجموعة من الذرات المرتبطة بروابط تساهمية قوية وتميل إلى العمل في التفاعلات الكيميائية كوحدة واحدة. يمكنك التفكير في المجموعات الوظيفية على أنها «مجموعات» متماسكة بإحكام من غير المحتمل أن ينفصل أعضاؤها. خمس مجموعات وظيفية مهمة في فسيولوجيا الإنسان؛ هذه هي مجموعات الهيدروكسيل والكربوكسيل والأمينو والميثيل والفوسفات (الجدول\(\PageIndex{1}\)).
طاولة\(\PageIndex{1}\)
| المجموعات الوظيفية المهمة في علم وظائف الأعضاء البشرية | ||
|---|---|---|
| مجموعة وظيفية | صيغة هيكلية | الأهمية |
| هيدروكسيل | —O—H | مجموعات الهيدروكسيل قطبية. إنها مكونات جميع الأنواع الأربعة من المركبات العضوية التي تمت مناقشتها في هذا الفصل. إنهم يشاركون في تخليق الجفاف وتفاعلات التحلل المائي. |
| كربوكسيل | أو—ك—أوه | توجد مجموعات الكربوكسيل داخل الأحماض الدهنية والأحماض الأمينية والعديد من الأحماض الأخرى. |
| أمينو | —إن—إتش 2 | توجد المجموعات الأمينية داخل الأحماض الأمينية، وهي اللبنات الأساسية للبروتينات. |
| ميثيل | —سي — إتش 3 | توجد مجموعات الميثيل داخل الأحماض الأمينية. |
| الفوسفات | —P—O 4 2— | توجد مجموعات الفوسفات داخل الفسفوليبيدات والنيوكليوتيدات. |
إن تقارب الكربون للترابط التساهمي يعني أن العديد من الجزيئات العضوية المتميزة والمستقرة نسبيًا مع ذلك تشكل بسهولة جزيئات أكبر وأكثر تعقيدًا. يُشار إلى أي جزيء كبير بالجزيء الكبير (macro-= «large»)، والمركبات العضوية في هذا القسم تتناسب جميعها مع هذا الوصف. ومع ذلك، تتكون بعض الجزيئات الكبيرة من عدة «نسخ» من وحدات مفردة تسمى المونومر (mono- = «one»؛ -mer = «part»). مثل الخرز في قلادة طويلة، ترتبط هذه المونومرات بروابط تساهمية لتكوين بوليمرات طويلة (بولي- = «كثيرة»). هناك العديد من الأمثلة على المونومرات والبوليمرات بين المركبات العضوية.
تشكل المونومرات البوليمرات من خلال الانخراط في تخليق الجفاف (انظر [الرابط]). كما لوحظ سابقًا، يؤدي هذا التفاعل إلى إطلاق جزيء من الماء. يساهم كل مونومر: أحدهما يتخلى عن ذرة الهيدروجين والآخر يتخلى عن مجموعة الهيدروكسيل. تنقسم البوليمرات إلى مونومرات بواسطة التحلل المائي (-التحلل = «التمزق»). يتم كسر الروابط بين المونومرات الخاصة بهم، عن طريق التبرع بجزيء من الماء، مما يساهم بذرة الهيدروجين في المونومر ومجموعة الهيدروكسيل للآخر.
الكربوهيدرات
مصطلح الكربوهيدرات يعني «الكربون المائي». تذكر أن الجذر المائي- يشير إلى الماء. الكربوهيدرات هي جزيء يتكون من الكربون والهيدروجين والأكسجين. في معظم الكربوهيدرات، يوجد الهيدروجين والأكسجين بنفس النسب النسبية التي تتراوح بين اثنين إلى واحد في الماء. في الواقع، الصيغة الكيميائية لجزيء «عام» من الكربوهيدرات هي (CH 2 O) n.
