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The molecules of ribose and deoxyribose differ by a single oxygen atom.

ure 2.21


The molecules of ribose and deoxyribose differ by a single oxygen atom.

cific protein molecules, which have a wide variety of functions. RNA molecules help to synthesize proteins.

DNA molecules have a unique ability to make copies of, or replicate, themselves. They replicate prior to cell division, and each newly formed cell receives an exact copy of the original cell's DNA molecules. Chapter 4 (p. 122) discusses the storage of information in nucleic acid molecules, use of the information in the manufacture of protein molecules, and how these proteins control metabolic reactions.

Table 2.8 summarizes the four groups of organic compounds. Figure 2.22 shows three-dimensional (space-filling) models of some important molecules, illustrating their shapes. Clinical Application 2.3 describes two techniques used to view human anatomy and physiology.

H Compare the chemical composition of carbohydrates, lipids, proteins, and nucleic acids.

How does an enzyme affect a chemical reaction?

What is likely to happen to a protein molecule that is exposed to intense heat or radiation?

What are the functions of DNA and RNA?

|m1|i organic Compounds in Cells


Elements Present

Building Blocks



Carbohydrates Lipids


C,H,O (often P) C,H,O,N (often S) C,H,O,N,P

Simple sugar Glycerol, fatty acids, phosphate groups Amino acids


Provide energy, cell structure Provide energy, cell structure

Provide cell structure, enzymes, energy Store information for the synthesis of proteins, control cell activities

Glucose, starch Triglycerides, phospholipids, steroids Albumins, hemoglobin


CT Scanning and PET Imaging

Physicians use two techniques — computerized tomography (CT) scanning and positron emission tomography (PET imag-ing)—to paint portraits of anatomy and physiology, respectively.

In CT scanning, an X-ray emitting device is positioned around the region of the body being examined. At the same time, an X-ray detector is moved in the opposite direction on the other side of the body. As these parts move, an X-ray beam passes through the body from hundreds of different angles. Because tissues and organs of varying composition absorb X rays differently, the intensity of X rays reaching the detector varies from position to position. A computer records the measurements made by the X-ray detector and combines them mathematically. This creates on a viewing screen a sectional image of the internal body parts (fig. 2D).

Ordinary X-ray techniques produce two-dimensional images known as radiographs or X rays or films. A CT scan provides three-dimensional information. The CT scan can also clearly differentiate between soft tissues of slightly different densities, such as the liver and kidneys, which cannot be seen in a conventional X-ray image. Thus, a CT scan can often spot abnormal tissue, such as a tumor. For example, a CT scan can tell whether a sinus headache that does not respond to antibiotic therapy is caused by a drug-resistant infection or a tumor.

PET imaging uses radioactive isotopes that naturally emit positrons, which are atypical positively charged electrons, to detect biochemical activity in a specific body part. Useful isotopes in PET imaging include carbon-11, nitrogen-13, oxygen-15, and fluorine-18. When one of these isotopes releases a positron, it interacts with a nearby negatively charged electron. The two particles destroy each other, an event called annihilation. At the moment of destruction, two gamma rays appear and move away from each other in opposite directions. Special equipment detects the gamma radiation.

To produce a PET image of biochemically active tissue, a person is injected with a metabolically active compound that includes a bound positron-emitting isotope. To study the brain, for example, a person is injected with glucose-containing fluorine-18. After the brain takes up the isotope-tagged compound, the person rests her head within a circular array of radiation detectors. A device records each time two gamma rays are emitted simultaneously and travel in opposite directions (the result of annihilation). A computer collects and combines the data and generates a cross-sectional image. The image indicates the location and relative concentration of the radioactive

Figure 2D

Figure 2D

isotope in different regions of the brain and can be used to study those parts metabolizing glucose.

PET images reveal the parts of the brain that are affected in such disorders as Huntington disease, Parkinson disease, epilepsy, and Alzheimer disease, and they are used to study blood flow in vessels supplying the brain and heart. The technology is invaluable for detecting the physiological bases of poorly understood behavioral disorders, such as obsessive-compulsive disorder. In this condition, a person repeatedly performs a certain behavior, such as washing hands, showering, locking doors, or checking to see that the stove is turned off. PET images of people with this disorder reveal intense activity in two parts of the brain that are quiet in the brains of unaffected individuals. Knowing the site of altered brain activity can help researchers develop more directed drug therapy.

In addition to highlighting biochemical activities behind illness, PET scans allow biologists to track normal brain physiology. Figure 2E shows that different patterns of brain activity are associated with learning and with reviewing something already learned.

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