يشار إلى الكربوهيدرات باسم السكريات، وهي كلمة تعني «السكريات». هناك ثلاثة أشكال مهمة في الجسم. السكريات الأحادية هي مونومرات الكربوهيدرات. تتكون السكريات الثنائية (di- = «two») من مونومرين. السكريات هي البوليمرات، ويمكن أن تتكون من مئات الآلاف من المونومرات.
السكريات الأحادية
أحادي السكاريد هو مونومر الكربوهيدرات. خمسة سكريات أحادية مهمة في الجسم. ثلاثة منها هي السكريات السداسية، وتسمى بذلك لأن كل منها يحتوي على ست ذرات من الكربون. هذه هي الجلوكوز والفركتوز والجالاكتوز، كما هو موضح في الشكل\(\PageIndex{1.a}\). السكريات الأحادية المتبقية هي سكريات البنتوس، التي تحتوي كل منها على خمس ذرات من الكربون. وهما الريبوز والديوكسيريبوز، كما هو موضح في الشكل\(\PageIndex{1.b}\).
السكاريد
ثنائي السكاريد هو زوج من السكريات الأحادية. تتكون السكريات الثنائية عن طريق تخليق الجفاف، ويشار إلى الرابطة التي تربط بينها باسم الرابطة الغليكوزيدية (glyco- = «السكر»). ثلاثة سكريات (كما هو موضح في الشكل\(\PageIndex{2}\)) مهمة للبشر. وهي السكروز، الذي يشار إليه عادةً بسكر المائدة؛ واللاكتوز، أو سكر الحليب؛ والمالتوز، أو سكر الشعير. كما يمكنك معرفة أسمائهم الشائعة، فإنك تستهلكها في نظامك الغذائي؛ ومع ذلك، لا يمكن لجسمك استخدامها مباشرة. بدلاً من ذلك، في الجهاز الهضمي، يتم تقسيمها إلى السكريات الأحادية المكونة لها عن طريق التحلل المائي.
شاهد هذا الفيديو لمراقبة تكوين السكاريد. ماذا يحدث عندما يواجه الماء رابطة جليكوسيد؟
السكريات
يمكن أن تحتوي السكريات على عدد قليل إلى ألف أو أكثر من السكريات الأحادية. ثلاثة منها مهمة للجسم (الشكل 1)\(\PageIndex{3}\)):
- Starches are polymers of glucose. They occur in long chains called amylose or branched chains called amylopectin, both of which are stored in plant-based foods and are relatively easy to digest.
- Glycogen is also a polymer of glucose, but it is stored in the tissues of animals, especially in the muscles and liver. It is not considered a dietary carbohydrate because very little glycogen remains in animal tissues after slaughter; however, the human body stores excess glucose as glycogen, again, in the muscles and liver.
- Cellulose, a polysaccharide that is the primary component of the cell wall of green plants, is the component of plant food referred to as “fiber”. In humans, cellulose/fiber is not digestible; however, dietary fiber has many health benefits. It helps you feel full so you eat less, it promotes a healthy digestive tract, and a diet high in fiber is thought to reduce the risk of heart disease and possibly some forms of cancer.
Functions of Carbohydrates
The body obtains carbohydrates from plant-based foods. Grains, fruits, and legumes and other vegetables provide most of the carbohydrate in the human diet, although lactose is found in dairy products.
Although most body cells can break down other organic compounds for fuel, all body cells can use glucose. Moreover, nerve cells (neurons) in the brain, spinal cord, and through the peripheral nervous system, as well as red blood cells, can use only glucose for fuel. In the breakdown of glucose for energy, molecules of adenosine triphosphate, better known as ATP, are produced. Adenosine triphosphate (ATP) is composed of a ribose sugar, an adenine base, and three phosphate groups. ATP releases free energy when its phosphate bonds are broken, and thus supplies ready energy to the cell. More ATP is produced in the presence of oxygen (O2) than in pathways that do not use oxygen. The overall reaction for the conversion of the energy in glucose to energy stored in ATP can be written:
In addition to being a critical fuel source, carbohydrates are present in very small amounts in cells’ structure. For instance, some carbohydrate molecules bind with proteins to produce glycoproteins, and others combine with lipids to produce glycolipids, both of which are found in the membrane that encloses the contents of body cells.
Lipids
A lipid is one of a highly diverse group of compounds made up mostly of hydrocarbons. The few oxygen atoms they contain are often at the periphery of the molecule. Their nonpolar hydrocarbons make all lipids hydrophobic. In water, lipids do not form a true solution, but they may form an emulsion, which is the term for a mixture of solutions that do not mix well.
Triglycerides
A triglyceride is one of the most common dietary lipid groups, and the type found most abundantly in body tissues. This compound, which is commonly referred to as a fat, is formed from the synthesis of two types of molecules (Figure \(\PageIndex{4}\)):
- A glycerol backbone at the core of triglycerides, consists of three carbon atoms.
- Three fatty acids, long chains of hydrocarbons with a carboxyl group and a methyl group at opposite ends, extend from each of the carbons of the glycerol.
Triglycerides form via dehydration synthesis. Glycerol gives up hydrogen atoms from its hydroxyl groups at each bond, and the carboxyl group on each fatty acid chain gives up a hydroxyl group. A total of three water molecules are thereby released.
Fatty acid chains that have no double carbon bonds anywhere along their length and therefore contain the maximum number of hydrogen atoms are called saturated fatty acids. These straight, rigid chains pack tightly together and are solid or semi-solid at room temperature (Figure \(\PageIndex{5.a}\)). Butter and lard are examples, as is the fat found on a steak or in your own body. In contrast, fatty acids with one double carbon bond are kinked at that bond (Figure \(\PageIndex{5.b}\)). These monounsaturated fatty acids are therefore unable to pack together tightly, and are liquid at room temperature. Polyunsaturated fatty acids contain two or more double carbon bonds, and are also liquid at room temperature. Plant oils such as olive oil typically contain both mono- and polyunsaturated fatty acids.
Whereas a diet high in saturated fatty acids increases the risk of heart disease, a diet high in unsaturated fatty acids is thought to reduce the risk. This is especially true for the omega-3 unsaturated fatty acids found in cold-water fish such as salmon. These fatty acids have their first double carbon bond at the third hydrocarbon from the methyl group (referred to as the omega end of the molecule).
Finally, trans fatty acids found in some processed foods, including some stick and tub margarines, are thought to be even more harmful to the heart and blood vessels than saturated fatty acids. Trans fats are created from unsaturated fatty acids (such as corn oil) when chemically treated to produce partially hydrogenated fats.
As a group, triglycerides are a major fuel source for the body. When you are resting or asleep, a majority of the energy used to keep you alive is derived from triglycerides stored in your fat (adipose) tissues. Triglycerides also fuel long, slow physical activity such as gardening or hiking, and contribute a modest percentage of energy for vigorous physical activity. Dietary fat also assists the absorption and transport of the nonpolar fat-soluble vitamins A, D, E, and K. Additionally, stored body fat protects and cushions the body’s bones and internal organs, and acts as insulation to retain body heat.
Fatty acids are also components of glycolipids, which are sugar-fat compounds found in the cell membrane. Lipoproteins are compounds in which the hydrophobic triglycerides are packaged in protein envelopes for transport in body fluids.
Phospholipids
As its name suggests, a phospholipid is a bond between the glycerol component of a lipid and a phosphorous molecule. In fact, phospholipids are similar in structure to triglycerides. However, instead of having three fatty acids, a phospholipid is generated from a diglyceride, a glycerol with just two fatty acid chains (Figure \(\PageIndex{6.a}\)). The third binding site on the glycerol is taken up by the phosphate group, which in turn is attached to a polar “head” region of the molecule. Recall that triglycerides are nonpolar and hydrophobic. This still holds for the fatty acid portion of a phospholipid compound. However, the phosphate-containing group at the head of the compound is polar and thereby hydrophilic. In other words, one end of the molecule can interact with oil, and the other end with water. This makes phospholipids ideal emulsifiers, compounds that help disperse fats in aqueous liquids, and enables them to interact with both the watery interior of cells and the watery solution outside of cells as components of the cell membrane.
Steroids
A steroid compound (referred to as a sterol) has as its foundation a set of four hydrocarbon rings bonded to a variety of other atoms and molecules (see Figure \(\PageIndex{6.b}\)). Although both plants and animals synthesize sterols, the type that makes the most important contribution to human structure and function is cholesterol, which is synthesized by the liver in humans and animals and is also present in most animal-based foods. Like other lipids, cholesterol’s hydrocarbons make it hydrophobic; however, it has a polar hydroxyl head that is hydrophilic. Cholesterol is an important component of bile acids, compounds that help emulsify dietary fats. In fact, the word root chole- refers to bile. Cholesterol is also a building block of many hormones, signaling molecules that the body releases to regulate processes at distant sites. Finally, like phospholipids, cholesterol molecules are found in the cell membrane, where their hydrophobic and hydrophilic regions help regulate the flow of substances into and out of the cell.
Prostaglandins
Like a hormone, a prostaglandin is one of a group of signaling molecules, but prostaglandins are derived from unsaturated fatty acids (see Figure \(\PageIndex{6.c}\)). One reason that the omega-3 fatty acids found in fish are beneficial is that they stimulate the production of certain prostaglandins that help regulate aspects of blood pressure and inflammation, and thereby reduce the risk for heart disease. Prostaglandins also sensitize nerves to pain. One class of pain-relieving medications called nonsteroidal anti-inflammatory drugs (NSAIDs) works by reducing the effects of prostaglandins.
Proteins
You might associate proteins with muscle tissue, but in fact, proteins are critical components of all tissues and organs. A protein is an organic molecule composed of amino acids linked by peptide bonds. Proteins include the keratin in the epidermis of skin that protects underlying tissues, the collagen found in the dermis of skin, in bones, and in the meninges that cover the brain and spinal cord. Proteins are also components of many of the body’s functional chemicals, including digestive enzymes in the digestive tract, antibodies, the neurotransmitters that neurons use to communicate with other cells, and the peptide-based hormones that regulate certain body functions (for instance, growth hormone). While carbohydrates and lipids are composed of hydrocarbons and oxygen, all proteins also contain nitrogen (N), and many contain sulfur (S), in addition to carbon, hydrogen, and oxygen.
Microstructure of Proteins
Proteins are polymers made up of nitrogen-containing monomers called amino acids. An amino acid is a molecule composed of an amino group and a carboxyl group, together with a variable side chain. Just 20 different amino acids contribute to nearly all of the thousands of different proteins important in human structure and function. Body proteins contain a unique combination of a few dozen to a few hundred of these 20 amino acid monomers. All 20 of these amino acids share a similar structure (Figure \(\PageIndex{7}\)). All consist of a central carbon atom to which the following are bonded:
- a hydrogen atom
- an alkaline (basic) amino group NH2 (see Table)
- an acidic carboxyl group COOH (see Table)
- a variable group
Notice that all amino acids contain both an acid (the carboxyl group) and a base (the amino group) (amine = “nitrogen-containing”). For this reason, they make excellent buffers, helping the body regulate acid–base balance. What distinguishes the 20 amino acids from one another is their variable group, which is referred to as a side chain or an R-group. This group can vary in size and can be polar or nonpolar, giving each amino acid its unique characteristics. For example, the side chains of two amino acids—cysteine and methionine—contain sulfur. Sulfur does not readily participate in hydrogen bonds, whereas all other amino acids do. This variation influences the way that proteins containing cysteine and methionine are assembled.
Amino acids join via dehydration synthesis to form protein polymers (Figure \(\PageIndex{8}\)). The unique bond holding amino acids together is called a peptide bond. A peptide bond is a covalent bond between two amino acids that forms by dehydration synthesis. A peptide, in fact, is a very short chain of amino acids. Strands containing fewer than about 100 amino acids are generally referred to as polypeptides rather than proteins.
The body is able to synthesize most of the amino acids from components of other molecules; however, nine cannot be synthesized and have to be consumed in the diet. These are known as the essential amino acids.
Free amino acids available for protein construction are said to reside in the amino acid pool within cells. Structures within cells use these amino acids when assembling proteins. If a particular essential amino acid is not available in sufficient quantities in the amino acid pool, however, synthesis of proteins containing it can slow or even cease.
Shape of Proteins
Just as a fork cannot be used to eat soup and a spoon cannot be used to spear meat, a protein’s shape is essential to its function. A protein’s shape is determined, most fundamentally, by the sequence of amino acids of which it is made (Figure \(\PageIndex{9.a}\)). The sequence is called the primary structure of the protein.
Although some polypeptides exist as linear chains, most are twisted or folded into more complex secondary structures that form when bonding occurs between amino acids with different properties at different regions of the polypeptide. The most common secondary structure is a spiral called an alpha-helix. If you were to take a length of string and simply twist it into a spiral, it would not hold the shape. Similarly, a strand of amino acids could not maintain a stable spiral shape without the help of hydrogen bonds, which create bridges between different regions of the same strand (see Figure \(\PageIndex{9.b}\)). Less commonly, a polypeptide chain can form a beta-pleated sheet, in which hydrogen bonds form bridges between different regions of a single polypeptide that has folded back upon itself, or between two or more adjacent polypeptide chains.
The secondary structure of proteins further folds into a compact three-dimensional shape, referred to as the protein’s tertiary structure (see Figure \(\PageIndex{9.c}\)). In this configuration, amino acids that had been very distant in the primary chain can be brought quite close via hydrogen bonds or, in proteins containing cysteine, via disulfide bonds. A disulfide bond is a covalent bond between sulfur atoms in a polypeptide. Often, two or more separate polypeptides bond to form an even larger protein with a quaternary structure (see Figure \(\PageIndex{9.d}\)). The polypeptide subunits forming a quaternary structure can be identical or different. For instance, hemoglobin, the protein found in red blood cells is composed of four tertiary polypeptides, two of which are called alpha chains and two of which are called beta chains.
When they are exposed to extreme heat, acids, bases, and certain other substances, proteins will denature. Denaturation is a change in the structure of a molecule through physical or chemical means. Denatured proteins lose their functional shape and are no longer able to carry out their jobs. An everyday example of protein denaturation is the curdling of milk when acidic lemon juice is added.
The contribution of the shape of a protein to its function can hardly be exaggerated. For example, the long, slender shape of protein strands that make up muscle tissue is essential to their ability to contract (shorten) and relax (lengthen). As another example, bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are deposited. These elongated proteins, called fibrous proteins, are strong and durable and typically hydrophobic.
In contrast, globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic. The hemoglobin proteins packed into red blood cells are an example (see Figure \(\PageIndex{9.d}\)); however, globular proteins are abundant throughout the body, playing critical roles in most body functions. Enzymes, introduced earlier as protein catalysts, are examples of this. The next section takes a closer look at the action of enzymes.
Proteins Function as Enzymes
If you were trying to type a paper, and every time you hit a key on your laptop there was a delay of six or seven minutes before you got a response, you would probably get a new laptop. In a similar way, without enzymes to catalyze chemical reactions, the human body would be nonfunctional. It functions only because enzymes function.
Enzymatic reactions—chemical reactions catalyzed by enzymes—begin when substrates bind to the enzyme. A substrate is a reactant in an enzymatic reaction. This occurs on regions of the enzyme known as active sites (Figure \(\PageIndex{10}\)). Any given enzyme catalyzes just one type of chemical reaction. This characteristic, called specificity, is due to the fact that a substrate with a particular shape and electrical charge can bind only to an active site corresponding to that substrate.
Binding of a substrate produces an enzyme–substrate complex. It is likely that enzymes speed up chemical reactions in part because the enzyme–substrate complex undergoes a set of temporary and reversible changes that cause the substrates to be oriented toward each other in an optimal position to facilitate their interaction. This promotes increased reaction speed. The enzyme then releases the product(s), and resumes its original shape. The enzyme is then free to engage in the process again, and will do so as long as substrate remains.
Other Functions of Proteins
Advertisements for protein bars, powders, and shakes all say that protein is important in building, repairing, and maintaining muscle tissue, but the truth is that proteins contribute to all body tissues, from the skin to the brain cells. Also, certain proteins act as hormones, chemical messengers that help regulate body functions, For example, growth hormone is important for skeletal growth, among other roles.
As was noted earlier, the basic and acidic components enable proteins to function as buffers in maintaining acid–base balance, but they also help regulate fluid–electrolyte balance. Proteins attract fluid, and a healthy concentration of proteins in the blood, the cells, and the spaces between cells helps ensure a balance of fluids in these various “compartments.” Moreover, proteins in the cell membrane help to transport electrolytes in and out of the cell, keeping these ions in a healthy balance. Like lipids, proteins can bind with carbohydrates. They can thereby produce glycoproteins or proteoglycans, both of which have many functions in the body.
The body can use proteins for energy when carbohydrate and fat intake is inadequate, and stores of glycogen and adipose tissue become depleted. However, since there is no storage site for protein except functional tissues, using protein for energy causes tissue breakdown, and results in body wasting.
Nucleotides
The fourth type of organic compound important to human structure and function are the nucleotides (Figure \(\PageIndex{11}\)). A nucleotide is one of a class of organic compounds composed of three subunits:
- one or more phosphate groups
- a pentose sugar: either deoxyribose or ribose
- a nitrogen-containing base: adenine, cytosine, guanine, thymine, or uracil
Nucleotides can be assembled into nucleic acids (DNA or RNA) or the energy compound adenosine triphosphate.
Nucleic Acids
The nucleic acids differ in their type of pentose sugar. Deoxyribonucleic acid (DNA) is nucleotide that stores genetic information. DNA contains deoxyribose (so-called because it has one less atom of oxygen than ribose) plus one phosphate group and one nitrogen-containing base. The “choices” of base for DNA are adenine, cytosine, guanine, and thymine. Ribonucleic acid (RNA) is a ribose-containing nucleotide that helps manifest the genetic code as protein. RNA contains ribose, one phosphate group, and one nitrogen-containing base, but the “choices” of base for RNA are adenine, cytosine, guanine, and uracil.
The nitrogen-containing bases adenine and guanine are classified as purines. A purine is a nitrogen-containing molecule with a double ring structure, which accommodates several nitrogen atoms. The bases cytosine, thymine (found in DNA only) and uracil (found in RNA only) are pyramidines. A pyramidine is a nitrogen-containing base with a single ring structure
Bonds formed by dehydration synthesis between the pentose sugar of one nucleic acid monomer and the phosphate group of another form a “backbone,” from which the components’ nitrogen-containing bases protrude. In DNA, two such backbones attach at their protruding bases via hydrogen bonds. These twist to form a shape known as a double helix (Figure \(\PageIndex{12}\)). The sequence of nitrogen-containing bases within a strand of DNA form the genes that act as a molecular code instructing cells in the assembly of amino acids into proteins. Humans have almost 22,000 genes in their DNA, locked up in the 46 chromosomes inside the nucleus of each cell (except red blood cells which lose their nuclei during development). These genes carry the genetic code to build one’s body, and are unique for each individual except identical twins.
In contrast, RNA consists of a single strand of sugar-phosphate backbone studded with bases. Messenger RNA (mRNA) is created during protein synthesis to carry the genetic instructions from the DNA to the cell’s protein manufacturing plants in the cytoplasm, the ribosomes.
Adenosine Triphosphate
The nucleotide adenosine triphosphate (ATP), is composed of a ribose sugar, an adenine base, and three phosphate groups (Figure \(\PageIndex{13}\)). ATP is classified as a high energy compound because the two covalent bonds linking its three phosphates store a significant amount of potential energy. In the body, the energy released from these high energy bonds helps fuel the body’s activities, from muscle contraction to the transport of substances in and out of cells to anabolic chemical reactions.
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When a phosphate group is cleaved from ATP, the products are adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis reaction can be written:
Removal of a second phosphate leaves adenosine monophosphate (AMP) and two phosphate groups. Again, these reactions also liberate the energy that had been stored in the phosphate-phosphate bonds. They are reversible, too, as when ADP undergoes phosphorylation. Phosphorylation is the addition of a phosphate group to an organic compound, in this case, resulting in ATP. In such cases, the same level of energy that had been released during hydrolysis must be reinvested to power dehydration synthesis.
Cells can also transfer a phosphate group from ATP to another organic compound. For example, when glucose first enters a cell, a phosphate group is transferred from ATP, forming glucose phosphate (C6H12O6—P) and ADP. Once glucose is phosphorylated in this way, it can be stored as glycogen or metabolized for immediate energy.
Chapter Review
Organic compounds essential to human functioning include carbohydrates, lipids, proteins, and nucleotides. These compounds are said to be organic because they contain both carbon and hydrogen. Carbon atoms in organic compounds readily share electrons with hydrogen and other atoms, usually oxygen, and sometimes nitrogen. Carbon atoms also may bond with one or more functional groups such as carboxyls, hydroxyls, aminos, or phosphates. Monomers are single units of organic compounds. They bond by dehydration synthesis to form polymers, which can in turn be broken by hydrolysis.
Carbohydrate compounds provide essential body fuel. Their structural forms include monosaccharides such as glucose, disaccharides such as lactose, and polysaccharides, including starches (polymers of glucose), glycogen (the storage form of glucose), and fiber. All body cells can use glucose for fuel. It is converted via an oxidation-reduction reaction to ATP.
Lipids are hydrophobic compounds that provide body fuel and are important components of many biological compounds. Triglycerides are the most abundant lipid in the body, and are composed of a glycerol backbone attached to three fatty acid chains. Phospholipids are compounds composed of a diglyceride with a phosphate group attached at the molecule’s head. The result is a molecule with polar and nonpolar regions. Steroids are lipids formed of four hydrocarbon rings. The most important is cholesterol. Prostaglandins are signaling molecules derived from unsaturated fatty acids.
Proteins are critical components of all body tissues. They are made up of monomers called amino acids, which contain nitrogen, joined by peptide bonds. Protein shape is critical to its function. Most body proteins are globular. An example is enzymes, which catalyze chemical reactions.
Nucleotides are compounds with three building blocks: one or more phosphate groups, a pentose sugar, and a nitrogen-containing base. DNA and RNA are nucleic acids that function in protein synthesis. ATP is the body’s fundamental molecule of energy transfer. Removal or addition of phosphates releases or invests energy.
Interactive Link Questions
Watch this video to observe the formation of a disaccharide. What happens when water encounters a glycosidic bond?
Answer: The water hydrolyses, or breaks, the glycosidic bond, forming two monosaccharides.
Review Questions
Q. C6H12O6 is the chemical formula for a ________.
A. polymer of carbohydrate
B. pentose monosaccharide
C. hexose monosaccharide
D. all of the above
Answer: C
Q. What organic compound do brain cells primarily rely on for fuel?
A. glucose
B. glycogen
C. galactose
D. glycerol
Answer: A
Q. Which of the following is a functional group that is part of a building block of proteins?
A. phosphate
B. adenine
C. amino
D. ribose
Answer: C
Q. A pentose sugar is a part of the monomer used to build which type of macromolecule?
A. polysaccharides
B. nucleic acids
C. phosphorylated glucose
D. glycogen
Answer: B
Q. A phospholipid ________.
A. has both polar and nonpolar regions
B. is made up of a triglyceride bonded to a phosphate group
C. is a building block of ATP
D. can donate both cations and anions in solution
Answer: A
Q. In DNA, nucleotide bonding forms a compound with a characteristic shape known as a(n) ________.
A. beta chain
B. pleated sheet
C. alpha helix
D. double helix
Answer: D
Q. Uracil ________.
A. contains nitrogen
B. is a pyrimidine
C. is found in RNA
D. all of the above
Answer: D
Q. The ability of an enzyme’s active sites to bind only substrates of compatible shape and charge is known as ________.
A. selectivity
B. specificity
C. subjectivity
D. specialty
Answer: B
Critical Thinking Questions
Q. If the disaccharide maltose is formed from two glucose monosaccharides, which are hexose sugars, how many atoms of carbon, hydrogen, and oxygen does maltose contain and why?
A. Maltose contains 12 atoms of carbon, but only 22 atoms of hydrogen and 11 atoms of oxygen, because a molecule of water is removed during its formation via dehydration synthesis.
Q. Once dietary fats are digested and absorbed, why can they not be released directly into the bloodstream?
A. All lipids are hydrophobic and unable to dissolve in the watery environment of blood. They are packaged into lipoproteins, whose outer protein envelope enables them to transport fats in the bloodstream.
Glossary
- adenosine triphosphate (ATP)
- nucleotide containing ribose and an adenine base that is essential in energy transfer
- amino acid
- building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-chain
- carbohydrate
- class of organic compounds built from sugars, molecules containing carbon, hydrogen, and oxygen in a 1-2-1 ratio
- denaturation
- change in the structure of a molecule through physical or chemical means
- deoxyribonucleic acid (DNA)
- deoxyribose-containing nucleotide that stores genetic information
- disaccharide
- pair of carbohydrate monomers bonded by dehydration synthesis via a glycosidic bond
- disulfide bond
- covalent bond formed within a polypeptide between sulfide groups of sulfur-containing amino acids, for example, cysteine
- functional group
- group of atoms linked by strong covalent bonds that tends to behave as a distinct unit in chemical reactions with other atoms
- lipid
- class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water
- macromolecule
- large molecule formed by covalent bonding
- monosaccharide
- monomer of carbohydrate; also known as a simple sugar
- nucleotide
- class of organic compounds composed of one or more phosphate groups, a pentose sugar, and a base
- peptide bond
- covalent bond formed by dehydration synthesis between two amino acids
- phospholipid
- a lipid compound in which a phosphate group is combined with a diglyceride
- phosphorylation
- addition of one or more phosphate groups to an organic compound
- polysaccharide
- compound consisting of more than two carbohydrate monomers bonded by dehydration synthesis via glycosidic bonds
- prostaglandin
- lipid compound derived from fatty acid chains and important in regulating several body processes
- protein
- class of organic compounds that are composed of many amino acids linked together by peptide bonds
- purine
- nitrogen-containing base with a double ring structure; adenine and guanine
- pyrimidine
- nitrogen-containing base with a single ring structure; cytosine, thiamine, and uracil
- ribonucleic acid (RNA)
- ribose-containing nucleotide that helps manifest the genetic code as protein
- steroid
- (also, sterol) lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and molecules
- substrate
- reactant in an enzymatic reaction
- triglyceride
- lipid compound composed of a glycerol molecule bonded with three fatty acid chains